Effect of soil pH on the transport, fractionation, and oxidation of chromium(III)

Quantifying the mitigating effect of organic matter on heavy metal availability in soils with different manure applications: A geochemical modelling study

Manure is one of the main sources of heavy metal (HM) pollution on farmlands. It has become the focus of global ecological research because of its potential threat to human health and the sustainability of food systems. Soil pH and organic matter are improved by manure and play pivotal roles in determining soil HM behavior. Geochemical modeling has been widely used to assess and predict the behavior of soil HMs; however, there remains a research gap in manure applications. In this study, a geochemical model (LeachXS) coupled with a pH-dependent leaching test with continuously simulations over a broad pH range was used to determine the effects and pollution risks of pig or cattle manure separate application on soil HMs distribution. Both pig and cattle manure applications led to soil pH reduction in alkaline soils and increased organic matter content. Pig manure application resulted in a potential 90.5-156.0 % increase in soil HM content. Cattle manure did not cause significant HM contamination. The leaching trend of soil HMs across treatments exhibited a V-shaped change, with the lowest concentration at pH = 7, gradually increasing toward strong acids and bases. The dissolved organic matter-bound HM content directly increased the HM availability, especially for Cu (up to 8.4 %) after pig manure application. However, more HMs (Cr, Cu, Zn, Ni) were in the particulate organic matter-bound state than in other solid phases (e.g., Fe-Al(hydr) oxides, clay minerals), which inhibited the HMs leaching by more than 19.3 % after cattle manure application. Despite these variations, high HM concentrations introduced by pig manure raised the soil contamination risk, potentially exceeding 40 times at pH ±1. When manure is returned to the field, reducing its HM content and mitigating possible pollution is necessary to realize the healthy and sustainable development of circular agriculture.

Spatial distribution, chemical fractionation and risk assessment of Cr in soil from a typical industry smelting site in Hunan Province, China

The closure or relocation of many industrial enterprises has resulted in a significant number of abandoned polluted sites enriched in heavy metals to various degrees, causing a slew of environmental problems. Therefore, it is essential to conduct research on heavy metal contamination in the soil of industrial abandoned sites. In this study, soils at different depths were collected in a smelting site located in Hunan Province, China, to understand the Cr distribution, speciation and possible risks. The results revealed that the high-content Cr and Cr(VI) contamination centers were mainly concentrated near S1 (Sample site 1) and S5. The longitudinal migration law of chromium was relatively complex, not showing a simply uniform trend of decreasing gradually with depth but presenting a certain volatility. The vertical distribution characteristics of chromium and Cr(VI) pollution suggest the need for attention to the pollution from chromium slag in groundwater and deep soil layers. The results of different speciation of Cr extracted by the modified European Community Bureau of Reference (BCR) method showed that Cr existed primarily in the residual state (F4), with a relatively low content in the weak acid extraction state (F1). The correlation analysis indicated that Cr was affected by total Cr, pH, organic matter and total carbon during the longitudinal migration process. The RSP results revealed that the smelting site as a whole had a moderate level of pollution. Soil at depths of 2-5 m was more polluted than other soil layers. Consequently, it is necessary to treat the site soil as a whole, especially the subsoil layer (2-5 m). Health risk assessment demonstrated that the soil chromium pollution was hazardous to both adults and children, and the probability of carcinogenic and non-carcinogenic risk was relatively high in the latter group. As a result, children should be a group of special concern regarding the assessment and remediation of soil contaminated with Cr. This study can provide some insight into the contamination characteristics, ecological and health risks of chromium in contaminated soils and offer a scientific basis for the prevention and control of chromium pollution at abandoned smelting sites.

Nano-enabled agrochemicals: mitigating heavy metal toxicity and enhancing crop adaptability for sustainable crop production

The primary factors that restrict agricultural productivity and jeopardize human and food safety are heavy metals (HMs), including arsenic, cadmium, lead, and aluminum, which adversely impact crop yields and quality. Plants, in their adaptability, proactively engage in a multitude of intricate processes to counteract the impacts of HM toxicity. These processes orchestrate profound transformations at biomolecular levels, showing the plant's ability to adapt and thrive in adversity. In the past few decades, HM stress tolerance in crops has been successfully addressed through a combination of traditional breeding techniques, cutting-edge genetic engineering methods, and the strategic implementation of marker-dependent breeding approaches. Given the remarkable progress achieved in this domain, it has become imperative to adopt integrated methods that mitigate potential risks and impacts arising from environmental contamination on yields, which is crucial as we endeavor to forge ahead with the establishment of enduring agricultural systems. In this manner, nanotechnology has emerged as a viable field in agricultural sciences. The potential applications are extensive, encompassing the regulation of environmental stressors like toxic metals, improving the efficiency of nutrient consumption and alleviating climate change effects. Integrating nanotechnology and nanomaterials in agrochemicals has successfully mitigated the drawbacks associated with traditional agrochemicals, including challenges like organic solvent pollution, susceptibility to photolysis, and restricted bioavailability. Numerous studies clearly show the immense potential of nanomaterials and nanofertilizers in tackling the acute crisis of HM toxicity in crop production. This review seeks to delve into using NPs as agrochemicals to effectively mitigate HM toxicity and enhance crop resilience, thereby fostering an environmentally friendly and economically viable approach toward sustainable agricultural advancement in the foreseeable future.

Is Cr(III) re-oxidation occurring in Cr-contaminated soils after remediation: Meta-analysis and machine learning prediction

Whether Cr(III) in Cr(III)-containing sites formed after Cr(VI) reduction and stabilization remediation are re-oxidized and pose toxicity risks again has been a growing concern. In this study, 1030 data were collected to perform a meta-analysis to clarify the effects of various factors (oxidant type, soil and Cr(III) solid compound properties, aging conditions, and testing methods) on Cr(III) oxidation. We observed that the soil properties of clay, pH ≥ 8, the lower CEC capacity, easily reducible Mn content, and Cr(III) content, and the higher Eh value and Fe content can promote the re-oxidation of Cr(III). Publication bias and sensitivity analyses confirmed the stability and reliability of the meta-analysis. Subsequently, we used five machine learning algorithms to construct and optimize the models. The prediction results of the RF model (RMSE <1.36, R2 >0.71) with good algorithm performance showed that after ten years of remediation, the extractable Cr(VI) concentration in the soil was 0.0087 mg/L, indicating a negligible secondary pollution risk of Cr(III) re-oxidation. This study provides theoretical support for subsequent risk management and control after Cr(VI) soil remediation and provides a solution for the quantitative prediction of Cr(III) re-oxidation.

Environmental pollution by heavy metals within the area influenced by the Tungurahua volcano eruption - Ecuador

Volcanic eruptions can have long-lasting negative effects on nearby environments and communities, especially those relying on agriculture. The Tungurahua and Sangay volcanoes in Ecuador's highlands pose a significant risk to the region's agricultural economy and inhabitants. The most recent eruption of the Tungurahua volcano spanned from September 1999 to March 2016. Volcanic ash is known to contain both non-essential and essential elements for plant growth, but excessive amounts of the latter can also be toxic and disrupt physiological processes. Additionally, the use of pesticides and fertilizers introduces additional elements to agricultural soils, which can potentially transfer to edible plants and raise health concerns for consumers. Despite this, little is known about the impact of the Tungurahua volcano's latest eruption on soil deposition of micronutrient and heavy metals and their potential transfer to the food chain. To address this knowledge gap, we conducted a study to explore the impact of the Tungurahua volcano's latest eruption on soil deposition of Cd, Cr, Ni, Pb, As, Fe, Cu, Mn, and Zn, as well as the physicochemical characteristics of cultivated and uncultivated soil samples and vegetables in the cantons most affected by volcanic ash. According to our findings, the physicochemical properties, micronutrient, and heavy metal concentrations of both cultivated and uncultivated soils differ and, in some cases, exceeded the maximum limits established, which could affect the health of the soil as well as of human beings through the trophic chain. This study provides valuable information for monitoring the chemical and physical properties of soil and vegetables in areas affected by the Tungurahua volcanic activity and agriculture, aiding in understanding their impact on the environment, agriculture, and potential health risks associated with locally grown crops in rural Ecuadorian communities.

