New insights into persulfate decomposition by soil minerals: radical and non-radical pathways.

Persulfate (PS)-based in situ chemical oxidation (ISCO) has been widely used for pollutant remediation in soil and groundwater. However, the underlying mechanism of interactions between mineral and PS was not fully explored. In this study, several soil model minerals including goethite, hematite, magnetite, pyrolusite, kaolin, montmorillonite, and nontronite were selected to investigate their potential effects on PS decomposition and free radical evolution. It was found the decomposition efficiency of PS by these minerals varied significantly, and both the radical and non-radical decomposition processes were included. Pyrolusite has the highest reactivity for PS decomposition. However, PS decomposition is prone to form SO42- through non-radical pathway, and thus, the amounts of free radicals (e.g., •OH and SO4•-) produced are relatively limited. However, PS mainly decomposed to produce free radicals in the presence of goethite and hematite. In the presence of magnetite, kaolin, montmorillonite, and nontronite, PS both decomposed to produce SO42- and free radicals. Furthermore, the radical process exhibited the high degradation performance for model pollutant such as phenol with relatively high utilization efficiency of PS, while non-radical decomposition has limited contribution to phenol degradation with extremely low utilization efficiency of PS. This study deepened the understanding of interactions between PS and minerals during the PS-based ISCO in soil remediation.

Comparative evaluation and prioritization of key influences on biodegradation of 2,2',4,4'-tetrabrominated diphenyl ether by bacterial isolate B. xenovorans LB400.

Polybrominated diphenyl ethers (PBDEs) are a class of persistent organic pollutants being widely distributed and harmful to human health and wildlife, and the development of sustainable rehabilitation strategies including microbial degradation is of great concern. Although the increasing number of bacteria, especially the broad-spectrum and potent aerobes have been isolated for the efficient removal of PBDEs, the external influences and the corresponding influential mechanism on biodegradation are not fully understood yet. Given the wide-spectrum biodegradability of aerobic bacterial isolate, B. xenovorans LB400 for PBDEs, the dual impacts of many pivotal factors including pH, temperature, presence of dissolved organic matter (DOM) and cadmium ion etc. were comprehensively revealed on biodegradation of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47). Due to the structural resemblance and stimulation of specific enzyme activity in bacteria, the biphenyl as substrates showed the greater capacity than non-aromatic compounds in improving biodegradation. The individual adaptation to neutrality and cultivation at about 30 °C was beneficial for biodegradation since the bacterial cellular viability and enzyme activity was mostly preserved. Although it was possibly good for the induction of hormesis and favorable to enhance the permeability or bioavailability of pollutant, the exceeding increase of Cd2+ or DOM may not give the profitable increase of biodegradation yet for the detrimental effect. For biodegradation, the mechanistic relationship that took account of the integrative correlation with the influential factors was artfully developed using partial least square (PLS) regression technique. Relative to the most significant influence of culture time and initial concentration of BDE-47, the larger relevance of other factors primarily marked as pH and DOM was consecutively shown after the quantitative prioritization. This may not only help understand the influential mechanism but provide a prioritizing regulation strategy for biodegradation of BDE-47. The PLS-derived relationship was validated with the certain predictability in biodegradation, and could be used as an alternative to accelerate a priori evaluation of suitability or improve the feasibility of such bacteria in remediation of PBDEs in the environment.

Congener-specificity, dioxygenation dependency and association with enzyme binding for biodegradation of polybrominated diphenyl ethers by typical aerobic bacteria: Experimental and theoretical studies.

