Cadmium Isotope Fractionation during Adsorption onto Edge Sites and Vacancies in Phyllomanganate.

Cadmium (Cd) geochemical behavior is strongly influenced by its adsorption onto natural phyllomanganates, which contain both layer edge sites and vacancies; however, Cd isotope fractionation mechanisms at these sites have not yet been addressed. In the present work, Cd isotope fractionation during adsorption onto hexagonal (containing both types of sites) and triclinic birnessite (almost only edge sites) was investigated using a combination of batch adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, surface complexation modeling, and density functional theory (DFT) calculations. Light Cd isotopes are preferentially enriched on solid surfaces, and the isotope fractionation induced by Cd2+ adsorption on edge sites (Δ114/110Cdedge-solution = -1.54 ± 0.11‰) is smaller than that on vacancies (Δ114/110Cdvacancy-solution = -0.71 ± 0.21‰), independent of surface coverage or pH. Both Cd K-edge EXAFS and DFT results indicate the formation of double corner-sharing complexes on layer edge sites and mainly triple cornering-sharing complexes on vacancies. The distortion of both complexes results in the negative isotope fractionation onto the solids, and the slightly longer first Cd-O distances and a smaller number of nearest Mn atoms around Cd at edge sites probably account for the larger fractionation magnitude compared to that of vacancies. These results provide deep insights into Cd isotope fractionation mechanisms during interactions with phyllomanganates.

Heavy metal-mediated adsorption of antibiotic tetracycline and ciprofloxacin on two microplastics: Insights into the role of complexation.

Microplastics (MPs) have recently become an emerging environmental concern. Nevertheless, limited information is known about the adsorption of MPs for organic contaminants under combined heavy metals pollution, with an emphasis on the role of complexation. Thus, this study aims to comprehensively compare and investigate the adsorption performance of antibiotic tetracycline (TC) and ciprofloxacin (CIP) on two polar MPs (polyamide (PA) and polyvinyl chloride (PVC)) affected by Cu(II) and Cd(II) with contrasting complexation abilities. Batch adsorption experiments were used in combination with speciation calculation, zeta potential determination, FTIR spectroscopy characterization and investigation of the affinity of MPs for heavy metals. Results showed that the sorption kinetics and isotherms of TC and CIP on PA and PVC could be well fitted to pseudo-second-order and Langmuir models, respectively, both in the absence and presence of Cu and Cd, suggesting that multiple interactions and monolayer adsorption played an important role in the adsorption process. The presence of Cu substantially improved TC and CIP adsorption and obviously changed the pH dependence of their adsorption onto both MPs, which may result from the Cu-induced strong complexation with TC and CIP. The presence of Cd slightly enhanced TC adsorption on both MPs while reduced CIP adsorption especially on PVC, which may be ascribed to the Cd-induced cationic bridging effects in TC adsorption and the competitive adsorption of Cd in CIP adsorption. Therefore, the heavy metal-mediated complexation effects may play a dominant role in antibiotic adsorption by MPs only in the presence of heavy metals with strong complexation ability while the adsorption performance in the presence of heavy metals with negligible complexation capacity may be influenced by effects other than complexation. This study helps further understand the heavy metal-mediated adsorption behavior of organic contaminants on polar MPs and the role of complexation reactions therein.

Prediction of Cd Accumulation in Wheat (Triticum aestivum L.) and Simulation Calculation of Lime or Zn Fertilizer Remediated Soil.

Soil Cd contamination to wheat raise wide concerns over food safety. It is essential to find the key factors affecting Cd accumulation in wheat and to establish a predictive model. The effects of pH, Zn, Ca, and DOM on the accumulation of Cd in wheat were investigated using hydroponic experiments. The results showed that Zn was the most important factor inhibiting Cd uptake in wheat. Models were developed to predict the Cd contents in wheat tissues based on the ion concentration. Meanwhile, the available Cd contents in soil were predicted using a geochemical multi-surface model (MSM) which is suitable for various soils and conditions. The combination of the hydroponic accumulation model and MSM exhibits good predictions of wheat-Cd (R2 = 0.822-0.862, RMSE = 0.317-0.533). The results of this study can quantitatively predict the accumulation of Cd in wheat and provide a reference for soil remediation and safe wheat production.

