Importance of chemical binding type between As and iron-oxide on bioaccessibility in soil: Test with synthesized two line ferrihydrite

Organic amendments temporarily change arsenic speciation and bioaccessibility in a lead and arsenic co-contaminated urban soil

Organic amendments often reduce the bioaccessibility of soil lead (Pb) but not that of soil arsenic (As). The effect of Pb on As bioaccessibility is rarely studied in co-contaminated soils. In a field study, we assessed the effect of mushroom compost, leaf compost, noncomposted biosolids, and composted biosolids amendments on As speciation in a co-contaminated (As and Pb) soil at 7, 349, and 642 days after amending soils and the change of As speciation during an in vitro bioaccessibility extraction (gastric solution, pH 2.5) using bulk X-ray absorption near-edge structure spectroscopy. Soil was contaminated by coal combustion and other diffuse sources and had low As bioaccessibility (7%-12%). Unamended soil had As(III) sorbed onto pyrite (As(III)-pyrite; ∼60%) and As(V) adsorbed onto Fe oxy(hydr)oxides (As(V)-Fh; ∼40%). In amended soils, except in composted biosolids-amended soils, at 7 days, As(V)-Fh decreased to 15%-26% and redistributed into As(III)-Fh and/or As(III)-pyrite. This transformation was most pronounced in mushroom compost amended soil resulting in a significant (46%) increase of As bioaccessibility compared to the unamended soil. Composted biosolids-amended soils had relatively stable As(V)-Fh. Lead arsenate formed during the in vitro extraction in amended soils, except in composted biosolids-amended soils. Arsenic speciation and bioaccessibility were similar in 349- and 642-day in all the amended and unamended soils. Reduction of As(V)-Fh to As(III) forms in the short term in three of the amended soils showed the potential to increase As bioaccessibility. The formation of stable lead arsenate during the in vitro extraction would counteract the short-term increase of As bioaccessibility in those amended soils.

Simultaneous removal of Cd(II) and As(V) by ferrihydrite-biochar composite: Enhanced effects of As(V) on Cd(II) adsorption

The coexistence of cadmium (Cd(II)) and arsenate (As(V)) pollution has long been an environmental problem. Biochar, a porous carbonaceous material with tunable functionality, has been used for the remediation of contaminated soils. However, it is still challenging for the dynamic quantification and mechanistic understanding of the simultaneous sequestration of multi-metals in biochar-engineered environment, especially in the presence of anions. In this study, ferrihydrite was coprecipitated with biochar to investigate how ferrihydrite-biochar composite affects the fate of heavy metals, especially in the coexistence of Cd(II) and As(V). In the solution system containing both Cd(II) and As(V), the maximum adsorption capacities of ferrihydrite-biochar composite for Cd(II) and As(V) reached 82.03 µmol/g and 531.53 µmol/g, respectively, much higher than those of the pure biochar (26.90 µmol/g for Cd(II), and 40.24 µmol/g for As(V)) and ferrihydrite (42.26 µmol/g for Cd(II), and 248.25 µmol/g for As(V)). Cd(II) adsorption increased in the presence of As(V), possibly due to the changes in composite surface charge in the presence of As(V), and the increased dispersion of ferrihydrite by biochar. Further microscopic and mechanistic results showed that Cd(II) complexed with both biochar and ferrihydrite, while As(V) was mainly complexed by ferrihydrite in the Cd(II) and As(V) coexistence system. Ferrihydrite posed vital importance for the co-adsorption of Cd(II) and As(V). The different distribution patterns revealed by this study help to a deeper understanding of the behaviors of cations and anions in the natural environment.