Polybrominated diphenyl ethers (PBDEs) are a group of organic pollutants that have attracted much concerns of scientific community over the ubiquitous distribution, chemical persistence and toxicological risks in the environment. Though a great number of aerobic bacteria have been isolated for the rapid removal of PBDEs, the knowledge about biodegradation characteristics and mechanism is less provided yet. Herein, the congener-specificity of aerobic biodegradation of PBDEs by typical bacteria, i.e. B. xenovorans LB400 was identified with the different biodegradation kinetics, of which the changes were largely hinged on the bromination pattern. The more bromination isomerically at ortho-sites other than meta-sites or the single bromination at one of aromatic rings might always exert the positive effect. The biodegradation of PBDEs should be thermodynamically constrained to some extent because the calculated Gibbs free energy changes of initial dioxygenation by quantum chemical method increased with the increase of bromination. Within the transition state theory, the high correlativity between the apparent biodegradation rates and Gibbs free energy changes implied the predominance and rate-limiting character of initial dioxygenation, while the regioselectivity of dioxygenation at the ortho/meta-sites was also manifested for the more negative charge population. The molecular binding with the active domain of dioxygenase BphA1 in aerobe was firstly investigated using docking approach. As significantly illustrated with the positive relationship, the higher binding affinity with BphA1 should probably signify the more rapid biodegradation. Besides the edge-on π-π stacking of PBDEs with F227 or Y277 and π-cation formulation with histidines (H233, H239) in BphA1, the reticular hydrophobic contacts appeared as the major force to underpin the high binding affinity and rapid biodegradation of PBDEs. Overall, the experimental and theoretical results would not only help understand the aerobic biodegradation mechanism, but facilitate enhancing applicability or strategy development of engineering bacteria for bioremediation of PBDEs in the environment.

Mechanistic insight into hydroxylation of 2,2',4,4'-tetrabromodiphenyl ether during biodegradation by typical aerobic bacteria: Experimental and computational studies.

Polybrominated diphenyl ethers (PBDEs) are a group of persistent pollutants in the environment. Though aerobic biodegradation of PBDEs have been extensively studied, the involved hydroxylation mechanism decisive for whole biotransformation is not clear yet. During the effective biodegradation of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) by B. xenovorans LB400, the depletion of endogenous ∙OH by scavenger could bring about the significant decrease of biodegradation efficiency whereas ·O2- was nearly not influential. Given the importance of ∙OH in hydroxylation, the reaction mechanisms along major pathways of electrophilic addition and hydrogen abstraction were theoretically examined by density functional theory (DFT). For the less demand of activation energy, the relative preference of electrophilic addition was shown at aromatic C3-site. When the secondary reaction was considered after addition at C4-site, the barrierless association of ∙OH at C3-site and deprotonation by H2O was validated as the energetically-favorable pathway that may cause dihydroxylation of BDE-47 into 3,4-dihydroxyl-BDE-17. The electrophilic addition followed by seconary barrierless trans-association of ∙OH and then dehydration seemed favorable for monohydroxylation as regards energetic barrier merely up to 194.01 kJ mol-1, while the hydrogen abstraction by ∙OH from C5-site was more privileged actually. The theoretical insights would help well understand the hydroxylation mechanism of PBDEs by aerobes.

Study on active response of superoxide dismutase and relevant binding interaction with bioaccumulated phthalates and key metabolites in Eisenia fetida.

Phthalic acid esters (PAEs) are a group of widespread persistent organic pollutants in the environment. Though the harmful effect of PAEs including activity inhibition of superoxide dismutase (SOD) to arouse oxidative stress were well documented, the deep insights into mechanisms that are relevant with SOD activity are still lacking. By 7d-cultivation of Eisenia fetida in artificially-polluted soil, the different active responses of SOD in earthworm were shown to PAE congeners. Despite the less bioaccumulation and bioavailability, the di-butyl phthalate (DBP) etc. structurally coupled with longer ester-chains appeared more effective to trigger the up-regulation and then the slight decline of SOD activity. Given the remarkable biotransformation especially for short-chain PAEs, the SOD activity response in earthworm should be regarded as joint effect with their metabolites, e.g. monophthalates (MAEs) and phthalic acid (PA). The in vitro SOD activity was shown with the obvious inhibition of 21.31% by DBP, 88.93% by MBP, and 58.57% by PA respectively when the concentrations were elevated up to 0.03 mM. The SOD activity inhibition confirmed the molecular binding with pollutants as an essential event besides the biological regulation for activity. The binding interaction was thermodynamically exothermic, spontaneous and strengthened primarily by Van der Waals force and hydrogen bonds, and was spectrally diagnosed with the conformational changes including diminution of α-helix content and spatial reorientation of fluorophore tryptophan. As coherently illustrated with the larger fluorescence quenching constants (3.65*104-4.47*104/mol) than DBP, the metabolites should be the priority concern due to stronger activity inhibition and toxicological risks.