Enhanced remediation of Cr(VI)-contaminated soil by modified zero-valent iron with oxalic acid on biochar

Hexavalent chromium (Cr(VI)) is carcinogenic and widely presented in soil. In this study, modified zero-valent iron (ZVI) with oxalic acid on biochar (OA-ZVI/BC) was prepared using wet ball milling method for the remediation of Cr(VI)-contaminated soil. Microscopic characterizations showed that ZVI were distributed on the biochar uniformly and confirmed the enhanced interface interaction between biochar and ZVI by wet ball milling. Electrochemical analysis indicated the strong electron transfer ability and enhanced corrosion behavior of OA-ZVI/BC. Moreover, inhibitory efficiencies of Cr(VI) removal with the addition of 1,10-phenanthroline suggested abundant Fe2+ generation in OA-ZVI/BC, which might facilitate the reduction of Cr(VI) to Cr(III). Theory calculation further demonstrated the ZVI modified by oxalic acid was more susceptible to solid-solid interfacial reactions with Cr(VI), and more electrons were transferred to Cr(VI). When applied to Cr(VI)-contaminated soil, OA-ZVI/BC could passivate 96.7 % total Cr(VI) and maintained for 90 days. The toxicity characteristic leaching procedure (TCLP) and simple based extraction test (SBET) were used to evaluate the leaching toxicity and bioaccessibility of Cr(VI), respectively. The TCLP-Cr(VI) decreased to 0.11 mg·L-1 after OA-ZVI/BC treatment, much lower than that of soils with ZVI/BC and OA-ZVI remediation (1.5 mg·L-1 and 4.1 mg·L-1). The bioaccessibility of Cr(VI) reduced by 93.5 % after 3-month remediation. Sequential extraction showed that Cr fractions in the soil after OA-ZVI/BC remediation was converted from acetic acid-extractable (HOAc-extractable) to more stable forms (e.g., residual and oxidizable forms). Benefiting from the synergies of oxalic acid, biochar and wet ball milling, OA-ZVI/BC exhibited an excellent performance on the remediation of Cr(VI)-contaminated soil, whose mechanisms involved adsorption, reduction (Fe0/Fe2+, Fe2+/Fe3+) and co-precipitation. This study herein develops a promising ZVI technology in the remediation of heavy metal-contaminated soil.

Floristic survey, trace element transfers between soil and vegetation and human health risk at an urban industrial wasteland

This study aimed to determine the trace element accumulation in the soil and plants in an industrial wasteland and to estimate the extent of transfer to humans to measure the effects on and risks to vegetation and human health and find bioindicator plants representative of the levels of the main contaminants. In areas with the highest extractable trace element levels, we observed decreases in plant biodiversity explained by the disappearance of several families, favouring the coverage of tolerant species, such as Urtica dioica and Hedera helix. Trace elements were also found in the leaves of several plants, especially in a dominant species that is poorly studied, Alliaria petiolata. Indeed, this species had the highest contents of Zn (1750 mg.kg-1 DW), Ni (13.1 mg.kg-1 DW), and Cd (18 mg.kg-1 DW) found at the site and is a potential Zn bioindicator since its leaf contents were also representative of the Zn extractable contents in soil (R² = 0.94). The hazard quotient and carcinogen risk revealed that most of the site had an identified or possible risk, mainly due to Pb and As. Native species, especially A. petiolata, could be used in phytoextraction to manage and limit these human and environmental risks.

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.

A Study on Fire Retardant and Soundproof Properties of Stainless Steel EAF Reducing Slag Applied to Fiber Reinforced Cement Boards

In recent years, cases of the improper utilization of steel furnace slag have been widely reported, resulting in a crisis of nowhere for recycled resources such as inorganic slag. The misplacement of resource materials that originally had sustainable-use value not only has a great impact on society and the environment but also greatly reduces industrial competitiveness. To solve the dilemma of steel furnace-slag reuse, it is critical to find solutions to the stabilization of steelmaking slag under the innovative thinking of the circular economy. In addition to enhancing the reuse value of recycled resources, the balance between economic development and environmental impact is also crucial. The high-performance building material could provide a solution based on a high-value market. With the development of society and the increasing requirements for quality of life, the requirements for the soundproof and fireproof performance of lightweight decorative panels common in cities have gradually become popular. Therefore, the high performance of fire retardant and soundproofing could be the main development focus of high-value building materials to ensure circular economic feasibility. This study continues the research results of the application of inorganic re-cycled engineering materials in recent years, and the application of electric-arc furnace (EAF)-reducing slag to the development of base materials for reinforced cement boards, in order to complete the development of high-value panels with fireproof and sound-insulation properties in line with the engineering characteristics of the boards. The research results showed the optimization of the proportions of the cement boards with EAF-reducing slag as a raw material. The proportions of EAF-reducing slag to fly ash at ratios of 70:30 and 60:40 all met the requirements of ISO 5660-1 Class I flame resistance; the sound transmission loss in the overall frequency band can reach more than 30 dB, which is higher by 3-8 dB or more than the same board with similar specifications (such as 12 mm gypsum board) in the present building-materials market The products could be developed into building partitions and ceiling decoration boards with high performance in terms of fire retardant and soundproofing values, and also reduce the use of natural raw materials by more than 35%. The results of this study could meet environmental compatibility targets and contribute towards greener buildings. This model of circular economics would achieve energy reduction, emissions reductions, and be environmentally friendly.

A Study on the Fire-Retardant and Sound-Proofing Properties of Stainless Steel EAF Oxidizing Slag Applied to the Cement Panel

Because of incomplete recycling resource management and technology development, inorganic sludge and slag has been misused in Taiwan. The recycling of inorganic sludge and slag is a pressing crisis. Resource materials with a sustainable use value are misplaced and have a significant impact on society and the environment, which greatly reduces industrial competitiveness. To solve the dilemma of EAF oxidizing slag recycled from the steel-making process, it is important to find solutions to improve the stability of EAF oxidizing slags based on the innovative thinking of the circular economy. We can improve the value of recycling resources and solve the contradiction between economic development and environmental impact. The project team intends to investigate the development and application of reclaiming EAF oxidizing slags blended with fire-retardant materials, which will integrate R&D work from four different aspects. First, a verification mechanism is carried out to establish stainless steel furnace materials. Suppliers must be assisted in conducting quality management for EAF oxidizing slags to ensure the quality of the materials provided. Next, high-value building materials must be developed using slag stabilization technology, and fire-retardant tests must be conducted on the recycled building materials. A comprehensive evaluation and verification of the recycled building materials must be undertaken, and high-value green building materials must be produced with fire-retardant and sound-proofing characteristics. Integration with national standards and regulations can drive the market integration of high-value building materials and the industrial chain. On the other hand, the applicability of existing regulations to facilitate the legal use of EAF oxidizing slags will be explored.

