Combined biochar and wheat-derived endophytic bacteria reduces cadmium uptake in wheat grains in a metal-polluted soil.

In this study, two wheat-derived cadmium (Cd)-immobilizing endophytic Pseudomonas paralactis M14 and Priestia megaterium R27 were evaluated for their effects on wheat tissue Cd uptake under hydroponic conditions. Then, the impacts of the biochar (BC), M14+R27 (MR), and BC+MR treatments on wheat Cd uptake and the mechanisms involved were investigated at the jointing, heading, and mature stages of wheat plants under field-plot conditions. A hydroponic experiment showed that the MR treatment significantly decreased the above-ground tissue Cd content compared with the M14 or R27 treatment. The BC+MR treatment reduced the grain Cd content by 51.5%-67.7% and Cd translocation factor at the mature stage of wheat plants and increased the organic matter-bound Cd content by 31%-75% in the rhizosphere soils compared with the BC or MR treatment. Compared with the BC or MR treatment, the relative abundances of the biomarkers associated with Gemmatimonas, Altererythrobacter, Gammaproteobacteria, Xanthomonadaceae, Phenylobacterium, and Nocardioides in the BC+MR-treated rhizosphere microbiome decreased and negatively correlated with the organic matter-bound Cd contents. In the BC+MR-treated root interior microbiome, the relative abundance of the biomarker belonging to Exiguobacterium increased and negatively correlated with the Cd translocation factor, while the relative abundance of the biomarker belonging to Pseudonocardiaceae decreased and positively correlated with the Cd translocation factor. Our findings suggested that the BC+MR treatment reduced Cd availability and Cd transfer through affecting the abundances of these specific biomarkers in the rhizosphere soil and root interior microbiomes, leading to decreased wheat grain Cd uptake in the contaminated soil.

Remediation mechanism of high concentrations of multiple heavy metals in contaminated soil by Sedum alfredii and native microorganisms.

Pollution accident of nonferrous metallurgy industry often lead to serious heavy metal pollution of the surrounding soil. Phytoremediation of contaminated soil is an environmental and sustainable technology, and soil native microorganisms in the process of phytoremediation also participate in the remediation of heavy metals. However, the effects of high concentrations of multiple heavy metals (HCMHMs) on plants and native soil microorganisms remain uncertain. Thus, further clarification of the mechanism of phytoremediation of HCMHMs soil by plants and native soil microorganisms is required. Using the plant Sedum alfredii (S. alfredii) to restore HCMHM-contaminated soil, we further explored the mechanism of S. alfredii and native soil microorganisms in the remediation of HCMHM soils. The results showed that (i) S. alfredii can promote heavy metals from non-rhizosphere soil to rhizosphere soil, which is conducive to the effect of plants on heavy metals. In addition, it can also enrich the absorbed heavy metals in its roots and leaves; (ii) native soil bacteria can increase the abundance of signal molecule-synthesizing enzymes, such as trpE, trpG, bjaI, rpfF, ACSL, and yidC, and promote the expression of the pathway that converts serine to cysteine, then synthesize substances to chelate heavy metals. In addition, we speculated that genes such as K19703, K07891, K09711, K19703, K07891, and K09711 in native bacteria may be involved in the stabilization or absorption of heavy metals. The results provide scientific basis for S. alfredii to remediate heavy metals contaminated soils, and confirm the potential of phytoremediation of HCMHM contaminated soil.

Priority sources identification and risks assessment of heavy metal(loid)s in agricultural soils of a typical antimony mining watershed.

Heavy metal(loid) (HM) pollution in agricultural soils has become an environmental concern in antimony (Sb) mining areas. However, priority pollution sources identification and deep understanding of environmental risks of HMs face great challenges due to multiple and complex pollution sources coexist. Herein, an integrated approach was conducted to distinguish pollution sources and assess human health risk (HHR) and ecological risk (ER) in a typical Sb mining watershed in Southern China. This approach combines absolute principal component score-multiple linear regression (APCS-MLR) and positive matrix factorization (PMF) models with ER and HHR assessments. Four pollution sources were distinguished for both models, and APCS-MLR model was more accurate and plausible. Predominant HM concentration source was natural source (39.1%), followed by industrial and agricultural activities (23.0%), unknown sources (21.5%) and Sb mining and smelting activities (16.4%). Although natural source contributed the most to HM concentrations, it did not pose a significant ER. Industrial and agricultural activities predominantly contributed to ER, and attention should be paid to Cd and Sb. Sb mining and smelting activities were primary anthropogenic sources of HHR, particularly Sb and As contaminations. Considering ER and HHR assessments, Sb mining and smelting, and industrial and agricultural activities are critical sources, causing serious ecological and health threats. This study showed the advantages of multiple receptor model application in obtaining reliable source identification and providing better source-oriented risk assessments. HM pollution management, such as regulating mining and smelting and implementing soil remediation in polluted agricultural soils, is strongly recommended for protecting ecosystems and humans.

