Treating reverse osmosis brine of petrochemical wastewater using preparative vertical-flow electrophoresis (PVFE) with multi-objective optimization by response surface method.

Electrochemical desalination is an effective method for recovering salts from reverse osmosis (RO) brine. However, traditional technologies like bipolar membrane technology often face challenges related to membrane blockage. To overcome this issue, a preparative vertical-flow electrophoresis (PVFE) system was used for the first time to treat RO brine of petrochemical wastewater. In order to optimize the PVFE operation and maximize acids and bases production while minimizing energy consumption, the response surface method was employed. The independent variables selected were the electric field intensity (E) and flow rate (v), while the dependent variables were the acid-base concentration and energy consumption (EC) for acid-base production. Using the central composite design methodology, the operation parameters were optimized to be E = 154.311 V/m and v = 0.83 mL/min. Under these conditions, the base concentrations of the produced bases and acids reached 3183.06 and 2231.63 mg/L, respectively. The corresponding base EC and acid EC were calculated to be 12.57 and 11.62 kW·h/kg. In terms of the acid-base concentration and energy consumption during the PVFE process, the electric field intensity was found to have a greater influence than the flow rate. These findings provide a practical and targeted solution for recycling waste salt resources from RO brine.

Preparation of ceramsite using solid residue from anaerobic digestion of waste activated sludge and its enhancing effect on catalytic ozonation.

Anaerobic digestion is an environmentally friendly method for reclaiming waste activated sludge. However, it cannot be overlooked that the solid residue generated from this process can still pose environmental risks and impose economic pressure on society. To mitigate and recycle the solid residue, this study utilized it as a primary raw material for manufacturing ceramsite with potential applications in wastewater treatment. The optimal ratio of solid residue to fly ash was demonstrated to be 6:4 with an additional 15% of clay supplementing the raw ceramsite materials. Furthermore, the optimal sintering process was established as preheating at 300 °C for 25 min followed by sintering at 1085 °C for 10 min, as determined through an L16 (44) Orthogonal test. The prepared ceramsite demonstrated advantageous performance parameters that exceeded the standards outlined in the Chinese industry standard CJ/T 299-2008 for water treatment artificial ceramsite. When utilized in an ozonation system, the ceramsite exhibited remarkable catalytic activity for phenol degradation by promoting the decomposition of molecular O3 into hydroxyl radicals. Additionally, it displayed minimal leaching of heavy metals and lower application costs. These findings emphasize its attractiveness in water and wastewater treatment processes and present a practical strategy for reclaiming this solid residue.

Alkaline-thermal hydrolysate of waste activated sludge as a co-metabolic substrate enhances biodegradation of refractory dye reactive black 5.

Aromatic azo dyes possess inherent resistance and are known to be carcinogenic, posing a significant threat to human and ecosystems. Enhancing the biodegradation of azo dyes usually requires the presence of co-metabolic substrates to optimize the process. In addressing the issue of excessive waste activated sludge (WAS) generation, this study explored the potential of utilizing alkaline-thermal hydrolysate of WAS as a co-metabolic substrate to boost the degradation of reactive black 5 (RB5) dyes. The acclimated microbial consortium, when supplemented with the WAS hydrolysate obtained at a hydrolysis temperature of 30 °C, achieved an impressive RB5 decolorization efficiency of 90.3% (pH = 7, 35 °C) with a corresponding COD removal efficiency of 45.0%. The addition of WAS hydrolysate as a co-substrate conferred the consortium with a remarkable tolerance to high dye concentration (1500 mg/L RB5) and salinity levels (4-5%), surpassing the performance of conventional co-metabolic sugars in RB5 degradation. 3D-EEM analysis revealed that protein-like substances rich in tyrosine and tryptophan, present in the WAS hydrolysate, played a crucial role in promoting RB5 biodegradation. Furthermore, the microbial consortium community exhibited an enrichment of dye-degrading species, including Acidovorax, Bordetella, Kerstersia, and Brevundimonas, which dominated the community. Notably, functional genes associated with dye degradation and intermediates were also enriched during the RB5 decolorization and biodegradation process. These findings present a practical strategy for the simultaneous treatment of dye-containing wastewater and recycling of WAS.

