Effect of aging on bioaccessibility of arsenic and lead in soils

Impact of long-term irrigation practices on distribution and speciation of arsenic in agricultural soil

To better understand the impact of long-term irrigation practices on arsenic (As) accumulation in agricultural soils, 100 soil samples from depths of 0-20 cm were collected from the Datong basin, where the As-contaminated groundwater has been used for irrigation for several decades. Soil samples were analyzed for major elements, trace elements, and As, Fe speciation. Results reveal As content ranging from 4.00 to 14.5 mg/kg, an average of 10.2 ± 2.05 mg/kg, consistent with surveys conducted in 1998 and 2007. Arsenic speciation ranked in descending order as follows: As associated with silicate minerals (AsSi, 29.70 ± 7.53 %) > amorphous Fe-minerals associated As (AsFeox1, 26.40 ± 3.27 %) > crystalline Fe-minerals associated As (AsFeox2, 24.02 ± 4.60 %) > strongly adsorbed As (AsSorb, 14.29 ± 2.81 %) > As combined with carbonates and Fe-carbonates (AsCar, 2.30 ± 0.44 %) > weakly adsorbed As (AsDiss, 2.59 ± 1.00 %). The anomalous negative correlation between As and Fe content reflects the primary influence of soil provenance. Evidence from major element compositions and rare earth element patterns indicates that total As and Fe contents in soils are controlled by parent materials, exhibiting distinct north-south differences (As: higher levels in the north, lower levels in the south; Fe: higher levels in the south, lower levels in the north). Evidence from the Chemical Index of Alteration (CIA) and As/Ti ratio suggests that chemical weathering has led to As enrichment in the central basin. Notably, relationships such as AsDiss/Ti, AsSorb/Ti with CIA and total Fe content indicate significant influences of irrigation practices on adsorbed As (both weakly and strongly adsorbed) contents, showing a pattern of higher levels in the central basin and lower levels in the Piedmont. However, total As content remained stable after long-term irrigation, potentially due to the re-release of accumulated As via geochemical pathways during non-irrigated periods. These findings demonstrate that the soil systems can naturally remediate exogenous As contamination induced by irrigation practices. Quantitative assessment of the balance between As enrichment and re-release in soil systems is crucial for preventing soil As contamination, highlighting strategies like water-saving techniques and fallow periods to manage As contamination in agricultural areas using As-contaminated groundwater for irrigation.

Oral bioaccessibility of PTEs in soils: A review of data, influencing factors and application in human health risk assessment

Understanding the behavior of metal(loi)ds transported from soil to humans is critical for human health risk assessment (HHRA). In the last two decades, extensive studies have been conducted to better assess human exposure to potentially toxic elements (PTEs) by estimating their oral bioaccessibility (BAc) and quantifying the influence of different factors. This study reviews the common in vitro methods used to determine the BAc of PTEs (in particular As, Cd, Cr, Ni, Pb, and Sb) under specific conditions (particularly in terms of the particle size fraction and validation status against an in vivo model). The results were compiled from soils derived from various sources and allowed the identification of the most important influencing factors of BAc (using single and multiple regression analyses), including physicochemical soil properties and the speciation of the PTEs in question. This review presents current knowledge on integrating relative bioavailability (RBA) in calculating doses from soil ingestion in the HHRA process. Depending on the jurisdiction, validated or non-validated bioaccessibility methods were used, and risks assessors applied different approaches: (i) using default assumptions (i.e., RBA of 1); (ii) considering that bioaccessibility value (BAc) accurately represents RBA (i.e., RBA equal to BAc); (iii) using regression models to convert BAc of As and Pb into RBA as proposed by the USA with the US EPA Method 1340; or (iv) applying an adjustment factor as proposed by the Netherlands and France to use BAc from UBM (Unified Barge Method) protocol. The findings from this review should help inform risk stakeholders about the uncertainties surrounding using bioaccessibility data and provide recommendations for better interpreting the results and using bioaccessibility in risk studies.

Geochemical fractionation, bioaccessibility and ecological risk of metallic elements in the weathering profiles of typical skarn-type copper tailings from Tongling, China

Nonferrous metal tailings have long posed a significant threat to the surrounding environment and population. Previous studies have primarily focused on heavy metal pollution in the vicinity of sulfide tailings, while little attention was given to metal mobility and bioavailability within skarn-type tailings profile during weathering. Therefore, this study aimed to investigate the fractionation, bioaccessibility, and ecological risk associated with metallic elements (MEs, including Pb, Cd, Cr, Zn, and Cu) in two representative weathering copper-tailings profiles of Tongling mine (China). This was achieved through the use of mineralogical analyses, BCR extractions (F1: exchangeable, F2: reducible, F3: oxidizable, F4: residual fraction), in-vitro gastrointestinal simulation test (PBET) and risk assessment models. The mineral compositions of two weathering profiles were similar, with quartz and calcite being the dominant minerals, along with minor amounts of siderite, hematite and spangolite. The mean concentration in the tailings profile was approximately 0.31 (Cr), 1.8 (Pb), 12 (Zn), 33 (Cd) or 34 (Cu) times of the local background values (LBVs). The mean content of the bottom weakly-weathering layer in profile was about 0.36 (Cr), 0.91 (Pb), 1.91 (Cd), 2.73 (Zn) or 2.68 (Cu) times of the surface oxide layer, indicating a strong weathering-leaching effect. The average proportion of BCR-F1 fraction for Cd (30.94 %) was the highest among the five MEs, possibly due to its association with calcite. The PBET-extracted fractions for Cd, Zn and Cu were significantly positively correlated with the F1, F2 and F3 fractions of BCR, suggesting that these elements have higher bioavailability/bioaccessibility. The assessment results indicated that Cd posed a higher health risk, while the risk of Cu, Zn, and Pb is relatively low and Cr is safe. In conclusion, this study provides valuable insights into the environmental geochemical behavior and potential risks of MEs in skarn-type non-ferrous metal tailings ponds.

