Free Radicals Produced from the Oxidation of Ferrous Sulfides Promote the Remobilization of Cadmium in Paddy Soils During Drainage

Dissolved organic matter (DOM) - Driven variations of cadmium mobility and bioavailability in waterlogged paddy soil

Cadmium (Cd) mobility and bioavailability in paddy soils are strongly influenced by dissolved organic matter (DOM), yet the mechanisms remain unclear. This study conducted a 90-day waterlogged soil incubation with DOM / sulfate amendments under varying Cd levels. Key parameters, including dissolved organic carbon (DOC), pe+pH, Fe/S - related parameters, alongside indicators of Cd mobility and bioavailability, were monitored. Results revealed that DOM addition increased Cd mobility on the 3rd day of incubation (DOI), irrespective of sulfate application, due to Cd desorption from iron oxides and DOM-Cd complexation. After the 10th DOI, DOM addition reduced Cd mobility and bioavailability mainly due to facilitation of sulfide-mediated Cd sequestration driven by Fe-S related reducing bacteria. The combined application with sulfate strengthened this effect. However, in low-Cd soils, DOM addition increased Cd bioavailability since the 45th DOI, likely due to the low Cd/DOM ratio, which limited sulfide immobilization. Nevertheless, sulfate application mitigated this effect. Furthermore, DOM supplementation generally decreased Cd mobility, but increased Cd availability at the 45th DOI in high-Cd soils due to competitive adsorption and Fe transformation. This study demonstrates the dual role of DOM in regulating Cd dynamics and its interaction with sulfate, offering insights for Cd contamination management in paddy soils.

Adding carbon quantum dots to enhance microbial extracellular electron transfer for accelerating aniline degradation in simulated wetlands

Microbial extracellular respiration is relatively weak in natural water systems because of the low electroactivity of environmental media, which limits the production of extracellular respiration-based reactive oxygen species (ROS) and the removal of pollutant. In this study, carbon quantum dots (CDs) were added to a simulated wetland to enhance extracellular respiration and accelerate the microbial oxidation of aniline under intermittent aeration conditions. The results indicated that CDs promoted extracellular electron transfer to increase the production of Fe2+ and ROS, and led to a 23.5 % higher aniline removal rate than that of the control group without CD addition. Multiple lines of evidence indicated that CDs bind to microorganisms in wetlands to induce the polarization of respiratory enzymes, thereby enhancing microbial electroactivity and the activity of respiratory chain enzymes. In addition, CDs enhanced proton efflux to form a higher membrane potential for adenosine triphosphate (ATP) production, which facilitated ROS generation. This study demonstrated a simple and efficient approach to enhance microbial extracellular electron transfer for ROS production and remediation of polluted wetlands.

Ferrihydrite sulfidation transformation and coupled As(V) and Cd(II) mobilization under anoxic conditions

Ferrihydrite sulfidation is an important process influencing the environmental behavior of co-existent arsenate (As(V)) and cadmium (Cd(II)) pollutants in mining-impacted environments. However, the mineral evolution of ferrihydrite and the coupled mobilization behavior of co-existent As(V) and Cd(II) remain unclear. In this study, we have investigated As(V)-Cd(II)-bearing ferrihydrite conversion behavior induced by environmentally relevant concentrations of S(-II) (1 and 5 mM). PXRD, HR-TEM, and XAS results demonstrate that the co-existent As(V) and Cd(II) inhibit the conversion of ferrihydrite to secondary lepidocrocite (γ-FeO(OH)) and subsequently to goethite (α-FeO(OH)) at different S(-II) concentrations. Elevated As(V) and Cd(II) levels promote the formation of amorphous mackinawite (FeS) and pyrite (FeS2). Lepidocrocite and greenockite (CdS) are the predominant secondary phases at 1 mM S(-II) but lepidocrocite and pyrite are dominant at 5 mM S(-II) when the As(V) and Cd(II) levels are low. These sulfidation transformation pathways reduce the mobilization of the co-existent As(V) and Cd(II). Cs-TEM and chemical extraction results reveal that substantial portions of Cd(II) and As(V) are incorporated into secondary pyrite and lepidocrocite, in addition to surface adsorption and greenockite precipitation. These findings not only enhance our understanding of the geochemical cycling of Fe(III), As(V), and Cd(II) in natural anoxic sulfidic environments but also may provide guidelines for developing effective remediation methods for As-Cd co-contaminated settings.

New insights into efficient iron sulfide oxidation for arsenic immobilization by microaerophilic and acidophilic Fe(II)-oxidizing bacteria under micro-oxygen and acidic conditions

Microbial-mediated FeS oxidation to Fe(Ⅲ) minerals via chemolithoautotrophic Fe(Ⅱ) oxidizers under pH/O₂ limitations engages As immobilization. However, this process is constrained under the dual stress of micro-oxygen and acidic conditions due to the critically diminished Fe(Ⅱ) oxidation capacity. Therefore, the interplay between Fe(Ⅱ) oxidation, carbon metabolism, and As immobilization in Fe(Ⅱ)-oxidizing bacteria under micro-oxygen and acidic conditions remains unclear. This study presents the first successful enrichment of microaerophilic and acidophilic Fe(II)-oxidizing bacteria (MAFeOB). These bacteria are capable of oxidizing FeS to Fe(III) minerals and immobilizing up to 27,835 mg/kg of As(Ⅴ) under micro-oxygen content (below 3.2 mg/L) and acidic pH (4.5-6.2). Through comprehensive metagenomic analysis, it was speculated that MAFeOB harbor a suite of genes potentially participating in critical processes, including carbon fixation, Fe(II) oxidation, and arsenic detoxification. Notably, a potential electron transfer pathway from Cyc2_repCluster2 to Cytochrome cbb3-type oxidases facilitates Fe(II) oxidation. Furthermore, As(Ⅲ) efflux pump (arsA, arsB, acr3) and As(Ⅲ) oxidase (aioA) genes indicate MAFeOB's potential for As immobilization. Our findings underscore the pivotal role of MAFeOB in overcoming limitations associated with Fe(III) mineral formation, thereby enhancing arsenic immobilization under micro-oxygen and acidic water.

Effect of organic materials and sulfate addition on cadmium bioavailability and accumulation of rice in cadmium sulfide nanoparticles- and cadmium chloride-treated soil

Water-soluble and extractable cadmium (Cd) govern Cd bioavailability. However, cadmium sulfide nanoparticles (CdSnp) bioavailability remains unclear. Pot experiments investigated how organic materials (1 % wheat straw and pine needles) and sulfate (30 mg S/kg) affect Cd bioavailability and accumulation in rice in CdCl₂- and CdSnp-treated soils. Sulfate addition alone immobilized Cd through sulfate-reducing bacteria-mediated Cd-precipitation in CdCl₂-treated soil, thereby reducing Cd accumulation in brown rice. However, it promoted CdSnp dissolution and Cd transfer, increasing Cd accumulation in CdSnp-treated soils. Organic materials alone promoted Cd re-release from immobilized Cd through sulfur-oxidizing bacteria (SOB)-induced oxidation and dissolved organic matter (DOM)-assisted complexation to increase Cd availability in CdCl₂-treated soil at the maturity stage. Consequently, organic materials alone increased brown rice Cd accumulation in CdCl₂-treated soil, while combination with sulfate had no effect. In CdSnp-treated soils, organic materials promoted CdSnp dissolution through driving SOB, decreasing pH, and increasing DOM complexation, which increased Cd availability and transfer, promoting brown rice Cd accumulation. CdSnp exhibited lower Cd bioavailability and accumulation than CdCl2 due to lower solubility and slower Cd release, suggesting CdSnp smaller contributions to rice Cd accumulation. Therefore, sulfate addition alone is recommended to reduce Cd accumulation, with organic materials only used in combination with sulfate.

Decoupling soil pH and geography: Universal drivers of cadmium bioavailability in rice across terrains

With the accelerating global industrialization, Cadmium (Cd) pollution in rice has become a significant threat to both ecological safety and human health. But the universal factors influencing Cd content across different terrains remain under-investigated. Hence, 300 groups of root system-rice samples were collected from typical rice planting areas in the plains and hills of Southern China to investigate the driving factors of Cd content in rice at a large scale. Moreover, a Cd content prediction model in rice was built. Results showed that although total Cd (T-Cd) and available Cd (DTPA-Cd) contents in rice soils from hilly areas were significantly lower than those in plains, the Cd content in rice was significantly higher (P < 0.05). In a geographical distribution analysis, there was a weak correlation between geographical distance and Cd content in soil (∣R∣<0.30, P < 0.05), although this showed evidence of gradual geographical changes. In addition, this study found that DTPA-Cd (positive correlation, feature importance score of 42.67) and soil pH (negative correlation, 38.91) were the most critical factors that influenced Cd content in rice from different terrains using a network diagram and random forest model computation. In summary, there was evidence of a complicated interaction between terrain and soil pH in rice-Cd pollution at a large regional scale. Soil pH and DTPA-Cd were dominant influencing factors of Cd content in rice when compared to geographical distribution. These results provide an important scientific reference for large-scaled Cd pollution monitoring, control, and risk evaluation.