Insights into the mechanisms underlying efficient Rhizodegradation of PAHs in biochar-amended soil: From microbial communities to soil metabolomics.

The combined effects of biochar amendment and the rhizosphere on the soil metabolic microbiome during the remediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil remain unknown. In this study, we attempted to characterize a PAH degradation network by coupling the direct PAH degradation with soil carbon cycling. From microbial community structure and functions to metabolic pathways, we revealed the modulation strategies by which biochar and the rhizosphere benefited PAH degradation in soil. Firstly, some PAH degraders were enriched by biochar and the rhizosphere, and their combination promoted the cooperation among these PAH degraders. Simultaneously, under the combined effects of biochar and the rhizosphere, the functional genes participating in upstream PAH degradation were greatly upregulated. Secondly, there were strong co-occurrences between soil microbial community members and metabolites, in particular, some PAH degraders and the metabolites, such as PAH degradation products or common carbon resources, were highlighted in the networks. It shows that the overall downstream carbon metabolism of PAH degradation was also greatly upregulated by the combined effects of biochar and plant roots, showing good survival of the soil microbiome and contributing to PAH biodegradation. Taken together, both soil carbon metabolism and direct contaminant biodegradation are likely to be modulated by the combined effects of biochar and plant roots, jointly benefitting to PAH degradation by soil microbiome. Our study is the first to link PAH degradation with native carbon metabolism by coupling sequencing and soil metabolomics technology, providing new insights into a systematic understanding of PAH degradation by indigenous soil microbiome and their networks.

Correlations between soil metabolomics and bacterial community structures in the pepper rhizosphere under plastic greenhouse cultivation.

Microbial communities in the plant rhizosphere are critical drivers of soil organic matter conversion and thus affect plant growth, especially under plastic greenhouse vegetable cultivation (PGVC). By high-throughput sequencing and soil metabolomics, we investigated the differential structures and functions of the soil bacterial community in pepper rhizosphere and bulk soils under PGVC. It was found that the soil properties, including the total dissolved organic carbon content, did not show significant differences between the rhizosphere and bulk soil. However, the soil metabolite profiles and the soil bacterial community structures changed in pepper rhizosphere. Eleven differential metabolites were detected between rhizosphere and bulk soils, including organic acids and sugars, which were positively or negatively correlated with the relative abundances of the differential bacteria. Pathway enrichment analysis indicated that the most differentially expressed metabolic pathway was starch and sucrose metabolism, and the main functional genes participating in this pathway were predicted to be downregulated in rhizosphere soil. This study linked the differential soil microbe and soil metabolite profiles in the pepper rhizosphere under PGVC and provided new insights into plant-microbe interactions in soil.

Theoretical investigation of AhR binding property with relevant structural requirements for AhR-mediated toxicity of polybrominated diphenyl ethers.

Polybrominated diphenyl ethers (PBDEs) are more frequently suspected with the induction of toxicity via signal transduction pathway of cytosolic aryl hydrocarbon receptor (AhR), the initial binding to which is assumed to be an essential prerequisite during the ligand-dependent activation. However, the AhR binding property and associated toxicity of PBDEs is yet to be clearly known for lacking insights into the structural requirements at molecular level. To understand the AhR binding property of PBDEs, the ligand binding domain (LBD) of AhR was simulatively developed on homologous protein after basic validation of geometrical rationality and the binding interaction profile was visually described using molecular docking approach. For AhR binding, the offset or edge-on π-π stackings with aromatic motifs including Phe289, Phe345 and His285 were shown to be structurally required whereas the electrostatic attraction validated for AhR binding to dioxins might be less effective for 2,2',3,4,4'-pentabromodiphenyl ether (BDE-85). Besides the demands of less steric hindrance from alanines and weak formulation of hydrogen bonds, the dispersion force through large contact and polarization of S-π electrons seemed to be impactful when BDE-85 were closer to Cys327, Met334 or Met342. With theoretical computation of AhR binding energies, the more significant correlativity with bioassays was derived especially for the lowly/moderately brominated congeners, and could be used to predict the AhR binding affinity on certain degree. The informative results would thus not only help well understand the molecular basis of AhR-mediated toxicity but give an approach for accelerative evaluation of AhR binding and toxicity of PBDEs.