Metal(loid) speciation in dermal bioaccessibility extracts from contaminated soils and permeation through synthetic skin

Dermal exposure to metal(loid)s from contaminated soils can contribute to health risk. Metal(loid) speciation will influence their bioaccessibility in sweat and subsequent permeation across the skin. Therefore, the speciation of the bioaccessible fraction of metal(loid)s in two synthetic sweat formulations (sweat A (pH 6.5) and B (pH 4.7)) was assessed using chemical equilibrium modelling (Visual MINTEQ). Permeation through synthetic skin and the influence of sebum in the permeation of As, Cr, Cu, Ni, Pb, and Zn were also investigated using Franz cells. Following dermal bioaccessibility tests for five Chromated Copper Arsenate (CCA)-contaminated soils and one certified soil (SQC001), mean metal(loid) bioaccessibility (%) was higher in sweat B (2.33-18.8) compared to sweat A (0.12-7.53). Arsenic was almost entirely found as As(V) in both sweats. In sweat A, comparable concentrations of Cr(III) and Cr(VI) were found whereas in sweat B, Cr was primarily present as Cr(III). Copper was primarily found as Cu2+. Bioaccessible Cr extracted from nearly all soils permeated through the Strat-M membrane when it was coated with sebum. The Cr permeation coefficient (Kp) ranged between 0.004 and 0.13 cm/h and the Kp for Cu was higher (0.024-0.52 cm/h). As, Ni, Pb, and Zn did not permeate the synthetic skin.

Influence of sebum proportion in synthetic sweat on dermal bioaccessibility and on permeation of metal(loid)s from contaminated soils

Even if dermal exposure to metal(loid)s from contaminated soils has received less attention than oral and inhalation exposure, the human health risk can be significant for some contaminants and exposure scenarios. The purpose of this study was to assess the influence of sebum proportion (1% v/v and 3% v/v) in two synthetic sweat formulations (EN 1811, pH 6.5 (sweat A) and NIHS 96-10, pH 4.7 (sweat B)) on As, Cr, Cu, Ni, Pb, and Zn dermal bioaccessibility and on subsequent diffusion through synthetic skin. A Franz cell with a Strat-M® membrane was used to quantify permeation parameters of bioaccessible metal(loid)s. Sebum's presence in synthetic sweat formulations significantly modified bioaccessibility percentages for As, Cr, and Cu. However, sebum proportion in both sweats did not influence the bioaccessibility of Pb and Zn. Some metal(loid)s, namely As and Cu, permeated the synthetic skin membrane during permeation tests when sebum was added to sweat while no permeation was observed without sebum in sweat formulations. Depending on sweat formulation, the addition of sebum (1% v/v) increased or decreased the Cr permeation coefficients (Kp). In all cases, bioaccessible Cr was no longer permeable when extracted with 3% sebum. Ni transdermal permeation was not influenced by the presence of sebum, and no permeation was observed for Pb and Zn. Further studies on the speciation of metal(loid)s in bioaccessible extracts in the presence of sebum are recommended.

A global meta-analysis of the correlation between soil physicochemical properties and lead bioaccessibility

Soil physiochemical properties play a vital role in bioaccessibility-based health risk assessment as it can determine the bioaccessibility and the true risk of potentially toxic elements in soil. However, the effects of soil properties on bioaccessibility still remains unclear. In this paper, 17 of the 1454 literatures with 474 samples were identified, screened and reviewed for exploring the correlation between soil physicochemical properties and lead bioaccessibility (BAc_Pb) through a meta-analysis approach. Five soil physicochemical parameters including pH, SOM, Clay, CEC and T-Pb were systematically analyzed using Principal component analysis, Pearson correlation analysis and survival analysis. The results showed that pH of simulated gastric juice is a major source of heterogeneity of the correlation between soil pH and BAc_Pb. In the gastric phase, the effect of alkaline soil on high BAc_Pb (BAc >50%) is more sensitive, and the effect of acidic soil on low BAc_Pb (BAc <50%) is more sensitive. However, in the small intestinal phase, soil pH displays little impacts on BAc_Pb in acidic, alkaline and neutral soils. Although three principal components explained 66.2% and 64.9% of the total variance of the urban, agricultural, and mining soils in gastric and small intestinal phases, respectively, there was no strong evidence that soil type can influence the BAc_Pb. The results of present study provide insights into the correlation between soil properties and BAc_Pb, and prediction of the bioaccessibility and bioavailability of Pb in different types of soil.

Changes in molybdenum bioaccessibility in four spiked soils with respect to soil pH and organic matter

Investigation of the inherent relationship between soil physicochemical properties and pollutant's bioaccessibility (BAc) by analyzing different soil types may produce erroneous results or bias, owing to the complexity of natural soil characteristics. However, use of single factor analysis (e.g., soil pH, organic matter) facilitates evaluation of the transition. In this study, the inherent relationship between soil properties and the BAc of molybdenum (Mo) was evaluated in two typical variable-charge soils (Ferralosol and Ferrosol) and constant-charge soils (Alfisol and Inceptisol) spiked with Mo after adjusting their pH and organic carbon content. The Unified Bioaccessibility Research Group of Europe (BARGE) Method (UBM) was applied to evaluate the BAc of Mo in the gastric and intestinal phase (GP and IP, respectively). Isothermal adsorption experiment, Tessier sequential extraction, and field emission scanning electron microscope-energy dispersive spectroscopy (FESEM-EDS) analysis were conducted on these spiked soils. The results indicated that the BAc of Mo in IP (27.42-80.41%) was significantly higher than that in GP (2.52-28.53%). A significantly lower level of BAc of Mo was found in the variable-charge soils, when compared with that in the constant-charge soils. Furthermore, significant negative correlations were identified between the BAc and adsorption of Mo, which decreased with soil pH. These negative correlations can be attributed to the increase in soil negative charge density and enhancement of Mo desorption by hydroxyl, which reinforce the repulsion between Mo and soil particles with increasing soil pH; this was further confirmed by the decrease in Mo adsorption with Alfisol pH. The Mo fractions and FESEM-EDS patterns confirmed that the BAc of Mo in GP was negatively correlated with soil organic carbon (SOC) content, possibly owing to an increase in Mo retention by SOC. These findings indicated that the health risk of Mo contamination in low pH and SOC-rich variable-charge soil is relatively low, thus providing references for rationalizing risk assessment and remediating Mo-polluted soil.