Assessing the impact of lime on chromium migration in soil caused by basic chromium sulfate in tannery

Chromium (Cr) pollution is the primary pollution problem of the soil in tannery. However, the effect of tanning chemicals on Cr migration in soil has not been clearly elucidated. Column leaching tests were designed in this study to reveal the transport and transformation of Cr from basic chromium sulfate (BCS) into soil and the effects of lime on Cr migration and transformation. The results showed that BCS was mainly leached out in the state of Cr(VI) after entering the soil, and the Cr concentration in leachate decreased with the increase of the bulking thickness of the BCS. Compared with the soil absent of lime, the concentration of total Cr in the leachate from soil with lime decreased by 8.80-88.1%. The proportions of Cr in the residual fraction were generally increased in the soil with lime, whereas other fractions were decreased. The presence of lime can reduce the migration and toxicity of BCS in soil to a certain extent. The analysis of soil bacterial community showed that the relative abundance of Proteobacteria increased significantly with the exposure to BCS and the Burkholderiaceae was the dominant bacteria family in the BCS contaminated soil. Understanding the mobility of BCS and lime and the bacterial community in BCS contaminated soil is conducive to the risk assessment of the tannery site.

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.

Pollution and health-risk assessments of Cr-contaminated soils from a tannery waste lagoon, Hebei, north China: With emphasis on Cr speciation

In this paper, heavy metals (i.e., V, Cr, Co, Cu, Zn, Cd, Pb, and Sb) in soils from a tannery waste lagoon, Hebei, north China were investigated. Element concentrates were determined by a portable X-ray fluorescence in situ and an inductively coupled plasma mass spectrometry in the lab. Two sets of indexes, including geological accumulation index, contamination factor, and pollution load index, and hazard quotient and total carcinogenic risk were adopted to evaluate the pollution and health-risk of heavy metals. A scanning electron microscopy in conjunction with an energy dispersive X-ray spectroscopy and an X-ray photoelectron spectroscopy was used to observe chromium occurrence and speciation. With an average of 6493.11 mg/kg, chromium contents in the lagoon soils reached up to 12971.19 mg/kg, 211-times higher than the threshold of Chinese soils (61.00 mg/kg). Elevated Cr contents resulted in significantly high pollution and noncarcinogenic and carcinogenic risks in the studied area. Chromium in most soils occurred predominately as Cr³⁺ (60-74%), and to a lesser extent, Cr⁶⁺. The mechanism responsible for decreasing Cr⁶⁺ percentages in soils with increasing depth was summarized: Cr⁶⁺ favors aqueous environment; soil moisture decreased with increasing depth; in soils especially in the lower portion, Cr⁶⁺ was reduced by Fe⁰ and Fe², transforming into Cr³⁺ and Fe³⁺. In addition, the alkaline condition promoted Cr³⁺ to precipitate, resulting more Cr³⁺ absorbing in soils. The intimate association of Cr and Fe in soils (i.e., Cr mainly occurred in Fe oxides and dolomite) further confirmed our assumptions. A combined application of microorganism (e.g., Aeromonas hydrophila) and biochar (prepared from maize stalk or peanut shells) were recommended to alleviate Cr pollution in the soils.

Chromium toxicity and tolerance mechanisms in plants through cross-talk of secondary messengers: An overview of pathways and mechanisms

Environmental sources of chromium (Cr) such as solid waste, battery chemicals, industrial /waste, automotive exhaust emissions, mineral mining, fertilizers, and pesticides, have detrimental effects on plants. An excessive amount of Cr exposure can lead to toxic accumulations in human, animal, and plant tissues. In plants, diverse signaling molecules like hydrogen sulfide (H₂S) and nitric oxide (NO) play multiple roles during Cr stress. Consequently, the molecular mechanisms of Cr toxicity in plants, such as metal binding, modifying enzyme activity, and damaging cells are examined by several studies. The reactive oxygen species (ROS) that are formed when Cr reacts with lipids, membranes, DNA, proteins, and carbohydrates are all responsible for damage caused by Cr. ROS regulate plant growth, programmed cell death (PCD), cell cycle, pathogen defense, systemic communication, abiotic stress responses, and growth. Plants accumulate Cr mostly through the root system, with very little movement to the shoots. The characterization of stress-inducible proteins and metabolites involved in Cr tolerance and cross-talk messengers has been made possible due to recent advances in metabolomics, transcriptomics, and proteomics. This review discusses Cr absorption, translocation, subcellular distribution, and cross-talk between secondary messengers as mechanisms responsible for Cr toxicity and tolerance in plants. To mitigate this problem, soil-plant systems need to be monitored for the biogeochemical behavior of Cr and the identification of secondary messengers in plants.

Fate and transport of chromium in industrial sites: Dynamic simulation on soil profile

Direct discharge of chromium-containing waste water and improper disposal of waste residues in industrial sites may lead to the vertical migration of metals into aquifers, posing serious threat to soil-groundwater system. The heterogeneity in soil profile further aggravates the complexity and unpredictability of this transport process. However, topsoil was the main focus of most studies. Herein, the vertical transport and transformation of Cr in soils at different depths in three industrial sites (i.e., Shijiazhuang, Zhuzhou, and Guangzhou) were investigated to delineate Cr transport and retention characteristics under complex conditions. Regional and vertical differences in soil properties led to the specificity in Cr migration behaviors among these three sites. Correlation analysis showed that soil pH (r = -0.909, p < 0.05) and Fe content (r = 0.949, p < 0.01) were the major controlling factors of Cr(VI) migration and transformation in aquifers. Furthermore, the soil of Zhuzhou site showed the maximum adsorption capacity for Cr(VI) (0.225 mol/kg), and the strongest reduction ability of Cr(VI) was observed in the Guangzhou soil. Results of model-based long-term forecast indicated that the Cr(III) concentration in the liquid phase of Guangzhou subsoil could reach 0.08 mol/m³ within 20 years. Heavier rainfall condition exacerbated the contamination due to an increased pollutant flux and enhanced convection. Specially, Cr was fixed in the topsoil of Zhuzhou site with the formation of PbCrO₄ and presented least vertical migration risk. The conclusions above can provide scientific theoretical guidance for heavy metal pollution prevention and control in industrial contaminated regions.

Chromium toxicity, speciation, and remediation strategies in soil-plant interface: A critical review

In recent decades, environmental pollution with chromium (Cr) has gained significant attention. Although chromium (Cr) can exist in a variety of different oxidation states and is a polyvalent element, only trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)] are found frequently in the natural environment. In the current review, we summarize the biogeochemical procedures that regulate Cr(VI) mobilization, accumulation, bioavailability, toxicity in soils, and probable risks to ecosystem are also highlighted. Plants growing in Cr(VI)-contaminated soils show reduced growth and development with lower agricultural production and quality. Furthermore, Cr(VI) exposure causes oxidative stress due to the production of free radicals which modifies plant morpho-physiological and biochemical processes at tissue and cellular levels. However, plants may develop extensive cellular and physiological defensive mechanisms in response to Cr(VI) toxicity to ensure their survival. To cope with Cr(VI) toxicity, plants either avoid absorbing Cr(VI) from the soil or turn on the detoxifying mechanism, which involves producing antioxidants (both enzymatic and non-enzymatic) for scavenging of reactive oxygen species (ROS). Moreover, this review also highlights recent knowledge of remediation approaches i.e., bioremediation/phytoremediation, or remediation by using microbes exogenous use of organic amendments (biochar, manure, and compost), and nano-remediation supplements, which significantly remediate Cr(VI)-contaminated soil/water and lessen possible health and environmental challenges. Future research needs and knowledge gaps are also covered. The review's observations should aid in the development of creative and useful methods for limiting Cr(VI) bioavailability, toxicity and sustainably managing Cr(VI)-polluted soils/water, by clear understanding of mechanistic basis of Cr(VI) toxicity, signaling pathways, and tolerance mechanisms; hence reducing its hazards to the environment.

