Organic matter-solid phase interactions are critical for predicting arsenic release and plant uptake in Bangladesh paddy soils

Differential Toxicity of Arsenic in Daphnia pulex Under Phosphorus and Food Limitation

The on-going anthropogenic degradation of freshwater habitats has drastically altered the environmental supply of both nutrients and common pollutants. Most organisms living in these altered habitats experience interactive effects of various stressors that can initiate adjustments at multiple levels impacting their fitness. Hence, studies measuring response to a single environmental parameter fail to capture the complexities of the status quo. We tested both the individual and the interactive effect of arsenic (As) exposure, food quantity, and dietary phosphorus (P)-supply on six life-history traits (Juvenile Growth Rate; Adult Growth Rate; Age and Size at Maturity, Lifespan, and Fecundity) as surrogates for organismal fitness in the keystone aquatic grazer Daphnia pulex. We also tested the effect of food quantity and P-supply on somatic As accumulation in Daphnia. Our results indicated an influence of P-supply on neonatal growth and an influence of As and food quantity on growth and maintenance later in life. Maturation was strongly influenced by all three variables, with no reproduction observed in the presence of two or more environmental stressors. We found a strong interaction between As and dietary P, with increased P-supply intensifing the toxicity effect of As. No such effects were seen between As and food quantity, indicating a differential role of quantity versus quality on As toxicity. We found a nominal effect of diet on somatic As accumulation. The results from the present study emphasize the importance of considering such interactions between co-occurring environmental stressors and the dietary status of organisms, to better predict and manage impacts and risks associated with common environmental toxicants in highly vulnerable ecosystems. Environ Toxicol Chem 2024;00:1-13. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Effects of citric acid on arsenic transformation and microbial communities in different paddy soils

Root exudate is a major source of soil organic matter and can significantly affect arsenic (As) migration and transformation in paddy soils. Citric acid is the main component of rice root exudate, however, the impacts and rules of citric acid on As bioavailability and rhizobacteria in different soils remains unclear. This study investigated the effects of citric acid on As transformation and microbial community in ten different paddy soils by flooded soil culture experiments. The results showed that citric acid addition increased total As and arsenate (As(V)) in the soil porewater by up to 41-fold and 65-fold, respectively, after 2-h incubation. As(V) was the main As species in soil porewater within 10 days with the addition of citric acid. Non-specifically sorbed As of soils, total Fe and total As were the main environmental factors affecting the soil microbial communities. High-throughput sequencing analysis demonstrated that citric acid addition significantly altered the soil microbial community structure, shifting the Proteobacteria-related reducing bacteria to Firmicutes-related reducing bacteria in different paddy soils. The relative abundance of Firmicutes was promoted by 174-196%. Clostridium-related bacteria belonging to Firmicutes became the dominant genera, which is believed to regulate As release through the reductive dissolution of iron oxides or the direct reduction of As(V) to arsenite (As(III)). However, citric acid addition significantly decreased the relative abundance of Geobacter and Anaeromyxobacter, which are also typical active As(V)- and ferric-reducing bacteria. Real-time quantitative polymerase chain reaction (qPCR) also revealed that the addition of citric acid significantly decreased the relative abundances of Geobacter in the different soils by 8-28 times while the relative abundances of Clostridium increased by 2-5 times. These results provide significant insight on As transformation in different types of rice rhizospheric soils and guidance for the application of rice varieties with low citric acid exuding to restrict As accumulation.

Effects of biodegradable microplastics on arsenic migration and transformation in paddy soils: a comparative analysis with conventional microplastics

The combined pollution of microplastics (MPs) and arsenic (As) in paddy soils has attracted more attention worldwide. However, there are few comparative studies on the effects of biodegradable and conventional MPs on As migration and transformation. Therefore, conventional (polystyrene, polyethylene, polyvinyl chloride) and biodegradable (polybutadiene styrene, polylactic acid, polybutylene adipate terephthalate) MPs were selected to explore and demonstrate their influences and mechanism on As migration from paddy soils to overlying water and As speciation transformation through microcosmic experiment with measuring the changes of As chemical distribution, physicochemical indexes and microbial community in paddy soils. The results showed that biodegradable MPs enhanced As migration and transformation more effective than conventional MPs during 60 d. Biodegradable MPs indirectly increased the content of As(Ⅲ) and bioavailable As by changing the microbial community structure and affecting the biogeochemical cycles of carbon, nitrogen, sulfur and iron in soils, and promoted the As migration and transformation. PBS showed the strongest promoting effect, transforming to more As(Ⅲ) (11.43%) and bioavailable As (4.28%) than control. This helps to a better understanding of the effects of MPs on As biogeochemical cycle and to clarify the ecological and food safety risks of their combined pollution in soils.

Impact of meteorological factors on Cd availability and average concentration prediction in rice growth cycle

During the rice growth cycle, the average available cadmium concentration (CA-Cd) in the soil determines the Cd content in rice plant. Given defined soil properties and rice varieties, the meteorological factors play a crucial role in soil's available cadmium concentration (CCd) during the rice growth cycle. Thus, it is significant to investigate the influence of meteorological factors in CCd during the rice growth cycle and develop a predictive model for CA-Cd. The rice was cultivated under seven different sowing dates in Cd and As-contaminated soil in Hunan Province. Studied the impact of meteorological factors on paddy soil. The results showed that accumulated temperature (AT) and total precipitation (TP) were key factors affecting the soil CCd. The correlation coefficients between AT and TP with soil CA-Cd were 0.98 and -0.94 (p < 0.01), respectively. However, there was no significant correlation with CAs. AT mainly influenced the CCd during the grouting and maturity stages. A straightforward empirical prediction model was developed, capable of accurately forecasting CA-Cd during the rice growth cycle by considering meteorological factors and the initial soil CCd. This study supported a novel foundation for the precise prediction of Cd content in rice.

Modelling the impact of different irrigation regimes and mulching on strawberry crop growth and water use in the arsenic-contaminated Bengal basin

Replacement of water-intensive winter rice with strawberry (Fragaria × ananassa Duch.) may restrict groundwater extraction and improve water productivity and sustainability of agricultural production in the arsenic-contaminated Bengal basin. The potential of strawberry cultivation in terms of yield obtained and water use efficiency need to be evaluated under predominant soil types with mulch applications. Water-driven model AquaCrop was used to predict the canopy cover, soil water storage and above-ground biomass of strawberry in an arsenic-contaminated area in the Bengal basin. After successful calibration and validation over three seasons, AquaCrop was used over a range of management scenarios (nine drip-irrigation × three soil types × four mulch materials) to identify the best irrigation options for a drip-irrigated strawberry crop. The most appropriate irrigation of 176 mm for clay loam soil in lowland and 189 mm for sandy clay loam in medium land rice areas and the use of organic mulch from locally available jute agrotextile improved 1.4 times higher yield and 1.7 times higher water productivity than that of without mulch. Strawberry can be introduced as an alternative crop replacing rice in non-traditional upland and medium land areas of the arsenic-contaminated Bengal basin with 88% lower groundwater extraction load and better economic return to farmers.

An extensive review of arsenic dynamics and its distribution in soil-aqueous-rice plant systems in south and Southeast Asia with bibliographic and meta-data analysis

Millions of people worldwide are affected by arsenic (As) contamination, particularly in South and Southeast Asian countries, where large-scale dependence on the usage of As-contaminated groundwater in drinking and irrigation is a familiar practice. Rice (Oryza sativa) cultivation is commonly done in South and Southeast Asian countries as a preferable crop which takes up more As than any other cereals. The present article has performed a scientific meta-data analysis and extensive bibliometric analysis to demonstrate the research trend in global rice As contamination scenario in the timeframe of 1980-2023. This study identified that China contributes most with the maximum number of publications followed by India, USA, UK and Bangladesh. The two words 'arsenic' and 'rice' have been identified as the most dominant keywords used by the authors, found through co-occurrence cluster analysis with author keyword association study. The comprehensive perceptive attained about the factors affecting As load in plant tissue and the nature of the micro-environment augment the contamination of rice cultivars in the region. This extensive review analyses soil parameters through meta-data regression assessment that influence and control As dynamics in soil with its further loading into rice grains and presents that As content and OM are inversely related and slightly correlated to the pH increment of the soil. Additionally, irrigation and water management practices have been found as a potential modulator of soil As concentration and bioavailability, presented through a linear fit with 95% confidence interval method.

