Multi-step column leaching using low-molecular-weight organic acids for remediating vanadium- and chromium-contaminated soil

The trigger mechanisms and the gene regulatory pathways of organic acid secretion during the vanadium-titanium magnetite tailing bioleaching

The long-term mining of vanadium-titanium (V-Ti) magnetite has generated a large accumulation of tailings, which can lead to metal pollution via microbial bioleaching. Current research has focused on the bioleaching of minerals, and a few studies have explored microbial responses to metals only through limited metabolite concentrations. However, the trigger mechanisms of metal release during the V-Ti magnetite tailing bioleaching and key gene regulatory pathways for organic acid metabolism are still unclear. This study screened a bioleaching fungus from the V-Ti magnetite tailing pond groundwater. The fungus promoted tailing dissolution by secreting more organic acids (808.99 mg L-1) than without tailings (671.11 mg L-1). The released metals were responsible for the difference in organic acid metabolism. The tailing-released Fe, Zn, and V were the triggers for the organic acid secretion via up-regulating the functional genes of citric, formic, and succinic acids in the TCA cycle, Methane metabolism, and D-arginine and D-ornithine metabolisms. Fe and V also led to the accumulation of malic acid through up-regulating functional genes during the conversion of phenylalanine, tyrosine, and glycine. Ni and Cu were the inhibitors by up-regulating related functional genes and promoting the conversion of acetyl-CoA to acetoacetyl-CoA, resulting in a decrease in organic acid concentrations. This study demonstrated the triggering metals of bioleaching and fungal gene regulation pathways, which provide a novel strategy for fungi domestication by considering the up-regulating metals to improve the bioleaching efficiency.

Synergistic Effects of Unmodified Tea Leaves and Tea Biochar Application on Remediation of Cr-Contaminated Soil

Hexavalent chromium (Cr(VI)) contamination in soil presents significant risks due to its high toxicity to both the environment and human health. Renewable, low-cost natural materials offer promising solutions for Cr(VI) reduction and soil remediation. However, the effects of unmodified tea leaves and tea-derived biochar on chromium-contaminated soils remain inadequately understood. In this study, tea tree pruning waste was converted into biochar at various temperatures, and the impacts of both unmodified tea leaves and tea biochar on soil Cr(VI) content, chromium fractionation, and soil biochemical properties were assessed using a soil incubation experiment. The results showed that the combined treatment of tea and tea biochar produced at 500 °C reduced Cr(VI) content by up to 49.30% compared to the control. Chromium fractionation analysis revealed a significant increase in the residual chromium fraction, accounting for 32.97% of total chromium, substantially reducing its bioavailability and mobility. Soil properties were markedly improved, with notable increases in pH (14.89%), cation exchange capacity (CEC; up to 100.24%), and organic matter content (up to 167.12%) under the combined treatments. Correlation analysis confirmed that Cr(VI) content reductions were positively correlated with increases in pH, nutrient retention, and enzyme activities, highlighting their role in chromium stabilization. This study underscores the synergistic potential of unmodified tea leaves and tea biochar as an innovative, eco-friendly strategy for Cr(VI) remediation, enhancing both soil quality and heavy metal stabilization.

Study on the enhancement of citric acid chemical leaching of contaminated soil by modified nano zero-valent iron

This study aimed to evaluate the effect of modified nanoscale zero-valent iron (SAS-nZVI) on chemical leaching of lead and cadmium composite contaminated soil by citric acid (CA). The synthesized SAS-nZVI was used as a leaching aid to improve the removal rate of soil heavy metals (HMs) by CA chemical leaching. The effects of various factors such as SAS-nZVI dosage, elution temperature and elution time were studied. At the same time, the effect of chemical leaching on the basic physical and chemical properties of soil and the morphology of HMs was evaluated. The results show that when the SAS-nZVI dosage is 2.0 g/L, the leaching temperature is 25 °C, and the leaching time is 720 min, the maximum removal rates of Pb and Cd in the soil are 77.64% and 97.15% respectively. The experimental results were evaluated using elution and desorption kinetic models (Elovich model, double constant model, diffusion model). The elution and desorption process of Pb and Cd in soil by SAS-nZVI-CA fitted well with the double-constant model, indicating that the desorption kinetic process of Pb and Cd is a heterogeneous diffusion process, and the elution process is controlled by diffusion factors. After leaching with SAS-nZVI-CA, the physical and chemical properties of the soil changed little, the mobility and toxicity of HMs in the soil were reduced, and the HMs content in the leaching waste liquid was reduced. It can be concluded that SAS-nZVI enhances the efficiency of CA in extracting Pb and Cd from soil, minimizes soil damage resulting from chemical leaching technology, and alleviates the challenges associated with treating leaching waste liquid.