Can biochar and oxalic acid alleviate the toxicity stress caused by polycyclic aromatic hydrocarbons in soil microbial communities?

It remains unclear whether biochar amendment can mediate changes in soil microbial communities caused by organic contaminants in the rhizosphere. In this study, phenanthrene-contaminated soil was amended with biochar and oxalic acid (OA) alone or in combination and incubated for 21 days. Phospholipid fatty acids (PLFAs) and high-throughput sequencing were used to evaluate shifts in bacterial and fungal community structure. Phenanthrene stress led to significant shifts in both soil bacterial and fungal community structure, in particularly, 82% of microbial phyla decreased in abundance. Biochar and/or OA improved the phenanthrene-polluted soil by positively mediating shifts in soil microbial communities stressed by phenanthrene. Specifically, biochar and/or OA led to the survival of certain microbial taxa that were inhibited by phenanthrene stress. In addition, many functional microbial individuals and genes participating in polycyclic aromatic hydrocarbon (PAH) degradation were positively stimulated by high phenanthrene stress and further stimulated by the simultaneous application of biochar and OA. Based on these findings, tandem biochar and rhizoremediation may be a feasible strategy for relieving PAH toxicity to soil microbial communities.

Does Biochar Induce Similar Successions of Microbial Community Structures Among Different Soils?

In this study, the responses of soil bacterial communities to biochar amendment in different soils were investigated. Biochar amendment had not significantly changed the bacterial richness and diversity in black soil, fluvo-aquic soil and red soil, but shifted all the soil bacterial community structures. Biochar amendment mainly increased the growth of low-abundance bacteria in fluvo-aquic soil and that of high-abundance bacteria in red soil. The most abundant bacterial phylum in black soil and fluvo-aquic soil, Proteobacteria, increased after biochar addition, while Chloroflexi, the most abundant phylum in red soil, decreased after biochar addition. Some bacterial phyla responded consistently to biochar amendment. However, many more bacterial phyla responded differently to biochar amendment in different soils, especially those phyla present at low abundances. Therefore, our study confirmed that the responses of soil bacterial communities to the same biochar were specific to both soil type and bacterial phylum.

Meso-/microporous carbon as an adsorbent for enhanced performance in solid-phase microextraction of chlorobenzenes.

There is urgent demand for the design of advanced coating materials for solid-phase microextraction (SPME) for water quality monitoring and assessment because of the global occurrence of chlorobenzenes (CBs). In this study, we proposed a dual-order activation method in which potassium hydroxide is used to modify pre-activated calcium citrate to synthesize a highly developed meso-/microporous carbon (MMC). The as-obtained MMC presented well-developed porosity with a super-high specific surface area (2638.09 m2 g-1), abundant meso-/micropores (0.5-10 nm), high hydrophobicity, excellent thermal stability (>720 °C), and a partly graphitized structure. As a coating material for headspace-SPME, the MMC-coated fiber exhibited outstanding extraction capability for CBs (up to 48.5 times higher than that of commercial fibers), which may be attributed to multiple interactions between the MMC and the pollutants, including size selectivity, micropore filling, π-π stacking and hydrophobicity. Finally, a satisfactory method using an MMC-coated fiber coupled with gas chromatography and electron capture detection was developed with good linearity (1-1000 ng L-1, R2 > 0.9982), high enrichment efficiencies (enrichment factors, 861-7819), low limits of detection (0.003-0.072 ng L-1), excellent repeatability (0.7-5.3%) and reproducibility (1.7-5.1%), and outstanding recoveries (90.18-103.02%) when applied to determine trace CBs in real water samples. These results suggest that MMC is a promising coating material for the SPME of CBs.

Green conversion of crop residues into porous carbons and their application to efficiently remove polycyclic aromatic hydrocarbons from water: Sorption kinetics, isotherms and mechanism.