Facile preparation of coprecipitates between iron oxides and dissolved organic matter for efficient Fenton-like degradation of norfloxacin.

As two important components of dissolved organic matter (DOM), dissolved black carbon (DBC) and humic acid (HA) possess different chemical and structural properties, which might influence their activities like metal complexation and mediating electron transfer. In this study, a series of coprecipitates of iron oxides (FeOx) and DOM (HA or DBC) having different C/Fe molar ratios (0.2-3.0) was prepared under ambient conditions, which exhibited excellent catalytic efficiencies upon Fenton-like degradation of norfloxacin (NOR). Pseudo-first-order rate constant of NOR oxidation catalyzed by DBC-FeOx (C/Fe=3.0, 1.13 h-1) was 30.5, 4.3-14.2, and 1.3-15.7 folds higher than those mediated by FeOx alone, HA-FeOx and DBC-FeOx coprecipitates having C/Fe molar ratios of 0.2 and 1.6, respectively. Due to the higher concentrations of surface-bound Fe(III)/Fe(II) in the DBC-FeOx mediated systems, improved Fe(III)/Fe(II) cycling rates, •OH accumulation and NOR degradation were observed as compared with those of counterpart systems mediated by HA-FeOx. Besides functioning in Fe-C complexation to accelerate FeOOH cleavage, carbonyl/carboxyl groups of the coprecipitates also serve as electron shuttles, both of which improved Fe(III)/Fe(II) cycling and •OH production. Our findings emphasized the influence of DOM source and compositions on Fe(III)/Fe(II) cycling and provided a facile approach of preparing Fe-C catalyst for contaminants elimination.

Amino acid preparation and recovery from refractory sludge by the oxidative acid hydrolysis process.

After the anaerobic digestion of excess sludge, dissolved organic matter is absorbed and used, but the treatment of refractory sludge is a headache. The oxidative acid (performic acid and hydrochloric acid) hydrolysis process can effectively prepare amino acids from refractory sludge. During the preparation process, insoluble proteins in sludge were turned into soluble proteins and peptides. All of them eventually hydrolyse into amino acids. The optimum conditions in the single-factor experiment were as follows: a temperature of 110°C, a reaction time of 24 h, and a hydrochloric acid (HCl) concentration of 6 M. The results showed that the maximum total yield of amino acids from refractory sludge was 94.76%. In the orthogonal experiment, the maximum total yield of amino acids was 97.20% under the optimum conditions of a temperature of 113.45°C, a reaction time of 26.79 h, and 5.92 M HCl. The recovery rate of purity amino acids was 17.16 g per 100 g of dry sludge. The recovery rate of the hydrochloric acid was approximately 70%. There were 17 kinds of amino acids in the hydrolysate, which could be used as deodorants, food additives, preservatives, and corrosion inhibitors. This new technology is expected to be very effective in the treatment of refractory sludge.

Role of extracellular polymeric substances in the immobilization of hexavalent chromium by Shewanella putrefaciens CN32 unsaturated biofilms.

Microbial remediation provides a promising avenue for the management and restoration of heavy metal-contaminated soils. Microorganisms in soils usually exist within unsaturated biofilms, however, their response to heavy metals is still limited compared to saturated biofilms. This work investigated the Cr(VI) immobilization by Shewanella putrefaciens CN32 unsaturated biofilms, and explored the underlying mechanisms of Cr(VI) complexation. Results reveal a dose-dependent toxicity of Cr(VI) to the growth of the unsaturated biofilms. During the early growth stage, the Cr(VI) addition stimulated more extracellular polymeric substances (EPS) production. In the meantime, the EPS were demonstrated to be the primary components for Cr(VI) immobilization, which accounted for more than 60% of the total adsorbed Cr(VI). The Fourier transform infrared spectra and X-ray photoelectron spectra corroborated that the binding sites for immobilizing Cr(VI) were hydroxyl, carboxyl, phosphoryl and amino functional groups of the proteins and polysaccharides in EPS. However, for the starved unsaturated biofilms, EPS were depleted and the EPS-bound Cr(VI) were released, which caused approximately 60% of the adsorbed Cr(VI) onto cell components and further aggravated the Cr(VI) stress to cells. This work extends our understanding about the Cr(VI) immobilization by unsaturated biofilms, and provides useful information for remediation of heavy metal-contaminated soils.