Decreased in vitro bioaccessibility of Cd and Pb in an acidic Ultisol through incorporation of crop straw-derived biochar

Studies analyzing the in vitro bioaccessibility (BAc) of heavy metals in biochar-amended soils are currently lacking. The present study aimed to assess the metal BAc in Cd- and Pb-spiked acidic Ultisol samples treated individually with 2% (w/w) maize, rice, wheat, soybean, and pea straw-derived biochar. The results indicate that the Cd-BAc simulated in gastric phase (GP) decreased from 78.4% to 66.5-72.3% and the Pb-BAC decreased from 74.3% to 67.2-69.2%; however, the Cd-BAc in the intestinal phase (IP) decreased from 35.6% to 27.9-33.5% and the Pb-BAc decreased from 34.7% to 29.7-32.9% after 120 d of incubation with biochar application compared to the un-amended Ultisol. The Cd- and Pb-BAc in both GP and IP were significantly negatively correlated with soil pH, CEC, and organic carbon (P < 0.05), which increased after biochar application. The soybean straw-derived biochar amendment has the greatest potential to decrease the BAc of Cd and Pb in the GP and IP, owing to the highest level of CEC, SOC, TC and TN among all soil samples. Moreover, the BAc was positively correlated with the exchangeable, and exchangeable + carbonate-bound Cd and Pb fractions (P < 0.05), indicating these fractions had a dominant influence on the BAc of cationic heavy metals. Therefore, crop straw-derived biochar amendment can decrease the BAc of Cd and Pb in acidic Ultisol, and thus mitigate the health risks posed by these metals from incidental ingestion.

Microwave-enhanced simultaneous immobilization of lead and arsenic in a field soil using ferrous sulfate

Remediation of soil contaminated by mixed heavy metals and metalloids has been a major challenge in the global environmental field. To address this critical issue, we tested a new technology for simultaneous immobilization of lead (Pb) and arsenic (As) in a field contaminated soil using a microwave-assisted FeSO₄·7H₂O treatment process. The process was able to rapidly reduce the TCLP-based leachability of Pb from 12.74 to 0.1 mg L^-1 and As from 2.704 to 0.002 mg L^-1 (MW power = 800 W, Irradiation time = 20 min, and FeSO₄·7H₂O = 4 wt%). The effects of FeSO₄·7H₂O dosage, microwave power, and irradiation time were determined and optimized. After 365 days of curing under atmospheric conditions, the TCLP-leached concentration of Pb and As in the treated soil remained below the regulatory limits of 0.1 and 0.002 mg L^-1, respectively. The microwave irradiation promoted the formation of insoluble PbSO₄(s) and Fe₃(AsO₄)₂·8H₂O(s), resulting in the long-term stability of Pb and As in the soil. The technology offers an effective alternative for remediation of Pb- and/or As-contaminated soil and groundwater.

Nickel bioaccessibility in soils with high geochemical background and anthropogenic contamination

Abnormally high concentrations of metals including nickel (Ni) in soils result from high geochemical background (HB) or anthropogenic contamination (AC). Metal bioaccessibility in AC-soils has been extensively explored, but studies in HB-soils are limited. This study examined the Ni bioaccessibility in basalt and black shale derived HB-soils, with AC-soils and soils without contamination (CT) being used for comparison. Although HB- and AC-soils had similar Ni levels (123 ± 43.0 vs 155 ± 84.7 mg kg^-1), their Ni bioaccessibility based on the gastric phase of the Solubility Bioaccessibility Research Consortium (SBRC) in vitro assay was different. Nickel bioaccessibility in HB-soils was 6.42 ± 3.78%, 2-times lower than the CT-soils (12.0 ± 9.71%) and 6-times lower than that in AC-soils (42.6 ± 16.3%). Based on the sequential extraction, a much higher residual Ni fractionation in HB-soils than that in CT- and AC-soils was observed (81.9 ± 9.52% vs 68.6 ± 9.46% and 38.7 ± 16.0%). Further, correlation analysis indicate that the available Ni (exchangeable + carbonate-bound + Fe/Mn hydroxide-bound) was highly correlated with Ni bioaccessibility, which was also related to the organic carbon content in soils. The difference in co-localization between Ni and other elements (Fe, Mn and Ca) from high-resolution NanoSIMS analysis provided additional explanation for Ni bioaccessibility. In short, based on the large difference in Ni bioaccessibility in geochemical background and anthropogenic contaminated soils, it is important to base contamination sources for proper risk assessment of Ni-contaminated soils.

Double-edged effects of elevating temperature on the aging of exogenous arsenic in flooded paddy soils

Temperature variation can have a significant impact on arsenic (As) bioavailability in paddy soils. However, details regarding the transformation of exogenous As during the aging process in paddy soils at various temperatures remain unclear. This work investigated the effects of temperature on the As extractability and As species transformation of three paddy soils spiked with exogenous arsenate at 60 mg kg^-1 under flooded aging and explored the related chemical and microbial mechanisms. The results showed that 0.05 M NH₄H₂PO₄-extractable As decreased over time during flooded aging for 192 days, and it decreased by approximately one-third at 35 °C compared with 15 °C and 25 °C at the same aging time, indicating that higher temperatures facilitated the decrease in As extractability. As(V) reduction mainly occurred at 35 °C because the abundance and As(V)-reducing capacity of the predominant indigenous bacteria, the Bacillus sp strains, and the abundance of the arrA gene were significantly higher than those at 15 °C and 25 °C. The reduction of As(V) to As(III) and aging occurred simultaneously. The kinetic models were established, and the rate constants of the reduction and aging processes were obtained. Soil properties significantly affected the aging and reduction processes of extractable As(V). Our study indicated that elevating temperature had dual effects on the environmental risk of As in the flooded aging process. The previous definition of "aging" based on cationic metals needs to be updated according to the transformation characteristics of As species in flooded conditions. Our results addressed the necessity of impeding the reduction of As(V) in paddy soils under global warming.

Correlating soil nutrient test lead with bioaccessible lead in highly-contaminated soils receiving lead-immobilizing amendments

Lead (Pb) is one of the most common metals exceeding human health risk guidelines for soil concentrations worldwide. Pb bioaccessibility is known to vary depending on soil physiochemical characteristics and, as a result, in vitro and in vivo tests exist that are used to estimate bioaccessible Pb in contaminated soils. Although in vitro tests such as the relative bioaccessibility leaching procedure (RBALP) present simpler and more cost-effective risk assessments than in vivo methods, soil tests such as Mehlich-3, Modified Morgan, and ammonium bicarbonate-diethylenetriamine pentaacetate (AB-DTPA) extractions are extremely routine and even more cost-effective. Currently, there are few comparisons examining the viability of common soil nutrient tests for assessing Pb bioaccessibility in soils from contaminated sites with extremely high total Pb concentrations or for sites that have received amendments, such as those containing compost, iron, and/or phosphorus, intended to immobilize Pb. Here, we examine the correlation between RBALP Pb and Pb as determined using three commonly utilized soil tests, Mehlich-3, Modified Morgan, and AB-DTPA, in archived samples from one Pb-contaminated site receiving compost amendment (Seattle, WA, USA) and one extremely Pb-contaminated site receiving mixtures of compost, P, and Fe (Joplin, MO, USA). At both the Seattle and Joplin sites separately, RBALP Pb was significantly correlated with all three soil nutrient test values, regardless of soil amendment. However, RBALP was only significantly correlated with Modified Morgan and total Pb when examining the Joplin and Seattle data together, likely resulting from different factors controlling Pb solubility at the two sites. These findings suggest that a diverse suite of relatively inexpensive and accessible soil nutrient test methods correlate with bioaccessible Pb at a specific site, regardless of whether Pb-immobilizing amendments have been used.

