Dechlorination Rate Research Articles

Enhanced dechlorination of 2,4-dichlorophenol using nanorod palygorskite-loaded Fe/Ni nanoparticles: Performance evaluation, influencing factors and action mechanism of Fe and Ni

The basic structural unit of palygorskite (P) consists of rod-shaped crystals with a diameter ranging from 20 to 70 nm. However, due to strong hydrogen bonding and electrostatic forces, most palygorskite rod crystals tend to aggregate, limiting its nanomaterial properties in practical application. To address this limitation, the study focused on dissociating palygorskite aggregates and incorporating them into nanorod palygorskite-loaded Fe/Ni bimetallic nanoparticles (nP-nFe/Ni) synthesis. This approach facilitated the dissociation of palygorskite aggregates and enabled the dispersion of Fe/Ni nanoparticles by palygorskite nanorods. The results revealed that the BET specific surface area of nP-nFe/Ni was significantly higher at 86.17 m2/g compared to 33.62 m2/g for nFe/Ni. Consequently, the dechlorination efficiency of nP-nFe/Ni for 2,4-dichlorophenol (2,4-DCP) at 120 min was 22.4% higher than that of nFe/Ni, despite nP-nFe/Ni containing only 35.1% of the Fe content found in nFe/Ni at the same dosage. Moreover, it was observed that acidic conditions favor maximum dichlorination, while higher temperatures enhance the dechlorination rate. The action mechanism of Fe was identified as the generation of H2, which is catalyzed by Ni to form active hydrogen (H*) that attack the C-Cl bonds of 2,4-DCP for hydrodechlorination. Ni played a vital role in catalyzing H2 conversion to H* , accelerating H2 production by Fe, and inhibiting Fe passivation. These findings are crucial for the application of nano iron technology and nano-scale palygorskite in water and wastewater treatment.

Substrate-dependent strategies to mitigate sulfate inhibition on microbial reductive dechlorination of polychlorinated biphenyls

Sulfate widely co-exists with polychlorinated biphenyls (PCBs) at various concentrations in the subsurface environment. Previous studies have suggested that sulfate often hampers microbial degradation of aliphatic chlorinated solvents such as chloroethenes. However, the impact of sulfate on microbial reductive dechlorination of aromatic PCBs and the underlying mechanisms have received limited attention. Likewise, strategies to mitigate such inhibition remain scarce. Here we found that the mechanisms and mitigation strategies of sulfate inhibition on PCB dechlorination were substrate-dependent. Under electron donor-limiting conditions, even a low concentration of sulfate (2 mM) resulted in a decreased PCB dechlorination rate by 88.7% in a co-culture comprising Dehalococcoides mccartyi CG1 and the sulfate-reducing bacterium Desulfovibrio desulfuricans F1, an inhibition which was attributed to the competition for electron donor between sulfate reduction and PCB dechlorination. As expected, re-amendment of 5 mM lactate effectively re-initiated PCB dechlorination. However, in the presence of a higher concentration of sulfate (5 mM), the PCB dechlorination rate in the co-culture was 77.7% lower than in the control, even with excessive electron donor supply. This inhibition was linked to high concentration of sulfide (∼5 mM) produced from sulfate reduction, as suggested by high availability of electron donor, recovery of dechlorination activity after removal of sulfide, and negligible influence of sulfate on PCB dechlorination in the axenic culture of D. mccartyi CG1. Indeed, sulfide (>5 mM) was found to directly suppress expression of PCB-dechlorinating reductive dehalogenase gene. The highest transcriptional level of pcbA1 was 2.9 ± 0.3 transcripts·cell−1 in the presence of ∼5 mM sulfide, which was increased to 37.4 ± 5.0 transcripts·cell−1 when sulfide was removed. Under this scenario, introduction of ferrous salts (5 mM) efficiently alleviated sulfide inhibition on PCB dechlorination. Interestingly, the augmentation of methanogens in the co-culture was also effective in mitigating sulfide inhibition on PCB dechlorination, offering a new approach to protect Dehalococcoides under sulfide stress. Collectively, these findings deepen our understanding of the influence of sulfate on microbial reductive dechlorination of PCBs and contribute to developing appropriate strategies based on geochemical conditions to alleviate sulfate inhibition during bioremediation of PCB-contaminated sites.

