Modulation of hexavalent chromium toxicity on Οriganum vulgare in an acidic soil amended with peat, lime, and zeolite

Pollution Characteristics and Risk Assessment of Heavy Metals in the Sediments of the Inflow Rivers of Dianchi Lake, China

To explore the contamination status and identify the source of the heavy metals in the sediments in the major inflow rivers of Dianchi Lake in China, sediment samples were collected and analyzed. Specifically, the distribution, source, water quality, and health risk assessment of the heavy metals were analyzed using correlation analysis (CA), principal component analysis (PCA), the heavy metal contamination factor (Cf), the pollution load index (PLI), and the potential ecological risk index (PERI). Additionally, the chemical fractions were analyzed for mobility characteristics. The results indicate that the average concentration of the heavy metals in the sediment ranked in the descending order of Zn > Cr > Cu > Pb > As > Ni > Cd > Hg, and most of the elements existed in less-mobile forms. The Cfwas in the order of Hg > Zn > Cd > As > Pb > Cr > Ni; the accumulation of Hg, Zn, Cd, and As was obvious. Although the spatial variability of the heavy metal contents was pronounced, the synthetical evaluation index of the PLI and PERI both reached a high pollution level. The PCA and CA results indicate that industrial, transportation, and agricultural emissions were the dominant factors causing heavy metal pollution. These results provide important data for improving water resource management efficiency and heavy metal pollution prevention in Dianchi Lake.

Adsorption enrichment integrated with paper-based devices for detection of trace levels of hexavalent chromium in water samples

In the present study, a sensitive microfluidic paper-based analytical device (μ-PADs) integrated with adsorption enrichment procedure was developed to analyze Cr(VI) in water samples. The affecting factors, including pH and amounts of reagents were optimized. The limit of detection of 0.0015 mg L-1 and linear range of 0.005-2 mg L-1 were achieved with good intra- and inter-day precision of 5.1 and 7.6% RSD, respectively. The results obtained by the proposed method were validated by inductively coupled plasma-optical emission spectrometry (ICP-OES). The recoveries of the present method and ICP-OES were ranged from 96.3 to 109.0‬% and 106.0 to 109.7%, respectively. The two sets of (μ-PADs and ICP-OES) results were in a good agreement as paired t-test indicated no significant differences. The proposed method could be utilized for analyzing trace levels of Cr(VI) in water samples in the absence of conventional analytical instruments.

The potential of zeolite nanocomposites in removing microplastics, ammonia, and trace metals from wastewater and their role in phytoremediation

Nanocomposites are emerging as a new generation of materials that can be used to combat water pollution. Zeolite-based nanocomposites consisting of combinations of metals, metal oxides, carbon materials, and polymers are particularly effective for separating and adsorbing multiple contaminants from water. This review presents the potential of zeolite-based nanocomposites for eliminating a range of toxic organic and inorganic substances, dyes, heavy metals, microplastics, and ammonia from water. The review emphasizes that nanocomposites offer enhanced mechanical, catalytic, adsorptive, and porosity properties necessary for sustainable water purification techniques compared to individual composite materials. The adsorption potential of several zeolite-metal/metal oxide/polymer-based composites for heavy metals, anionic/cationic dyes, microplastics, ammonia, and other organic contaminants ranges between approximately 81 and over 99%. However, zeolite substrates or zeolite-amended soil have limited benefits for hyperaccumulators, which have been utilized for phytoremediation. Further research is needed to evaluate the potential of zeolite-based composites for phytoremediation. Additionally, the development of nanocomposites with enhanced adsorption capacity would be necessary for more effective removal of pollutants.

Remediation materials for the immobilization of hexavalent chromium in contaminated soil: Preparation, applications, and mechanisms

Hexavalent chromium is a toxic metal that can induce severe chromium contamination of soil, posing a potential risk to human health and ecosystems. In recent years, the immobilization of Cr(VI) using remediation materials including inorganic materials, organic materials, microbial agents, and composites has exhibited great potential in remediating Cr(VI)-contaminated soil owing to the environmental-friendliness, short period, simple operation, low cost, applicability on an industrial scale, and high efficiency of these materials. Therefore, a systematical summary of the current progress on various remediation materials is essential. This work introduces the production (sources) of remediation materials and examines their characteristics in detail. Additionally, a critical summary of recent research on the utilization of remediation materials for the stabilization of Cr(VI) in the soil is provided, together with an evaluation of their remediation efficiencies toward Cr(VI). The influences of remediation material applications on soil physicochemical properties, microbial community structure, and plant growth are summarized. The immobilization mechanisms of remediation materials toward Cr(VI) in the soil are illuminated. Importantly, this study evaluates the feasibility of each remediation material application for Cr(VI) remediation. The latest knowledge on the development of remediation materials for the immobilization of Cr(VI) in the soil is also presented. Overall, this review will provide a reference for the development of remediation materials and their application in remediating Cr(VI)-contaminated soil.