Comparison of struvite and K-struvite for Pb and Cr immobilisation in contaminated soil

Struvite is a value-added by-product recovered from phosphorus-rich wastewater treatment by adding magnesium. Struvite is mainly used as slow-release fertilisers containing phosphate that can form insoluble salts with certain heavy metals. Hence, struvite may have potential application as a phosphate remediation agent for the immobilisation of heavy metals in contaminated soil, while the related study is limited. Similarly, an analogue compound of struvite, K-struvite, may also have this value but has not been reported elsewhere. This study investigated the effect of struvite and K-struvite on the remediation of Cr-spiked and Pb-spiked soil. To evaluate the feasibility, the agent dosage and two quality parameters (particle size and purity) of struvite and K-struvite were considered for the experimental design and statically analysed by principal component analysis (PCA) and partial least squares (PLS). The results show that the dosage significantly impacts the immobilisation process, while the effect of particle size and purity are negligible. Struvite and K-struvite have similar performance on heavy metals immobilisation, and both are significant in Pb immobilisation (up to 96% of F5, stable fraction) and are beneficial for reducing the most mobilised fractions (F1 and F2) of Cr to lesser than 3%. Struvite and K-struvite share similar performance due to their similar atomic radius, and the different performance between Cr and Pb immobilisation can be explained by the strong hydrolysis trend of chromium ion, which may inhibit the binding of the phosphate and chromium. The kinetic study finds that all three variables positively impact the free chromium ion, and the immobilisation process is fast so unlikely to be kinetically limited. These findings of this project will provide insight into how the immobilisation process changes in response to the dosage and quality of struvite compounds.

Electrochemical conversion of chromium from tannery effluents for potential reuse in industrial applications

Electrochemical oxidation of trivalent chromium from leather tanning mud waste leachates (containing ca 6 g^.L^-1 Cr(III)) to its hexavalent form was carried out using a PbO_x/Pb anode electrode in a prototype small (0.4 L) cylindrical batch electrochemical reactor. The PbO_x/Pb anode was prepared by electrochemical anodization at constant current (75 mA cm^-2 for 30 min) in a sulfuric acid solution and characterized by the cyclic voltammetry technique to investigate the effect of pH on the process. It was found that at pH = 3, Cr(III) oxidation prevails over the competing water oxidation-oxygen evolution reaction (OER), hence increasing the efficiency of the process. A detailed study of pH (0-3), current density (12-24 mA cm^-2), and cell type (divided-undivided) effects on bulk electrolysis of Cr(III) leachates in the batch prototype reactor resulted in process optimization. At pH = 3, 12 mA cm^-2 and a cathode inserted in a porous diaphragm envelope, nearly 70% conversion was achieved at a nearly 60% current efficiency, among the highest in the previously reported literature. The method (further optimized with an ion-selective membrane separator) could offer an attractive route for tannery Cr(III) conversion to Cr(VI) for reuse as an etchant or electroplating agent.

Chromium Contamination and Health Risk Assessment of Soil and Agricultural Products in a Rural Area in Southern China

With the rapid development of industry, chromium (Cr) pollutants accumulate constantly in the soil, causing severe soil Cr pollution problems. Farmland Cr pollution hurts the safety of agricultural production and indirectly affects human health and safety. However, the current situation of Cr pollution in farmland soil and crops has not been detailed enough. In this study, the evaluation of Cr potential risk in soil-crop systems was conducted in a rural area that was affected by industry and historic sewage irrigation. Ten different crops and rhizosphere soils were sampled from four fields. The results showed that Cr contents in farmland soil exceeded the national standard threshold in China (>21.85%), and the Cr content in edible parts of some agricultural products exceeded that too. According to the PCA and relation analysis, the Cr accumulation in edible parts showed a significant correlation with soil Cr contents and available potassium contents. Except for water spinach, the target hazard quotient (THQ) of the other crops was lower than 1.0 but the carcinogenic health risks all exceeded the limits. The carcinogenic risks (CR) of different types of crops are food crops > legume crops > leafy vegetable crops and root-tuber crops. A comprehensive assessment revealed that planting water spinach in this area had the highest potential risk of Cr pollution. This study provided a scientific and reliable approach by integrating soil environmental quality and agricultural product security, which helps evaluate the potential risk of Cr in arable land more efficiently and lays technical guidelines for local agricultural production safety.

Sensitivity of Zea mays and Soil Microorganisms to the Toxic Effect of Chromium (VI)

Chromium is used in many settings, and hence, it can easily enter the natural environment. It exists in several oxidation states. In soil, depending on its oxidation-reduction potential, it can occur in bivalent, trivalent or hexavalent forms. Hexavalent chromium compounds are cancerogenic to humans. The aim of this study was to determine the effect of Cr(VI) on the structure of bacteria and fungi in soil, to find out how this effect is modified by humic acids and to determine the response of Zea mays to this form of chromium. A pot experiment was conducted to answer the above questions. Zea mays was sown in natural soil and soil polluted with Cr(VI) in an amount of 60 mg kg^-1 d.m. Both soils were treated with humic acids in the form of HumiAgra preparation. The ecophysiological and genetic diversity of bacteria and fungi was assayed in soil under maize (not sown with Zea mays). In addition, the following were determined: yield of maize, greenness index, index of tolerance to chromium, translocation index and accumulation of chromium in the plant. It has been determined that Cr(VI) significantly distorts the growth and development of Zea mays, while humic acids completely neutralize its toxic effect on the plant. This element had an adverse effect on the development of bacteria of the genera Cellulosimicrobium, Kaistobacter, Rhodanobacter, Rhodoplanes and Nocardioides and fungi of the genera Chaetomium and Humicola. Soil contamination with Cr(VI) significantly diminished the genetic diversity and richness of bacteria and the ecophysiological diversity of fungi. The negative impact of Cr(VI) on the diversity of bacteria and fungi was mollified by Zea mays and the application of humic acids.

Synergistic roles of montmorillonite and organic matter in reducing bioavailable state of chromium in tannery sludge

Organic matter (OM) has an excellent retention effect on stabilizing chromium (Cr), and functional groups on OM play a predominant role in this process. Based on this result, it is found that a considerable amount of Cr in tannery sludge is immobilized from ion exchangeable species into bound species, benefiting from complexing reaction with functional groups. Especially, the mentioned immobilizing process is enhanced in way of adding with montmorillonite (MMT) which performs adsorption reaction with Cr, as well as plays interaction with functional groups. The result is confirmed by employing density functional theory (DFT) analysis, suggesting the binding ability among Cr, functional groups, and MMT is stronger (- 77.36503 eV) than that of the system of Cr and MMT (- 61.29942 eV), indicating the synergetic roles of OM and MMT. This synergetic role could also be illustrated by a new peak (Cr-OH 20.1%) shown in XPS result. Meanwhile, DFT analysis emphasizes that functional groups on OM give the response for binding with Cr in the order of hydroxyl (-OH) > carboxyl (-COOH) > epoxy (-COC), and all the functional groups tend to donate electron to bind with Cr. In addition, the stabilizing process shows a better fitting effect with pseudo second-order kinetic model (R² > 0.94), indicating that exchangeable Cr mass transfer and chemical adsorption occur simultaneously.