Effects of biomass co-pyrolysis and herbaceous plant colonization on the transformation of tailings into soil like substrate.

Enhancing soil organic matter characteristics, ameliorating physical structure, mitigating heavy metal toxicity, and hastening mineral weathering processes are crucial approaches to accomplish the transition of tailings substrate to a soil-like substrate. The incorporation of biomass co-pyrolysis and plant colonization has been established to be a significant factor in soil substrate formation and soil pollutant remediation. Despite this, there is presently an absence of research efforts aimed at synergistically utilizing these two technologies to expedite the process of mining tailings soil substrate formation. The current study aimed to investigate the underlying mechanism of geochemical changes and rapid mineral weathering during the process of transforming tailings substrate into a soil-like substrate, under the combined effects of biomass co-smoldering pyrolysis and plant colonization. The findings of this study suggest that the incorporation of smoldering pyrolysis and plant colonization induces a high-temperature effect and biological effects, which enhance the physical and chemical properties of tailings, while simultaneously accelerating the rate of mineral weathering. Notable improvements include the amelioration of extreme pH levels, nutrient enrichment, the formation of aggregates, and an increase in enzyme activity, all of which collectively demonstrate the successful attainment of tailings substrate reconstruction. Evidence of the accelerated weathering was verified by phase and surface morphology analysis using X-ray diffraction and scanning electron microscopy. Discovered corrosion and fragmentation on the surface of minerals. The weathering resulted in corrosion and fragmentation of the surface of the treated mineral. This study confirms that co-smoldering pyrolysis of biomass, combined with plant colonization, can effectively promote the transformation of tailings into soil-like substrates. This method has can effectively address the key challenges that have previously hindered sustainable development of the mining industry and provides a novel approach for ecological restoration of tailings deposits.

Bioaccessibility and health risk assessment of hydrophobic organic pollutants in soils from four typical industrial contaminated sites in China.

There have been reports of potential health risks for people from hydrophobic organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated hydrocarbons (PCHs), and organophosphate flame retardants (OPFRs). When a contaminated site is used for residential housing or public utility and recreation areas, the soil-bound organic pollutants might pose a threat to human health. In this study, we investigated the contamination profiles and potential risks to human health of 15 PAHs, 6 PCHs, and 12 OPFRs in soils from four contaminated sites in China. We used an in vitro method to determine the oral bioaccessibility of soil pollutants. Total PAHs were found at concentrations ranging from 26.4 ng/g to 987 ng/g. PCHs (0.27‒14.3 ng/g) and OPFRs (6.30‒310 ng/g) were detected, but at low levels compared to earlier reports. The levels of PAHs, PCHs, and OPFRs released from contaminated soils into simulated gastrointestinal fluids ranged from 1.74% to 91.0%, 2.51% to 39.6%, and 1.37% to 96.9%, respectively. Based on both spiked and unspiked samples, we found that the oral bioaccessibility of pollutants was correlated with their logKow and molecular weight, and the total organic carbon content and pH of soils. PAHs in 13 out of 38 contaminated soil samples posed potential high risks to children. When considering oral bioaccessibility, nine soils still posed potential risks, while the risks in the remaining soils became negligible. The contribution of this paper is that it corrects the health risk of soil-bound organic pollutants by detecting bioaccessibility in actual soils from different contaminated sites.

A review of functions and mechanisms of clay soil conditioners and catalysts in thermal remediation compared to emerging photo-thermal catalysis.

