Arsenic release dynamics of paddy field soil during groundwater irrigation and natural flooding

Prediction and verification of arsenic phytoavailability in paddy soil based on CD-MUSIC model in the Xiangjiang River Basin

Arsenic (As) accumulation in rice grains is primarily governed by its phytoavailability, which is major influenced by soil physicochemical properties and the surface activity of iron oxides, particularly goethite and ferrihydrite. Soil pH and the ionic strength of the soil solution are determinants of surface activity on iron oxides, which controlled the As adsorption and desorption dynamics. However, As activity is highly variable during the rice reproductive cycle, there is still a lack of approach to accurately predict and to the phytoavailability of As due to the paddy soil heterogeneity and complexity. The Charge Distribution-Multisite Complexation (CD-MUSIC) is a promising method to simulate ion adsorption behavior on iron oxide surfaces. This study applied the CD-MUSIC model to optimize soil pH and ionic strength parameters based on different scenario simulation. Subsequent linear regression analysis revealed a strong correlation (R² = 0.7664) between model-predicted dissolved As concentrations and As accumulation in rice grains. The R2 between predicted rice As with pot rice As increasing to 0.9208 after BCF (Bioconcentration Factor) corrected, demonstrating high homology between model-predicted dissolved As and bioavailable As assimilated by rice plants. To evaluate regional applicability, the model was simplified and validated across 293 sampling sites spanning the upper, middle, and lower reaches of the Xiangjiang River Basin, achieving a robust regional-scale prediction accuracy (R² = 0.6958). These results provide a feasible model for predicting rice As accumulation, which supports the safe development of regional agriculture and risk management.

Long-term effectiveness of heavy metal(loid) stabilization: Development of an assessing method

In-situ stabilization technology offers a cost-effective solution for the remediation of heavy metal(loid) (HM) contaminated soils. However, the lack of a reliable method to assess the long-term effectiveness of HM stabilization significantly impedes the practical application of this technology. To address this gap, we have devised an innovative method that integrates acid rain leaching with dry-wet alternation to evaluate the long-term effectiveness of HM stabilization. We initiate the acid rain leaching process by adding 200 mL of a H2SO4 and HNO3 solution, with a pH of 3.20, to 20 g of tested soil and stirring at 30 ± 2 rpm for 2 h. After decanting the supernatant, we dried the soil in a water bath at 60 °C. Then repeat this leaching and drying cycle until HM in the leachate either exceed the preset thresholds or become stable. The time-dependent effectiveness of the stabilization is calculated based on the annual average rainfall, and the number of cycles. By using multiple types of soils contaminated with various HM, we demonstrated that this method is versatile and not limited by the types of soil or HM, and exhibits excellent multi-laboratory precision. The method exhibited excellent multi-laboratory precision, with over 82% of samples having a relative standard deviation (RSD) of less than 30%. This method is of significance for not only mitigating the risk of re-contamination from HM reactivation post-remediation, but also broadening the disposal options for remediated soils beyond landfill, thereby fostering environmentally sustainable practices.

A review on As-contaminated soil remediation using waste biomass feedstock-based biochar and metal-modified biochar

Arsenic (As) is a carcinogen that threatens ecosystems and human health. Due to its high adsorption, and microporosity, biochar is widely available for soil remediation. This review significantly summarizes the current status of waste biomass feedstock-based biochar and metal-modified biochar for As-contaminated soil remediation. Firstly, this paper briefly describes the sources and hazards of As in soil, and secondly, lists eleven feedstocks for preparing biochar. Agricultural, domestic, and forestry wastes provide a plentiful source for biochar preparation. Single or multi-metal modifications such as iron (Fe), manganese (Mn), and cerium (Ce) can effectively improve the Arsenite [As(III)] and arsenate [As(V)] adsorption capacity of biochar. The primary mechanisms of As removal by waste biomass feedstock-based biochar and metal-modified biochar include ion exchange, electrostatic attraction, surface complexation, redox transformation, and H-bond formation. In conclusion, this review presents an in-depth discussion on both waste biomass feedstocks and metal modification, providing constructive suggestions for the future development of biochar to remediate As-contaminated soil.