The Molecular Mechanism of the Response of Rice to Arsenic Stress and Effective Strategies to Reduce the Accumulation of Arsenic in Grain

Rice (Oryza sativa L.) is the staple food for more than 50% of the world's population. Owing to its growth characteristics, rice has more than 10-fold the ability to enrich the carcinogen arsenic (As) than other crops, which seriously affects world food security. The consumption of rice is one of the primary ways for humans to intake As, and it endangers human health. Effective measures to control As pollution need to be studied and promoted. Currently, there have been many studies on reducing the accumulation of As in rice. They are generally divided into agronomic practices and biotechnological approaches, but simultaneously, the problem of using the same measures to obtain the opposite results may be due to the different species of As or soil environments. There is a lack of systematic discussion on measures to reduce As in rice based on its mechanism of action. Therefore, an in-depth understanding of the molecular mechanism of the accumulation of As in rice could result in accurate measures to reduce the content of As based on local conditions. Different species of As have different toxicity and metabolic pathways. This review comprehensively summarizes and reviews the molecular mechanisms of toxicity, absorption, transport and redistribution of different species of As in rice in recent years, and the agronomic measures to effectively reduce the accumulation of As in rice and the genetic resources that can be used to breed for rice that only accumulates low levels of As. The goal of this review is to provide theoretical support for the prevention and control of As pollution in rice, facilitate the creation of new types of germplasm aiming to develop without arsenic accumulation or within an acceptable limit to prevent the health consequences associated with heavy metal As as described here.

Study on arsenic speciation, bioaccessibility, and gut microbiota in realgar-containing medicines by DGT technique and artificial gastrointestinal extraction (PBET) combine with simulated human intestinal microbial ecosystem (SHIME)

It is well-known that several Chinese patent medicines use realgar as a specific component. People are more aware of the health dangers associated with realgar since it includes arsenic. Previous research overstated the arsenic toxicity of realgar-containing Chinese prescription medications because little thought was given to the influence of arsenic bioaccessibility by gut microbiota. In light of this, this study examined the total content, bioaccessibility and speciation of targeted medications while also examining intestinal epithelial transit utilizing the diffusive gradients in thin-films (DGT). All samples contained arsenic, and the bioaccessibilities of the colon, intestine and gastric regions ranged from 0.19% to 1.73%, 0.25-1.88% and 0.21-1.70% respectively. The range of DGT-bioaccessibility is 0.01-0.0018%. Three steps of analysis were conducted on inorganic As(III) and As(V). In health risk assessment, the ADDs and HQs of DGT-bioaccessibility were below the threshold levels when compared to computing average daily intake dose (ADD) and hazard quotient (HQ) by bioaccessibility of gastric, intestinal and colon. Additionally, Proteobacteria and Firmicutes were discovered to be the two predominant kinds of gut microbes in this study. Under arsenic exposure, the abundance of Christensenellaceae, Desulfovibrionaceae and Akkermansiaceae increased, but the quantity of Rikenellaceae decreased. These findings revealed that alterations in gut microbiota had an impact on host metabolism.

Exploring the potential: Can arsenic (As) resistant silicate-solubilizing bacteria manage the dual effects of silicon on As accumulation in rice?

Rice (Oryza sativa L.) cultivation in regions marked by elevated arsenic (As) concentrations poses significant health concerns due to As uptake by the plant and its subsequent entry into the human food chain. With rice serving as a staple crop for a substantial share of the global population, addressing this issue is critical for food security. In flooded paddy soils, where As availability is pronounced, innovative strategies to reduce As uptake and enhance agricultural sustainability are mandatory. Silicon (Si) and Si nanoparticles have emerged as potential candidates to mitigate As accumulation in rice. However, their effects on As uptake exhibit complexity, influenced by initial Si levels in the soil and the amount of Si introduced through fertilization. While low Si additions may inadvertently increase As uptake, higher Si concentrations may alleviate As uptake and toxicity. The interplay among existing Si and As availability, Si supplementation, and soil biogeochemistry collectively shapes the outcome. Adding water-soluble Si fertilizers (e.g., Na2SiO3 and K2SiO3) has demonstrated efficacy in mitigating As toxicity stress in rice. Nonetheless, the expense associated with these fertilizers underscores the necessity for low cost innovative solutions. Silicate-solubilizing bacteria (SSB) resilient to As hold promise by enhancing Si availability by accelerating mineral dissolution within the rhizosphere, thereby regulating the Si biogeochemical cycle in paddy soils. Promoting SSB could make cost-effective Si sources more soluble and, consequently, managing the intricate interplay of Si's dual effects on As accumulation in rice. This review paper offers a comprehensive exploration of Si's nuanced role in modulating As uptake by rice, emphasizing the potential synergy between As-resistant SSB and Si availability enhancement. By shedding light on this interplay, we aspire to shed light on an innovative attempt for reducing As accumulation in rice while advancing agricultural sustainability.

Risk assessment of toxic and hazardous metals in paddy agroecosystem by biochar-for bio-membrane applications

Toxic and carcinogenic metal (loid)s, such arsenic (As) and cadmium (Cd), found in contaminated paddy soils pose a serious danger to environmental sustainability. Their geochemical activities are complex, making it difficult to manage their contamination. Rice grown in Cd and As-polluted soils ends up in people's bellies, where it can cause cancer, anemia, and the deadly itai sickness. Solving this issue calls for research into eco-friendly and cost-effective remediation technology to lower rice's As and Cd levels. This research delves deeply into the origins of As and Cd in paddy soils, as well as their mobility, bioavailability, and uptake mechanisms by rice plants. It also examines the current methods and reactors used to lower As and Cd contamination in rice. Iron-modified biochar (Fe-BC) is a promising technology for reducing As and Cd toxicity in rice, improving soil health, and boosting rice's nutritional value. Biochar's physiochemical characteristics are enhanced by the addition of iron, making it a potent adsorbent for As and Cd ions. In conclusion, Fe-BC's biomembrane properties make them an attractive option for remediating As- and Cd-contaminated paddy soils. More efficient mitigation measures, including the use of biomembrane technology, can be developed when sustainable agriculture practices are combined with these technologies.

The key roles of Fe oxyhydroxides and humic substances during the transformation of exogenous arsenic in a redox-alternating acidic paddy soil

Arsenic (As) from mine wastewater is a significant source for acidic paddy soil pollution, and its mobility can be influenced by alternating redox conditions. However, mechanistic and quantitative insights into the biogeochemical cycles of exogenous As in paddy soil are still lacking. Herein, the variations of As species in paddy soil spiking with As(III) or As(V) were investigated in the process of 40 d of flooding followed 20 d of drainage. During flooding process, available As was immobilized in paddy soil spiking As(III) and the immobilized As was activated in paddy soil spiking As(V) owing to deprotonation. The contributions of Fe oxyhydroxides and humic substances (HS) to As immobilization in paddy soil spiking As(III) were 80.16% and 18.64%, respectively. Whereas the contributions of Fe oxyhydroxides and HS to As activation in paddy soil spiking As(V) were 47.9% and 52.1%, respectively. After entering drainage, available As was mainly immobilized by Fe oxyhydroxides and HS and adsorbed As(III) was oxidized. The contribution of Fe oxyhydroxides to As fixation in paddy soil spiking As(III) and As(V) was 88.82% and 90.26%, respectively, and of HS to As fixation in paddy soil spiking As(III) and As(V) was 11.12% and 8.95%, respectively. Based on the model fitting results, the activation of Fe oxyhydroxides and HS bound As followed with available As(V) reduction were key processes during flooding. This may be because the dispersion of soil particles and release of soil colloids activated the adsorbed As. Immobilization of available As(III) by amorphous Fe oxyhydroxides followed with adsorbed As(III) oxidation were key processes during drainage. This may be ascribe to the occurrence of coprecipitation and As(III) oxidation mediated by reactive oxygen species from Fe(II) oxidation. The results are beneficial for a deeper understanding of As species transformation at the interface of paddy soil-water as well as an estimation pathway for the impacts of key biogeochemical cycles on exogenous As species under a redox-alternating condition.

Geochemical characteristics and ecotoxicological risk of arsenic in water-level-fluctuation zone soils of the Three Gorges Reservoir, China

The Three Gorges Reservoir (TGR) has formed the water-level-fluctuation zone (WLFZ) due to reservoir regulation. However, as a sensitive zone in reservoir, little is known about the geochemical process and ecotoxicological risk of arsenic (As) in WLFZ soils under the anti-seasonal flow regulation. Hence, the anthropogenic contamination, mobility and ecotoxicological risks of As in WLFZ soils of the TGR were comprehensively assessed using the geochemical baseline concentration (GBC), chemical fractions, diffusive gradients in thin films (DGT) and toxicity data. The As concentrations in WLFZ soils showed a trend of increasing at the early stage of water impoundment and then stabilizing in recent years, which presented a low ecological risk of As according to the assessment by pollution indices. Based on GBC calculations, the average anthropogenic contribution of As was 13.95 %, indicating a slight influence of human activities. The distribution of labile As measured by DGT in WLFZ soils was mainly controlled by the Fe/Mn oxides, pH and organic matter. The DGT-induced fluxes in soils (DIFS) model further implied that resupply of As to soil solution was partially sustained by the soil solid phase, in which the resupply capacity was low and limited by the adsorption and desorption kinetics. In addition, the DGT was combined with toxicity data to obtain the risk quotient (RQ) and probabilistic risk assessment. The RQ value was lower than 1, indicating a low toxicity risk in WLFZ soils. Furthermore, the As in WLFZ soils had a low probability (5.97E-3 % and 7.77E-2 % in the mainstream and tributary, respectively) of toxic effects toward the aquatic biota. This study provides a comprehensive evaluation for the mobility and toxicity risk of As in WLFZ soils, which is beneficial to the prevention and control of heavy metals pollution in the riparian soils of lakes and reservoirs.