Fate of microplastics in soil-water systems: View from free radicals driven by global climate change

Microplastics are ubiquitously distributed and persistently present in soil-water systems, posing potential ecological and health risks worldwide. Free radicals are highly reactive in soil-water systems, particularly at soil-water-air interface. The dynamic changes of free radicals sensitive to environmental conditions may greatly impact the fate of microplastics. However, the pathways, reaction kinetics, or transformation products of microplastic degradation by free radicals in soil-water systems remains unclear. Climate change alters the physical and chemical environment of soil-water systems and this transformation can directly affect the degradation of microplastics, or indirectly influence it by altering the generation and species of free radicals. Here, we summarized and analyzed the impact of fluctuations in free radicals (such as superoxide radicals, hydrogen peroxide, peroxyl radicals, and hydroxyl radicals) in soil-water systems on the degradation of microplastics and their derivants. We also discussed how changes in free radicals driven by climate change affect the fate of microplastics. By integrating aspects such as climate change, free radical chemistry, and microplastic pollution, this work delineates the critical issues of microplastic pollution exacerbated by environmental condition changes. In response to the existing challenges and deficiencies in current research, feasible countermeasures are proposed. This work offers valuable insights for future research on predicting and controlling ecotoxicity and health risks caused by microplastics associated with global climate change.

Iron, Sulfur, and Carbon Dynamics Collectively Regulate the Fate of Cadmium over the Sulfidation-Reoxidation Cycle

Cadmium bioavailability is sensitive to redox fluctuations, with its fate linked to the coupled dynamics of Fe, S, and C. This study examines the behavior of Cd-loaded ferrihydrite (Fh) with/without organic matter (OM) undergoing S(-II)-induced reduction followed by O2-induced reoxidation. During sulfidation, S(-II) was fully consumed, and Fh was partially reduced to Fe(II) species, with some OM released from the Fh surface. Meanwhile, Cd initially adsorbed on Fh was completely converted to CdS, regardless of Cd loading or the presence of OM. Upon reoxidation, Fe(II) species were reoxidized to Fe(III) oxides, which recaptured OM, while solid-phase S(-II) was oxidized to S0 and sulfate. Concurrently, partial oxidation of CdS occurred, mainly driven by H2O2 generated during Fe(II) oxidation, with minor contributions from •OH and O2, but OM inhibited CdS oxidation, primarily by scavenging H2O2. Released Cd from CdS oxidation was predominantly readsorbed on Fe(III) oxides. Additionally, released Cd was partially structurally incorporated into newly formed Fe(III) oxides while some CdS was encapsulated within Fe(III) oxide aggregates. However, OM interactions with Fe(III) oxides reduced the formation of these Cd species. These findings provide insights into the molecular-scale mechanisms governing Cd dynamics in redox-dynamic environments.

Hydroxyl radicals produced from oxidation of ferrous sulfides promote mobilization of mercuric sulfide in soil-water system

Mercuric sulfide nanoparticles (HgS-NPs) are recognized as a significant source of bioavailable mercury in paddy fields. The factors influencing the mobilization and bioavailability of HgS-NPs formed in flooded or drained paddy field-like systems are complicated and remain unexplored to date. Here, we show that ferrous sulfide (FeS) as an important mineral substance plays a crucial role in the dissolution and transformation of HgS-NPs in overlying water or during the drainage stage, as well as their bioavailability toward rice. Specifically, we found that oxidation of FeS significantly enhances the dissolution of HgS-NPs, with the degree of activation intensified with increasing FeS concentrations. This activation was further evidenced to be driven by the generation of hydroxyl radicals (•OH) during FeS oxidation, leading to the release of Hg(Ⅱ). The enhanced dissolution of HgS-NPs increases its bioavailability, as verified by the augmented accumulation of Hg in rice upon FeS oxidation. This study underscores the overlooked yet important role of FeS in affecting the fate of HgS-NPs and offers valuable insights for pollution control of Hg-contaminated paddy fields and wetlands.

Agricultural amendments enhanced the redox cycling of iron species and hydroxyl radical formation during redox fluctuation of paddy soil

Hydroxyl radical (•OH) plays a critical role in accelerating organic contaminant attenuation during water-table decline in paddy soil, but the impacts of widely applied agricultural amendments (e.g., organic manure, rice straw, and biochar) on these processes have been rarely explored. Hence, the effects of agricultural amendments on •OH formation and pollutant degradation were examined based on field experiments. Compared with control, organic fertilizer (supplying more organic carbon (OC) and bioavailable elements that promoted Fe(II) formation by microorganisms) enhanced •OH production by 0.8-1.3 times, while straw returning and biochar have negligible effects, probably due to the decreased pH and inhibition of microorganisms. The increased oxidation of active Fe(II) species (e.g., exchangeable Fe(II) and Fe(II) in lower-crystallinity minerals) mainly contributed to •OH production. Further analyses showed that organic fertilizers significantly enhanced the redox cycling of Fe species mainly through increasing the contents of soil organic carbon and relative abundances of Fe(III)-reducing microorganisms. In addition, the increased •OH formation markedly enhanced imidacloprid degradation by 24.3-42.4 %, with the toxicity of intermediates increased versus the parent compound. This study systematically examined the effects of typical agricultural amendments on the •OH formation and organic contaminant attenuation in paddy soil, which probably provides promising strategies for regulating contaminant remediation in agricultural fields.

Heavy metals pollution and the associated ecological risks along the Luanhe River basin in North China

Luanhe River is one of the important parts in Northern China hydrographic nets, which has been seriously impacted by industrialization. However, the heavy metal pollution caused by the production activities across the basin remains poorly understood, which may pose health risks to local residents. The study aims to address this gap by focusing on heavy metal pollution in sediments from 41 sampling sites along the Luanhe River basin evaluating the potential ecological risks. The results showed that, due to varying local industries, geography, and human activities, different sections exhibited distinct contamination patterns. The upper reach showed predominant pollution by Mn (up to 4305.59 mg kg-1) and As (59.92 mg kg-1), the middle reach by Mn (4305.59 mg kg-1), Cu (692.88 mg kg-1), As, and Hg (3.48 mg kg-1), and the lower reach by Mn and Hg. Thereinto, the particular concern was the moderate to serious ecological risk posed by Hg across most of the river basin, as indicated by the potential ecological risk indices. For most of the identified heavy metals along the Luanhe River basin, the ecological risk assessed by the geo-accumulation index and potential ecological risk index indicated a moderate to serious pollution level. Overall, this work systematically depicted the heavy metal pollution map across the Luanhe River basin, revealed and discussed the relationship between industrialization and heavy metal pollution in this district. This study highlights that practical policy and technical measures should be taken to cope with the heavy metal pollution and associated ecological risks in this area.

Deciphering the mechanism of rhizosphere microecosystem in modulating rice cadmium accumulation via integrating metabolomics and metagenomics

Cadmium (Cd) accumulation in rice poses significant risks to human health. The Cd accumulation levels vary widely among cultivars and are strongly associated with the rhizosphere microecosystem. However, the underlying mechanisms remain poorly understood. Here, we conducted a field experiment in Cd-contaminated areas with 24 popular regional cultivars. These cultivars were categorized into high Cd accumulation (HA) and low Cd accumulation (LA) groups based on their grain Cd content. Rhizosphere soil physicochemical properties were monitored, and key metabolites, microbiomes, and their interaction contributing to Cd accumulation were analyzed using omics-sequencing technologies and bioinformatics analysis. Metabolomic analysis identified distinct rhizosphere metabolite profiles between the HA and LA groups, with key metabolites showing strong correlations with Cd accumulation. Key metabolites in the LA group were linked to reduced Cd uptake and enhanced antioxidant defense mechanisms, while those in the HA group were associated with increased Cd mobility and uptake. Metagenomic analysis of the rhizosphere soil showed that the LA group harbored a more diverse and interconnected microbial community, with tax such as Syntrophaceae, Anaerolineae, Thermoflexales, and Syntrophales, along with metabolite such as disopyramide, playing central roles in Cd immobilization and detoxification. Additionally, the enhanced carbon, nitrogen, and phosphorus cycling in the LA group suggests a more robust nutrient assimilation process that supports plant growth and reduces Cd uptake. This study highlights the critical role of the rhizosphere microecosystem in regulating Cd accumulation and underscores the potential of selecting rice cultivars with favorable rhizosphere traits as a strategy for reducing Cd uptake.

