Sorption-desorption of Sb(III) in different soils: Kinetics and effects of the selective removal of hydroxides, organic matter, and humic substances

Environmental quality standards for agricultural land in China: What should be improved on derivation methodology?

Soil pollution has caused increasingly widespread attention in China. The environmental risk threshold of pollutants is a yardstick to measure soil environmental quality. For decades, plenty of research on soil environmental quality standards (SEQSs) has been carried out, providing scientific basis for the investigation and supervision of soil environmental quality. This paper summaries the development of SEQSs in China, the corresponding influencing factors and methodology of SEQSs derivation. In the current version of SEQSs (GB15618-2018), the thresholds of soil pollutants are derived by the methods of environmental risk assessment, which are more methodologically scientific than geochemical method and ecological effect method used in the previous version (GB15618-1995). Abundant toxicology data on related species is required for risk assessment of soil pollution using extrapolation; however, basic toxicological data is insufficient and few valid data is available at present. Besides, the inadequate consideration on influencing factors for the derivation of soil pollutant threshold would affect the scientificity and rationality of SEQSs, such as biotic factors (species type, test endpoint etc.) and abiotic factors (aging effect, leaching effect, synergistic or antagonistic effects of elements etc.). These problems should be paid close attention in future research on soil environmental quality standards. The contents summarized in this review may provide reference for decision-making on supervision of soil environmental quality and point out important directions for future studies.

Speciation, transportation, and pathways of cadmium in soil-rice systems: A review on the environmental implications and remediation approaches for food safety

Cadmium (Cd) contamination in paddy fields is a serious health concern because of its high toxicity and widespread pollution. Recently, much progress has been made in elucidating the mechanisms involved in Cd uptake, transport, and transformation from paddy soils to rice grains, aiming to mitigate the associated health risk; however, these topics have not been critically reviewed to date. Here, we summarized and reviewed the (1) geochemical distribution and speciation of Cd in soil-rice systems, (2) mobilization, uptake, and transport of Cd from soil to rice grains and the associated health risks, (3) pathways and transformation mechanisms of Cd from soil to rice grains, (4) transporters involved in reducing Cd uptake, transport, and accumulation in rice plants, (5) factors governing Cd bioavailability in paddy, and (6) comparison of remediation approaches for mitigating the environmental and health risks of Cd contamination in paddy fields. Briefly, this review presents the state of the art about the fate of Cd in paddy fields and its transport from soil to grains, contributing to a better understanding of the environmental hazards of Cd in rice ecosystems. Challenges and perspectives for controlling Cd risks in rice are thus raised. The summarized findings in this review may help to develop innovative and applicable methods for controlling Cd accumulation in rice grains and sustainably manage Cd-contaminated paddy fields.

Comparing desorption properties of pollutants on bentonite particles and in compacted bentonite

Desorption is one of the main factors causing groundwater and soil pollution. Therefore, the study of clay desorption characteristics is important for the prediction of groundwater and soil pollution. In previous studies, batch tests and column tests were used to study the desorption characteristics of pollutants on clay. However, the desorption parameters obtained via the two test methods were often quite different. To investigate differences in the desorption characteristics of different pollutants on clay particles and in compacted clay, batch and column desorption tests were conducted using cadmium chloride, fulvic acid, and sodium phosphate as the adsorbates and bentonite as the adsorbent. It was found that the unit particle surface area desorption distribution coefficients of pollutants on bentonite particles were approximately equal to the unit pore surface area distribution coefficients of pollutants in compacted bentonite. This indicates that the desorbed amount per unit of surface area is basically consistent, regardless of whether they are sorbed on particles or in compacted bentonite. A simple formula for determining the desorption retardation factor of pollutants in compacted bentonite is presented. The results of this study provide a reference for the prediction and evaluation of groundwater and soil pollution.