High temperatures and providing sufficient time for the thermal desorption of persistent organic pollutants (POPs) from contaminated clay soils can lead to intensive energy consumption. Therefore, this article provides a critical review of the potential additives which can improve soil texture and increase the volatility of POPs, and then discusses their enhanced mechanisms for contributing to a green economy. Ca-based additives have been used to reduce plasticity of bentonite clay, absorb water and replenish system heat. In contrast, non-Ca-based additives have been used to decrease the plasticity of kaolin clay. The soil structure and soil plasticity can be changed through cation exchange and flocculation processes. The transition metal oxides and alkali metal oxides can be applied to catalyze and oxidize polycyclic aromatic hydrocarbons, petroleum and emerging contaminants. In this system, reactive oxygen species (•O2- and •OH) are generated from thermal excitation without strong chemical oxidants. Moreover, multiple active ingredients in recycled solid wastes can be controlled to reduce soil plasticity and enhance thermal catalysis. Alternatively, the alkali, nano zero-valent iron and nano-TiN can catalyze hydrodechlorination of POPs under reductive conditions. Especially, photo and photo-thermal catalysis are discussed to accelerate replacement of fossil fuels by renewable energy in thermal remediation.

Predicting the efficiency of arsenic immobilization in soils by biochar using machine learning.

Arsenic (As) pollution in soils is a pervasive environmental issue. Biochar immobilization offers a promising solution for addressing soil As contamination. The efficiency of biochar in immobilizing As in soils primarily hinges on the characteristics of both the soil and the biochar. However, the influence of a specific property on As immobilization varies among different studies, and the development and application of arsenic passivation materials based on biochar often rely on empirical knowledge. To enhance immobilization efficiency and reduce labor and time costs, a machine learning (ML) model was employed to predict As immobilization efficiency before biochar application. In this study, we collected a dataset comprising 182 data points on As immobilization efficiency from 17 publications to construct three ML models. The results demonstrated that the random forest (RF) model outperformed gradient boost regression tree and support vector regression models in predictive performance. Relative importance analysis and partial dependence plots based on the RF model were conducted to identify the most crucial factors influencing As immobilization. These findings highlighted the significant roles of biochar application time and biochar pH in As immobilization efficiency in soils. Furthermore, the study revealed that Fe-modified biochar exhibited a substantial improvement in As immobilization. These insights can facilitate targeted biochar property design and optimization of biochar application conditions to enhance As immobilization efficiency.

Synthesis of ternary geopolymers using prediction for effective solidification of mercury in tailings.

This study used steel slag, fly ash, and metakaolin as raw materials (SFM materials) to create silica-alumina-based geopolymers that can solidify Hg2+ when activated with sodium-based water glass. The experiments began with a triangular lattice point mixing design experiment, and the results were fitted, analyzed, and predicted. The optimum SFM material mass ratio was found to be 70% steel slag, 25% fly ash, and 5% metakaolin. The optimum modulus of the activator was identified by comparing the unconfined compressive strength and solidifying impact on Hg2+of geosynthetics with different modulus. The SFM geopolymer was then applied in the form of potting to cure the granulated mercury tailings. The inclusion of 50% SFM material generated a geosynthetic that reduced mercury transport to the surface soil by roughly 90%. The mercury concentration of herbaceous plant samples was also reduced by 78%. It indicates that the SFM material can effectively attenuate the migration transformation of mercury. Finally, characterization methods such as XPS and FTIR were used to investigate the mechanism of Hg2+ solidification by geopolymers generated by SFM materials. The possible solidification mechanisms were proposed as alkaline environment-induced mercury precipitation, chemical bonding s, surface adsorption of Hg2+ and its precipitates by the geopolymer, and physical encapsulation.

Simultaneous immobilization of multiple heavy metal(loid)s in contaminated water and alkaline soil inoculated Fe/Mn oxidizing bacterium.

Two strains of Fe/Mn oxidizing bacteria tolerant to high concentrations of multiple heavy metal(loid)s and efficient decontamination for them were screened. The surface of the bio-Fe/Mn oxides produced by the oxidation of Fe(II) and Mn(II) by Pseudomonas taiwanensis (marked as P4) and Pseudomonas plecoglossicida (marked as G1) contains rich reactive oxygen functional groups, which play critical roles in the removal efficiency and immobilization of heavy metal(loid)s in co-contamination system. The isolated strains P4 and G1 can grow well in the following environments: pH 5-9, NaCl 0-4%, and temperature 20-30°C. The removal efficiencies of Fe, Pb, As, Zn, Cd, Cu, and Mn are effective after inoculation of the strains P4 and G1 in the simulated water system (the initial concentrations of heavy metal(loid) were 1 mg/L), approximately reaching 96%, 92%, 85%, 67%, 70%, 54% and 15%, respectively. The exchangeable and carbonate bound As, Cd, Pb and Cu are more inclined to convert to the Fe-Mn oxide bound fractions in P4 and G1 treated soil, thereby reducing the phytoavailability and bioaccessible of heavy metal(loid)s. This research provides alternatives method to treat water and soil containing high concentrations of multi-heavy metal(loid)s.