Performance and degradation pathway of florfenicol antibiotic by nitrogen-doped biochar supported zero-valent iron and zero-valent copper: A combined experimental and DFT study

Fluorinated compounds are a class of organic substances resistant to degradation. Although zero-valent iron (Fe0) has a promising reducing capability, it still fails to degrade fluorine-containing antibiotics (i.e., florfenicol) efficiently. In this study, we applied a simple one-pot pyrolytic approach to synthesize nitrogen-doped biochar supported Fe0 and zero-valent copper (Cu0) composite (Fe/Cu@NBC) and investigated its performance on florfenicol removal. The results clearly showed that approximately 91.4% of florfenicol in the deionized water was removed by Fe/Cu@NBC within 8 h. As the reaction time was extended to 15 d, the total degradation rate of florfenicol reached 96.6%, in which the defluorination and dechlorination rates were 73.2% and 82.1%, respectively. Both experimental results and density functional theory calculation suggested that ∙OH and ·O2− triggered β-fluorine elimination, resulting in defluorination prior to dechlorination. This new finding was distinct from previous viewpoints that defluorination was more difficult to occur than dechlorination. Fe/Cu@NBC also had a favorable performance for removal of florfenicol in surface water. This study provides a new insight into the degradation mechanism and pathway of florfenicol removal in the Fe/Cu@NBC system, which can be a promising alternative for remediation of fluorinated organic compounds in the environment.

Tetrachloroethane (TeCA) removal through sequential graphite-mixed metal oxide electrodes in a bioelectrochemical reactor

Electro-bioremediation offers a promising approach for eliminating persistent pollutants from groundwater since allows the stimulation of biological dechlorinating activity, utilizing renewable electricity for process operation and avoiding the injection of chemicals into aquifers. In this study, a two-chamber microbial electrolysis cell has been utilized to achieve both reductive and oxidative degradation of tetrachloroethane (TeCA). By polarizing the graphite granules cathodic chamber at −650 mV vs the standard hydrogen electrode and employing a mixed metal oxide (MMO) counter electrode for oxygen production, the reductive and oxidative environment necessary for TeCA removal has been established. Continuous experiments were conducted using two feeding solutions: an optimized mineral medium for dechlorinating microorganisms, and synthetic groundwater containing sulphate and nitrate anions to investigate potential side reactions. The bioelectrochemical process efficiently reduced TeCA to a mixture of trans-dichloroethylene, vinyl chloride, and ethylene, which were subsequently oxidized in the anodic chamber with removal efficiencies of 37 ± 2%, 100 ± 4%, and 100 ± 5%, respectively. The introduction of synthetic groundwater with nitrate and sulphate stimulated reductions in these ions in the cathodic chamber, leading to a 17% decrease in the reductive dechlorination rate and the appearance of other chlorinated by-products, including cis-dichloroethylene and 1,2-dichloroethane (1,2-DCA), in the cathode effluent. Notably, despite the lower reductive dechlorination rate during synthetic groundwater operation, aerobic dechlorinating microorganisms within the anodic chamber completely removed VC and 1,2-DCA. This study represents the first demonstration of a sequential reductive and oxidative bioelectrochemical process for TeCA mineralization in a synthetic solution simulating contaminated groundwater.

Chlorine Distribution in Soil and Vegetation in Boreal Habitats along a Moisture Gradient from Upland Forest to Lake Margin Wetlands.