The evaluation of potentially toxic elements using artificial neural networks and fuzzy linear regression analysis methods in cappadocian volcanic ash soils of Turkey

The aim of this study is to determine the relationships between some potentially toxic elements (PTE) (Cu, Mn, Ni, Zn) in human stomach and intestinal tissues and toxic element contents in soil, vegetables and fruits. This study was conducted in the eastern of Erciyes Strato volcano, an area of 2400 km2 in Turkey. Tissue samples taken from the stomach and intestines of people living in the study area, soils, vegetables, and fruits were used as material. In the study, tissue samples of 26 people's stomach and intestines, 576 soil samples from 192 points and 3 different depths (0-30 cm, 30-60 cm, and 60-90 cm) and vegetable and fruit samples from 137 sampling points were taken. Cu, Mn, Ni, and Zn contents of human tissue samples, soil samples, vegetable and fruit samples were determined. Artificial Neural Networks method (ANN) and Fuzzy Linear Regression Analysis (FLRA) methods were used to determine the relationships between PTE contents in human tissue samples and soils, vegetables, and fruits. Root Mean squared error (RMSE) and coefficient of determination (R2) indices were used as the test criteria for goodness of fit. When compared with ANN method, it was determined that PTE values in stomach and intestinal tissue estimated by FLRA method had the lowest error and high R2 values. It was found that the most effective variable in estimating the average PTE value in stomach and intestinal tissue is PTE values in soil. It was determined that the FLRA regression analysis method has a better predictive power than the ANN method. Using FLRA and ANN regression methods, it was determined that there is a statistically high relationship between PTE contents in soils and stomach and intestinal tissues. It is recommended to make the study findings more meaningful with effective and reliable service planning by using different regression analysis methods in ecological and clinical studies.

Aquaculture sediments amended with biochar improved soil health and plant growth in a degraded soil

Sustainable and safe management of aquaculture sediments is of great concern. Biochar (BC) and fishpond sediments (FPS) are rich in organic carbon and nutrients and thus can be used as soil amendments; however, it is not fully explored how the biochar amended fishpond sediments can affect soil properties/fertility and modulate plant physiological and biochemical changes, particularly under contamination stress. Therefore, a comprehensive investigation was carried out to explore the effects of FPS and BC-treated FPS (BFPS) on soil and on spinach (Spinacia oleracea L.) grown in chromium (Cr) contaminated soils. Addition of FPS and BFPS to soil caused an increase in nutrients content and reduced Cr levels in soil, which consequently resulted in a significant increase in plant biomass, chlorophyll pigments, and photosynthesis, over the control treatment. The most beneficial effect was observed with the BFPS applied at 35 %, which further increased the antioxidant enzymes (by 2.75-fold, at minimum), soluble sugars by 24.9 %, and upregulated the gene expression activities. However, the same treatment significantly decreased proline content by 74.9 %, Malondialdehyde by 65.6 %, H₂O₂ by 65.1 %, and Cr concentration in spinach root and shoot tissues. Moreover, the average daily intake analysis showed that BFPS (at 35 %) could effectively reduce human health risks associated with Cr consumption of leafy vegetables. In conclusion, these findings are necessary to provide guidelines for the reutilization of aquaculture sediments as an organic fertilizer and a soil amendment for polluted soils. However, more future field studies are necessary to provide guidelines and codes on aquaculture sediments reutilization as organic fertilizer and soil amendment for polluted soils, aiming for a more sustainable food system in China and globally, with extended benefits to the ecosystem and human.

A Comprehensive Review of the Current Progress of Chromium Removal Methods from Aqueous Solution

Chromium (Cr) exists in aqueous solution as trivalent (Cr³⁺) and hexavalent (Cr⁶⁺) forms. Cr³⁺ is an essential trace element while Cr⁶⁺ is a dangerous and carcinogenic element, which is of great concern globally due to its extensive applications in various industrial processes such as textiles, manufacturing of inks, dyes, paints, and pigments, electroplating, stainless steel, leather, tanning, and wood preservation, among others. Cr³⁺ in wastewater can be transformed into Cr⁶⁺ when it enters the environment. Therefore, research on Cr remediation from water has attracted much attention recently. A number of methods such as adsorption, electrochemical treatment, physico-chemical methods, biological removal, and membrane filtration have been devised for efficient Cr removal from water. This review comprehensively demonstrated the Cr removal technologies in the literature to date. The advantages and disadvantages of Cr removal methods were also described. Future research directions are suggested and provide the application of adsorbents for Cr removal from waters.