In this study, rape straw- and corn cob-derived porous carbons (PCs) were fabricated by hydrothermal treatment (250 °C, 4 h) and subsequent activation (850 °C, 1 h) using a non-corrosive agent, potassium bicarbonate. The PCs exhibited a very large specific surface area (1069-1281 cm2 g-1), high pore volume (0.55-0.72 cm3 g-1), wide pore size distribution (from micropores to macropores), high hydrophobicity, and partly graphitized structure. These properties contributed to highly efficient performance for the sorption of polycyclic aromatic hydrocarbons (PAHs), with maximum sorption capacities of 592.97, 480.27, and 692.27 mg g-1 towards naphthalene, acenaphthene, and phenanthrene, respectively. A three-step sorption process with pore filling, hydrophobic effects, and π-π stacking interactions on the heterogeneous surface is a possible mechanism for the sorption of PAHs onto PCs. This study presents an environmentally friendly strategy for the reuse of crop residues in the field of organic micropollutant-contaminated water treatment.

Reducing plant uptake of a brominated contaminant (2,2',4,4'­tetrabrominated diphenyl ether) by incorporation of maize straw into horticultural soil.

Application of crop residues is a conventional practice that contributes to crop production through nutrient returns and other benefits to soil health: driving soil physicochemical and biological functions. However, little is known about the impacts of straw residue incorporation on the bioavailability of organic pollutants and associated changes in microbial community structure in contaminated soils. In this study, maize straw was added to a soil contaminated with a model polybrominated diphenyl ether (BDE-47). A pot experiment was conducted and planted with carrot (Daucus carota L.). We found that straw addition greatly reduced the bioavailability of BDE-47, changed the bacterial community structure and affected a range of soil physiochemical properties. Moreover, the amount of BDE-47 that had accumulated in carrot roots and aboveground tissues was significantly reduced. This study may therefore describe an effective agronomic strategy to reduce the bioavailability of polybrominated diphenyl ethers (PBDEs) in a soil used to grow high value vegetable crops. This strategy draws on traditional wisdom and shows promise as a practical method to support horticultural production systems, remediate soils, and help to ensure food safety.

Optimization of Sample Pretreatment based on Graphene Oxide Dispersed Acid Silica Gel for Determination of Polybrominated Diphenyl Ethers in Vegetables near an E-waste Recycling Plant.

Polybrominated diphenyl ethers (PBDEs) derived from e-waste dismantling, tend to easily bioaccumulate in vegetables. In this study, an optimized sample pretreatment method based on graphene oxide (GO) dispersed acid silica gel was used to determine PBDEs levels in vegetables. The recovery efficiency of the optimized method ranged between 90.3%-107.5% with the detection limit (LOD) being within 0.17-1.8 ng g-1. Vegetable samples were grown nearby an e-waste recycling plant in Nanjing, China, and analyzed using the optimized method. The concentrations of ΣPBDEs in the samples ranged from 12.1 to 20.1 ng g-1. This study developed an optimized sample pretreatment method to determine PBDEs in vegetables nearby e-waste contaminated sites and provides insights on the potential risks derived from e-waste dismantling to the surrounding environment.

Comparative evaluation of influence of aging, soil properties and structural characteristics on bioaccessibility of polychlorinated biphenyls in soil.

Though bioaccessibility commonly recognized as a guideline for risk assessment is closely related with pollution occurrence and chemical species of compounds, the mechanistic links are barely evaluated particularly for widespread polychlorinated biphenyls (PCBs) in soil. With the biomimetic extraction of hydroxypropyl-ß-cyclodextrin (ß-HPCD), the temporal and spatial influences of soil properties, aging and structural characteristics, e.g. polarity of PCB congeners on bioaccessibility were investigated for PCBs. Sensitive variation of bioaccessibility with aging, soil organic matter (SOM), particle size and soil moisture were clearly evidenced for different PCB congeners. Due to aging, the bioaccessibility decreased in the long term after stabilization for 36 h. In concert with the first-order kinetics, the decay rates of bioaccessibility were shown with congener-specificity and were well correlated with dipoles of PCBs. The increment of SOM diminished the bioaccessibility for the strengthened adsorption while the increased particle size and soil moisture elevated it possibly due to the less adsorption on soil particles and more accommodation of PCBs in soil pore water. Except the positive correlations with particle size, soil moisture and dipole moment, the greater dependency on aging and SOM was highlighted for bioaccessibility by partial least squares (PLS) analysis. The mutual relationship with influential factors was quantitatively formulated for accelerative prediction of bioaccessibility, and the comparative evaluation and detailed insights into the mechanistic links would thus help enhance the precise determination of bioaccessibility and risk assessment of PCBs in soil.

