Reduction kinetics of hexavalent chromium in soils and its correlation with soil properties

Characteristics of iron (hydr)oxides and Cr(VI) retention mechanisms in soils from tropical and subtropical areas of China

Though both iron (hydr)oxides and soil organic matter (SOM) significantly influence heavy metal behaviors in soils, studies on the characteristics of natural minerals and the synergic effects of the two on Cr(VI) transformation are limited. This study investigated Cr(VI) retention mechanisms in four soils from tropical and subtropical regions of China based on a comprehensive characterization of Fe (hydr)oxides. These soils exhibited varying quantities of hematite, ferrihydrite and goethite, with distinct Al substitution levels and varied exposed crystallographic facets. Adsorption experiments revealed a positive correlation between Fe (hydr)oxide content and Cr(VI) fixation amount on colloid, which was influenced by the mineral types, Al substitution levels and facet exposures. Further, Cr(VI) was sequestered on soil by adsorption and reduction. In soils enriched with crystalline Fe (hydr)oxides, Cr(VI) reduction was primarily governed by SOM, while in soils enriched with poorly crystalline Fe (hydr)oxides, mineral-associated Fe(II) also contributed to Cr(VI) reduction. Aging experiments demonstrated that SOM and mineral-associated Fe(II) expedited Cr (VI) passivation and diminished the Cr leaching. These results improve our understanding of natural Fe (hydr)oxide structures and their impact on Cr(VI) behavior in soils, and shed light on complex soil-contaminant interactions and remediation of Cr(VI) polluted soils.

Nano zero-valent iron enhances the absorption and transport of chromium in rice (Oryza sativa L.): Implication for Cr risks management in paddy fields

Chromium (Cr) accumulating in soil caused serious pollution to cultivated land. At present, nano zero-valent iron (nZVI) is considered to be a promising remediation material for Cr-contaminated soil. However, the nZVI impact on the behavior of Cr in the soil-rice system under high natural geological background value remains unknown. We studied the effects of nZVI on the migration and transformation of Cr in paddy soil-rice by pot experiment. Three different doses of nZVI (0, 0.001 % and 0.1 % (w/w)) treatments and one dose of 0.1 % (w/w) nZVI treatment without plant rice were set up. Under continuous flooding conditions, nZVI significantly increased rice biomass compared with the control. At the same time, nZVI significantly promoted the reduction of Fe in the soil, increased the concentration of oxalate Fe and bioavailable Cr, then facilitated the absorption of Cr in rice roots and the transportation to the aboveground part. In addition, the enrichment of Fe(III)-reducing bacteria and sulfate-reducing bacteria in soil provided electron donors for Cr oxidation, which helps to form bioavailable Cr that is easily absorbed by plants. The results of this study can provide scientific basis and technical support for the remediation of Cr -polluted paddy soil with high geological background.

Alleviation of chromium stress in plants using metal and metal oxide nanoparticles

Chromium (Cr), one of the hazardous pollutants, exists predominantly as Cr(VI) and Cr(III) in the environment. Cr(VI) is more toxic than Cr(III) due to its high mobility and solubility. Elevated levels of Cr in agricultural soil due to various anthropogenic activities cause Cr accumulation in plants, resulting in a significant reduction in plant yield and quality due to Cr-induced physiological, biochemical and molecular alterations. It can infiltrate the food chain through crop plants and cause harmful effects in humans via biomagnification. Cr(VI) is linked to cancer in humans. Therefore, mitigation strategies are required to remediate Cr-polluted soils and limit its accumulation in plants for safe food production. Recent research on metal and metal oxide nanoparticles (NPs) has shown that they can effectively reduce Cr accumulation and phytotoxicity. The effects of these NPs are influenced by their type and dose, exposure method, plant species and experimental settings. In this review, we present an up-to-date compilation and comprehensive analysis of the existing literature regarding the process of uptake and distribution of Cr and impact and potential mechanisms of metal and metal oxide nanoparticles led mitigation of Cr-induced stress in plants. We have also discussed recent developments, existing research gaps and future research directions in the field of Cr stress mitigation by NPs in plants. Overall, this review can provide valuable insights in reducing Cr accumulation and toxicity using metal and metal oxide nanoparticles, thereby promoting safe and sustainable cultivation of food and phytostabilization of Cr-polluted soil.

Effects of soil pH and organic carbon content on in vitro Cr bioaccessibility in Ultisol, Alfisol, and Inceptisol

Hexavalent chromium (Cr(VI)) is a toxic heavy metal and its mobility and bioaccessibility in soils are influenced by soil properties. In this study, the soil pH and organic carbon contents of Ultisol, Alfisol, and Inceptisol were adjusted before they were polluted with 230 mg kg^-1 Cr(VI). Alkaline digestion, sequential extraction, and an in vitro experiment were conducted to study the valence state, species, and bioaccessibility of Cr in the soils. The results showed that a high soil pH was not favorable for reduction of Cr(VI); therefore the Cr(VI) and exchangeable Cr contents were positively related to soil pH. Soil organic carbon promoted the reduction of Cr(VI). Almost all Cr(VI) was reduced to Cr(III) when the soil organic carbon content reached 10 g kg^-1. Chromium bioaccessibility in simulated gastric and intestinal phase solutions was influenced by Cr(VI) and Cr(III) adsorption/desorption, dissolution/precipitation, and redox reactions. Chromium bioaccessibility differences between the gastric and intestinal phases were associated with the Cr(VI)/Cr(III) ratio. Acidic conditions and a high organic carbon content promoted the conversion of Cr(VI) to Cr(III). When soil pH was increased from 4.01 to 5.85, Cr(VI) in Alfisol without the addition of humic acid increased from 96.38 to 174.78 mg kg^-1, the exchangeable Cr proportion increased from 9.7% to 22.6%, and Cr bioaccessibility increased from 41.29% to 49.14% in the gastric phase and from 41.32% to 48.24% in the intestinal phase. When the organic content increased from 3.95 to 9.28 g kg^-1 in Alfisol, Cr(VI) content decreased from 167.66 to 20.52 mg kg^-1, which led to a decrease in Cr bioaccessibility from 49.15% to 13.8% in the gastric phase and from 45.85% to 7.67% in the intestinal phase. Therefore, acidic conditions and increasing soil organic carbon levels can reduce the health risk posed by Cr in soils.