The interactions of Cr (VI) concentrations and amendments (biochar and manure) on growth and metal accumulation of two species of Salicornia in contaminated soil

Heavy metals are among the most dangerous contaminants in the environment. Organic components and plant species that can accumulate and stabilize heavy metals in their organs are a good option for soil remediation of these elements. Therefore, this study aimed to investigate the effects of manure and biochar on the accumulation of heavy metals by Salicornia species. Salicornia persica Akhani and Salicornia perspolitana Akhani were cultivated outdoor in experimental pots. The effects of experimental treatments, including Cr (VI) concentrations, manure, and biochar on the two studied species, were investigated. The results indicated a significant effect (p < 0.05) of biochar on the accumulation of heavy metals by two species, S. persica and S. perspolitana, so that Cr concentrations in the roots and shoots were 258 and 5.41 mg/kg, respectively. In addition, Cr accumulations under manure treatments in the roots and shoots were 334.34 and 9.79 mg/kg, respectively. The content of photosynthetic pigments in both S. persica and S. perspolitana species under biochar treatment was higher than in control and manure treatments. In general, one can conclude that the accumulation of Cr in S. perspolitana was higher than in S. persica. Applying biochar and manure amendments could stabilize Cr in soil and reduce Cr accumulation in both S. persica and S. perspolitana species.

Soil dynamics of Cr(VI) and responses of Portulaca oleracea L. grown in a Cr(VI)-spiked soil under different nitrogen fertilization regimes

The reduction potential of the highly toxic Cr(VI) to the inert Cr(III) in an alkaline soil was studied during a 50-day experiment with Portulaca oleracea L. grown in pots. We aimed at assessing whether our test species can be a phytoremediation candidate for Cr(VI)-contaminated soils. We measured the Cr(VI) reduction rate in soil, determined the Cr(VI) and Cr(III) concentrations in aerial and root P. oleracea tissues, and calculated the transfer coefficient (TC = metal in plant over metal in soil) and the translocation factor (TF = metal in aerial biomass over metal in roots) in order to assess Cr(VI) uptake and distribution in plant tissues, while we also studied the effect of added nitrogen in the studied parameters. We added five different Cr(VI) levels (from the unamended T-0 to the treatment of T-4 = 150 mg Cr(VI) kg^-1 soil) and also had two N levels (equivalent to 0 and 200 kg ha^-1). The results indicated that Cr in plant tissues was mainly found in its reduced form (Cr(III)) and only a minor fraction of Cr was detected in its oxidized form (Cr(VI)), with only 1.04% of plant Cr being hexavalent at T-4 with no added N and 1.30% at T-4 with added N. The main remediation mechanism was found to be that of the naturally occurring Cr(VI) reduction that effectively produced Cr(III), followed by the uptake of Cr(VI) from our test plants (at T-4 with no N, 58% of soil added Cr(VI) was reduced and 0.1% absorbed, while at T-4 with added N, 63% was reduced and only 0.4% absorbed by plant). We also found that Cr(VI) in P. oleracea tissues was mainly found in roots and relatively low Cr(VI) concentrations were found in the above-ground tissues. We concluded that P. oleracea is a tolerant plant species, especially if assisted with a sufficient level of N fertilization, although it failed to approach the threshold of being categorized as an accumulator species. However, as this is a rather preliminary experiment, before reaching more conclusive suggestions about P. oleracea as a potential phytoremediation species, further investigation is necessary in order to verify the gained results with naturally contaminated soils with Cr under field conditions.

Nitrogen Effect on Growth-Related Parameters and Evaluation of Portulaca oleracea as a Phytoremediation Species in a Cr(VI)-Spiked Soil

In a pot experiment, we assessed the potential of purslane (Portulaca oleracea) as a phytoremediation species in Cr(VI)-contaminated soils. We focused on the evaluation of phytotoxic Cr(VI) effects at concentrations reaching 150 mg Cr(VI) kg−1 and the possible stress amelioration effect of nitrogen on Cr(VI)-stressed plants. Treatments were T-0 (control), T-1 (25 mg Cr(VI) kg−1), T-2 = 50 mg kg−1, T-3 = 100 mg kg−1, and T-4 = 150 mg kg−1. We measured Cr(VI) concentration in aerial and root tissues, a series of parameters related to photosynthesis and plant growth, phosphorus aerial plant tissue content, and we also calculated indices (ratios) related to leaf growth and above ground tissue water content. Cr(VI) almost exclusively was found in root tissues; all physiological and growth parameters studied were severely affected and plants selectively accumulated phosphorus in aerial plant tissues with increasing Cr(VI) soil concentrations. On the other hand, N amendment resulted in improved plant features in some of the measured parameters: chlorophyll index was improved with added N at T-2, plant height was significantly higher at T-0, T-1, and T-2, and aerial dry weight and leaf area was higher at T-0; these effects indicate that added N did increase P. oleracea potential to ameliorate Cr(VI) toxic effects. We conclude that purslane showed a potential as a possible species to be successfully introduced to Cr(VI)-laden soils, but more research is certainly necessary.

