Interactions between arsenic adsorption/desorption and indigenous bacterial activity in shallow high arsenic aquifer sediments from the Jianghan Plain, Central China

Release characteristics of arsenic from sediments and its source or sink competition with phosphorus: A case of a great lake with grass-algae alternation

The arsenic (As) release from sediments in great lakes is affected by various factors. In this study, the characteristics of As release from sediments was investigated, and the As sources and sinks with the strengths in sediments from different areas (grass-type, algae-type, and grass-algae alternation areas) in great shallow lakes (Taihu Lake, China) were analyzed, and the influence of P competition in the process of As release was also studied. The results showed that changing trend of the values of equilibrium As concentration in sediments were consistent with the regional changes (0 to 28.12 µg/L), and the sediments from algae-type areas had the higher values. The sediments from western lake and northwest lake bay were a strong As and a weak P source, and the north lake bay had the opposite trend of these two regions. Intense P source competition with As from the sediments occurred in algae-type areas. The grass-type areas had strong As and P retention capacities, indicating a sink role of sediment with high As and P sorption capacities. The degree of As and P saturation had similar trend in sediments, and the grass-type areas had the higher values, 18.3%-21.4% and 15.31%-20.34%, respectively. Contribution analysis results showed that most of As release contribution was from the bottom (30-50 cm) sediments, and the surface (0-10 cm) sediments from algae-type areas contributed more to the overlying water than other region.

Extracellular electron shuttles induced transformation and mobilization of Fe/As with the occurrence of biogenic vivianite

Microorganisms that utilize organic matter to reduce Fe oxides/hydroxides constitute the primary geochemical processes controlling the formation of high-arsenic (As) groundwater. Biogenic secondary iron minerals play a significant role in As migration. However, the influence of quinone electron shuttles and competitive anionic phosphate on this process has not been thoroughly studied. In this study, 10 mM phosphate effectively increased the growth and reproduction of the indigenous metal-reducing bacterium Bacillus D2201, ensuring high biomass participation in goethite reduction. Three forms of goethite (pure goethite [Gt], goethite with coprecipitated As [Gt-As], and goethite with adsorbed As [Gt*As]) were synthesized and reduced by strain D2201 to investigate the fate of As/Fe. The results showed that the amount of Fe(II) released and precipitated in the Gt-As group with the addition of 9,10-anthraquinone-2,6-disulfonic acid (AQDS) and phosphate was the highest. Various solid-phase analytical techniques revealed that a significant amount of dissolved Fe(II) precipitated and formed the secondary mineral vivianite owing to phosphate input. Vivianite formation was pH-dependent, with high pH levels inhibiting vivianite development. As migration in the Gt-As system exhibited desorption and re-adsorption phenomena. The total As content decreased by 59.0 %, 53.7 %, and 49.4 %, at pH 6.0, 7.0, and 8.0, respectively, compared to the maximum As content values. The As re-adsorption percentage in the Gt*As group was lower than that in the Gt-As group, with decreases of 30.2 %, 16 %, and 10.3 % at pH, 6.0, 7.0, and 8.0, respectively. The results indicated that phosphate and AQDS enhanced goethite bioreduction and facilitated the migration of As and Fe. However, the subsequent formation of secondary vivianite resulted in the re-fixation of As and Fe. Our research suggested that metal-reducing bacteria may not universally facilitate As migration from sediments to groundwater, as previously assumed. This study highlights the effects of phosphate, As doping methods, and pH levels on As migration and transformation and refines theories on microbiologically induced high-As groundwater formation.