Benefit-risk assessment of metal bioavailability in edible fungi by biomimetic whole digestive tracts with digestion, metabolism, and absorption functions

As worldwide edible fungi, Lentinula edodes and Agaricus bisporus accumulate both essential and harmful metals. Metal bioavailability is important for metal benefit-risk assessment. A full functional model of digestive tracts (including digestion, metabolism, and absorption) is established. Under the digestive tract functions, the bioaccessible and bioavailable metals are released from edible fungi and absorbed by intestinal tract, respectively. Based on bioavailable metal contents in the intestine, safe dosage and maximum consumption are 43.52 g/d and 248.7 g/d for Agaricus bisporu, 20.59/328.9 g/d (for males/ female) and 132.9 g/d for Lentinus edodes; V, Co, Ni, Cu, Zn, Se, Cr, Cd and Pb in Agaricus bisporus and Lentinula edodes are absorbed mainly in the large intestine; Fe is mainly absorbed in small intestine; edible fungi species-specificity in metal bioavailability is observed for As and Mn, which are mainly absorbed by small and large intestine for Agaricus bisporus and Lentinus edodes, respectively; and then metal toxicity on small and large intestine is disclosed. Metal benefit-risk is assessed by the content of monolayer liposome-extracted metal in the chyme from small and large intestine, which is controlled by the gastrointestinal functions, metal and edible fungi species.

Comparison of Synthetic Sweat and Influence of Sebum in the Permeation of Bioaccessible Metal(loid)s from Contaminated Soils through a Synthetic Skin Membrane

Dermal exposure to metal(loid)s from contaminated soils has received less attention than oral and inhalation exposure. Still, it can be a relevant pathway for some contaminants. Comparison of synthetic sweats (donor solutions), the influence of sebum, and the characterization of diffusion parameters through a synthetic membrane (acting as skin surrogate) in the permeation of metal(loid)s (As, Cr, Cu, Ni, Pb, and, Zn) from polluted soils is missing. The dermal bioaccessibility tests were performed using two sweat compositions [EN 1811, pH 6.5 (sweat A) and NIHS 96-10, pH 4.7 (sweat B)]. Diffusion parameters of soluble metal(loid)s using the Franz cell methodology were calculated using the Strat-M membrane. The influence of synthetic sebum in the permeation of metal(loid)s was also investigated. The metal(loid) bioaccessibility percentage was higher for sweat B (pH 4.7) compared to sweat A (pH 6.5), attributed to lower pH of sweat B. Among the six elements tested, only chromium and copper permeated the membrane. Permeation coefficient (K_p) was higher for chromium in sweat A (0.05-0.11 cm h^-1) than sweat B (0.0007-0.0037 cm h^-1) likely due to a higher pH and thus more permeable Cr species. The presence of sebum increased lag times for copper permeation. Additional studies regarding speciation of metal(loid)s following extractions in synthetic sweat and comparison of synthetic membrane Strat-M and human skin in the permeation of metal(loid)s are recommended.

The influence of long-term ageing on arsenic ecotoxicity in soil

The ageing of a contaminant in soil influences the bioavailability and toxicity of environmental pollutants. Yet, despite arsenic (As) being an important terrestrial contaminant, the effect of As ageing on phytotoxicity has received relatively little research. Research to date has reported predominantly short term (< 0.5 years) experiments. Here, we studied the influence of ageing over 0.25 and 5 years on the phytotoxicity of As (as arsenate) on Cucumis sativus L. (cucumber). The study showed that increasing ageing time of As from 0.25 to 5 years increased the EC₁₀ and EC₅₀ values by 4.0 and 1.76 fold, respectively. The dependence of ageing on soil properties was also examined, although only Freundlich sorption parameters were correlated to the ageing factor (r = 0.68, P = 0.028). Soils with high adsorption capacity also showed the greatest change in toxicity over 5 years. In addition, data was compiled from relevant literature to develop a model for As ecotoxicity. The combined model (n = 54) showed no relationship with pH but was correlated to the oxalate extractable iron content and %clay. Arsenate ecotoxicity (EC₅₀, mg/kg) in the multivariate model was related to oxalate iron content, %clay and ageing time. Thus, the results of this study have significant implications for risk assessment of long-term As contaminated soils.

Selenium bioaccessibility in native seleniferous soil and associated plants: Comparison between in vitro assays and chemical extraction methods

Selenium (Se) bioaccessibility in soil and crops from seleniferous areas is closely relevant to Se intake risks of local residents. The current in vitro digestion methods used for Se bioaccessibility evaluation are single and inconsistent, and most of them are only for food and neglect soil. In this study, 14 Se-contaminated soils and their corresponding crops in Naore Village (seleniferous area) were used as the research objects. Four in vitro digestion assays, including Solubility Bioaccessibility Research Consortium method (SBRC), physiologically-based extraction test (PBET), in vitro gastrointestinal method (IVG), and Unified Bioaccessibility Method (UBM) were used to determine the bioaccessible Se concentration in soil and edible parts of crops. Results showed that the Se in natural seleniferous soil mainly existed in relatively stable forms, i.e., residual and Fe-Mn oxide-bound Se (average of 80%). Only 10.6% of the total Se was distributed in water-soluble and exchangeable Se fractions. The Se content in crops was significantly positively correlated with the organic-bound and phosphate-extractable Se contents in the corresponding soil (p < 0.05). The organic-bound Se was clearly a potentially bioavailable Se source in soil. The Se bioaccessibility in soil and crops measured using the four in vitro methods in gastric/intestinal digestions were in the same order, which was PBET > UBM > SBRC > IVG. Similar to the absorption and utilization of soil Se fractions by crops, the water-soluble, organic-bound and exchangeable Se in soil were the main contributors of bioaccessible Se in the digestive juices in various in vitro methods. Furthermore, the bioaccessible Se in crops and soil measured via PBET method demonstrated the most significant correlation between the total Se in crops and the phosphate-extractable Se in soil. Therefore, the PBET method was the optimum in vitro method for the evaluation of Se bioaccessibility in crops and soil.