Hybrid and enhanced electrokinetic system for soil remediation from heavy metals and organic matter.

The electrokinetic (EK) process has been proposed for soil decontamination from heavy metals and organic matter. The advantages of the EK process include the low operating energy, suitability for fine-grained soil decontamination, and no need for excavation. During the last three decades, enhanced and hybrid EK systems were developed and tested for improving the efficiency of contaminants removal from soils. Chemically enhanced-EK processes exhibited excellent efficiency in removing contaminants by controlling the soil pH or the chemical reaction of contaminants. EK hybrid systems were tested to overcome environmental hurdles or technical drawbacks of decontamination technologies. Hybridization of the EK process with phytoremediation, bioremediation, or reactive filter media (RFM) improved the remediation process performance by capturing contaminants or facilitating biological agents' movement in the soil. Also, EK process coupling with solar energy was proposed to treat off-grid contaminated soils or reduce the EK energy requirements. This study reviews recent advancements in the enhancement and hybrid EK systems for soil remediation and the type of contaminants targeted by the process. The study also covered the impact of operating parameters, imperfect pollution separation, and differences in the physicochemical characteristics and microstructure of soil/sediment on the EK performance. Finally, a comparison between various remediation processes was presented to highlight the pros and cons of these technologies.

Efficient remediation of different concentrations of Cr-contaminated soils by nano zero-valent iron modified with carboxymethyl cellulose and biochar.

Nano zero-valent iron (nZVI) is widely used in soil remediation due to its high reactivity. However, the easy agglomeration, poor antioxidant ability and passivation layer of Fe-Cr coprecipitates of nZVI have limited its application scale in Cr-contaminated soil remediation, especially in high concentration of Cr-contaminated soil. Herein, we found that the carboxymethyl cellulose on nZVI particles could increase the zeta potential value of soil and change the phase of nZVI. Along with the presence of biochar, 97.0% and 96.6% Cr immobilization efficiency through CMC-nZVI/BC were respectively achieved in high and low concentrations of Cr-contaminated soils after 90-days remediation. In addition, the immobilization efficiency of Cr(VI) only decreased by 5.1% through CMC-nZVI/BC treatment after 10 weeks aging in air, attributing to the strong antioxidation ability. As for the surrounding Cr-contaminated groundwater, the Cr(VI) removal capacity of CMC-nZVI/BC was evaluated under different reaction conditions through column experiments and COMSOL Multiphysics. CMC-nZVI/BC could efficiently remove 85% of Cr(VI) in about 400 hr when the initial Cr(VI) concentration was 40 mg/L and the flow rate was 0.5 mL/min. This study demonstrates that uniformly dispersed CMC-nZVI/BC has an excellent remediation effect on different concentrations of Cr-contaminated soils.

Nitrogen-cycling processes under long-term compound heavy metal(loids) pressure around a gold mine: Stimulation of nitrite reduction.

Mining and tailings deposition can cause serious heavy metal(loids) pollution to the surrounding soil environment. Soil microorganisms adapt their metabolism to such conditions, driving alterations in soil function. This study aims to elucidate the response patterns of nitrogen-cycling microorganisms under long-term heavy metal(loids) exposure. The results showed that the diversity and abundance of nitrogen-cycling microorganisms showed negative feedback to heavy metal(loids) concentrations. Denitrifying microorganisms were shown to be the dominant microorganisms with over 60% of relative abundance and a complex community structure including 27 phyla. Further, the key bacterial species in the denitrification process were calculated using a random forest model, where the top three key species (Pseudomonas stutzei, Sphingobium japonicum and Leifsonia rubra) were found to play a prominent role in nitrite reduction. Functional gene analysis and qPCR revealed that nirK, which is involved in nitrite reduction, significantly accumulated in the most metal-rich soil with the increase of absolute abundance of 63.86%. The experimental results confirmed that the activity of nitrite reductase (Nir) encoded by nirK in the soil was increased at high concentrations of heavy metal(loids). Partial least squares-path model identified three potential modes of nitrite reduction processes being stimulated by heavy metal(loids), the most prominent of which contributed to enhanced nirK abundance and soil Nir activity through positive stimulation of key species. The results provide new insights and preliminary evidence on the stimulation of nitrite reduction processes by heavy metal(loids).

