Oxidizable Forms Research Articles

Adsorption mechanism and remediation of heavy metals from soil amended with hyperthermophilic composting products: Exploration of waste utilization

Due to high humification, hyperthermophilic composting products (HP) show potential for remediating heavy metal pollution. However, the interaction between HP and heavy metals remains unclear. This study investigated the adsorption mechanism and soil remediation effect of HP on heavy metals. The results showed that the maximum adsorption capacity of HP increased by an average of 30.74 % compared to conventional composting products. HP transformed 34.87 % of copper, 42.55 % of zinc, and 35.63 % of lead from exchangeable and reducible forms into residual and oxidizable forms, thus reducing the soil risk level. In conclusion, HP significantly enhanced the adsorption of heavy metals and their transformation from unstable to stable forms, primarily due to the higher content of hydroxyl and carboxyl groups. This study aims to demonstrate the effectiveness of HP for remediating heavy metal pollution and to enhance the understanding of the underlying mechanism, which lays a foundation for waste utilization.

Redox-active vitamin C suppresses human osteosarcoma growth by triggering intracellular ROS-iron–calcium signaling crosstalk and mitochondrial dysfunction

Pharmacological vitamin C (VC) has gained attention for its pro-oxidant characteristics and selective ability to induce cancer cell death. However, defining its role in cancer has been challenging due to its complex redox properties. In this study, using a human osteosarcoma (OS) model, we show that the redox-active property of VC is critical for inducing non-apoptotic cancer cell death via intracellular reactive oxygen species (ROS)-iron-calcium crosstalk and mitochondrial dysfunction. In both 2D and 3D OS cell culture models, only the oxidizable form of VC demonstrated potent dose-dependent cytotoxicity, while non-oxidizable and oxidized VC derivatives had minimal effects. Live-cell imaging showed that only oxidizable VC caused a surge in cytotoxic ROS, dependent on iron rather than copper. Inhibitors of ferroptosis, a form of iron-dependent cell death, along with classical apoptosis inhibitors, were unable to completely counteract the cytotoxic effects induced by VC. Further pharmacological and genetic inhibition analyses showed that VC triggers calcium release through inositol 1,4,5-trisphosphate receptors (IP3Rs), leading to mitochondrial ROS production and eventual cell death. RNA sequencing revealed down-regulation of genes involved in the mitochondrial electron transport chain and oxidative phosphorylation upon pharmacological VC treatment. Consistently, high-dose VC reduced mitochondrial membrane potential, oxidative phosphorylation, and ATP levels, with ATP reconstitution rescuing VC-induced cytotoxicity. In vivo OS xenograft studies demonstrated reduced tumor growth with high-dose VC administration, concomitant with the altered expression of mitochondrial ATP synthase (MT-ATP). These findings emphasize VC's potential clinical utility in osteosarcoma treatment by inducing mitochondrial metabolic dysfunction through a vicious intracellular ROS-iron-calcium cycle.

Speciation distribution and leaching behavior of heavy metals in coal gasification fine ash: Influence of particle size, carbon content and mineral composition

In this study, the occurrence and distribution of heavy metals in coal gasification fine ash (CGFA) with different particle sizes were investigated to ensure safer disposal and utilization strategies for CGFA. These measures are critical to sustainable industrial practices. This study investigates the distribution and leachability of heavy metals in CGFA, analyzing how these factors vary with particle size, carbon content, and mineral composition. The results demonstrated that larger CGFA particles (>1 mm) encapsulated up to 70 % more heavy metals than smaller particles (<0.1 mm). Cr and Zn were present in higher concentrations in larger CGFA particles, whereas volatile elements such as Zn, Hg, Se, and Pb were found in relatively higher contents in finer CGFA particles. At least 70 % of Hg in CGFA was present in an acid-soluble form of speciation, whereas Cd, Zn, and Pb were mostly present in a reducible form of speciation, which could be attributed to the presence of franklinite. More than 40 % of Cd and Zn in fine CGFA particles exist in an acid-soluble form. With the exception of CGFA_1.18, Se in CGFA mainly existed in an oxidizable form at a ratio of 60 %–80 %. This could be attributed to the presence of bassanite particles as well as the higher affinity of Se for S. In contrast, Cr, Cu, and As were mostly present in residual speciation forms owing to their parasitism in quartz, sillimanite, and amorphous Fe solid solution in CGFA. Additionally, the study revealed that there was no significant relationship between heavy metal content, leaching behavior, and carbon content in CGFA. Based on combined analyses using toxicity characteristic leaching procedure (TCLP) leaching concentrations and risk assessment code (RAC) results, it is recommended to focus on the environmental risks posed by Cd, Cr, Pb, Zn, and Hg in CGFA during their modification and utilization processes.

