Arsenic(V) adsorption-desorption in agricultural and mine soils: Effects of organic matter addition and phosphate competition

Mechanistic studies of adsorption and ion exchange of Si(OH)4 molecules on the surface of scorodites

Scorodites are commonly used for arsenic immobilization, and it is also the main component of arsenic bearing tailings. Alkali-activated geopolymers are commonly used to landfill arsenic-bearing minerals. However, there no previous studies have explored the interaction between geopolymer molecules and the surface of scorodite. In this paper, Si(OH)4 as a monomer molecule of geopolymer, the mechanism of adsorption and 'ion exchange' between Si(OH)4 molecule and the surface of scorodite during alkali-activation is studied. Results show that the Fe-terminated scorodite (010) surface has high stability. Si(OH)4 are more easily adsorbed on the hollow site of an Fe-terminated scorodite (010) surface, which is described as chemisorption. Compared with Si(OH)4, NaOH is easier to adsorb on an Fe-terminated scorodite (010) surface. The co-adsorption of NaOH and Si(OH)4 on the Fe-terminated scorodite (010) surface was studied, and also belongs to chemical adsorption. When the hydroxyl binds to the As atom, the adsorbed Si(OH)4 is more likely to undergo an 'ion exchange' reaction with the surface, and the reaction is barrierless. The intermediate As(OH)4 produced by the 'ion exchange' reaction can be deprotonated to form an arsenate molecule, which can occur spontaneously. This work reveals that the interaction mechanism of geopolymer molecules on surface of scorodite.

Determining soil conservation strategies: Ecological risk thresholds of arsenic and the influence of soil properties

The establishment of ecological risk thresholds for arsenic (As) plays a pivotal role in developing soil conservation strategies. However, despite many studies regarding the toxicological profile of As, such thresholds varying by diverse soil properties have rarely been established. This study aims to address this gap by compiling and critically examining an extensive dataset of As toxicity data sourced from existing literature. Furthermore, to augment the existing information, experimental studies on As toxicity focusing on barley-root elongation were carried out across various soil types. The As concentrations varied from 12.01 to 437.25 mg/kg for the effective concentrations that inhibited 10% of barley-root growth (EC10). The present study applied a machine-learning approach to investigate the complex associations between the toxicity thresholds of As and diverse soil properties. The results revealed that Mn-/Fe-ox and clay content emerged as the most influential factors in predicting the EC10 contribution. Additionally, by using a species sensitivity distribution model and toxicity data from 21 different species, the hazardous concentration for x% of species (HCx) was calculated for four representative soil scenarios. The HC5 values for acidic, neutral, alkaline, and alkaline calcareous soils were 80, 47, 40, and 28 mg/kg, respectively. This study establishes an evidence-based methodology for deriving soil-specific guidance concerning As toxicity thresholds.

Arsenic stress on soil microbial nutrient metabolism interpreted by microbial utilization of dissolved organic carbon

In a 120-day microcosm incubation experiment, we investigated the impact of arsenic contamination on soil microbial nutrient metabolism, focusing on carbon cycling processes. Our study encompassed soil basal respiration, key enzyme activities (particularly, β-1,4-N-acetylglucosaminidase and phosphatases), microbial biomass, and community structure. Results revealed a substantial increase (1.21-2.81 times) in β-1,4-N-acetylglucosaminidase activities under arsenic stress, accompanied by a significant decrease (9.86%-45.20%) in phosphatase activities (sum of acid and alkaline phosphatases). Enzymatic stoichiometry analysis demonstrated the mitigation of microbial C and P requirements in response to arsenic stress. The addition of C-sources alleviated microbial C requirements but exacerbated P requirements, with the interference amplitude increasing with the complexity of the C-source. Network analysis unveiled altered microbial nutrient requirements and an increased resistance process of microbes under arsenic stress. Microbial carbon use efficiency (CUE) and basal respiration significantly increased (1.17-1.59 and 1.18-3.56 times, respectively) under heavy arsenic stress (500 mg kg-1). Arsenic stress influenced the relative abundances of microbial taxa, with Gemmatimonadota increasing (5.5-50.5%) and Bacteroidota/ Nitrospirota decreasing (31.4-47.9% and 31.2-63.7%). Application of C-sources enhanced microbial resistance to arsenic, promoting cohesion among microorganisms. These findings deepen our understanding of microbial nutrient dynamics in arsenic-contaminated areas, which is crucial for developing enzyme-based toxicity assessment systems for soil arsenic contamination.

Enhanced attenuation of arsenic by Quaternary agricultural soils of Eastern Punjab, India upon anionic clays and gypsum amendment

Agricultural soil of the Sutlej River basin was evaluated for its natural attenuation efficacy for arsenic (As) under the field variables of pH, competitive anions, contact time and varied As contents. The role of layered double hydroxides (HTLDH) and gypsum on uptake efficiency and long-term stability of entrapped As demonstrates rapid As uptake by both geosorbents without mineral structure altering. Arsenic retention by gypsum is poorer than that by HTLDH and greater uptake (∼100% within 2 h) was achieved in the co-precipitation process than adsorption on HTLDH. Freundlich isotherm and pseudo-second-order kinetic model fits of the data demonstrate the multilayer rate-limiting sorption process. NO3- and PO43- hardly affected As retention capacity of HTLDH and gypsum with greater retention at pH 6 and high sorbate concentrations. Studied soil shows a strong potential for As (0.68 g kg-1) which enhanced upon adding HTLDH, while gypsum lowered As retention efficiency of soil except at pH 6.0. Gypsum exhibited relatively greater desorption than HTLDH where almost no As was desorbed in the latter case within seven days of exposure, but ∼30% sorbed As gets desorbed from gypsum which was further enhanced by NO3-+PO43- and soil mixing. Identical behaviour was observed from the soil and HTLDH/gypsum mixture at variable ratios as well. This study shows that MgFe-based HTLDH can efficiently retard arsenic mobilization from the soil with competitive anions and wide pH ultimately limiting As bioavailability in the environment and can be successfully used as a potential scavenger for As remediation purposes.

Photo-aged non-biodegradable and biodegradable mulching film microplastics alter the interfacial behaviors between agricultural soil and inorganic arsenic

The impacts of microplastics (MPs) prevalent in soil on the transport of pollutants were urged to be addressed, which has important implications for ecological risk assessment. Therefore, we investigated the influence of virgin/photo-aged biodegradable polylactic acid (PLA) and non-biodegradable black polyethylene (BPE) mulching films MPs on arsenic (As) transport behaviors in agricultural soil. Results showed that both virgin PLA (VPLA) and aged PLA (APLA) enhanced the adsorption of As(Ⅲ) (9.5%, 13.3%) and As(Ⅴ) (22.0%, 6.8%) due to the formation of abundant H-bonds. Conversely, virgin BPE (VBPE) reduced the adsorption of As(Ⅲ) (11.0%) and As(Ⅴ) (7.4%) in soil owing to the "dilution effect", while aged BPE (ABPE) improved arsenic adsorption amount to the level of pure soil due to newly generated O-containing functional groups being feasible to form H-bonds with arsenic. Site energy distribution analysis indicated that the dominant adsorption mechanism of arsenic, chemisorption, was not impacted by MPs. The occurrence of biodegradable VPLA/APLA MPs rather than non-biodegradable VBPE/ABPE MPs resulted in an increased risk of soil accumulating As(Ⅲ) (moderate) and As(Ⅴ) (considerable). This work uncovers the role of biodegradable/non-biodegradable mulching film MPs in arsenic migration and potential risks in the soil ecosystem, depending on the types and aging of MPs.

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

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

Pyrolysis temperature influences the capacity of biochar to immobilize copper and arsenic in mining soil remediation

Biochar is a promising material used for multiple remediation approaches, mainly in polluted soils. Its properties can differ depending on feedstock and pyrolysis temperature. In this context, we tested the capacity of three biochar products made from corncob, pyrolyzed at different temperatures (350, 500, and 650 °C), to remediate a mining soil affected by high levels of Cu and As. We performed an exhaustive characterization of the biochar. We found that biochar showed a higher surface area with increasing pyrolysis temperature, whereas high molecular weight PAHs were detected in biochar produced at the maximum temperature, thus indicating potential ecotoxicological risks. After the application of biochar to the soil, Cu was partially immobilized, especially when using that obtained at 500 °C. This effect is attributed to the structure of this material and an increase in soil pH and organic matter content. Conversely, As was increased in the soluble fraction for all three types of biochar but in a proportion that lacks relevance. On the whole, given its lower PAH content, higher Cu immobilization ratio, and an almost negligible increase in As availability, biochar obtained at 500 °C outperformed the other two products with respect to soil recovery. Of note, data on Cu and As availability were doubled-checked using two extraction methodologies. We propose that this operational approach for determining the most suitable pyrolysis temperature will find application in other soil remediation actions.