The assumed dominance of chloride (Cl-) in terrestrial ecosystems is challenged by observations of extensive formation of organically bound Cl (Clorg), resulting in large soil Cl storage and internal cycling. Yet, little is known about the spatial distribution of Cl in ecosystems. We quantified patterns of Cl distribution in different habitats along a boreal hillslope moisture gradient ranging from relatively dry upland coniferous forests to wet discharge areas dominated by alder. We confirmed that dry habitats are important for Cl storage but found that Cl pools tended to be larger in moist and wet habitats. The storage of Clorg was less important in wet habitats, suggesting a shift in the balance between soil chlorination and dechlorination rates. Cl concentrations in the herb layer vegetation were high in wet and moist sites attributed to a shift in plant species composition, indicating plant community-dependent ecosystem Cl cycling. Mass-balance calculations showed that internal Cl cycling increased overall ecosystem Cl residence times at all sites and that plant uptake rates of Cl- were particularly high at wet sites. Our results indicate that habitat characteristics including plant communities and hydrology are key for understanding Cl cycling in the environment.

Bioelectrochemical system accelerates reductive dechlorination through extracellular electron transfer networks

Bioelectrochemical system is considered as a promising approach for enhanced bio-dechlorination. However, the mechanism of extracellular electron transfer in the dechlorinating consortium is still a controversial issue. In this study, bioelectrochemical systems were established with cathode potential settings at −0.30 V (vs. SHE) for trichloroethylene reduction. The average dechlorination rate (102.0 μM Cl·d−1) of biocathode was 1.36 times higher than that of open circuit (74.7 μM Cl·d−1). Electrochemical characterization via cyclic voltammetry illustrated that electrostimulation promoted electrochemical activity for redox reactions. Moreover, bacterial community structure analyses indicated electrical stimulation facilitated the enrichment of electroactive and dechlorinating populations on cathode. Metagenomic and quantitative polymerase chain reaction (qPCR) analyses revealed that direct electron transfer (via electrically conductive pili, multi-heme c-type cytochromes) between Axonexus and Desulfovibrio/cathode and indirect electron transfer (via riboflavin) for Dehalococcoides enhanced dechlorination process in BES. Overall, this study verifies the effectiveness of electrostimulated bio-dechlorination and provides novel insights into the mechanisms of dechlorination process enhancement in bioelectrochemical systems through electron transfer networks.

Unlocking the potential of resuscitation-promoting factor for enhancing anaerobic microbial dechlorination of polychlorinated biphenyls

Microbial dechlorination of polychlorinated biphenyls (PCBs) is limited by the slow growth rate and low activity of dechlorinators. Resuscitation promoting factor (Rpf) of Micrococcus luteus, has been demonstrated to accelerate the enrichment of highly active PCB-dechlorinating cultures. However, it remains unclear whether the addition of Rpf can further improve the dechlorination performance of anaerobic dechlorination cultures. In this study, the effect of Rpf on the performance of TG4, an enriched PCB-dechlorinating culture obtained by Rpf amendment, for reductive dechlorination of four typical PCB congeners (PCBs 101, 118, 138, 180) was evaluated. The results indicated that Rpf significantly enhanced the dechlorination of the four PCB congeners, with residual mole percentages of PCBs 101, 118, 138 and 180 in Rpf-amended cultures being 16.2–29.31 %, 13.3–20.1 %, 11.9–14.4 % and 9.4–17.3 % lower than those in the corresponding cultures without Rpf amendment after 18 days of incubation. Different models were identified as appropriate for elucidating the dechlorination kinetics of distinct PCB congeners, and it was observed that the dechlorination rate constant is significantly influenced by the PCB concentration. The supplementing Rpf did not obviously change dechlorination metabolites, and the removal of chlorines occurred mainly at para- and meta- positions. Analysis of microbial community and functional gene abundance suggested that Rpf-amended cultures exhibited a significant enrichment of Dehalococcoides, Dehalogenimonas and Desulfitobacterium, as well as non-dechlorinators belonging to Desulfobacterota and Bacteroidetes. These findings highlight the potential of Rpf as an effective additive for enhancing PCB dechlorination, providing new insights into the survival of functional microorganisms involved in anaerobic reductive dechlorination.