Removal of V(V) from a Mixed Solution Containing Vanadium and Chromium Using a Micropocrous Resin in a Column: Migration Regularity of the Mass Transfer Zone and Analysis of Dynamic Properties

In China, both vanadium(V) and chromium(VI) are present in wastewater resulting from vanadate precipitation (AVP wastewater) and from leaching vanadium-chromium reduction slag. Addressing environmental preservation and the comprehensive utilization of metal resources necessitates the extraction and separation of V(V) and Cr(VI) from these mixed solutions. However, their separation is complicated by very similar physicochemical properties. This study establishes a method for the dynamic selective adsorption of V(V) from such mixtures. It evaluates the impact of various operating conditions in columns on dynamic adsorption behavior. This study examines the migration patterns of the mass transfer zone (MTZ) and forecasts its effective adsorption capacity through multivariate polynomial regression and a neural network (NN) model. The NN model's outcomes are notably more precise. Its analysis reveals that C 0 is the most critical factor, with Q and H following in importance. Furthermore, the dynamic properties were analyzed using two established models, Thomas and Klinkenberg, revealing that both intraparticle and liquid film diffusion influence the rates of exchange adsorption, with intraparticle diffusion being the more significant factor. Using 3 wt % sodium hydroxide as the eluent to elute V(V)-loaded resin at a flow rate of 4 mL/min resulted in a chromium concentration of less than 3 mg/L in the V(V) eluate, indicating high vanadium-chromium separation efficiency in this method. These findings offer theoretical insights and economic analysis data that are crucial for optimizing column operation processes.

Effects of secondary release of chromium and vanadium on soil properties, nutrient cycling and bacterial communities in contaminated acidic paddy soil

Although the contamination situation of chromium (Cr) and vanadium (V) have been revealed, the effects of their re-release on ecological risk in contaminated acidic paddy soil are unclear. To evaluate the effects, we assigned soil microcosms across three different concentration (100, 200, 300 mg/L) and introduced Cr and V alone or combination into an already slightly contaminated acidic soil. We found that Cr and V alone or interacted to increased soil bioavailable-metals, changed soil properties and nutrients to varying degrees. Meanwhile, soil ammoniacal nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N) contents, nitrogen (N) -cycling enzyme activities, microbial mass N were significantly influenced by Cr addition. Which demonstrated that Cr re-release may disturb soil N cycle. However, V alone significantly improved soil NO₃^--N contents, cellulase and dehydrogenase activities, soil respiration intensity and microbial mass carbon: nitrogen. Meanwhile, V addition also decreased bacterial diversity while Cr addition increased bacterial diversity and shaped new bacterial community, some V(V) and Cr (VI) reducing bacteria were identified. Heatmap of Pearson correlation and Redundancy analysis showed that NH₄⁺-N, NO₃^--N, Potassium, Phosphorus, and Cr played an important role in bacterial community structure. These findings suggested that re-release of Cr and V disturbed soil function and raised ecological risks, and the power to destroy the ecosystem stability originated from Cr was much stronger than V. This study was contributed to understand the effects of Cr and V re-release on microecology in contaminated acidic agricultural soil.