The influence of diverse fertilizer regimes on the phytoremediation potential of Pteris vittata in an abandoned nonferrous metallic mining site

Organic waste comprises a large amount of hydrocarbon containing organic substances, which is regarded as a potential resource rather than simply a waste. A field experiment was conducted in a poly-metallic mining area to investigate the potential of organic waste to facilitate the soil remediation process. Different organic wastes and a commonly used commercial fertilizer were added to heavy metal contaminated soil, which was under phytoremediation using the As hyperaccumulator Pteris vittata. The influence of diverse fertilizer regimes on the biomass of P. vittata and heavy metal removal by P. vittata, was investigated. The soil properties were analyzed after the application of phytoremediation with or without the addition of organic wastes. Results indicated that sewage sludge compost is an appropriate amendment to improve the phytoremediation efficiency. Compared to the control, the application of sewage sludge compost significantly reduced the extractability of As in soil by 26.8 %, and increased the removal of As and Pb by 26.9 % and 186.5 %, respectively. The highest removal of As and Pb reached 33 and 34 kg/ha, respectively. The sewage sludge compost-strengthened phytoremediation improved soil quality. And the diversity and richness of the bacterial community were improved, as represented by the increase in Shannon and Chao index. With improved efficiency and acceptable cost, the organic waste-strengthened phytoremediation can be used to control the risks posed by high concentrations of heavy metals in mining areas.

Short-term arsenic mobilization, labilization, and microbiological aspects after gasoline and diesel addition in tropical soils

The effect of the presence of gasoline and diesel on the speciation and mobility of inorganic arsenic species in tropical topsoils was investigated. Topsoil samples (n = 25) were contaminated with gasoline and diesel (500 mg kg^-1) in laboratory and were incubated under unsaturated conditions and regular aeration for 21 days. Speciation analysis and chemical fractionation were performed in the pore water from control, gasoline, and diesel-contaminated soil samples. Arsenic concentrations were compared to microbiological parameters (microbial metabolic quotient and soil basal breathing) and the presence of ArsM-harboring bacteria. The spike of gasoline and diesel to the topsoils increased pore water As³⁺ (H₃AsO₃) concentration. Arsenic mobilization was lower compared to previously reported data for other sources of organic matter (biochar, litter, and a mixture of sphagnum peat moss and composted poultry manure). However, gasoline or diesel addition mobilized As fractions that were adsorbed to the solid phase, in approximately 60% of the soils. Methylation presented an important role in the As³⁺ regulation in control soils, which was no longer observed after gasoline or diesel addition. The quantification of the labile fractions sampled by the diffusive gradients in thin films technique showed that the increased As concentration in the gasoline or diesel-contaminated soils mostly included inert species. Dissolved organic carbon content seems to be an important control mechanism of the labile As concentration. The increase in As mobility seems to pose a more concerning scenario due to As leaching than to plant uptake.

Enhancement of phytoextraction efficiency coupling Pteris vittata with low-dose biochar in arsenic-contaminated soil

Phytoremediation of arsenic (As) by Pteris vittata (P. vittata) is a cost-effective and environmentally friendly method for restoring As-contaminated sites. However, the phytoextraction efficiency is low in some cases, such as clay soil, thus biochar was applied to enhance the efficiency of As extraction. The paper investigated the effect of biochar on soil characteristic, As mobility, and As uptake in P. vittata with a 90-day greenhouse experiment. Biochar derived from rice straw was added at rates of 0.5, 1.5, and 4% (w/w). The results showed that, under biochar amendment, soil pH raised from 5.24 to 6.03 and 4.91 to 5.85, soil dissolved organic carbon (DOC) increased 11.1-46.1% and 2.8-11.2%, respectively, in rhizosphere and bulk soils. Biochar also increased soil catalase (CAT) activity significantly, especially for the rhizosphere soil. Besides, biochar increased the labile As in the soils and transfer coefficient from roots to aboveground, thereby enhancing As accumulation by P. vittata tissues. The accumulation of As in fronds of P. vittata was up to 350 mg kg-1 in 1.5% biochar, which was more than twice the control and far beyond other biochar treatments. The results indicate that biochar addition is favorable to improve phytoremediation of P. vittata in As-contaminated soil and 1.5% (w/w) biochar may be a reasonable application ratio, thus providing an effective solution to enhance the efficiency of As phytoextraction.

Simulation and risk assessment of arsenic by Hydrus-3D and CalTOX in a typical brownfield site

Accurate quantification of arsenic migration and accumulation in brownfield site is critical for environmental management and soil remediation. However, the researches simulating arsenic in brownfield site in China are limited due to sparse data and complex migration behaviors. In this study, we simulated historic arsenic contamination using Hydrus-3D in an abandoned brownfield site in Hebei, China, from 1972 to 2019. Atmospheric discharge, wastewater leakage, solid waste discharge and tank leakage were calculated according to the factory processes for model simulation. Based on the results of Hydrus-3D, we assessed health risk of arsenic in this site. The results showed that total arsenic input to the soil surface from 4 pathways was 24.6 tons, the solid waste discharge was the highest contributor. The accumulation process mainly occurred in the unsaturated zone due to clay and silty clay absorbed arsenic and thus slow down the migration process. While in the saturation zone, abundant groundwater promoted migration of arsenic, resulting in widespread distribution of contaminated area. The model results represented good performance between simulated and measured values. Sensitivity analysis indicated that adsorption constant and water conductivity were the most influential parameters. Heath risk assessment showed that arsenic contamination continues to threaten resident health.

Insight into the effect of natural organic matter on the photooxidation of arsenite induced by colloidal ferric hydroxides in water

Surface complexation of arsenite (As(III)) on colloidal ferric hydroxide (CFH) plays an important role not only in the adsorptive immobilization of As(III) but also in the subsequent oxidation of As(III) to arsenate (As(V)) through light-induced ligand-to-metal charge transfer (LMCT) in water at near-neutral pH. However, the effects of natural organic matter (NOM), especially humic substances (HSs) and low molecular weight carboxylic acids (CAs), on the photochemistry of the CFH-As(III) system have not been sufficiently understood. In this work, the inhibition of photooxidation of As(III) in terms of the observed apparent rate constant (k_obs) by six HSs (below 16 mg L^-1) and seven CAs (below 2.5 mM) has been observed in water containing 66 μM Fe(III) and 5 μM As(III) at pH 7 under simulated solar irradiation consisting of UVA (λ_max 365 nm) and UVB (λ_max 313 nm) lights. Total inhibition factors (T) have been determined from the combined effect of light-screening factor (S) and competitive complexation factor (C), wherein both S and C varied with NOM concentration. S was obtained by determining the absorbance of NOM, and C was obtained by fitting modified Langmuir or Freundlich models to the amount of As(III) desorbed from CFH upon the addition of NOM. Statistical analysis between the experimental T_exp and the calculated one according to T_cal = S × C showed that the Freundlich model (RMSE for HS 0.1609 and for CA 0.1771) was better than the Langmuir model and was statistically robust (Q_LOO²= 0.691 > 0.5). This work provided an estimation method for the effects of NOM on As(III) photooxidation in the presence of CFH as well as a deeper understanding of the transformation of arsenic species in sunlit water.

A new DGT technique based on nano-sized Mg2Al layered double hydroxides with DTPA for sampling of eight anionic and cationic metals

In this work, a new resin gel incorporated with layered double hydroxide nanoparticles modified with diethylenetriaminepentaacetic acid is developed for application in diffusive gradients in thin-film devices (abbreviated as LDHs DGT) to monitor eight anions and cations (such as Fe, Mn, Co, Ni, Cu, Cd, Pb, and As) in natural waters and soils. The accumulated anions and cations were quantitatively recovered by one-step elution using 0.5 mol·L^-1 HNO₃ with an optimized elution time of 30 min. The performance of the LDHs DGT was independent of solution pH (5-8) and ionic strengths (5-100 mmol·L^-1). The capacities of the LDHs DGT for Mn(II), Fe(II), Co(II), Ni(II), Cu(II), As(V), Cd(II), and Pb(II) individually are determined to be 202.9, 363.6, 246.9, 88.8, 99.5, 75.3, 159.8, and 671.7 μg·cm^-2. During the field deployments in a nature river, LDHs DGT measured concentrations of cations and anions were almost like those measured by the traditional sampling method (except Fe(II), Cd(II), and Co(II)). In addition, bioavailable Cd measured by LDHs DGT correlated well with Cd in rice grains (R² = 0.55), indicating that LDHs DGT is a reliable tool for assessing the risk of Cd.