Chromium transformation driven by iron redox cycling in basalt-derived paddy soil with high geological background values

The flooding and drainage of paddy fields has great effects on the transformation of heavy metals, however, the transformation of Cr in basalt-derived paddy soil with high geological background values was less recognized. The typical basalt-derived paddy soil was incubated under alternating redox conditions. The Cr fractions and the dynamics of Fe/N/S/C were examined. The HCl-extractable Cr increased under anaerobic condition and then decreased during aerobic stage. The UV-vis spectra of the supernatant showed that amounts of colloids were released under anaerobic condition, and then re-aggregated during aerobic phase. The scanning transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) revealed that Fe oxides were reduced and became dispersed during anaerobic stage, whereas Fe(II) was oxidized and recrystallized under aerobic condition. Based on these results, a kinetic model was established to further distinguish the relationship between the transformation of Cr and Fe. During anaerobic phase, the reduction of Fe(III) oxides not only directly released the structurally bound Cr, but also enhanced the breakdown of soil aggregation and dissolution of organic matter causing indirect mobilization of Cr. During aerobic phase, the oxidation of Fe(II) and further recrystallization of newly formed Fe(III) oxides might induce the re-aggregation of soil colloids and further incorporation of Cr. In addition, the kinetic model of Cr and Fe transformation was further verified in the pot experiment. The model-based findings demonstrated that the Cr transformation in the basalt-derived paddy soil with high geological background values was highly driven by redox sensitive iron cycling.

Aging factor and its prediction models of chromium ecotoxicity in soils with various properties

Aging of pollutants determines bioavailability and toxicity thresholds of environmental pollutants in soil. However, the ecotoxicity of chromium (Cr) rarely considers the effect of aging as well as soil properties. In order to explore the aging characteristics and establish their quantitative relationship with different soil properties, this study selected 7 soils with different properties through exogenous addition of Cr and determined its toxicity on barley root elongation. From 14d to 540d, EC₁₀ and EC₅₀ of barley root elongation ranged from 21.40 to 312.52 (mg·kg^-1) and 50.15 to 883.88 (mg·kg^-1) respectively. The hormesis appeared in the dose-response curve of acid soil as relative barley root elongation reached >110 % compared with the control. Extended aging time of Cr from 14d to 540d was associated with the attenuation of the toxicity of Cr, as the aging factor increased from 1.26 to 6.09 for EC₅₀, from 0.88 to 4.98 for EC₁₀. The prediction model of AF_EC50 and soil properties is lg (AF_360d) = 0.306lg Clay+0.026lg CEC + 0.240 (R² = 0.872, P < 0.01). The results demonstrated that with the extension of aging time, the toxicity of Cr decreased at 360d and reached a slow reaction stage, after that soil OC, Clay and CEC could well explain the aging procedure of Cr (VI). These results are beneficial for risk assessment of Cr contaminated soils and establishment of a soil environmental quality criteria for Cr.

Hexavalent chromium reduction and bioremediation potential of Fusarium proliferatum S4 isolated from chromium-contaminated soil

Microbial remediation, utilizing reduction of Cr(VI) to Cr(III), is considered a promising method for lowering toxic environmental chromium levels. In this study, a Cr(VI)-resistant fungal strain, Fusarium proliferatum S4 (F. proliferatum), was isolated from seriously chromium-polluted soil at Haibei Chemical Plant, China. This strain for treatment chromium-containing solution resulted in 100.00%, 93%, and 74% removal at initial concentrations of 10, 30, and 50 mg L^-1 Cr(VI), respectively, after 12 days of treatment in a batch mode. Contributions of different cell fractions to Cr(VI) removal were explored. The Cr(VI) removal capacity of various cell components from strong to weak was as follows: cytoplasm, cell secretions, and cell debris. Observations obtained by scanning electron microscopy and transmission electron microscopy with energy dispersive X-ray spectroscopy revealed that not only the cell surfaces but also the intracellular contents were involved Cr through adsorption, reduction, or accumulation. Fourier transform infrared spectra indicated that a large number of functional groups (amino, carbonyl, carboxyl, and phosphate groups) participated in chromium binding on the cell surface. X-ray photoelectron spectroscopy confirmed the presence of Cr on the cell surface only as Cr(III). The results have important implications for an in-depth understanding of microbial chromate reduction by F. proliferatum. This study provides an insight into the microbial Cr(VI) bioreduction efficiency, and mechanisms in the chromium-contaminated environment.

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.

The toxicity of hexavalent chromium to soil microbial processes concerning soil properties and aging time

Chromium (Cr) pollution has attracted much attention due to its biological toxicity. However, little is known regarding Cr toxicity to soil microorganisms. The present study assesses the toxicity of Cr(VI) on two microbial processes, potential nitrification rate (PNR) and substrate-induced respiration (SIR), in a wide range of agricultural soils and detected the abundance of soil bacteria, fungi, ammonia-oxidizing bacteria and archaea. The toxicity thresholds of 10% and 50% effective concentrations (EC₁₀ and EC₅₀) for PNR varied by 32.18- and 38.66-fold among different soils, while for SIR they varied by 391.21- and 16.31-fold, respectively. Regression model analysis indicated that for PNR, CEC as a single factor explained 27% of the variation in EC₁₀, with soil clay being the key factor explaining 47.3% of the variation in EC₅₀. For SIR, organic matter and pH were found to be the most vital predictors for EC₁₀ and EC₅₀, explaining 34% and 61.1% of variation, respectively. In addition, extended aging time was found to significantly attenuate the toxicity of Cr on PNR. SIR was mainly driven by total bacteria rather than fungi, while PNR was driven by both AOA and AOB. These results were helpful in deriving soil Cr toxicity threshold based on microbial processes, and provided a theoretical foundation for ecological risk assessments and establishing a soil environmental quality criteria for Cr.