The effects of arsenic on dechlorination of trichloroethene by consortium DH: Microbial response and resistance

Inorganic arsenic and organochlorines are frequently co-occurring contaminants in anoxic groundwater environments, and the bioremediation of their composite pollution has long been a rigorous predicament. Currently, the dechlorination behaviors and stress responses of microbial dechlorination consortia to arsenic are not yet fully understood. This study assessed the reductive dechlorination performance of a Dehalococcoides-bearing microcosm DH under gradient concentrations of arsenate [As(V)] or arsenite [As(III)] and investigated the response patterns of different functional microorganisms. Our results demonstrated that although the dechlorination rates declined with increasing arsenic concentrations in both As(III/V) scenarios, the inhibitory impact was more pronounced in As(III)-amended groups compared to As(V)-amended groups. Moreover, the vinyl chloride (VC)-to-ethene step was more susceptible to arsenic exposure compared to the trichloroethene (TCE)-to-dichloroethane (DCE) step, while high levels of arsenic exposure [e.g. As(III) > 75 μM] can induce significant accumulation of VC. Functional gene variations and microbial community analyses revealed that As(III/V) affected reductive dechlorination by directly inhibiting organohalide-respiring bacteria (OHRB) and indirectly inhibiting synergistic populations such as acetogens. Metagenomic results indicated that arsenic metabolic and efflux mechanisms were identical among different Dhc strains, and variations in arsenic uptake pathways were possibly responsible for their differential responses to arsenic exposures. By comparison, fermentative bacteria showed high potential for arsenic resistance due to their inherent advantages in arsenic detoxification and efflux mechanisms. Collectively, our findings expanded the understanding of the response patterns of different functional populations to arsenic stress in the dechlorinating consortium and provided insights into modifying bioremediation strategies at co-contaminated sites for furtherance.

Impact of different zero valent iron-based particles on anaerobic microbial dechlorination of 2,4-dichlorophenol: Comparison of dechlorination performance and the underlying mechanism

The integration of iron-based materials and anaerobic microbial consortia has been extensively studied owing to its potential to enhance pollutant degradation. However, few studies have compared how different iron materials enhance the dechlorination of chlorophenols in coupled microbial systems. This study systematically compared the combined performances of microbial community (MC) and iron materials (Fe0/FeS2 +MC, S-nZVI+MC, n-ZVI+MC, and nFe/Ni+MC) for the dechlorination of 2,4-dichlorophenol (DCP) as one representative of chlorophenols. DCP dechlorination rate was significantly higher in Fe0/FeS2 +MC and S-nZVI+MC (1.92 and 1.67 times, with no significant difference between two groups) than in nZVI+MC and nFe/Ni+MC (1.29 and 1.25 times, with no significant difference between two groups). Fe0/FeS2 had better performance for the reductive dechlorination process as compared with other three iron-based materials via the consumption of any trace amount of oxygen in anoxic condition and accelerated electron transfer. On the other hand, nFe/Ni could induce different dechlorinating bacteria as compared to other iron materials. The enhanced microbial dechlorination was mainly due to some putative dechlorinating bacteria (Pseudomonas, Azotobacter, Propionibacterium), and due to improved electron transfer of sulfidated iron particles. Therefore, Fe0/FeS2 as a biocompatible as well as low-cost sulfidated material can be a good alternative for possible engineering applications in groundwater remediation.

Dithionite promoted microbial dechlorination of hexachlorobenzene while goethite further accelerated abiotic degradation by sulfidation in paddy soil

It is of great scientific and practical importance to explore the mechanisms of accelerated degradation of Hexachlorobenzene (HCB) in soil. Both iron oxide and dithionite may promote the reductive dechlorination of HCB, but their effects on the microbial community and the biotic and abiotic mechanisms behind it remain unclear. This study investigated the effects of goethite, dithionite, and their interaction on microbial community composition and structure, and their potential contribution to HCB dechlorination in a paddy soil to reveal the underlying mechanism. The results showed that goethite addition alone did not significantly affect HCB dechlorination because the studied soil lacked iron-reducing bacteria. In contrast, dithionite addition significantly decreased the HCB contents by 44.0–54.9%, while the coexistence of dithionite and goethite further decreased the HCB content by 57.9–69.3%. Random Forest analysis suggested that indicator taxa (Paenibacillus, Acidothermus, Haliagium, G12-WMSP1, and Frankia), Pseudomonas, richness and Shannon’s index of microbial community, and immobilized Fe content were dominant driving factors for HCB dechlorination. The dithionite addition, either with or without goethite, accelerated HCB anaerobic dechlorination by increasing microbial diversity and richness as well as the relative abundance of the above specific bacterial genera. When goethite and dithionite coexist, sulfidation of goethite with dithionite could remarkably increase FeS formation and then further promote HCB dechlorination rates. Overall, our results suggested that the combined application of goethite and dithionite could be a practicable strategy for the remediation of HCB contaminated soil.