The influence of different antimony (Sb) compounds and ageing on bioavailability and fractionation of antimony in two dissimilar soils

Assessing the bioavailability of various Sb substances plays a crucial role in human health and the ecological risk assessment of contaminated soils. However, fate, behaviour and bioavailability of different Sb compounds in soils are insufficiently known. Therefore, in this present study, the effects of soil properties and ageing on bioavailability of four different Sb compounds (C₈H₄K₂O₁₂Sb₂, Sb₂S₃, Sb₂O₃ and Sb₂O₃ nanoparticles) were evaluated during 120 days ageing time. A black soil (BS) with approximately 12% organic matter (OM) and a red soil (RS) with less than 1% OM were amended with 1000 mg Sb kg^-1 of different Sb compounds and subjected to single extractions with distilled (DI) water, 2M HNO₃, Simplified Bioaccessibility Extraction Test (SBET) and a modified Community Bureau of Reference (BCR) sequential extraction method. The results revealed that there are substantial variations in dissolution rate of various Sb sources, depending upon soil type and Sb compound. The amounts of DI water extractability of Sb during the incubation time varied between <1% and 2%, whereas HNO₃ extractable fractions and Sb bioaccessibility at the end of ageing time ranged between about 1%-3% and <1%-9% of the total Sb, with maximum bioaccessibility observed in BS contaminated with C₈H₄K₂O₁₂Sb₂. The residual and labile fractions accounted for 77-93% and 0.1-4% of the total Sb, respectively, indicating that Sb is mostly associated with recalcitrant fractions of the soils. The results of single and sequential extraction studies revealed that source of Sb, ageing time and soil properties can greatly affect the bioavailability of Sb in soils. The findings of this research provide a deeper understanding of the potential risks associated with Sb compounds and highlights the role of site-specific considerations for improving the robustness of toxicity guidelines and long-term management of Sb contaminated sites.

Influences of soil properties and long-time aging on phytotoxicity of antimony to barley root elongation

Antimony (Sb) is a toxic element of global concern. To date, the most previous researches about phytotoxicity of Sb failed to fully consider the effects of soil properties and long-time aging. To address this, the toxicity of exogenous Sb(III) and Sb(V) were studied using the standardized barley root elongation bioassay. The results indicated that in ten soils aged only for 1 d, the EC₁₀ (concentrations causing 10% inhibition) values were 221-3164 mg kg^-1 and 135-4260 mg kg^-1 in Sb(III)- and Sb(V)-treated soils, respectively. The EC₅₀ values (concentrations causing 50% inhibition) were more than the setting highest concentration of 6400 mg kg^-1 in half of ten soils. The regression analysis showed that the amorphous Fe oxide and pH were the most foremost single soil factor explaining above-mentioned variance in EC₁₀, respectively, which suggested that the dominant soil factors were related to Sb forms. The inclusion of amorphous Mn oxide in above these two simple regression model could best explain the toxicity variance. After aged for 116 and 365 d, the phytotoxicity of Sb in Sb-treated soils significantly decreased and the phytotoxicity were even not found in the majority of Sb(V)-treated soils. The extent of aging varied with soils, and correlation analysis indicated that the aging effects negatively correlated with soil pH and positively correlated with clay and amorphous Al oxide in the Sb(III)-treated test soils.

Effect of pH on the adsorption of arsenic(V) and antimony(V) by the black soil in three systems: Performance and mechanism

Arsenic (As) and antimony (Sb) are listed as the priority pollutants by the U.S. Environmental Protection Agency (EPA) and the European Union (EU) due to their toxicity and potential carcinogenicity. It is necessary to investigate their adsorption over soil as such a behavior affects their mobility and bioavailability. In this study, the effect of pH on the adsorption of As(V) and Sb(V) by the black soil was investigated with three systems: the Single system, Binary system, and Sequence system. The operating pH was set at 4.0, 7.0 and 10.0. Based on the Langmuir isothermal and the pseudo-second-order kinetic models, the adsorption for As(V) was always better than Sb(V) in the whole pH range; the best adsorption performance for the two sorbates was achieved at pH of 4.0, followed by 7.0 and 10.0 in the three systems. The reasons could be that the atomic radius of arsenic is smaller than that of antimony, and the positively charged functional groups carried by the inorganic colloids in the soil contributed to binding with the negatively charged As(V)/Sb(V). A lower pH promoted the inorganic colloids to carry more positive charges. Compared to Single system, the maximum adsorption capacity (q_m) and the initial adsorption rates (k₂q_e,cal²) of As(V) and Sb(V) in Binary system decreased obviously, suggesting competitive adsorption occurred when As(V) and Sb(V) coexisted. The findings of this workimprove the understanding of As(V)/Sb(V) adsorption behavior in soil under different situations and would facilitate a comprehensive evaluation on the risk assessment of arsenic and antimony.