Extracellular electron transfer influences the transport and retention of ferrihydrite nanoparticles in quartz sand coated with Shewanella oneidensis biofilm.

Microbial biofilm has been found to impact the mobility of nanoparticles in saturated porous media by altering physicochemical properties of collector surface. However, little is known about the influence of biofilm's biological activity on nanoparticle transport and retention. Here, the transport of ferrihydrite nanoparticles (FhNPs) was studied in quartz sands coated with biofilm of Shewanella oneidensis MR-1 that is capable of reducing Fe(III) through extracellular electron transfer (EET). It was found that MR-1 biofilm coating enhanced FhNPs' deposition under different pH/ionic strength conditions and humic acid concentrations. More importantly, when the influent electron donor (glucose) concentration was increased to promote biofilm's EET activity, the breakthrough of FhNPs in biofilm-coated sands was inhibited. A lack of continuous and stable supply of electron donor, on the contrary, led to remobilization and release of the originally retained FhNPs. Column experiments with biofilm of EET-deficient MR-1 mutants (ΔomcA/ΔmtrC and ΔcymA) further indicated that the impairment of EET activity decreased the retention of FhNPs. It is proposed that the effective surface binding and adhesion of FhNPs that is required by direct EET cannot be neglected when evaluating the transport of FhNPs in sands coated with electroactive biofilm.

Energy Taxis toward Redox-Active Surfaces Decreases the Transport of Electroactive Bacteria in Saturated Porous Media.

The fate and transport of bacteria in porous media are essential for bioremediation and water quality control. However, the influence of biological activities like extracellular electron transfer (EET) and swimming motility toward granular media on cell transport remains unknown. Here, electroactive bacteria with higher Fe(III) reduction abilities were found to demonstrate greater retention in ferrihydrite-coated sand. Increasing the concentrations of the electron donor (1-10 mM lactate), shuttle (0-50 µM anthraquinone-2,6-disulfonate), and acceptor (ferrihydrite, MnO2, or biochar) under flow conditions significantly reduced Shewanella oneidensis MR-1's mobility through redox-active porous media. The deficiency of EET ability or flagellar motion and inhibition of intracellular proton motive force, all of which are essential for energy taxis, enhanced MR-1's transport. It was proposed that EET could facilitate MR-1 to sense, tactically move toward, and attach on redox-active media surface, eventually improving its retention. Positive linear correlations were established among parameters describing MR-1's energy taxis ability (relative taxis index), cell transport behavior (dispersion coefficient and relative change of effluent percentage), and redox activity of media surface (reduction potential or electron-accepting rate), providing novel insights into the critical impacts of bacterial microscale motility on macroscale cell transport through porous media.

Improving waste activated sludge dewaterability with sodium periodate pre-oxidation on extracellular polymeric substances.

The efficiency of sludge dewatering is affected by the structure and composition of hydrated extracellular polymeric substances (EPS). Degrading EPS can improve the sludge dewatering performance. As an oxidizing agent, sodium periodate (NaIO4 ) has ability to oxidize organics, which is expected to decompose the protein and polysaccharide in EPS and improve the efficiency of sludge dewaterability. This study adopted NaIO4 , for the first time, as an advanced oxidation agent to regulate EPS of waste activated sludge and was combined with anionic polyacrylamide (APAM) as a flocculant to subsequently enhance sludge dewatering. Response surface methodology (RSM) was used to determine the optimal conditions of pH, NaIO4 , and APAM. The results showed that the composite conditioner's specific resistance of filtration (SRF) and the water content of the vacuum-filtered cake (Wc) were highly enhanced compared with those of the raw sludge (RS) under pH 6.5, a NaIO4 concentration of 50 mg/g dry solids (DS), and an APAM concentration of 5 mg/g DS. Owing to the pre-oxidation achieved by NaIO4 under a mildly acid environment, sludge flocs were broken. Subsequently, chemical coagulation (APAM) agglomerated the smaller particles into larger flocs of sludge by adsorption and bridging, thus improving sludge dewaterability. PRACTITIONER POINTS: A novel conditioner, pH/NaIO4 /APAM, was explored for sludge dewatering. IO3 • and HO• oxidized extracellular polymeric substances (EPS). Degradation of the protein content of EPS released bound water. Highly enhanced sludge dewaterability was achieved under optimal conditions.