Ball-milled PET plastic char as an electron shuttle accelerated anaerobic degradation of 2,4,6-trichlorophenol in water environment

11The number behind BMPC denotes the pyrolysis temperature.The combination of carbonaceous materials and microbial degradation is an attractive measure in improving the removal efficiency of organic pollutants in water environment. In this study, the anaerobic dechlorination in a coupled system of ball-milled plastic chars (BMPCs) and the microbial consortium were investigated. The anaerobic microorganism cultured from raw sludge (CAM) contributed to the dechlorination of the 2,4,6-trichlorophenol (2,4,6-TCP) into 4-chlorophenol (4-CP) as the final product via ortho-dechlorination in all testing groups. The dechlorination rate was accelerated in different BMBC plus CAM groups than that in only CAM group (0.048 d−1), of which, it was greater in BMPC-500 plus CAM group (0.375 d−1) than that in BMPC-700 plus CAM group (0.171 d−1). The electron exchange capacity (EEC) of BMPCs decreased with the increase of pyrolysis temperature (0.053 mmol e−/g for BMPC-500 and 0.037 mmol e−/g for BMPC-700), which directly affected anaerobic dechlorination. Direct interspecies electron transfer (DIET) of BMPCs also boosted the biogas yield by 1.5 times compared to that without BMPCs. Microbial community analysis illustrated that BMPCs helped to enrich the putative dechlorinating bacteria. The abundance of Clostridium_aenus_stricto_12, as a dominant dechlorinator, significantly increased from 0.02 % to 11.3 % (without BMPCs), 39.76 % (BMPC-500) and 9.3 % (BMPC-700), and followed by， Prevotella and Megaspheara, which was reported to take part in anaerobic dechlorination and digestion as H2 producers, also increased in the presence of BMPC. This study contributes to the realization of 2,4,6-TCP in-situ reduction technology and provides a scientific reference for anaerobic dechlorination by cultured anaerobes combined with BMPCs.

Reactive membranes for groundwater remediation of chlorinated aliphatic hydrocarbons: Competitive dechlorination and cost aspects

A nanocomposite membrane incorporating reactive Pd-Fe nanoparticles (NPs) was developed to remediate chlorinated aliphatic hydrocarbons (CAHs) from groundwater. Other than recapturing the produced Fen+ for in-situ regeneration, the functionalized polyanions prevented NPs agglomeration and resulting in a spherical Fe0 core (55 nm, O/Fe = 0.05) and an oxidized shell (4 nm, O/Fe = 1.38). The reactive membranes degraded 92% of target CAHs with a residence time of 1.7 s. After long-term treatment and regeneration, reusability was confirmed through recovered reactivity, recurrence of Fe0 in X-ray photoelectron spectroscopy, and >96% remaining of Fe and Pd. The total cost (adjusted present value for 20 years) was estimated to be 13.9% lower than the granular activated carbon system, following an EPA work breakdown structure-based cost model. However, non-target CAHs from groundwater can compete for active sites, leading to decreased surface-area normalized dechlorination rate (ksa) by 28.2–79.9%. A hybrid nanofiltration (NF)/reactive membrane was proposed to selectively intercept larger competitors, leading to 54% increased dechlorination efficiency and 1.3 to 1.9-fold enlarged ksa. Overall, the practical viability of the developed reactive membranes was demonstrated by the stability, reusability, and cost advantages, while the optional NF strategy could alleviate competitive degradation towards complex water chemistry.