Influence of compost application rate on nutrient and heavy metal mobility: Implications for stormwater management

Amending soils with compost has become increasingly common in stormwater management practices. Compost can be a source and sink for nutrients and heavy metals, and it is important to understand the effect of compost on pollutant leaching under different hydrologic conditions. The objectives of this study were (a) to quantify the distribution coefficient (K_d ) of PO₄ -P and metals (Cd, Cr, Cu, Ni, Pb, Zn) for compost-soil blends and (b) to examine how compost rate alters leaching patterns of nutrients (NH₄ -N, NO₃ -N, PO₄ -P) and metals from compost-soil blends. Material consisted of a sandy loam subsoil, a yard-waste compost, and compost-soil blends at 20 or 50% compost by volume. Materials were tested in sorption-desorption experiments using simulated stormwater (SW); columns with the materials were also leached with either SW or deionized (DI) water. As compost rate increased, the K_d decreased for PO₄ -P and Cr but increased for Cd, Cu, Ni, and Zn. The addition of compost reduced the sorption of PO₄ -P and Cr, potentially making it a source of these pollutants. Simulated stormwater did not increase the amount of pollutants retained compared with DI water for compost blends, except for 100% compost columns. Nitrate was the only constituent that had a negative removal efficiency, suggesting the compost was a source of NO₃ -N. Column media retained >70% of the metals from the added stormwater solution. These results suggest that yard-waste compost blends at ≤50% have the potential to retain certain pollutants from infiltrating stormwater, but this effect may decline after several storm events.

Interaction mechanism between chlorinated polyfluoroalkyl ether potassium sulfonate (F-53B) and chromium on different types of soil surfaces

The coexistence of per- and polyfluoroalkyl substances (PFASs) and heavy metals have been found in soils. However, the interaction between the combined pollutants in soils remains unclear. In this study, the adsorption processes of single and combined Cr(VI) and chlorinated polyfluoroalkyl ether potassium sulfonate (F-53 B) in red, yellow and black soils were simulated. When compared with the single F-53 B and Cr(VI), the adsorption amount of the combined F-53 B and Cr(VI) on soils changed with the types of soils. The interactions between F-53 B and Cr(VI) in soils affected their adsorption behavior. The adsorption of the combined F-53 B and Cr(VI) best fit second-order kinetics and the Freundlich equation. Moreover, aluminum and iron oxides are highly correlated with adsorption of F-53 B and Cr(VI). Both F-53 B and Cr(VI) can form complexes with aluminum and iron oxides through electrostatic interactions, but PFOS could be bridged with iron oxides to form an inner sphere complex and with aluminum oxides to form an outer sphere complex. The coexistence of F-53 B and Cr(VI) could change the fluorescent group of dissolved organic matter (DOM) in soils due to the complexation between F-53 B and DOM. In addition, F-53 B increased the acid-soluble portion of Cr and decreased its residual form, which promoted the environmental risk of Cr in soils.

Brassinosteroid Supplementation Alleviates Chromium Toxicity in Soybean (Glycine max L.) via Reducing Its Translocation

Chromium (Cr) phytotoxicity severely inhibits plant growth and development which makes it a prerequisite to developing techniques that prevent Cr accumulation in food chains. However, little is explored related to the protective role of brassinosteroids (BRs) against Cr-induced stress in soybean plants. Herein, the morpho-physiological, biochemical, and molecular responses of soybean cultivars with/without foliar application of BRs under Cr toxicity were intensely investigated. Our outcomes deliberated that BRs application noticeably reduced Cr-induced phytotoxicity by lowering Cr uptake (37.7/43.63%), accumulation (63.92/81.73%), and translocation (26.23/38.14%) in XD-18/HD-19, plant tissues, respectively; besides, improved seed germination ratio, photosynthetic attributes, plant growth, and biomass, as well as prevented nutrient uptake inhibition under Cr stress, especially in HD-19 cultivar. Furthermore, BRs stimulated antioxidative defense systems, both enzymatic and non-enzymatic, the compartmentalization of ion chelation, diminished extra production of reactive oxygen species (ROS), and electrolyte leakage in response to Cr-induced toxicity, specifically in HD-19. In addition, BRs improved Cr stress tolerance in soybean seedlings by regulating the expression of stress-related genes involved in Cr accumulation, and translocation. Inclusively, by considering the above-mentioned biomarkers, foliar spray of BRs might be considered an effective inhibitor of Cr-induced damages in soybean cultivars, even in Cr polluted soil.

Modified and pristine biochars for remediation of chromium contamination in soil and aquatic systems

Chromium (Cr) contamination in soil and water poses high toxicity risks to organisms and threatens food and water security worldwide. Biochar has emerged as a promising material for cleaning up Cr contamination owing to biochar's strong capacity to immobilize Cr. This paper synthesizes information on biochar modification for the efficient remediation of Cr contamination in soil and water, and critically reviews mechanisms of Cr adsorption on pristine and modified biochars. Biochar modification methods include physical activation via ball milling or ultraviolet irradiation, chemical activation via magnetization, alkali/acid treatment, nano-fabrication or loading of reductive agents, and biological activation via integrating biochars with microorganisms and their metabolites. Modified biochars often have multi-fold enhancement in Cr adsorption/reduction capacity than pristine biochars. Iron (Fe)-supported magnetic biochars have the most promising Cr removal abilities with high reusability of the biochars. Pre-pyrolysis modification with Fe could load Fe₃O₄ micro-/nanoparticles on biochars, and increase the surface area and electrostatic attraction between chromate anions and biochar surfaces, and reduce Cr(VI) to Cr(III). Post-pyrolysis modification could enrich oxygen-containing functional groups such as CO and -OH on biochar surfaces and promote Cr reduction and adsorption. Future research directions for Cr mitigation using advanced biochar products are discussed in this review.

Roles of humic substances redox activity on environmental remediation

Humic substances (HS) as representative natural organic matters and the most common organic compounds existing in the environment, has been applied to the treatment and remediation of environmental pollution. This review systematically introduces and summarizes the redox activity of HS for the remediation of environmental pollutants. For inorganic pollutants (such as silver, chromium, mercury, and arsenic), the redox reaction of HS can reduce their toxicity and mobilization, thereby reducing the harm of these pollutants to the environment. The concentration and chemical composition of HS, environmental pH, ionic strength, and competing components affect the degree and rate of redox reactions between inorganic pollutants and HS significantly. With regards to organic pollutants, HS has photocatalytic activity and produces a large number of reactive oxygen species (ROS) under the light which reacts with organic pollutants to accelerate the degradation of organic pollutants. Under the affection of HS, the redox of Fe(III) and Fe(II) can enhance the efficiency of Fenton-like reaction to degrade organic pollutants. Finally, the research direction of HS redox remediation of environmental pollution is prospected.