Migration, transformation of arsenic, and pollution controlling strategies in paddy soil-rice system: A comprehensive review

Arsenic pollution in paddy fields has become a public concern by seriously threatening rice growth, food security and human health. In this review, we delve into the biogeochemical behaviors of arsenic in paddy soil-rice system, systemically revealing the complexity of its migration and transformation processes, including the release of arsenic from soil to porewater, uptake and translocation of arsenic by rice plants, as well as transformation of arsenic species mediated by microorganism. Especially, microbial processes like reduction, oxidation and methylation of arsenic, and the coupling of arsenic with carbon, iron, sulfur, nitrogen cycling through microbes and related mechanisms were highlighted. Environmental factors like pH, redox potential, organic matter, minerals, nutrient elements, microorganisms and periphyton significantly influence these processes through different pathways, which are discussed in this review. Furthermore, the current progress in remediation strategies, including agricultural interventions, passivation, phytoremediation and microbial remediation is explored, and their potential and limitations are analyzed to address the gaps. This review offers comprehensive perspectives on the complicated behaviors of arsenic and influence factors in paddy soil-rice system, and provides a scientific basis for developing effective arsenic pollution control strategies.

Vertical distribution of arsenic and bacterial communities in calcareous farmland amending by organic fertilizer and iron-oxidizing bacteria: Field experiment on concomitant remediation

The migration and transformation mechanisms of arsenic (As) in soil environments necessitate an understanding of its influencing processes. Here, we investigate the subsurface biogeochemical transformation of As and iron (Fe) through amended in the top 20 cm with iron oxidizing bacteria (FeOB) and organic fertilizer (OF). Our comprehensive 400-day field study, conducted in a calcareous soil profile sectioned into 20 cm increments, involved analysis by sequential extraction and assessment of microbial properties. The results reveal that the introduction of additional OF increased the release ratio of As/Fe from the non-specific adsorption fraction (136.47 %) at the subsoil depth (40-60 cm), underscoring the importance of sampling at various depths and time points to accurately elucidate the form, instability, and migration of As within the profile. Examination of bacterial interaction networks indicated a disrupted initial niche in the bottom layer, resulting in a novel cooperative symbiosis. While the addition of FeOB did not lead to the dominance of specific bacterial species, it did enhance the relative abundance of As-tolerant Acidobacteria and Gemmatimonadetes in both surface (39.2 % and 38.76 %) and deeper soils (44.29 % and 23.73 %) compared to the control. Consequently, the amendment of FeOB in conjunction with OF facilitated the formation of poorly amorphous Fe (hydr)oxides in the soil, achieved through abiotic and biotic sequestration processes. Throughout the long-term remediation process, the migration coefficient of bioavailable As within the soil profile decreased, indicating that these practices did not exacerbate As mobilization. This study carries significant implications for enhancing biogeochemical cycling in As-contaminated Sierozem soils and exploring potential bioremediation strategies. ENVIRONMENTAL IMPLICATION: The long-term exposure of sewage irrigation has potential adverse effects on the local ecosystem, causing serious environmental problems. Microorganisms play a vital role in the migration and transformation of arsenic in calcareous soil in arid areas, which highlights the necessity of understanding its dynamics. The vertical distribution, microbial community and fate of arsenic in calcareous farmland soil profile in northwest China were studied through field experiments. The results of this work have certain significance for the remediation of arsenic-contaminated soil in arid areas, and provide new insights for the migration, transformation and remediation of arsenic in this kind of soil.

Fundamentals and application in phytoremediation of an efficient arsenate reducing bacterium Pseudomonas putida ARS1

Arsenic is a ubiquitous environmental pollutant. Microbe-mediated arsenic bio-transformations significantly influence arsenic mobility and toxicity. Arsenic transformations by soil and aquatic organisms have been well documented, while little is known regarding effects due to endophytic bacteria. An endophyte Pseudomonas putida ARS1 was isolated from rice grown in arsenic contaminated soil. P. putida ARS1 shows high tolerance to arsenite (As(III)) and arsenate (As(V)), and exhibits efficient As(V) reduction and As(III) efflux activities. When exposed to 0.6 mg/L As(V), As(V) in the medium was completely converted to As(III) by P. putida ARS1 within 4 hr. Genome sequencing showed that P. putida ARS1 has two chromosomal arsenic resistance gene clusters (arsRCBH) that contribute to efficient As(V) reduction and As(III) efflux, and result in high resistance to arsenicals. Wolffia globosa is a strong arsenic accumulator with high potential for arsenic phytoremediation, which takes up As(III) more efficiently than As(V). Co-culture of P. putida ARS1 and W. globosa enhanced arsenic accumulation in W. globosa by 69%, and resulted in 91% removal of arsenic (at initial concentration of 0.6 mg/L As(V)) from water within 3 days. This study provides a promising strategy for in situ arsenic phytoremediation through the cooperation of plant and endophytic bacterium.