Nano-remediation of toxic heavy metal contamination: Hexavalent chromium [Cr(VI)]

The inexorable industrialization and modern agricultural practices to meet the needs of the increasing population have polluted the environment with toxic heavy metals such as Cr(VI), Cu²⁺, Cd²⁺, Pb^2+, and Zn²⁺. Among the hazardous heavy metal(loid)s contamination in agricultural soil, water, and air, hexavalent chromium [Cr(VI)] is the most virulent carcinogen. The metallurgic industries, tanneries, paint manufacturing, petroleum refineries are among various such human activities that discharge Cr(VI) into the environment. Various methods have been employed to reduce the concentration of Cr(VI) contamination with nano and bioremediation being the recent advancement to achieve recovery at low cost and higher efficiency. Bioremediation is the process of using biological sources such as plant extracts, microorganisms, and algae to reduce the heavy metals while the nano-remediation uses nanoparticles to adsorb heavy metals. In this review, we discuss the various activities that liberate Cr(VI). We then discuss the various conventional, nano-remediation, and bioremediation methods to keep Cr(VI) concentration in check and further discuss their efficiencies. We also discuss the mechanism of nano-remediation techniques for better insight into the process.

Evaluating Cr behaviour in two different polluted soils: Mechanisms and implications for soil functionality

This work investigates the mechanisms determining Cr speciation and availability in two different soils polluted with two chromium sources (an industrial sludge, highly polluted with Cr, and Cr(VI) solution) and the influence of these parameters on the recovery of the soil functions related with biological quality and plant growth. The experiment was carried out in greenhouse conditions using 36 pots of 17 kg for the growth of Silene vulgaris for 21 months. Logistic Regression Model using Lasso estimator shows that soil organic matter (SOM) and pH control Cr availability in studied soils. In soils treated with the sludge, X ray Absorption spectroscopy showed that Cr was present as Cr(III), biological quality indicators increased and plants were able to grow. However, in soils polluted with Cr(VI), Cr availability was significantly different in the two soils. In the alkaline and poor in organic matter soil, 12% of Cr(VI) remained in the soil leading to the decrease of soil quality indicators and the total inhibition of plant growth. In the neutral soil, Cr(VI) was totally reduced to Cr(III) by soil organic matter (SOM), quality indicators were not affected and plants grown properly. Infrared Spectroscopy showed that different functional groups reacted with Cr in the two soils. This study highlights the importance to understand the mechanisms underlaying Cr redox and adsorption reactions in Cr polluted soils as they determine the potential recovery of the functions related with biological quality indicators and plant growth. The methodology proposed allows this study in complex soil samples at realistic concentrations and may be useful for risk assessment and for the planning of managing strategies in Cr polluted soils.

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 the application of an organic amendment and nanoscale zero-valent iron particles on soil Cr(VI) remediation

Chromium is considered an environmental pollutant of much concern whose toxicity depends, to a great extent, on its valence state, with Cr(VI) being more soluble, bioavailable, and toxic, compared to Cr(III). Nanoremediation is a promising strategy for the remediation of metal pollutants by changing their valence state. However, among other aspects, its effectiveness for soil remediation is seriously hampered by the interaction of nanoparticles with soil organic matter. In this study, soil was (i) amended with two doses of a municipal solid organic waste and (ii) artificially polluted with 300 mg Cr(VI) kg-1 DW soil. After a period of aging, a nanoremediation treatment with nanoscale zero-valent iron particles (1 g nZVI kg-1 DW soil) was applied. The efficiency of the remediation treatment was assessed in terms of Cr(VI) immobilization and recovery of soil health. The presence of the organic amendment caused (i) a decrease of redox potential, (ii) Cr(VI) immobilization via its reduction to Cr(III), (iii) a stimulation of soil microbial communities, and (iv) an improvement of soil health, compared to unamended soil. By contrast, nZVI did not have any impact on Cr(VI) immobilization nor on soil health. It was concluded that, unlike the presence of the organic amendment, nanoremediation with nZVI was not a valid option for soils polluted with Cr(VI) under our experimental conditions.

Gentle remediation options for soil with mixed chromium (VI) and lindane pollution: biostimulation, bioaugmentation, phytoremediation and vermiremediation

Gentle Remediation Options (GROs), such as biostimulation, bioaugmentation, phytoremediation and vermiremediation, are cost-effective and environmentally-friendly solutions for soils simultaneously polluted with organic and inorganic compounds. This study assessed the individual and combined effectiveness of GROs in recovering the health of a soil artificially polluted with hexavalent chromium [Cr(VI)] and lindane. A greenhouse experiment was performed using organically-amended vs. non-amended mixed polluted soils. All soils received the following treatments: (i) no treatment; (ii) bioaugmentation with an actinobacteria consortium; (iii) vermiremediation with Eisenia fetida; (iv) phytoremediation with Brassica napus; (v) bioaugmentation + vermiremediation; (vi) bioaugmentation + phytoremediation; and (vii) bioaugmentation + vermiremediation + phytoremediation. Soil health recovery was determined based on Cr(VI) and lindane concentrations, microbial properties and toxicity bioassays with plants and worms. Cr(VI) pollution caused high toxicity, but some GROs were able to partly recover soil health: (i) the organic amendment decreased Cr(VI) concentrations, alleviating toxicity; (ii) the actinobacteria consortium was effective at removing both Cr(VI) and lindane; (iii) B. napus and E. fetida had a positive effect on the removal of pollutants and improved microbial properties. The combination of the organic amendment, B. napus, E. fetida and the actinobacteria consortium was the most effective strategy.