Adsorption and desorption characteristics of arsenic in calcareous soils as a function of time; equilibrium and thermodynamic study

Irrigation of carbonate-rich agricultural soils with arsenic (As)-contaminated water leads to the accumulation of As in these soils. In this regard, there is an opportunity to adsorb and fix the As in soil and decrease the As transportation to the plants and subsequently the human food chain. So, the present study aimed to investigate the adsorption-desorption characteristics of As in calcareous soils and the potential of As fixation over time. First, to achieve this purpose, 53 soil samples were gathered from the study site and after the laboratory analysis, the soils were categorized into four groups based on their physicochemical properties. Then, four representative samples of these groups were selected, namely soil 1, soil 2, soil 3, and soil 4. Afterward, the As adsorption-desorption was investigated in a lab-scale batch experiment. Next, the effect of age was assessed by incubating the As-adsorbed soils for 60 days, and to study the impact of temperature, the adsorption was performed at four temperature levels (10, 20, 30, and 40 °C). Finally, the isotherm models were fitted to experimental data, and the amount of loosely and tightly held As was quantified. Results revealed that the As adsorption isotherms were L-type, in which As adsorption increased with the increase of As loading. The double-site Langmuir (DSL) estimated that a limited amount of As was adsorbed on high-energy surfaces and a large amount of As was adsorbed on low-energy surfaces. Desorption results showed that a significant amount of As desorbed immediately; however, the desorption significantly decreased with the increase of age, especially at low equilibrium concentrations. By aging the loosely held As transformed into non-labile forms so that in soils 1, 2, 3, and 4, the fraction of As adsorbed on high-energy surfaces increased from 72.5, 93.2, 63.2, and 123 mg/kg to 167, 141, 70.6, and 196 mg/kg, respectively, and the fraction of As adsorbed on low-energy surfaces decreased from 397, 256, 202, and 317 mg/kg to 182, 238, 173, and 172 mg/kg, respectively (after aging for 60 days). Aging proved to be a promising solution for decreasing As transport into the human food chain and could be employed for crops with longer irrigation cycles. ΔH_ad values were positive and varied from 9.26 to 13.0 kJ/mol, confirming the endothermic nature of adsorption. ΔG_ad values were negative and varied from - 18.8 to - 22.8 kJ/mol at all temperatures, demonstrating the spontaneous nature of adsorption.

Antimony release and volatilization from rice paddy soils: Field and microcosm study

The fate of antimony (Sb) in submerged soils and the impact of common agricultural practices (e.g., manuring) on Sb release and volatilization is understudied. We investigated porewater Sb release and volatilization in the field and laboratory for three rice paddy soils. In the field study, the porewater Sb concentration (up to 107.1 μg L^-1) was associated with iron (Fe) at two sites, and with pH, Fe, manganese (Mn), and sulfate (SO₄^2-) at one site. The surface water Sb concentrations (up to 495.3 ± 113.7 μg L^-1) were up to 99 times higher than the regulatory values indicating a potential risk to aquaculture and rice agriculture. For the first time, volatile Sb was detected in rice paddy fields using a validated quantitative method (18.1 ± 5.2 to 217.9 ± 160.7 mg ha^-1 y^-1). We also investigated the influence of two common rice agriculture practices (flooding and manuring) on Sb release and volatilization in a 56-day microcosm experiment using the same soils from the field campaign. Flooding induced an immediate, but temporary, Sb release into the porewater that declined with SO₄^2-, indicating that SO₄^2- reduction may reduce porewater Sb concentrations. A secondary Sb release, corresponding to Fe reduction in the porewater, was observed in some of the microcosms. Our results suggest flooding-induced Sb release into rice paddy porewaters is temporary but relevant. Manuring the soils did not impact the porewater Sb concentration but did enhance Sb volatilization. Volatile Sb (159.6 ± 108.4 to 2237.5 ± 679.7 ng kg^-1 y^-1) was detected in most of the treatments and was correlated with the surface water Sb concentration. Our study indicates that Sb volatilization could be occurring at the soil-water interface or directly in the surface water and highlights that future works should investigate this potentially relevant mechanism.

Study of arsenic adsorption in calcareous soils: Competitive effect of phosphate, citrate, oxalate, humic acid and fulvic acid

Arsenic (As) bio-availability in the soil is influenced by different organic and inorganic anions. In the present study, the effects of various competitive agents, including phosphate, citrate, oxalate, humic acid (HA), and fulvic acid (FA), on the adsorption of As in calcareous soils were investigated. The results revealed the presence of phosphate, citrate, and oxalate in soil has a significant impact on the arsenic retention (adsorption) in soil which increases the As bio-availability. The negative impact of the competing anions was increased at higher concentrations. The Double Site Langmuir (DSL) isotherm was best fitted to the adsorption data, which indicates that most of the As adsorbed on the low-energy surfaces (non-specific adsorption by oxides, clays, and clay-size calcite). Accordingly, in soil 1, the DSL predicted that, due to phosphate, citrate, and oxalate competition (at a concentration of 10 mM), the adsorption capacity of the high- and low-energy surfaces decreased from 86.2 to 33.5, 82.1 and 61.3 mg/kg and from 663 to 659, 335.8, and 303.5 mg/kg, respectively, Moreover, after addition of phosphate, citrate, and oxalate to the soil-As system, the Langmuir constant of high-energy surfaces decreased from 0.686 to 0.074, 0.261, and 0.301 L/mg, respectively. No regular trend was observed for the Langmuir constant of low-energy surfaces. Similarly, in soils 2, 3, and 4, the adsorption capacities of both high- and low-energy surfaces as well as the Langmuir constant of high-energy surfaces decreased by the addition of phosphate, citrate, and oxalate to the soil-As system. HA and FA did not have a significant effect on the As adsorption behavior. Phosphate, citrate, and oxalate, as interfering oxyanions, increased the As bio-availability in the calcareous soils by decreasing the As adsorption.

Adsorption and desorption behavior of arsenite and arsenate at river sediment-water interface

The adsorption of inorganic arsenic (As) plays an important role in the mobility and transport of As in the river environment. In this work, the adsorption and desorption of arsenite [As(III)] and arsenate [As(V)] on river sediment were conducted under different pH, initial As concentrations, river water and sediment composition to assess As adsorption behavior and mechanism. Both adsorption kinetics and equilibrium results showed higher adsorption capacity of sediment for As(V) than As(III). Adsorption of As(III) and As(V) on river sediment was favored in acidic to neutral conditions and on finer sediment particles, while sediment organic matter marginally reduced adsorption capacity. In addition, higher adsorption affinity of As(III) and As(V) in river sediment was observed in deionised water than in river water. For the release process, the desorption of both As(III) and As(V) followed nonlinear kinetic models well, showing higher amount of As(III) release from sediment than As(V). Adsorption isotherm was well described by both Langmuir and Freundlich models, demonstrating higher maximum adsorption capacity of As(V) at 298.7 mg/kg than As(III) at 263.3 mg/kg in deionised water, and higher maximum adsorption capacity of As(III) of 234.3 mg/kg than As(V) of 206.2 mg/kg in river water. The XRD showed the changes in the peaks of mineral groups of sediment whilst FTIR results revealed the changes related to surface functional groups before and after adsorption, indicating that Fe-O/Fe-OH, Si(Al)-O, hydroxyl and carboxyl functional groups were predominantly involved in As(III) and As(V) adsorption on sediment surface. XPS analysis evidenced the transformation between these As species in river sediment after adsorption, whilst SEM-EDS revealed higher amount of As(V) in river sediment than As(III) due to the lower signal of Al.