Effects of tannic acid on the transport behavior of trivalent chromium in soils and its mechanism

Trivalent chromium [Cr(III)] and tannins serve as necessary substances in leather processing and coexist in tannery site, which lead to the chromium contamination in site soil when disposed improperly. However, coexisting tannins are very likely to complex with Cr(III) and affect its properties, ultimately changing the mobility of chromium in soil. In this study, tannic acid (TA) was selected to investigate the complexation with Cr(III) and the influence on the solubility and sorption of Cr(III) in soils. Then, the transport behavior and mechanism of Cr(III)-TA complexes in soil was clarified. Dialysis results showed that the increase of TA concentration and solution pH promoted the formation of complexed Cr(III). The results of UV-Vis absorption spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations indicated that the adjacent ionized phenolic hydroxyls in TA functioned as the binding sites with Cr(III) to form the Cr-O bonds and the degree of complexation increased with pH. The Cr(III)-TA complexes had higher solubility than free Cr(III) at pH ≥ 6.0. Batch sorption experiments demonstrated that the sorption capacity of Cr(III)-TA to soils with different pH was always lower than that of free Cr(III). These reasons led to the stronger mobility of Cr(III)-TA in soil columns than Cr(III). Our research reveals that the enhanced mobility of Cr(III) in soils coexisting with TA.

Impact of δ-MnO2 on the chemical speciation and fractionation of Cr(III) in contaminated soils

Oxidation of Cr(III) by birnessite (δ-MnO₂) was an important geochemical reaction determining the toxicity and mobility of dissolved Cr(III) in soils. Herein, changes of Cr speciation and fractionation were systematically studied in Cr(III)-contaminated soils with δ-MnO₂ in soil aging process. The results showed that Cr(III) could be rapidly oxidized to Cr (VI) by δ-MnO₂, and the coating of Fe and Al oxides on δ-MnO₂ had a strong hindering effect on the oxidation of Cr(III). The Cr(III) oxidation process by δ-MnO₂ followed a two-phase model of pseudo first-order kinetics. The rapid decrease of oxidation rate constant in second phase was due to the coverage of adsorbed Cr(III) and newly generated Cr(VI) and Mn(II) on the active sites of δ-MnO₂. X-ray photoelectron spectroscopy analysis further confirmed that the diffusion and adsorption of Cr(III) on the electron-accepting sites were important factors affecting the Cr(III) oxidation by δ-MnO₂. Compared with the soils without δ-MnO₂, high contents of Cr(VI) were generated in silt (22.30 mg/kg) and sandy soil (70.95 mg/kg) with 2 wt% δ-MnO₂ after the addition of Cr(III) wastewater within 2 days, and the Cr(VI) contents were above 1 mg/kg in these two soils during the whole incubation process. Moreover, the total Cr proportion of the exchangeable fraction increased by 12.8% in silt and 5.2% in sandy soil with 2 wt% δ-MnO₂ after soil aging for 120 days. The presence of δ-MnO₂ markedly increased the oxidation potential and mobility of exogenous Cr(III) in soils.

Metal accumulation and morphofunctional damage in coelomocytes of earthworm collected from industrially contaminated soil of Kolkata, India

The current study is aimed to assess the ecotoxicological effects of toxic metals and seasonal shift of the physicochemical characteristics of soil in an endogeic earthworm Metaphire posthuma of industrially contaminated soil of Calcutta leather complex. The accumulation of cadmium, chromium, lead and mercury was quantitated in whole earthworms and coelomocytes. The accumulation of metals was derived to be high in the coelomocytes than whole earthworms. Morphofunctional shift in coelomocytes indicated a high level of metal toxicity in soil inhabitants. The shift in differential coelomocyte count and cellular damage including intense cytoplasmic vacuolation and membrane blebbing of coelomocytes of M. posthuma of contaminated soil were suggestive to a state of immunocompromisation in the same species. Shift in the generation of nitric oxide and activity of inducible nitric oxide synthase indicated a possible immunosuppression in earthworm. Depletion in the acetylcholinesterase activity of coelomocytes indicated neurotoxicity of metals leached from the dumped wastes in Calcutta leather complex. Integrated biomarker response based analysis was carried out to assess the biomarker potential of experimental endpoints of M. posthuma to monitor metal toxicity in soil.

Accumulation of chromium in plants and its repercussion in animals and humans

The untreated effluents released from industrial operations have adverse impacts on human health, environment and socio-economic aspects. Environmental pollution due to chromium is adversely affecting our natural resources and ecosystem. Chromium is hazardous carcinogenic element released from spontaneous activities and industrial procedures. Chromium toxicity, mobility and bioavailability depend mainly on its speciation. Chromium mainly exists in two forms, first as an immobile, less soluble trivalent chromium [Cr(III)] species under reducing conditions whereas hexavalent chromium [Cr(VI)] as a mobile, toxic and bioavailable species under oxidizing conditions. Hexavalent chromium is more pernicious in comparison to trivalent form. Chromium negatively affects crop growth, total yield and grain quality. Exposure of chromium even at low concentration enhances its accretion in cells of human-beings and animals which may show detrimental health effects. Many techniques have been utilized for the elimination of chromium. The selection of the green and cost-efficient technology for treatment of industrial effluent is an arduous task. The present review highlights the problems associated with chromium pollution and need of its immediate elimination by suitable remediation strategies. Further, investigations are required to fill the gaps to overcome the problem of chromium contamination and implementation of sustainable remediation strategies with their real-time applicability on the contaminated sites.

Application of exogenous glutathione decreases chromium translocation and alleviates its toxicity in soybean (Glycine max L.)

Chromium is considered one of the most severe toxic elements affecting agriculture. Soybean seedlings under chromium stress were treated with glutathione and buthionine sulfoximine. The effects of exogenous glutathione on the physiological effects of two different chromium-resistant soybean seedlings and the expression levels of expression levels related genes were studied. This study tested the seedling weight and SPAD values, detected enzymatic antioxidants (i.e., superoxide dismutase, peroxidase, catalase, catalase, ascorbate peroxidase), and non-enzymatic antioxidants (i.e., glutathione, proline, soluble sugars, and soluble phenols) that attenuate chromium-induced reactive oxygen species, and quantified several genes associated with glutathione-mediated chromium stress. The results showed that exogenous glutathione could improve the physiological adaptability of soybean seedlings by regulating photosynthesis, antioxidant, and related enzyme activities, osmotic system, the compartmentalization of ion chelation, and regulating the transcription level of related genes, thereby increasing the chromium accumulation of soybean seedlings, enhancing the tolerance of chromium stress, and reducing the toxicity of chromium. Overall, the application of glutathione alleviates chromium toxicity in soybeans, and this strategy may be a potential farming option for soybean bioremediation in chromium-contaminated soils.

Immobilizing chromium in tannery sludge via adding collagen protein waste: an in-depth study on mechanism

Owing to containing high fraction of organic matter, the tannery sludge seemed to be fit for composting. Actually, it was intensively harmful to the environment, due to containing chromium (Cr). So it might undergo a long time of storage until finding a proper way to dispose it. In the storage period, it would expose the surrounding environment a risk via releasing Cr. In this study, an approach was proposed to minimize the amount of released Cr, and reveal the mechanism on immobilizing Cr. Collagen protein waste (CPW) was adopted to immobilize Cr, and it was evaluated via leaching experiment. The lowest leaching concentration of Cr was 12 mg/L, meeting the limits of related standard in China (GB 5085.3-2007, Tcr < 15 mg/L). Moreover, the compositions and functional groups of the optimum sample (12 mg/L) were also characterized, confirming that the dominant functional groups cross-linking with Cr were hydroxyl (-OH), carboxyl (-COOH), and epoxy (-COC). Importantly, density functional theory (DFT) calculation was also employed, suggesting that Cr was restrained by accepting electrons from O atoms donating by functional groups.