Synergistic catalytic Fenton-like degradation of sulfanilamide by biosynthesized goethite-reduced graphene oxide composite.

A series of goethite (Gt)-graphene (rGO) composites (Gt-rGO) having different rGO contents (2%-10%) was biologically prepared under mild conditions with Acidovorax sp. BoFeN1 and exhibited comparable or even higher catalytic efficiencies upon sulfonamides degradation than most known chemically synthesized catalysts. Pseudo-first-order rate constant of sulfanilamide degradation (60 µM, 0.971 h-1) in the system mediated by Gt-rGO with the optimal rGO content of 6% was 6.7, 15.4 and 168.1 folds higher than those in the control rGO/H2O2, Gt/H2O2 and H2O2 systems, respectively. Excellent synergistic catalytic effects between Gt and rGO in Gt-rGO were identified in four continuous cycles. The Gt-rGO systems exhibited more efficient •OH generation, H2O2 decomposition and Fe(II) accumulation rates than the control Gt or rGO systems. Fast Fe(III)/Fe(II) cycling was obtained in the Gt-rGO systems, which might be due to the strong Fe-C coordination and the decrease of rGO aggregation and Gt particle sizes. Additionally, Gt particles in Gt-rGO exposed more defects as active sites for H2O2 activation. High-performance liquid chromatography-mass spectrometer analysis suggested that sulfanilamide was gradually degraded through hydroxylation, C-N cleavage and benzene ring opening. The results provided a new approach for the tailored design of eco-friendly, cost-effective and efficient iron (oxyhydr)oxides-graphene catalysts for contaminants elimination.

Interaction between hexavalent chromium and biologically formed iron mineral-biochar composites: Kinetics, products and mechanisms.

Biogenic Fe(II) is a dominant natural reductant to convert carcinogenic Cr(VI) to less toxic Cr(III). Field-applied biochar could promote microbial production of Fe(II) and form iron-biochar composites. Although there have been mounting research on the interactions of biochar or Fe(II) with Cr(VI), their coupling effects on Cr(VI) immobilization have been largely neglected. Here, iron mineral-biochar composite (IMBC) was prepared via biochar-mediated dissimilatory reduction of ferrihydrite or goethite by Shewanella oneidensis MR-1, and its reaction with Cr(VI) was investigated. IMBC was able to effectively remove aqueous Cr(VI) via reductive transformation by adsorbed Fe(II). The removal process nicely followed pseudo-second-order kinetics and Langmuir isotherm model. The removal ability of IMBC decreased with increasing pH (5.5-8.0) but was independent of ionic strength changes (0-100 mM). After reaction, the Fe-Cr coprecipitates formed on IMBC exhibited slightly higher Fe/Cr ratios (0.93-0.96) than those on corresponding iron mineral controls (0.88-0.94). For IMBC, while the presence of biochar decreased the reactivity of adsorbed Fe(II), their removal capacities were ~30% higher than those of iron minerals alone, due to the enhanced yields of adsorbed Fe(II). These findings improved our knowledge of interactions among biochar, iron mineral and iron-reducing bacteria and their contribution to chromium immobilization.

Goethite-humic acid coprecipitate mediated Fenton-like degradation of sulfanilamide: The role of coprecipitated humic acid in accelerating Fe(III)/Fe(II) cycle and degradation efficiency.