Aging effects on fractionation and speciation of redox-sensitive metals in artificially contaminated soil

Artificially contaminated soil is often used in laboratory experiments as a substitute for actual field contaminated soils. In the preparation and use of laboratory contaminated soils, questions remain as to how much and how long metals remain in labile form and in their oxidation state during the contamination process. Therefore, the objectives of this study were to determine if the speciation of added contaminants can be retained in the original form and to observe the change in lability of each element with aging time. In this study, natural soil was artificially polluted with five redox-sensitive toxic elements in their oxidized or reduced forms, i.e., As(III)/As(V), Sb(III)/Sb(V), Cr(III)/Cr(VI), Mo(VI), and W(V). Metal distribution was measured in progressive chemical fractionation using sequential extraction methods in contaminated soils after 3, 100, and 300 days of aging. The results indicated that the more strongly bound fraction of metals increased by day 100; whereas the fractions were not significantly different from those in the 300-day-aged soil. Among five metals, the ratio of weakly-bound fractions remained highest in As- and lowest in Cr-contaminated soils. The W(VI)-contaminated soil showed strong sorption without changes in speciation during aging. The oxidized or reduced metal species converged to occur as a single species under given soil conditions, regardless of the initial form of metal used to spike the soil. Both As and Sb existed as their oxidized form while Cr existed as its reduced form. The results of this study may provide a useful and practical guideline for artificial soil contamination.

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

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

Aging Process of Cadmium, Copper, and Lead under Different Temperatures and Water Contents in Two Typical Soils of China

Aging process of exogenous heavy metals in soil is significant for reducing their environmental risk due to the redistribution of species of soil heavy metals. A red soil (ultisol) and a brown soil (alfisol) were selected to investigate the aging process of cadmium (Cd), copper (Cu), and lead (Pb) under different regimes of temperature and water content. Most introduced heavy metals were all transformed from dissolved fraction to more stable fractions within 5 days of incubation. During incubation, most Pb existed in the fraction bound to Fe/Mn oxides, while exchangeable and carbonate-associated fraction was the dominant portion for Cd and Cu, suggesting that the transformation rate followed the order: Pb > Cu > Cd. The exchangeable and carbonate-associated fraction in red soil, which was characterized with higher pH and Fe/Al/Mn oxides and lower organic matter (OM), was significantly higher than that in brown soil, implying that soil OM was the important factor affecting the aging process of soil heavy metals in the present study. In addition, increases of temperature and soil water content can accelerate the transformation of most introduced Cd, Cu, and Pb to more stable forms in the soils. The results indicated that soil properties, environmental factors (i.e., temperature and water content), types of heavy metals, and pollution time can significantly affect the aging process of exogenous heavy metals.

Uptake of vegetable and soft drink affected transformation and bioaccessibility of lead in gastrointestinal track exposed to lead-contaminated soil particles

Accidental ingestion of Pb-contaminated soil particles by direct hand-to-mouth activity or by swallowing airborne dust particles is important pathway of human exposure to Pb. Appropriate evaluation of Pb risk to human is important in determining whether the soil needs remediation or not, however, there is paucity of data about the dietary influences on Pb bioaccessibility (Pb-BA) and transformation in humans. This study chose two typical foods, spinach and cola, representing vegetable and soft drink, respectively, and investigated their effects on Pb species in gastrointestinal tract using the physiologically based extraction test. Results showed that ingestion of spinach and cola decreased the Pb-BA by 52%-94% in the gastric phase and by 38%-95% in the intestinal phase, respectively. The reduction of Pb-BA by spinach was attributed to the precipitation of Pb with phosphorus in spinach and the sorption of Pb by the generated hydrolysate and un-hydrolysate from spinach in gastrointestinal tract. Cola decreased Pb-BA mainly via formation of insoluble Pb phosphates precipitates. Analysis of X-ray diffraction and MINTEQ modeling demonstrated that the dissolved Pb was transformed to precipitated or sorbed Pb with intake of cola or spinach. Our findings suggest that dietary habit greatly influence the speciation and subsequent Pb-BA in the gastrointestinal tract, which should be incorporated into human health risk assessment of Pb-contaminated soil.

The Suitability of Short Rotation Coppice Crops for Phytoremediation of Urban Soils

This experiment was aimed at verifying the usefulness of phytoremediation using Short Rotation Coppice (SRC) in an urban Zn-contaminated site. Besides elemental uptake and reclamation, the SRC method was applied to evaluate the additional benefits of a green infrastructure. Nine different plants with rapid growth and large biomass production were selected: three Populus clones, three Salix hybrids, and three Robinia genotypes. Annual and biennial coppicing were evaluated. Poplar clones were more productive using annual coppicing, while Salix and Robinia produced higher biomass in blocks not coppiced. Poplar had the highest phytoextraction rate during the second year, with 1077 g/ha. Salix clones S1 and S3 extracted similar quantities using biennial coppicing. After two years, the bioavailable fraction of Zn decreased significantly using all species, from the 26% decrease of Robinia to the 36% decrease of Salix. The short rotation coppice method proved to be useful in an urban context, for both landscape and limiting the access to the contaminated area. Improving the biomass yield through the phytomanagement options (fertilization, irrigation, coppicing, etc.) could make SRC phytoremediation an economic and effective solution to manage urban contaminated areas, coupling the added values of biomass production to the landscape benefits.

Heavy metal behaviour at mineral-organo interfaces: Mechanisms, modelling and influence factors

The mineral-organo composites control the speciation, mobility and bioavailability of heavy metals in soils and sediments by surface adsorption and precipitation. The dynamic changes of soil mineral, organic matter and their associations under redox, aging and microbial activities further complicate the fate of heavy metals. Over the past decades, the wide application of advanced instrumental techniques and modelling has largely extended our understanding on heavy metal behavior within mineral-organo assemblages. In this review, we provide a comprehensive summary of recent progress on heavy metal immobilization by mineral-humic and mineral-microbial composites, with a special focus on the interfacial reaction mechanisms of heavy metal adsorption. The impacts of redox and aging conditions on heavy metal speciations and associations with mineral-organo complexes are discussed. The modelling of heavy metals adsorption and desorption onto synthetic mineral-organo composites and natural soils and sediments are also critically reviewed. Future challenges and prospects in the mineral-organo interface are outlined. More in-depth investigations are warranted, especially on the function and contribution of microorganisms in the immobilization of heavy metals at the complex mineral-organo interface. It has become imperative to use the state-of-the-art methodologies to characterize the interface and develop in situ analytical techniques in future studies.