Removal and recycling of hexavalent chromium from alkaline wastewater via a new ferrite process to produce the valuable chromium ferrite

Current technologies for removal of Cr(VI) are generally fit for acidic wastewater. In this study, a new ferrite process for removal and recycling of Cr(VI) from alkaline wastewater to produce the valuable chromium ferrite has been developed. The results show that this new ferrite method is a one-step process which can be divided into two successive reactions including Cr(VI) reduction to form coprecipitation (Cr_0.25Fe_0.75(OH)₃) and subsequently magnetic conversion of Cr_0.25Fe_0.75(OH)₃ induced by Fe²⁺ under the same alkaline condition. The total Fe/Cr mole ratio of 5:1 is at least required for the chromium ferrite transformation. Increasing temperature and pH can enhance the interaction of Fe²⁺ with Cr_0.25Fe_0.75(OH)₃ and further promote the formation of chromium ferrite, while suppressing the generation of nonmagnetic by-product goethite. Almost pure chromium ferrite is formed under proposed optimum conditions (Fe/Cr = 7:1, 65 °C and pH of 9) with Cr(VI) removal ratio around 100%. The Cr(VI) remained in the filtrate can be reduced to 0.01 mg/L which is much lower than the limits concentration for surface water (≤0.05 mg/L). The chromium ferrite product whose molecular formula can be expressed as Cr_0.5-xFe_2.5+xO₄ (where 0 ≤ x < 0.5) presents good magnetic properties and has the potential to be recycled as a useful material.

Chromium biogeochemical behaviour in soil-plant systems and remediation strategies: A critical review

Chromium (Cr) is a toxic heavy metal that is heavily discharged into the soil environment due to its widespread use and mining. High Cr levels may pose toxic hazards to plants, animals and humans, and thus have attracted global attention. Recently, much progress has been made in elucidating the mechanisms of Cr uptake, transport and accumulation in soil-plant systems, aiming to reduce the toxicity and ecological risk of Cr in soil; however, these topics have not been critically reviewed and summarised to date. Accordingly, based on available data-especially from the last five years (2017-2021)-this review traces a plausible link among Cr sources, levels, chemical forms, and phytoavailability in soil; Cr accumulation and translocation in plants; and Cr phytotoxicity and detoxification in plants. Additionally, given the toxicity and hazard posed by Cr(VI) in soils and the application of reductant materials to reduce Cr(VI) to Cr(III) for the remediation of Cr(VI)-contaminated soils, the reduction and immobilisation mechanisms by organic and inorganic reductants are summarised. Finally, some priority research challenges concerning the biogeochemical behaviour of Cr in soil-plant systems are highlighted, as well as the environmental impacts resulting from the application of reductive materials and potential research prospects.

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.

Chromium Distribution, Leachability and Speciation in a Chrome Plating Site

Hexavalent chromium (Cr(VI)) waste produced by chrome plating activities pollutes the surrounding environment and harms human health. However, information about the chromium (Cr) pollution characteristics of actual electroplating sites is still lacking. In this study, the concentration, leachability and speciation of Cr in soils from a typical chrome plating site were analyzed. Our results showed that this site was severely contaminated by Cr (7.2 to 7735.2 mg/kg) and Cr(VI) reached the mean concentration of 138.7 mg/kg. The spatial distribution of Cr(VI) was related to the plating processes. Chrome plating and sewage treatment areas could be considered as the hot spots of contaminated sites. The vertical distribution of Cr(VI) was mainly affected by soil properties, where the loam layer retained and reduced a large amount of Cr(VI) due to its high content of iron minerals and finer particle fractions. Additionally, the chemical extraction results showed that Cr was mainly in non-residual fractions and the existence of Cr(VI) led to a high leaching toxicity based on the toxicity characteristic leaching procedure (TCLP) results. Moreover, X-ray photoelectron spectroscopy (XPS) results revealed the speciation of Cr in the long-term contaminated soils. A large amount of Cr(VI) was reduced into Cr(III) and mainly existed as Cr(OH)3 and Cr2O3. Furthermore, Cr(VI) tended to precipitate as CaCrO4 and persisted in soils. Therefore, it is necessary to find appropriate strategies to remediate these contaminated soils. Overall, these findings strengthen our understanding of Cr(VI) behaviors and lay a foundation for the future pollution investigation, ecological remediation and risk assessment of sites contaminated by electroplating.

Spatial distribution and morphological transformation of chromium with coexisting substances in tannery landfill

The prosperity and development of tannery industry have brought about rapid economic growth. However, the tannery landfill without anti-seepage measures in the early stage has generated masses of environmental hazards owing to the lack of awareness in environmental protection. Therefore, it is imperative to pay much attention to the understanding of environmental hazards from tannery waste. In this study, solid samples and groundwater samples were collected from a tannery landfill to study the effect of the characteristic pollutants produced by tanning on chromium distribution with other coexisting substances. The results showed that significant correlations were demonstrated between multiple coexisting substances (total organic carbon, total petroleum hydrocarbons, total nitrogen, Cr, F, Ca, Cu and Pb), indicating the possible same source or they coming from the same tannery production stage. The weights of positive effects and negative effects of coexisting substances on total Cr distribution in the profile decreased in the order: total nitrogen > Cu > Ca > Pb > total organic carbon > F > SO₄^2-> Cd, and Ni > Cl > Hg, respectively. Moreover, the simulation of Visual MINTEQ showed that the cations were mainly bound to Cr as CrO₄^2-, while the anions were bound to Cr³⁺. This study provided a new perspective on the selection of remediation strategies for Cr-contaminated sites to avoid secondary environmental pollution caused by the release of coexisting heavy metals.

Microscopic understanding about adsorption and transport of different Cr(VI) species at mineral interfaces

Cr(VI) ranks as one of the most toxic heavy metals and herein, microscopic mechanisms for adsorption and transport of different Cr(VI) oxyanions (Cr₂O₇^2- and CrO₄^2-) at kaolinite interfaces are addressed by dispersion-corrected periodic density functional theory calculations. Cr(VI) oxyanions adsorb favorably at both tetrahedral and octahedral surfaces, and K⁺ ions serve as bridge for Cr(VI) oxyanions and tetrahedral surfaces while Cr(VI) oxyanions serve as bridge for K⁺ ions and octahedral surfaces. Adsorption structures are altered significantly by pH variation, and stability trends at different pH ranges are deciphered by the dominant interaction force with clay surfaces: Electrostatic interaction with K⁺ ions at tetrahedral surfaces whereas combined action of electrostatic and H-bonding interactions with Cr(VI) oxyanions at octahedral surfaces. Electron transfers are strongly pH-dependent, and clay surfaces serve as electron reservoirs. CrO₄^2- rather than Cr₂O₇^2- is dominant at clay interfaces, and HCrO₄^- can co-exist under acidic conditions. Cr₂O₇^2- transformation to CrO₄^2- is kinetically blocked at pH ≈ PZC while preferred at pH < PZC. Cr(VI) removal and reclamation should proceed at pH > 7.0 and pH < PZC, respectively. Results greatly promote the understanding about Cr(VI) bioavailability and fate in surficial environments and are also useful for Cr(VI) removal and reclamation.