Impacts of climate change on metal leaching and partitioning for submarine mine tailings disposal

At present, there are no standardised tests to assess metal leaching during submarine tailings discharge. In this study the influence of variables known to affect metal mobility and availability (dissolved organic carbon (DOC), pH, salinity, temperature, aerated/anoxic conditions) along with variables affected by the discharge conditions (flocculant concentration, suspension) were studied in bench-scale experiments. The leaching tests were developed based on the case of a copper mine by Repparfjorden, northern Norway, which is planned to re-open in 2022. The experiments, which had three week duration, revealed low (<6 %) leaching of metals. Multivariate analysis showed that all variables, apart from DOC, highly influenced leaching and partitioning of at least one metal (Ba, Cr, Cu, and/or Mn). The high quantity of the planned annual discharge of mine tailings to the fjord (1-2 million tonnes) warranted estimation of the leached quantity of metals. Multivariate models, using present-day conditions in the fjord, estimated leaching of up to 124 kg Ba, 154 kg Cu and 2400 kg Mn per year during discharge of tailings. Future changes in the fjord conditions caused by climate change (decreased pH, increased temperature) was predicted by the multivariate models to increase the leaching up to 55 %, by the year 2065. The bench-scale experiments demonstrated the importance of including relevant variables (such as pH, salinity, and temperature) for metal leaching and -partitioning in leaching tests. The results showed that metal leaching during discharge is expected and will increase in the future due to the changed conditions caused by the foreseen climate change, and thereby underline the importance of monitoring metal concentrations in water during operations to determine the fate of metals in the fjord.

Will Vanadium-Based Electrode Materials Become the Future Choice for Metal-Ion Batteries?

Metal-ion batteries have emerged as promising candidates for energy storage system due to their unlimited resources and competitive price/performance ratio. Vanadium-based compounds have diverse oxidation states rendering various open-frameworks for ions storage. To date, some vanadium-based polyanionic compounds have shown great potential as high-performance electrode materials. However, there has been a growing concern regarding the cost and environmental risk of vanadium. In this Review, all links in the industry chain of vanadium-based electrodes were comprehensively summarized, starting with an analysis of the resources, applications, and price fluctuation of vanadium. The manufacturing processes of the vanadium extraction and recovery technologies were discussed. Moreover, the commercial potentials of some typical electrode materials were critically appraised. Finally, the environmental impact and sustainability of the industry chain were evaluated. This critical Review will provide a clear vision of the prospects and challenges of developing vanadium-based electrode materials.

Adsorption and reduction from modified polypyrrole enhance electrokinetic remediation of hexavalent chromium-contaminated soil

Toxic metal pollutant Cr(VI) in the environment will pose a severe threat to animal and human health. In this work, Fe₃O₄@PPy, Arg@PPy, and Arg/Fe₃O₄@PPy were prepared to enhance adsorption of Cr(VI) by doping Fe₃O₄ nanoparticles and amino radicals into the original PPy structure. Their characteristics were investigated by FTIR, SEM, EDS, BET analysis, and batch adsorption experiments. And they were used as permeable reaction barriers (PRB) to combine with electrokinetic remediation (EKR) to remediate Cr-contaminated soil. Adsorption experiment results showed that the maximum adsorption capacities of PPy, Fe₃O₄@PPy, Arg@PPy, and Arg/Fe₃O₄@PPy for Cr(VI) were 60.43 mg/g, 67.12 mg/g, 159.86 mg/g, and 141.50 mg/g, respectively. They all followed the kinetic pseudo-second-order model and the Langmuir isothermal model with a monolayer adsorption behavior. In the EKR/PRB system, the presence of Fe₃O₄@PPy, Arg@PPy, and Arg/Fe₃O₄@PPy obtained the higher Cr(VI) removal efficiency near the anode than that of the PPy, increasing by 74.60%, 26.04%, and 68.64%, respectively. A strong electrostatic attraction between anion contaminants and protonated modified PPy and a reduction from Cr(VI) to Cr(III) appeared in the EKR remediation process under acid conditions. This study opened up a prospect for applying modified PPy composites to treat toxic metal-contaminated soil.