Water incubation-induced fluctuating release of heavy metals in two smelter-contaminated soils

The soil moisture regime can affect the release of heavy metals in soil. In the previous studies, slightly polluted soils or artificially contaminated soil samples were considered to investigate the effect of soil moisture. We used highly smelter-contaminated and aged soils to study the release of typical heavy metals (Cu, Zn, Cd and Pb) induced by water incubation in batch experiments with characterization via speciation and X-ray diffraction analyses (XRD). The results show that the leachable concentrations of the heavy metals increased slightly in the first 30 days, decreased drastically between 30 and 90 days, and immobilized relatively constant thereafter. The fluctuation was ascribed to the changes of soil Eh and pH, the reductive dissolution of crystalline iron oxides, the formation of new amorphous iron oxides, the absorption of dissolved organic matter and the precipitation of metal sulfide. Speciation analysis indicated that a proportion of the soil heavy metals was transformed from an exchangeable fraction to a less labile fraction after water incubation. And the presence of a lead iron oxide phase and the peak increasing of zinc sulfide were observed via XRD analyses. Finally, water incubation restrained the release of heavy metals after 180 days of incubation, and reduced the leachability of Cu, Zn, Cd and Pb by as much as 1.61%-7.21% for soil A and 0.43%-3.36% for soil B, respectively. The study findings have implications for the formulation of risk control and management strategies for heavy metals in smelter-contaminated soils.

Chemical fractionation of heavy metals in fine particulate matter and their health risk assessment through inhalation exposure pathway

Samples of PM_2.5 were collected from an urban area close to a national highway in Agra, India and sequentially extracted into four different fractions: water soluble (F1), reducible (F2), oxidizable (F3) and residual fraction (F4) for chemical fractionation of arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), nickel (Ni) and lead (Pb). The metals were analyzed by inductively coupled plasma optical emission spectroscopy in each fraction. The average mass concentration of PM_2.5 was 93 ± 24 μg m^-3.The total concentrations of Cr, Pb, Ni, Co, As and Cd in fine particle were 192 ± 54, 128 ± 25, 108 ± 34, 36 ± 6, 35 ± 5 and 8 ± 2 ng m^-3, respectively. Results indicated that Cd and Co had the most bioavailability indexes. Risk Assessment Code and contamination factors were calculated to assess the environmental risk. The present study evaluated the potential Pb hazard to young children using the Integrated Exposure Uptake Biokinetic Model. From the model, the probability density of PbB (blood lead level) revealed that at the prevailing atmospheric concentration, 0.302 children are expected to have PbB concentrations exceeding 10 μg dL^-1 and an estimated IQ (intelligence quotient) loss of 1.8 points. The predicted blood Pb levels belong to Group 3 (PbB < 5 μg dL^-1). Based on the bioavailable fractions, carcinogenic and non-carcinogenic risks via inhalation exposure were assessed for infants, toddlers, children, males and females. The hazard index for potential toxic metals was 2.50, which was higher than the safe limit (1). However, the combined carcinogenic risk for infants, toddlers, children, males and females was marginally higher than the precautionary criterion (10^-6).

The toxicity of exogenous arsenic to soil-dwelling springtail Folsomia candida in relation to soil properties and aging time

Arsenic (As) is a toxic metalloid, but studies on As toxicity to soil-dwelling springtails are fairly limited, and did not consider the effects of various soil properties and long aging time. To address this, the toxicity of As to model organism-Folsomia candida were evaluated in the laboratory studies. The results showed that compared to the soils aged only for 15 d, the concentrations inhibiting 50% reproduction (EC₅₀) significantly increased by 1.3- to 2.0-fold in four soils aged for 150 d, the concentrations causing 50% mortality (LC₅₀) were higher than the highest test concentration in the most soils. Furthermore, the aging effects correlated significantly with soil free Fe oxides contents. The toxicity of As differed in ten soils aged for 150 d, the LC₅₀ were 320-> 1280 mg/kg in acute test and the EC₅₀ were 67-580 mg/kg in chronic test. Regression analysis indicated that soil clay was the most important single factor predicting soil As toxicity to reproduction, explaining 89% of the variance in EC₅₀ values. Soil pH, free Fe oxides and Al oxides could also well explain the toxicity variance (> 65%), indicating that As sorption was a key factor controlling its toxicity.

Relationship of bioaccessibility and fractionation of cadmium in long-term spiked soils for health risk assessment based on four in vitro gastrointestinal simulation models

The bioavailability and bioaccessibility of heavy metals mainly depend on their speciation in soils; however, the relationship between bioaccessibility and its speciation as determined as fractionation remains to be better characterized. Therefore, Cd fractionations in three types of long-term Cd-spiked soils were determined using a modified BCR method, and compared with Cd bioaccessibility data obtained from four in vitro gastrointestinal simulation assays including the IVG, PBET, SBRC, and UBM. The results shows that the majority of Cd were found in the exchangeable/acid soluble (B1) and reducible (B2) fractions (total percentage>97%) after being spiked and aged for three years, indicating high Cd bioavailability; the bioaccessibility of Cd ranged from 57.7±1.8% to 99.3±2.8% in the gastric phase, and from 5.8±2.0% to 35.9±1.8% in the intestinal phase, respectively. Among the four assays, the strongest positive correlation was observed between Cd bioaccessibility based on the PBET assay and its B1 fraction in the spiked soils (r² gastric: 0.62 and intestinal: 0.52), suggesting that the PBET assay maybe more suitable for determining Cd bioaccessibility in aged contaminated soils.

Environmental risk assessment of steel-making slags and the potential use of LD slag in mitigating methane emissions and the grain arsenic level in rice (Oryza sativa L.)

Over the past decades, with increasing steel manufacturing, the huge amount of by-products (slags) generated need to be reused in an efficient way not only to reduce landfill slag sites but also for sustainable and eco-friendly agriculture. Our preliminary laboratory study revealed that compared to blast furnace slag, electric arc furnace slag and ladle furnace slag, the Linz-Donawitz converter (LD) slag markedly decreased CH₄ production rate and increased microbial activity. In the greenhouse experiment, the LD slag amendment (2.0 Mg ha^-1) significantly (p < 0.05) increased grain yield by 10.3-15.2%, reduced CH₄ emissions by 17.8-24.0%, and decreased inorganic As concentrations in grain by 18.3-19.6%, compared to the unamended control. The increase in yield is attributed to the increased photosynthetic rates and increased availability of nutrients to the rice plant. Whereas, the decrease in CH₄ emissions could be due to the higher Fe availability in the slag amended soil, which acted as an alternate electron acceptor, thereby, suppressed CH₄ emissions. The more Fe-plaque formation which could adsorb more As and the competitive inhibition of As uptake with higher availability of Si could be the reason for the decrease in As uptake by rice cultivated with LD slag amendment.