Optimizing hydrothermal dechlorination of PVC in a SS-316 reactor: From chemistry knowledge to material considerations

PVC remains one of the most challenging wastes for environmental activists and industrial recyclers because its decomposition products cause severe corrosion to equipment and our planet. The current study was designed to systematically investigate PVC dechlorination using the well-known Response Surface Methodology (RSM) to mature the process chemistry and assess the corrosion rate in the common SS-316 alloy. Undercover non-catalytic experiments, Cl removal, reactor wall corrosion rate, and H/C molar ratio of the products made the most argument to comprehend the chemistry and find the optimum operating condition. Based on data collected from different analytic techniques, like FESEM, SEM-EDX, TG, FT-IR, CHN, and AAS, it was found that water drove Cl removal through a combination of SN2 and Elimination pathways where the in-situ production of hydrochloric acid at high Severity Factor (SF) resulted in a desirably high dechlorination rate of 89.42%. However, significant metal leakage into the aqueous phase, e.g., 278.01 mg of Fe, in the Aqueous Phase (AP) necessitated the valorization of different approaches, including multistage dechlorination and application of various hetero and homogeneous catalysts, dealing with chloride and metal salts inside the reaction chamber. Among the solutions applied, the dual-stage and Ni-assisted dechlorination showed promising performances receptively with a corrosion rate as low as 0.034 g and the highest Cl removal efficiency of 96.18%.

Sustained detoxification of 1,2-dichloroethane to ethylene by a symbiotic consortium containing Dehalococcoides species

1,2-Dichloroethane (1,2-DCA) is a ubiquitous volatile halogenated organic pollutant in groundwater and soil, which poses a serious threat to the ecosystem and human health. Microbial reductive dechlorination has been recognized as an environmentally-friendly strategy for the remediation of sites contaminated with 1,2-DCA. In this study, we obtained an anaerobic microbiota derived from 1,2-DCA contaminated groundwater, which was able to sustainably convert 1,2-DCA into non-toxic ethylene with an average dechlorination rate of 30.70 ± 11.06 μM d−1 (N = 6). The microbial community profile demonstrated that the relative abundance of Dehalococcoides species increased from 0.53 ± 0.08% to 44.68 ± 3.61% in parallel with the dechlorination of 1,2-DCA. Quantitative PCR results showed that the Dehalococcoides species 16S rRNA gene increased from 2.40 ± 1.71 × 108 copies∙mL−1 culture to 4.07 ± 2.45 × 108 copies∙mL−1 culture after dechlorinating 110.69 ± 30.61 μmol of 1,2-DCA with a growth yield of 1.55 ± 0.93 × 108 cells per μmol Cl− released (N = 6), suggesting that Dehalococcoides species used 1,2-DCA for organohalide respiration to maintain cell growth. Notably, the relative abundances of Methanobacterium sp. (p = 0.0618) and Desulfovibrio sp. (p = 0.0001995) also increased significantly during the dechlorination of 1,2-DCA and were clustered in the same module with Dehalococcoides species in the co-occurrence network. These results hinted that Dehalococcoides species, the obligate organohalide-respiring bacterium, exhibited potential symbiotic relationships with Methanobacterium and Desulfovibrio species. This study illustrates the importance of microbial interactions within functional microbiota and provides a promising microbial resource for in situ bioremediation in sites contaminated with 1,2-DCA.

Cathode potential regulates the microbiome assembly and function in electrostimulated bio- dechlorination system.