Long-Term Plastic Greenhouse Cultivation Changes Soil Microbial Community Structures: A Case Study.

Plastic greenhouse vegetable cultivation (PGVC) has been widely developed around the world and has resulted in great changes in soil properties and potential contamination by phthalate esters (PAEs). Using high-throughput sequencing, this study investigated the succession and potential factors impacting soil microbial community structures over 20 years of PGVC. The results showed that the pH of soils under PGVC were significantly lower, while the nutrient contents of soils were higher, relative to those of open field soil. The residue concentrations of PAEs in soil under PGVC increased with increasing periods of PGVC. The fungal community diversity, rather than the bacterial community diversity, was significantly reduced in soils under PGVC. However, both the soil bacterial and fungal community structures were changed by long-term PGVC. Among the tested soil physicochemical properties, soil pH and clay were the top two factors affecting the soil bacterial community, while pH and phosphorus (P) mainly affected the soil fungal community structures. No relationship between the changes of microbial communities and PAE residues in soil was observed. This study indicates that the soil acidification and nutrient accumulation under PGVC mainly shifted the changes of soil microbial community structures, which could occur after only 5 years of PGVC.

Facile synthesis of hierarchical porous carbon from crude biomass for high-performance solid-phase microextraction.

Porous materials have great prospective applications for solid-phase microextraction (SPME) technology because of their large specific surface area and pore volume. In this study, a hierarchical porous carbon (HPC) was synthesized by simple hydrothermal reaction and potassium hydroxide (KOH) activation of crude biomass and found to be an efficient adsorbent for SPME of organic pollutants. Results show that the as-prepared HPC has a partly graphitic amorphous-like structure with ultrahigh specific surface area (2551 m2/g), high pore volume (1.53 cm3/g), good pore size distribution (PSD) (mesopore/micropore ratio of 68%), and great thermal stability (>450 °C). When we utilized it as SPME fiber coating, the extraction capacities for chlorobenzenes (CBs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and phthalates (PAEs) were, respectively, 1.13-39.46, 2.40-7.78, 1.34-36.02, and 1.50-1.83 times higher than those of a commercial polydimethylsiloxane (PDMS) fiber. Under the optimized extraction conditions, an analytical method for CBs with low detection limits (0.01-0.24 ng/L), good repeatability (1.00%-4.93% for intra-day, 1.11%-6.94% for inter-day), and great reproducibility (1.48%-8.91%, n = 3) was developed. Moreover, we evaluated the practicality of the developed method for environmental water sample and obtained satisfactory recoveries (86.21%-104.34%). The findings provide a novel and promising HPC from crude biomass using a low-cost and facile synthetic route for SPME applications.

Mechanisms of biochar reducing the bioaccumulation of PAHs in rice from soil: Degradation stimulation vs immobilization.

This study aimed to elucidate the mechanisms by which biochar reduces the bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) in rice under anaerobic conditions. Corn straw- or bamboo-derived biochar pyrolyzed at 300 °C and 700 °C (CB300 or BB700), respectively, was amended into flooded PAH-contaminated soil. After harvest, 2% CB300, 0.5% BB700 or 2% BB700 amendments reduced the bioaccumulation of PAHs in rice root, especially that of high-molecular-weight PAHs (p < .05). Total PAH concentrations were higher, and their bioavailable concentrations were lower in BB700-amended soils than the control. The stimulation of PAH desorption from BB by low-molecular-weight organic acids (LMWOAs) was gentle and did not significantly retard the adsorption of PAHs on BB700, indicating that BB700 reduced PAH bioavailability primarily via immobilization. The total and bioavailable concentrations of PAHs were both lower in the 2% CB300-treated soils than the control. LMWOAs facilitated PAH release from CB300-amended soils, thus increasing the bioavailability of immobilized PAHs. The relative abundances of the bacteria, functional genes, and methanogens involved in PAH anaerobic degradation were significantly higher in the 2% CB300 treatment than other treatments. Fast PAH dissipation in soil amended with 2% CB300 may be attributed to the increased bioavailability of immobilized PAHs and enhanced biodegradation, both of which were induced by LMWOAs and CB. In summary, biochar types and root presence jointly affected the mechanisms by which biochar reduced the bioaccumulation of PAHs in rice under anaerobic conditions.