Enhanced dissolution of arsenic in anaerobic soils upon organic amendment application: acid detergent-soluble organic matter as a potential indicator

Application of organic amendments (OAMs) often enhances arsenic (As) dissolution in paddy soils. Therefore, understanding the properties of OAMs that determine the extent of As dissolution is essential for appropriate soil management. Since As dissolution increases with decrease in soil redox potential caused by microbial respiration, the decomposability of OAMs might be a critical factor controlling As dissolution in amended soils. We hypothesized that contents of acid detergent-soluble organic matter (ADSOM, mainly composed of non-fiber organic matter and hemicellulose) in OAMs can help estimate the potential of OAMs in accelerating As dissolution in soils with added OAMs. Therefore, two contrasting soil types, Andosol and Fluvisol, were mixed with 24 different OAMs and subjected to anaerobic incubation for 14 weeks. Changes in soil Eh and dissolved As contents were monitored throughout the incubation period, and As species in solid phases and ferrous iron (Fe(II)) contents in soils were measured after 2 and 6 weeks of incubation. The higher the ADSOM content in soils with OAMs, the higher the dissolved As contents in soils and the lower the Eh values. Dissolved As also positively correlated with the proportion of As(III) in solid phases and Fe(II) content after 2 and 6 weeks of incubation, indicating that decomposition of ADSOM led to reducing soil conditions, thereby promoting the reduction of As(V) and As-bearing Fe oxides and subsequent As dissolution. The results were consistent between the two types of soils, despite dissolved As content in the Andosol being two orders lower than that in Fluvisol. This is the first study to demonstrate that ADSOM can be a prominent indicator of the potential of OAMs, for promoting As dissolution, when applied to paddy soils.

Distinct response of arsenic speciation and bioavailability to different exogenous organic matter in paddy soil

Land application of organic waste has been increasingly encouraged since it could sequester carbon to mitigate climate change. Considering the susceptibility of arsenic (As) bioavailability in soils to organic matter, understanding the influence of different exogenous organic matter on As biogeochemical behavior in rice-soil system is crucial to reasonably recycle organic waste on soils and ensure the food safety. In this study, impacts of two typical organic matter amendments, rice straw and humic substance, on the As speciation and bioavailability in paddy soil were investigated. Results showed that addition of both rice straw and humic substance could increase the dissolved organic carbon (DOC) content in soil solution by 16.4%-34.4% and 21.7%-53.2%, respectively, but the response of As speciation and bioavailability was quite different, showing the decoupling between As release and DOC. Rice straw addition increased As release to porewater by 28.0%-28.4%, particularly at the initial 0-18 days after the soil was flooded, but humic substance presented the opposite effect, decreasing As release by 27.4%-43.1% which was mainly attributable to the As^III immobilization. This study suggests that the organic matter with high contents of labile heteroaliphatic/aliphatic carbon, being easily to be biodegraded, should not be applied on As contaminated soils.

Arsenic as hazardous pollutant: Perspectives on engineering remediation tools

Arsenic (As) is highly toxic metal (loid) that impairs plant growth and proves fatal towards human population. It disrupts physiological, biochemical and molecular attributes of plants associated with water/nutrient uptake, redox homeostasis, photosynthetic machineries, cell/membrane damage, and ATP synthesis. Numerous transcription factors are responsive towards As through regulating stress signaling, toxicity and resistance. Additionally, characterization of specific genes encoding uptake, translocation, detoxification and sequestration has also explained their underlying mechanisms. Arsenic within soil enters the food chain and cause As-poisoning. Plethora of conventional methods has been used since decades to plummet As-toxicity, but the success rate is quite low due to environmental hazards. Henceforth, exploration of effective and eco-friendly methods is aimed for As-remediation. With the technological advancements, we have enumerated novel strategies to address this concern for practicing such techniques on global scale. Novel strategies such as bioremediation, phytoremediation, mycorrhizae-mediated remediation, biochar, algal-remediation etc. possess extraordinary results. Moreover, nitric oxide (NO), a signaling molecule has also been explored in relieving As-stress through reducing oxidative damages and triggering antioxidative responses. Other strategies such as role of plant hormones (salicylic acid, indole-3-acetic acid, jasmonic acid) and micro-nutrients such as selenium have also been elucidated in As-remediation from soil. This has been observed through stimulated antioxidant activities, gene expression of transporters, defense genes, cell-wall modifications along with the synthesis of chelating agents such as phytochelatins and metallothioneins. This review encompasses the updated information about As toxicity and its remediation through novel techniques that serve to be the hallmarks for stress revival. We have summarised the genetic engineering protocols, biotechnological as well as nanotechnological applications in plants to combat As-toxicity.

Increasing temperature and flooding enhance arsenic release and biotransformations in Swiss soils

Reductive dissolution is one of the main causes for arsenic (As) mobilisation in flooded soils while biomethylation and biovolatilisation are two microbial mechanisms that greatly influence the mobility and toxicity of As. Climate change results in more extreme weather events such as flooding and higher temperatures, potentially leading to an increase in As release and biotransformations. Here, we investigated the effects of flooding and temperature on As release, biomethylation and biovolatilisation from As-rich soils with different pH and source of As (one acidic and anthropogenic (Salanfe) and one neutral and geogenic (Liesberg)). Flooded soils incubated at 23 °C for two weeks showed a ~ 3-fold (Liesberg site) and ~ 7-fold (Salanfe site) increase in the total As concentration of soil solution compared to those incubated at 18 °C. Methyl- and thio-As species were found in the acidic soil and soil solution. High temperatures enhanced thiolation and methylation although inorganic As was predominant. We also show that volatile As fluxes increased more than 4-fold between treatments, from 18 ± 5 ng/kg/d at 18 °C to 75 ± 6 ng/kg/d at 23 °C from Salanfe soil. Our results suggest that high As soils with acidic pH can become an important source of As to the surrounding environment according to realistic climatic scenarios, and that biovolatilisation is very sensitive to increases in temperature. This study provides new data and further justifies further investigations into climate-induced changes on As release and speciation and its links to important factors such as microbial ecology and sulfate or iron biogeochemistry. SYNOPSIS: In the studied Swiss soils, elevated temperature increases arsenic mobility through volatilisation and methylation.

Hydrogeochemical analysis of groundwater quality for drinking and irrigation with elevated arsenic and potential impact on agro-ecosystem in the upper Brahmaputra plain, India

The upper Brahmaputra floodplain is considerably enriched with geogenic contaminants, mainly Arsenic (As) exhibiting high spatial heterogeneity. With this concern, the present study was conducted in Lakhimpur district located in the upper Brahmaputra floodplain aiming to assess the groundwater (GW) suitability for drinking and irrigation. Chemical analysis of 78 GW samples revealed significantly high As (max 0.12 mgL^-1, mean 0.02 mgL^-1) followed by iron (max 62.2 mgL^-1, mean 19.94 mgL^-1) showing uneven distribution. Hydrogeochemical analysis of the GW samples revealed that majority of the water samples belong to mixed Ca²⁺- Na⁺- HCO₃^- type with weathering and forward ion exchange as the dominant processes in the aquifer system. Positive correlation among iron (Fe) and As, together with close grouping in hierarchical cluster analysis and principal components analysis, indicated the involvement of reductive hydrolysis process. Calculation of saturation indices indicated that, precipitation of Fe minerals may lead to de-coupling of Fe and As contributing to the enrichment of both the elements in the GW. Further, assessment of key parameters for irrigation water quality including water quality index indicated the GW to be suitable for irrigation purpose. Finally, toxicity implications of crops produced from As contaminated water indicated higher accumulation potential of As in the food grains implying significant impacts on the agro-ecosystem and associated health hazards.

Impact of different manure-derived dissolved organic matters on the fate of arsenic-antibiotic in co-contaminated paddy soils

Manure application increases the transfer risk of antibiotic resistance to farmland. Especially, its impact remains unclear when it occurs in arsenic (As)-contaminated paddy soils, which is considered as a global environmental problem. In this work, we investigated the fate of antibiotic resistance genes (ARGs) in As-antibiotic co-contaminated paddy soils under the application of manure from different sources (pig manure, cow dung, and chicken manure). Differences in the aliphatic carbon and electron-donating capacities of these dissolved organic matters (DOM) regulated the transformation of iron and As by both biotic and abiotic processes. The regulation by pig manure was stronger than that by cow dung and chicken manure. DOM regulation increased the abundance of As-related functional genes (arsC, arrA, aioA, and arsM) in the soil and accelerated the transformation of As speciation, the highest proportion of As(III) being 45%-61%. Meanwhile, the continuous selection pressure provided by the highly toxic As(III) increased the risk of ARGs and mobile genetic elements (MGEs) via horizontal gene transfer. As-resistant bacteria, including Bacillus, Geobacter, and Desulfitobacterium, were finally considered as potential host bacteria for ARGs and MGEs. In summary, this study clarified the synergistic mechanism of As-antibiotic on the fate of ARGs in co-contaminated paddy soils, and provided practical guidance for the proper application of organic fertilizers.