Zea mays cultivation, biochar, and arbuscular mycorrhizal fungal inoculation influenced lead immobilization.

Plant cultivation can influence the immobilization of heavy metals in soil. However, the roles of soil amendments and microorganisms in crop-based phytoremediation require further exploration. In this study, we evaluated the impact of Zea mays L. cultivation, biochar application, and arbuscular mycorrhizal fungi (AMF) inoculation on soil lead (Pb) immobilization. Our results indicated that biochar addition resulted in a significant, 42.00%, reduction in AMF colonization. Plant cultivation, AMF inoculation, and biochar addition all contributed to enhanced Pb immobilization, as evidenced by decreased levels of diethylenetriaminepentaacetic acid- and CaCl2-extractable Pb in the soil. Furthermore, soil subjected to plant cultivation with AMF and biochar displayed reduced concentrations of bioavailable Pb. Biochar addition altered the distribution of Pb fractions in the soil, transforming the acid-soluble form into the relatively inert reducible and oxidizable forms. Additionally, biochar, AMF, and their combined use promoted maize growth parameters, including height, stem diameter, shoot and root biomass, and phosphorus uptake, while simultaneously reducing the shoot Pb concentration. These findings suggest a synergistic effect in Pb phytostabilization. In summary, despite the adverse impact of biochar on mycorrhizal growth, cultivating maize with the concurrent use of biochar and AMF emerges as a recommended and effective strategy for Pb phytoremediation.IMPORTANCEHeavy metal contamination in soil is a pressing environmental issue, and phytoremediation has emerged as a sustainable approach for mitigating this problem. This study sheds light on the potential of maize cultivation, biochar application, and arbuscular mycorrhizal fungi (AMF) inoculation to enhance the immobilization of Pb in contaminated soil. The findings demonstrate that the combined use of biochar and AMF during maize cultivation can significantly improve Pb immobilization and simultaneously enhance maize growth, offering a promising strategy for sustainable and effective Pb phytoremediation practices. This research contributes valuable insights into the field of phytoremediation and its potential to address heavy metal pollution in agricultural soils.

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.

Chemical Speciation Distribution and Thermal Stability of Heavy Metals along Flue Gas Cleaning Systems in a Hazardous Waste Incinerator

Hazardous heavy metals may release from incineration residues in the utilization concerning heat treatment or landfills, which would cause great harm on the environment. The evaluation of chemical speciation and thermal stability of heavy metals are of great significance for the control of heavy metal pollution during heat treatments. In this study, chemical speciation distribution and thermal stability of heavy metals (As, Cd, Cr, Cu, Pb, Se, Zn, and Hg) along flue gas cleaning systems were investigated in a hazardous waste incinerator located in Zhejiang, China. The chemical speciation and thermal stability of heavy metals, environmental risk evaluation, and morphological and mineralogical characteristics of incineration residues were assessed before and after heat treatments. Results indicated that the chemical speciation of heavy metals in the bottom slag, burn-out chamber ash, boiler ash, and bag filter ash varied significantly. The thermal stability of heavy metals in the incineration residues was significantly influenced by chemical speciation, and the thermal stability of the chemical speciation decreased basically in the order of residual form > reducible form > oxidizable form > acid soluble form. As, Cr, and Se showed high thermal stability due to the fact that they were involved in the crystallization reaction, and heat treatment would promote the transformation from mobile fractions into the residual fraction. Hg and Cd displayed the worst thermal stability, and high-temperature heat treatment would contribute to secondary volatilization from the incineration residues. More attention should be paid to the secondary volatilization of Hg and Cd during thermal disposal. Cu, Pb, and Zn had moderate thermal stability, and heat treatment would cause the volatilization of mobile fractions and the transformation from mobile fractions into the residual fraction. Pretreatments such as water-washing or adding a chemical stabilizer would be conducive to reducing the volatilization of heavy metals during thermal disposal.

Synergistic Effects of Earthworms and Plants on Chromium Removal from Acidic and Alkaline Soils: Biological Responses and Implications.