Mechanism of microbiome assembly and function driven by cathode potential in electro-stimulated microbial reductive dechlorination system remain poorly understood. Here, core microbiome structure, interaction, function and assembly regulating by cathode potential were investigated in a 2,4,6-trichlorophenol bio-dechlorination system. The highest dechlorination rate (24.30μM/d) was observed under -0.36V with phenol as a major end metabolite, while, lower (-0.56V) or higher (0.04V or -0.16V) potentials resulted in 1.3-3.8 times decreased of dechlorination kinetic constant. The lower the cathode potential, the higher the generated CH4, revealing cathode participated in hydrogenotrophic methanogenesis. Taxonomic and functional structure of core microbiome significantly shifted within groups of -0.36V and -0.56V, with dechlorinators (Desulfitobacterium, Dehalobacter), fermenters (norank_f_Propionibacteriaceae, Dysgonomonas) and methanogen (Methanosarcina) highly enriched, and the more positive interactions between functional genera were found. The lowest number of nodes and links and the highest positive correlations were observed among constructed sub-networks classified by function, revealing simplified and strengthened cooperation of functional genera driven by group of -0.36V. Cathode potential plays one important driver controlling core microbiome assembly, and the low potentials drove the assembly of major dechlorinating, methanogenic and electro-active genera to be more deterministic, while, the major fermenting genera were mostly governed by stochastic processes.

Enhancing the dechlorination of CuCl residue using a microwave sulfation-roasting method: Process optimization and mechanism investigation

Spent CuCl residue produced from zinc hydrometallurgy is a typical toxic and hazardous solid waste as well as a secondary resource owing to its high contents of chlorine and valuable metals (Pb, Cu, Zn). The removal of chlorine from the CuCl residue and recycling of valuable metals are critical issues for the copper slag resource treatment process. In this study, a novel method of microwave sulfation-roasting under an oxygen atmosphere is used to improve the dechlorination process, and various parameters, such as roasting temperature, duration, sulfuric acid concentration, and liquid/solid ratio, are optimized. First, according to thermodynamic calculations, the addition of H2SO4 can significantly decrease both the roasting temperature and time needed for microwave oxidation-roasting. The dechlorination experimental results reveal that the dechlorination rate of the CuCl residue is as high as 91.91% under a temperature of 250 °C, holding time of 120 min, sulfuric acid concentration of 70%, and liquid-solid ratio of 1.42. The microstructure, chemical composition, and phase changes of the CuCl residue before and after roasting are characterized using scanning electron microscopy coupled with energy-dispersive spectroscopy, X-ray fluorescence and X-ray diffraction; further, the mass contents of the main elements in the roasting product and tail gas absorption solution are determined by chemical analysis, Evidently, the selective heating property of microwaves produces more holes and cracks in the copper slag, which can provide a new reaction channel for H2SO4 and CuCl slag reactions, and almost all CuCl in the copper slag is completely removed by transformation into CuO, CuSO4 and Cu2O(SO4). Simultaneously, a large number of gas by-products produced in the dechlorination process, such as SO2 and Cl2, can be absorbed by the NaOH solution. Therefore, no pollution is generated in the environment. These findings illustrate that microwave sulfation-roasting is an effective method for promoting the treatment and recycling of CuCl waste residues.

The role of NaOH in the hydrothermal dehydrochlorination of polyvinyl chloride

The role of NaOH in polyvinyl chloride (PVC) dehydrochlorination was studied over a concentration range that yielded the maximum reaction rate. In addition to limiting autocatalysis through HCl scavenging, NaOH also influences the physicochemical evolution of PVC during hydrothermal treatment. In 3 M NaOH, the dechlorination rate peaked because NaOH effectively prevented particle agglomeration and autocatalysis. This is attributed to the action of sodium cations in producing short-range hydration repulsive forces that prevent the adhesion of particles and facilitate significant size reduction. The apparent activation energy was 138 ± 3.0 kJmol−1. In 2 M NaOH, particle agglomeration and pronounced autocatalysis reduced the activation energy to 105 ± 7.2 kJmol−1. Therefore, NaOH influences both the morphological evolution and dechlorination reaction kinetics.

Iron nitride nanoparticles for rapid dechlorination of mixed chlorinated ethene contamination.