Water Quality Analysis of the Yangtze and the Rhine River: A Comparative Study Based on Monitoring Data from 2007 to 2018.

Monitoring water environmental quality of large rivers is the basis of river evaluation and local environmental protection worldwide. This study compared the water quality of the Rhine and the Yangtze River, two of the world's most important arteries, based on monitoring data collected from their monitor stations from 2007 to 2018. Dissolved oxygen (DO), pH, chemical oxygen demand (COD)/dissolved organic carbon (DOC) and ammonia-nitrogen (NH3-N) were used to evaluate their water quality. The changing trend, the temporal and special patterns were also analyzed. It was found that the overall water quality of the Rhine in the last decade (p = 0.95) was better than the Yangtze (p = 1.01). Notably, CODMn and NH3-N were identified as main pollution factors of the Rhine and the Yangtze, respectively. This study provided information for water quality modelling, therefore might be helpful for the water quality management of China.

Environmental Governance of Western Europe and Its Enlightenment to China: In Context to Rhine Basin and the Yangtze River Basin.

The pollution control of the Rhine is a successful management mode for comprehensive cooperation in environmental governance, which could provide an important reference for the Yangtze River governance. This study explored river basin management and various pollution problems faced by these two rivers at different times, and also includes governance and management effects. The changes in water quality and aquatic life before and after policy implementation had also been compared. The results showed that the International Commission for the Protection of the Rhine (ICPR) made outstanding contributions to the water-quality improvement, demonstrating its management mode was efficient and effective. Cooperative communication and targeted policies are effective ways to improve the water quality of large river basins. Clear division of labor system and phased cooperation-governance objectives are also significant for the inter-provincial cooperative governance of Yangtze River pollution.

Technology and Concept of Wastewater Treatment: Differences Between the Rhine Basin and the Yangtze Basin.

Based on field visits and literature research, the situations of several typical wastewater treatment plants (WWTPs) in the Rhine basin and the Yangtze basin were investigated, to compare the technology and concept of wastewater treatment in these two areas. Our results showed that WWTPs in the Rhine performed well in pollutant removal, and have shifted their focus to energy production and nutrient recovery; While in the Yangtze basin, most WWTPs still operate on the sole concept of pollution treatment. Though China's WWTPs attach importance to water reclamation, the related technologies are still under development. In years to come, the construction of New Concept WWTPs is expected by Chinese famous experts, to integrate sustainable wastewater treatment and energy/nutrient recovery. To better plan its future avenue in wastewater treatment, China is suggested to learn from the successful practice of energy production and nutrient recovery of WWTPs in the Rhine.

Comparison of Four Amendments for Arsenic and Cadmium Combined Contaminated Soil.

Arsenic (As) and cadmium (Cd) are common soil pollutants whose opposing geochemical behaviors must be taken into account in the development of cost-effective, environmentally friendly remediation strategies. In this study, a pot experiment with lettuce and a field experiment with wheat were performed to examine the impacts of zeolite, biochar, MnO2, zero-valent iron (ZVI) individually and in binary combinations thereof on As-Cd pollution. The results of the pot experiment showed that biochar, MnO2 and ZVI had good passivation effects on As and Cd when provided individually, but the effects of a combination of 0.2% ZVI/0.5% biochar or 0.2% MnO2/0.5% ZVI were even better. These amendments were further investigated in a field experiment, which confirmed the positive effect of 0.2% MnO2/0.5% ZVI. Therefore, ZVI/biochar and MnO2/ZVI mixtures may offer effective solutions to the remediation of farmland soil contaminated with both As and Cd.

Rapid Hydrolysis of Penicillin Antibiotics Mediated by Adsorbed Zinc on Goethite Surfaces.