While extensive studies found that dissociative and iron mineral-adsorbed humic acid (HA) could either stimulate or inhibit Fenton-like processes, little was known about the influence of iron mineral-coprecipitated HA on Fenton-like reactions. Here, goethite and HA (Gt-HA) coprecipitates having different C:Fe molar ratios (C:Fe = 0.16-0.99) were biologically prepared, and for the first time, investigated for their abilities of H2O2 activation and catalytic degradation of sulfanilamide. For system containing Gt-HA with the optimal C:Fe ratio of 0.30, over 91.1% of sulfanilamide (10 mg/L) was removed in 2 h, which was 46.2% higher than that of the control Gt system. Additionally, H2O2 decomposition, •OH production, and organic carbon removal in Gt-HA systems were all more efficient than those in Gt system. Higher carbon moieties stability and lower micropore surface area of Gt-HA decreased the competition for •OH and H2O2, thus helped to improve degradation efficiency. Electrochemical analysis, quenching experiments, and Fe species detection showed that the coprecipitated HA could serve as electron shuttle and complex with Fe(III) mainly via carboxyl groups at octahedral sites to improve Fe(III)/Fe(II) transformation. This study improved our understanding of Fe(III)/Fe(II) cycling in Fe‒C coprecipitates and demonstrated the potential of developing Fe‒C coprecipitates as efficient catalysts in Fenton-like processes.

Acceleration of goethite-catalyzed Fenton-like oxidation of ofloxacin by biochar.

While carbon materials have been well studied to stimulate the homogeneous Fenton-like processes, little was known about their impacts on iron mineral-catalyzed heterogeneous Fenton-like reactions. Here, it was found that biochar prepared at 300 °C or 600 °C (BC300 or BC600) greatly stimulated the degradation of ofloxacin (OFX) in a goethite (Gt)-mediated Fenton-like system. In 4 h, while only 38.4 % and 48.4 % OFX were removed in Gt/H2O2 and BC600/H2O2 systems, the removal efficiency reached over 94.0 % in Gt/BC600/H2O2 system. And the pseudo-first-order rate constant of Gt/H2O2, BC600/H2O2 and Gt/BC600/H2O2 systems were 0.12, 0.16 and 0.72 h-1, respectively, indicating the occurrence of synergistically catalytic degradation. •OH was identified as the major oxidant. Both the •OH yield and the H2O2 utilization efficiency of Gt/BC600/H2O2 system were higher than those of Gt/H2O2 and BC600/H2O2 systems. BC600 showed better stimulation effects than BC300. The persistent free radicals (PFRs) of BC could activate H2O2 and partly contribute to •OH production in the Gt/BC/H2O2 system. While BC could not directly reduce Fe(III) in Gt, it improved the cycling of Fe(III)/Fe(II) through complexing Fe(III) with its carboxyl group. Potential pathways were proposed for OFX degradation in the Gt/BC/H2O2 system.

Roles of molecular weight-fractionated extracellular polymeric substance in transformation of Au(III) to Au nanoparticles in aqueous environments.

Extracellular polymeric substance (EPS) is widely distributed in natural environments and plays important roles in the biogeochemical cycling of heavy metal. Earlier works reported that EPS could reduce metal ions such as Au(III) and Ag(I) to corresponding metal nanoparticles (NPs). EPS is a complex mixture of microbiogenic polymers with wide molecular weight (MW) distribution, and the specific components of EPS responsible for Au(III) reduction and AuNPs stabilization are still not well understood. In this study, the EPS of Shewanella oneidensis MR-1 was divided into six fractions with MW of <3, 3-10, 10-30, 30-50, 50-100, and >100 kDa, respectively through the ultrafiltration method and the roles of MW-fractionated EPS in the reduction of Au(III) to AuNPs were investigated. It was found that the low MW (<3 kDa) EPS was the major reducing agent in EPS but the fraction itself could not convert high concentration (>25 mg/L) of Au(III) to stable AuNPs due to its inferior AuNPs-stabilizing capacity. The high MW (>50 kDa) EPS could act as coating reagents to increase the stability of the formed AuNPs with sizes of 20-50 nm, but had low Au(III)-reducing activity. The carboxyl-containing substances in EPS may play crucial roles in stabilizing AuNPs. This finding is important for a better understanding of the differential roles of MW-fractionated EPS in the transformation and fate of Au(III) and AuNPs, as well as other metal ions and metal NPs in natural environments.

Facilitated bioreduction of nitrobenzene by lignite acting as low-cost and efficient electron shuttle.