Time effect on the stabilization of technogenic copper compounds in solid phases of Haplic Chernozem

We studied the regularities of Cu stabilization in Haplic Chernozem contaminated with easily soluble (acetates, nitrates, sulfates, chlorides) and poorly soluble (acetates, nitrates, sulfates, chlorides) metal compounds in a five-year model experiment were studied. A low contents of Cu loosely bound compounds (LBC) (no >5% of total content) unchanging over time were revealed in the uncontaminated soil, which is indicative of the stability of soil processes. The Cu LBC content decreased over time in the soils contaminated with easily soluble compounds and increased in the soils contaminated with poorly soluble compounds. In both cases, the contents of Cu LBCs after 5 years of incubation remained higher than in the original uncontaminated soil. The effect of the attendant anions on the Cu LBC content was more apparent under the Cu application of 2000 mg kg^-1 than at 300 mg kg^-1. An inverse process characterized by an increase in the concentration of Cu LBCs over time was observed in the soil contaminated with Cu orthophosphate and oxide. Soil contaminated with different Cu compounds results in soil acidification. According to the effect on the decrease in soil pH, the attendant anions form the following series: SO₄^2- ≈ Cl^- > NO₃^- > Ac^- > PO₄^2- > O^2-, which correlates with the Cu LBC content. Thus, the stabilization of Cu compounds in the soil is affected by the attendant anions and the interaction time of the metal with the soil solid phases.

As and Sb are more labile and toxic to water spinach (Ipomoea aquatica) in recently contaminated soils than historically co-contaminated soils

Elevated concentrations of As and Sb impact environmental quality and human health. In this study total and bioavailable As and Sb were measured from recently and historically contaminated soils and the phytotoxicity of these soils was evaluated with Ipomoea aquatica (35-d exposure from germination) using biomass, length of plant tissues and photosynthetic efficiency. As and Sb were both present within the soil (co-contaminated). The bioavailable As and Sb in soils were determined by a Sequential Extraction Procedure (SEP) and compared to total soil concentrations and bioaccumulation in the edible parts of I. aquatica. For both As and Sb, bioavailable concentrations increased proportionally with the total soil concentrations and greater bioavailability in recently contaminated soil was observed. Tissue dry mass and length drastically reduced with increasing total and SEP-bioavailable As and Sb soil concentrations. The total soil concentration was a less sensitive measure of the phytotoxicity of As and Sb than the bioavailable fraction. Shoot length was inhibited by 50% (EC50) at bioavailable As concentrations of 80-96 mg kg-1 in both recently and historically contaminated soils; however, bioavailable Sb EC50 for shoot length was achieved at lower bioavailable concentrations, 96 (42-219) and 12 (7-19) mg kg-1 in recently contaminated soils and historically contaminated soils, respectively. Shoot biomass was inhibited by 50% (EC50) at bioavailable As concentrations of 11 (4-30) and 49 (37-65) mg kg-1 in recently and historically contaminated soils, respectively whereas this occurred at much lower bioavailable Sb concentrations, 2-5 mg kg-1 in both recently and historically contaminated soils. Aging is important in contaminated soils, it decreases the lability of As and Sb and hence their bioavailability to agricultural plants, thus posing a lower risk of exposure of these metalloids to humans through agricultural plants grown in contaminated soils.

Accumulation, fractionation, and risk assessment of mercury and arsenic in the soil-wheat system from the wastewater-irrigated soil in Baiyin, northwest China

Wastewater irrigation can increase metal concentrations in soil and wheat, thereby posing metal-associated health risk via food ingestion. We investigated levels of mercury (Hg) and arsenic (As) in roots, husks, stems, leaves, and grains of wheat and their fractionations in farmland soil from Baiyin City, an industrial and mining city, northwest China. Results show that the mean concentrations of Hg in soils from Dongdagou and Xidagou stream in Baiyin were 8.5 times and three times higher than local soil background values, respectively. Those of As were 4.5 times and 1.6 times higher, respectively. Most Hg and As were mainly accumulated in wheat leaves. The spatial distributions of As in soils and grains exhibit a very similar pattern, which suggest that As pollution in soils might be predicted by its level in wheat grains. Residual fractions for Hg (RES-Hg) and As (RES-As) are the highest compared to other fractions, indicating weak mobility of Hg and As in soil. The crop oral intake hazard quotients of both Hg and As for children were approximately two times higher than that for adults, indicating that children have higher exposure risks to Hg- and As-contaminated wheat. The crop oral intake was the main route of exposure causing non-carcinogenic and carcinogenic risk for local residents.

Kinetic release of arsenic after exogenous inputs into two different types of soil

The mobility of arsenic (As) in soil depends on its sorption/desorption processes on soil particles. Plant uptake locally lowers As concentration in soil pore water, which would trigger resupplies of As from soil solid phase. To better understand the fate of As in soil system after its inputs into soil and its subsequent dynamic processes, diffusive gradients in thin films (DGT) technique along with DGT-induced fluxes in soils (DIFS) model were introduced to study the kinetic information of As in soils, including its response time (T_C) and resupply rate constant (k_-1). To achieve a series of soils with gradient As level, two different types of soils with similar As level (total As in soil JL is 7.4 mg kg^-1, while in soil BJ is 6.5 mg kg^-1) were collected and amended with exogenous As. Then, DGT deployments were carried out following a period of 90-day soil incubation. The simulated T_C values in non-amended soil JL and soil BJ were 0.036 and 0.001 s^-1, respectively. The difference may due to the properties of these two soils, including pH values and contents of adsorption materials, such as Fe and Al compounds. After As inputs into soils, the intrinsic rate of As release from the solid phase to the solution phase in As-amended JL soil was much higher than that in non-amended soil. While for soil BJ, a decreasing trend was observed after As spiking. The redistribution of As may responsible for the different variation trends of As kinetics in these two soils after As spiking. The results indicated that the distribution coefficient of As (K_d) in soil was mainly affected by soil Olsen-P content due to an ubiquitous competition between P and As on soil particles.