Kinetics of Chromium Reduction Associated with Varying Characteristics of Agricultural Soils

Chromium (Cr)(VI) is carcinogenic; thus, the excessive presence of Cr(VI) in soils can pose potential risks to water quality, food safety, and human health. The kinetics of Cr(VI) reduction in soils are important for assessing the fate of Cr in the environment. The present study tested physio-chemical and microbial properties in twenty-eight agricultural soils collected in Taiwan to evaluate the relationship between the reduction rate of Cr(VI) and soil properties, using 49-day incubation at 25 °C. At the beginning of incubation, 100 mg Cr(VI) kg−1 was spiked into the soils. The reduction of Cr(VI) was described by first-order kinetics at a significant level (p < 0.05) for the tested soils. The rate constant (k) of Cr(VI) reduction ranged from 0.01 to 4.21 day−1. In addition, the k value significantly increased with organic carbon (OC) and cation exchange capacity, but significantly decreased with increasing pH and dithionite-citrate-bicarbonate extractable Mn (Mnd). However, a predictive model using stepwise regression analysis indicated that the k value of the kinetics was controlled by OC, dissolved organic carbon (DOC), and Mnd, thereby identifying the complex interactions between Cr(VI) reduction and soil factors in the humid tropics.

The pH-sensitive sorption governed reduction of Cr(VI) by sludge derived biochar and the accelerating effect of organic acids

Reduction coupling immobilization is one of the most commonly adopted strategies for the remediation of Cr(VI) contamination. Biochar is a carbon-rich material with abundant active functional groups for sorption and reduction reactions. In previous reports, phytomass derived biochars and organic functional groups have been emphasized, while the performance of sludge derived biochar (SBC) has often been understated. In the present study, a 30 d kinetic study proved that the removal route involved the sorption of Cr(VI), reduction to Cr(III) and immobilization of Cr(III), and that the sorption process was the primary and rate determining step. As a result of the SBC alkalinity, the solution pH increased, and sorption was largely inhibited, which then governed the overall removal ratio. The FTIR spectra suggested the involvement of hydroxyls in these processes. Low molecular weight organic acids accelerated the removal process in the early phase and improved the reduction process.

Aging shapes Cr(VI) speciation in five different soils

To make sound decisions regarding management of heavy metal contamination in soils, it is necessary to understand contaminant transformations over extended periods. In this study, sequential extraction methods were applied to quantify the changes of Cr fractions [available Cr(VI), immobile Cr(VI) and immobile Cr(III)] in five contrasting soils spiked with Cr(VI) over a 240-day incubation. Results showed that available Cr(VI) in soils continually decreased during aging, with a sharp decline occurring in the first 30 days. The best fit of available Cr(VI) data was obtained using an Elovich model for Brunisol and Anthrosol-1, a fractional power model for Anthrosol-2, and a pseudo first-order kinetic model for Luvisol-1 and Luvisol-2. After aging for 240 days, immobile Cr(VI) increased by 4.5-31% and immobile Cr(III) increased by 68-95% of total spiked Cr(VI) in Brunisol, Anthrosol-1 and Anthrosol-2. The two Luvisol soils had relatively high reduction rates with no Cr(VI) immobilized. A multireaction model was developed in MATLAB Simulink toolbox to describe transformation flow rates among soluble Cr(VI), adsorbed Cr(VI), immobilized Cr(VI) and immobilized Cr(III) in soils with aging. We conclude that (i) Cr(VI) reduction and immobilization were occurring concurrently in soils and competing for available Cr(VI) species; (ii) Cr(VI) reduction is favored by low soil pH and high organic carbon, while Cr(VI) immobilization occurs with cations (such as Ca²⁺) and Fe oxides.

Continuous flooding stimulates root iron plaque formation and reduces chromium accumulation in rice (Oryza sativa L.)

Chromium (Cr) contamination in rice poses a serious threat to human health. Therefore, we conducted pot experiments to investigate the influence of water management regimes on the formation of iron plaque on rice roots, and its effect on the accumulation and translocation of Cr in rice grown on contaminated soil. The results showed that water management regimes, including continuous and intermittent flooding, exerted notable effects on soil solution concentrations of Cr(VI) and Cr(III) through changes in redox potential, pH, and dissolved Fe(II) concentrations. In particular, 69.2%-71.8% of Cr(VI) was reduced to Cr(III) under continuous flooding, whereas only 33.3%-38.6% was reduced under intermittent flooding conditions. Additionally, continuous flooding created a rhizosphere environment favorable to the formation of iron plaque. The amount of iron plaque formed increased by 28.2%-47.2% under continuous flooding conditions as compared with that under intermittent flooding conditions. Moreover, compared with intermittent flooding, under continuous flooding, more Cr (18.0%-23.9%) was adsorbed in the iron plaque, thereby sequestering Cr and reducing its mobility. The Cr concentrations in rice root, straw, husk, and grain under continuous flooding conditions were, respectively, 32.0%-36.5%, 32.7%-36.3%, 34.2%-46.9%, and 25.4%-37.7% lower than those under intermittent flooding conditions. Therefore, continuous flooding caused a substantial decrease in the Cr concentrations in rice tissues, as well as an increased distribution of Cr in the iron plaque that acted as a barrier to reduce Cr transfer to the rice roots. These results indicate that continuous flooding irrigation was effective in minimizing the accumulation of Cr in rice plants, as it not only enhanced Cr(VI) reduction in the soil but also improved the blocking capacity of the iron plaque.

Remediation of soil contaminated with high levels of hexavalent chromium by combined chemical-microbial reduction and stabilization

In order to solve the problem of re-oxidation after chemical remediation of soil contaminated with high levels of hexavalent chromium (Cr(VI)), we investigated the use of chemical reduction combined with microbial stabilization to remediate soils contaminated with high Cr(VI) concentration. The leaching toxicity and microbial diversity of Cr(VI)-contaminated soil and the leaching toxicity of remediated soil oxidized by potassium permanganate (KMnO₄) were measured. The results indicate that the conversion rate of Cr(VI) reached 97 %, and the concentration of Cr(VI) in toxic solutions leaching can be reduced by 95 % after 40 days of microbial stabilization. Sterilization experiments showed that the reduction of Cr(VI) by microorganisms is stable. The results of microbial diversity analysis indicate that bacterial community changed more than fungal community during the reduction process of Cr(VI), and the species abundance and species evenness of bacteria decreased. Bacillus spp. and Halomonas spp. were the dominant species in this study.