Exploring the significance of nanomaterials and organic amendments - Prospect for phytoremediation of contaminated agroecosystem

Emerging micro-pollutants have rapidly contaminated the agro-ecosystem, posing serious challenges to a sustainable future. The vast majority of them have infiltrated the soil and damaged agricultural fields and crops after being released from industry. These pollutants and their transformed products are also transported in vast quantities which further exacerbate the damage. Sustainable remediation techniques are warranted for such large amounts of contaminants. As aforementioned, many of them have been detected at very high concentrations in soil and water which adversely affect crop physiology by disrupting different metabolic processes. To combat this situation, nanomaterials and other organic amendments assisted phytoremediation ware considered as a viable alternative. It is a potent synergistic activity between the biological system and the supplied organic or nanomaterial material to eliminate emerging contaminants and micropollutants from crop fields. This can be effectively be applied to degraded crop fields and could potentially embody a green technology for sustainable agriculture.

Sorption of arsenic by composts and biochars derived from the organic fraction of municipal solid wastes: Kinetic, isotherm and oral bioaccessibility study

The historic contamination of water and soils by arsenic (As) is an extremely alarming environmental and public health issue worldwide. This study investigated the relationship between As sorption and physicochemical properties of composts and biochars derived from the organic fraction of municipal solid wastes (OFMSW) towards the development of promising sorbents with value-added solid wastes management solutions. The sorbents were characterized and their effectiveness on the As sorption was tested. Several isothermal and kinetic sorption models were used for the prediction of sorption. Composts did not show promising sorption capacities, and in some cases, the As immobilization was practically null. In contrast, biochars achieved higher sorption performance, and the experimental data fitted well on Dubinin-Rabushkevich and Langmuir models, with higher R² values. The maximum sorption capacities of BC700 estimated by such models were 6.495 and 170.252 mg g^-1, respectively, whereas those of BC500 estimated by D-R and Langmuir models were only 0.066 and 0.070 mg g^-1, respectively. In sorption kinetics, As was retained onto biochars at a faster first stage, reaching equilibrium after approximately 1 h and 2 h for initial concentrations of 10 and 100 mg L^-1. The pseudo-second-order, Ritchie's second-order, Ritchie's, and Elovich models more adequately described the sorption kinetics of As onto biochars with high R² values. Overall, the complexation and precipitation were predominant mechanisms for As sorption by OFMSW-derived biochars. Furthermore, the mathematical models indicated contributions arise from physisorption and external and internal diffusion mechanisms. Although BC700 can immobilize large As amounts, the gastric phase of the oral bioaccessibility test revealed more than 80% of the sorbed As could be released under conditions similar to a human stomach (pH~1.2). Such conclusions have given important insights about the refining of effective and eco-friendly remediation technologies for the management and rehabilitation of As-contaminated soil and water, particularly in developing countries.

Establishing the state-of-the-art on the adsorption of coexisting pnictogens in water: A literature review

The occurrence of pnictogens, namely phosphorus, arsenic, and antimony, can be observed in soils, sediments and mining areas, and their coexistence requires a multifaceted approach to the design of adsorption systems to maximize their simultaneous removal efficiency. Therefore, this work aims to provide an extensive literature review of P, As, and Sb adsorption in multicomponent systems and the statistical treatment of the quantitative results. Binary As-P systems have been the most studied in the literature. The oxidation state did not significantly affect the P influence in As adsorbed amount (p = 0.955), but this influence was correlated with the As:P ratio (p < 0.05). A few works have explored As-Sb and Sb-P systems, demonstrating that effective treatments for As do not always reveal a good removal efficiency of the other pnictogens. The Sb adsorbed amount was significantly less affected in the trivalent than in the pentavalent state in both As-Sb and Sb-P systems (p < 0.05). Most of the interactions were competitive, with a few studies reporting synergistic effects for Sb due to the presence of the other elements. Many topics have been identified as lacking in-depth research: ternary As-Sb-P systems, the effect of concentration ratios, pH, and redox conditions (namely those that lead to trivalent species' prevalence), the surface interactions with materials other than iron oxides, and the influence of other aqueous components. This review provides a first step in gathering the relevant literature and approaching the study of adsorption treatment methodology as a complex subject involving many factors.

Impacts of Redox Conditions on Arsenic and Antimony Transformation in Paddy Soil: Kinetics and Functional Bacteria

Arsenic (As) and antimony (Sb) are known carcinogens and are present as contaminants in paddy soils. However, the complicated dynamics of the mobility of these metalloids have not been well understood due to changing redox conditions in paddy soils. Herein, the kinetics of dissolved As and Sb, and functional bacteria/genes were examined in a paddy soil cultured under aerobic and anaerobic conditions. Under aerobic condition, dissolved As(V) and Sb(V) increased constantly due to sulfide oxidation by O₂ and bound As and Sb were released. Under anaerobic condition, the reduction of As(V) and Sb(V) occurred, and the mobility of As and Sb were affected by soil redox processes. The bacteria with functional genes aioA and arrA were responsible for the direct As/Sb transformation, while Fe- and N-related bacteria had an indirect effect on the fate of As/Sb via coupling with the redox processes of Fe and N. These findings improve understanding of the mobility of As and Sb in paddy soil systems under different redox conditions.

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.

Short-term experiment for the in situ stabilization of a polluted soil using mining and biomass waste

Mining and biomass waste were used to remediate a brownfield affected by As, Cd, Cu, Pb and Zn pollution in a pilot scale experiment, and a plant used for phytoremediation purposes was used as an indicator of possible toxicological effects. To carry out the experiments, plots in field conditions were treated with magnesite (mining waste), magnesite-sludge compost, and magnesite-biochar respectively, while untreated soil was used as a control. The plots were then irrigated and left for one week, after which seeds of the ryegrass Lolium perenne L. were sown. Soil properties such as metal(loid) availability, pH, phosphorus availability, total nitrogen, organic carbon, and nutrients were monitored for two months. Finally, the ryegrass was harvested and pollutant concentrations were analyzed in the aerial parts. Magnesite proved to be an excellent amendment for metal(loid) immobilization, although the notable increase in soil pH and Mg content inhibited plant growth. However, the application of magnesite in combination with the sludge compost (rich in N and P) favored plant growth and also immobilized metals, although As availability increased. In contrast, the analysis of plants in this treatment revealed lower As and metal concentrations than those grown in the untreated soil. In turn, the application of magnesite and biochar was also effective in reducing metal(loid) availability; however, the plants did not grow under these conditions, probably due to the low N and P content of biochar. In this regard, the combined application of mining waste and sludge compost emerges as a useful nature-based solution for soil remediation in the context of the circular economy.

Plumbojarosite formation in contaminated soil to mitigate childhood exposure to lead, arsenic and antimony

In this study, a novel method for lead (Pb) immobilization was developed in contaminated soils using iron (III) (Fe³⁺) in conjunction with 0.05 M H₂SO₄. During method optimization, a range of microwave treatment times, solid to solution ratios, and Fe₂(SO₄)₃/H₂SO₄ concentrations were assessed using a mining/smelting impacted soil (BHK2, Pb: 3031 mg/kg), followed by treatment of additional Pb contaminated soils (PP, Pb: 1506 mg/kg, G10, Pb: 2454 mg/kg and SoFC-1, Pb: 6340 mg/kg) using the optimized method. Pb bioaccessibility was assessed using USEPA Method 1340, with Pb speciation determined by X-ray Absorption (XAS) spectroscopy. Treatment efficacy was also validated using an in vivo mouse assay, where Pb accumulation in femur, kidney and liver was assessed to confirm in vitro bioaccessibility outcomes. Results showed that Pb bioaccessibility could be reduced by 77.4-97.0% following treatment of soil with Fe₂(SO₄)₃ (0.4-1.0 M), H₂SO₄ (0.05 M) at 150 °C for 60 min in a closed microwave system. Results of bioavailability assessment demonstrated treatment effect ratio of 0.06-0.07 in femur, 0.06-0.27 in kidney and 0.06-0.11 in liver (bioavailability reduction between 73% and 93%). Formation of plumbojarosite in treated soils was confirmed by XAS analysis.