Tetracycline-Induced Release and Oxidation of As(III) Coupled with Concomitant Ferrihydrite Transformation

Cocontamination with tetracycline (TC) and arsenic (As) is very common in paddy fields. However, the process and underlying mechanism of arsenite (As(III)) transformation on iron mineral surfaces in the presence of antibiotic contaminants remain unclear. In this study, the release and oxidation of As(III) on ferrihydrite (Fh) surfaces and Fh transformation in the presence of TC under both aerobic and anaerobic conditions were investigated. Our results indicated that the TC-induced reductive dissolution of Fh (Fe(II) release) and TC competitive adsorption significantly promote the release of As, especially under anaerobic conditions. The release of As was increased with increasing TC concentration, whereas it decreased with increasing pH. Interestingly, under both aerobic and anaerobic conditions, the addition of TC enhanced the oxidation of As(III) by Fh and induced the partial transformation of Fh to lepidocrocite. Under aerobic conditions, the adsorbed Fe(II) activated the production of reactive oxygen species (·OH and ¹O₂) from dissolved O₂, with Fe(IV) being responsible for As(III) oxidation. Under anaerobic conditions, the abundant oxygen vacancies of Fh affected the oxidation of As(III) during Fh recrystallization. Thus, this study provided new insights into the role of TC on the migration and transformation of As coupled with Fe in soils.

Synergistic use of siderophores and weak organic ligands during zinc transport in the rhizosphere controlled by pH and ion strength gradients

Citrate (Cit) and Deferoxamine B (DFOB) are two important organic ligands coexisting in soils with distinct different affinities for metal ions. It has been theorized that siderophores and weak organic ligands play a synergistic role during the transport of micronutrients in the rhizosphere, but the geochemical controls of this process remain unknown. Here we test the hypothesis that gradients in pH and ion strength regulate and enable the cooperation. To this end, first we use potentiometric titrations to identify the dominant Zn(II)–Cit and Zn(II)–DFOB complexes and to determine their ionic strength dependent stability constants between 0 and 1 mol dm⁻³. We parametrise the Extended Debye-Hückel (EDH) equation and determine accurate intrinsic association constants (logβ⁰) for the formation of the complexes present. The speciation model developed confirms the presence of [Zn(Cit)]⁻, [Zn(HCit)], [Zn₂(Cit)₂(OH)₂]⁴⁻, and [Zn(Cit)₂]⁴⁻, with [Zn(Cit)]⁻ and [Zn₂(Cit)₂(OH)₂]⁴⁻ the dominant species in the pH range relevant to rhizosphere. We propose the existence of a new [Zn(Cit)(OH)₃]⁴⁻ complex above pH 10. We also verify the existence of two hexadentate Zn(II)–DFOB species, i.e., [Zn(DFOB)]⁻ and [Zn(HDFOB)], and of one tetradentate species [Zn(H₂DFOB)]⁺. Second, we identify the pH and ionic strength dependent ligand exchange points (LEP) of Zn with citrate and DFOB and the stability windows for Zn(II)–Cit and Zn(II)–DFOB complexes in NaCl and rice soil solutions. We find that the LEPs fall within the pH and ionic strength gradients expected in rhizospheres and that the stability windows for Zn(II)–citrate and Zn(II)–DFOB, i.e., low and high affinity ligands, can be distinctly set off. This suggests that pH and ion strength gradients allow for Zn(II) complexes with citrate and DFOB to dominate in different parts of the rhizosphere and this explains why mixtures of low and high affinity ligands increase leaching of micronutrients in soils. Speciation models of soil solutions using newly determined association constants demonstrate that the presence of dissolved organic matter and inorganic ligands (i.e., bicarbonate, phosphate, sulphate, or chlorides) do neither affect the position of the LEP nor the width of the stability windows significantly. In conclusion, we demonstrate that cooperative and synergistic ligand interaction between low and high affinity ligands is a valid mechanism for controlling zinc transport in the rhizosphere and possibly in other environmental reservoirs such as in the phycosphere. Multiple production of weak and strong ligands is therefore a valid strategy of plants and other soil organisms to improve access to micronutrients.

Biochar mitigates bioavailability and environmental risks of arsenic in gold mining tailings from the eastern Amazon

Artisanal gold mining has generated tailings highly contaminated by arsenic (As) in Cachoeira do Piriá, eastern Amazon, leading to severe risks to the environment. Such risks should be mitigated considering the bioavailable concentration of the element, since it implies immediate damage to the ecosystem. The objective of this study was to evaluate the potential of biochars in mitigating the environmental risks of bioavailable As concentrations in gold mining tailings from underground and cyanidation exploration. The biochar addition increased mineral components, cation retention, phosphorus in all fractions, and organic and inorganic carbon. The bioavailability of As was reduced after adding the biochars, following the order palm kernel cake biochar > Brazil nut shell biochar > açaí seed biochar, with reductions of up to 13 mg kg-1 in the underground mining tailings and 17 mg kg-1 in the cyanidation mining tailings. These results contributed to the statistically significant reduction of the environmental risks in both mining tailings (6-17% in the underground mining tailings and 9-20% in the cyanidation mining tailings), which was emphasized by Pearson's correlation and multivariate analyzes. The incorporation of the bioavailable fractions of As (from sequential extraction) in the environmental risk assessment was a promising method for evaluating the efficiency of biochars in mitigating the damage caused by this metalloid in gold mining tailings.

Field observations to establish the impact of fluvial flooding on potentially toxic element (PTE) mobility in floodplain soils

Inundation of river water during flooding deposits contaminated sediments onto floodplain topsoil. Historically, floodplains were considered an important sink for potentially toxic elements (PTEs). With increasing flood frequency and duration, due to climate change and land use change, it is important to understand the impact that further flooding may have on this legacy contamination. In this study a field-based approach was taken, extracting soil pore waters by centrifugation of soils sampled on multiple occasions from multiple locations across a floodplain site, which lies adjacent to the River Loddon in southeast England. Flooding generally decreased pore water PTE concentrations and significantly lower pore water concentrations of Cd, Cu, and Cr were found post-flood compared to pre-flood. The dominant process responsible for this observation was precipitation with sulphides resulting in PTE removal from the pore water post-flood. The changes in pH were found to be associated with the decreased pore water concentration of Cu, which suggests the pH rise may have aided adsorption mechanisms or precipitation with phosphates. The impact of flooding on the release and retention of PTEs in floodplain soils is the net effect of several key processes occurring concurrently. It is important to understand the dominant processes that drive mobility of individual PTEs on specific floodplains so that site-specific predictions can determine the impact of future floods on the environmental fate of legacy contaminants.

Assessing the fractionation and bioavailability of heavy metals in soil-rice system and the associated health risk

This study developed a method to build relationships between chemical fractionations of heavy metals in soils and their accumulations in rice and estimate the respective contribution of each geochemical speciation in the soils from the Yangtze River Delta, China. In contaminated areas, residue and humic acid-bound fractions in soils were the main phases for most heavy metals. The mobility of heavy metals was in this following order: Cd > Pb ≈ Zn > Ni > As ≈ Cr > Hg. Transfer factors calculated by the ratios of specific fractionations of heavy metals in the soil-rice system were used to assess the capability of different metal speciation transfer from soil to rice. The carbonate and Fe/Mn oxyhydroxides bound phase had significant positive correlations with total metal concentrations in rice. Hg uptake by rice might be related to the exchangeable and carbonate-bound fractions of soil Hg. Results of PCA analysis of transfer factors estimated that the labile fractions (i.e. water soluble, exchangeable and carbonate bound) contributed more than 40% of the heavy metal accumulations in rice. Effect of organic matter and residue fraction on metals transfer was estimated to be ~ 25 to  ~ 30% while contribution of humic acid and Fe/Mn oxyhydroxides-bound fractions was estimated to be ~ 20 to  ~ 30%. Modified risk assessment code (mRAC) and ecological contamination index (ECI) confirmed that the soil samples were polluted by heavy metals. Soil Cd contributed more than 80% of mRAC. Contrarily, the main contributors to ECI were identified as As, Hg, Pb and Zn. The average values of total target hazard quotient (TTHQ) and Risk_total were above 1 and 10^-4 respectively, implying people living in the study area were exposed to both non-carcinogenic and carcinogenic risk. As and Pb were the main contributor to high TTHQ value while As, Cd and Cr in rice contributed mostly to Risk_total value. Spatial changes of ecological risk indexes and human health risk indexes showed that the samples with high TTHQ values distributed in the area with high values of mRAC. Likewise, the area with high ECI values and with high carcinogenic risk overlapped.

The chemical-microbial release and transformation of arsenic induced by citric acid in paddy soil

Citric acid (CA) is the major exudate of rice roots, yet the effects of CA on arsenic (As) transformation and microbial community in flooded paddy soil have not been clearly elucidated. In this study, microcosms were established by amending CA to As contaminated paddy soils, mimicking the rhizosphere environment. Results showed that 0.5% CA addition significantly enhanced As mobilization after one-hour incubation, increased total As in porewater by about 20-fold. CA addition induced arsenate release into porewater, and subsequently formed ternary complex of As, iron and organic matters, inhibiting further As transformation (including arsenate reduction and arsenite methylation). Furthermore, the results of linear discriminant analysis (LDA) effect size (LEfSe) and network analysis revealed that CA addition significantly enriched bacteria associated with arsenic and iron reductions, such as Clostridium (up to 35-fold) and Desulfitobacterium (up to 4-fold). Our results suggest that CA exhibits robust ability to mobilize As through both chemical and microbial processes, increasing the risk of As accumulation by rice. This study sheds light on our understanding of As mobilization and transformation in rhizosphere soil, potentially providing effective strategies to restrict As accumulation in food crops by screening or cultivating varieties with low CA exuding.