The role of interface interaction between iron/sulfate-reducing bacteria (ISRB) and goethite in sulfur (S) redox cycling couple with Cd immobilization

Microbial sulfate reduction leads to the formation of various chalcophile trace metal sulfides, thereby immobilizing chalcophile trace metals in sediments. Iron/sulfate-reducing bacteria (ISRB) are ubiquitous in soils and sediments, its ability to reduce Fe(III) (oxyhydr)oxides and biogeochemical significance have attracted much attention. This research investigated the effect of the goethite and ISRB induced S cycle on cadmium mobility. The experiment demonstrated that the removal of Cd(II) in coexistence of ISRB19 and goethite was more efficiently than their individual components. Combined with X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), Raman spectra and X-ray photoelectron spectroscopy (XPS), conclusions can be drawn that goethite enhanced Cd(II) retention by ISRB, which was attributed to the formation of metabolism product during interaction between ISRB19 (Enterobacter chengduensis) and goethite. Our results revealed the interaction of goethite and ISRB in S cycling under anaerobic conditions with its implications for Cd(II) remediation.

Production of Reactive Oxygen Species during Redox Manipulation and Its Potential Impacts on Activated Sludge Wastewater Treatment Processes

Reactive oxygen species (ROS) are ubiquitous in redox-fluctuating environments, exerting profound impacts on biogeochemical cycles. However, whether ROS can be generated during redox manipulation in activated sludge wastewater treatment processes (AS-WTPs) and the underlying impacts remain largely unknown. This study demonstrates that ROS production is ubiquitous in AS-WTPs due to redox manipulation and that the frequency and capacity of ROS production depend on the operating modes. The anaerobic/oxic continuous-flow reactor showed persistent ROS generation (0.8-2.1 μM of instantaneous H2O2), whereas the oxic/anoxic sequencing batch reactor (0.21-0.28 mM of H2O2 per cycle) and the anaerobic/anoxic digestion reactor (0.27-0.29 mM of H2O2 per cycle) exhibited periodic ROS production. Our results illustrated that ROS generated during redox manipulation can contribute to the removal of organic micropollutants. Due to their high activity, ROS can directly accelerate the abiotic oxidation of organic phenolics and Fe(II) minerals in sludges. ROS could also affect biotic nitrification by changing the microbial community composition and regulating the relative expression of functional genes, such as amoA, nrxA, and nrxB. This research demonstrates the ubiquitous production of ROS during redox manipulation in AS-WTPs, which provides new insights into pollutant removal and the abiotic and biotic elemental transformation in AS-WTPs.

Novel magnetic adsorbents based on oyster and clam shells for the removal of cadmium in soil

Magnetic adsorbents can effectively remove heavy metals from soil. However, the magnetization process may reduce availability of adsorption sites, making it challenging to balance magnetic and adsorption properties. In this study, oyster shell (OS) and clam shell (CS) material was magnetized by an improved chemical co-precipitation method. The organic matter in the shells was destroyed by calcination modification to expose new active sites, and calcinated ferro-magnetic adsorbent was produced with either ferrosodium EDTA (giving CEOS and CECS) or with iron citrate (for CCOS and CCCS). All four modified adsorbents reached adsorption equilibrium for Cd2+ in solution within 120 min, with maximum adsorption capacities ranging from 115.5 to 266.5 mg/g, giving high removal efficiencies for Cd2+. Adsorption by precipitation and cation exchange mechanisms was dominant, together contributing >60 % of all adsorption capacity, followed by complexation. When used for remediation of Cd-contaminated soil, CEOS demonstrated the best Cd removal efficiency, achieving removal rates of 46 % and 58 % for total and available Cd, respectively. This was mainly because CEOS had the highest magnetic recovery rate, of 98 %. CEOS maintained removal rates of 34 % for total Cd after regeneration and reuse three cycles, with recovery rates remaining above 90 %. Contaminated soil was treated with the novel adsorbents and in pot experiments with water spinach cultivation it was shown that both CEOS and CECS treatment significantly reduced Cd content (by up to 56 %). The magnetic adsorbents presented here demonstrate excellent performance to remove Cd from water and soil, and have promising application prospects.

Simultaneous removal of PAHs and heavy metals from soil by combining electrokinetic-assisted delivery of persulfate and ultrasound activation

Herein, we proposed and demonstrated a novel approach of combining electrokinetics (EK)-assisted delivery of persulfate (PS) and ultrasound-induced thermal activation of PS and release of heavy metals, to synchronously remove organic pollutants and heavy metals from soil. Results showed that in tested soil, the mass transfer efficiency of PS induced by electromigration was approximately 3.36 times that by electroosmotic flow and 1416 times that by diffusion. Compared to the PS injection alone, EK-assisted delivery of PS significantly increased the degradation rate of phenanthrene, improving from 21.60 % to 76.23 %. Due to the applied ultrasound, the temperature of soil was observed to reach target temperatures∼50℃ for effective PS activation, and hydroxyl radicals and alkyl-like radicals were detected in the soil. Moreover, the decomposition of PS on the cathode electrode inhibited the electrolytic water reaction, avoiding the focusing effect and potential flattening. Concurrently, ultrasound thermally activated PS promoted the release of heavy metals from soil. As a result, the removal rates of Pb, Zn and Cd near the anode region increased from 0.00 %, 5.49 % and 11.72 % to 11.89 %, 82.64 % and 86.30 %, respectvcely. These results indicated the novel approach holds the potential for simultaneous removal of organic pollutants and heavy metals.

A holistic field experimental inquiry into cadmium's migration and translocation dynamics across the entire growth spectrum of five Japonica rice cultivars

The contamination of farmland soils with cadmium (Cd) poses a substantial threat to agricultural productivity, food security and safety, and ultimately human health. However, little research has been done on the Cd transport mechanisms in highly Cd polluted soil via field experiment. This study, from a field-scale perspective, examines the migration and transformation features of Cd throughout the growth cycle of five (C1, C2, C3, C4, H1) Japonica rice cultivars in Jiangsu Province, China. Analysis of pH, SOM, total Cd, DTPA-Cd, and microbial communities were conducted. C1 ~ C3 were classified as High Cd-accumulating rice (HC), while C4 and H1 were considered as low Cd-accumulating rice (LC) based on the Cd levels in their brown rice. Phloem was confirmed as the main pathway for Cd into rice grains in high-Cd soil. For the HC group, the Cd concentration in brown and polished rice was positively correlated with the Cd concentration in the leaves and spikes; while for the LC group, they were significantly positively correlated with the Cd concentration in both stem and spike (p < 0.05). The husks of the LC group were more effective in intercepting and sequestering Cd. It was revealed that 6 % ~ 9.09 % of the Cd content detected in the rice grains could be attributed to the internal translocation processes occurring within the plant itself, and approximately 90.91 % ~ 93.84 % of the Cd was traced back to the roots' absorption during grouting. Rice polishing decreased the Cd content from the level in the brown rice by 18 % ~ 47 %. Distinct microbial profiles separated rice rhizosphere from bulk soil, with the former favouring copiotrophs in nutrient-rich zones and the latter oligotrophs in lean conditions. This study delivers crucial data support from a field perspective for a deeper understanding and control of Cd migration and transformation processes in highly Cd-contaminated soil.

Cadmium distribution in rice: Understanding the role of plant nodes and growth stages

Cadmium (Cd) contamination in farmland poses a significant threat to food security in staple crops, especially rice. Using a mix of hydroponic and soil culture methods, stable isotope tracers, and advanced analytical techniques, this study elucidated the mechanisms of Cd uptake, translocation, and accumulation in rice throughout different growth stages. Despite a notable linear correlation between soil DTPA (diethylene-triaminepentaacetic acid)-Cd and the total Cd concentration of rice, our findings showed that the influence of soil Cd level on the proportion of Cd in grain was negligible. The study highlighted the dynamic response of Cd distribution within plant nodes to changes in DTPA-extractable Cd. Heading stage (HS) and mature stage (MS) were critical for Cd uptake and upward transport in rice, and the contribution of Cd absorption in brown rice was 28.61% and 40.16%, respectively. Moreover, the distribution of Cd in nodes showed how important nodes are for controlling and redistributing Cd in rice. In the HS, the lower node had a function in re-transporting, whereas in the MS, there was a considerable redistribution of Cd in the upper node. These insights can help us understand rice Cd dynamics and develop agronomic techniques and rice cultivars that minimize Cd accumulation.

Arsenic Methylation by a Sulfate-Reducing Bacterium from Paddy Soil Harboring a Novel ArsSM Fusion Protein

Microbial arsenic (As) methylation is an important process of As biogeochemistry. Only a few As-methylating microorganisms have been isolated from paddy soil, hindering the mechanistic understanding of the process involved. We isolated 54 anaerobic and 32 aerobic bacteria from paddy soil with a high As methylation potential. Among the 86 isolates, 14 anaerobes, including 7 sulfate-reducing bacteria (SRB), but none of the aerobes were able to methylate arsenite [As(III)] or monomethylarsenite [MMA(III)] or both, suggesting that the As-methylating ability is much more prevalent in anaerobes than in aerobes. We performed a detailed investigation on As methylation by a SRB isolate, Solidesulfovibrio sp. TC1, and identified a novel bifunctional enzyme consisting of a fusion of As(III) S-adenosylmethionine (SAM) methyltransferase (ArsM) and a radical SAM protein. The enzyme (ArsSM) can catalyze As(III) methylation to MMA and DMA and subsequent adenosylation of DMA to form 5'-deoxy-5'-dimethylarsinoyl-adenosine (DDMAA), which is a key intermediate in the biosynthesis of arsenosugars. High concentrations of sulfide produced by SRB did not affect As(III) methylation to MMA but inhibited MMA methylation to DMA. Genes encoding ArsSM fusion proteins are widespread in anaerobes, particularly SRB, suggesting that ArsSM-carrying anaerobes may play an important role in As methylation in an anoxic environment.