A study on the effects of anion, cation, organic compounds, and pH on the release behaviors of As and Sb from sediments

The trace element of As and Sb have resulted in serious threats to ecological sustainability and human health in the Xijiang River basin, China. The geoaccumulation index (I_geo) results showed the sediment of Xijiang River was heavily contaminated by Sb as well as moderately contaminated by As. The effect of inorganic anions on the released As and Sb from the sediment increases in the following sequence: Cl^- < SO₄^2- < CO₃^2- < PO₄^3-. The presence of PO₄^3- and CO₃^2- had the greater effect over As mobility from the sediment compared to Sb. The effect of Ca(II), Al(III), and Mg(II) on the released amount of Sb from the sediment is negligible. Meanwhile, in the case of As, Ca(II) and Mg(II) had small impacts, but the released amount of As increased slightly with an increase of Al(III) concentration. The stability of As and Sb in the sediment was found to be the best at pH 5. The effect of organic compounds (≤ 0.05 mol/L) on the dissolved As percentages from the sediment increased in the following sequence: ethylenediaminetetraacetate (EDTA) < oxalate < citrate, and the effect on Sb release was oxalate < EDTA < citrate at concentrations (≤ 0.025 mol/L). This study provides the basic theoretical basis to manage the mobilization of co-contamination of sediments with As and Sb under the influence of external perturbations.

The Effects of Forest Litter and Waterlogging on the Ecotoxicity of Soils Strongly Enriched in Arsenic in a Historical Mining Site

This study examined the effects of waterlogging and forest litter introduced to soil on chemical properties of soil pore water and ecotoxicity of soils highly enriched in As. These effects were examined in a 21-day incubation experiment. Tested soil samples were collected from Złoty Stok, a historical centre of arsenic and gold mining: from a forested part of the Orchid Dump (19,600 mg/kg As) and from a less contaminated site situated in a neighboring forest (2020 mg/kg As). An unpolluted soil was used as control. The concentrations of As, Fe and Mn in soil pore water were measured together with a redox potential Eh. A battery of ecotoxicological tests, including a bioassay with luminescence bacteria Vibrio fischeri (Microtox) and several tests on crustaceans (Rapidtox, Thamnotox and Ostracodtox tests), was used to assess soil ecotoxicity. The bioassays with crustaceans (T. platyurus, H. incongruens) were more sensitive than the bacterial test Microtox. The study confirmed that the input of forest litter into the soil may significantly increase the effects of toxicity. Waterlogged conditions facilitated a release of As into pore water, and the addition of forest litter accelerated this effect thus causing increased toxicity.

Fractionation of carbon isotopes of dissolved organic matter adsorbed to goethite in the presence of arsenic to study the origin of DOM in groundwater

Natural dissolved organic matter (DOM) in groundwater plays a crucial role in mobilizing arsenic (As). The complex contribution of DOM sources makes it hard to predict how the variation of environmental conditions would affect the distribution of As concentrations. Identifying the carbon isotope fractionation of DOM is the key to quantify DOM sources based on stable carbon isotopes. To understand the magnitude and variability in the carbon isotopic fractionation of DOM in competitive adsorption with As(V), this study investigated the δ¹³C values of fulvic acid (FA) and DOM during adsorption to goethite in the presence of As(V), at a specific pH and temperature. The carbon isotopic enrichment factor (ε) of FA in the adsorption to goethite was 0.65 ± 2.11‰ at pH 4.1, 25 °C, suggesting that FA molecules containing ¹³C were more easily adsorbed to goethite. An increasing temperature increased ε_FA from 0.32 ± 1.17‰ to 0.82 ± 5.39‰ at 15-35 °C. For dissolved sediment organic matter (DSOM) cases, molecules containing ¹³C were more easily adsorbed to goethite. However, enrichment factors were not detected due to a reduction in DSOM adsorption and the diversity of natural humic substances or groups. The findings provide basic data for accurately ascertaining DOM sources through carbon isotopes, which is significant for predicting As fluctuation in aquifers affected by monsoon climate and/or human activities.

Ternary metal composite membrane FCMNCM enhances the separation of As(Ⅲ) in water through the multifunctional cooperation

More cases of arsenic contamination are reported globally, making the restoration of arsenic in water an active area of research. Especially, As(Ⅲ) is more difficult to remove than negatively charged As(Ⅴ) due to the presence of neutral H₃AsO₃ in the water, so to achieve efficient separation of As(Ⅲ) in water, it is very important to pre-oxidize As(Ⅲ) to As(Ⅴ). Herein, Fe-coated Cu⁰ doped MnO₂ nanowire membrane (FCMNCM) was successfully prepared to enhance the oxidation of As(Ⅲ) to As(Ⅴ) through the combination of superoxide anion (O₂^·-) and MnO₂ oxidation. Experimental results show that Cu⁰ activates oxygen to generate O₂^·-, the generated O₂^·- not only significantly enhances the conversion efficiency of As(Ⅲ) to As(Ⅴ) but also oxidize the Mn(Ⅱ)/Mn(Ⅲ) produced by the reduction of MnO₂ by As(Ⅲ) to Mn(Ⅳ) again to realize multi-channel oxidation of As(Ⅲ), and the maximum separation efficiency of As(Ш) can reach 99.34%. Acidic conditions are favorable for the separation of As(Ш), and carbonate and phosphate have a serious negative effect on As(Ⅲ) separation by competing for the active site. Anti-fouling and repeatability experimental show that FCMNCM is an environmentally friendly material with long service life and excellent reusability, it provides a new platform for As(Ⅲ)-containing sewage treatment.

Nucleotide Adsorption on Iron(III) Oxide Nanoparticle Surfaces: Insights into Nano-Geo-Bio Interactions Through Vibrational Spectroscopy

Molecular processes at geochemical interfaces impact many environmental processes that are critical to the fate and transport of contaminants in water systems. Often these interfaces are coated with natural organic matter, oxyanions, or biological components, yet little is understood about these coatings. Herein, we are interested in better understanding the interaction of biological components with nanoscale iron oxide minerals. In particular, we use attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to investigate the adsorption behavior of deoxyadenosine monophosphate (dAMP) on hematite nanoparticle surfaces as a function of pH and in the presence and absence of adsorbed phosphate. These results show that fewer nucleotides adsorb at higher pH. Additionally, when phosphate anions are preadsorbed, nucleotide adsorption is significantly limited due to site-blocking by adsorbed inorganic phosphate. The pH dependence provides insights into the adsorption process and the importance of electrostatic interactions. Preadsorbed phosphate affects the binding mode of dAMP, suggesting synergistic interactions between the coadsorbates. Two-dimensional correlation spectroscopy was used to further analyze the infrared spectra. Based on this analysis, a dAMP adsorption pathway onto a preadsorbed phosphate-hematite surface was proposed, suggesting the displacement of adsorbed phosphate by dAMP. Overall, this study provides some insights into geochemical-biological interactions on nanoscale iron oxide surfaces using vibrational spectroscopy.

Use of Brassica juncea and Dactylis glomerata for the phytostabilization of mine soils amended with compost or biochar

Phytostabilization of mine soils contaminated by potentially toxic elements (PTEs) requires plants tolerant to PTE toxicity and to the poor soil physico-chemical characteristics of these areas. A pot experiment was carried out to assess the phytostabilization potential of Brassica juncea and Dactylis glomerata in mine soils amended with compost and biochar. Furthermore, the Environmental Risk of the soils and the effects of the phytostabilization process on the microbiological population size and activity in the soils were also determined. According to the Ecological Risk Index (ERI) the soils studied presented "very high risk" and As, Cd and Pb were the target elements for phytostabilization. Both amendments improved soil conditions (e.g., increasing total-N and total organic-C concentrations) and contributed to PTE (Cd, Pb and Zn) immobilization in the soil. Compost showed a more marked effect on soil microbial biomass and nutrients release in soil, which led to higher B. juncea and D. glomerata biomass in compost treated soils. Biochar treatment showed a positive effect only on D. glomerata growth, despite it provoked strong PTE immobilization in both soils. The addition of both amendments resulted in an overall reduction of PTE concentration in the plants compared to the control treatment. In addition, both plant species showed higher accumulation of PTE in the roots than in the shoots (transfer factor<1) independently of the treatment received. Therefore, they can be considered as good candidates for the phytostabilization of PTE contaminated mine soils in combination with organic amendments like biochar and compost.