Polystyrene and low-density polyethylene pellets are less effective in arsenic adsorption than uncontaminated river sediment

The adsorption process of inorganic arsenic (As) plays an important role in its mobility, bioavailability, and toxicity in the river environment. In this work, the adsorption of dissolved arsenite (As(III)) and arsenate (As(V)) by microplastics (MPs) pellets (polystyrene (PS) and low-density polyethylene (LDPE)), river sediment, and their mixture were investigated to assess the adsorption affinities and mechanism. The adsorption kinetics showed slow and mild rising zones from the natural behavior of the chemical adsorption. The results indicated that both MP characteristics and water properties played a significant role in the adsorption behavior of inorganic As species. The As adsorption equilibrium was modeled well by both Langmuir and Freundlich isotherms and partly fitted with the Sips model suggesting that both mono-layer and multi-layer adsorption occurred during adsorption The spontaneous adsorption process for both As(III) and As(V) was evidenced by the adsorption thermodynamics. The maximum adsorption capacities of As(III) and As(V) reached 143.3 mg/kg and 109.8 mg/kg on PS in deionized water, which were higher than those on sediment-PS mixture (119.3 mg/kg, 99.2 mg/kg), which were all lower than on sediment alone (263.3 mg/kg, 398.7 mg/kg). The Fourier transform infrared spectroscopy analysis identified that As(III) and As(V) interaction with sediment surface functional groups was the main adsorption mechanism from surface complexation and coordination. Two functional groups of polystyrene (-NH2, -OH) were mainly involved in the adsorption of inorganic As species on PS, while -COO- and -OH functional groups contributed to the adsorption mechanism of inorganic As species on LDPE. The findings provide valuable insight on the adsorption behavior and mechanisms of As(III) and As(V) in river systems in the presence of MPs particles. Both PS and LDPE were shown to be less effective than river sediment in the adsorption of As species from water, which provides a different perspective in understanding the scale of MPs impact in pollutant transport in the aquatic environment.

Hydrochemical characteristics and quality assessment of shallow groundwater in Yangtze River Delta of eastern China

Water resource is in high demand within the Yangtze River Delta, given its developed economy. Long-term exploitation of this resource has posed risks of artificial pollution and seawater intrusion to the shallow groundwater. This study aims to reveal the hydrochemical characteristics and health risks of shallow groundwater in the coastal plain of the Yangtze River Delta, as well as to discuss the possible factors affecting groundwater quality. Standard methods for hydrochemical parameter measurements, water quality assessment, and health risk models were applied to fulfill the objectives of the study. The results showed that the shallow groundwater was slightly alkaline, and the average values of total dissolved solids (TDS) and total hardness (TH) were 930.74 mg/L and 436.20 mg/L, respectively. The main hydrochemical types of groundwater were HCO₃-Ca·Mg and HCO₃-Ca·Na, accounting for 44.3% and 47.5%, respectively. In addition, As concentration was generally high, with a mean value of 0.0115 mg/L. The principal factors affecting the groundwater components include water-rock interactions (especially silicate), cation exchange, seawater intrusion, and human activities. The data also showed that As is strongly influenced by the redox of Fe, Mn, and NO₃^-. The results of the groundwater quality evaluation indicated that the shallow groundwater in some regions was unsuitable for drinking and agricultural irrigation. Health risk assessment showed that 44.3% of the water samples had significant health risks, which was attributed to the high As concentration. Therefore, it is urgent to establish long-term As monitoring to maintain sustainable groundwater management and drinking water safety. The results of this study provide essential data for water resource management and human health security in the Yangtze River Delta.