Risk assessment of antimony-arsenic contaminated soil remediated using zero-valent iron at different pH values combined with freeze-thaw cycles

Soil in mining wastelands is seriously polluted with heavy metals. Zero-valent iron (ZVI) is widely used for remediation of heavy metal-polluted soil because of its excellent adsorption properties; however, the remediation process is affected by complex environmental conditions, such as acid rain and freeze-thaw cycles. In this study, the effects of different pH values and freeze-thaw cycles on remediation of antimony (Sb)- and arsenic (As)-contaminated soil by ZVI were investigated in laboratory simulation experiments. The stability and potential human health risks associated with the remediated soil were evaluated. The results showed that ZVI has a significant stabilizing effect on Sb and As in both acidic and alkaline soils contaminated with dual levels of Sb and As, and the freeze-thaw process in different pH value solution systems further enhances the ability of ZVI to stabilize Sb and As, especially in acidic soils. However, it should be noted that apart from the pH=1.0 solution environment, ZVI's ability to stabilize As is attenuated under other circumstances, potentially leading to leaching of its unstable form and thereby increasing contamination risks. This indicates that the F1 (2% ZVI+pH=1 solution+freeze-thaw cycle) processing exhibits superior effectiveness. After F1 treatment, the bioavailability of Sb and As in both soils also significantly decreased during the gastric and intestinal stages (about 60.00%), the non-carcinogenic and carcinogenic risks of Sb and As in alkaline soils are eliminated for children and adults, with a decrease ranging from 60.00% to 70.00%, while in acidic soil, the non-carcinogenic and carcinogenic risks of As to adults and children is acceptable, but Sb still poses non-carcinogenic risks to children, despite reductions of about 65.00%. These findings demonstrate that soil pH is a crucial factor influencing the efficacy of ZVI in stabilizing Sb and As contaminants during freeze-thaw cycles. This provides a solid theoretical foundation for utilizing ZVI in the remediation of Sb- and As-contaminated soils, emphasizing the significance of considering both pH levels and freeze-thaw conditions to ensure effective and safe treatment.

Effects of in situ Fe oxide precipitation on As stabilization and soil ecological resilience under salt stress

Iron (Fe) oxide precipitation is a promising method for stabilizing arsenic (As) in contaminated soils; however, the addition of salts during the process can negatively affect soil functions. This study investigated the effects of in situ Fe oxide precipitation on As stabilization and the impact of salt stress on soil functions and microbial communities. Fe oxide precipitation reduced the concentration of bioaccessible As by 84% in the stabilized soil, resulting in the formation of ferrihydrite and lepidocrocite, as confirmed by XANES. Nevertheless, an increase in salt stress reduced barley development, microbial enzyme activities, and microbial diversity compared to those in the original soil. Despite this, the stabilized soil exhibited natural resilience and potential for enhanced microbial adaptations, with increased retention of salt-tolerant bacteria. Washing the stabilized soil with water restored EC1:5 to the level of the original soil, resulting in increased barley growth rates and enzyme activities after 5-d and 20-week incubation periods, suggesting soil function recovery. 16 S rRNA sequencing revealed the retention of salt-tolerant bacteria in the stabilized soil, while salt-removed soil exhibited an increase in Proteobacteria, which could facilitate ecological functions. Overall, Fe oxide precipitation effectively stabilized soil As and exhibited potential for restoring the natural resilience and ecological functions of soils through microbial adaptations and salt removal.

P/Pb transport at the interface of water and Al-substituted ferrihydrite: Effect of P/Pb loading sequence