The soil environment is an important sink for penicillin antibiotics released from animal manure and wastewater, but the mineral-catalyzed transformation of penicillins in soil has not been well studied. To simulate this environmental process, we systematically investigated the behavior of penicillin G and amoxicillin, the two most widely-used penicillin antibiotics, in the presence of goethite and metal ions. The results demonstrated that Zn ions significantly promoted the hydrolysis of penicillins in goethite suspensions, as evidenced by the degradation rate nearly 3 orders of magnitude higher than that of the non-Zn-containing control. The spectroscopic analysis indicated that the specific complexation between penicillins, adsorbed Zn, and goethite was responsible for the enhanced degradation. Metastable interactions, involving hydrogen bonds between carbonyl groups in the ß-lactam ring and the double/triple hydroxyl groups on goethite surface, and coordination bonding between carboxyl groups and surface irons were proposed to stabilize the ternary reaction intermediates. Moreover, the surface zinc-hydroxide might act as powerful nucleophile to rapidly rupture the ß-lactam ring in penicillins. This study is among the first to identify the synergic roles of Zn ion and goethite in facilitating penicillin degradation and provides insights into ß-lactam antibiotics to assess their environmental risk in soil.

Metabolomics Reveals the "Invisible" Responses of Spinach Plants Exposed to CeO2 Nanoparticles.

Engineered nanoparticles (NPs) with activities that mimic antioxidant enzymes have good prospects in agriculture because they can increase photosynthesis and improve stress tolerance. Here, the interaction between cerium oxide NPs with spinach plants ( Spinacia oleracea) was investigated by integrating phenotypic and metabolomic analyses. Soil-grown, four-week-old spinach plants were foliar exposed for 3 weeks to CeO2 NPs at 0, 0.3, and 3 mg per plant. Phenotypic parameters (chlorophyll fluorescence, photosynthetic pigment contents, plant biomass, lipid peroxidation, and membrane permeability) were not affected. However, metabolomics analysis revealed that both doses of CeO2 NPs induced metabolic reprogramming in leaves and roots in a non-dose-dependent manner. The low dose of CeO2 NPs (0.3 mg per plant) induced stronger metabolic reprogramming in spinach leaves than high dose of CeO2 NPs. However, the high dose of CeO2 NPs triggered more metabolic changes in roots, compared to the low dose. Foliar spray of CeO2 NPs at 3 mg/plant induced marked down-regulation of a number of amino acids (threonine, tryptophan, l-cysteine, methionine, cycloleucine, aspartic acid, asparagine, tyrosine, and glutamic acid). In addition, Zn decreased by 44% and 54% in leaves and Ca decreased by 38% and 32% in roots under exposure to CeO2 NPs at 0.3 and 3 mg/plant, respectively. These results provide better understanding of the intrinsic phenotypic and metabolic changes imposed by CeO2 NPs in spinach plants.

Threshold Concentrations of Silver Ions Exist for the Sunlight-Induced Formation of Silver Nanoparticles in the Presence of Natural Organic Matter.

Sunlight-induced photoformation of silver nanoparticles (nAg), mediated by natural organic matter (NOM), is significantly affected by the concentration of Ag(I) and chloride. The initial photoformation rates of nAg in Suwannee River humic acid (SRHA) and Suwannee River natural organic matter (SRNOM) solutions were examined under simulated sunlight irradiation. A critical induction concentration (CIC) of Ag(I) (10 mg/L for SRHA and 5 mg/L for SRNOM, respectively) was observed, below which the nAg formation was minimal. The threshold is attributed to the interplay of reduction and oxidation reactions mediated by NOM, reflecting the need to achieve sufficiently fast growth of silver clusters to outcompete oxidative dissolution. The CIC can be reduced by scavenging oxidative radicals or be increased by promoting singlet oxygen and hydrogen peroxide generation. The presence of chloride effectively reduced the CIC by forming AgCl, which facilitates reduction reactions and provides deposition surfaces. SRNOM is more efficient in mediating photoformation of nAg than SRHA, owing to their differed phototransient generation. These results highlight prerequisites for the photoformation of nAg mediated by NOM, in which the photochemistry and solution chemistry are both important.

Development and validation of abiotic ligand model for nickel toxicity to wheat (Triticum aestivum).