Effect of ferrolysis and organic matter accumulation on chromate adsorption characteristics of an Oxisol-derived paddy soil

How paddy cultivation influences the adsorption isotherms, envelopes, and the kinetics of hexavalent chromate (Cr(VI)) on Fe (hydro)oxide-rich paddy soil, as well as the mechanisms involved, remain largely unaddressed. To this end, the Cr(VI) adsorption characteristics on a paddy soil, in comparison with its parent upland Oxisol, were studied. The results showed that Cr(VI) adsorption capacities (Q_m^ad) were higher in the surface Oxisol than in the same layer of paddy soil. The Q_m^ad increased by 18.0% and 41.3% after removal of soil organic matter (SOM) from the surface Oxisol and paddy soil layers, respectively, indicating that Cr(VI) adsorption was considerably inhibited by SOM. The adsorption and desorption isotherms demonstrated that non-electrostatic adsorption was mainly responsible for Cr(VI) adsorption, accounting for 59.37%-83.42% of Cr(VI) adsorption capacities. The negative shift of the zeta potential-pH curves with Cr(VI) loading further corroborated the finding that non-electrostatic adsorption is largely responsible for Cr(VI) retention. Cr(VI) adsorption at equilibrium, obtained by the stirred flow chamber technique, and the free Fe (hydro)oxides (Fe_d) contents were in the same order, suggesting that Fe_d are the main adsorbents for Cr(VI). Therefore, paddy cultivation has had a profound impact on the electrochemical properties of the Oxisol and on subsequent Cr(VI) adsorption characteristics.

Application of biochar, compost and ZVI nanoparticles for the remediation of As, Cu, Pb and Zn polluted soil

Here we tested the capacity of zero valent iron nanoparticles (nZVI) combined with two organic amendments, namely, compost and biochar, to immobilize metal(oid)s such as As, Cu, Pb, and Zn. In addition, the effects of the amendments on the development of Brassica juncea L., a plant widely used for phytoremediation purposes, were also examined. To perform the experiments, pots containing polluted soil were treated with nZVI, compost-biochar, or a blend of compost-biochar-nZVI. Metal(oid)s availability and soil properties were evaluated after 15 and 75 days, and the height and weight of the plants were measured to determine development. The compost-biochar amendment showed excellent capacity to immobilize metals, but As availability was considerably increased. However, the addition of nZVI to the mixture corrected this effect considerably. In addition, soil treatment with nZVI alone led to a slight increase in Cu availability, which was not observed for the mixture with organic amendments. With respect to soil properties, the CEC and pH were enhanced by the compost-biochar amendment, thereby favoring plant growth. Nevertheless, the nanoparticles reduced the concentration of available P, which impaired plant growth to a certain extent. In conclusion, Fe-based nanoparticles combined with organic amendments emerge as powerful approaches to remediate soils contaminated by metals and metalloids.

Arsenic Uptake by Two Tolerant Grass Species: Holcus lanatus and Agrostis capillaris Growing in Soils Contaminated by Historical Mining

The study focused on two grass species Holcus lanatus and Agrostis capillaris abundant in the sites of former As mining and processing in the Sudetes. Arsenic uptake from soils was examined to assess a risk associated with its accumulation in grass shoots and to check its dependence on soil fertilization. The research involved a field study and greenhouse experiment. In the field study, soil and plant samples were collected from 33 sites with 72-98,400 mg/kg total soil As. Arsenic uptake by grasses differed widely. Both species indicated a strategy typical for eliminators, although As concentrations in more than 50% of the shoot samples exceeded 4 mg/kg, a maximum permissible value for fodder. In the greenhouse experiment, commercial cultivars of both species were grown in five soils containing 394-19,600 mg/kg, untreated and fertilized. All seedlings died in the soil with highest total As, and considerable phytotoxicity was observed in other soils, particularly in nonfertilized ones. Fertilization resulted in the improvement of plant growth and reduction of As uptake except for Agrostis capillaris fertilized with manure. Further research should focus on identifying tolerant genotypes growing in extremely enriched sites and analysis of factors that will efficiently reduce As phytoaccumulation.

Mobility and accessibility of Zn, Pb, and As in abandoned mine tailings of northwestern Mexico

Generation, storage, and management of waste coming from industrial processes are a growing worldwide problem. One of the main contributors is the mining industry, in particular tailings generated by historical mining, which are barely maintained, especially in developing countries. Assessing the impact of a mining site to surrounding soils and ecosystems can be complex, especially when determining mobility and accessibility of the contaminants is required to perform ecological and human health risk assessment. As an effort to obtain information regarding mobility and accessibility of some potentially toxic elements (Zn, Pb, and As) from an historical mining site of northwestern Mexico, the abandoned mine tailings of San Felipe de Jesús in central Sonora and adjacent agricultural soils were investigated. Mobility and accessibility were assessed by means of sequential extraction procedures and using simulated physiological media. Additionally, an assessment of accidental oral intake was calculated considering the bioaccessible fractions. Results show that higher concentrations of contaminants were found in sulfide-rich tailings (Zn = 92,540; Pb = 21,288; As = 19,740 mg kg^-1) compared with oxide-rich tailings (Zn = 43,240; Pb = 14,763; As = 13,401 mg kg^-1). Concentrations in agricultural soils were on average Zn = 4755, Pb = 2840, and As = 103 mg kg^-1. Zinc was mainly recovered from labile fractions in oxide-rich tailings (~ 60%) and in a lower amount from sulfide-rich tailings (~ 30%). Pb and As were mainly associated with residual fractions (80-95%) in both types of tailings. The percentage of mobile fractions (sum of water-soluble, exchangeable, and bound to carbonate fractions) in agricultural soils was as follows: Zn ~ 60%, Pb ~ 15%, and As ~ 70%. Regarding the phytoaccessible fraction, the studied elements in mine tailings and agricultural soil samples exceeded the threshold limits, except for As in agricultural soils. According to data obtained, toxic effects were also calculated. As for daily oral intake for non-carcinogenic effects in adults and children, only Pb and As exceeded reference dose values, especially in children exposed to sulfide-rich tailings and agricultural soils. Regarding carcinogenic effects of Pb and As, most of the samples were above acceptable risk values.

Arsenic adsorption by innovative iron/calcium in-situ-impregnated mesoporous activated carbons from low-temperature water and effects of the presence of humic acids

Innovative iron/calcium in-situ-impregnated mesoporous activated carbons (GL100 and GL200) have been prepared by iron/calcium in-situ-impregnation and Multistage Depth-Activation. Arsenic adsorption kinetics, isotherms, thermodynamics, and re-usability were investigated. Effects of surface-absorbed (ST-HA) and dissolved states humic acid (DHA) on the arsenic adsorption were also determined. Results suggested in-situ iron/calcium impregnation caused the well-development of mesoporous structures during ranges of 2.0-5.0 nm in GL100 and 5.0-50 nm in GL200, respectively. The increase of iron/calcium ensured surface basicity and high ash contents on GL100/GL200, and As(III)/As(V) can be better adsorbed in neutral conditions with higher kinetics in comparison with regular mesoporous carbon XHIT. Maximum adsorption capacities of As(III)/As(V) by GL100 and GL200 were 2.985/3.385 mg/g and 2.516/2.807 mg/g, respectively. Arsenic desorption and carbon re-usability of GL100/200 was improved. As(III)(As (V)) adsorption capacities by GL100 and GL200 were 2.437(1.672) mg/g and 1.740(1.308) mg/g, respectively, after eight cycles. Arsenic adsorption capacities on GL100 were proved to be promoted with the presence of low-level of ST-HA or DHA, and be inhibited at a high-level. As(V) was bound more strongly than As(III) in the presence of ST-HA. As(III)/As(V) uptakes increased slightly and decrease gradually to 1.75/1.86 mg/g in the presence of DHA (0-10 mg DOC/L). Physisorption and chemisorption mechanisms dominant in arsenic adsorption on GL100 in presence of humic acid, forming inner-sphere complexation with metallic oxide, functional groups on carbon surface and humic acid structure, or ternary surface complexation via cationic metal ions as cation bridge.