Nitrogen and sulfur codoped micro-mesoporous carbon sheets derived from natural biomass for synergistic removal of chromium(VI): adsorption behavior and computing mechanism

We reported the effective removal of chromium(VI) (Cr(VI)) from wastewater with nitrogen and sulfur codoped micro-mesoporous carbon sheets (N,S-MMCSs), which were fabricated by pyrolysis of natural biomass (luffa sponge) followed by chemical activation and hydrothermal treatment. N,S-MMCSs possessed a hierarchical micro-mesoporous sheet-like framework, large specific surface area (1525.45 m² g^-1), high pore volume (1.21 cm³ g^-1), and appropriate N (1.81 wt%) and S (1.01 wt%) co-doping. Batch adsorption experiments suggested that Cr(VI) adsorption by the N,S-MMCSs increased with increase the solution acidity, adsorbent dosage, Cr(VI) concentration, temperature, and time. The Cr(VI) adsorption was mainly controlled by the chemisorptions and could be well interpreted by the Langmuir isotherm and pseudo-second-order kinetic models. The maximum adsorption capacities of Cr(VI) were 217.39, 277.78, and 312.50 mg g^-1 at 298, 308, and 318 K, respectively. The Cr(VI) adsorption procedure was spontaneous, endothermic, and randomness. The Cr(VI) adsorption mechanism followed the physical adsorption, electrostatic attraction, in situ reduction, and surface chelation. Besides, the density functional theory (DFT) calculation demonstrated that the N and S co-doping could decrease the adsorption energy and enhance the attractive interaction between N,S-MMCSs and Cr(VI) through the synergistic effect, and thus significantly improve the Cr(VI) adsorption property.

Effects of heavy metals on organic matter decomposition in inundated soils: Microcosm experiment and field examination

Microcosm and field investigation were conducted to examine the effects of heavy metals on the decomposition and accumulation of organic carbon in contaminated Mersey estuarine floodplain, northwest England. The results show that inhibition of microbially mediated decomposition of organic matter occurred in the water-inundated soils. However, individual heavy metals had differential effects on the inhibition of soil organic matter decomposition with arsenic and copper being much stronger, as compared to other investigated heavy metals. The weak inhibitory effects of chromium on organic matter decomposition was due to the conversion of highly toxic Cr(VI) to less toxic Cr(III) under reducing conditions. Lead also had a weaker capacity to inhibit organic matter decomposition due to its low solubility. It was surprising that the same phenomenon was not clearly observed during the field examination. The inhibitory effects of heavy metals on soil organic matter decomposition could be curtained under field conditions. pH, Eh and EC played more important roles, as compared to soil-borne heavy metals, in affecting the soil carbon dynamics in the contaminated Mersey estuarine floodplain.

Characteristics and applications of biochar for remediating Cr(VI)-contaminated soils and wastewater

Chromium (Cr) is a common environmental contaminant due to industrial processes and anthropogenic activities such as mining of chrome ore, electroplating, timber treatment, leather tanning, fertilizer and pesticide, etc. Cr exists mainly in both hexavalent [Cr(VI)] and trivalent [Cr(III)] form, being Cr(VI) with non-degradability and potential to be hidden, thereby affecting surrounding environment and being toxic to human health. Therefore, researches on remediation of Cr pollution in the environment have received much attention. Biochar is a low-cost adsorbent, which has been identified as a suitable material for Cr(VI) immobilization and removal from soil and wastewater. This review incorporates existing literature to provide a detailed examination into the (1) Cr chemistry, the source and current status of Cr pollution, and Cr toxicity and health; (2) feedstock and characterization of biochar; (3) processes and mechanisms of immobilization and removal of Cr by biochar, including oxidation-reduction, electrostatic interactions, complexation, ion exchange, and precipitation; (4) applications of biochar for Cr(VI) remediation and the modification of biochar to improve its performance; (5) factors affecting removal efficiency of Cr(VI) with respect to its physico-chemical conditions, including pH, temperature, initial concentration, reaction time, biochar characteristics, and coexisting contaminants. Finally, we identify current issues, challenges, and put forward recommendations as well as proposed directions for future research. This review provides a thorough understanding of using biochar as an emerging biomaterial adsorbent in Cr(VI)-contaminated soils and wastewater.