A terrestrial biotic ligand model (t-BLM) was developed to predict nickel toxicity to wheat (Triticum aestivum) root elongation in hydroponic solutions. The competitive effects of five major cations (Ca2+, Mg2+, Na+, K+ and H+) on Ni toxicity were investigated and Mg2+was found to be a strong competitor, while H+ showed less competing effect. Besides free Ni2+, the toxicity induced by the species NiHCO3+ was non-neglect able at pH>7 because NiHCO3+ occupied a significant fraction of total Ni under such condition. Thus, a t-BLM including Ni2+, NiHCO3+, Mg2+, and H+ could successfully predict the nickel toxicity to wheat root elongation and it performed better prediction than the conventional free ion activity model. In addition, the model was examined with two sets of independent experiments, which contained multiple cations and low-molecular-weight organic acids to mimic the rhizosphere condition. The developed t-BLM well predicted nickel toxicity in both experiments since it can account in both complexation and competition effects, suggesting its potential to be used in a complicated matrix like soil solution. This study provides direct evidence that the t-BLM is a reliable method for the risk assessment of nickel in terrestrial system.

Stimulation of Tetrabromobisphenol A Binding to Soil Humic Substances by Birnessite and the Chemical Structure of the Bound Residues.

Studies have shown the main fate of the flame retardant tetrabromobisphenol A (TBBPA) in soils is the formation of bound residues, and mechanisms on it are less-understood. This study investigated the effect of birnessite (δ-MnO2), a naturally occurring oxidant in soils, on the formation of bound residues. (14)C-labeled TBBPA was used to investigate the pH dependency of TBBPA bound-residue formation to two soil humic acids (HAs), Elliott soil HA and Steinkreuz soil HA, in the presence of δ-MnO2. The binding of TBBPA and its transformation products to both HAs was markedly increased (3- to 17-fold) at all pH values in the presence of δ-MnO2. More bound residues were formed with the more aromatic Elliott soil HA than with Steinkreuz soil HA. Gel-permeation chromatography revealed a uniform distribution of the bound residues within Steinkreuz soil HA and a nonuniform distribution within Elliott soil HA. (13)C NMR spectroscopy of (13)C-TBBPA residues bound to (13)C-depleted HA suggested that in the presence of δ-MnO2, binding occurred via ester and ether and other types of covalent bonds besides HA sequestration. The insights gained in this study contribute to an understanding of the formation of TBBPA bound residues facilitated by δ-MnO2.

Joint toxicity of tetracycline with copper(II) and cadmium(II) to Vibrio fischeri: effect of complexation reaction.

Co-contamination of antibiotic and heavy metals commonly occurs in the environment. Tetracycline (TC), a common antibiotic, can behave as an efficient organic ligand to complex with cations. In this paper, the joint toxicity of TC with two commonly existing metals, copper(II) and cadmium(II), towards a luminescent bacteria, Vibrio fischeri, are investigated. Results showed that coexistence of TC and Cu(II) showed a significant antagonistic effect, while TC and Cd(II) showed a synergistic effect. The aqueous speciation of TC with two metal cations was calculated using a chemical equilibrium software Visual MINTEQ and results indicated that a strong complexation exist between TC and Cu(II), while much weaker interaction between TC and Cd(II). Traditional joint toxicity prediction model based on independent action failed to predict the combined toxicity of TC with metals. A new method based on speciation calculation was used to evaluate the joint toxicity of ligands and cations. It is assumed that the metal-ligand complexes are non-toxic to V. fischeri and the joint toxicity is determined by the sum of toxic unit of free metal-ions and free organic ligands. It explained the joint toxicity of the mixed systems reasonably well. Meanwhile, citric acid (CA) and fulvic acid (FA) were also introduced in this study to provide a benchmark comparison with TC. Results showed it is also valid for mixed systems of CA and FA with metals except for the Cd-CA mixture.

Comparison of the arsenic protective effects of four nanomaterials on pakchoi in an alkaline soil.