Arsenic accumulation by red fescue (Festuca rubra) growing in mine affected soils - Findings from the field and greenhouse studies

Soils strongly enriched in arsenic in historical mining sites pose the environmental risk. Phytostabilization is a reasonable method for their remediation. A suitability of red fescue (Festuca rubra L.) for this purpose was examined. Plant and soil material was collected from four study objects: mine dumps in Złoty Stok and Czarnów and two areas formerly flooded by tailings. Total As in soils ranged 72-48900 mg/kg, while the shoots and roots of red fescue contained 1.5-65.5 and 2.3-824 mg/kg As, respectively. Bioaccumulation BAF and translocation TF factors were typical for excluders, however, in most cases, As in shoots exceeded 4 mg/kg, an EU threshold for As in fodder. A greenhouse experiment, that involved treatment with mineral fertilizers, manure, and forest litter, was performed to closer examine the factors governing As uptake by red fescue. A stress-resistant cultivar Leo-Pol was used as a test plant. Grass shoots were harvested after 6 and 12 weeks. Manure treatment increased strongly As extractability but did not increase As uptake by plants. Though, As concentrations in plants were in the pot experiment by manifold higher than those in the field. Particularly high (66.5-1580 mg/kg) was As in the second shoot harvest. Differences between the field and greenhouse data indicate that the populations of red fescue, that develop in As rich sites, are specifically As-tolerant. Possible mechanisms of tolerance are discussed. The conclusion is that the commercial cultivar, despite declared stress-resistance, cannot be used for phytostabilization of barren As-rich soils.

Effects of Different In Situ Remediation Strategies for an As-Polluted Soil on Human Health Risk, Soil Properties, and Vegetation

The demand for soils for recreational uses, gardening, or others in urban and periurban areas is increasing, and thus the presence of polluted technosols in these areas requires nature-based in situ remediation technologies. In this context, the capacity of three amendments, namely zero valent iron nanoparticles (nZVI), compost and a mixture of compost and biochar, to immobilise As in a polluted technosol simultaneously cultivated with Lolium perenne L. were tested and compared. The characteristics of the soil were comprehensively characterised by chemical and X-ray analysis to determine As contents, distribution, and mineralogy. As mobility was evaluated by the RBA methodology and then potential human health risks, both carcinogenic and non-carcinogenic, were assessed in all treatments. The nZVI treatment reduced risks due to the As immobilisation obtained (41% As decrease, RBA test), whereas the organic amendments did not imply any significant reduction of the RBA values. As to soil properties, the organic treatments applied lowered the pH values, increasing cation exchange capacity, and carbon and nutrient contents. To determine impacts over plant production, fresh biomass, As, Ca, Fe, K, Mg, Na and P were measured in Lolium under the different treatments. Notably, organic amendments improved As extraction by plants (57% increase), as well as fresh biomass (56% increase). On the contrary, nZVI diminished As extraction (65% decrease) and promoted a fresh biomass decrease of 57% due to nutrients immobilisation (61% decrease of P in plants tissues).

Influence of Soil Phosphate on the Accumulation and Toxicity of Arsenic and Antimony in Choy Sum Cultivated in Individually and Co-contaminated Soils

Fertilizers containing phosphate (PO₄^3- ) are commonly used within the agricultural industry and are known to increase the bioavailability and mobility of metalloids like arsenic (As). This may increase plant uptake of As and hence pose a risk to human health. Arsenic and antimony (Sb) often co-occur in contaminated soils; however, little is known about the interactions between As and Sb with PO₄^3- on their bioavailability, accumulation, and toxicity in plants. The present study investigated individual and combined As and Sb-contaminated soils across 2 soil PO₄^3- concentrations using a commonly consumed leafy vegetable, choy sum (Brassica chinensis var. parachinensis). Increased soil PO₄^3- had no clear influence on the bioavailability of As or Sb (derived from a sequential extraction procedure). At high PO₄^3- concentration, B. chinensis accumulated higher amounts of As in the shoots and roots in both individual and co-contaminated soil, whereas Sb accumulation increased only when Sb was the only contaminant. When As was the only contaminant, the translocation of As from roots to shoots decreased as soil PO₄^3- increased. Increased soil PO₄^3- had no influence on Sb translocation from root to shoot. Although As was toxic (impaired growth) at low PO₄^3- soil concentration, no toxicity was observed in the high-PO₄^3- soil. No toxicity was observed for Sb in either low- or high-PO₄^3- soils. Increased soil PO₄^3- concentration ameliorated or masked As toxicity to plant growth and led to higher As concentration in the plant's edible parts. The addition of high soil PO₄^3- concentrations ameliorated or masked As toxicity to plant growth in both individually and As + Sb co-contaminated soil; however, the plant's edible parts accumulated higher As and Sb concentrations. Environ Toxicol Chem 2020;39:1233-1243. © 2020 SETAC.

Aided phytostabilisation over two years using iron sulphate and organic amendments: Effects on soil quality and rye production

An outdoor macrocosm experiment using Fe-based and organic amendments over 2 years was set up to evaluate the effectiveness of aided-phytostabilisation. For that, a soil contaminated with As- and Cu-rich waste material (∼13000 mg As kg^-1 and ∼500 mg Cu kg^-1) was treated with combinations of iron sulphate (Fe) with lime, paper mill sludge (PS), holm-oak biochar (BC), olive mill waste compost (OMWC) or green waste compost (GWC). Rye (Secale cereale L.) was grown in the treated and non-treated soils 16 months after addition of the amendments. Arsenic and Cu dynamics in soil were assessed throughout the experiment and soil quality parameters (soil nutrients, organic matter and soil biology) were measured almost two years after addition of the amendments. All treatments resulted in a reduction of soluble and extractable Cu during the experiment and, despite the increase in soil pH (from 5 to 68) and DOC (from 10 up to 50 mg DOC L^-1) provoked by the amendments, As was not significantly mobilised in the treated soils. Treatments combining Fe sulphate with the organic materials, especially biochar and both composts, resulted in an increase in soil available nutrients and enhanced rye growth. In this semi-field scale experiment, the combination of Fe sulphate with holm-oak biochar showed the most promising results in terms of soil fertility (nutrient availability), plant As and Cu uptake and soil C sequestration. Further research should focus on monitoring long-term effects of the soil amendments on crops, following repeated applications.

The role of organic amendment in soils affected by residual pollution of potentially harmful elements

The addition of organic amendment in soils affected by residual pollution of potentially harmful elements (PHEs) is evaluated. The area was polluted twenty years ago and remediation actions were intensively applied, but evidence of pollution are still detected in some sectors. The amendment application produces significant changes in the main soil properties and modifies the mobility and availability of the pollutants. In general, Cu, Zn, Cd and Pb, showed a significant reduction in soluble and exchangeable forms after the vermicompost addition (percentage of reduction ranging from 59% for soluble Pb to 95% for exchangeable Zn), both in highly (UVS) as in moderately (VS1) polluted soils. This reduction is strongly related to the rise in OC content and pH. Arsenic presented no significant reduction or even an increase in soluble forms in moderately polluted soils (VS1), where the competing effects of OC and phosphorous could be responsible for this increase. Pb also showed an increase in availability after vermicompost application, probably related to the competing effect of Mg²⁺ coming from the organic amendment. The less mobile forms (those extracted with oxalic-oxalate, pyrophosphate and EDTA), indicate that vermicompost application reduce medium-long term mobility to similar values of those found in less polluted soils (VS2); anyway, an increase in available forms of Pb and As was detected in some cases, indicating a potential risk of toxicity that should be monitored over time.