Soil heavy metal pollution has become one of the major environmental issues of global concern and solving this problem is a major scientific and technological need for today's socio-economic development. Environmentally friendly bioremediation methods are currently the most commonly used for soil heavy metal pollution remediation. Via controlled experiments, the removal characteristics of chromium from contaminated soil were studied using earthworms (Eisenia fetida and Pheretima guillelmi) and plants (ryegrass and maize) at different chromium concentrations (15 mg/kg and 50 mg/kg) in acidic and alkaline soils. The effects of chromium contamination on biomass, chromium bioaccumulation, and earthworm gut microbial communities were also analyzed. The results showed that E. fetida had a relatively stronger ability to remove chromium from acidic and alkaline soil than P. guillelmi, and ryegrass had a significantly better ability to remove chromium from acidic and alkaline soil than maize. The combined use of E. fetida and ryegrass showed the best effect of removing chromium from contaminated soils, wih the highest removal rate (63.23%) in acidic soil at low Cr concentrations. After soil ingestion by earthworms, the content of stable chromium (residual and oxidizable forms) in the soil decreased significantly, while the content of active chromium (acid-extractable and reducible forms) increased significantly, thus promoting the enrichment of chromium in plants. The diversity in gut bacterial communities in earthworms decreased significantly following the ingestion of chromium-polluted soil, and their composition differences were significantly correlated with soil acidity and alkalinity. Bacillales, Chryseobacterium, and Citrobacter may have strong abilities to resist chromium and enhance chromium activity in acidic and alkaline soils. There was also a significant correlation between changes in enzyme activity in earthworms and their gut bacterial communities. The bacterial communities, including Pseudomonas and Verminephrobacter, were closely related to the bioavailability of chromium in soil and the degree of chromium stress in earthworms. This study provides insights into the differences in bioremediation for chromium-contaminated soils with different properties and its biological responses.

Chemodestructive fractionation of humus as an indicator of the functional state of soil organic matter

The content, composition, and properties of humus are the most important characteristics that reflect the features of modern soil formation, the genesis and evolution of soils, and their agrogenic transformation. The main criteria for assessing soil fertility are traditionally the content and reserves of organic matter in soils. At the same time, the decrease in soil fertility is associated not so much with a decrease in the total content of humus, but with the loss of its labile forms, which determine the life of the soil, its most important agronomic properties and crop yields. One of the integral indicators characterizing the state and functioning of soil organic matter (SOM) is the ratio of stable and labile forms of organic compounds obtained by chemodestructive analysis. The determination of the bichromate oxidizability of humus showed that the virgin soils of the Southern Cis-Baikal region contain an equal amount of stable and labile forms of organic compounds. Accordingly, the humus in them is a stable and balanced system in relation to external influences. Plowing led to a significant decrease in the easily and hardly hydrolysable humus fractions. In the fallow areas, the upper part of the humus horizon, in terms of the ratio of humus fractions, approaches virgin soils, and the lower part approaches the arable horizons of agricultural soils. The method of permanganate oxidizability of humus revealed a noticeable enrichment in easily oxidizable forms of virgin and, especially, fallow soils of the region. Compared to them, agricultural soils contained noticeably less labile humus, which indicates a sharp decrease in their fertility.

The Influence of the Reduction in Clay Sediments in the Level of Metals Bioavailability-An Investigation in Liujiang River Basin after Wet Season.

The seasonal elevation of metals' bioavailability can aggravate the threat of metal contamination in the aquatic environment. Nevertheless, their regulations have rarely been studied, particularly the connections between metals' transformation and environmental variations. Therefore, the catchment area of Liujiang River was taken as an example in this study, their seasonal variations in metals' bioavailability in sediments, especially during the wet season, was investigated to recover the processes associated with metals' speciations and multiple environmental factors. The results revealed that the concentration of metals in sediments were high overall in the wet season, but low in the dry season. The significantly reduced ratio of metals in non-residual forms was largely related to the overall reduction in metals in oxidizable and reducible forms after the wet season. However, the elevated BI indexes of most metals suggested their increased bioavailability in the dry season, which should be closely related to their corresponding elevations in carbonate-bound and exchangeable forms after the wet season. The variations in metals' bioavailability were primarily related to their predominance of exchangeable and carbonate-bound form. The higher correlation coefficients suggested the destabilization of the oxidizable form should be treated as a critical approach to the impact of metals' bioavailability after the wet season. In view of that, sediments' coarsening would pose the impacts on the destabilization of exogenous metals in sediments, the reduction in clay sediments should be responsible for the elevation of metals bioavailability after the wet season. Therefore, the monitoring of metals' bioavailability in sediments should be indispensable to prevent metal contamination from enlarging the scope of their threat to the aquatic environment of the river, especially after the wet season.