Accurately applying engineered nanoparticles (NPs) in farmland stress management is important for sustainable agriculture and food safety. We investigated the protective effects of four engineered NPs (SiO2, CeO2, ZnO, and S) on pakchoi under arsenic (As) stress using pot experiments. The results showed that CeO2, SiO2, and S NPs resulted in biomass reduction, while ZnO NPs (100 and 500 mg kg-1) significantly increased shoot height. Although 500 mg kg-1 S NPs rapidly dissolved to release SO42-, reducing soil pH and pore water As content and further reducing shoot As content by 21.6 %, the growth phenotype was inferior to that obtained with 100 mg kg-1 ZnO NPs, probably due to acid damage. The addition of 100 mg kg-1 ZnO NPs not only significantly reduced the total As content in pakchoi by 23.9 % compared to the As-alone treatment but also enhanced plant antioxidative activity by increasing superoxide dismutase (SOD) and peroxidase (POD) activities and decreasing malondialdehyde (MDA) content. ZnO NPs in soil might inhibit As uptake by roots by increasing the dissolved organic carbon (DOC) by 19.12 %. According to the DLVO theory, ZnO NPs were the most effective in preventing As in pore water from entering plant roots due to their smaller hydrated particle size. Redundancy analysis (RDA) further confirmed that DOC and SO42- were the primary factors controlling plant As uptake under the ZnO NP and S NP treatments, respectively. These findings provide an important basis for the safer and more sustainable application of NP-conjugated agrochemicals.

How Fe-bearing materials affect soil arsenic bioavailability to rice: A meta-analysis.

Arsenic (As) contamination is widespread in soil and poses a threat to agricultural products and human health due to its high susceptibility to absorption by rice. Fe-bearing materials (Fe-Mat) display significant potential for reducing As bioavailability in soil and bioaccumulation in rice. However, the remediation effect of various Fe-Mat is often inconsistent, and the response to diverse environmental factors is ambiguous. Here, we conducted a meta-analysis to quantitatively assess the effects of As in soils, rice roots, and grains based on 673, 321, and 305 individual observations from 67 peer-reviewed articles, respectively. On average, Fe-Mat reduced As bioavailability in soils, rice roots, and grains by 28.74 %, 33.48 %, and 44.61 %, respectively. According to the analysis of influencing factors, the remediation efficiency of Fe-Mat on As-contaminated soil was significantly enhanced with increasing Fe content in the material, in which the industry byproduct was the most effective in soils (-42.31 %) and rice roots (-44.57 %), while Fe-biochar was superior in rice grains (-54.62 %). The efficiency of Fe-Mat in minimizing soil As mobility was negatively correlated with soil Fe content, CEC, and pH. In addition, applying Fe-Mat in alkaline soils with higher silt, lower clay and available P was more effective in reducing As in rice grains. A higher efficiency of applying Fe-Mat under continuous flooding conditions (27.39 %) compared with alternate wetting and drying conditions (23.66 %) was also identified. Our results offer an important reference for the development of remediation strategies and methods for various As-contaminated paddy soils.

Alleviation of arsenic stress in pakchoi by foliar spraying of engineered nanomaterials.

Addressing heavy metal contamination in leafy vegetables is critically important due to its adverse effects on human health. In this study, we investigated the inhibitory effects of foliar spraying with four nanoparticles (CeO2, ZnO, SiO2, and S NPs) on arsenic (As) stress in pakchoi (Brassica rapa var. Chinensis). The findings reveal that foliar application of ZnO NPs at 1 ~ 2.5 mg plant-1 and CeO2 NPs at 5 mg plant-1 significantly reduces As in shoots by 40.9 ~ 47.3% and 39.4%, respectively. Moreover, 5 mg plant-1 CeO2 NPs increased plant height by 6.06% and chlorophyll a (Chla) content by 30.2% under As stress. Foliar spraying of CeO2 NPs at 0.2-5 mg plant-1 also significantly enhanced superoxide dismutase (SOD) activity in shoots by 9.4 ~ 13.9%, lowered H2O2 content by 42.4 ~ 53.25%, and increased root protein contents by 79 ~ 109.2%. CeO2 NPs regulate the As(III)/As(V) ratio, aiding in As efflux from roots and thereby reducing As toxicity to plants. In vitro digestion experiments reveal that the consumption of CeO2 NPs carries the lowest health risk of As. In addition, foliar spraying of ZnO NPs at 1 ~ 2.5 mg plant-1 can suppress plant As uptake by modulating enzyme activity, reducing leaf damage, and enhancing chlorophyll content. The study demonstrates that high CeO2 NP concentrations and suitable ZnO NP concentrations can alleviate As toxicity in pakchoi, consequently reducing human health risks.