Combined effects of rice straw-derived biochar and water management on transformation of chromium and its uptake by rice in contaminated soils

Chromium (Cr) pollution in soil is a global problem owing to its wide industrial use. The mobility, toxicity, and crop uptake of Cr depends on its valence state. Cr(VI) is highly mobile and toxic whereas Cr(III) is generally considered immobile and less toxic. We performed a pot experiment to investigate the combined effects of rice straw-derived biochar and water management on transformation of Cr and its uptake by rice in contaminated soils. The main plots had water management treatments of alternating wetting and drying (AWD) and continuous flooding (CF), and the subplots had three levels of straw biochar (0, 5, and 10 g kg^-1). The results showed that water management and the addition of biochar had a significant effect on the dynamics of soil redox potential (Eh), pH, dissolved organic carbon (DOC), and Fe(II) concentration. As these parameters are important factors affecting Cr transformation in paddy soils, the dynamics of the Cr(III) and Cr(VI) concentrations were clearly different under different treatments. The highest reduction of Cr(VI) was observed in the treatment with CF water management in combination with 10 g kg^-1 of biochar amendment, which resulted in a 62% reduction of Cr(VI) to Cr(III) in soil. The alterations in the oxidation state of Cr greatly affected its accumulation in the rice grains. The CF combined with 10 g kg^-1 of biochar treatment, caused the Cr concentration in rice grains to be 66.2% lower compared with that of the unamended control under AWD water management. Possibly owing to the reduction in phytotoxic effects of Cr(VI), the combined treatment showed an improvement in rice grain weight. In conclusion, the combination of 10 g kg^-1 of biochar amendment and CF water management may potentially be used in Cr-contaminated soil to mitigate the impacts of Cr contamination on rice production.

Enhanced biogeogenic controls on dichromate speciation in subsoil containment

In general, lab-based Cr (VI) reduction studies do not often corroborate the prevailing biogeochemical controls for on-site pollution abatement. To promulgate its importance, herein, we investigate the existing biogeogenic parameters of a contaminated site to attenuate the underground Cr (VI) toxicity. This study significantly assesses the speciation of dichromate by biogenic agents that are inherent and self-sustaining to treat the contaminated soil. Herein, a group of bacteria exposed to high concentrations of chromium (≥3500 mg/L) plays a vital role as an enhanced biogeogenic control for the detoxification of toxic Cr (VI). All identified bacteria were screened based on their ability to differentiate from extracellular speciation and harvested in a Cr (VI)-enriched molasses to achieve dichromate concentrations as low as 0.05 mg/L in 168 h. Under low O₂ condition, the bacterial growth rate and doubling time were monitored to establish the half-life period of Cr (VI) for adequate containment treatment. Furthermore, to understand the soil decontamination, Cr (VI) reactive transport was demonstrated to facilitate the contaminant reduction under both saturated and unsaturated groundwater conditions. Herein, Cr (VI) speciation to Cr (III) by the influence of abiogenic factors are unlikely or less probable as studied in existing geogenic conditions. Moreover, the evidence of biogenic reduction of Cr (VI) in microcosm suggests its effectiveness in enhanced detoxification of Cr (VI) up to ≤ 0.1 mg/L, within the reaction period of 144 h and 192 h, for saturated and unsaturated flow conditions, respectively.

Use of carbon stable isotopes to monitor biostimulation and electron donor fate in chromium-contaminated groundwater

Hexavalent chromium Cr(VI) is a common inorganic contaminant in industrial areas and represents a serious threat to human health due its toxicity. Here we report experimental results from a field-scale investigation of Cr(VI) bio-immobilization at Hanford 100H reservation, a U.S Department of Energy facility (Washington State, USA). Microbial Cr(VI) reduction was stimulated via injection of a¹³C-labeled sodium lactate solution into the high-permeability aquifer consisting of gravel and coarse sand sediments. Concentrations and carbon isotope ratios of metabolites, including dissolved inorganic carbon and total organic carbon, and compound-specific analysis of acetate and propionate, together with phospholipid fatty acids (biomass) have been analyzed to help provide an understanding of the predominant redox processes accompanying Cr(VI) reduction. Results of our study indicate that the injection of an electron donor caused a sharp decrease of Cr(VI) concentration from ∼32 to ∼10 nM. Cr(VI) reduction was associated with a decrease in the concentration of carboxylic acids, such as lactate (∼6 mM to undetectable), propionate (∼9 mM to undetectable), and acetate (∼6 mM to undetectable), as well as dissolved inorganic carbon (30-10 mM C). Carbon isotope data indicate carbon transfers from the original substrate to organic byproducts and mineralized carbon. Concentrations of metabolites and stable isotope data as well as carbon isotope mass balance calculations were used to monitor biologically mediated reduction of Cr(VI).

The toxicity thresholds of metal(loid)s to soil-dwelling springtail Folsomia candida-A review

Increasing concentrations of metals in soil have posed a serious threat to the soil environment. The control and evaluation of soil metal hazards demand the establishment of soil ecological criteria, which is mainly based on the obtainment of toxicity thresholds. As the most typical representative of soil-dwelling springtails, Folsomia candida performs numerous essential ecological functions in soil and has been extensively used to investigate metal toxicity effects and thresholds. This review outlined the current state of knowledge on the metal toxicity thresholds to Folsomia candida, including (1) toxicity thresholds of soil metals for the different endpoints, (2) the influence factors of metal toxicity thresholds including the test conditions, the chemical forms of metal, the soil physicochemical properties, aging time and leaching, (3) the bioavailable fractions predicting metal toxicity thresholds, (4) the internal threshold of metals. To conclude, several recommendations for future research are given to obtain the more reliable toxicity thresholds and further supplement the toxicity data of metals to Folsomia candida.

The Kinetics of Aging and Reducing Processes of Cr(VI) in Two Soils

To investigate the aging process and reduction of Cr(VI) in two soils. The adsorption behavior of the soils demonstrated that the paddy soil had higher adsorption capacity for Cr(VI), but the capacity was lower for Cr(III), which contrasted the results for fluro-aquic soil. The mobilizable Cr was assessed using EDTA extraction. The results suggested that the reduction of Cr(VI) to Cr(III) and the aging process occurred simultaneously. A simplified kinetic model was established and the rate constants of the reduction and aging processes were obtained. The aging process and reduction of Cr(VI) were faster in the paddy soil, due to a higher adsorption capacity and stronger reducing ability, as indicated by the organic matter and amorphous Fe oxides. The Cr(III) aging was faster in the fluro-aquic soil due to the low solubility of Cr(III) at a high pH. The modeling study provides a fundamental understanding of the dynamics of Cr mobility in a complicated soil system.