Oxygen atmosphere enhances ball milling remediation of petroleum-contaminated soil and reuse as adsorptive/catalytic materials for wastewater treatment.

Ball milling is an environmentally friendly technology for the remediation of petroleum-contaminated soil (PCS), but the cleanup of organic pollutants requires a long time, and the post-remediation soil needs an economically viable disposal/reuse strategy due to its vast volume. The present paper develops a ball milling process under oxygen atmosphere to enhance PCS remediation and reuse the obtained carbonized soil (BCS-O) as wastewater treatment materials. The total petroleum hydrocarbon removal rates by ball milling under vacuum, air, and oxygen atmospheres are 39.83%, 55.21%, and 93.84%, respectively. The Langmuir and pseudo second-order models satisfactorily describe the adsorption capacity and behavior of BCS-O for transition metals. The Cu2+, Ni2+, and Mn2+ adsorbed onto BCS-O were mainly bound to metal carbonates and metal oxides. Furthermore, BCS-O can effectively activate persulfate (PDS) oxidation to degrade aniline, while BCS-O loaded with transition metal (BCS-O-Me) shows better activation efficiency and reusability. BCS-O and BCS-O-Me activated PDS oxidation systems are dominated by 1O2 oxidation and electron transfer. The main active sites are oxygen-containing functional groups, vacancy defects, and graphitized carbon. The oxygen-containing functional groups and vacancy defects primarily activate PDS to generate 1O2 and attack aniline. Graphitized carbon promotes aniline degradation by accelerating electron transfer. The paper develops an innovative strategy to simultaneously realize efficient remediation of PCS and sequential reuse of the post-remediation soil.

Enhancing microbial superoxide generation and conversion to hydroxyl radicals for enhanced bioremediation using iron-binding ligands.

Harnessing bacteria for superoxide production in bioremediation holds immense promise, yet its practical application is hindered by slow production rates and the relatively weak redox potential of superoxide. This study delves into a cost-effective approach to amplify superoxide production using an Arthrobacter strain, a prevalent soil bacterial genus. Our research reveals that introducing a carbon source along with specific iron-binding ligands, including deferoxamine (DFO), diethylenetriamine pentaacetate (DTPA), citrate, and oxalate, robustly augments microbial superoxide generation. Moreover, our findings suggest that these iron-binding ligands play a pivotal role in converting superoxide into hydroxyl radicals by modulating the electron transfer rate between Fe(III)/Fe(II) and superoxide. Remarkably, among the tested ligands, only DTPA emerges as a potent promoter of this conversion process when complexed with Fe(III). We identify an optimal Fe(III) to DTPA ratio of approximately 1:1 for enhancing hydroxyl radical production within the Arthrobacter culture. This research underscores the efficacy of simultaneously introducing carbon sources and DTPA in facilitating superoxide production and its subsequent conversion to hydroxyl radicals, significantly elevating bioremediation performance. Furthermore, our study reveals that DTPA augments superoxide production in cultures of diverse soils, with various soil microorganisms beyond Arthrobacter identified as contributors to superoxide generation. This emphasizes the universal applicability of DTPA across multiple bacterial genera. In conclusion, our study introduces a promising methodology for enhancing microbial superoxide production and its conversion into hydroxyl radicals. These findings hold substantial implications for the deployment of microbial reactive oxygen species in bioremediation, offering innovative solutions for addressing environmental contamination challenges.

Impact of residual antibiotics on microbial decomposition of livestock manures in Eutric Regosol: Implications for sustainable nutrient recycling and soil carbon sequestration.