Vanadium: A Review of Different Extraction Methods to Evaluate Bioavailability and Speciation

The excessive input of heavy metals such as vanadium (V) into the environment has been one of the consequences of global industrial development. Excessive exposure to V can pose a potential threat to ecological safety and human health. Due to the heterogeneous composition and reactivity of the various elements in soils and sediments, quantitative analysis of the chemical speciation of V in different environmental samples is very complicated. The analysis of V chemical speciation can further reveal the bioavailability of V and accurately quantify its ecotoxicity. This is essential for assessing for exposure and for controlling ecological risks of V. Although the current investigation technologies for the chemical speciation of V have grown rapidly, the lack of comprehensive comparisons and systematic analyses of these types of technologies impedes a more comprehensive understanding of ecosystem safety and human health risks. In this review, we studied the chemical and physical extraction methods for V from multiple perspectives, such as technological, principle-based, and efficiency-based, and their application to the evaluation of V bioavailability. By sorting out the advantages and disadvantages of the current technologies, the future demand for the in situ detection of trace heavy metals such as V can be met and the accuracy of heavy metal bioavailability prediction can be improved, which will be conducive to development in the fields of environmental protection policy and risk management.

Method and mechanism of chromium removal from soil: a systematic review

Heavy metal pollution has increasingly affected human life, and the treatment of heavy metal pollution, especially chromium pollution, is still a major problem in the field of environmental governance. As a commonly used industrial metal, chromium can easily enter the environment with improperly treated industrial waste or wastewater, then pollute soil and water sources, and eventually accumulate in the human body through the food chain. Many countries and regions in the world are threatened by soil chromium pollution, resulting in the occurrence of cancer and a variety of metabolic diseases. However, as a serious threat to agriculture, food, and human health. Notwithstanding, there are limited latest and systematic review on the removal methods, mechanisms, and effects of soil chromium pollution in recent years. Hence, this article outlines some of the methods and mechanisms for the removal of chromium in soil, including physical, chemical, biological, and biochar methods, which provide a reference for the treatment and research on soil chromium pollution drawn from existing publications.

Rare earth element migration and impact of Dicranopteris dichotoma at mines in south China

The migration of rare earth elements (REEs) has a major influence on the environment. However, little is known about REEs migration control by Dicranopteris dichotoma at REEs mines. Therefore, this study examined the horizontal and vertical migration of REEs using artificial rainfall and naive soil column experiments, respectively, and the REEs migration control of Dicranopteris dichotoma at REEs mines in south China. The REEs contents in the runoff and sediment were found to be 32.42 μg and 3.39 mg, respectively, in the artificial rainfall experiment. The mean REEs content and amount in the leaching liquor were 0.23 μg L^-1 and 7.75 μg, respectively. The REEs content after leaching was significantly lower than that before leaching in the 0-5 cm layer, was significantly higher than that before leaching in the 5-10 cm layer. However, the REEs content of the Fe-Mn oxide form showed significant variation among the sites with different Dicranopteris dichotoma covers. Moreover, the runoff reducing rate rose with increasing Dicranopteris dichotoma cover, and the sediment reducing rate increased to the relatively stable stage when Dicranopteris dichotoma cover was equal or greater than 70%. Thus, REEs migration by sediment is the dominant migration method, and the risk of REEs vertical migration in the soil is low and might have been overestimated. Further, Dicranopteris dichotoma does not control REEs migration by altering the REEs forms, and Dicranopteris dichotoma cover should reach and be maintained at ≥ 70% in order to control REEs migration at REEs mines in south China.

Remediation Techniques for Cadmium-Contaminated Dredged River Sediments after Land Disposal

This paper examines the remediation techniques of cadmium (Cd)-contaminated dredged river sediments after land disposal in a city in East China. Three remediation techniques, including stabilization, soil leaching, and phytoremediation, are compared by analyzing the performance of the techniques for Cd-contaminated soil remediation. The experimental results showed that the stabilization technique reduced the leaching rate of soil Cd from 33.3% to 14.3%, thus effectively reducing the biological toxicity of environmental Cd, but the total amount of Cd in soil did not decrease. Leaching soil with citric acid and oxalic acid achieved Cd removal rates of 90.1% and 92.4%, respectively. Compared with these two remediation techniques, phytoremediation was more efficient and easier to implement and had less secondary pollution, but it took more time, usually several years. In this study, these three remediation techniques were analyzed and discussed from technical, economic, and environmental safety perspectives by comprehensively considering the current status and future plans of the study site. Soil leaching was found to be the best technique for timely treatment of Cd contamination in dredged river sediments after land disposal.