INFLUENCE OF FERTILIZERS APPLICATION ON MOBILITY OF METALS

Metals play an important role in the life of plants and animals, and in small quantities, most of them are necessary for the normal functioning of living organism. However, increased concentrations can also harm the same functions of a living organism. In the soil cover, metals can be contained in various species, which determine their mobility and toxicity. Metals are transferred from soil to plants and then to animal and human organisms only if they are mobile. The mobile fractions include the water-soluble, exchangeable, carbonate, reducible and oxidizable forms and they can migrate from one environment to another and cause a negative cumulative effect. An increase in anthropogenic load leads to an increase in the mobility of metals. A special place is occupied by agricultural activity, which, using various fertilizers, increases the proportion of geochemically mobile species of metals. This work is aimed at studying the effect of fertilization on the mobility of some metals. As part of the work, the soils of the Almaty region (Baiterek village) of the Republic of Kazakhstan were studied by applying various fertilizers (ammonium nitrate and monopotassium phosphate), followed by studying the species of metals and the most toxic radionuclides (polonium-210 and lead-210) according to the method of A. Tessier. Using the fertilizers under investigation insignificantly affects the species of alkali metals, polonium-210, and lead-210. While application of monopotassium phosphate increases the geochemical mobility of sodium and alkaline earth elements due to the transition to the water-soluble and exchangeable fractions, and heavy metals due to the transition to the exchangeable fraction. The obtained data are useful as the basis for the development of recommendations for the application of certain types of fertilizers to reduce the risk of impact on the life and health of the local population without reducing the productivity of agricultural crops.

Speciation Characteristics and Risk Assessment of Soil Heavy Metals from Puding Karst Critical Zone, Guizhou Province

The Community Bureau of Reference (BCR) sequential extraction method was used to analyze the distribution and chemical fractions of soil heavy metals (i.e., Cd, Cr, Zn, Fe, Ni, and Mn) under different land uses from the Puding karst critical zone, and the bioavailability and potential ecological risk of these heavy metals were evaluated. The results showed that the Fe, Zn, Cr, and Ni mainly were mainly concentrated in residual fractions and not likely to be absorbed by organisms, whereas the available fractions of Cd and Mn were in higher concentration, which retained strong potential migration and bioavailability. The richer organic matter in Puding soil might have promoted the transformation of oxidizable speciation of Fe, Ni, Cr, and Cd. The oxidizable forms of Fe, Mn, and Ni tended to accumulate in macro-aggregate soil. The risk assessment of RAC and RSP showed that the Puding karst soil was at slight ecological risk, and most heavy metals (except Cd) were at low risk to the ecological environment. Among the five land use types, the environmental risk of Cd in cropland and abandoned farmland was higher, which was mainly related to the input of Cd caused by fertilization, spraying pesticides, and other agricultural activities.

Evaluation of Land Use Adaptation by Sequential Extraction of Soil Trace Elements at an Abandoned Gold and Copper Refinery Site in Northern Taiwan

This study site is located at an abandoned factory of mining, smelting, and refining of gold and copper in north Taiwan for more than one hundred years. The present study used soil background investigation out of the site and the sequential extraction procedures for arsenic and copper to assess the reutilization potential of brownfields at the site. The upper limit of background concentration out of the site was 300 mg/kg for arsenic and 700 mg/kg for copper. The soil arsenic within the site was mainly in the immobile fraction, such as forms fixed by layer silicates, that were very low risk for environmental releases. The soil copper in the abandoned sedimentation basin, gold refinery, and copper refinery was in the mobile fractions such as acid extractable, reducible, and oxidizable forms with higher release risk; therefore, except merely those three zones in the entire site with higher risk for environmental releases of copper, the release risks of trace elements are quite low in the rest of the areas, and land reuse without contact with soil or plant non-edible plants is possible. Therefore, in response to public demand for opening part of the site to promote local tourism development, appropriate control and isolation measures can be implemented to prevent the toxic elements from affecting human health through soil ingestion, skin contact, and other exposure pathways. In terms of pollution control, reducing dust inhalation is also an option to efficiently reduce health risks to an acceptable level and achieve the goal of sustainable land use at the contaminated site.