Sediment mineral such as Al-substituted ferrihydrite plays a critical role for contaminant transport in the river systems. Heavy metals and nutrient pollutants often coexisted in the natural aquatic environment, and they may enter the river at different time frames, altering the fate and transport of each other subsequently discharged into the river. However, most studies focused on the simultaneous adsorption of co-existing pollutants instead of their loading sequence. In this study, the transport of P and Pb at the interface of Al-substituted ferrihydrite and water was investigated under different P and Pb loading sequences. The results showed that preloaded P provided additional adsorption sites for the following adsorption of Pb, with enhanced Pb adsorption amount and accelerated adsorption process. Moreover, Pb preferred to be bounded with the preloaded P to form P-O-Pb ternary complexes rather than directly reacted with Fe-OH. The formation of the ternary complexes effectively prevented the release of Pb once adsorbed. However, the adsorption of P was slightly affected by the preloaded Pb, and most of the P were adsorbed onto Al-substituted ferrihydrite directly with the formation of Fe/Al-O-P. Moreover, the release process of the preloaded Pb was significantly inhibited by the following adsorbed P due to the formation of Pb-O-P. Meanwhile, the release of P was not detected from all P and Pb loaded samples of different adding sequence due to the high affinity between P and the mineral. Thus, the transport of Pb at the interface of Al-substituted ferrihydrite was seriously influenced by the adding sequence of Pb and P, while the transport of P was not sensitive to the adding sequence. The results provided important information for the transport of heavy metal and nutrients in river system with different discharging sequence, and offered new insights to further understand the secondary pollution in multi-contaminated river.

The novel chitosan-amphoteric starch dual flocculants for enhanced removal of Microcystis aeruginosa and algal organic matter

A novel flocculation strategy for simultaneously removing Microcystis aeruginosa and algal organic matter (AOM) was proposed using chitosan-amphoteric starch (C-A) dual flocculants in an efficient, cost-effective and ecologically friendly way, providing new insights for harmful algal blooms (HABs) control. A dual-functional starch-based flocculant, amphoteric starch (AS) with high anion degree of substitution (DS_A) and cation degree of substitution (DS_C), was prepared using a cationic moiety of 3-chloro-2-hydroxypropyltrimethylammonium chloride (CTA) coupled with an anion moiety of chloroacetic acid onto the backbone of starch simultaneously. In combination of the results of FTIR, XPS, ¹H NMR, ¹³C NMR, GPC, EA, TGA and SEM, it was evidenced that the successfully synthesized AS with excellent structural characteristics contributed to the enhanced flocculation of M. aeruginosa. Furthermore, the novel C-A dual flocculants could achieve not only the removal of >99.3 % of M. aeruginosa, but also the efficacious flocculation of algal organic matter (AOM) at optimal concentration of (0.8:24) mg/L, within a wide pH range of 3-11. The analysis of zeta potential and cellular morphology revealed that the dual effects of both enhanced charge neutralization and notable netting-bridging played a vital role in efficient M. aeruginosa removal.

New insights into surface behavior of dimethylated arsenicals on montmorillonite using X-ray absorption spectroscopy

Although recent studies have revealed the occurrence of dimethylated arsenicals, little is known about their behavior in environment. This study investigates the adsorption behavior of dimethylarsinic acid (DMA^V), dimethyldithioarsinic acid (DMDTA^V), and dimethylmonothioarsinic acid (DMMTA^V) on montmorillonite. Complicated transformations among arsenicals under normal environmental conditions were also considered. Our results clearly demonstrate that DMDTA^V was oxidized to DMMTA^V, which was relatively stable but partially transformed to DMA^V when exposed to air during adsorption. The transformed DMA^V exhibited high adsorption affinities for montmorillonite, while DMMTA^V and DMDTA^V were not appreciably retained by montmorillonite for 48 h. This is the first study to provide insights into DMDTA^V oxidation under environmental conditions. X-ray absorption near edge structure and extended X-ray absorption fine structure studies confirmed that most of the adsorbed arsenicals on montmorillonite were DMA^V. The significantly different bonding characteristics of each adsorbed DMA^V provide direct evidence for the transformation of DMA^V from DMDTA^V and DMMTA^V. Our study suggests the importance of incorporating the DMMTA^V in the realistic risk management for soil environments because it is highly toxic, easily transformed from DMDTA^V, and stable in the environment.