Effects of Methane Fermentation on Spectral Properties of Fulvic Acid Extracted from Peat through Liquid Acid Precipitation

The effects of acid precipitation with different liquid acids on spectral properties of fulvic acid extracted from peat was studied to select the most appropriate liquid acid for the acid precipitation step of the alkali solution acid precipitation method, and the new process mechanism of peat utilization by coupling methane fermentation and extraction of fulvic acid was analyzed. The alkali solution acid precipitation method was adopted to extract fulvic acid from the methane-fermented group and methane-unfermented group. Different liquid acids were used to conduct acid precipitation during the extraction. And then, the characterizations of fulvic acid samples were conducted through Fourier infrared spectroscopy, UV-Vis spectroscopy, and fluorescence spectroscopy. The yield and content of fulvic acid decreased significantly after methane fermentation. During the methane fermentation process, some fulvic acid was consumed and utilized by microorganisms to promote the methane fermentation process, resulting both in the decrease of methyl, hydroxyl, and ether bonds and in the increase of methylene, carbonyl, conjugated double bond, benzene rings, and other groups. The E4/E6 ratio shows that the E4/E6 ratio of fulvic acid was decreased after methane fermentation, and the fermentation consumed the functional group with simpler structure such as aliphatic chain hydrocarbon of fulvic acid while the structure with a higher degree of aromatization and conjugate cannot be consumed. When conducting acid precipitation with different liquid acids, the yield of fulvic acid was the highest through acid precipitation with phosphoric acid. The fulvic acid obtained through acid precipitation with nitric acid has a higher content, more benzene rings, and the highest degree of aromatization.

Concentrations of porewater heavy metals, their benthic fluxes and the potential ecological risks in Daya Bay, South China

Heavy metal (Cr, Mn, Fe, Ni, Cu, Zn, Cd, Pb) concentrations in surface sediment porewater and their benthic fluxes were investigated in Daya Bay, South China, to study their accumulation and transfer at the sediment-water interface, as well as the impact of human activities on heavy metals. Heavy metals in porewater displayed different patterns in three partitions (top, center and inlet), which was mainly attributed to the difference in the biogeochemical conditions, hydrodynamic force inner the bay and the human activities along the bay. Ecological risk assessment results showed that heavy metals in porewater dramatically exceeded the background values. The average release of heavy metals from sediment were (6.1 ± 3.3) × 10⁴-(2.7 ± 1.6) × 10⁸ g a^-1 in the bay, so they had potential risks to the water environment, and sediment should be paid more attention to as the endogenesis of contamination.

Comparable effects of manure and its biochar on reducing soil Cr bioavailability and narrowing the rhizosphere extent of enzyme activities

Chromium (Cr) contamination is especially hazardous to soil biota. Application of manure and biochar has been frequently proposed to remediate Cr-contaminated soil. However, the understanding of mechanisms behind manure and biochar impacts on soil enzyme activities requires advanced visualization technologies. For the first time, we compared manure and its biochar influence on the spatial distribution of β-glucosidase, N-acetyl-glucosaminidase and phosphomonoesterase activities in Cr-contaminated soil using direct zymography. Maize was planted for 45 days in (a) soil mixed with manure, (b) soil mixed with manure-derived biochar and (c) soil without any addition. Soil pH decreased over 45 days, inducing an increase in acid soluble Cr. The concomitant decrease in β-glucosidase and N-acetyl-glucosaminidase activities explained the narrowing rhizosphere extent of enzyme activities by 13-44%, indicating that increased Cr bioavailability decreases microbial activities. A larger maize performance index and the greatest plant shoot/root ratio after biochar application suggested enhanced maize growth (p < 0.05). In contrast, manure induced the narrowest extent of β-glucosidase and phosphomonoesterase activities due to the addition of labile organic compounds and nutrients following its application. Our study emphasizes the importance of pH on Cr bioavailability and enzyme activities and demonstrates that biochar application is more ideally suited for remediating Cr-contaminated soil.

The applicability of compost, zeolite and calcium oxide in assisted remediation of acidic soil contaminated with Cr(III) and Cr(VI)

The effect of soil amendments, i.e., compost, zeolite, and calcium oxide, on the chemical properties of soil contaminated with Cr(III) and Cr(VI) and the uptake of selected heavy metals by spring barley (Hordeum vulgare L.) and maize (Zea mays L.) was determined in a pot experiment. The content of all investigated heavy metals in the tested plants varied significantly in response to the tested soil amendments and increasing concentrations of Cr(III) and Cr(VI). Compost, zeolite, and calcium oxide contributed to an increase in the average yield of the aerial parts of maize plants only in treatments contaminated with Cr(III). The concentrations of Cr, Zn, and Ni in the aerial parts of spring barley and maize were higher in treatments contaminated with Cr(III) than in treatments contaminated with Cr(VI). Calcium oxide induced a significant increase in soil pH relative to the control treatment. In treatments without soil amendments, the average Cr content of soil was higher in pots contaminated with Cr(VI). The concentrations of Zn and Cu in non-amended treatments were negatively correlated with increasing doses of Cr(III) and Cr(VI). Calcium oxide decreased the average content of Cr, Cu, and Ni in all experimental variants. Compost increased the average content of Zn in treatments contaminated with Cr(III) and Cr(IV) relative to non-amended soil.