Review on the interactions of arsenic, iron (oxy)(hydr)oxides, and dissolved organic matter in soils, sediments, and groundwater in a ternary system

High concentrations of arsenic (As) in groundwater threaten the environment and public health. Geogenically, groundwater As contamination predominantly occurs via its mobilization from underground As-rich sediments. In an aquatic ecosystem, As is typically driven by several underlying processes, such as redox transitions, microbially driven reduction of iron (Fe) oxide minerals, and release of associated As. Notably, dissolved As mobilized from soils and sediments exhibits high affinity for dissolved organic matter (DOM). Thus, high DOM concentrations can increase As mobility. Therefore, it is crucial to understand the complex interactions and biogeochemical cycling of As, DOM, and Fe oxides. This review collates knowledge regarding the fate of As in multicomponent As-DOM-Fe systems, including ternary complexes involving both Fe and DOM. Additionally, the release mechanisms of As from sediments into groundwater in the presence of both Fe and DOM have been discussed. The mechanisms of As mobilization/sorption at the solid-water interface can be affected by negatively charged DOM competing for sorption sites with As on Fe (oxy)(hydr)oxides and may be further modified by other anionic ubiquitous species such as phosphate, silicic acid, or sulfur. This review emphasizes the need for a comprehensive understanding of the impact of DOM, Fe oxides, and related biogeochemical processes on As mobilization to aquifers. The review identifies important knowledge gaps that may aid in developing applicable practices for preventing the spread of As contamination in aquatic resources and traditional soil management practices.

Arsenic contamination, impact and mitigation strategies in rice agro-environment: An inclusive insight

Arsenic (As) contamination and its adverse consequences on rice agroecosystem are well known. Rice has the credit to feed more than 50% of the world population but concurrently, rice accumulates a substantial amount of As, thereby compromising food security. The gravity of the situation lays in the fact that the population in theAs uncontaminated areas may be accidentally exposed to toxic levels of As from rice consumption. In this review, we are trying to summarize the documents on the impact of As contamination and phytotoxicity in past two decades. The unique feature of this attempt is wide spectrum coverages of topics, and that makes it truly an interdisciplinary review. Aprat from the behaviour of As in rice field soil, we have documented the cellular and molecular response of rice plant upon exposure to As. The potential of various mitigation strategies with particular emphasis on using biochar, seed priming technology, irrigation management, transgenic variety development and other agronomic methods have been critically explored. The review attempts to give a comprehensive and multidiciplinary insight into the behaviour of As in Paddy -Water - Soil - Plate prospective from molecular to post-harvest phase. From the comprehensive literature review, we may conclude that considerable emphasis on rice grain, nutritional and anti-nutritional components, and grain quality traits under arsenic stress condition is yet to be given. Besides these, some emerging mitigation options like seed priming technology, adoption of nanotechnological strategies, applications of biochar should be fortified in large scale without interfering with the proper use of biodiversity.

Heavy metals and metalloids concentrations across UK urban horticultural soils and the factors influencing their bioavailability to food crops

Urban horticulture (UH) has been proposed as a solution to increase urban sustainability, but the potential risks to human health due to potentially elevated soil heavy metals and metalloids (HM) concentrations represent a major constraint for UH expansion. Here we provide the first UK-wide assessment of soil HM concentrations (total and bioavailable) in UH soils and the factors influencing their bioavailability to crops. Soils from 200 allotments across ten cities in the UK were collected and analysed for HM concentrations, black carbon (BC) and organic carbon (OC) concentrations, pH and texture. We found that although HM are widespread across UK UH soils, most concentrations fell below the respective UK soil screening values (C4SLs): 99 % Cr; 98 % As, Cd, Ni; 95 % Cu; 52 % Zn. However, 83 % of Pb concentrations exceeded C4SL, but only 3.5 % were above Pb national background concentration of 820 mg kg^-1. The bioavailable HM concentrations represent a small fraction (0.01-1.8 %) of the total concentrations even for those soils that exceeded C4SLs. There was a significant positive relationship between both total and bioavailable HM and soil BC and OC concentrations. This suggest that while contributing to the accumulation of HM concentrations in UH soils, BC and OC may also provide a biding surface for the bioavailable HM concentrations contributing to their immobilisation. These findings have implications for both management of the risk to human health associated with UH growing in urban soils and with management of UH soil. There is a clear need to understand the mechanisms driving soil-to-crop HM transfer in UH to improve potentially restrictive C4SL (e.g. Pb) especially as public demand for UH land is growing. In addition, the UH community would benefit from education programs promoting soil management practices that reduce the risk of HM exposure - particularly in those plots where C4SLs were exceeded.

Arsenic behavior in soil-plant system and its detoxification mechanisms in plants: A review

Arsenic (As) is one of the most toxic and cancer-causing metals which is generally entered the food chain via intake of As contaminated water or food and harmed the life of living things especially human beings. Therefore, the reduction of As content in the food could be of great importance for healthy life. To reduce As contamination in the soil and food, the evaluation of plant-based As uptake and transportation mechanisms is critically needed. Different soil factors such as physical and chemical properties of soil, soil pH, As speciation, microbial abundance, soil phosphates, mineral nutrients, iron plaques and roots exudates effectively regulate the uptake and accumulation of As in different parts of plants. The detoxification mechanisms of As in plants depend upon aquaporins, membrane channels and different transporters that actively control the influx and efflux of As inside and outside of plant cells, respectively. The xylem loading is responsible for long-distance translocation of As and phloem loading involves in the partitioning of As into the grains. However, As detoxification mechanism based on the clear understandings of how As uptake, accumulations and translocation occur inside the plants and which factors participate to regulate these processes. Thus, in this review we emphasized the different soil factors and plant cell transporters that are critically responsible for As uptake, accumulation, translocation to different organs of plants to clearly understand the toxicity reasons in plants. This study could be helpful for further research to develop such strategies that may restrict As entry into plant cells and lead to high crop yield and safe food production.

Fate of arsenic in living systems: Implications for sustainable and safe food chains

Arsenic, a group 1 carcinogen for humans, is abundant as compared to other trace elements in the environment and is present mainly in the Earth's crust and soil. The arsenic distributions in different geographical regions are dependent on their geological histories. Anthropogenic activities also contribute significantly to arsenic release into the environment. Arsenic presents several complications to humans, animals, and plants. The physiology of plants and their growth and development are affected by arsenic. Arsenic is known to cause cancer and several types of organ toxicity, such as cardiotoxicity, nephrotoxicity, and hepatotoxicity. In the environment, arsenic exists in variable forms both as inorganic and organic species. From arsenic containing compartments, plants can absorb and accumulate arsenic. Crops grown on these contaminated soils pose several-fold higher toxicity to humans compared with drinking water if arsenic enters the food chain. Information regarding arsenic transfer at different trophic levels in food chains has not been summarized until now. The present review focuses on the food chain perspective of arsenic, which affects all components of the food chain during its course. The circumstances that facilitate arsenic accumulation in flora and fauna, as components of the food chain, are outlined in this review.

A predictive model for arsenic accumulation in rice grains based on bioavailable arsenic and soil characteristics

Arsenic (As) is a well-known human carcinogen, and rice consumption is the main way Chinese people are exposed to As. In this study, 14 kinds of paddy soils were collected from the main rice-producing areas in China. The results showed that rice roots and leaves accumulated more As than stems and grains in the following sequence: As_root> As_leaf> As_stem> As_grain. The accumulation of As by rice grains mainly depends on the total As and bioavailable As (0.43 mol/L HNO₃ extractable As), which explained 32.2% and 22.2% of the variation in the grain As, respectively. In addition, soil pH, organic matter (OM) and clay contents were the major factors affecting grain As, explaining 13.1%, 7.9% and 5.3% of the variation, respectively. An effective prediction model was established via multiple linear regression as As_grain= 0.024 BAs - 0.225 pH+ 0.013 OM+ 0.648 EC - 0.320 TN - 0.088 TP - 0.002 AS+ 2.157 (R² =0.68, P < 0.01). Through the verification of the samples from both pot experiments and paddy fields, the model successfully provided accurate predictions for rice grain As.

Arsenic biogeochemical cycling in paddy soil-rice system: Interaction with various factors, amendments and mineral nutrients

Arsenic (As) contamination is a well-recognized environmental and health issue, threatening over 200 million people worldwide with the prime cases in South and Southeast Asian and Latin American countries. Rice is mostly cultivated under flooded paddy soil conditions, where As speciation and accumulation by rice plants is controlled by various geo-environmental (biotic and abiotic) factors. In contrast to other food crops, As uptake in rice has been found to be substantially higher due to the prevalence of highly mobile and toxic As species, arsenite (As(III)), under paddy soil conditions. In this review, we discussed the biogeochemical cycling of As in paddy soil-rice system, described the influence of critical factors such as pH, iron oxides, organic matter, microbial species, and pathways affecting As transformation and accumulation by rice. Moreover, we elucidated As interaction with organic and inorganic amendments and mineral nutrients. The review also elaborates on As (im)mobilization processes and As uptake by rice under the influence of different mineral nutrients and amendments in paddy soil conditions, as well as their role in mitigating As transfer to rice grain. This review article provides critical information on As contamination in paddy soil-rice system, which is important to develop suitable strategies and mitigation programs for limiting As exposure via rice crop, and meet the UN's key Sustainable Development Goals (SDGs: 2 (zero hunger), 3 (good health and well-being), 12 (responsible consumption and production), and 13 (climate action)).