Migration and fate of characteristic pollutants migration from an abandoned tannery in soil and groundwater by experiment and numerical simulation

The tannery industry is an integral part of economic development in many developing countries, and the environmental pollution caused by the tannery industry cannot be ignored. In this study, soil and groundwater samples at different depths were collected from an abandoned tannery to investigate the temporal and spatial distribution of characteristic pollutants produced by tanning. The concentrations of Cr, Cl, F and NH₄⁺-N in the soil from the sludge temporary storage area were higher than those from the liming and unhairing workshop, chrome tanning workshop, wastewater outlet, and around wastewater pond. The concentration of Cr(VI) in all sampling sites was below the detection limit. The main species of Cr in the groundwater were Cr(NH₃)₆Cl²⁺ and CrO₄^2- based on the simulation of Visual MINTEQ. The saturation index was negative and changed with time indicating that Cr existed in the dissolved phase. The proportion of Cr(VI) to total Cr was negatively correlated with the saturation index in village 1 and village 3. The simulation results from Visual MODFLOW and MT3DMS showed that the migration of Cr, NH₄⁺-N, Cl^- and F^- mainly occurred in the Quaternary system. The coverage of the pollution plume of pollutants in villages 1 and 3 was as follows: Cr > NH₄⁺-N > Cl^- > F^-. Two decay rate calculation methods of pollutants with migration time and distance were put forward to provide a basis for the actual investigation of the pollution migration scope and time determination.

Remediation of Chromium-Contaminated Soil Based on Bacillus cereus WHX-1 Immobilized on Biochar: Cr(VI) Transformation and Functional Microbial Enrichment

Microorganisms are applied to remediate chromium (Cr)-contaminated soil extensively. Nevertheless, the microbial loss and growth inhibition in the soil environment restrain the application of this technology. In this study, a Cr(VI)-reducing strain named Bacillus cereus WHX-1 was screened, and the microbial aggregates system was established via immobilizing the strain on Enteromorpha prolifera biochar to enhance the Cr(VI)-reducing activity of this strain. The mechanism of the system on Cr(VI) transformation in Cr-contaminated soil was illuminated. Pot experiments indicated that the microbial aggregates system improved the physicochemical characteristics of Cr-contaminated soil obviously by increasing organic carbon content and cation exchange capacity, as well as decreasing redox potential and bulk density of soil. Moreover, 94.22% of Cr(VI) was transformed into Cr(III) in the pot, and the content of residue fraction Cr increased by 63.38% compared with control check (CK). Correspondingly, the physiological property of Ryegrass planted on the Cr-contaminated soil was improved markedly and the main Cr(VI)-reducing microbes, Bacillus spp., were enriched in the soil with a relative abundance of 28.43% in the microbial aggregates system. Considering more active sites of biochar for microbial aggregation, it was inferred that B. cereus WHX-1 could be immobilized by E. prolifera biochar, and more Cr(VI) was transformed into residue fraction. Cr stress was decreased and the growth of plants was enhanced. This study would provide a new perspective for Cr-contaminated soil remediation.

Oxidation of trivalent chromium induced by unsaturated oils: A pathway for hexavalent chromium formation in soil

The kinetics and mechanisms of the oxidation of Cr(III) in soil contaminated by unsaturated oils were investigated. Batch experiments were performed with unsaturated oils, namely, fish oil, hydrogenated lard oil, rapeseed oil, and caster oil. Impacts of environmental parameters, including temperature, soil pH, UV irradiation, oil content, and soil moisture content were examined. Results showed that oxidation of Cr(III) in oil-treated soils was accompanied by the formation of Cr(VI), which first increased and then decreased. Changes in the peroxide values of oils and the production of hexanal in the soil indicated that hydroperoxide was closely related to the formation of Cr(VI). tert-Butylhydroperoxide, as a model molecule of hydroperoxide, significantly enhanced the oxidation of Cr(III) in water. This result further showed that hydroperoxides were responsible for the oxidation of Cr(III). Native soil substances, such as organic matter, Fe(II), and microbes, and the decomposition products of hydroperoxides, such as aldehydes, could reduce Cr(VI). The change in Cr(VI) content in the soil resulted from the competition between the oxidation of Cr(III) and the reduction of Cr(VI). High temperature, high soil pH, UV irradiation, and low soil moisture content could facilitate the oxidation of Cr(III), which is of environmental significance.

Remediation of fluoride contaminated soil with nano-hydroxyapatite amendment: Response of soil fluoride bioavailability and microbial communities

Nano-hydroxyapatite (NHAP), possessing high defluoridation capacity, has been widely used to remove fluoride (F) from polluted water, but little is known about how it affects the bioavailability and toxicity of soil F towards plants. Here, the impact of NHAP (2% w/w) amendment on immobilization, speciation and accumulation of F was studied in a soil-plant system. The results revealed that the NHAP amendment worked effectively to reduce levels of water-soluble F (37.3%-87.8%) and increase available P (76.6%-147%). X-ray photoelectron spectroscopy analysis indicated that the formation of insoluble CaF₂ and the ion exchange of F^- with OH^- into NHAP might be involved in the mechanism of F immobilization and soil pH elevation. Exposure to NHAP significantly decreased the abundance of Cyanobacteria in tested soils, and Gemmatimonadetes abundance in bulk soil was significantly higher than that in rhizosphere soil at 1,000 mg kg^-1 F spiked level. Additionally, NHAP amendment decreased F accumulation in wheat shoots (9.10%-18.7%) and roots (3.88%-22.4%), which could mainly be attributed to the reduction of soil bioavailable F and the supplement of Ca from NHAP. These results suggest that NHAP could be a promising amendment to be applied to acidic soil contaminated with F.

Metal contaminants in rice from Cuba analyzed by ICP-MS, ICP-AES and CVAAS

Mercury, lead, cadmium and arsenic are considered hazardous pollutants in the environment, which can result in a risk for human health. This study aimed to determine the contents of As, Cd, Cr, Cu, Hg, Ni, Pb and Zn by ICP-MS, CVAAS and ICP-AES in paddy and brown rice, grown in different soils of Cuba. The interest to include Cr, Ni, Cu and Zn was because these elements are found in very high levels in agricultural soils. To assess the safety of the dietary intake of these metals, the estimated weekly intake was calculated. The contents of Cd and As were below the maximum limits. A higher value was obtained for Pb when compared to Codex Alimentarius limits. The weekly intake of Cr and Ni was higher than the maximum weekly intake recommended by FAO/WHO.

Cd Adsorption by Iron-Organic Associations: Implications for Cd Mobility and Fate in Natural and Contaminated Environments

The mobility and fate of Cd in soil are mainly controlled by active substances such as iron minerals and organic matter. Iron minerals and organic matter often coexist in the form of iron-organic associations (IOA), which have large specific surface areas and many functional groups, potentially affecting Cd adsorption. However, little is known about Cd adsorption by IOA. This study investigated Cd adsorption by the synthetic IOA under different conditions. The results indicate Cd adsorption increased with the increasing amount of IOA, while the adsorption efficiency decreased gradually. pH significantly affects Cd adsorption, because the Cd speciation and the surface charge of IOA changed under different pH conditions. Under alkaline condition, part of Cd would form hydroxide precipitate, facilitating Cd adsorption by IOA. The composition of organic matter in IOA didn't significantly affect Cd adsorption.