Spatial heterogeneity of soil moisture caused by drainage and its effects on cadmium variation in rice grain within individual fields

Paddy drainage is the critical period for rice grain to accumulate cadmium (Cd), however, its roles on spatial heterogeneity of grain Cd within individual fields are still unknown. Herein, field plot experiments were conducted to study the spatial variations of rice Cd under continuous and intermittent (drainage at the tillering or grain-filling or both stages) flooding conditions. The spatial heterogeneity of soil moisture and key factors involved in Cd mobilization during drainages were further investigated to explain grain Cd variation. Rice grain Cd levels under continuous flooding ranged from 0.16 to 0.22 mg kg-1 among nine sampling sites within an individual field. Tillering drainage slightly increased grain Cd levels (0.19-0.31 mg kg-1) with little change in spatial variation. However, grain-filling drainage greatly increased grain Cd range to 0.33-0.95 mg kg-1, with a huge spatial variation observed among replicated sites. During two drainage periods, soil moisture decreased variously in different monitoring sites; greater variation (mean values ranged from 0.14 to 0.27 m3 m-3) was observed during grain-filling drainage. Accordingly, 2.9-3.3-fold variation in soil Eh and 0.55-0.67-unit variation in soil pH were observed among those sites. In the soil with low moisture, ferrous fractions such as ferrous sulfide (FeS) were prone to be oxidized to ferric fractions; meanwhile, the followed generation of hydroxyl radicals involved in Cd remobilization was enhanced. Consequently, soil dissolved Cd changed from 2.97 to 8.92 μg L-1 among different sampling sites during grain-filling drainage; thus, large variation was observed in grain Cd levels. The findings suggest that grain-filling drainage is the main process controlling spatial variation of grain Cd, which should be paid more attention in paddy Cd evaluation.

Sulfidation-reoxidation enhances heavy metal immobilization by vivianite

Iron minerals in nature are pivotal hosts for heavy metals, significantly influencing their geochemical cycling and eventual fate. It is generally accepted that, vivianite, a prevalent iron phosphate mineral in aquatic and terrestrial environments, exhibits a limited capacity for adsorbing cationic heavy metals. However, our study unveils a remarkable phenomenon that the synergistic interaction between sulfide (S2-) and vivianite triggers an unexpected sulfidation-reoxidation process, enhancing the immobilization of heavy metals such as cadmium (Cd), copper (Cu), and zinc (Zn). For instance, the combination of vivianite and S2- boosted the removal of Cd2+ from the aqueous phase under anaerobic conditions, and ensured the retention of Cd stabilized in the solid phase when shifted to aerobic conditions. It is intriguing to note that no discrete FeS formation was detected in the sulfidation phase, and the primary crystal structure of vivianite largely retained its integrity throughout the whole process. Detailed molecular-level investigations indicate that sulfidation predominantly targets the Fe(II) sites at the corners of the PO4 tetrahedron in vivianite. With the transition to aerobic conditions, the exothermic oxidation of CdS and the S sites in vivianite initiates, rendering it thermodynamically favorable for Cd to form multidentate coordination structures, predominantly through the Cd-O-P and Cd-O-Fe bonds. This mechanism elucidates how Cd is incorporated into the vivianite structure, highlighting a novel pathway for heavy metal immobilization via the sulfidation-reoxidation dynamics in iron phosphate minerals.

Radial Oxygen Loss Triggers Diel Fluctuation of Cadmium Dissolution in the Rhizosphere of Rice

Cadmium (Cd) contamination poses a significant global threat to human health, primarily through dietary intake, with rice serving as a major source. While Cd predominantly resides in bound states in soil, the physiological processes by which rice facilitates Cd absorption in the rhizosphere remain largely elusive. This study delves into the mechanisms governing Cd uptake by rice plants in the rhizosphere, emphasizing the impact of daytime and nighttime fluctuations in microenvironmental conditions. Employing a microfluidic chip setup, the research reveals that radial oxygen loss from rice roots triggers dissolution of Cd in the rhizosphere. Notably, Cd mobility exhibits distinct diurnal fluctuations, peaking at 44.0 ± 4.1 nM during the daytime and dropping to 8.3 ± 1.3 nM during the nighttime. Further investigations reveal that variations in dissolved oxygen and hydroxyl radical concentrations influence Cd release, while pH changes and microbial reduction reactions play crucial roles in Cd immobilization. These findings provide insights into the intricate processes governing Cd mobilization in the rice rhizosphere, highlighting the importance of regulating these processes for effective Cd adsorption control in rice crops and safeguarding public health.

Controlling exposure to As and Cd from rice via irrigation management

Irrigation management controls biogeochemical cycles in rice production. Under flooded paddy conditions, arsenic becomes plant-available as iron-reducing conditions ensue, while oxic conditions lead to increased plant availability of Cd in acidic soils. Because Cd enters rice through Mn transporters, we hypothesized that irrigation resulting in intermediate redox could simultaneously limit both As and Cd in rice grain due to As retention in soil and Mn competition for Cd uptake. In a 2 year field study, we used 6 irrigation managements that varied in extent and frequency of inundation, and we observed strong effects of irrigation management on porewater chemistry, soil redox potentials, plant As and Cd concentrations, plant nutrient concentrations, and methane emissions. Plant As decreased with drier irrigation management, but in the grain this effect was stronger for organic As than for inorganic As. Grain organic As, but not inorganic As, was strongly and positively correlated with cumulative methane emissions. Conversely, plant Cd increased under more aerobic irrigation management and grain Cd was negatively correlated with porewater Mn. A hazard index approach showed that in the tested soil with low levels of As and Cd (5.4 and 0.072 mg/kg, respectively), irrigation management could not simultaneously decrease grain As and Cd. Many soil properties, such as reducible As, available Cd, soil pH, available S, and soil organic matter should be considered when attempting to optimize irrigation management when the goal is decreasing the risk of As and Cd in rice grain.

Direct evidence for the occurrence of indigenous cadmium-based nanoparticles in paddy soils

Speciation of heavy metal-based nanoparticles (NPs) in paddy soils greatly determines their fate and potential risk towards food safety. However, quantitative understanding of such distinctive species remains challenging, because they are commonly presented at trace levels (e.g., sub parts-per-million) and extremely difficult to be fractionated in soil matrices. Herein, we propose a state-of-art non-destructive strategy for effective extraction and quantification of cadmium (Cd)-NPs - the most widespread heavy metal in paddy soils - by employing single particle inductively coupled plasma mass spectrometry (spICP-MS) and tetrasodium pyrophosphate (TSPP) as the extractant. Acceptable extraction efficiencies (64.7-80.4 %) were obtained for spiked cadmium sulfide nanoparticles (CdS-NPs). We demonstrate the presence of indigenous Cd-NPs in all six Cd-contaminated paddy soils tested, with a number concentration ranging from 2.20 × 108 to 3.18 × 109 particles/g, representing 17.0-50.4 % of the total Cd content. Furthermore, semi-spherical and irregular CdS-NPs were directly observed as an important form of the Cd-NPs in paddy soils, as characterized by transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (TEM-EDX). This research marks a significant step towards directly observing indigenous Cd-NPs at trace levels in paddy soil, offering a useful tool for quantitative understanding of the biogeochemical cycling of heavy metal-based NPs in complex matrices.

Sulfur enhances iron plaque formation and stress resistance to reduce the transfer of Cd and As in the soil-rice system

Sulfur plays an essential role in agricultural production, but few studies have been reported on how sulfur simultaneously impacts the transformation of cadmium (Cd) and arsenic (As) in the soil-rice system. This research selected two soils co-contaminated with both Cd and As, varying in acidity and alkalinity levels, to study the impacts of elemental sulfur (S) and calcium sulfate (CaSO4) on the migration and accumulation of Cd and As by rice. Results indicated that two types of sulfur had a substantial (P < 0.05) impact on decreasing the contents of Cd (28.3-50.4 %) and As (20.1-38.6 %) in brown rice in acidic and alkaline soils. They also increased rice biomass (29.3-112.8 %) and reduced Cd transport coefficient (27.2-45.6 %) significantly (P < 0.05). Notably, sulfur augmented the generation of iron plaque on rice root surfaces, which increased the fixation of Cd (17.6-61.0 %) and As (14.0-45.9 %). SEM-EDS results also indicated that the rice root surface exhibited significant enrichment of Fe, Cd, and As. The mechanism of simultaneous Cd and As immobilization by sulfur application was mainly ascribed to the contribution of iron plaque. Additionally, sulfur reduced the contents of Cd and As in soil porewater and promoted the transformation of As(III) to As(V) to reduce the toxicity of As. The K-edge XAFS of As in iron plaque also confirmed that sulfur application significantly promoted As(III) oxidation. Sulfur also promoted the activities of antioxidant enzymes and the contents of NPT, GSH, and PCs in rice plants. In general, this study establishes a foundation for sulfur to lower As and Cd bioavailability in paddy soils, enhance iron plaque and rice resistance, and reduce heavy metal accumulation.