Sulfidation and, more recently, nitriding have been recognized as promising modifications to enhance the selectivity of nanoscale zero-valent iron (nZVI) particles for trichloroethene (TCE). Herein, we investigated the performance of iron nitride (FexN) nanoparticles in the removal of a broader range of chlorinated ethenes (CEs), including tetrachloroethene (PCE), cis-1,2-dichloroethene (cis-DCE), and their mixture with TCE, and compared it to the performance of sulfidated nZVI (S-nZVI) prepared from the same precursor nZVI. Two distinct types of iron nitride (FexN) nanoparticles, containing γ'-Fe4N and ε-Fe2-3N phases, exhibited substantially higher PCE and cis-DCE dechlorination rates compared to S-nZVI. A similar effect was observed with a CE mixture, which was completely dechlorinated by both types of FexN nanoparticles within 10 days, whereas S-nZVI was able to remove only about half of the amount, most of which being TCE. Density functional theory calculations further revealed that the cleavage of the first C-Cl bond was the rate-limiting step for all CEs dechlorinated on the γ'-Fe4N(001) surface, with the reaction barriers of PCE and cis-DCE being 29.9, and 40.8kJmol-1, respectively. FexN nanoparticles proved to be highly effective in the remediation of PCE, cis-DCE, and mixed CE contamination.

Revisiting UV/sulfite exposed to air: A redox process for reductive dechlorination and oxidative mineralization

2,4-Dichlorophenol (2,4-DCP) widely exists in many industrial wastewaters and is considered a priority pollutant due to its high toxicity. In this work, we proposed a UV/sulfite process without aeration for high-efficiency dechlorination and enhanced mineralization. The UV/sulfite system significantly improved the removal of 2,4-DCP (93.33 % in 60 min) and dechlorination rate (85.13 % in 60 min) compared with UV alone and sulfite alone, and the synergistic factor was 6.59. The UV/sulfite system without aeration improved the mineralization rate (49.80 %) compared with the nitrogen aeration system. UV/sulfite was a reduction-oxidation coupled process and was more suitable for neutral and alkaline conditions. Reducing species (hydrated electrons and H•) made 2,4-DCP dechlorinate to form dechlorination products such as p-chlorophenol (4-CP), o-chlorophenol (2-CP), and phenol, which were further mineralized by oxidized species (SO4•-). UV/sulfite remained highly efficient in the presence of coexisting ions and under different water quality conditions. This process was also suitable for removing a wide range of chlorinated organic compounds. The UV/sulfite process without aeration can achieve high dechlorination and enhanced mineralization with simple operation and low cost (1.78 $·m−3 order−1), which has a broad and cost-effective application prospect in removing refractory halogenated organic pollutants.

A New Process of Direct Zinc Oxide Production by Carbothermal Reduction of Zinc Ash.

Zinc ash is a by-product of the hot-dip galvanizing process and the electrolytic zinc process, which is classified as a hazardous waste consisting predominately of zinc oxide that could be recovered as the useful main resource for ZnO preparation. In this work, in order to reduce the energy consumption of the direct reduction process and improve the resource-recovery rate. A new technology for zinc oxide production, by a carbothermal reduction of zinc ash, is proposed. This process includes two steps: high-temperature roasting of zinc ash for dechlorination and a carbothermal reduction of dechlorination ash. Zn in zinc ash is mainly presented in the form of zinc oxide (ZnO), basic zinc chloride (Zn5(OH)8Cl2H2O), and metallic zinc (Zn). Basic zinc chloride can be roasted and decomposed to reduce the chlorine content in zinc ash. The results of a chloride ion removal test show that the optimal roasting temperature is 1000 °C, with a holding time of 60 min. Under the modified conditions, the chloride content in the roasted zinc ash is reduced to 0.021 wt.%, and the dechlorination rate is more than 99.5%, which can meet the requirements of zinc oxide production. The best process conditions for zinc oxide production by carbothermic reduction are as follows: reduction temperature of 1250 °C, reduction time of 60 min, and reduction agent addition of 22 wt.%. Under the best reduction process, the purity of zinc oxide product is 99.5%, and the recovery of zinc is more than 99.25%. Needle-like zinc oxide obtained by carbothermic reduction has high purity and can replace zinc oxide produced by an indirect process.