Adsorption Characteristics of Dimethylated Arsenicals on Iron Oxide-Modified Rice Husk Biochar

In this study, the adsorption characteristics of dimethylated arsenicals to rice husk biochar (BC) and Fe/biochar composite (FeBC) were assessed through isothermal adsorption experiments and X-ray absorption spectroscopy analysis. The maximal adsorption capacities (q_m) of inorganic arsenate, calculated using the Langmuir isotherm equation, were 1.28 and 6.32 mg/g for BC and FeBC, respectively. Moreover, dimethylated arsenicals did not adsorb to BC at all, and in the case of FeBC, q_m values of dimethylarsinic acid (DMA(V)), dimethylmonothioarsinic acid (DMMTA(V)), and dimethyldithioarsinic acid (DMDTA(V)) were calculated to be 7.08, 0.43, and 0.28 mg/g, respectively. This was due to the formation of iron oxide (i.e., two-line ferrihydrite) on the surface of BC. Linear combination fitting using As K-edge X-ray absorption near edge structure spectra confirmed that all chemical forms of dimethylated arsenicals adsorbed on the two-line ferrihydrite were DMA(V). Thus, FeBC could retain highly mobile and toxic arsenicals such as DMMTA(V) and DMDTA(V)) in the environment, and transform them into DMA(V) with relatively low toxicity.

Remediation of cadmium and lead contaminated soils using Fe-OM based materials

Iron oxide-lignin composites (GLS) were prepared based on the significant role of Fe-OM in the environmental behaviour of heavy metals and lignin binding with iron oxide preferentially in soil. GLS was applied in Cd/Pb immobilization and the stability under acid rain was investigated. The results show that the iron oxide appeared weakly crystalline or amorphous similar to 2-line ferrihydrite after the addition of lignin. Agglomerates of nanoparticles with higher adsorption capacity were observed for GLS. The mobility factor (MF) of Cd/Pb in the soil decreased rapidly after adding GLS. At the 3% dosage, the MF of Cd and Pb in the soil was decreased by 58.94% and 78.15% respectively, which was approximately 5 times that of goethite (GE). The mobile and exchangeable Cd/Pb were converted to organic, amorphous Fe oxide-bound and residue fractions. Under acid rain conditions, MF continues to decline for the GLS group, increasing the organic and amorphous Fe oxide-bound fractions, while for control group (CK) and GE, the trend was the opposite. Lignin could inhibit iron oxide dissolution and stabilize the combination of Cd/Pb and iron oxides in soil. The better stability performance of GLS for Cd/Pb may be related to the higher adsorption capacity and microstructural difference after iron oxide combined with lignin.

Effect of soil particle size and extraction method on the oral bioaccessibility of arsenic

Recent findings indicate that incidental ingestion of soil by humans primarily involves soil particles <150 µm, rather than <250 µm-sized fraction previously used for most oral bioaccessibility and bioavailability studies. It was postulated that a greater soil surface area in the finer fraction (<150 versus <250 µm) might increase oral bioaccessibility of arsenic (As) in soil. Bioaccessibility and concentrations of As were compared in <150 and <250 µm fractions of 18 soil samples from a variety of arsenic-contaminated sites. The two methods used to measure bioaccessibility were compared - EPA Method 1340 and the California Arsenic Bioaccessibility (CAB) method. Arsenic concentrations were nearly the same or higher in the <150 fraction compared with <250 µm. EPA Method 1340 and the CAB method presented significantly different bioaccessibility results, as well as estimated relative oral bioavailability (RBA) based upon algorithms specific to the methods, but there was no marked difference for <150 and <250 µm soil fractions within either method. When compared with RBA determined previously for these soil samples in vivo in non-human primates, EPA Method 1340 was generally more predictive than the CAB method. Data suggest that soil- or site-specific factors control bioaccessibility under either method and that the test method selected is more important than the particle size fraction (<150 or <250) in using these in vitro methods to predict As RBA for use in risk assessment.