Zeolite facilitates sequestration of heavy metals via lagged Fe(II) oxidation during sediment aeration

Aeration of sediments could induce the release of endogenous heavy metals (HMs) into overlying water. In this study, experiments involving FeS oxygenation and contaminated sediment aeration were conducted to explore the sequestering role of zeolite in the released HMs during sediment aeration. The results reveal that the dynamic processes of Fe(II) oxidation play a crucial role in regulating HMs migration during both FeS oxygenation and sediment aeration in the absence of zeolite. Based on the release of HMs, Fe(II) oxidation can be delineated into two stages: stage I, where HMs (Mn2+, Zn2+, Cd2+, Ni2+, Cu2+) are released from minerals or sediments into suspension, and stage II, released HMs are partially re-sequestered back to mineral phases or sediments due to the generation of Fe-(oxyhydr) oxide. In contrast, the addition of zeolite inhibits the increase of HMs concentration in suspension during stage I. Subsequently, the redistribution of HMs between zeolite and the newly formed Fe-(oxyhydr) oxide occurs during stage II. This redistribution of HMs generates new sorption sites in zeolite, making them available for resorbing a new load of HMs. The outcomes of this study provide potential solutions for sequestering HMs during the sediment aeration.

Rice straw returning enhances cadmium activation by accelerating iron cycling thus hydroxyl radical production in paddy soils during drainage

Straw returning is widely found elevating the bioavailability of cadmium (Cd) in paddy soils with unclear biogeochemical mechanisms. Here, a series of microcosm incubation experiments were conducted and spectroscopic and microscopic analyses were employed. The results showed that returning rice straw (RS) efficiently increased amorphous Fe and low crystalline Fe (II) to promote the production of hydroxyl radicals (OH) thus Cd availability in paddy soils during drainage. On the whole, RS increased OH and extractable Cd by 0.2-1.4 and 0.1-3.3 times, respectively. While the addition of RS effectively improved the oxidation rate of structural Fe (II) mineral (i.e., FeS) to enhance soil Cd activation (up to 38.5 %) induced by the increased OH (up to 69.2 %). Additionally, the existence of CO32- significantly increased the efficiency level on OH production and Cd activation, which was attributed to the improved reactivity of Fe (II) by CO32- in paddy soils. Conclusively, this study emphasizes risks of activating soil Cd induced by RS returning-derived OH, providing a new insight into evaluating the safety of straw recycling.

Microbially Induced Soil Colloidal Phosphorus Mobilization Under Anoxic Conditions

Understanding the behavior of colloidal phosphorus (Pcoll) under anoxic conditions is pivotal for addressing soil phosphorus (P) mobilization and transport and its impact on nutrient cycling. Our study investigated Pcoll dynamics in acidic floodplain soil during a 30-day flooding event. The sudden oxic-to-anoxic shift led to a significant rise in pore-water Pcoll levels, which exceeded soluble P levels by more than 2.7-fold. Colloidal fractions transitioned from dispersed forms (<220 nm) to colloid-associated microaggregates (>220 nm), as confirmed by electron microscopy. The observed increase in colloidal sizes was paralleled by their heightened ability to form aggregates. Compared to sterile control conditions, anoxia prompted the transformation of initially dispersed colloids into larger particles through microbial activity. Curiously, the 16S rRNA and ITS microbial diversity analysis indicated that fungi were more strongly associated with anoxia-induced colloidal release than bacteria. These microbially induced shifts in Pcoll lead to its higher mobility and transport, with direct implications for P release from soil into floodwaters.

Investigation on the role of ·OH for BPA removal in coastal sediments: The important mediation of low reactivity Fe(II)

Bisphenol A (BPA) in seawater tends to be deposited in coastal sediments. However, its degradation under tidal oscillations has not been explored comprehensively. Hydroxyl radicals (·OH) can be generated through Fe cycling under redox oscillations, which have a strong oxidizing capacity. This study focused on the contribution of Fe-mediated production of ·OH in BPA degradation under darkness. The removal of BPA was investigated by reoxygenating six natural coastal sediments, and three redox cycles were applied to prove the sustainability of the process. The importance of low reactivity Fe(II) in the production of ·OH was investigated, specifically, Fe(II) with carbonate and Fe(II) within goethite, hematite and magnetite. The degradation efficiency of BPA during reoxygenation of sediments was 76.78-94.82%, and the contribution of ·OH ranged from 36.74% to 74.51%. The path coefficient of ·OH on BPA degradation reached 0.6985 and the indirect effect of low reactivity Fe(II) on BPA degradation by mediating ·OH production reached 0.5240 obtained via partial least squares path modeling (PLS-PM). This study emphasizes the importance of low reactivity Fe(II) in ·OH production and provides a new perspective for the role of tidal-induced ·OH on the fate of refractory organic pollutants under darkness.

Soil carbon and nitrogen cycles driven by iron redox: A review

Soil carbon and nitrogen cycles affect agricultural production, environmental quality, and global climate. Iron (Fe), regarded as the most abundant redox-active metal element in the Earth's crust, is involved in a biogeochemical cycle that includes Fe(III) reduction and Fe(II) oxidation. The redox reactions of Fe can be linked to the carbon and nitrogen cycles in soil in various ways. Investigating the transformation processes and mechanisms of soil carbon and nitrogen species driven by Fe redox can provide theoretical guidance for improving soil fertility, and addressing global environmental pollution as well as climate change. Although the widespread occurrence of these coupling processes in soils has been revealed, explorations of the effects of Fe redox on soil carbon and nitrogen cycles remain in the early stages, particularly when considering the broader context of global climate and environmental changes. The key functional microorganisms, mechanisms, and contributions of these coupling processes to soil carbon and nitrogen cycles have not been fully elucidated. Here, we present a systematic review of the research progress on soil carbon and nitrogen cycles mediated by Fe redox, including the underlying reaction processes, the key microorganisms involved, the influencing factors, and their environmental significance. Finally, some unresolved issues and future perspectives are addressed. This knowledge expands our understanding of the interconnected cycles of Fe, carbon and nitrogen in soils.

Structural Fe(II)-induced generation of reactive oxygen species on magnetite surface for aqueous As(III) oxidation during oxygen activation

Magnetite is a reductive Fe(II)-bearing mineral, and its reduction property is considered important for degradation of contaminants in groundwater and anaerobic subsurface environments. However, the redox condition of subsurface environments frequently changes from anaerobic to aerobic owing to natural and anthropogenic disturbances, generating reactive oxygen species (ROS) from the interaction between Fe(II)-bearing minerals and O2. Despite this, the mechanism of ROS generation induced by magnetite under aerobic conditions is poorly understood, which may play a crucial role in As(III) oxidation. Herein, we found that magnetite could activate O2 and induce the oxidative transformation of As(III) under aerobic conditions. As(III) oxidation was attributed to the ROS generated via structural Fe(II) within the magnetite octahedra oxygenation. The electron paramagnetic resonance and quenching tests confirmed that O2•-, H2O2, and •OH were produced by magnetite. Moreover, density function theory calculations combined with experiments demonstrated that O2•- was initially formed via single electron transfer from the structural Fe(II) to the adsorbed O2; O2•- was then converted to •OH and H2O2 via a series of free radical reactions. Among them, O2•-and H2O2 were the primary ROS responsible for As(III) oxidation, accounting for approximately 52 % and 19 % of As(III) oxidation. Notably, As(III) oxidation mainly occurred on the magnetite surface, and As was immobilized further within the magnetite structure. This study provides solid evidence regarding the role of magnetite in determining the fate and transformation of As in redox-fluctuating subsurface environments.