Natural aging and adsorption/desorption behaviors of polyethylene mulch films: Roles of film types and exposure patterns.

Polyethylene (PE) mulch films are an important source of microplastics (MPs) in agricultural soils, which may further affect the bioavailability of coexisting pollutants. In this study, white (WM), black (BM), and silver-black (SM) PE mulch films were aged on the soil surface and under soil burial to simulate the two exposure patterns of abandoned mulch films in the field. Results indicated that the soil-surface exposure induced more pronounced aging characteristics, and WM seemed the most susceptible. Serious surface deterioration by aging led to a drastic decrease in the tensile properties of the films, suggesting the tendency to fragment. Oxygen-containing functional groups were generated on the film surfaces, with oxygen/carbon ratios increasing by up to 29 times, which contributed to the prominent increase in Pb adsorption on the film-derived MPs. Additionally, the film surface became more hydrophobic when exposed to the soil surface but more hydrophilic in the soil-burial exposure, which was in agreement with the change in triclosan adsorption, i.e., promotion and suppression, respectively. Aging generally decreased the desorption potential of the adsorbed pollutants in simulated gastrointestinal solutions due to increased interactions. By comparison, exposure patterns were revealed to be the critical factor for these changes, regardless of film types.

Effects of nZVI on the migration and availability of Cr(VI) in soils under simulated acid rain leaching conditions.

Hexavalent chromium, Cr(VI), is a ubiquitous toxic metal that can be reduced to Cr(III) by nano-zero-valent iron (nZVI). Finding out effects of continuous rainfall leaching on the Cr(VI) release and availability remains a problem, needing to be addressed. Whether the Cr(VI) reduction by nZVI and continuous rainfall leaching lead to localized heterogeneity in soil is unclear. Therefore, two in situ high-resolution (HR) techniques of the diffusive gradients in thin-films (DGT) and planar optode were combined with ex situ sampling experiments here. Results demonstrate that nZVI decreased Cr(VI) leaching by 5.60-8.50 % compared to control soils. DGT-measured concentrations of Cr(VI), CDGT-Cr(VI), ranged from 7.31 to 19.4 µg L-1 in the control soils, increasing with depth while CDGT-Cr(VI) in nZVI-treated soils (2.41-6.18 µg L-1) decreased or remained stable with depth. However, simulated acid-rain leaching increases CDGT-Cr(VI) by 1.61-fold in nZVI-treated soils, negatively affecting the remediation. DGT measurements in bulk soils using disc devices are better at capturing the change of Cr(VI) availability at different conditions, whereas 2D-HR DGT mappings did not characterize significant mobilization of Cr(VI) at the micro-scale. These findings emphasize the importance of monitoring Cr(VI) release and availability in remediated soil under acid-rain leaching conditions for effective environment management.

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.

Antimony and arsenic migration in a heterogeneous subsurface at an abandoned antimony smelter under rainfall.

While antimony (Sb) and arsenic (As) co-contamination in subsurface soil systems due to the legacy of Sb smelting wastes has been documented, the role of inherent heterogeneity on pollutant migration is largely overlooked. Herein this study investigated Sb and As migration in a slag impacted, vertically stratified subsurface at an abandoned Sb smelter. A 2-dimensional flume was assembled as a lab-scale analogue of the site and subject to rainfall and stop-rain events. Reactive transport modeling was then performed by matching the experimental observations to verify the key factors and processes controlling pollutant migration. Results showed that rainfall caused Sb and As release from the shallow slag layer and promoted their downward movement. Nevertheless, the less permeable deeper layers limited physical flow and transport, which led to Sb and As accumulation at the interface. The re-adsorption of Sb and As onto iron oxides in the deeper, more acidic layers further retarded their migration. Because of the large difference between Sb and As concentrations, Sb re-adsorption was much less effective, which led to higher mobility. Our findings overall highlight the necessity of understanding the degree and impacts of physicochemical heterogeneity for risk exposure assessment and remediation of abandoned Sb smelting sites.