Enhanced Cr(VI) stabilization in soil by chitosan/bentonite composites

In this study, chitosan/bentonite composites (CSBT) was synthesized and applied to the immobilization of chromium in the soil. The influence of passivating agents on various forms of chromium was investigated by batch experiment. The results showed that CSBT could reduce the content of exchangeable form and oxidizable form, while increase the content of residual form of chromium. The addition of 0.2 g·kg−1 CSBT had the best effect, with the concentration of exchangeable, reducible and oxidizable form decreased by 46.74%, 8.15%, and 14.46%, respectively. During the experiment time, the passivation effect increased rapidly within 14 days, and the content of residual form in the total Cr increased from 0.76% to 14.23%, the equilibrium was reached at the 28th day and was basically maintained in the subsequent period. CSBT had little impact on soil pH, and soil pH maintained constant during the experiment period. The amino, carboxyl and hydroxyl groups of CSBT promoted the conversion of available chromium to residual state in soil, and reduced the bioavailability of chromium in soil.

Behavior of iron and other heavy metals in passivated sediments and the coupling effect on phosphorus

In situ passivation, which is easy to operate and affordable, is one of the most commonly used methods for sediment phosphorus (P) remediation. Understanding the behavior of iron and other heavy metals in passivated sediments is important for alleviating lake eutrophication and for ensuring drinking water safety. In this study, we investigated the behavior of P, Fe, Mn, Cd, Co, and Pb in lanthanum modified bentonite (LMB, Phoslock®) and polyaluminum chloride (PAC)-passivated sediments using intact sediment cores. Rhizon sampler and diffusive gradients in thin films technology (DGT) were respectively used to collect soluble and labile substances in sediment; a modified sequential selective extraction method was used to characterize metal forms. Results showed that LMB reduced soluble reactive phosphorus (SRP) at sediment depths of 0 ~ −15 mm and DGT-labile P flux at 0 ~ −50 mm. Correlation between DGT-labile P and Fe (R2 = 0.71) indicated that P mobility in the LMB group was affected by the behavior of Fe. PAC decreased SRP at sediment depths of 0, −5, −10, −15, −20, −25, and −50 mm with removal rates of 100%, 90%, 45%, 35%, 81%, 89%, and 100%, respectively. DGT-labile P flux was decreased by PAC at 0 ~ −10 mm and −50 ~ −110 mm, but increased at −10 ~ −50 mm; this is a result of synthetical effect by Al flocs adsorption and Fe(III) reductive dissolution. LMB decreased Cd, Co, and Pb in LMB layer in carbonate, reducible, and oxidizable forms. PAC decreased Cd mobility but caused the transformation of Co and Pb from reducible to other forms because of Fe(III) reductive dissolution. Those results indicate that sedimentary Fe plays an important role in in situ passivation. We suggest modifying passivators to Fe(II) adsorbents and increasing DO permeability of sediment to promote the formation of an Fe(III) passivation layer and hence the effectiveness of P control.

Effect of biochar on the form transformation of heavy metals in paddy soil under different water regimes

ABSTRACT To ascertain the effect of biochar on remediation of paddy soil contaminated by heavy metals under different irrigation regimes (flooding irrigation (FI), intermittent irrigation (II) and wet irrigation (WI)), rice straw biochar (RSB) and rice husk biochar (RHB) were applied into soil contaminated by Cd and Cu. The concentrations of available Cd and Cu were significantly reduced by RSB, and decreased by 6.8–15.9%, 11.1–20.0% and 6.8–18.2%, and 6.2–12.9%, 7.0–14.8% and 5.6–17.7% under FI, II and WI regimes, respectively. The application of RHB generally reduced the content of acid extractable and reducible Cd under FI and II regimes, and promoted its transformation to oxidizable and residual form. Similarly, both biochars reduced the acid extractable Cu in paddy soil and promoted its transformation to reducible and oxidizable form under three irrigation regimes (especially in II). This is significantly related to the increase of soil pH, organic matter content and cation exchange capacity by biochar. The preliminary results of this study indicate that biochar application combined with intermittent irrigation could be an effective measure to enhance the paddy soil quality. However, future studies are still needed to evaluate the long-term impact of biochar on heavy metal bioavailability under different water conditions.