Simultaneous adsorption of As(III) and Cd(II) by ferrihydrite-modified biochar in aqueous solution and their mutual effects

A simply synthetic ferrihydrite-modified biochar (Fh@BC) was applied to simultaneously remove As(III) and Cd(II) from the aqueous solution, and then to explore the mutual effects between As(III) and Cd(II) and the corresponding mechanisms. The Langmuir maximum adsorption capacities of As(III) and Cd(II) in the single adsorbate solution were 18.38 and 18.18 mg g⁻¹, respectively. It demonstrated that Fh@BC was a potential absorbent material for simultaneous removal of As(III) and Cd(II) in aqueous solution. According to the XRF, SEM–EDS, FTIR, XRD, and XPS analysis, the mechanisms of simultaneous removal of As(III) and Cd(II) by Fh@BC could be attributable to the cation exchange, complexation with R-OH and Fe-OH, and oxidation. Moreover, the mutual effect experiment indicated that Cd(II) and As(III) adsorption on Fh@BC in the binary solution exhibited competition, facilitation and synergy, depending on their ratios and added sequences. The mechanisms of facilitation and synergy between Cd(II) and As(III) might include the electrostatic interaction and the formation of both type A or type B ternary surface complexes on the Fh@BC.

Stabilization of fluorine-contaminated soil in aluminum smelting site with biochar loaded iron-lanthanide and aluminum-lanthanide bimetallic materials

Trivalent metals-modified-biochar (BC) has been widely used for the removal of fluorine (F) in water, but little is known about its effects on the stability and mobility of F-contaminated soil. Two types of modified-BC materials (BC-loaded iron-lanthanide (BC/Fe-La) and BC-loaded aluminum-lanthanide (BC/Al-La)) were synthesized and used for the remediation of F-contaminated soil. The forms of BC/La_xFe_3x(OH)_y in BC/Fe-La and BC/La_xAl_3x(OH)_y in BC/Al-La were identified by spectroscopy, X-ray dispersion, thermogravimetric, and pore diameter/volume analyses. Following application (4-12%, w/w) to F-contaminated soil for 30 d, water soluble fluoride (WSF) decreased significantly. The modified-BC with a 1:1:1 molar ratio (BC: Al³⁺ or Fe³⁺: La³⁺) were more effective than those at 1:0.5:0.5. The BC/Al-La were the most effective to stabilize F. In particular, the highest decrease in WSF (by 91.75%) was obtained with the application of 12% BC/Al-La-2, while 8% BC/Al-La-2% and 12% BC/Al-La-1 reduced the WSF by 87.58% and 90.17%, respectively; all values obtained were lower than the national standard of China (< 1.5 mg/L). In addition, the sequential extraction results showed that modified-BC promoted the transformation of the other chemical speciation to the Fe/Mn-F.

The role of soil arsenic fractionation in the bioaccessibility, transformation, and fate of arsenic in the presence of human gut microbiota

Soil arsenic (As) fractionation and its bioaccessibility are two important factors in human health risk assessment. However, data related to the impact of As minerals on the bioaccessibility with human gut microbiota involvement are scarce. In this study, speciation analysis was determined using HPLC-ICP-MS and XANES after incubation with colon microbiota from human origin, in combination with sequential extraction. Significant increase of colon As bioaccessibility was contributed primarily from As associated with amorphous and crystalline Fe/Al (hydr)oxides. We found a high degree of transformation at higher bioaccessibility (ave. 40 % of total As), which was predominantly present as liquid-phase As. In contrast, As transformation occurred mainly in the solid phase at lower bioaccessibility (< 5%), especially for soils containing As-S species. XANES spectroscopy revealed that As(III) increased by about 20 % in soil residues. Finally, the excreted As may be predominantly in association with (alumino)silicate minerals by SEM-EDX. It inferred that the priority sequence in As transformation by human gut microbiota was dissolved As(V), As(V) sorbed to mineral surfaces, crystalline As(V)-bearing minerals and As sulfides. This study will shed new light on the role of As-bearing minerals in evaluating health risks from soil As exposure.