Quantification of the effect of biochar application on heavy metals in paddy systems: Impact, mechanisms and future prospects

Biochar (BC) has shown great potential in remediating heavy metal(loid)s (HMs) contamination in paddy fields. Variation in feedstock sources, pyrolysis temperatures, modification methods, and application rates of BC can result in great changes in its effects on HM bioavailability and bioaccumulation in soil-rice systems and remediation mechanisms. Meanwhile, there is a lack of application guidelines for BC with specific properties and application rates when targeting rice fields contaminated with certain HMs. To elucidate this topic, this review focuses on i) the effects of feedstock type, pyrolysis temperature, and modification method on the properties of BC; ii) the changes in bioavailability and bioaccumulation of HMs in soil-rice systems applying BC with different feedstocks, pyrolysis temperatures, modification methods, and application rates; and iii) exploration of potential remediation mechanisms for applying BC to reduce the mobility and bioaccumulation of HMs in rice field systems. In general, the application of Fe/Mn modified organic waste (OW) derived BC for mid-temperature pyrolysis is still a well-optimized choice for the remediation of HM contamination in rice fields. From the viewpoint of remediation efficiency, the application rate of BC should be appropriately increased to immobilize Cd, Pb, and Cu in rice paddies, while the application rate of BC for immobilizing As should be <2.0 % (w/w). The mechanism of remediation of HM-contaminated rice fields by applying BC is mainly the direct adsorption of HMs by BC in soil pore water and the mediation of soil microenvironmental changes. In addition, the application of Fe/Mn modified BC induced the formation of iron plaque (IP) on the root surface of rice, which reduced the uptake of HM by the plant. Finally, this paper describes the prospects and challenges for the extension of various BCs for the remediation of HM contamination in paddy fields and makes some suggestions for future development.

Rice straw biochar and lime regulate the availability of heavy metals by managing colloid-associated- but dissolved-heavy metals

Heavy metal (HM) pollution has extensively spread in agricultural soils, posing potential threats to food safety and human health. Biochar and lime are two amendments used to remediate the soils contaminated with HMs. However, colloids have been shown to increase the mobility of HMs in paddy soils. Nevertheless, limited investigations have been made into the impact of biochar and lime on the formation of colloid-associated (colloidal) HMs in paddy soils. In this study, column and microcosm incubation experiments were conducted to examine how biochar and lime affected the availability of HMs (arsenic, cadmium, copper, iron, manganese, lead, and zinc) in different layers of paddy soils. The results revealed that biochar significantly inhibited the formation of colloidal HMs in the soil flooding phase, whereas the lime increased the colloidal HMs. These colloids containing HMs were identified as poorly dissolved metal sulfides. When the soil was drained, colloidal HMs transformed into dissolved forms, thereby improving the availability of HMs. Biochar decreased HM availability by reducing colloidal- but dissolved- HMs, whereas lime had the opposite effect. Hence, biochar demonstrated a stable and reliable remediation ability to decrease HM availability in paddy soil during flooding and drainage processes. In conclusion, this study highlighted that biochar efficiently reduced HM availability by mitigating the formation of colloidal HMs during flooding and their transformation into dissolved HMs during drainage in paddy soils.

Effect of metal fractions on rice grain metal uptake and biological parameters in mica mines waste contaminated soils

Heavy metals from mica waste not only deteriorate the soil quality but also results in the uptake of metals in the crop. The present investigation was conducted to evaluate the effects of different fractions of metals on the uptake in rice, soil microbial and biochemical properties in mica waste-contaminated soils of Jharkhand, India. From each active mine, soil samples were randomly collected at distances of < 50 m (zone 1), 50-100 m (zone 2), and >100 m (zone 3). Sequential metal extraction was used to determine the fractions of different metals (nickel (Ni), cadmium (Cd), chromium (Cr) and lead (Pb)) including water-soluble (Ws) and exchangeable metals (Ex), carbonate-bound metals (CBD), Fe/Mn oxide (OXD) bound metals, organically bound metals (ORG), and residues (RS). The Ni, Cr, Cd and Pb in rice grain were 0.83±0.41, 0.41±0.19, 0.21±0.14 and 0.17±0.08 mg/kg respectively. From the variable importance plot of the random forest (RF) algorithm, the Ws fraction of Ni, Cr and Cd and Ex fraction of Pb was the most important predictor for rice grain metal content. Further, the partial dependence plots (PDP) give us an insight into the role of the two most important metal fractions on rice grain metal content. The microbial and enzyme activity was significantly and negatively correlated with Ws and Ex metal fractions, indicating that water-soluble and exchangeable fractions exert a strong inhibitory effect on the soil microbiological parameters and enzyme activities.

Soil moisture dynamics regulates the release rates and lability of copper in contaminated paddy soils

The climate changes have caused more extreme precipitation and drought events in the field and have exacerbated the severity of wet-dry events in soils, which will inevitably lead to severe fluctuations in soil moisture content. Soil moisture content has been recognized to influence the distribution of heavy metals, but how temporal changes of soil moisture dynamics affect the release rates and lability of heavy metals is still poorly understood, which precludes accurate prediction of environmental behavior and environmental risk of heavy metals in the field. In this study, we combined experimental and modeling approaches to quantify copper release rates and labile copper fractions in two paddy soils from southern China under different moisture conditions. Our kinetic data and models showed that the release rates and lability of copper were highly associated with the soil moisture contents, in which, surprisingly, high soil moisture contents effectively reduced the release rates of copper even with little changes in the reactive portions of copper in soils. A suite of comprehensive characterization on soil solid and solution components along the incubation suggested that soil microbes may regulate soil copper lability through forming microbially derived organic matter that sequestered copper and by increasing soil particle aggregation for protecting copper from release. This study highlights the importance of incorporating soil moisture dynamics into future environmental models. The experimental and modeling approaches in this study have provided basis for further developing predictive models applicable to paddy soils with varying soil moisture under the impact of climate change.

Heavy Metals in Agricultural Soils: Sources, Influencing Factors, and Remediation Strategies

Soil heavy metal pollution is a global environmental challenge, posing significant threats to eco-environment, agricultural development, and human health. In recent years, advanced and effective remediation strategies for heavy metal-contaminated soils have developed rapidly, and a systematical summarization of this progress is important. In this review paper, first, the anthropogenic sources of heavy metals in agricultural soils, including atmospheric deposition, animal manure, mineral fertilizers, and pesticides, are summarized. Second, the accumulation of heavy metals in crops as influenced by the plant characteristics and soil factors is analyzed. Then, the reducing strategies, including low-metal cultivar selection/breeding, physiological blocking, water management, and soil amendment are evaluated. Finally, the phytoremediation in terms of remediation efficiency and applicability is discussed. Therefore, this review provides helpful guidance for better selection and development of the control/remediation technologies for heavy metal-contaminated agricultural soils.

Cadmium accumulation in brown rice (Oryza sativa L.) depends on environmental factors and nutrient transport: A three-year field study

Cadmium (Cd) accumulation in brown rice is a complex process in agroecosystems and is influenced by multiple factors, such as climate, soil properties, and nutrient transport. However, during the Cd transport process (soil-root-straw-brown rice), it remains unclear how Cd concentration in brown rice (BCd) is causal relationship to environmental factors and nutrient transport. The differences in precipitation, soil properties, nutrient transport, and Cd transport were studied through a three-year fixed-point field trial and linked them to the standard of Cd and nutrient absorption and transport processes. The results showed that the available Cd concentration (ACd), and BCd in 2020 were lower than those in 2019 and 2021, but monthly precipitation (MP) was higher in 2020 than in 2019 and 2021. The MP and niche metrics were significantly negatively associated with ACd and BCd. However, the relationship between the form and location of different nutrient elements and Cd in roots, Cd in straws, and BCd also varied during the transport of nutrient elements and Cd from soil to root to straw to brown rice. Structural equation modelling analysis showed that nitrogen (N 15.5 %), phosphorus (P 14.1 %), silicon (Si 4.2 %), and iron (Fe 7.6 %) transport were more closely related to BCd than to potassium (K), calcium (Ca), magnesium (Mg), and manganese (Mn). The increase in MP significantly inhibited the increase in BCd, whereas the MP led to a decrease in BCd by affecting the transport of N and Fe. Among them, Si, Fe, and BCd had indirect causal relationships, whereas N, P, and BCd had direct causal relationships. Particularly, P is a crucial nutrient in reducing BCd in the Cd transport process. Our results highlight a strong causal relationship between environmental factors and nutrient transport and BCd, and provide a theoretical basis for fertiliser application in Cd-contaminated agroecosystems.

Degradation of Nitrobenzene by Mackinawite through a Sequential Two-Step Reduction and Oxidation Process

Mackinawite (FeS) has gained increasing interest due to its potential application in contaminant removal by either reduction or oxidation processes. This study further demonstrated the efficiency of FeS in degrading nitrobenzene (ArNO2) via a sequential two-step reduction and oxidation process under neutral conditions. In the reduction stage, FeS rapidly reduced ArNO2 to aniline (ArNH2), with nitrosobenzene (ArNO) and phenylhydroxylamine (ArNHOH) serving as the intermediates. X-ray photoelectron spectroscopy (XPS) analysis indicated that both Fe(II) and S(II) in FeS contributed electrons to the reduction of ArNO2. In the subsequent oxidation stage with oxygen, by addition of 0.5 mM tripolyphosphate (TPP), ArNH2 generated in the reduction process could be effectively oxidized to aminophenols by hydroxyl radicals (•OH), which would undergo eventual mineralization via ring-cleavage reactions. TPP exerted a favorable role in enhancing •OH production for ArNH2 degradation by promoting the formation of the dissolved Fe(II)-TPP complex, thus enhancing the homogeneous Fenton reaction. Additionally, TPP adsorption inhibited the surface oxidation reactivity of FeS due to the change of Fe(II) coordination. Finally, the effective degradation of ArNO2 by FeS in actual groundwater was demonstrated by using this sequential reduction and oxidation approach. These research findings provide a theoretical basis for a new FeS-based remediation approach, offering an alternative way for comprehensive removal of ArNO2.