The variation of metal fractions and potential environmental risk in phytoremediating multiple metal polluted soils using Noccaea caerulescens assisted by LED lights

Different light combinations can improve phytoremediation efficiency by increasing the biomass yield and metal concentrations of plants. However, there has been rare research of using hyperaccumulators to change metal fractions and its possible leaching risk during phytoremediation. It was investigated in this study the impacts of different intensities of blue and red light mixed on the biomass production and metal uptake of Noccaea caerulescens and the changes of water soluble and exchangeable metal fractions in soil. The biomass of N. caerulescens increased with light intensity. The increment was relatively slow at 50 m^-2 s^-1, dramatically increased at 200 m^-2 s^-1 and decreased significantly when beyond. Under optimal light condition, N. caerulescens produced less biomass than Thlaspi arvense, but the former is significantly more efficient in phytoremediation than the latter because it can accumulate significantly more metals per unit biomass. Without light irradiation, N. caerulescens can deteriorate the potential leaching risk of Cu and Pb by increasing their water soluble and exchangeable fractions in soil comparing with T. arvense. The proportions of bioavailable fractions did not change under the treatment of light at an intensity of 50 m^-2 s^-1, but decreased obviously when the intensity exceeded 100 m^-2 s^-1. Therefore, using hyperaccumulator for multiple metal contaminated soil remediation should be conducted with caution since the species can mobilize all metals in soil but only hyperaccumulate part of them, and proper intensity of light can improve the phytoremediation effect and alleviate the leaching risk through decreasing bioactive metal fractions in soil.

Enhancing phytoextraction of potentially toxic elements in a polluted floodplain soil using sulfur-impregnated organoclay

Enhancing metals phytoextraction using gentile mobilizing agents might be an appropriate approach to increase the phytoextraction efficiency and to shorten the phytoremediation duration. The effect of sulfur-impregnated organoclay (SIOC) on the redistribution of potentially toxic elements (PTEs) among their geochemical fractions in soils and their plant uptake has not yet been studied. Therefore, our aim is to investigate the role of different SIOC application doses (1%, 3% and 5%) on operationally defined geochemical fractions (soluble + exchangeable; bound to carbonate; manganese oxide; organic matter; sulfide; poorly- and well-crystalline Fe oxide; and residual fraction) of Cd, Cr, Cu, Ni, Pb, and Zn, and their accumulation by pea (Pisum sativum) and corn (Zea mays) in a greenhouse pot experiment using a polluted floodplain soil. The SIOC caused a significant decrease in soil pH, and an increase in organic carbon and total sulfur content in the soil. The addition of SIOC increased significantly the soluble + exchangeable fraction and bioavailability of the metals. The SIOC leads to a transformation of the residual, organic, and Fe-Mn oxide fractions of Cd, Cu, Ni, and Zn to the soluble + exchangeable fraction. The SIOC addition increased the potential mobile (non-residual) fraction of Cr and Pb. The SIOC increased the sulfide fraction of Cr, Ni, and Zn, while it decreased the same fraction for Cd, Cu, and Pb. The effect of SIOC on the redistribution of metal fractions increased with enhancing application dosages. Pea accumulated more metals than corn with greater accumulation in the roots than shoots. Application of the higher dose of SIOC promoted the metals accumulation by roots and their translocation to shoots of pea and corn. Our results suggest the potential suitability of SIOC for enhancing the phytomanagement of PTEs polluted soils and reducing the environmental risk of these pollutants.

Goethite catalyzed Cr(VI) reduction by tartaric acid via surface adsorption

The surface catalysis of goethite on the Cr(VI) reduction by tartaric acid was examined together with its adsorption characteristics towards the two reactants. The results showed the adsorption of tartaric acid by goethite was favorable at low pH and adsorption isotherm could be properly described by Langmuir model. The adsorption kinetic curves for both reactants obeyed the pseudo second-order rate model (R² >0.99). The FTIR spectrum suggested the formation of bidentate binuclear surface complexes between tartaric acid and goethite. At pH 4.50, the reduction percentage of 0.1 mM Cr(VI) by 1.0 mM tartaric acid alone was about 12% after 72 h, while which was increased to 100% in the presence of goethite within 24 h. Kinetic results revealed the Cr(VI) reduction only occurred between the adsorbed tartaric acid and the aqueous Cr(VI) since the Cr(VI) adsorption was completely inhibited under the examined conditions. Meanwhile, the catalysis of aqueous Fe(III) released from the goethite surfaces was excluded due to its low concentration (<5 μM). With the initial concentration of tartaric acid decreased to 0.1 mM, Cr(VI) reduction could be completed within 4 h, confirmed by the XPS result that only Cr(III) species existed on the goethite surfaces. In this case, electron transfer was suggested to occur directly between the two adsorbed reactants or goethite was believed to serve as an ideal channel to allow electron excited from the adsorbed tartaric acid to transfer to the adsorbed Cr(VI). The findings above were helpful for us to understand the Cr(VI) reduction by organic compounds in soils with rich contents of Fe-oxides.