Reduction of bioaccessibility of As in soil through in situ formation of amorphous Fe oxides and its long-term stability

The bioaccessibility of As in soil, rather than its total concentration, is closely related to its potential risk. In this study, the in situ formation of amorphous Fe oxides was applied to As-contaminated soil to induce As-Fe coprecipitates that can withstand the gastric digestion condition of human beings. To promote the formation of Fe oxides, 2% ferric nitrate (w/w) and 30% water (v/w) were introduced, and the pH was adjusted to ~7. The chemical extractability of As in soil was determined using the solubility/bioavailability research consortium method and five-step sequential extraction. In situ formation of Fe oxides resulted in a remarkable increase in the As associated with amorphous Fe oxides, decreasing most of the exchangeable As (i.e., the sum of SO₄^2- and PO₄^3- extractable As), and thereby reducing the bioaccessibility of As. The types of association between As and Fe oxides were investigated using X-ray absorption spectroscopy analysis. Linear combination fit (LCF) analysis demonstrated that As bound to amorphous Fe oxides could exist as coprecipitates with ferrihydrite and schwertmannite after stabilization. The bioaccessibility of the coprecipitated As in soil further decreased as amorphous Fe oxides transformed to crystalline form with time, which was supported by the LCF results showing an increase of goethite in aged soil.

Effect of neutralizing agents on the type of As co-precipitates formed by in situ Fe oxides synthesis and its impact on the bioaccessibility of As in soil

The bioaccessibility of heavy metals in soil is closely related to their potential risk. Therefore, developing techniques for reducing it needs considerable attention. In this study, we aimed to co-precipitate soil As(V) through an in situ formation of Fe oxides, thereby reducing its bioaccessibility. Soil As(V) was co-precipitated by introducing 2% Fe-nitrate (w/w) and 30% water (v/w) into soil at pH ~7. Two different neutralizing agents (NaOH and CaO) were used to induce the precipitation of Fe oxides, and their effects on the speciation of As were investigated. In all the stabilized soils, the exchangeable As fraction decreased, and the fraction of As bound to amorphous Fe oxides increased by a factor of more than 1.4. In contrast, a marked decrease in bioaccessibility of As was achieved using NaOH (40% to 7%). X-ray absorption spectroscopy analysis demonstrated that highly bioaccessible forms of calcium iron arsenate (yukonite and arseniosiderite) could be generated in CaO-stabilized soil. Our study found that neutralizing agents may play an important role in stabilizing As(V) and lowering its bioaccessibility through determining the type of formed Fe oxides in soil.

Effect of lignosulfonate on the adsorption performance of hematite for Cd(II)

Lignin is a precursor of humus in soil and sediment. Lignin can be separated from vascular plants in the form of lignosulfonate via pulping processes. On the other hand, composites of iron oxide and organic matter can adsorb heavy metals, and thus influence the migration of these heavy metals in the environment. In this paper, a hematite/lignosulfonate composite (HLS) was prepared via coprecipitation to compare the adsorption performance of hematite (α-Fe₂O₃) toward Cd(II) before and after the incorporation of lignosulfonate (LS). The HLS is found to exhibit a weakly crystalline structure and possess a large number of nanoscale particles. Specific surface area of HLS (291.97 m²/g) is about 11 times that of α-Fe₂O₃, and the pore volume of HLS (0.22 cm³/g) is twice that of α-Fe₂O₃. The adsorption of Cd(II) is well illustrated by the pseudo-second-order adsorption kinetics and the initial adsorption rate (h) of HLS is 13.83 times that of α-Fe₂O₃. The maximum adsorption capacities are significantly improved from 4.89-6.35 mg/g (α-Fe₂O₃) to 39.03-53.65 mg/g (HLS). A greater affinity and more favorable association between Cd(II) and HLS is observed via fitting models. The incorporation of LS provides HLS with significantly better adsorption properties toward Cd(II) than α-Fe₂O₃, as is further confirmed by FT-IR and XPS characterization. Fe-O-O-H and Fe-O-H structures as well as more hydroxyl groups are observed, which promote the adsorption performance since the process are mainly influenced by complexation via coordination bonds.