The land application of livestock manure has been widely acknowledged as a beneficial approach for nutrient recycling and environmental protection. However, the impact of residual antibiotics, a common contaminant of manure, on the degradation of organic compounds and nutrient release in Eutric Regosol is not well understood. Here, we studied, how oxytetracycline (OTC) and ciprofloxacin (CIP) affect the decomposition, microbial community structure, extracellular enzyme activities and nutrient release from cattle and pig manure using litterbag incubation experiments. Results showed that OTC and CIP greatly inhibited livestock manure decomposition, causing a decreased rate of carbon (28%-87%), nitrogen (15%-44%) and phosphorus (26%-43%) release. The relative abundance of gram-negative (G-) bacteria was reduced by 4.0%-13% while fungi increased by 7.0%-71% during a 28-day incubation period. Co-occurrence network analysis showed that antibiotic exposure disrupted microbial interactions, particularly among G- bacteria, G+ bacteria, and actinomycetes. These changes in microbial community structure and function resulted in decreased activity of urease, ß-1,4-N-acetyl-glucosaminidase, alkaline protease, chitinase, and catalase, causing reduced decomposition and nutrient release in cattle and pig manures. These findings advance our understanding of decomposition and nutrient recycling from manure-contaminated antibiotics, which will help facilitate sustainable agricultural production and soil carbon sequestration.

Pristine/magnesium-loaded biochar and ZVI affect rice grain arsenic speciation and cadmium accumulation through different pathways in an alkaline paddy soil.

Cadmium (Cd) and arsenic (As) co-contamination has threatened rice production and food safety. It is challenging to mitigate Cd and As contamination in rice simultaneously due to their opposite geochemical behaviors. Mg-loaded biochar with outstanding adsorption capacity for As and Cd was used for the first time to remediate Cd/As contaminated paddy soils. In addition, the effect of zero-valent iron (ZVI) on grain As speciation accumulation in alkaline paddy soils was first investigated. The effect of rice straw biochar (SC), magnesium-loaded rice straw biochar (Mg/SC), and ZVI on concentrations of Cd and As speciation in soil porewater and their accumulation in rice tissues was investigated in a pot experiment. Addition of SC, Mg/SC and ZVI to soil reduced Cd concentrations in rice grain by 46.1%, 90.3% and 100%, and inorganic As (iAs) by 35.4%, 33.1% and 29.1%, respectively, and reduced Cd concentrations in porewater by 74.3%, 96.5% and 96.2%, respectively. Reductions of 51.6% and 87.7% in porewater iAs concentrations were observed with Mg/SC and ZVI amendments, but not with SC. Dimethylarsinic acid (DMA) concentrations in porewater and grain increased by a factor of 4.9 and 3.3, respectively, with ZVI amendment. The three amendments affected grain concentrations of iAs, DMA and Cd mainly by modulating their translocation within plant and the levels of As(III), silicon, dissolved organic carbon, iron or Cd in porewater. All three amendments (SC, Mg/SC and ZVI) have the potential to simultaneously mitigate Cd and iAs accumulation in rice grain, although the pathways are different.

Soil colloids can significantly enhance spreading of polybromodiphenyl ethers in groundwater by serving as an effective carrier.

Polybromodiphenyl ethers (PBDEs), the widely used flame retardants, are common contaminants in surface soils at e-waste recycling sites. The association of PBDEs with soil colloids has been observed, indicating the potential risk to groundwater due to colloid-facilitated transport. However, the extent to which soil colloids may enhance the spreading of PBDEs in groundwater is largely unknown. Herein, we report the co-transport of decabromodiphenyl ester (BDE-209) and soil colloids in saturated porous media. The colloids released from a soil sample collected at an e-waste recycling site in Tianjin, China, contain high concentration of PBDEs, with BDE-209 being the most abundant conger (320 ± 30 mg/kg). The colloids exhibit relatively high mobility in saturated sand columns, under conditions commonly observed in groundwater environments. Notably, under all the tested conditions (i.e., varying flow velocity, pH, ionic species and ionic strength), the mass of eluted BDE-209 correlates linearly with that of eluted soil colloids, even though the mobility of the colloids varies markedly depending on the specific hydrodynamic and solution chemistry conditions involved. Additionally, the mass of BDE-209 retained in the columns also correlates strongly with the mass of retained colloids. Apparently, the PBDEs remain bound to soil colloids during transport in porous media. Findings in this study indicate that soil colloids may significantly promote the transport of PBDEs in groundwater by serving as an effective carrier. This might be the reason why the highly insoluble and adsorptive PBDEs are found in groundwater at some PBDE-contaminated sites.

Characterization and utilization potential of typical molybdenum tailings in Shaanxi Province, China.