Toxicity of exogenous hexavalent chromium to soil-dwelling springtail Folsomia candida in relation to soil properties and aging time

Chromium (Cr) is a well-known toxic metal, but studies on Cr 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 chronic toxicity of Cr(VI) to survival and reproduction of model organism-Folsomia candida were evaluated in the laboratory studies. The results showed that compared to the soils aged only for 2 and 21 d, the concentrations inhibiting 50% reproduction (EC₅₀) significantly increased by 2.8-5.2 fold and 1.7-2.6 fold, the concentrations causing 50% mortality (LC₅₀) were higher than the highest test concentration in four soils aged for 150 d. Furthermore, the aging effects correlated significantly with soil amorphous Fe oxides. The EC₅₀ values of Cr significantly differed in ten soils aged for 150 d, ranging from 27 to 512 mg kg^-1, which were associated with the variations in reduction and sorption capacity in different soils. Regression analysis indicated that soil clay was the most important single factor predicting soil Cr toxicity to reproduction, and the inclusion of cation exchange capacity in the clay regression could best explain the toxicity variance (87.2%). Additionally, soil pH, organic matter and amorphous Fe oxides could also well explain the toxicity variance (>55%).

A Bacillus strain TCL isolated from Jharia coalmine with remarkable stress responses, chromium reduction capability and bioremediation potential

Microbial reduction of Cr(VI) to Cr(III) can mitigate environmental chromium toxicity. A chromium, cadmium and nickel tolerating strain TCL with 97% 16S rRNA gene sequence homology to Bacillus cereus was isolated from a derelict open-cast, Tasra Coalmine Lake of Jharia, India. It could tolerate up to Cr₂₀₀₀ [2,000 mg L^-1 Cr(VI)] and completely reduce Cr₂₀₀ within 16 h under heterotrophic condition. TCL grown in ≥ Cr₅₀₀ exhibited multifarious stress responses particularly in its prolonged lag-phase, like cell aggregation, up to two-fold elongation, increased exopolysaccharide production, and stress enzyme activities. These were relieved by increasing inoculum size or nutrient content. Chromium reduction was constitutive, with maximum activities detected in loosely-bound exopolysaccharides and membrane fractions, followed by cytoplasm and spent media. Cr(VI) was efficiently reduced to Cr(III) and >90% was released in spent media. Cells also expressed Cr-induced active efflux pumps. Growing cells or its crude enzyme extracts could efficiently reduce Cr(VI) in diverse temperatures (15-45 °C), pH (5-9); and in presence of other metals (Cd, Cu, Mo, Ni, Pb), oxyanions (SO₄^-2, NO₂^-), and metabolic inhibitors (phenol, NaN₃, EDTA). Growth and reduction were also detected in nutrient-limited minimal salt media, and contaminated leather industry effluent thereby making TCL a potential candidate for bioremediation.

Wetland plant microbial fuel cells for remediation of hexavalent chromium contaminated soils and electricity production

The plant microbial fuel cell (PMFC) is a novel technology which integrates plants, microbes, and electrochemical elements together to create renewable energy. However, information regarding using the PMFC system to remediate metal-contaminated soils is still limited. In this study, we evaluate the potential of PMFC systems to remediate soils polluted by Cr(VI). We compare different plants and different electrode materials with regard to their electricity generation and Cr(VI) removals under different soil Cr(VI) concentrations. In PMFC systems, the soil pH was transformed from slightly acidic to neutral, and the electrical conductivity was reduced during operation. The removal efficiency of Cr(VI) in soils could reach 99%, and the total Cr of soils could also be reduced. The closed circuit voltage of PMFC systems of Chinese pennisetum using the graphite carbon felt as the electrodes could reach the daily average value of 469.21 mV. PMFC systems have successfully demonstrated the ability to remove Cr(VI) from soils collected from actual metal-contaminated sites. Our results suggest that using PMFCs to remediate contaminated soils is promising, and the effects of decontamination are mostly contributed by bioelectrochemical processes and plant uptake.

Molecular structure-reactivity correlations of humic acid and humin fractions from a typical black soil for hexavalent chromium reduction

Different soil humus fractions are structurally distinct from each other molecularly, however, the relationship between their microscopic molecular structures and the macroscopic reduction of Cr(VI) is still unknown, especially for the humin fraction. In this study, different humus fractions (HA, humic acid; HMi, humin linked to iron oxides; HMc, humin linked to clay; and HMr, humin residue) were sequentially extracted from a typical black soil and well characterized. It was found that HA, HMi and HMc were the same type of humus with similar molecular structures, while HMr was structurally different from the other fractions with a high cellulose content. The removal rate of Cr(VI) in solution decreased with progressive humus fractionation, namely, HA > HMi > HMc > HMr. Based on the two-dimensional correlation spectroscopic analysis (2DCOS) of the FTIR data, the changing functional groups of all humus fractions during reacting with Cr(VI) followed a similar order: carboxyl > phenol > hydroxyl > methyl > methylene. According to the correlation analysis, Cr(VI) reduction rates by different humus fractions were mainly determined by the content of phenol (R² = 0.99) instead of carboxyl (R² = 0.28). Except for HMr, the Cr(VI) reduction rates of different humus fractions were also positively correlated with surface and bulk polarity (R² = 0.98 and 0.99) but not with aromaticity or aliphaticity (R² = 0.21).