Factors and Mechanisms Affecting Arsenic Migration in Cultivated Soils Irrigated with Contained Arsenic Brackish Groundwater

Contained arsenic (As) and unsafe brackish groundwater irrigation can lead to serious As pollution and increase the ecological risk in cultivated soils. However, little is known about how Fe oxides and microbes affect As migration during soil irrigation processes involving arsenic-contaminated brackish groundwater. In this study, the samples (porewater and soil) were collected through the dynamic soil column experiments to explore the As migration process and its effect factors during soil irrigation. The results showed that the As concentration in porewater samples from the topsoil was enriched compared to that in the subsoil, and the main solid As fractions were strongly adsorbed or bound to amorphous and crystalline Fe oxides. The aqueous As concentration and the solid As fractions indicated that reductive dissolution and desorption from amorphous Fe oxides were the primary mechanisms of As release at the topsoil and subsoil, respectively. Meanwhile, Sphingomonas_sp., Microvirga_ossetica and Acidobacteriota_bacterium were the dominant microbes affecting As biotransformation by arsenate reductase gene (arsC) expression. Accompanied by the Eh and competitive ions concentration change, amorphous Fe oxide dissolution increased to facilitate the As release, and the changes in the microbial community structure related to As reduction may have enhanced As mobilization in soils irrigated by As-containing brackish groundwater.

The Mechanism of Arsenic Release in Contaminated Paddy Soil with Added Biochar: The Role of Dissolved Organic Matter, Fe, and Bacteria

The addition of biochar inevitably modifies the acidity (pH), redox potential (Eh), and dissolved organic matter (DOM) level in the soil. These alterations also have coupled effects on the cycling of iron (Fe) and the composition of bacterial communities, thereby impacting the speciation and availability of arsenic (As) in the soil. This study explored the potential mechanisms through which biochar affects As in paddy soil during flooded cultivation with different pyrolysis temperature biochars (300 °C, 400 °C, and 500 °C) added. The results revealed that the TAs concentration increased in the initial 15 days of soil cultivation with SBC300 or SBC400 addition because increasing the concentration of DOM induced the mobility of As though the formation of As-DOM complexes. Meanwhile, biochar addition elevated the pH, decreased the Eh, and promoted the transformation of specific adsorbed As (A-As) and amorphous iron oxide-bound As (Amo-Fe-As) to supernatant As through enhancing the reductive dissolution of Fe(oxy)(hydr)oxides. Moreover, the biochar altered the relative abundance of As (V)-reducing bacteria (such as Firmicutes) and As (III)-oxidizing bacteria (such as Chloroflex), thereby affecting As speciation. However, these mechanistic effects varied depending on the pyrolysis temperature of the biochar. The microbial composition of SBC300 and SBC400 were similar, with both containing larger populations of Enterobacteriaceae (AsRB) and pseudomonas (FeRB) compared to CK and SBC500. It was proposed that lower pyrolysis temperatures (300 °C and 400 °C) are more favorable for the dissolution of Fe(oxy)(hydr)oxides and the reduction of As (V). However, the biochar from the higher pyrolysis temperature (500 °C) showed environmental impacts akin to the control group (CK). This study demonstrated potential mechanisms of biochar's effect on As and the role of pyrolysis temperature.

Optimizing cropland use to reduce groundwater arsenic hazards in a naturally arsenic-enriched grain-producing region

In the Hetao Basin, a grain-producing region plagued by naturally occurring arsenic (As) pollution, understanding the role of agricultural cultivation activities in mobilizing As in groundwater is worthwhile. Here we investigated the impact of cropland use characteristics on groundwater As hazards using a model that combines Random Forest (RF) classification with SHapley Additive exPlanation (SHAP). The analysis incorporated eight cropland use characteristics and three natural factors across 1258 groundwater samples as independent variables. Additionally, an optimized cropland use strategy to mitigate groundwater As hazards was proposed. The results revealed that crop cultivation area, especially within a 2500m-radius buffer around sampling points, most significantly influenced the probability of groundwater As concentrations exceeding an irrigation safety threshold of 50 μg/L, achieving an AUC of 0.86 for this prediction. The relative importance of crop areas on As hazards were as follows: sunflower > melon > wheat > maize. Specifically, a high proportion of sunflower area (>30%), particularly in regions with longer cropland irrigation history, tended to elevate groundwater As hazards. Conversely, its negative driving force on groundwater As hazards was more pronounced with the increase in the proportion of wheat area (>5%), in contrast to other crops. Transitioning from sunflower to wheat or melon cultivation in the northeast of the Hetao Basin may contribute to lower groundwater As hazards. This study provides a scientific foundation for balancing food production with environmental safety and public health considerations.