Simultaneous oxidation and sorption of highly toxic Sb(III) using a dual-functional electroactive filter

One of the topics gaining lots of recent attention is the antimony (Sb) pollution. We have designed a dual-functional electroactive filter consisting of one-dimensional (1-D) titanate nanowires and carbon nanotubes for simultaneous oxidation and sorption of Sb(III). Applying an external limited DC voltage assist the in-situ conversion of highly toxic Sb(III) to less toxic Sb(V). The Sb(III) removal kinetics and efficiency were enhanced with flow rate and applied voltage (e.g., the Sb(III) removal efficiency increased from 87.5% at 0 V to 96.2% at 2 V). This enhancement in kinetics and efficiency are originated from the flow-through design, more exposed sorption sites, electrochemical reactivity, and limited pore size on the filter. The titanate-CNT hybrid filters perform effectively across a wide pH range of 3-11. Only negligible inhibition was observed in the presence of nitrate, chloride, and carbonate at varying concentrations. Our analyses using STEM, XPS, or AFS demonstrate that Sb were mainly adsorbed by Ti. DFT calculations suggest that the Sb(III) oxidation kinetics can be accelerated by the applied electric field. Exhausted titanate-CNT filters can be effectively regenerated by using NaOH solution. Moreover, the Sb(III)-spiked tap water generated ∼2400 bed volumes with a >90% removal efficiency. This study provides new insights for rational design of continuous-flow filters for the decontamination of Sb and other similar heavy metal ions.

Antimony accumulation and iron plaque formation at different growth stages of rice (Oryza sativa L.)

To better understand the Sb phytoavailability in rice, we studied Sb accumulation in rice (Zhongjiazao-17, widely cultivated in Hunan province) at different growth stages based on adding Sb^III and Sb^V to waterlogged soils in 10, 50 and 100 mg kg^-1 treatment levels. Proportional exogenous Sb^III and Sb^V remained in the soil solution after equilibration. In Sb^III treatments, the iron plaque (IP) amounts and Sb in rice roots sharply increased from tillering to jointing stages and then reduced at the following stages. However, in Sb^V treatments, they increased continuously from tillering to maturing stages. The accumulation trends of Sb in straws, ears and grains were consistent in Sb^III and Sb^V treatments, rising from tillering to jointing stages followed with reducing from jointing to flowering stages slightly, and rising again significantly from flowering to maturing stages. The Tfsoil-grain values in all the Sb treatments were low (0.77 × 10^-3-5.1 × 10^-3), However, when Sb in waterlogged soils were higher than 50 mg kg^-1, it could pose human health risk for residents.

Antimonite Complexation with Thiol and Carboxyl/Phenol Groups of Peat Organic Matter

Peatlands and other wetlands with abundant natural organic matter (NOM) are important sinks for antimony (Sb). While formation of Sb(III) sulfide phases or Sb(III) binding to NOM are discussed to decrease Sb mobility, the exact binding mechanisms remain elusive. Here, we reacted increasing sulfide concentrations with purified model peat at pH 6, forming reduced organic sulfur species, and subsequently equilibrated the reaction products with 50 μM of antimonite under anoxic conditions. Sulfur solid-phase speciation and the local binding environment of Sb were analyzed using X-ray absorption spectroscopy. We found that 85% of antimonite was sorbed by untreated peat. Sulfide-reacted peat increased sorption to 98%. Shell-by-shell fitting of Sb K-edge X-ray absorption fine structure spectra revealed Sb in untreated peat bound to carboxyl or phenol groups with average Sb-carbon distances of ∼2.90 Å. With increasing content of reduced organic sulfur, Sb was progressively coordinated to S atoms at distances of ∼2.45 Å and Sb-carbon distances of ∼3.33 Å, suggesting increasing Sb-thiol binding. Iterative target factor analysis allowed exclusion of reduced inorganic Sb-sulfur phases with similar Sb-sulfur distances. In conclusion, even when free sulfide concentrations are too low for formation of Sb-sulfur precipitates, peat NOM can sequester Sb in anoxic, sulfur-enriched environments.