[Reduction and Oxidation of Mercury by Dissolved Organic Matter Under Anaerobic Conditions].

In sediments, wetlands, and flooded paddy soils, natural organic matter (DOM), such as humic acid (HA) and fulvic acid (FA), exist in a reduced state and mediate the reduction of Hg(Ⅱ) and affect the transformation of mercury and geochemical cycles. This study simulated a dark anaerobic environment to study the reduction of Hg(Ⅱ) by reduced HA and FA. Compared to oxidized HA and FA, the reduction capacity of the reduced HA and FA for Hg(Ⅱ) were higher. The optimal concentrations of reduced HA and FA to reduce Hg(Ⅱ) to Hg(0) were 0.2 mg·L-1 and 1.5 mg·L-1, respectively. The reduction quantity of Hg(Ⅱ) was enhanced with the increased concentration of reduced HA and FA at a lower than optimal concentration, whereas the reduction quantity of Hg(Ⅱ) decreased with increasing concentration of reduced HA and FA at a higher than optimal concentration due to the occurrence of thiol-competitive complexation. Hg(Ⅱ) was not reduced at a reduced HA concentration of 5 mg·L-1. The kinetics results of Hg(Ⅱ) reduction demonstrated that the reduction rates were IHSS-HA > FRC-HA > FRC-FA at a low molar ratio (dissolved organic carbon (DOC):Hg(Ⅱ)=400:1). The reduction of Hg(Ⅱ) by the reduced HA stopped or processed in the opposite direction at a high molar ratio (DOC:Hg(Ⅱ)=10000:1). The quantity of Hg(0) decreased with increasing HA concentration. Furthermore, as the concentrations of reduced HA and FA were increased to 5 mg·L-1 and 10 mg·L-1, respectively, no Hg(0) was detected in the samples. The experiment of the recovery confirmed that reduced HA and FA reacted with Hg(0). Additionally, reduced HA, FA, and Hg(0) induced oxidative complexation with sulfhydryl or disulfide bonds. Reduced DOM played a dual role in Hg redox reactions, affecting the availability of active Hg, which, in turn, affected the methylation of microbial Hg.

Sustainable synthesis of nanoporous carbons from agricultural waste and their application for solid-phase microextraction of chlorinated organic pollutants.

To guarantee the safety of water resources for humans, there is a high demand for the development of highly-efficient probes for solid-phase microextraction and analysis of trace organic pollutants. In this work, we greenly synthesized nanoporous carbons (NPCs) from oilseed rape straw via a facile hydrothermal treatment and potassium bicarbonate activation. Results showed that the NPCs had partly graphitic, amorphous-like structures with a high surface area (up to 1253 m2 g-1), large pore volume (up to 0.71 cm3 g-1), high mesopore to total pore volume ratio (up to 29%) and great thermal stability (>400 °C). When the NPCs were utilized as a solid-phase microextraction fiber coating, the extraction efficiencies for chlorinated organic pollutants (COPs) were higher (1-38 times) than with a common commercial polydimethylsiloxane coating because of high surface adsorption energy, strong π-π stacking interactions and large mass transfer capacity. Using the most efficient NPC-8 coating, under optimum extraction conditions (desorption temperature, 290 °C; extraction temperature, 80 °C; extraction time, 25 min), an analysis method for trace COPs in water was developed with good linearity (0.9991-0.9998), high sensitivity (limits of detections, 0.08-0.64 ng L-1), acceptable repeatability (RSDs of single fiber, 2.63-6.73%) and great reproducibility (RSDs of fiber-to-fiber, 2.22-7.12%). Finally, the NPC-8 coating was applied to a real environmental sample with satisfactory recoveries (86.66-103.27%).