The searching for efficient and economical redox mediators to promote the treatment of wastewater containing recalcitrant organic compounds is greatly needed. In this study, the redox mediator activities of four different lignite samples to facilitate the bioreduction of nitrobenzene by Shewanella oneidensis MR-1 were tested for the first time. The initial nitrobenzene reduction rate was increased by 40.4%-90.3% in the presence of 50 mg/L of different lignite samples. Lignite collected from Xinjiang (XJL) having more oxygenated groups performed better in enhancing nitrobenzene bioreduction. The stimulating effects increased with the increase of lignite dosage (0-200 mg/L) and the decrease of lignite particle size (150-0.1 µm). However, the pristine XJL samples with assorted sizes of particles exhibited better stimulating effects than size-fractionated ones, implying that different-sized XJL particles might have synergetic effects on the bioreduction process. When humic acid or iron was removed from XJL, its promoting effects were decreased, demonstrating the crucial roles of both components in lignite-enhanced nitrobenzene bioreduction. Nitric acid treatment could form more oxygenated moieties on lignite surface, which played vital roles in promoting nitrobenzene bioreduction. The initial nitrobenzene bioreduction rate in the presence of HNO3-treated XJL was 80.8% higher than that obtained with pristine XJL. This study proposed an effective and readily available redox mediator that could be applied to promote the bioreduction of recalcitrant electrophilic pollutants.

Effect on sludge disintegration by EDTA-enhanced thermal-alkaline treatment.

Sludge disintegration is an effective pretreatment to enhance the biodegradability of sludge. At present, the thermal-alkaline is one of the most commonly used methods, but it has a massive consumption of energy and chemical reagents. EDTA-enhanced thermal-alkaline treatment was used to strengthen the dewatered sludge disintegration at mid-low temperature in this study. Results showed that the dissolving-out quantity of soluble chemical oxygen demand and the volatile solid (VS) in residual sludge in the EDTA-added group were 14.7% higher and 7% lower than those in control system without EDTA, respectively, indicating that EDTA addition improved the performance of sludge disintegration. The addition of EDTA loosened the floc structure and enhanced the hydrolyzability of dissolved organic matters (DOM) with a narrower distribution of the relative molecular weight. The membrane damage of microbial cells in EDTA-added group reached 73.3% after 120 min, which was much higher than that in the control group (31.9%). EDTA contains a large number of hydrogen bond acceptors and could form hydrogen bonds with alcohols and phenols in solubilization products and DOM. It was speculated that the mechanism of EDTA-enhanced sludge disintegration was related to the formation of hydrogen bonds between EDTA and organic matter inside and outside the cell. PRACTITIONER POINTS: The addition of EDTA facilitated the thermal-alkali cracking of dewatered sludge. EDTA increased the particle size of sludge and enhanced the hydrolysis of DOM. The strengthening effect mainly occurred at the beginning of TB-EPS dissolving slowly. Hydrogen bond played important roles in the enhanced disintegration of sludge by EDTA.

Transformation of silver ions to silver nanoparticles mediated by humic acid under dark conditions at ambient temperature.

The conversion of silver materials in environments would impact their toxicity and risk. Previous studies have reported that silver ions (Ag+) could be reduced to silver nanoparticles (AgNPs) by natural organic matters (NOM) under sunlight or heating conditions. However, whether such reaction could occur in darkness at ambient temperature and the transformation mechanism were unclear. This study found that Ag+ at environmentally relevant concentrations (as low as 1 µg/L) could be reduced to AgNPs by Suwannee River humic acid (SRHA) in darkness at 30 °C. The reaction mechanism probed by X-ray absorption fine structure spectroscopy revealed that Ag+ was first bound to the carboxylic groups of SRHA to form Ag+-SRHA ligands, which were then reduced to metallic Ag. The increase of pH (6-9) and the coexistence of formate, acetate, carbonate, and sulfate promoted the formation of AgNPs. Besides, the formed AgNPs would coalesce to large aggregates under acidic conditions or in the presence of sulfate. These results suggest that the dark transformation of Ag+ to AgNPs mediated by NOM could occur in environments and are important for the better understanding of the natural origin of AgNPs.

Effects of reduced graphene oxide on humic acid-mediated transformation and environmental risks of silver ions.