Temporal Changes in Cadmium Speciation in Brazilian Soils Evaluated Using Cd LIII-Edge XANES and Chemical Fractionation

Chemical speciation of soil cadmium (Cd) dictates its mobility and potential toxicity in the environment. Our objective was to compare temporal changes in speciation of Cd(II) reacted with samples from six Brazilian soils having varying Cd(II) sorption capacities. Cadmium L-edge X-ray absorption near edge structure (XANES) analysis showed there were short-term changes in speciation after reaction with 4.45 mmol Cd kg for 0.5 and 6 h. Chemical fractionation evaluated changes in Cd extractability after reaction with 89 μmol Cd kg for up to 4 mo. The XANES spectral fits suggested that Cd(II) bound with organic matter was a dominant species in all samples, along with Cd(II) bound with iron and aluminum oxides or montmorillonite. In several samples, CdCl apparently precipitated from aqueous Cd(II) during drying. The XANES spectral fits typically showed <25% change in speciation between 0.5 and 6 h of reaction, and chemical fractionation showed significant ( < 0.05) temporal changes in Cd extractability over time in two samples. Our results suggest that Cd(II) discharged into these soils, such as that occurring as a release into the environment, would bind with soil organic matter and oxide minerals or remain dissolved, with little change in speciation in the months following release.

Factors Affecting the Bioaccessibility and Intestinal Transport of Difenoconazole, Hexaconazole, and Spirodiclofen in Human Caco-2 Cells Following in Vitro Digestion

This study examined how gastrointestinal conditions affect pesticide bioaccessibility and intestinal transepithelial transport of pesticides (difenoconazole, hexaconazole, and spirodiclofen) in humans. We used an in vitro model combining human gastric and intestinal digestion, followed with Caco-2 cell model for human intestinal absorption. Bioaccessibility of three tested pesticides ranged from 25.2 to 76.3% and 10.6 to 79.63% in the gastric and intestinal phases, respectively. A marked trend similar to the normal distribution was observed between bioaccessibility and pH, with highest values observed at pH 2.12 in gastric juice. No significant differences were observed with increasing digestion time; however, a significant negative correlation was observed with the solid-liquid (S/L) ratio, following a logarithmic equation. R² ranged from 0.9198 to 0.9848 and 0.9526 to 0.9951 in the simulated gastric and intestinal juices, respectively, suggesting that the S/L ratio is also a major factor affecting bioaccessibility. Moreover, significant dose- and time-response effects were subsequently observed for intestinal membrane permeability of difenoconazole, but not for hexaconazole or spirodiclofen. This is the first study to demonstrate the uptake of pesticides by human intestinal cells, aiding quantification of the likely effects on human health and highlighting the importance of considering bioaccessibility in studies of dietary exposure to pesticide residues.

In-vitro bioaccessibility of five pyrethroids after human ingestion and the corresponding gastrointestinal digestion parameters: A contribution for human exposure assessments

Bioaccessibility is a crucial parameter in assessing the absorption of contaminants during the human digestive process, but few studies have involved the differences in the bioaccessibilities of pesticides. To investigate the mode of using the in vitro bioaccessibility to refine estimates of dietary exposure to pesticide residues, this study measured the bioaccessibilities of five pyrethroids in apples, and then, it modelled physicochemical predictors (gastrointestinal pH, digestive times, and the solid-liquid (S/L) ratio) of the bioaccessibilities of pyrethroids. Apple samples of gastric and intestinal phase digestive juices were obtained from an in vitro simulated digestion model. Our survey of in vitro digestion models found that the bioaccessibilities ranged from 4.42% to 31.22% and 10.58%-35.63% in the gastric and intestinal phases, respectively. A sharp trend similar to a normal distribution was observed between the bioaccessibilities and pH values. The bioaccessibility reached its highest value at a pH of 1.91 in the simulated gastric juice and did not significantly change with an increase of the digestive time. A significant negative correlation occurred between the bioaccessibility and S/L ratio, which followed a logarithmic equation. The correlation coefficients (R²) ranged from 0.9259 to 0.9831 and 0.9077 to 0.9960 in the simulated gastric and intestinal juice, respectively, suggested that both the pH value and S/L ratio were the main factors affecting the bioaccessibility. Furthermore, a combination of the acceptable daily intake (ADI) and bioaccessibility for human exposure assessments indicated the implication that traditional risk assessment using ADI may seriously overestimate the actual risk.

Biogeochemistry of Ni and Pb in a periodically flooded arable soil: Fractionation and redox-induced (im)mobilization

The redox-induced (im)mobilization of nickel (Ni) and lead (Pb) under pre-definite redox conditions and their binding forms were studied in a periodically flooded, slightly acidic arable soil enriched with serpentine minerals at the Velika Morava River valley, Serbia. The total contents of Ni and Pb were 152 and 109 mg kg^-1, respectively. Geochemical fractionation of Ni, combined with mineralogical analysis, confirmed its geogenic origin in the soil. Potentially mobile fractions were the dominating binding forms of Pb; thus, indicating anthropogenic sources as prevailing. Risk assessment indicated a low risk of Ni and Pb transfer from soil to other environmental constituents. However, the results imply that geogenic metals might pose higher environmental risk than those from anthropogenic origin, in dependence of their total concentrations and contents in the specific solid-phase fractions. Flooding of the soil was simulated in an automated biogeochemical microcosm system, which allows a control and a continuous measurements of redox potential (E_H) and pH. Subsequently, the E_H was increased in steps of approximately 100 mV from anoxic to oxic conditions. Concurrently, the concentrations of soluble Ni, Pb, iron (Fe), manganese (Mn), dissolved organic carbon (DOC), and sulfates were measured. The E_H was brought from low to high values (-220 to 520 mV) and correlated negative with soluble Ni, Pb, Fe, Mn and DOC. Soluble Ni ranged from 125 to 228 μg l^-1 while Pb ranged from 3.0 to 21.4 μg l^-1. Concentrations of both metals in solution were high at low E_H and decreased with increasing E_H. Nickel immobilization may be attributed to sorption to or co-precipitation with re-oxidized Fe-Mn (hydr)oxides, whereas Pb, in addition, might be immobilized via precipitation with inorganic ligands, such as carbonates and phosphates. The results imply that Ni and Pb solubility might also be related to the formation of metal-DOC complexes. The detected dynamic and mechanisms might be useful in providing critical information for assessing the potential environmental risk and creating appropriate environmental management strategies for agricultural areas enriched with Ni and Pb.