Rice Rhizospheric Effects on the Bioavailability of Toxic Trace Elements during Land Application of Biochar

Land application of biochar, the product of organic waste carbonization, can improve soil fertility as well as sequester carbon to mitigate climate change. In addition, biochar can greatly influence the bioavailability of toxic trace elements (TTEs) in soils resulting from its large internal surface areas, abundance in organic carbon, and ability to modify soil pH. Most research to date employs batch leaching tests to predict how biochar addition impacts TTE bioavailability, but these ex situ tests rarely considered the rhizospheric effect which might offset or intensify the changes induced by organic residue addition. This is especially so in rice rhizospheres because of strong clines in localized redox conditions. In this study, we adopted in situ high-resolution (HR) diffusive gradients in thin films (DGT) as well as rhizo-bag porewater sampling experiments to depict an overall picture of the difference in TTE (As, Cd, Cu, Ni, and Pb) bioavailability between the rice rhizosphere and bulk soils during land application of biochar. Porewater sampling experiments revealed that biochar additions stimulated TTE release due to the increase of dissolved organic carbon (DOC) and H⁺ concentrations. In the rhizosphere, although biochar still promoted As, Cd, and Ni release into porewaters, the rhizospheric effect was one of dampening/reduction compared with the bulk soil. When we focused on the localized changes of TTE bioavailability in the rhizosphere using an in situ HR-DGT approach, on the contrary, flux maxima of Cd, Cu, and Ni occurred near/on the root surface, and hot spots of As can be observed at peripheries of the rooting zone, which demonstrated the high heterogeneity and complexity of the rhizosphere's influence on TTE bioavailability.

Influences on tidal channel and aquaculture shrimp pond water chemical composition in Southwest Bangladesh

Detailed geochemical studies of both major and minor elements in Bangladesh surface waters are sparse, particularly in shrimp aquaculture pond environments. Therefore, water samples from shrimp aquaculture ponds and tidal channels were collected in high precipitation (July) and low precipitation (May) months from 2018-2019 in Southwest Bangladesh and analyzed for complete water chemistry. Selenium (Se) and arsenic (As) were elevated above WHO guidelines in 50% and ~ 87% of samples, respectively, but do not show any recognizable spatial patterns. Shrimp pond and tidal channel water compositions in the dry season (May) are similar, illustrating their connectivity and minimal endogenous effects within shrimp ponds. Tidal channels are less saline in July than shrimp ponds still irrigated by tidal channels, suggesting that either farmers limit irrigation to continue farming saltwater shrimp, or the irrigation flux is low and leads to a lag in aquaculture-tidal channel compositional homogenization. δ¹⁸O and δ²H isotopic compositions from samples in May of 2019 reveal tidal channel samples are closer to the local meteoric water line (LMWL) than shrimp pond samples, because of less evaporation. However, evaporation in May shrimp ponds has a minimal effect on water composition, likely because of regular drainage/exchange of pond waters. Dissolved organic carbon (DOC) is positively correlated with both δ¹⁸O and δ²H in shrimp ponds, suggesting that as evaporation increases, DOC becomes enriched. Multiple linear regression reveals that As and Se can be moderately predicted (adjusted R² values between 0.4 and 0.7, p < 0.01) in surface waters of our study with only 3-4 independent predictor variables (e.g., Ni, V and DOC for Se prediction; Cu, V, Ni and P for As prediction). Thus, this general approach should be followed in other regions throughout the world when measurements for certain hazardous trace elements such as Se and As may be lacking in several samples from a dataset.

Periphyton enhances arsenic release and methylation at the soil-water interface of paddy soils

Periphyton is ubiquitous in rice paddy fields, however its role in paddy soil arsenic (As) biogeochemistry remains unexplored. In this study, microcosm incubations and extensive field sampling were used to better understand the roles of periphyton on As mobility and transformation at the soil-water interface. Microcosm incubations revealed that periphyton on the paddy soil surface enhanced As release to water and increased methylated As contents at the soil-water interface. Experimental additions of dissolved phosphate did not significantly affect these processes. The presence of periphyton increased the dissolved organic carbon (DOC) content of the surface soil which may have played a role in the increased As mobility. However, the increase in methylated As species at the soil-water interface is indicative of detoxification processes of As by periphyton. The results from the field study revealed a high abundance and diversity of As biotransformation and detoxification genes in periphyton. Genera of Kineosporia, Limisphaera, Ornatilinea, Ktedonosporobacter and Anaerolinea played key roles in shaping arsM harboring microbe communities in field periphyton. These results highlight the importance of periphyton in the behavior of As in paddy soils and can potentially facilitate improved management of As contamination in paddy soils.

Impact of Water Regimes and Amendments on Inorganic Arsenic Exposure to Rice

Rice-based diet faces an important public health concern due to arsenic (As) accumulation in rice grain, which is toxic to humans. Rice crops are prone to assimilate As due to continuously flooded cultivation. In this study, the objective was to determine how water regimes (flooded and aerobic) in rice cultivation impact total As and inorganic As speciation in rice on the basis of a field-scale trial in the post-monsoon season. Iron and silicon with NPK/organic manure were amended in each regime. We hypothesised that aerobic practice receiving amendments would reduce As uptake in rice grain with a subsequent decrease in accumulation of inorganic As species relative to flooded conditions (control). Continuously flooded conditions enhanced soil As availability by 32% compared to aerobic conditions. Under aerobic conditions, total As concentrations in rice decreased by 62% compared to flooded conditions. Speciation analyses revealed that aerobic conditions significantly reduced (p < 0.05) arsenite (68%) and arsenate (61%) accumulation in rice grains. Iron and silicon exhibited significant impact on reducing arsenate and arsenite uptake in rice, respectively. The study indicates that aerobic rice cultivation with minimum use of irrigation water can lead to lower risk of inorganic As exposure to rice relative to flooded practice.

Inorganic arsenic toxicity and alleviation strategies in rice

Direct or indirect exposure to inorganic arsenic (iAs) in the forms of AsIII (arsenite) and AsV (arsenate) through consumption of As-contaminated food materials and drinking water leads to arsenic poisoning. Rice (Oryza sativa L.) plant potentially accumulates a high amount of iAs from paddy fields than any other cereal crops. This makes it to be a major source of iAs especially among the population that uses it as their dominant source of diet. The accumulation of As in human bodies poses a serious global health risk to the human population. Various conventional methods have been applied to reduce the arsenic accumulation in rice plant. However, the success rate of these techniques is low. Therefore, the development of efficient and effective methods aimed at lowering iAs toxicity is a very crucial public concern. With the current advancement in technology, new strategies aimed at addressing this concern are being developed and utilized in various parts of the world. In this review, we discuss the recent advances in the management of iAs in rice plants emphasizing the use of nanotechnology and biotechnology approaches. Also, the prospects and challenges facing these approaches are described.

Arsenic behavior across soil-water interfaces in paddy soils: Coupling, decoupling and speciation

The sharp redox gradient at soil-water interfaces (SWI) plays a key role in controlling arsenic (As) translocation and transformation in paddy soils. When Eh drops, As is released to porewater from solid iron (Fe) and manganese (Mn) minerals and reduced to arsenite. However, the coupling or decoupling processes operating within the redox gradient at the SWI in flooded paddy soils remain poorly constrained due to the lack of direct evidence. In this paper, we reported the mm-scale mapping of Fe, As and other associated elements across the redox gradient in the SWI of five different paddy soils. The results showed a strong positive linear relationship between dissolved Fe, Mn, As, and phosphorus (P) in 4 out of the 5 paddy soils, indicating the general coupling of these elements. However, decoupling of Fe, Mn and As was observed in one of the paddy soils. In this soil, distinct releasing profiles of Mn, As and Fe were observed, and the releasing order followed the redox ladder. Further investigation of As species showed the ratio of arsenite to total As dropped from 100% to 75.5% and then kept stable along depth of the soil profile, which indicates a dynamic equilibrium between arsenite oxidization and arsenate reduction. This study provides direct evidence of multi-elements' interaction along redox gradient of SWI in paddy soils.