In Vitro Assessment of Arsenic Release and Transformation from As(V)-Sorbed Goethite and Jarosite: The Influence of Human Gut Microbiota

The importance of arsenic metabolism by gut microbiota has been evidenced in risk characterization from As exposures. In this study, we evaluated the metabolic potency of human gut microbiota toward As(V)-sorbed goethite and jarosite, presenting different behaviors of As release, and the solid-liquid transformation and partitioning. The release of As occurred mainly in the small intestinal phase for jarosite and in the colon phase for goethite, respectively. We found higher degree of As(V) and Fe(III) reduction by human gut microbiota in the colon digests of goethite than jarosite. Speciation analysis using high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry and X-ray absorption near-edge spectroscopy, revealed that 43.2% and 8.5% of total As was present as As(III) in the liquid and solid phase, respectively, after goethite incubation, whereas almost all generated As(III) was in the colon digests of jarosite. Therefore, As bioaccessibility in human gastrointestinal tract was predominantly contributed to Fe(III) dissolution in jarosite, and to microbial reduction of Fe(III) and As(V) in goethite. It expanded our knowledge on the role of Fe minerals in human health risk assessment associated with soil As exposures.

Interaction among soil physicochemical properties, bacterial community structure, and arsenic contamination: Clay-induced change in long-term arsenic contaminated soils

Pyrosequencing analyses to determine soil bacterial communities were conducted with forty-two soil samples collected from rice paddy and forest/farmland soils (Group A and B, respectively) at a long-term As-contaminated site. Soil physicochemical properties, such as the concentrations of As, Fe, Al, and Mn, pH, organic matter content, and clay content, were found to be significantly different with land use, and more importantly, strongly affected the bacterial community structure of the soil samples. When fitting the soil properties onto a nonmetric multidimensional scale plot of soil bacterial communities, clay content was found to be the most important factor in clustering the bacterial communities (R² = 0.4831, p-value = 0.001). Phylum Chloroflexi (-1.03 of bioplot score) and Planctomycetes (1.31 of bioplot score) showed a significant relationship with clay content in soil samples. Interestingly, thebacterial phylotypes linked to clay content were only found in the soil samples of group B with low clay content, and had a strong relationship to As contamination in the redundancy analysis and the correlation analysis.Our results suggest that clay content seems to be negatively related to As contamination in soils, which, in turn, strongly influences the structure of bacterial communities in As-contaminated soil.

Low arsenic bioaccessibility by fixation in nanostructured iron (Hydr)oxides: Quantitative identification of As-bearing phases

A new analytical protocol was developed to provide quantitative, single-particle identification of arsenic in heterogeneous nanoscale mineral phases in soil samples, with a view to establishing its potential risk to human health. Microscopic techniques enabled quantitative, single-particle identification of As-bearing phases in twenty soil samples collected in a gold mining district with arsenic concentrations in range of 8 to 6354 mg kg^-1. Arsenic is primarily observed in association with iron (hydr) oxides in fine intergrowth with phyllosilicates. Only small quantities of arsenopyrite and ferric arsenate (likely scorodite) particles, common in the local gold mineralization, were identified (e.g., 7 and 9 out, respectively, of app. 74,000 particles analyzed). Within the high-arsenic subgroup, the arsenic concentrations in the particle size fraction below 250μm ranges from 211 to 4304 mg kg^-1. The bioaccessible arsenic in the same size fraction is within 0.86-22 mg kg^-1 (0.3-5.0%). Arsenic is trapped in oriented aggregates of crystalline iron (hydr)oxides nanoparticles, and this mechanism accounts for the low As bioaccessibility. The calculated As exposure from soil ingestion is less than 10% of the arsenic Benchmark Dose Lower Limit - BMDL_0.5. Therefore, the health risk associated with the ingestion of this geogenic material is considered to be low.