Prediction of As and Cd dissolution in various soils under flooding condition

Although the mobility of arsenic (As) and cadmium (Cd) in soils during the flooding-drainage process has been intensively studied, predicting their dissolution among various soils still remains a challenge. After comprehensively monitoring multiple parameters related to As and Cd dissolution in 8 soils for a 60-day anaerobic incubation, the redundancy analysis (RDA) and structural equation model (SEM) were employed to identify the key factors and influencing pathways controlling the dynamic release of As and Cd. Results showed that pH alone explained 90.5 % Cd dissolution, while the dissolved-Fe(II) and 5 M-HCl extractable Fe(II) jointly only explained 50.6 % As dissolution. After data normalization, the ratio of Fe(II) to 5 M-HCl extracted total Fe (i.e. FetotII/Fetot) significantly improved the correlation to R2 = 0.824 (p < 0.001) with a fixed slope of 0.393 among the 8 soils. Our results highlight the crucial role played by the reduction degree of total iron contents in determining both the reduction and dissolution of As during flooding. In contrast, dissolved-Fe(II) was too vulnerable to soil properties to be a stable indicator of As dissolution. Therefore, we propose to replace the dissolved-Fe(II) with this novel ratio as the key index to quantitatively assess the kinetic change of As solubility potential across various soils under flooding conditions.

Predicting Redox Conditions in Groundwater at a National Scale Using Random Forest Classification

Redox conditions in groundwater may markedly affect the fate and transport of nutrients, volatile organic compounds, and trace metals, with significant implications for human health. While many local assessments of redox conditions have been made, the spatial variability of redox reaction rates makes the determination of redox conditions at regional or national scales problematic. In this study, redox conditions in groundwater were predicted for the contiguous United States using random forest classification by relating measured water quality data from over 30,000 wells to natural and anthropogenic factors. The model correctly predicted the oxic/suboxic classification for 78 and 79% of the samples in the out-of-bag and hold-out data sets, respectively. Variables describing geology, hydrology, soil properties, and hydrologic position were among the most important factors affecting the likelihood of oxic conditions in groundwater. Important model variables tended to relate to aquifer recharge, groundwater travel time, or prevalence of electron donors, which are key drivers of redox conditions in groundwater. Partial dependence plots suggested that the likelihood of oxic conditions in groundwater decreased sharply as streams were approached and gradually as the depth below the water table increased. The probability of oxic groundwater increased as base flow index values increased, likely due to the prevalence of well-drained soils and geologic materials in high base flow index areas. The likelihood of oxic conditions increased as topographic wetness index (TWI) values decreased. High topographic wetness index values occur in areas with a propensity for standing water and overland flow, conditions that limit the delivery of dissolved oxygen to groundwater by recharge; higher TWI values also tend to occur in discharge areas, which may contain groundwater with long travel times. A second model was developed to predict the probability of elevated manganese (Mn) concentrations in groundwater (i.e., ≥50 μg/L). The Mn model relied on many of the same variables as the oxic/suboxic model and may be used to identify areas where Mn-reducing conditions occur and where there is an increased risk to domestic water supplies due to high Mn concentrations. Model predictions of redox conditions in groundwater produced in this study may help identify regions of the country with elevated groundwater vulnerability and stream vulnerability to groundwater-derived contaminants.

Formulation of ferric/phosphorus composite coating on coal gangue as a novel fertilizer for enhancing slow-release of silicon and implication of As, Cr and Pb

Owing to the abundant silicon content in coal gangue, its conversion into fertilizer can help address large-scale storage. Nonetheless, the rapid release of silicon in coal gangue poses challenges for plants to fully utilize it. A slow-release fertilizer prepared by ferric/phosphorus composite coating on coal gangue (C@SP) was developed in the study. The findings revealed that the C@SP can facilitate slow release of Si and enhance the stabilization of As, Pb, and Cr in soil. C@SP can react with As and Cr to form stable Fe-As-PO4 and Fe-Cr-PO4 compounds. The -OH in C@SP can combine with Pb, transforming it into insoluble Pb, which was then integrated into the crystal structure with ferric/phosphorus composite or Fe(III)-oxyhydroxysulfate to create a more stable form. The silicon release was promoted by the conversion of the passivation film to iron oxides. Thus, the fertilizer holds promise for application in environmental activities.