Remediation of Cr(VI)-contaminated soil using combined chemical leaching and reduction techniques based on hexavalent chromium speciation

Hexavalent chromium [Cr(VI)] has strong mobility and it can enter into deep regions of soil. Cr(VI)-contaminated soil remediation is the process of removing Cr(VI) present in deep soils and any residual Cr(VI). In this study, the Cr(VI)-contaminated soil in Chongqing was investigated, and the remediation and economic feasibility of chemical leaching and reduction combined with a soil repairing approach was explored. The results showed that the leaching reagent, liquid-solid ratio, leaching time, reduction agent dosage, reduction temperature and reduction time had significant (P < 0.05) effects on the remediation of Cr(VI). At 0.02 mol/L oxalic acid and citric acid using a liquid-solid ratio of 5:1 and leaching time of 45 min, the removal rate of Cr(VI) was 62.7%, the residual Cr(VI) in soil was 126 mg/kg, and the soil pH was 4.09 after leaching. Between 25 and 90 °C, and at a molar ratio of 25:1 of FeSO₄•7 H₂O to Cr(VI), the reduction rate of Cr(VI) in soil after reduction was 54.0-98.4%, and the leaching concentration of Cr(VI) in soil was 0.01-0.29 mg/L. The optimal reduction was at 90 °C for 60 min, resulting in only 2.7 mg/kg of residual Cr(VI) in soil. The cost of this technology to treat the area studied was 826 ¥/ton of soil, which represents an economically feasible method for Cr(VI)-contaminated soil remediation.

Biochar-driven reduction of As(V) and Cr(VI): Effects of pyrolysis temperature and low-molecular-weight organic acids

Batch experiments were conducted to examine the differential effects of biochar pyrolysis temperature and low-molecular-weight organic acids on the reduction of As(V) and Cr(VI) driven by Pennisetum hydridum biochar. The results showed that pyrolysis temperature significantly affected the reducing strength of the biochar. Biochar produced at 500 °C had a stronger electron-donating capacity than did the biochars produced at 300 and 700 °C. In the co-presence of the biochar and a low-molecular-weight organic acid, arsenic and chromium behaved differently. Oxalic acid and malic acid tended to have better effects on enhancing biochar-driven Cr(VI) reduction, as compared to citric acid while the opposite was observed for biochar-driven As(V) reduction. Biochar produced at 300 °C was more favourable for Cr(VI) reduction, as compared to the higher-temperature biochars while the opposite was observed for As(V) reduction in the presence of low-molecular-weight organic acids. This may make the lower-temperature biochar ideal for remediating contaminated soils containing both As(V) and Cr(VI) since it could maximize Cr(VI) reduction while minimizing As(V) reduction.

Effects of endophytes inoculation on rhizosphere and endosphere microecology of Indian mustard (Brassica juncea) grown in vanadium-contaminated soil and its enhancement on phytoremediation

We investigated the effects of endophytes inoculation on ecological factors such as root morphology, rhizosphere soil properties, heavy metal speciation, and rhizosphere and endophytic bacterial communities and their role on phytoremediation. Indian mustards were grown for two months in V-contaminated soil with three treatments (control, inoculation with Serratia PRE01 or Arthrobacter PRE05). Inoculation with PRE01 and PRE05 increased organic matter content by 6.94% and 4.6% respectively and significantly increased bioavailability of heavy metals in rhizosphere soils. Despite the endophyte inocula failed to flourish as stable endophytes, they significantly affected the specific composition and diversity of endophytic bacterial communities in roots, with no significant effect on rhizosphere bacterial communities. The test strains could greatly increase plant growth promotion-related biomarkers in the endosphere, especially those associated with Pseudomonas and Microbacterium genera. PICRUSt analysis predicted high relative abundances of functional genes related to environmental information processing especially in the endophytic microbiota. More biomass production (12.0%-17.4%) and total metals uptake (24.2%-32.0%) were acquired in inoculated treatments. We conclude that endophyte PRE01 or PRE05 inoculation could effectively enhance phytoremediation of V-contaminated soil by improving the rhizosphere and endosphere microecology without causing any ecological damage.