Derivation of empirical model to predict the accumulation of Pb in rice grain

Lead contamination in soil has become a worldwide threat on food security and human health. To assess the Pb bioavailability and evaluate the safe use of low Pb polluted soil for food production, the speciation of Pb in 19 types of paddy soil were investigated by chemical extraction and X-ray absorption near-edge structure (XANES), and the uptake and accumulation characteristics of Pb in different soil-rice systems were investigated. Moreover, an empirical model was established to predict the content of Pb in rice grain, and field validation was conduct to evaluate model performance. Results showed that the proportion of available Pb in different soil satisfied normal distribution N (0.47, 0.23). Pb(CH₃COO)₂, GSH-Pb, PbO, PbHPO₄ and Pb₃(PO₄)₂ performed well in characterizing the speciation of Pb in different rhizosphere soils, and PbHPO₄ accounted for more than 70%. The exceedance of Pb in grain in CK, 0.5X and 1X treatment were 10.5%, 36.1% and 42.1%, respectively, and the accumulation of Pb in grain was significantly related with Pb content in root. Carbonate and organic bound Pb in rhizosphere soil were two major Pb species that influenced the accumulation of Pb in rice. Moreover, content of total Pb, clay and SOM performed well in predicting the Pb content in grain, both for pot and field samples. Above all, our predicting model worked well in evaluating Pb accumulation in rice grain among low polluted paddy farmland (Total Pb < 300 mg/kg).

Inhibition of arsenic transport from soil to rice grain with a sustained field-scale aerobic rice cultural practice

A field-scale investigation has been carried out to assess the uptake of Arsenic (As) in rice under aerobic practice. Two consecutive field experiments have been designed considering the rice cultivation system's variation in the comparison between aerobic and flooded practices during monsoon and post-monsoon seasons using the cultivars of Swarna masuri and Satabdi, respectively. Notwithstanding the impact of the rice cultivation systems, the implications of amendments like iron, silicon, and organic matter were also taken into account on As uptake by rice. We hypothesized that the application of amendments in combination with sustained aerobic practice would reduce the subsequent accumulation of As in rice as compared to flooded practice (control). However, regardless of the cultivation systems, the grain productivity of rice delivered a non-significant impact. Results revealed that the plant available As content in soil under aerobic practice was averaged 22% and 26% lower than flooded, during monsoon and post-monsoon seasons, respectively. Aerobic treatment significantly reduced accumulation of As in root and straw as compared to flooded (p < 0.05), which in accordance corresponded to lower translocation efficiency of As from root to straw. For Swarna masuri, the bioaccumulation of As in polished rice, husk and bran was reduced by 33%, 48% and 47%, respectively, under aerobic practice. On the contrary, Satabdi exhibited a reduction in As accumulation with 54% in polished rice, followed by 31% and 38% in husk and bran, respectively. The inhibition of As uptake in rice was notably impacted by iron, silicon, and organic matter. Following the treatments of rice cultivation system and amendment, the bioaccumulation of As in rice plant parts was arranged in the order of root > straw > grain > husk > bran > polished rice in both the cultivars. The health risk assessment was also considered to estimate the potential human health risk measuring the estimated dietary intake and the health hazard quotient. The results highlighted that the consumption of rice grown in aerobic practice was ensured to provide non-carcinogenic health risk as compared to rice grown in flooded practice. In the overall attempt, the present investigation corroborates the insinuation of specific management practices in quantifying the reduction of As bioavailability in rice with subject to the concern of reducing human health risk.

Arsenic dynamics in paddy soil under traditional manuring practices in Bangladesh

Fertilization with organic matter (farm yard manure and/or rice straw) is thought to enhance arsenic (As) mobilization into soil porewaters, with subsequent As assimilation by rice roots leading to enhanced translocation to the grain. Here, interlinked experiments (field manuring and soil batch culture) were conducted to find the effect of organic matter at a field application rate practiced in Bangladesh (5 t/ha) on As mobilization in soil for paddies impacted by As contaminated groundwater irrigation, a widespread phenomenon in Bangladesh where the experiments were conducted. Total As concentration in a paddy soil (Sonargaon) ranged from 21.9 to 8.1 mg/kg down the soil profile and strongly correlated with TOC content. Arsenic, Fe, Mn, and DOC release into soil solution, and As speciation, are intimately linked to OM amendment, soil depth and temporal variation. Organic matter amendments lead to increased mobilization of As into both soil porewaters and standing surface waters. The As speciation in the porewater was dominated by inorganic As (As_i) (arsenite and arsenate), with traces amounts of methylated species (DMA^V and MMA^V) only being found with OM amendment. It was noted in field trials that OM fertilization greatly enhanced As mobility to surface waters, which may have major implications for the fate of As in paddy agronomic ecosystems.

Geographical variations in arsenic contents in rice plants from Latin America and the Iberian Peninsula in relation to soil conditions

Arsenic is a ubiquitous, toxic element that is efficiently accumulated by rice plants. This study assessed the spatial variability in the total As (tAs) contents and organic and inorganic forms in different types of rice, plant parts (husk, stem, leaves and phytoliths) and residues. Samples were collected in different countries in Latin America (Ecuador, Brazil and Peru) and the Iberian Peninsula (Spain and Portugal). The tAs content in commercial polished rice from the Latin American countries was similar (0.130-0.166 mg kg^-1) and significantly lower than in the rice from the Iberian countries (0.191 ± 0.066 mg kg^-1), and together, the tAs concentration in brown rice (236 ± 0.093 mg kg^-1) was significantly higher than in polished and parboiled rice. The inorganic As (iAs) content in rice was similar in both geographical regions, and the aforementioned difference was attributed to dimethylarsinic acid (DMA). The relative abundance of organic species increased as the tAs content in rice grain increased. A meta-analysis of our and previously reported data confirmed the negative correlation between iAs/tAs and tAs. At low tAs concentrations, inorganic forms are dominant, while at higher values (tAs > 0.300 mg kg^-1) the concentration of organic As increases substantially and DMA becomes the dominant form in rice grain. On the contrary, inorganic arsenic was always the dominant form, mainly as arsenate [As(V)], in leaves and stems. The presence in soils of high concentrations of amorphous Fe and Al oxides and hydroxides, which are capable of strongly adsorbing oxyanions (i.e. arsenate), was associated with low concentrations of As in rice plants. In addition, the presence of high concentrations of As(V) in stems and leaves, low concentration of As in phytoliths, and the As associated with organic matter in stems and husk, together suggest that rice plants take up more As(V) than As(III).

Dissolved organic matter differentially influences arsenic methylation and volatilization in paddy soils

The effect of dissolved organic matter (DOM), derived from composted pig manure or rice straw, on arsenic methylation and subsequent biovolatilization in paddy soils was investigated. Arsine production following pig manure DOM application was 2.7- and 9.6-fold higher than that of soils treated with rice straw DOM and the control, respectively. Trimethylarsine was the dominant arsine at 54 %, followed by dimethylarsine at 22 %, arsine at 21 %, and monomethylarsine at 3 %. The copy numbers of the total and As-methylating bacteria were significantly enhanced in paddy soils treated with DOM. Pig manure DOM altered soil bacterial profile by increasing the OTU number of As methylation-inducing bacteria, such as Proteobacteria, Bacteroidetes, Geobacter, Sphingomonas, Streptomyces, and Rhodopseudomonas, thereby promoting As volatilization and methylation in paddy soils. The higher relative content of alkyl-C, N-alkyl C, and carboxyl-C in pig manure DOM was responsible for the increase in total and arsM-carrying bacteria in paddy soils, leading to enhanced As methylation. These observations will promote a better understanding of the role of DOM in mediating As methylation and volatilization, along with how organic fertilization affects straighthead disorder of rice, a condition caused by methylated arsenic species.

DGT and selective extractions reveal differences in arsenic and antimony uptake by the white icicle radish (Raphanus sativus)

Increasing soil contamination of arsenic (As) and antimony (Sb) is posing a serious concern to human health. Due to insufficient studies on Sb, the biogeochemical behaviour and plant uptake of Sb are assumed to be similar to that of As. As part of extensive research unravelling As and Sb biogeochemistry and plant uptake, the diffusive gradients in thin films (DGT) technique and sequential extraction procedure (SEP) were applied to evaluate As and Sb uptake by the white icicle radish (Raphanus sativus) cultivated in diluted cattle dip soils contaminated with As only and diluted mining soils contaminated with both As and Sb under agricultural conditions. Labile As and Sb in these soils measured by DGT (C_DGT), soil solution (C_sol), and SEP (C_SEP-labile), were compared with As and Sb bioaccumulation in R. sativus tissues. Regardless of contamination sources and measurement techniques, the results showed that As was consistently more labile than Sb although total As concentrations in two soil types were lower than total Sb. Labile As in cattle dip soils was higher than that in mining soils, although there were no significant differences in soil As concentrations. The analysis of R. sativus tissues revealed that the overall As bioaccumulation was 4.5-fold higher than for Sb, and that As translocation to shoots was limited. In contrast, considerable Sb translocation to shoots was observed. The As and Sb bioaccumulation were strongly correlated with their C_SEP-labile, C_DGT, and C_sol (R² = 0.87-0.99), demonstrating the effectiveness of these techniques in predicting As and Sb in the white icicle radish. Compared with the cherry bell radish previously studied, the white icicle radish exhibited higher bioaccumulation factors (BAF) for Sb, but lower BAF for As, and lower translocation of As and Sb to shoots, providing understanding of how As and Sb are accumulated by radish cultivars.