Inhibitory effect of exogenous mineral elements (Si, P, Zn, Ca, Mn, Se, Fe, S) on rice Cd accumulation and soil Cd bioavailability in Cd-contaminated farmlands: A meta-analysis

A promising strategy for safely remediating Cd-contaminated farmland has been the application of mineral elements, which can reduce Cd accumulation in rice and inhibit its bioavailability in Cd-contaminated farmlands. However, there is still a lack of systematic and quantitative evaluations regarding how different mineral elements affect rice Cd accumulation and soil Cd bioavailability. Here, a meta-analysis was conducted based on 1062 individual observations from 137 published works to explore the effects of Si, P, Zn, Ca, Mn, Se, Fe and S in rice Cd accumulation and soil Cd bioavailability, we aimed to identify key factors that control the reduction of Cd concentration in rice grains. The results showed that the presence of exogenous elements had dramatically reduced rice grains Cd concentrations in the following decreasing order: Fe (43.03%) > P (38.45%) > Si (33.24%) > Ca (31.90%) > Se (29.83%) > Zn (25.95%) > Mn (23.26%) > S (18.78%). The elements of Ca, P and Si had strongly reduced Cd bioavailability in soils by 29.87%, 27.80% and 22.70%, respectively. The effects of these elements on Cd bioavailability appeared to be controlled by soil physio-chemical properties, such as pH, soil organic carbon (SOC) but also water management, application amounts and elemental forms. Overall, this study provides valuable insights into the potential of using exogenous mineral elements to mitigate Cd contamination in rice and farmlands, and facilitates the selection and application of mineral elements for the safe utilization of Cd-contaminated farmlands, taking into account soil properties and other factors that affect their effect.

Influence of sulfate reducing bacteria cultured from the paddy soil on the solubility and redox behavior of Cd in a polymetallic system

As a toxic heavy metal, cadmium (Cd) easily enters into rice while rice grains greatly contribute to the dietary Cd intake in the populations consuming rice as a staple food. The availability of Cd in paddy soil determines the accumulation of grain Cd. Soil drainage leads to the remobilization of Cd, increasing bioavailability of Cd. In contrast, soil flooding results in little contribution of soil Cd to grain Cd, which is generally attributed to sulfate reduction induced by sulfate-reducing bacteria (SRB) in paddy soils. However, effects of SRB cultured from the paddy soil on the solubility and redox behavior of Cd have been seldom investigated before. Here, we used SRB enrichment cultures to investigate the temporal dynamics of Cd2+. The results showed that SRB enrichment cultures efficiently reduced solution redox potential (Eh) to less than -100 mV and gradually increased pH to neutral, demonstrating their ability to create a good anaerobic environment. The solubility of Cd obviously decreased in the anaerobic phase and Cd2+ was transformed into poorly dissolved CdS near the SRB cell wall edge. The addition of Zn2+ and/or Fe2+ further improved the decrease in Cd solubility and facilitated the formation of polymetallic sulfides as a consequence of promoting the production of S0 and dissolved sulfides (S2-/HS-) and the transformation of S0 into S2-/HS-. Little of Cd was detected in the media upon reoxidation, which was probably due to the high pH and the interaction between CdS and ZnS/FeS. Conclusively, these results demonstrate the detailed dynamic processes that explain the essential role of SRB in regulating the redox dynamics of chalcophile heavy metals and their bioavailability in paddy soils.

Quantitative relationship between soil pH and electrical conductivity values and cadmium phytoavailability for Chinese cabbage under simulated conditions

Pot experiments were conducted to investigate the impacts of continuous addition of different concentrations of calcium chloride (CaCl2) and/or low-molecular-weight organic acids (LMWOAs) on soil pH, electrical conductivity (EC), and cadmium (Cd) transformation. These factors subsequently affected Cd phytoavailability in a system consisting of Cd-contaminated soil and Chinese cabbage (Brassica chinensis L.). The results indicate that CaCl2 addition had a greater impact on reducing soil pH value, increasing soil EC value, and enhancing Cd phytoaccumulation in Chinese cabbage compared to LMWOAs. When soil pH dropped by 0.3 unit and the soil EC increased by 500 µS cm-1, the Cd concentration in the Chinese cabbage shoots was 3 times higher than that in the control group. Throughout two planting terms of Chinese cabbage, the addition of CaCl2 (1.6-3.2 g kg-1) and LMWOAs (≤ 1.0 g kg-1) led to phytoextracted Cd concentration exceeding exchangeable Cd concentration in soil samples before the pot experiment. Regarding phytoextracted Cd, desorption from carbonate-bound Cd contributes more than desorption from bound to organic matter Cd and adsorption to Fe/Mn oxide Cd. This study underscores the influence of soil pH and EC value variations and Cd transformation on Cd phytoavailability. Special attention should be given to leafy vegetables grown in Cd-contaminated soil, as the phytoavailable Cd concentration reaches approximately 2.0 µg kg-1, which may lead to Cd levels surpassing acceptable limits for Chinese cabbage.

Identification of cadmium phytoavailability in response to cadmium transformation and changes in soil pH and electrical conductivity

Owing to complex changes in the soil environment, determining cadmium (Cd) phytoavailability is challenging. We devised a soil-wheat system to monitor alterations in soil pH, electrical conductivity (EC), and Cd transformation under various rates of calcium chloride and/or low-molecular-weight organic acids (LMWOAs) addition. The findings indicate that decreasing soil pH value, increasing soil EC value, and Cd transformation affect the phytoextraction of Cd. The exchangeable Cd and transformation of Cd under shifts in soil pH and EC contribute differentially to the phytoextracted Cd. The level of potentially phytoavailable Cd was identified through complete wheat cultivation in which the soil pH decreased by 0.47 unit and soil EC increased by 600-1000 μS cm-1, resembling the concentration of 0.01 M LMWOAs extractable Cd, when transitioning from paddy to dryland soil. Based on considering the phytoextracted Cd as the phytoavailable Cd throughout a complete wheat growth term, the threshold for phytoavailable Cd in soil, ensuring the safety of wheat grain (limit: 0.1 mg kg-1), is determined to be 2.90 μg kg-1. Maintaining control over Cd phytoavailability in soil emerges as the key factor in ensuring the safety of wheat grain cultivation.

Dynamic Production of Hydroxyl Radicals during the Flooding-Drainage Process of Paddy Soil: An In Situ Column Study

Frequent cycles of flooding and drainage in paddy soils lead to the reductive dissolution of iron (Fe) minerals and the reoxidation of Fe(II) species, all while generating a robust and consistent output of reactive oxygen species (ROS). In this study, we present a comprehensive assessment of the temporal and spatial variations in Fe species and ROS during the flooding-drainage process in a representative paddy soil. Our laboratory column experiments showed that a decrease in dissolved O2 concentration led to rapid Fe reduction below the water-soil interface, and aqueous Fe(II) was transformed into solid Fe(II) phases over an extended flooding time. As a result, the •OH production capacity of liquid phases was reduced while that of solid phases improved. The •OH production capacity of solid phases increased from 227-271 μmol kg-1 (within 1-11 cm depth) to 500-577 to 499-902 μmol kg-1 after 50 day, 3 month, and 1 year incubation, respectively. During drainage, dynamic •OH production was triggered by O2 consumption and Fe(II) oxidation. ROS-trapping film and in situ capture revealed that the soil surface was the active zone for intense H2O2 and •OH production, while limited ROS production was observed in the deeper soil layers (>5 cm) due to the limited oxygen penetration. These findings provide more insights into the complex interplay between dynamic Fe cycling and ROS production in the redox transition zones of paddy fields.

A combined strategy to mitigate the accumulation of arsenic and cadmium in rice (Oryza sativa L.)

Arsenic and cadmium in rice grain are of growing concern in the global food supply chain. Paradoxically, the two elements have contrasting behaviors in soils, making it difficult to develop a strategy that can concurrently reduce their uptake and accumulation by rice plant. This study examined the combined impacts of watering (irrigation) schemes, different fertilizers and microbial populations on the bioaccumulation of arsenic and cadmium by rice as well as on rice grain yield. Compared to drain-flood and flood-drain treatments, continuously flooded condition significantly reduced the accumulation of cadmium in rice plant but the level of arsenic in rice grain remained above 0.2 mg/kg, which exceeded the China national food safety standard. Application of different fertilizers under continuously flooded condition showed that compared to inorganic fertilizer and biochar, manure addition effectively reduced the accumulation of arsenic over three to four times in rice grain and both elements were below the food safety standard (0.2 mg/kg) while significantly increasing the rice yield. Soil Eh was the critical factor in the bioavailability of cadmium, while the behavior of arsenic in rhizosphere was associated with the iron cycle. The results of the multi-parametric experiments can be used as a roadmap for low-cost and in-situ approach for producing safe rice without compromising the yield.