Removal behavior and chemical speciation distributions of heavy metals in sewage sludge during bioleaching and combined bioleaching/Fenton-like processes

The removal and chemical speciation changes of heavy metals in the sewage sludge during the single bioleaching and combined bioleaching/Fenton-like processes were compared in this study. The improvement in the dewaterability of the treated sludge was also investigated. The single bioleaching led to a removal of Zn, Cu, Cd, Cr, Mn, Ni, As and Pb of 67.28%, 50.78%, 64.86%, 6.32%, 56.15%, 49.83%, 20.78% and 10.52% in 10 days, respectively. The chemical speciation analysis showed that the solubilization of heavy metals in mobile forms (exchangeable/acid soluble and reducible forms) and oxidizable form was the main reason for their removal. Subsequent Fenton-like treatment was carried out at different bioleaching stages when the bioleached sludge dropped to certain pH values (4.5, 4.0 and 3.0), by adding H2O2 at different dosages. The highest removal ratio of Zn, Cu, Cd, Cr, Mn and Ni could reach 75.53%, 52.17%, 71.91%, 11.63%, 66.29% and 65.19% after combined bioleaching/Fenton-like process, respectively, with appropriate pH and H2O2 dosages in less than 6 days. The solubilization efficiencies of these heavy metals in mobile forms were further improved by Fenton-like treatment. The removal efficiencies of As and Pb decreased due to their transformation into insoluble forms (mostly residual fraction) after Fenton treatment. The capillary suction times (CST) of the raw sludge (98.7 s) decreased by 79.43% after bioleaching and 87.44% after combined process, respectively.

The Variation of Heavy Metals Bioavailability in Sediments of Liujiang River Basin, SW China Associated to Their Speciations and Environmental Fluctuations, a Field Study in Typical Karstic River.

The bioavailability of heavy metals (HMs) in sediments is closely related to the security of the aquatic environment, but their impacts are poorly researched, particularly in karstic rivers. Therefore, Liujiang River Basin was taken as an example in this study. Seven HMs were analyzed to determine the bioavailability and speciations of HMs in sediments. Moreover, the impacts of environmental factors on HMs were identified. The obtained results suggested that HMs in the sediments are all within their permissible exposure limit (PEL), but Cd and Zn are significantly higher than the soil baseline. Most HMs were found to be in a residual fraction, while their exchangeable fraction was found to be in an extremely low ratio. HMs in bioavailable parts are significantly higher than in the exchangeable and carbonate-bound phases but lower than in the non-residual phase, which demonstrated that HM bioavailability is not confined to the exchangeable and carbonate-bound phases. The correlation coefficients commonly decreased with decreasing speciation ratios, which suggested that the overall bioavailability of metals should be determined by speciation ratios instead of speciations themselves. Noteworthily, most HMs in the residual form were found to be significantly correlated with their overall bioavailability, which highlighted the potential bioavailability of residual form. The non-correlations between pH, electrical conductivity (EC), total dissolved solids (TDS), and HM bioavailability suggested that HMs in the carbonate-bound phase are stable and unsusceptible to environmental variations, while the significant correlations between redox potential (Eh), turbidity, organic matter (OM), main grain size (Mz), and HM bioavailability suggested that HMs in the reducible and oxidizable forms are susceptible to environmental fluctuations. Therefore, the variation of HM bioavailability in karstic rivers is largely regulated by their reducible and oxidizable forms instead of their carbonate-bound form.

Effects of heavy metals speciations in sediments on their bioaccumulation in wild fish in rivers in Liuzhou—A typical karst catchment in southwest China