The influence of pH, co-existing ions, ionic strength, and temperature on the adsorption and reduction of hexavalent chromium by undissolved humic acid

The retention of Cr(VI) in subsurface environment is highly dependent on humic acid (HA), however, the undissolved form is poorly investigated, the amount of which can be of two magnitude higher compared with the dissolved one in soils and sediments. In this study, the effects of time, initial concentration, pH, ionic strength, ion species and temperature on the adsorption and reduction respective processes by undissolved self-extracted peat soil HA from Northeast China (EHA) and from Sigma Aldrich (CHA) were investigated by batch experiments. Cr(VI) removal rates by EHA were higher than CHA and the maximum Cr(VI) removal amount for EHA and CHA were 0.77 (±0.01) and 0.61 (±0.02) mmol/g. Of these, 98% and 54% were reduced to Cr(III) by EHA and CHA respectively, which were related to the phenolic group content of HA. With time, the adsorbed Cr(VI) on HA increased to a maximum level (equilibrium) beyond which Cr(VI) reduction dominated the removal process. Cr(VI) adsorption and reduction by undissolved HA increased as pH decreased. Co-existing ion species had varying effect on Cr(VI) adsorption and indirectly on reduction especially divalent cations which was suggestive of cation bridging between Cr anions and ionized carboxyl group of HA. The positive effect of ionic strength (Ca²⁺) on Cr(VI) adsorption through complexation corroborated the cation bridge effect of divalent cations. Temperature increased both Cr(VI) adsorption (complexation) and reduction with enhancing reduction rate constants and partitioning. ΔH^o, ΔS^o, and ΔG⁰ parameters showed that Cr(VI) adsorption and reduction processes were endothermic, irreversible and spontaneous.

The role of different functional groups in a novel adsorption-complexation-reduction multi-step kinetic model for hexavalent chromium retention by undissolved humic acid

Undissolved humic acid (HA) has a great retention effect on the migration of hexavalent chromium [Cr(VI)] in soil, and HA functional groups play a predominant role in this process. However, the coupled mode between Cr(VI) retention and HA functional groups reaction is still unclear. In this study, it was found that a fair amount of Cr on HA existed in the forms of ion exchangeable and binding Cr(VI) during the reaction resulting from the ion exchange adsorption and complexation of Cr(VI). According to the results of two-dimensional correlation spectroscopic analysis (2DCOS), HA functional groups participated in the reaction with Cr(VI) in the order of carboxyl ≈ chelated carboxyl > phenol > polysaccharide > methyl, and all the functional groups were more likely to be located at aromatic domains. Based on the results of XPS spectra, rather than to be oxidized by Cr(VI), carboxyl more tended to be complexed by chromium, which is regarded as the precondition for Cr(VI) reduction. Phenol, polysaccharide and methyl with distinct reaction activities successively acted as major electron donors for Cr(VI) reduction in different reaction stages. Consequently, it was determined that the retention of Cr(VI) by undissolved HA followed an adsorption-complexation-reduction mechanism, and based on this, a multi-step kinetic model with multiple types of complexation/reduction sites was developed to simulate the retention processes resulting in a much better fitting effect (R² > 0.99) compared with traditional first-order and second-order kinetic models (R² < 0.95). This demonstrated that the multi-step kinetic model is of great potential in accurately simulating the migration and transformation of Cr(VI) in soil environment.

Reduction mechanism of hexavalent chromium by functional groups of undissolved humic acid and humin fractions of typical black soil from Northeast China

Soil organic matters (SOM) have a great retention effect on Cr(VI) migration in subsurface environment, which act as the main electron donors for Cr(VI) reduction; however, Cr(VI) reduction mechanism by different SOM fractions is still unclear, such as undissolved humic acid (HA) and humin (HM). In this study, HA and HM fractions extracted from typical black soil from Northeast China were used to investigate the reaction mechanism between humus functional groups and Cr(VI). According to the results, phenol and hydroxyl were determined as the main electron donors for Cr(VI) reduction by HA and HM instead of carboxyl and carbonyl, which were more likely involved in Cr complexation. Furthermore, Cr(VI) reduction was more dependent on aromatic carbon, rather than aliphatic carbon, and functional groups on the particle surfaces of HA and HM were much more active for Cr(VI) reduction than their interior part. Additionally, HM was found to have a relatively low Cr(VI) reduction capability compared with HA resulting from its high content of cellulose structures that are quite resistant to Cr(VI) oxidation. These results suggest that in the soil environment, undissolved HA tends to play a much more important role than HM in Cr(VI) reduction and retention in the condition that their mass contents are comparable.

A novel method of three-dimensional hetero-spectral correlation analysis for the fingerprint identification of humic acid functional groups for hexavalent chromium retention

Two-dimensional hetero-spectral correlation analysis has been widely used for the interpretation of spectral changes of humic substances involved in various environmental processes. However, when three different types of spectroscopies are utilised, only a pairwise correlation can be achieved. In order to overcome this problem, a novel method of three-dimensional hetero-spectral correlation analysis with scaling technique was developed in this study, which can further establish a direct correlation between three different types of spectroscopies, including FTIR, 13C CP/MAS NMR, and XPS. The proposed method was applied to the fingerprint identification of undissolved humic acid functional groups for Cr(vi) retention, which is one of the most important points for understanding the migration and transformation of Cr(vi) in a subsurface environment. The results indicated that mainly free and dissociated carboxylic groups, phenolic groups, and polysaccharide participated in the reaction with Cr(vi), and these functional groups were mainly located at aromatic domains. Besides, the variations of functional groups were related to the reduction of Cr(vi), and the reduced Cr(iii) mainly bound to aromatic domains. The successful application of the proposed method demonstrated that it can serve as a promising tool for further investigations concerning more complicated environmental processes and even other scientific fields by supplying more detailed, reliable and visualised spectral information.

Mechanism study of humic acid functional groups for Cr(VI) retention: Two-dimensional FTIR and 13C CP/MAS NMR correlation spectroscopic analysis

Undissolved humic acid (HA) is known to substantially effect the migration and transformation of hexavalent chromium [Cr(VI)] in soils. The mechanisms of Cr(VI) retention in soils by undissolved HA have been reported; however, past studies are inconclusive about the types of HA functional groups that are involved in Cr(VI) retention and the retention mechanisms. Utilizing a two-dimensional correlation spectroscopy (2DCOS) analysis for FTIR and ¹³C CP/MAS NMR, this study investigated the variations of HA function groups and molecular structures after reactions with aqueous Cr(VI) under different pH conditions. Based on the changing sequence of functional groups interpreted from the 2DCOS results, a four-step mechanism for Cr(VI) retention was determined as follows: (1) electrostatic adsorption of Cr(VI) to HA surface, (2) complexation of adsorbed Cr(VI) by carboxyl and ester, (3) reduction of complexed Cr(VI) to Cr(III) by phenol and polysaccharide, and (4) complexation of reduced Cr(III) by carboxylic groups. These functional groups that are involved in Cr(VI) retention were determined to occur in aromatic domains.