Shaanxi Province is located in the most important molybdenum ore district in the world, but a lot of molybdenum tailings have been released, polluting the environment and wasting resources seriously. Taking eleven tailing samples collected at the main molybdenum tailings ponds in Shaanxi Province as the research object, the physical, chemical, and mineralogical characteristics were studied through scanning electron microscope, X-ray fluorescence, X-ray diffraction, inductively coupled plasma mass spectrometer, and others. The ecological risk and utilization potential of molybdenum tailings were investigated through leaching test, geo-accumulation index, potential ecological risk assessment, and other methods. The results demonstrated that the main chemical and mineralogical composition of various molybdenum tailings in Shaanxi Province is similar, and the predominant mineral composition is muscovite, quartz, microcline, and calcite. The potential ecological risk of heavy metals in six molybdenum tailings is high, while Pb and Cd are the main pollution risk elements. Molybdenum tailings contain considerable amounts of critical minerals with huge potential economic value, and molybdenum tailings with high environmental hazards could be converted into a possible source for critical minerals by recovering the critical minerals and repurposing the secondary tailings as an additive or cement substitute. This study provides an innovative idea for the pollution treatment of molybdenum tailings and indicates the prospect of molybdenum tailings as a secondary source for critical minerals.

Toward a more realistic estimate of exposure to chromium and nickel in soils of geogenic and/or anthropogenic origin: importance of oral bioaccessibility.

To enhance risk assessment for contaminated sites, incorporating bioavailability through bioaccessibility as a corrective factor to total concentration is essential to provide a more realistic estimate of exposure. While the main in vitro tests have been validated for As, Cd, and/or Pb, their potential for assessing the bioaccessibility of additional elements remains underexplored. In this study, the physicochemical parameters, pseudototal Cr and Ni concentrations, soil phase distribution, and oral bioaccessibility of twenty-seven soil samples were analysed using both the ISO 17924 standard and a simplified test based on hydrochloric acid. The results showed wide variability in terms of the concentrations (from 31 to 21,079 mg kg-1 for Cr, and from 26 to 11,663 mg kg-1 for Ni) and generally low bioaccessibility for Cr and Ni, with levels below 20% and 30%, respectively. Bioaccessibility variability was greater for anthropogenic soils, while geogenic enriched soils exhibited low bioaccessibility. The soil parameters had an influence on bioaccessibility, but the effects depended on the soils of interest. Sequential extractions provided the most comprehensive explanation for bioaccessibility. Cr and Ni were mostly associated with the residual fraction, indicating limited bioaccessibility. Ni was distributed in all phases, whereas Cr was absent from the most mobile phase, which may explain the lower bioaccessibility of Cr compared to that of Ni. The study showed promising results for the use of the simplified test to predict Cr and Ni bioaccessibility, and its importance for more accurate human exposure evaluation and effective soil management practices.

Differential immobilization of cadmium and changes in soil surface charge in acidic Ultisol by chitosan and citric acid: effect of their functional groups.

Soil organic matter plays an important role in cadmium adsorption and immobilization. Since different organic matter components affect cadmium adsorption processes differently, selecting the right organic substrate and knowing how to apply it could improve cadmium remediation. This study compares the effects of two contrasting organic molecules; chitosan and citric acid, on cadmium adsorption and speciation in acidic Ultisol. The adsorption of chitosan to Ultisol significantly increased the soil positive charge while adsorption of citric acid increased the soil negative charge. At pH 5.0, the maximum amount of cadmium adsorbed in excess chitosan was 341% greater than that in excess citric acid. About 73-89% and 60-62% of adsorbed cadmium were bound to Fe/Mn oxides and organic matter/sulfide at pH 4.0 while this fraction was 77-100% and 57-58% for citric acid and chitosan at pH 5.0, respectively. This decrease in the complexing ability of chitosan was related to the destabilizing effect of high pH on chitosan's structure. Also, the sequence through which chitosan, citric acid, and cadmium were added into the adsorption system influenced the adsorption profile and this was different along a pH gradient. Specifically, adding chitosan and cadmium together increased adsorption compared to when chitosan was pre-adsorbed within pH 3.0-6.5. However, for citric acid, the addition sequence had no significant effect on cadmium adsorption between pH 3.0-4.0 compared to pH 6.5 and 7.5, with excess citric acid generally inhibiting adsorption. Given that the action of citric acid is short-lived in soil, chitosan could be a good soil amendment material for immobilizing cadmium.