Advances in As contamination and adsorption in soil for effective management

Arsenic (As) is a heavy metal that causes widespread contamination and toxicity in the soil environment. This article reviewed the levels of As contamination in soils worldwide, and evaluated how soil properties (pH, clay mineral, organic matter, texture) and environmental conditions (ionic strength, anions, bacteria) affected the adsorption of As species on soils. The application of the adsorption isotherm models for estimating the adsorption capacities of As(III) and As(V) on soils was assessed. The results indicated that As concentrations in contaminated soil varying significantly from 1 mg/kg to 116,000 mg/kg, with the highest concentrations being reported in Mexico with mining being the dominating source. Regarding the controlling factors of As adsorption, soil pH, clay mineral and texture had demonstrated the most significant impacts. Both Langmuir and Freundlich isotherm models can be well fitted with As(III) and As(V) adsorption on soils. The Langmuir adsorption capacity varied in the range of 22-42400 mg/kg for As(V), which is greater than 45-8901 mg/kg for As(III). The research findings have enhanced our knowledge of As contamination in soil and its underlying controls, which are critical for the effective management and remediation of As-contaminated soil.

Identification and Characterization of Arsenic Transforming Bacillus Species from Abandoned Mining Regions of Madhya Pradesh and Jharkhand

The arsenic (As) comprehensiveness in nature has aggravated the expansion of arsenic fortification and detoxification components in microorganisms. Many microorganisms discovered today with ability to oxidize arsenite (As3+) into arsenate (As5+) or reduce As5+ to As3+. In this study, two bacterial strains designated 3AB3 and 5AB2 was isolated from the soil samples collected from abandoned mining region of Madhya Pradesh and Jharkhand, India and arsenic concentration has been determined in both water and soil samples. Enrichment culturing method was employed for isolating bacteria and further they are screened for their redox ability. The isolated strains exhibited maximum growth at 30°C, at pH 7.0 in arsenic stressed Luria Bertani broth, checked through UV-Vis spectrophotometer at OD-620nm. Biochemical characterization of isolated strains was performed with various confirmation tests. Phylogenetic analysis of selected bacterial strains through MEGA-X confirmed their relationship to the genus Bacillus. Further, they are tested for transformation ability of arsenic (MSA method) and gene identification was done in selected isolated strains (PCR method). The result of this study shows that, even after abandoning the mining activities, concentration of arsenic increases in ground water by reducing ability of bacterial strains. PCR analysis depicted the presence of genes arsR, arsB and arsC in the strain 3AB3 and gene aoxB in 5AB2 respectively.

Bacterially mediated release and mobilization of As/Fe coupled to nitrate reduction in a sediment environment

Metal-reducing bacteria play an important role in the release and mobilization of arsenic from sediments into groundwater. This study aimed to investigate the influence of nitrate on arsenic bio-release. Microcosm experiments consisting of high arsenic sediments and indigenous bacterium Bacillus sp. D2201 were conducted and the effects of nitrate on the mobilization of As/Fe determined. The results show arsenic release is triggered by iron reduction, which is regulated by nitrate. Increasing the nitrate concentration from 0 to 1 and 3 mM decreased Fe(III) reduction by 62.5% and 16.9% and decreased As(V) bio-release by 41.5% and 85.5%, respectively. Moreover, the results of step-wise Wenzel sequential extractions indicate nitrate addition prevents the transformation of poorly crystalline iron oxides to well crystalline iron oxides. Overall, nitrate appears to have a dual effect, inhibiting both iron reduction and arsenic release by incubation strain D2201. This study offers new insights regarding the biogeochemistry of arsenic in groundwater systems.

Efficient removal of As(Ш) via the synergistic effect of oxidation and absorption by FeOOH@MnO2@CAM nano-hybrid adsorption membrane