Hexavalent chromium availability and phytoremediation potential of Cichorium spinosum as affect by manure, zeolite and soil ageing

Cichorium spinosum (spiny chicory) is a wild edible vegetable, and a possible suitable species for Cr(VI)-phytoremediation. There are three approaches for altering Cr(VI) dynamics: reduction to Cr(III) by organic matter addition, soil ageing, and Cr(VI) retention by high binding capacity materials added to soil, e.g., zeolite. Our aim was to assess spiny chicory as a phytoremediation species in relation to these three methods of altering Cr(VI) soil dynamics. There were 5 treatments: control (C); soil with 100 mg kg-1 Cr(VI) (S); soil with zeolite plus 100 mg kg-1 Cr(VI) (Z); soil with manure plus 100 mg kg-1 Cr(VI) (M); and soil added with 100 mg kg-1 Cr(VI) one year before this experiment (AS, "aged soil"). In soil, Cr(VI) was higher at S, while Z, M and AS were lower. In plant, Cr(VI) at Z, S, and AS were similar and significantly higher than M. This indicates that added manure decrease Cr(VI) availability to chicory due to the formation of organometallic complexes. However, chicory uptake amounted to 0.26-0.40 kg Cr(VI) ha-1 for Z, S, and AS, while uptake at M was lower. In conclusion, manure addition was more successful in decreasing Cr(VI) bioavailability, but it also slowed Cr(VI)-phytoremediation process.

Production of a microcapsule agent of chromate-reducing Lysinibacillus fusiformis ZC1 and its application in remediation of chromate-spiked soil

Lysinibacillus fusiformis ZC1 is an efficient Cr(VI)-reducing bacterium that can transform the toxic and soluble chromate [Cr(VI)] form to the less toxic and precipitated chromite form [Cr(III)]. As such, this strain might be applicable for bioremediation of Cr(VI) in soil by reducing its bioavailability. The study objective was to prepare a microcapsule agent of strain ZC1 for bioremediation of Cr(VI)-contaminated soil. Using a single-factor orthogonal array design, the optimal fermentation medium was obtained and consisted of 6 g/L corn flour, 12 g/L soybean flour, 8 g/L NH₄Cl and 6 g/L CaCl₂. After enlarged fermentation, the cell and spore densities were 5.9 × 10⁹ and 1.7 × 10⁸ cfu/mL, respectively. The fermentation products were collected and embedded with 1 % gum arabic and 1 % sorbitol as the microcapsule carriers and were subsequently spray-dried. Strain ZC1 exhibited viable cell counts of (3.6 ± 0.44) × 10¹⁰ cfu/g dw after 50-day storage at room temperature. In simulated soil bioremediation experiments, 67 % of Cr(VI) was reduced in 5 days with the inoculation of this microcapsule agent, and the Cr(VI) concentration was below the soil Cr(VI) standard level. The results demonstrated that the microcapsule agent of strain ZC1 is efficient for bioremediation of Cr(VI)-contaminated soil.

Reduction process of Cr(VI) by Fe(II) and humic acid analyzed using high time resolution XAFS analysis

The initial reduction behavior of Cr(VI) to Cr(III) has not been clearly understood due to its rapid reduction reaction. In order to study the reduction process of Cr(VI) in detail, we applied quick X-ray absorption fine structure (QXAFS) analysis to observe how Cr(VI) was reduced to Cr(III) by Fe(II) and humic acid (HA) with time. The Cr(VI) concentration was analyzed every 60s, and the plots of ln(Cr(VI)/Cr(VI)0) versus time were used to evaluate the reduction process based on their linearity. Reduction by Fe(II) showed a linear relation, whereas reduction by HA showed a nonlinear relation. With combined Fe(II) and HA, the linearity was unlike those of Fe(II) and HA individually. The reduction rate was not constant. The structure of Fe(II) produced by HA during the Cr(VI) reduction was investigated by using Mössbauer spectroscopy, which showed that Fe(II) produced by HA reduction of Fe(III) had the same structure as the initial Fe(II). HA can reduce Fe(III) back to Fe(II), and reproduced Fe(II) reduces Cr(VI). For Cr(VI) reduction by combined Fe(II) and HA, each reductant contribute differently: Fe(II) directly contributes to the Cr(VI) reaction, whereas HA reduces both Cr(VI) and Fe(III).

Effects of alternating wetting and drying versus continuous flooding on chromium fate in paddy soils

Anthropogenic chromium (Cr) pollution in soils poses a great threat to human health through the food chain. It is imperative to understand Cr fate under the range of conditions suitable for rice growth. In this study, the effects of irrigation managements on dynamics of porewater Cr(VI) concentrations in rice paddies and Cr distribution in rice were investigated with pot experiments under greenhouse conditions. Soil redox potential in continuous flooding (CF) treatments showed that reducing conditions remained for the whole duration of rice growing period, while soil redox potential in alternating wetting and drying (AWD) treatments showed that soil conditions alternately changed between reducing and oxic. As soil redox potential is an important factor affecting Cr(VI) reduction in paddy soils, dynamics of Cr(VI) concentration were clearly different under different irrigation managements. In CF treatments, porewater Cr(VI) concentrations decreased with time after planting, while in AWD treatments porewater Cr(VI) concentrations were increased and decreased alternately response to the irrigation cycles. Chromium(VI) concentrations in the CF treatments were lower than those in AWD treatments for most part of rice-growing season. Moreover, Cr concentrations in rice tissues were significantly influenced by irrigation with relatively higher values in the AWD treatments, which might be attributed to the higher porewater Cr(VI) concentrations in AWD treatments. Therefore, it would be better to use CF than AWD management in Cr-contaminated paddy soils to reduce Cr accumulation in rice, and thus to reduce the potential risk to human health.