Although fish are widely confirmed to be susceptible to heavy metals (HMs) contamination in sediments, this bioconversion haven’t been detailed. This is especially the case in karst areas, where HMs are less stably retained in the sediments and are more bioavailable. Therefore, we surveyed representative karst rivers in Liuzhou, China, in order to study the relationship between the speciations of seven HMs in the sediments with their bioaccumulation in wild fish. The results showed that the HMs in sediments are all below their permissible exposure limit (PEL), but Cd and Zn are significantly higher than soil basline. Most HMs are in residual fraction, while their exchangeable fractions are present in extremely low proportions. The concentration of Zn, Cr and Cd in some fish are above their maximum recommended limit (MRL). The concentrations of most of the HMs in the fish are significantly correlated with the levels in the sediments and given the higher correlation coefficients for their carbonate-bound phase, this phase can be seen to play a critical role in HMs bioconversion. However, the presence of this phase in low proportions enables other phases, especially oxidizable form, to play a greater role in HMs bioaccumulation. Apart from Do, HMs in the fish samples are significantly correlated with multiple environmental factors, demonstrating environmental fluctuations can manipulate HMs bioconversion from sediments; however, their significance depend heavily on the proportion of particular species. HMs in reducible and oxidizable fraction are more important in regulating, rather than promoting, their bioconversion during environmental fluctuations. Fluctuations in EC, TDS and pH can increase the impacts of HMs in carbonate-bound fraction on their bioconversion. Given the higher background values of EC and TDS and lower pH values during the monsoon period, careful attention should be paid to the increased bioconversion of HMs in karst rivers during this season.

Heavy metal pollution and potential ecological risk assessment for surficial sediments of Deepor Beel, India

The present study has been conducted for three objectives for the assessment of sediment pollution loadings and their ecological risk. Firstly, the hierarchical clustering of the raw sediment dataset was carried out, which categorized the sampling locations into three statistically significant clusters, depending on their similarities in behaviours. Clusters 1, 2, and 3 corresponded to high, moderate, and low pollution sites, respectively. The principal component analysis was then carried out on the dataset for three different seasons; pre-monsoon, monsoon and post-monsoon respectively, which showed significant temporal changes in the pollution sources. Various indices such as contamination factor, pollution load index, enrichment factor, and the geo-accumulation index showed that the wetland is most affected in the post-monsoon season as compared to other seasons. At the same time, monsoon remains the best. This was attributed to the significant increase in the wetland's water depth during the monsoon, coupled with the rise in precipitation capacity during the post-monsoon. Secondly, the potential ecological risk due to the contaminants displayed that the post-monsoon period has the greatest number of sites under the moderate risk category. In contrast, all the sites during the monsoon attributed to low risk. Also, Cd was found to be the primary contributor to ecological risk. Finally, chemical speciation analyses of all the heavy metals were conducted to determine their available forms in the sediment column. Cr, Mn, and Mg were observed to have a profoundly negative impact on aquatic ecology (available in F1 fraction in higher percentages). While Fe was predominant in reducible (F3) form, Cd was found to have equal contributions in reducible and oxidizable (F3 and F4, respectively) forms. Cu and Pb displayed typical complex characteristics of all the fractions in almost equal proportions. The results of the study indicated that although the anthropogenic interventions to the wetland have not been significant till yet, as far as the sediment contamination is concerned, it would be too late if swift and appropriate measures are not taken for restricting future probable contaminations.

Release and transformation mechanisms of hazardous trace elements in the ash and slag during underground coal gasification

Underground coal gasification (UCG) is a new coal utilization technology with advantages of high safety and high efficiency. However, the risk of hazardous trace elements (HTEs) in the UCG residue to the underground environment has always been an important issue. In this paper, the UCG sequential transformation products were prepared, and the chemical speciation of hazardous trace elements (Hg, As, Se, Pb, Cr, and Cd) in the UCG products were studied by using a combination of chemical analysis and thermodynamic calculation. Moreover, the risk assessment code (RAC) was introduced to evaluate the risk level of hazardous trace elements to underground environment. The results indicate that during the UCG pyrolysis process, Hg, Pb, Se, and Cd in the Ulanqab coal react with S to form HgS, PbS, SeS, and CdS, respectively. The oxidizable form of Hg (50.00 ± 0.18–66.67 ± 0.23%) and As (78.34 ± 0.37–83.36 ± 0.46%) is the main speciation in 900–1300 °C reduction ash. For the oxidation ash and slag samples, Hg is loaded on the mineral system of Na-Si-O in the form of HgO, and As is combined with the FeAl2O4. Based on the risk assessment code (RAC), Hg, Se and Pb have medium risk level in the coal and semi-coke. Cr and Se in the 900–1200 °C reduction ash can pose high risk on the underground environment. In the oxidation ash and slag, the risk level of HTEs to the environment decreases. The results can provide scientific guidance for the control and treatment of residues in the later-stage of underground coal gasification.