Mitigation of the mobilization and accumulation of toxic metal(loid)s in ryegrass using sodium sulfide

Remediation of soils contaminated with toxic metal(loid)s (TMs) and mitigation of the associated ecological and human health risks are of great concern. Sodium sulfide (Na2S) can be used as an amendment for the immobilization of TMs in contaminated soils; however, the effects of Na2S on the leachability, bioavailability, and uptake of TMs in highly-contaminated soils under field conditions have not been investigated yet. This is the first field-scale research study investigating the effect of Na2S application on soils with Hg, Pb and Cu contents 70-to-7000-fold higher than background values and also polluted with As, Cd, Ni, and Zn. An ex situ remediation project including soil replacement, immobilization with Na2S, and safe landfilling was conducted at Daiziying and Anle (China) with soils contaminated with As, Cd, Cu, Hg, Ni, Pb and Zn. Notably, Na2S application significantly lowered the sulfuric-nitric acid leachable TMs below the limits defined by Chinese regulations. There was also a significant reduction in the DTPA-extractable TMs in the two studied sites up to 85.9 % for Hg, 71.4 % for Cu, 71.9 % for Pb, 48.1 % for Cd, 37.1 % for Zn, 34.3 % for Ni, and 15.7 % for As compared to the untreated controls. Moreover, Na2S treatment decreased the shoot TM contents in the last harvest to levels lower than the TM regulation limits concerning fodder crops, and decreased the TM root-to-shoot translocation, compared to the untreated control sites. We conclude that Na2S has great potential to remediate soils heavily tainted with TMs and mitigate the associated ecological and human health risks.

Insight into the functional mechanisms of nitrogen-cycling inhibitors in decreasing yield-scaled ammonia volatilization and nitrous oxide emission: A global meta-analysis

Soil ammonia (NH3) volatilization and nitrous oxide (N2O) emission decrease nitrogen (N) utilization efficiency and cause some environmental problems. The N-cycling inhibitors are suggested to apply to enhance N utilization efficiency. Quantifying effects of N-cycling inhibitors on yield-scaled NH3 volatilization and N2O emission and functional genes could provide support for the optimal selection and application of N-cycling inhibitor. We conducted a meta-analysis to reveal the effects of N-cycling inhibitors on soil abiotic properties, functional genes and yield-scaled NH3 volatilization and N2O emission by extracting data from 166 published articles and linked their comprehensive relationships. The N-cycling inhibitors in this meta-analysis mainly includes nitrification inhibitors 3, 4-dimethyl pyrazole phosphate, dicyandiamide and 2-chloro-6-trichloromethylpyridine, urease inhibitor N-(n-butyl) thiophosphoric triamide and biological nitrification inhibitors methyl 4-hydroxybenzoate and 1, 9-decanediol. The N-cycling inhibitor applications significantly increased alkaline soil pH but significantly decreased acidic soil pH. The N-cycling inhibitors decreased soil AOB amoA gene abundances mostly under the condition of pH 4.5-6 (mean: 212%, 95% confidence intervals (CI): 249% and -176%) and significantly decreased nirS gene (mean: 39%; 95% CI: 72% and -6%). The yield-scaled NH3 volatilization was significantly decreased by the N-cycling inhibitors under the condition of soil pH = 7-8.5 (mean: 45%; 95% CI: 59% and -31%). The yield-scaled N2O emission was also significantly reduced by all N-cycling inhibitors and had negative correlations with the soil nirK and nirS gene abundances. The effects of N-cycling inhibitors on soil pH, ammonium-N, nitrate-N and nitrifying and denitrifying genes and yield-scaled NH3 volatilization and N2O emission were dominated by the inhibitor types, soil textures, crop species and environmental pH. Our study could provide technical support for the optimal selection and application of N-cycling inhibitor under different environmental conditions.