Dramatically enhanced phenol degradation upon FeS oxygenation by low-molecular-weight organic acids

Oxidizing potential of FeS for organic contaminants degradation due to hydroxyl radicals (•OH) production has been recently documented, but the oxidizing efficiency was limited. Here, we revealed that low-molecular-weight organic acids (LMWOAs) can immensely enhance phenol degradation during FeS oxygenation due to increased utilization efficiency of FeS electron for •OH production. Upon oxygenation of 0.5 g/L FeS, phenol degradation boosted from 7.1% without LMWOAs to 91.5%, 84.6% and 95.0% with the addition of 1 mM oxalate, citrate and EDTA, respectively. Electron utilization efficiency of Fe(II) for •OH production dramatically rose from 0.3% with FeS alone to respective 2.0%, 2.5% and 2.7% in the LMWOAs systems. An increase in oxalate concentrations benefited •OH formation and phenol degradation. Coexisting oxalate led to an additional •OH production pathway from Fe(II)-oxalate oxidation, which expanded the O2 reduction to H2O2 from a two- to one-electron transfer process. Meanwhile, electron transfer from FeS to dissolved Fe(III)-oxalate promoted the redox cycling of Fe(III)/Fe(II), thus supplying the Fe(II) oxidation for •OH production. Moreover, the presence of oxalate decreased the crystallinity and particles size of lepidocrocite generated from FeS oxidation. Consequently, this study shed lights on the LMWOAs-enhanced contaminant degradation in either natural or engineered FeS oxidation systems.

Coal gangue-based magnetic porous material for simultaneous remediation of arsenic and cadmium in contaminated soils: Performance and mechanisms

Remediation of arsenic (As) and cadmium (Cd) co-contaminated soil is a challenge in environmental remediation. In this study, coal gangue-based magnetic porous material (MPCG) was designed for simultaneous immobilization of As and Cd in contaminated soil. After the incubation experiment, the effects of CG and MPCG on the availability and fractions of As and Cd and the related microbial functional genes were analyzed to explore the potential remediation mechanisms of MPCG for As and Cd in contaminated soil. The results showed that the stabilization effect of MPCG on As and Cd was significantly higher than that of coal gangue. It reduced the available As and Cd by 17.94-29.81% and 14.22-30.41%, respectively, and transformed unstable As/Cd to stable. The remediation mechanisms of MPCG on As included adsorption, oxidation, complexation and precipitation/co-precipitation. Meanwhile, the remediation mechanisms of MPCG for Cd included adsorption, ion exchange, complexation and precipitation. In addition, MPCG increases the abundance of sulfate-reducing bacteria (dsrA) by 43.39-381.28%, which can promote sulfate reduction. The sulfide can precipitate with As and Cd to reduce the availability of As and Cd in soil. Thus, MPCG is a promising amendment for achieving the remediation of As and Cd co-contaminated soil.

Reciprocal Interactions of Abiotic and Biotic Dechlorination of Chloroethenes in Soil

Chloroethenes (CEs) as common organic pollutants in soil could be attenuated via abiotic and biotic dechlorination. Nonetheless, information on the key catalyzing matter and their reciprocal interactions remains scarce. In this study, FeS was identified as a major catalyzing matter in soil for the abiotic dechlorination of CEs, and acetylene could be employed as an indicator of the FeS-mediated abiotic CE-dechlorination. Organohalide-respiring bacteria (OHRB)-mediated dechlorination enhanced abiotic CEs-to-acetylene potential by providing dichloroethenes (DCEs) and trichloroethene (TCE) since chlorination extent determined CEs-to-acetylene potential with an order of trans-DCE > cis-DCE > TCE > tetrachloroethene/PCE. In contrast, FeS was shown to inhibit OHRB-mediated dechlorination, inhibition of which could be alleviated by the addition of soil humic substances. Moreover, sulfate-reducing bacteria and fermenting microorganisms affected FeS-mediated abiotic dechlorination by re-generation of FeS and providing short chain fatty acids, respectively. A new scenario was proposed to elucidate major abiotic and biotic processes and their reciprocal interactions in determining the fate of CEs in soil. Our results may guide the sustainable management of CE-contaminated sites by providing insights into interactions of the abiotic and biotic dechlorination in soil.

Effects of blast furnace slag on the immobilization, plant uptake and translocation of Cd in a contaminated paddy soil

The potential toxicity of Cd to soil and rice plant severely threaten human health. This study was conducted to investigate the remediation effects of blast furnace slag (BFS) on the bioavailability of Cd in a contaminated paddy soil from a perspective of soil solution chemistry. Batch experiments, pot culture experiments, and principal analysis (PCA) were used to study the effects and mechanisms of BFS addition changing Cd chemical behavior and Cd toxicity. Results indicated that BFS facilitated Cd adsorption in soils, increased pH, Eh, and EC values in soil solution, whereas reduced dissolved Cd content. BFS amendment was efficient in decreasing root Cd intake and Cd upward transport in rice plant, with the Cd translocation factor in brown rice decreased by ∼ 75% (BFS treatment, 6‰ wt) relative to Cd treatment, which in turn increased rice biomass and grain yield. PCA indicated that the dissolved Cd concentration had a close relationship with soil pH and metal concentration in soil solution. Results from this study indicated that BFS had potential ability for either immobilization or remobilization of Cd in soils, and the findings have important implications for Cd-polluted soil remediation or other resource utilization with slag-based materials.

Oxidative compensation mechanism of Fe-S synergetic inhibition of Cd activity in paddy field during flooding and drainage

It is known that the transformation of Fe and S forms in soil affects the migration and activity of Cd, but the coordinated regulation of Cd activity by Fe and S under different redox conditions is still unclear. Here, Diffusive gradients in thin films (DGT), an in-situ monitoring technique, is used to explore the difference of the regulation of Cd activity in paddy fields with ferrihydrite (FH) and ferrihydrite coprecipitated by sulfate (FH-S) under the flooding and drainage conditions. The addition of FH-S and FH significantly reduced the activity of Cd (Dissolved, Exchanged, and CDGT-Cd). Compared with pure FH, the adsorption extent of Cd in FH was enhanced by increasing concentrations of SO42- (i.e., S/Fe ratio), which is attributed to the decrease in the crystallinity of FH by sulfate. During soil flooding, the addition of FH-S promoted the production of metal sulfide (CdS and FeS/FeS2). The activity of Cd increased after drainage, while the FH-S treatment groups delayed the release of Cd. After 30 days of drainage, the concentration of Cd in FH-S treatment groups decreased by 28.9-44.1 % compared with the control group. The fresh FeS/FeS2 is not the main adsorbent for fixing Cd, and due to the existence of oxidation compensation mechanism, the preferential oxidation of FeS/FeS2 delays the release of Cd in the drainage stage. Our study shed new light on the mechanism of Fe-S synergistic regulation of Cd and remediation of Cd-contaminated soils.

Soil ridging combined with biochar or calcium-magnesium-phosphorus fertilizer application: Enhanced interaction with Ca, Fe and Mn in new soil habitat reduces uptake of as and Cd in rice

Reducing the bioavailability of both cadmium (Cd) and arsenic (As) in paddy fields is a worldwide challenge. The authors investigated whether ridge cultivation combined with biochar or calcium-magnesium-phosphorus (CMP) fertilizer effectively reduces the accumulation of Cd and As in rice grains. Field trial showed that applying biochar or CMP on the ridges was similar to the continuous flooding, which maintained grain Cd at a low level, but grain As was reduced by 55.6%, 46.8% (IIyou28) and 61.9%, 59.3% (Ruiyou 399). Compared with ridging alone, the application of biochar or CMP decreased grain Cd by 38.7%, 37.8% (IIyou28) and 67.58%, 60.98% (Ruiyou399), and reduced grain As by 38.9%, 26.9% (IIyou28) and 39.7%, 35.5% (Ruiyou 399). Microcosm experiment showed that applying biochar and CMP on the ridges decreased As in soil solution by 75.6% and 82.5%, respectively, and kept Cd at a comparably low level at 0.13-0.15 μg L^-1. Aggregated boosted tree (ABT) analysis revealed that ridge cultivation combined with soil amendments altered soil pH, redox state (Eh) and enhanced the interaction of Ca, Fe, Mn with As and Cd, which promoted the concerted reduction of As and Cd bioavailability. Application of biochar on the ridges enhanced the effects of Ca and Mn to maintain a low level of Cd, and enhanced the effects of pH to reduce As in soil solution. Similar to ridging alone, applying CMP on the ridges enhanced the effects of Mn to reduce As in soil solution, and enhanced the effects of pH and Mn to maintain Cd at a low level. Ridging also promoted the association of As with poorly/well-crystalline Fe/Al and the association of Cd on Mn-oxides. This study provides an effective and environmentally friendly method to decrease Cd and As bioavailability in paddy fields and mitigate Cd and As accumulation in rice grain.