Health risk to residents and stimulation to inherent bacteria of various heavy metals in soil

The toxicities and effects of various metals and metalloids would be misunderstood by health risks based on their concentrations, when their effects on bacterial and ecological functions in soil are disregarded. This study investigated the concentrations and health risks of heavy metals, soil properties, and bacterial 16S rRNA gene in soil around the largest fresh water lake in North China. The health risks posed by Mn and As were higher than those of other heavy metals and metalloids. Mn, As, and C were significantly correlated with the bacterial species richness indices. According to canonical correspondence analysis, species richness was mainly affected by Mn, Pb, As, and organic matter, while species evenness was mainly affected by Mn, pH, N, C, Cd, and Pb. Covariable analysis confirmed that most effects of metals on bacterial diversity were attributed to the combined effects of metals and soil properties rather than single metals. Most bacteria detected in (almost) all soil were identified as Gammaproteobacteria. Specific bacteria belonging to Proteobacteria (Gamma, Alpha, Epsilon, and Beta), Firmicutes, Actinobacteria, Cyanobacterium, Nitrospirae, and Fusobacterium were only identified in soil with high concentrations of Mn, Pb, and As, indicating their remediation potency. Bacterial abilities and mechanisms in pollutant resistance and element cycling in the region were also discussed.

Chromium resistance of dandelion (Taraxacum platypecidum Diels.) and bermudagrass (Cynodon dactylon [Linn.] Pers.) is enhanced by arbuscular mycorrhiza in Cr(VI)-contaminated soils

In a greenhouse pot experiment, dandelion (Taraxacum platypecidum Diels.) and bermudagrass (Cynodon dactylon[Linn.] Pers.), inoculated with and without arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis, were grown in chromium (Cr)-amended soils (0 mg/kg, 5 mg/kg, 10 mg/kg, and 20 mg/kg Cr[VI]) to test whether arbuscular mycorrhizal (AM) symbiosis can improve Cr tolerance in different plant species. The experimental results indicated that the dry weights of both plant species were dramatically increased by AM symbiosis. Mycorrhizal colonization increased plant P concentrations and decreased Cr concentrations and Cr translocation from roots to shoots for dandelion; in contrast, mycorrhizal colonization decreased plant Cr concentrations without improvement of P nutrition in bermudagrass. Chromium speciation analysis revealed that AM symbiosis potentially altered Cr species and bioavailability in the rhizosphere. The study confirmed the protective effects of AMF on host plants under Cr contaminations.

Chromium-resistant bacteria promote the reduction of hexavalent chromium in soils

Knowledge of the chromium (Cr) redox process in soil is important in addressing Cr bioavailability and risk assessment of contaminated soils. In this study, seven representative agricultural soils with different physicochemical properties were used to investigate the importance of microbially mediated Cr(VI) reduction and the response of soil bacterial community to Cr contamination. Chromium application increased soil bacterial diversity in Periudic Argosols, Calcaric Regosols, Stagnic Anthrosols, Mollisols, Typic Haplustalfs, and Ustic Cambosols, with an exception of Udic Ferrisols. The soil bacterial community responded to Cr contamination through changes in bacterial community structure, with Cr-resistant bacteria becoming the dominant species, and the percentage of Cr-resistant bacteria of total cultivable bacteria was 89.9, 75.2, 92.8, 65.3, 72.8, 77.3, and 65.4%, respectively, for Periudic Argosols, Udic Ferrisols, Calcaric Regosols, Stagnic Anthrosols, Mollisols, Typic Haplustalfs, and Ustic Cambosols. , , , , , , , , , and were identified as the Cr-resistant bacteria. Moreover, our results demonstrated that microbial reduction was an important Cr(VI) reduction pathway, and the relative contribution of microorganisms to Cr(VI) reduction was 14.4, 44.0, 20.6, 34.9, 21.9, 21.7, and 22.0%, respectively for Periudic Argosols, Udic Ferrisols, Calcaric Regosols, Stagnic Anthrosols, Mollisols, Typic Haplustalfs, and Ustic Cambosols. Soil properties, especially Fe(II) and soil particle distribution, affected the microbially mediated Cr(VI) reduction. These results provide useful information for the bioremediation of Cr-contaminated soils under a wide range of environmental conditions.

Prediction of the distribution coefficients of metals in Amazonian soils

Soil pollution with metals alters ecosystem structure and function, as well as poses risks to human health. In this study the distribution coefficients (Kd) were determined for Cd, Co, Cr, Cu, Hg, Ni, Pb, and Zn in 21 soils from eastern Amazonian via Freundlich adsorption isotherm. The stepwise multiple regression models were used to estimate the Kd values through the combined effect of soil attributes. Multiple regressions showed that Kd values were strongly correlated with the soil pH, CEC and Fe oxide contents. A close association between Kd of metals and soil properties was observed using three principal components (PCA) and Pearson's correlation. In the principal components analysis showed that concentration of metals was related to pH, Fe and Mn oxides contents, which in combination explained 90 percent of the variation. This probably happens because of greater affinity of those metals to Fe and Mn oxides formed predominantly under tropical conditions.

Model for evaluation of the phytoavailability of chromium (Cr) to rice (Oryza sativa L.) in representative Chinese soils

Anthropogenic chromium (Cr) pollution in soils poses a great threat to human health through the food chain. It is imperative to understand Cr phytoavailability to rice (Oryza sativa L.), which is a major staple food crop for the largest population of people on Earth. This study was aimed to establish a model for evaluation of the phytoavailability of Cr to rice in six representative Chinese soils based on soil properties. Simple correlation analysis indicated that Cr concentration in polished rice was significantly correlated with total Cr, Mehlich-3 extractable Cr, and Cr(VI) in soil. Stepwise multiple regression analysis also demonstrated that the Cr phytoavailability was strongly correlated with soil total Cr, Mehlich-3 extractable Cr, Cr(VI) concentration, soil organic matter, Fe(II), and particle size distribution. Critical Cr concentrations in the six soils were evaluated for rice based on the maximum safe level for daily intake of Cr. Mehlich-3 extractable Cr are the most suitable Cr thresholds for Periudic Argosols, Udic Ferrisols, Mollisols, and Ustic Cambosols with values of 1.54, 0.56, 0.42, and 2.18 mg kg(-1), respectively, while Cr(VI) are adequate thresholds for Calcaric Regosols and Stagnic Anthrosols with values of 0.68 and 0.84 mg kg(-1), respectively.