Ecotoxicity of pore water in soils developed on historical arsenic mine dumps: The effects of forest litter

Arsenic release from dump soils in historical mining sites poses the environmental risk. Decomposing forest litter can affect mobilization of As and other toxic elements, change their speciation in pore water and influence the toxicity to biota. This study examined the chemistry and ecotoxicity of pore water acquired from four soils that developed on the dumps in former As mining sites, in the presence and absence of forest litter collected from beech and spruce stands. Soils contained 1540-19600 mg/kg of As. Pore water was collected after 2, 7, 21 and 90 days of incubation, using MacroRhizon suction samplers. Its chemical analysis involved determination of pH, the concentrations of As, Cu and Pb (the elements with high enrichment factor Igeo>3), as well as metals considered most mobile: Cd, Zn and Mn. Ecotoxicity of pore water was examined in three bioassays: Microtox, MARA and Phytotox with Sinapis alba as test plant. The release of As, unlike heavy metals, was particularly intensive from the soils with neutral and alkaline pH. The concentrations of toxic elements in pore water were in broad ranges, up to dozens mg/L. The results of Phytotox had a poor precision, but their means correlated well with As concentrations in pore water, which indicates that As made a crucial factor of phytotoxicity. The outcomes of Microtox bioassay indicated poorer relationships between As concentrations and toxicity, and other factors contributed to ecotoxicity at very low and very high As concentrations. The highest toxicity was recorded from the soils treated with forest litter. MARA turned out to be not sensitive enough to give reproducible results in experimental conditions. The PCA analysis confirmed that the growth of microbes in MARA bioassay was poorly dependent on As and metals in pore water except for a yeast Pichia anomala (No 11). The results let us conclude that the bioassays Phytotox and Microtox can provide useful information on ecotoxicity of pore water in soils that develop on As-rich dumps whereas applicability of MARA in those conditions proved limited.

Combination of soil organic and inorganic amendments helps plants overcome trace element induced oxidative stress and allows phytostabilisation

Trace element (TE)-contaminated soils require the improvement of their physico-chemical properties in order to allow their restoration through phytostabilization technologies. This study aimed to determine the usefulness of oxidative stress related parameters to validate the suitability of two different combinations of organic (solid fraction of pig slurry) and inorganic (paper mill sludge or a commercial red mud derivative) amendments for the phytostabilization of an acidic (4.2) TE-contaminated mine soil from SE Spain. Two wild species (Silybum marianum and Piptatherum miliaceum) were greenhouse cultivated and the development of the plants, their ionome, and oxidative stress related parameters were determined. Both amendment combinations increased significantly soil pH (to 5-6) and soil/pore water total organic C and total N concentrations, allowing an adequate plant growth and development (plants did not grow in untreated soils). The combination of amendments significantly reduced metal availability and showed to be effective (specially the one including the red mud derivative) in limiting shoot TE concentrations, which were all within common ranges (exclusion based tolerance of these species). Both protein carbonylation and lipid peroxidation were significantly higher in S. marianum plants from phytostabilized soils than in those from non-contaminated soils, which confirms the oxidative stress these plants suffer despite their satisfactory growth in the treated soils. P. miliaceum plants showed no differences between phytostabilized and non-contaminated soils. Therefore, the combination of amendments and TE-tolerant autochthonous species would be a suitable option for the phytostabilisation of soils contaminated by mining activities, reducing TE solubility and allowing an adequate plant growth.

Effect of phosphate on amorphous iron mineral generation and arsenic behavior in paddy soils

Arsenic (As) contamination was detected in paddy soils of Guangdong, China due to mining and weathering processes. Furthermore, As may be released into the soil and irrigation water during the application of phosphate (P). In this study, As behavior was assessed in three paddy soils (S6, S8 and TR) along the Hengshi river using batch and circular flow experiments with different phosphate application doses (0, 1, 5, 10, 50, 100 mg/L). The results indicate that pH variation (3-7) and higher phosphate concentrations in solution, can induce the release of As, with total As release ranked in the order: S6 > S8 > TR. In addition, As^V was the main state affected by phosphate in the circular soil solution. In particular, after 7 days of P₁₀ application, the highest As concentration in S6, S8 and TR soil solutions reached 2298.4, 829.9 and 153.9 μg/L respectively, with the As^V state accounting for 93%, 97% and 18% of As. Some minerals were found to be generated in the middle container, most of which were amorphous iron or aluminum oxides and hydroxides, as confirmed by XRD. With mineral generation, the As concentration in soil solutions decreased to 314.2, 98.1 and 34.1 μg/L. The SEM results indicate that the minerals became more fine (<100 nm) when more P was applied. In addition, XPS, SEM-eds and elemental analysis results also revealed that As distribution was closely associated with iron minerals. Along with soil depth, P influenced the state and distribution of iron minerals and As in the topsoil, while phosphate increased the available As and reduced the amorphous iron mineral content in the soil. Therefore, it is essential to evaluate As behavior in paddy soils, to monitor and avoid potential food security risks.

Post-depositional changes in elemental leaching from recovered soils separated from disaster waste and tsunami deposits generated by the Great East Japan Earthquake and tsunami

The Great East Japan Earthquake and subsequent tsunami in 2011 generated massive amounts of disaster waste and tsunami deposits, one-third of which comprised soil and sediment, which are expected to be re-used; however, there has been no previous experience or knowledge of recovered soil. In this study, up-flow column leaching tests were conducted to investigate the elemental leaching behavior in this soil and sediment following its separation and treatment (hereafter termed "recovered soil") to assess whether it can be safely re-used without posing any environmental risk. The pH of the leaching water was slightly alkaline throughout the test period, regardless of the source of the recovered soil. Concentrations of calcium and sulfate ions in the leaching water varied in a similar way to the electrical conductivity (EC), with a stable state being observed initially followed by a rapid decrease until typical concentrations were reached, whereas sodium and chloride ions derived from seawater made a relatively small contribution to the EC. In terms of toxic elements, zinc and copper concentrations decreased as the volume of leaching water increased, whereas lead, fluoride, and arsenic concentrations increased as the concentration of calcium and sulfate ions decreased, indicating that the dissolution of large quantities of compounds such as calcium sulfate controlled the toxic element behavior in the recovered soils. Consequently, there is a need for continuous and careful monitoring of areas where recovered soil is re-used or treatment of the recovered soil prior to its re-use.

Field tests of in-well electrolysis removal of arsenic from high phosphate and iron groundwater

Subsurface arsenic (As) removal has been proposed for in situ immobilizing As in aquifers at a low cost and without post-disposal of As-containing wastes. However, the results reported for field tests are very limited, particularly when high As, phosphate (P) and iron (Fe) coexist in the groundwater. Herein the performance of single- and multiple-well operations was evaluated for in situ removing groundwater As in Jianghan Plain, central China. To enhance groundwater oxygenation, in-well electrolysis was employed in both operation modes. The groundwater in confined aquifer in Jianghan Plain contains elevated concentrations of As (272-606 μg/L), Fe²⁺ (4.7-14.3 mg/L) and P (0.90-1.58 mg/L). In the single-well operation with cycles of injection and abstraction, groundwater Fe²⁺ was completely removed but As cannot be reduced to below the World Health Organization guideline (10 μg/L) due to the high concentration and the competition of coexisting P. In-well electrolysis is cost-effective for boosting dissolved oxygen (DO) and Fe²⁺ removal in single-well operations. In the multiple-well operation with one abstraction well surrounded by 6 in-well electrolysis wells, removals of groundwater As, Fe, P and Mn were not sufficient because of clogging of treatment wells and incomplete capture of groundwater flowing to the abstraction well. In comparison, single-well operation is more simple and efficient for in situ treatment of groundwater As and Fe than multiple-well operation. This study provides a field example of in situ removing high As in groundwater by both single- and multiple-well operations, and underscores the difficulty in treating the groundwater with coexistence of elevated As and P.

Pronounced Antagonism of Zinc and Arsenate on Toxicity to Barley Root Elongation in Soil

Zinc (Zn) and arsenic (As) occur as mixed contaminants in soil and the interactions between them remain unclear. Here, we investigated a Zn2+ and H2AsO4− mixture interaction and their effects on plant growth. Three different soils were spiked with ZnCl2 and NaH2AsO4, each dosed singly or in combination. The soils were leached to remove excessive salt and were aged (>7 days), before toxicity testing using a 5-day root elongation of barley (Hordeum vulgare L.). In the single treatments, the 50% inhibitory effect concentrations in the soil (EC50, total measured concentration) were 2000–3800 mg Zn kg−1 and 96–620 mg As kg−1, depending on the soils. The mixture analyses based on the total concentrations showed overall and significant Zn–As antagonism in two soils, either based on the concentration addition (CA) or independent action (IA) model, whereas no significant interactions (either CA or IA) were found in one soil, which had the lowest content of Fe-oxides. The soil solution composition showed a decreased As concentration upon the addition of ZnCl2 at an equal soil As total concentration; however, the reverse was not found, in line with the cation–anion electrostatic interaction or formation of ternary surface complexes on Fe-oxides. The data revealed that the Zn–As antagonisms (total concentrations) are partially related to the increased Zn immobilizing As in soil.