Due to the neutral charge of As(III) oxy-ions that make approaching the traditional adsorbent very improbable compared to the As(V) case, making it harder to be separated. To enhance the adsorption of As(Ш), the FeOOH coated cellulose acetate (CA) membrane doped with MnO₂ nanoparticles (FeOOH@MnO₂@CAM) was fabricated and then to removes As(Ш) in water through the synergistic effect of oxidation and adsorption, and the maximum adsorption capacity can reach 50.34 mg/g. FeOOH@MnO₂@CAM was fabricated with CA as a substrate by dipping-precipitation phase inversion and hydrothermal method. Langmuir and pseudo-second-order model showed that As(Ш) was adsorbed by chemical interactions through the monolayer and thermodynamic showed that As(Ш) adsorption was an exothermic and spontaneous process. The results of the pH study showed that as the pH increases from 3 to 11, the adsorption capacity of As(Ш) decreases from 50.34 to 14.32 mg/g, which was attributed to the acidic environment promoting the protonation of the surface of FeOOH@MnO₂@CAM, which increases the electrostatic attraction, and the alkaline environment increases electrostatic repulsion due to deprotonation. The competitive ions exhibited the PO₄^3- significantly reduce the adsorption capacity of As(Ш)，and as the PO₄^3- content increases, the adsorption capacity of As(Ш) decreases from 29.76 to 18.57 mg/g, which was attributed to the similar chemical properties of PO₄^3- and arsenate. Importantly, FeOOH@MnO₂@CAM still maintains an adsorption capacity of 20.19 mg/g after seven cycles, demonstrating that it is a kind of environmentally friendly material to remove As(Ш) in the water environment.

Effects of dam construction on arsenic mobility and transport in two large rivers in Tibet, China

Construction of dams on the Singe Tsangpo (ST) and the Yarlung Tsangpo (YT) Rivers, the upper stretch of the Indus and the Brahmaputra Rivers, respectively, are expected to affect material transport. To evaluate the effects of dam construction on arsenic (As) mobility and transport in the ST River and the YT River in Tibet and the downstream river basins, water column and sediment core samples in the Shiquan Reservoir of the ST River and in the Zam Reservoir of the YT River were obtained in August 2017, and January and May 2018, and additionally, at the inflows and outfalls of the reservoirs. The seasonal variation of dissolved As contents in the inflow water of the Zam Reservoir and the Shiquan Reservoir was regulated by the mixing between the low-As river runoff and the high-As hot spring input. Water residence time (WRT) is a key variable regulating the variation of dissolved As contents in reservoirs and outflow waters with time. The absence of the oxic layer at the sediment-water interface reduced the accumulation of As in surface sediments under high-flow conditions. Arsenic mobility in sediment of the two reservoirs was mainly controlled by Mn oxides and organic matter. Reservoirs with long water residence time are more favorable for As retention. Sedimentation was the main mechanism of As retention. The Shiquan Reservoir with a longer WRT of 385 days can effectively retain 55% of the total arsenic load from upstream, while the Zam Reservoir has no effective retention of arsenic due to the very short WRT of 1.1 days. These have important implications on the geochemical and ecological environments of the downstream river basins.

An indigenous bacterium Bacillus XZM for phosphate enhanced transformation and migration of arsenate

A number of arsenate-reducing bacteria respire adsorbed As(V), producing As(III) and thus contributing to arsenic mobilization from the solid phase to the aqueous phase. Two arsenate reducing genes, arsC and arrA, were both amplified in an indigenous bacterium Bacillus XZM isolated from high arsenic aquifer sediments. The effect of phosphate input on this novel bacterium in terms of mediating the biogeochemical behavior of arsenic was investigated for the first time. The results show bacterial growth and arsenate reduction appear to increase with the addition of phosphate. Input of 1 mM phosphate reduced the negative effects of As(V) on bacterial growth, resulting in 55-60% greater biomass production compared to lower phosphate inputs (0.01 and 0.1 mM). The data of real-time quantitative PCR (qPCR) indicated arsenate was involved in the expressions of two arsenate reductase genes (arsC and arrA genes) in indigenous bacterium Bacillus XZM. Overall, the addition of phosphate (from 0.1 to 1 mM) resulted in a doubling of arsenate bio-desorption from the sediment into the aqueous medium. Oxidation-reduction potential, as an environmental indicator of the bacterial reduction of metals, declined to -200 mV in the presence of strain XZM and 1 mM phosphate in the microcosm. Phosphate input enhanced arsenic biomigration, indicating the effect of phosphate concentration should be considered when studying the biogeochemical behavior of arsenic.