The reduction of Ag+ mediated by natural organic matters has been demonstrated to be an important process of Ag+ transformation and would influence the risks of Ag+ and Ag-containing materials in aquatic environment. Considering the large production of carbon nanomaterials (CNMs) and their inevitable release into the environment, the effects of CNMs on Ag transformation are of considerable interest. This study demonstrated that the humic acid-mediated reduction of Ag+ to free Ag nanoparticles (AgNPs) in aqueous phase was suppressed by coexisting reduced graphene oxide (rGO). A large amount of Ag+ was reduced on rGO surface, resulting in the generation of AgNPs-rGO composites. rGO at concentrations of 1-2 orders of magnitude lower than those of Ag+ would exhibit significant effects. The X-ray absorption fine structure spectroscopy study showed that Ag+ was first adsorbed on rGO surface cooperatively with humic acid and then rapidly reduced to AgNPs. The hydroxylic-OH on rGO could participate in the AgNPs formation and was oxidized to carbonyl during the reduction of Ag+. Additionally, the formed AgNPs-rGO had a relatively lower environmental risk compared to AgNPs or rGO alone. Overall, these results improve our understanding of the interaction between CNMs and Ag+ in aquatic systems.

Accelerating effects of humin on sulfide-mediated azo dye reduction.

As an important fraction of humic substances, humin has been found capable of stimulating bioreduction reactions. However, whether humin could promote abiotic reduction and the effects of coexisting soluble humic substance and insoluble mineral remained unsolved. In this study, a humin sample was isolated from a paddy soil. Cyclic voltammetry, electron paramagnetic resonance, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses of the humin indicated the existence of redox-active quinone moieties and other oxygen-containing groups. The humin could be reduced by sulfide and its presence stimulated the abiotic reduction of acid red 27 (AR27) and four other azo dyes by sulfide. In the presence of 100-1000 mg/L intact humin, the sulfide-mediated AR27 reduction efficiency in 7 d was enhanced from 56.3% to 92.5%. The stimulating behavior of intact humin was observed for 100-300 mg/L AR27 and increased with the increase of sulfide concentration (1.2-3.0 mM). Much higher stimulating effects were found with the presence of humin pre-reduced by sulfide. Moreover, for sulfide-mediated AR27 reduction, the coexistence of humin (500 mg/L) and humic acid (10-30 mg/L) or Wyoming sodium-montmorillonite (SWy-2, 1-4 g/L) led to better promotion activities than the presence of single component. And synergistic promotion of sulfide-mediated AR27 reduction was observed with coexisting humin and SWy-2 due to enhanced Fe(II) production. These findings extended our understanding of the influence of humin on reductive transformation of pollutants in the environment.

Effects of sludge lysate for Cr(VI) bioreduction and analysis of bioaugmentation mechanism of sludge humic acid.

This study evaluated the effects of sludge lysate (SL) on the anaerobic bioreduction of Cr(VI) and the role of sludge humic acid (SHA) during this process. The results showed that supplement of SL significantly enhanced the efficiency of Cr(VI) bioreduction by 29.61%, in 12 h compared with that of the control without SL. Moreover, SHA exhibited promoting effects on bioreduction of Cr(VI), and the promotion increased with increasing SHA concentrations from 100 to 300 mg/L. In the presence of 300 mg/L SHA, Cr(VI) (98.21 mg/L) was completely reduced after 24 h with a removal rate increased by 34.3% compared with that of the control without SHA. Further investigation on the bioaugmentation mechanism of SHA by studying the nature of SHA and the reaction mechanism between SHA and Cr(VI) revealed that SHA exhibited a strong adsorption ability, which could adsorb and combine with Cr(VI). The adsorption capacity of Cr(VI) by SHA was calculated as 34.4 mg/g with 0.2 g of SHA and 10 mg/L of Cr(VI). It could also act as redox mediators to accelerate the electron transfer between microorganisms and Cr(VI) to promote reduction of Cr(VI). Furthermore, the effects of SL on the microbial community compositions of the anaerobic Cr(VI) bioreduction system were studied. Brachymonas was the primary bacteria at the genus level. The abundance of electroactive bacteria, such as Acinetobacter, Pseudomonas, and Arcobacter, increased in the SL-amended system. These findings expand the versatility of SL and justify wider use of residual activated sludge, which might contribute to the treatment of heavy metal-contaminated wastewater.