Review of interactions between phosphorus and arsenic in soils from four case studies

Arsenic is a non-essential element that poses risks in many environments, including soil, groundwater, and surface water. Insights into the environmental biogeochemistry of As can be gained by comparing As and P reaction processes. Arsenic and P are chemical analogues, and it is proposed that they have similar chemical behaviors in environmental systems. However some chemical properties of As and P are distinct, such as redox reactions, causing the biogeochemical behavior of the two elements to differ. In the environment, As occurs as either As(V) or As(III) oxyanions (e.g., AsO₄^3- or AsO₃^3-). In contrast, P occurs predominantly as oxidation state five plus; most commonly as the orthophosphate ion (PO₄^3-). In this paper, data from four published case studies are presented with a focus on P and As distribution and speciation in soil. The goal is show how analyzing P chemistry in soils can provide greater insights into As reaction processes in soils. The case studies discussed include: (1) soil developed from shale parent material, (2) mine-waste impacted wetland soils, (3) phosphate-amended contaminated soil, and (4) plants grown in biochar-amended, mine-contaminated soil. Data show that while P and As have competitive reactions in soils, in most natural systems they have distinct biogeochemical processes that create differing mobility and bioavailability. These processes include redox reactions and rhizosphere processes that affect As bioavailability. Results from these case studies are used as examples to illustrate how studying P and As together allows for enhanced interpretation of As biogeochemical processes in soils.

Arsenic adsorption and plant availability in an agricultural soil irrigated with As-rich water: Effects of Fe-rich amendments and organic and inorganic fertilisers

The use of As-rich water for irrigation in agricultural soils may result in As accumulation in soil and crops, with the consequent risk of its entry into the food chain. The effectiveness of three different Fe-based materials (a commercial iron oxide (Bayoxide^®), lamination slag (a by-product of the hot rolling of steel) and a commercial red mud derivative (ViroBind™)) used as soil amendments to minimise the impact of irrigation with As-rich water in an agricultural soil-plant system was evaluated in a pot experiment. Simultaneously, the influence of organic and inorganic fertilisation (olive oil mill waste compost versus NPK fertiliser) on the effectiveness of iron oxide in As adsorption processes was also assessed. The As adsorption capacity of the amendments was determined in a preliminary batch experiment using sorption isotherms. Then, a pot experiment was carried out in a growth chamber using an agricultural soil (arenosol) from Segovia province (central Spain), amended with the different materials, in which Lactuca sativa (lettuce) was grown for two months. The As adsorption capacity was higher in the commercial iron oxide and in the red mud derivative, which fitted the Freundlich model (no saturation), than in the lamination slag, which fitted the Langmuir model (limited adsorption). All the materials decreased the pore water As concentration compared to the control (by 29-80%), but only iron oxide reduced As availability in the soil, and none of the amendments decreased the As concentration in plant leaves. The combination of iron oxide and compost did not significantly improve plant growth, but increased nutrients (N, K, Ca, Na and Mg) concentrations and availability in the soil and their concentration in the plants, relative to the other treatments and the control. Therefore, this seems to be a viable option to prevent As leaching and improve the plant nutritional status.

Mechanism of arsenic resistance in endophytic bacteria isolated from endemic plant of mine tailings and their arsenophore production

Arsenic contamination is an important environmental problem around the world since its high toxicity, and bacteria resist to this element serve as valuable resource for its bioremediation. Aiming at searching the arsenic-resistant bacteria and determining their resistant mechanism, a total of 27 strains isolated from roots of Prosopis laevigata and Spharealcea angustifolia grown in a heavy metal-contaminated region in Mexico were investigated. The minimum inhibitory concentration (MIC) and transformation abilities of arsenate (As⁵⁺) and arsenite (As³⁺), arsenophore synthesis, arsenate uptake, and cytoplasmatic arsenate reductase (arsC), and arsenite transporter (arsB) genes were studied for these strains. Based on these results and the 16S rDNA sequence analysis, these isolates were identified as arsenic-resistant endophytic bacteria (AREB) belonging to the genera Arthrobacter, Bacillus, Brevibacterium, Kocuria, Microbacterium, Micrococcus, Pseudomonas, and Staphylococcus. They could tolerate high concentrations of arsenic with MIC from 20 to > 100 mM for As⁵⁺ and 10-20 mM for As³⁺. Eleven isolates presented dual abilities of As⁵⁺ reduction and As³⁺ oxidation. As the most effective strains, Micrococcus luteus NE2E1 reduced 94% of the As⁵⁺ and Pseudomonas zhaodongensis NM2E7 oxidized 46% of As³⁺ under aerobic condition. About 70 and 44% of the test strains produced arsenophores to chelate As⁵⁺ and As³⁺, respectively. The AREB may absorb arsenate via the same receptor of phosphate uptake or via other way in some case. The cytoplasmic arsenate reductase and alternative arsenate reduction pathways exist in these AREB. Therefore, these AREB could be candidates for the bioremediation process.

Insight into mineralizer modified and tailored scorodite crystal characteristics and leachability for arsenic-rich smelter wastewater stabilization

The addition of appropriate mineralizers is a potentially effective strategy for the control of crystal growth, and could be used in the disposal and stabilization of arsenic-rich non-ferrous effluents.

Aided phytostabilisation of As- and Cu-contaminated soils using white lupin and combined iron and organic amendments

An aided phytostabilisation strategy consisting of several composite amendments of iron sulphate and organic materials combined with Lupinus albus L. (white lupin) was evaluated for remediation of an As- and Cu-contaminated soil. Iron sulphate was combined with lime, paper mill sludge (PS), olive mill waste compost (OMWC) or holm oak biochar (BC) and applied to a slightly acidic soil with high concentration of As (∼2200 mg kg^-1) and Cu (∼150 mg kg^-1). White lupin was grown for 48 days in pots containing amended and non-amended soils and the effect of soil treatments on soluble and extractable trace elements, soil fertility and plant growth and composition was evaluated. The addition of the amendments raised soil pH and reduced soluble As (50-93%) and extractable As and Cu (50-89%). Despite the reduction of As- and Cu-extractable fractions, plant As and Cu uptake was not greatly affected by the amendments. Variations in soil pH and P-Olsen seemed to have influenced As dynamics in the treated soils, although they did not provoke its mobilisation with respect to the non-amended soil. Our results suggest that the freshly formed iron oxides resulting from addition of iron sulphate controlled As dynamics in the treated soils, avoiding its mobilisation due to application of organic materials. The combination of iron sulphate with OMWC and BC is shown as appropriate for aided phytostabilisation of metal(loid)s contaminated soils, as it improved soil fertility and plant nutrition while reduced As and Cu mobility.

Dynamic arsenic aging processes and their mechanisms in nine types of Chinese soils

Although specific soil properties controlling the arsenic (As) aging process have been studied extensively, few investigations have attempted to determine how soil types influence As bioavailability and fractionations in soils. Nine types of soil were selected from typical grain producing areas in China, and the bioavailability and fractionations of As during aging were measured. Results showed that available As in all soils rapidly decreased in the first 30 days and slowly declined thereafter. In spiked soils, As easily became less available and less toxic in low pH soils compared to high pH soils, demonstrating the importance of soil pH on As availability. Results from fitting kinetic equations revealed that the pseudo-second-order model described the As aging processes well in all soils (R² = 0.945-0.999, P < 0.01, SE = 0.09-4.25), implying that the mechanism for As aging combined adsorption, external diffusion, and internal diffusion. Fe oxides were more important than Al oxides for determining the As aging rate (|k|). Based on these results, we are the first to propose the approximate aging equilibrium time (T) for As, which was mainly influenced by soil clay content. The shortest time for approximate stabilization of As aging was 28 d in latosol soils (LS), while the longest approximate equilibrium time was 169 d in cinnamon soils (CS). Individual soil properties controlling the variation in different As fractionations further confirmed that the influences of soil types on As aging were the result of the combined effects of soil properties and a time-consuming redistribution process.