Varying concentrations of soil chromium (VI) for the exploration of tolerance thresholds and phytoremediation potential of the oregano (Origanum vulgare)

Varying concentrations of soil Cr(VI) were used in order to explore the tolerance thresholds and phytoremediation potential of Greek oregano (Origanum vulgare), in a pot experiment conducted outdoors. Oregano exhibited a rather exceptional capacity to bioaccumulate Cr in both the aerial part (up to 1200 mg of total Cr kg^-1 DM) and the root-reaching 4300 mg kg^-1 DM when grown in soil [Cr(VI)] of 150-200 mg kg^-1. Plant responses indicated that there was a threshold set at 100 mg Cr(VI) kg^-1 in the soil, above which the following results were recorded: (i) a restriction of Cr translocation from below- to above-ground plant part, (ii) a raise of the soil-to-root Cr transfer, and (iii) the Cr(III) evolution from the reduction of Cr(VI) was significantly decelerated in the root and accelerated in the aerial part. Soil [Cr] that surpassed this threshold challenged plant tolerance, resulting in a dose-dependent reduction of growth and antioxidant phenolics pool. Nonetheless, the significant Cr uptake capacity at plant level accounted for the considerably short remediation time (i.e., 29 years at soil [Cr(VI)] of 150 mg kg^-1) calculated according to these results. The overall performance of oregano indicated that phytoremediation would be feasible at sites with Cr contamination levels ranging within the above-defined thresholds.

Transcriptome analysis of silver, palladium, and selenium stresses in Pantoea sp. IMH

Heavy metal contamination is a significant environmental issue. Using bacteria for removal and reduction of heavy metals is an attractive alternative owing to its low-cost and eco-friendly properties. However, the mechanisms of resistance to and reduction of Ag(I), Pd(II), and Se(IV), especially in the same strain, remain unclear. Here, Pantoea sp. IMH was examed for its reduction of Ag(I), Pd(II), and Se(IV) to nanoparticles (NPs), and the molecular mechanism was investigated by transcriptome analysis. The results revealed that genes encoding binding, transport, catalytic activity, and metabolism were differentially expressed in cells exposed to Ag(I), Pd(II), and Se(IV). The same resistance mechanisms for all metals included multiple stress resistance protein BhsA and glutathione detoxification metabolism. However, zinc transport protein and sulfate metabolism played an important role in the resistance to cationic metals (Ag⁺ and Pd²⁺), while the oxalate transporter and arsenic resistance mechanisms were specifically involved in the resistance to and reduction of anion (SeO₃^2-). In addition, Ag(I) was speculated to be reduced to AgNPs by glucose and cytochrome CpxP was involved in Pd(II) reduction. Our results provided new clues on the mechanisms of resistance to and reduction of Ag(I), Pd(II), and Se(IV).

Enhanced sorption of hexavalent chromium [Cr(VI)] from aqueous solutions by diluted sulfuric acid-assisted MgO-coated biochar composite

Metal oxide-Carbon composites have aroused great interesting towards specific anionic contaminants removal from the polluted environment. In this study, aiming at removing toxic chromate ion [Cr(VI)] from aqueous solutions, a novel approach was developed to produce surface-enhanced MgO-coated biochar adsorbent from sugarcane harvest residue (SHR). It was found that sulfuric acid hydrolysis and MgO-coating both facilitated the removal of Cr(VI) by biochars, and the maximum sorption capacities for the pristine biochar (SHR₅₅₀), MgO-coated biochar (MgSHR₅₅₀), and acid-assisted MgO-coated biochar (MgASHR₅₅₀) that derived from the Langmuir isotherm model were 20.79, 54.64, and 62.89 mg g^-1, respectively. Additionally, the Cr(VI) removal was a pseudo-second-order kinetic model controlled process with equilibrium reached within 24 h. The mechanism investigation revealed that Cr(VI) ions was directly sorbed by the MgO-coated biochars via the chemical interaction between MgO and Cr(VI), whereas the sorption-coupled reduction of Cr(VI) to Cr(III) governed the sorption of Cr(VI) on the SHR₅₅₀. Although the increases of solution pH (>2.0) and KNO₃ concentration (>0.05 mol L^-1) reduced the Cr(VI) removal by biochars, while there were lower secondary pollution risks in MgO-coated biochar treatments due to the suppressed release of Cr(III) in solutions. This work could provide guidance for the production of efficient biochar for the removal of Cr(VI) from wastewater.