Toxic Chlorinated By-products Research Articles

Mechanism of anodic activation of chloride to generate singlet oxygen for fast organic removal using an innovative anode

Electrochemical persulfate activation (E-PS) has recently emerged as a highly effective advanced oxidation process in water decontamination. However, the presence of chloride ions (Cl−) in waters can accelerate anodic corrosion as well as lead to the formation of toxic chlorinated byproducts (i.e., ClO4−), limiting its practical application. In this study, we introduce a novel Nd/Bi@SnO2 anode to construct E-PS, which exhibits high stability in chloride-containing water with a long-expected service lifetime of 13.7 years. The Nd/Bi@SnO2 electrode can effectively convert Cl− to reactive chlorine with the assistance of PMS, triggering singlet oxygen (1O2) generation for superior organic removal while avoiding toxic chlorinated byproducts (i.e., ClO4−) generation as well as greatly reducing the energy consumption. Comprehensive structural and electrochemical characterization results demonstrate Nd/Bi co-doping introduces oxygen vacancy on Nd/Bi@SnO2, enabling the anode with high oxygen evolution potential, excellent conductivity and superior stability. Scavenging experiments and electron paramagnetic resonance illustrate the generation of various reactive species in the system, among which 1O2 predominantly contributes to organic removal and results in harmless intermediates. This innovative approach transforms Cl− into ROSs for eco-friendly, energy-efficient water decontamination.

Impact of supporting electrolyte on electrochemical performance of borophene-functionalized graphene sponge anode and degradation of per- and polyfluoroalkyl substances (PFAS)

Graphene sponge anode functionalized with two-dimensional (2D) boron, i.e., borophene, was applied for electrochemical oxidation of C4-C8 per- and polyfluoroalkyl substances (PFASs). Borophene-doped graphene sponge outperformed boron-doped graphene sponge anode in terms of PFASs removal efficiencies and their electrochemical degradation; whereas at the boron-doped graphene sponge anode up to 35% of the removed PFASs was recovered after the current was switched off, the switch to a 2D boron enabled further degradation of the electrosorbed PFASs. Borophene-doped graphene sponge anode achieved 32–77% removal of C4-C8 PFASs in one-pass flow-through mode from a 10 mM phosphate buffer at 230 A m−2 of anodic current density. Higher molarity phosphate buffer (100 mM) resulted in lower PFASs removal efficiencies (11–60%) due to the higher resistance of the graphene sponge electrode in the presence of phosphate ions, as demonstrated by the electrochemical impedance spectroscopy (EIS) analyses. Electro-oxidation of PFASs was more efficient in landfill leachate despite its high organic loading, with up to 95% and 75% removal obtained for perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), versus 77% and 57% removal in the 10 mM phosphate buffer, respectively. Defluorination efficiencies as determined relative to the electrooxidized fraction of PFASs indicated up to 69% and 82% of defluorination of PFOS and PFOA in 10 mM phosphate buffer, which was decreased to 16 and 29% defluorination, respectively, for higher buffer molarity (100 mM) due to the worsened electrochemical performance of the sponge. In landfill leachate, relative defluorination efficiencies of PFOS and PFOA were 33% and 45%, respectively, indicating the inhibiting effect of complex organic and inorganic matrix of landfill leachate on the C-F bond breakage. This study demonstrates that electrochemical degradation of PFASs is possible to achieve in complex and brackish streams using a low-cost graphene sponge anode, without forming toxic chlorinated byproducts even in the presence of >7 g L−1 of chloride.

New insights into the mechanism of Fered-Fenton treatment of industrial wastewater with high chloride content: Role of multiple reactive species

Hydroxyl radical (OH) is considered the dominant reactive species in the electro-Fenton (EF) and Fered-Fenton (EF-Fere) processes for wastewater treatment. However, in chloride-rich media, this is arguable due to the obscure mechanisms for the oxidant speciation and pollutant degradation. Herein, the role of active chlorine and Fe(IV)-oxo species (FeIVO2+) as primary oxidizing agents in HClO-mediated Fered-Fenton (EF-Fere-HClO) process is discussed, along with the dependence of their contribution on the pollutant structure. HClO generated from anodic oxidation of Cl− can be consumed by added H2O2 to form singlet oxygen (1O2), which is detrimental because this species is quickly deactivated by water. The reaction between HClO and Fe2+ was proved to generate FeIVO2+, rather than OH or Cl suggested in the literature. The yield of FeIVO2+ species was proportional to the Cl− concentration and barely affected by solution pH. The long-lived HClO and FeIVO2+ can selectively react with electron-rich compounds, which occurs simultaneously to the non-selective attack of OH formed from Fenton's reaction. The FeIVO2+ and OH concentration profiles were successfully modelled. Although the accumulation of toxic chlorinated by-products from HClO-mediated oxidation might cause new environmental concerns, the toxicity of pesticide wastewater with 508 mM Cl− was halved upon EF-Fere-HClO treatment.

Enhanced Photocatalytic Degradation of P-Chlorophenol by ZnIn2S4 Nanoflowers Modified with Carbon Quantum Dots

The removal of chlorophenol (CP) contaminants from water is a great challenge owing to their natural robustness and the toxic chlorinated by-products generated in degradation processes. In this work, a series of three-dimensional nanoflower-like structured photocatalysts (CQDs/ZnIn2S4-x, x = 1, 2, or 3 wt%) were fabricated via a facile hydrothermal approach. Excellent photocatalytic abilities toward 4-CP degradation under Xe lamp irradiation were achieved over the as-prepared composites. The removal efficiency of total organic carbon for 4-CP on the optimized CQDs/ZnIn2S4-2 was 49.1%, which was 16.0% higher than that of ZnIn2S4. The presence of CQDs could not only be used to adjust controllable band structures for enhancing light absorption, but it could also serve as an electron acceptor to promote the transition of electron–hole pairs. Moreover, a possible degradation mechanism of 4-CP was also proposed according to the analyses of active species, electron paramagnetic resonance characterization, degradation products, and attacked sites. Overall, this work unveils a superior function of an efficient photocatalyst for refractory organic pollutants.

Performance of anode materials in electro-Fenton oxidation of cefoperazone in chloride medium: New insight into simultaneous mineralization and toxic byproducts formation

Electrochemical oxidation of saline organic wastewater is always accompanied by the formations of toxic chlorinated byproducts, hence it is of great significance to control the potential toxic effect while ensuring the mineralization efficiency. In this study, the electro-Fenton (EF) degradation of 200 mg L−1 cefoperazone (CFPZ) has been investigated in chloride media, performed in 0–20 mM NaCl as well as 50 mM Na2SO4 comparatively. An active carbon fiber (ACF) was used as cathode, while an active RuO2/Ti, non-active SnO2–Sb2O5/Ti or boron-doped diamond supported on niobium (BDD/Nb) was selected as anode. In EF process, along with active chlorine originated from the anodic oxidation of Cl−, CFPZ decays always been promoted, but the mineralization extents got inhibited. Both the mineralization and the chlorinated byproducts formation increased in sequence of RuO2/Ti < SnO2–Sb2O5/Ti < BDD/Nb. Chlorinated byproducts were largely recalcitrant when RuO2/Ti or SnO2–Sb2O5/Ti anodes were used, leading to only a small portion of Cl− being converted to active chlorine and chlorate (ClO3−). When using BDD/Nb, Cl− was further oxidized to ClO3− and, mostly, to perchlorate (ClO4−), due to the action of the largely generated hydroxyl radical (•OH) and physically adsorbed •OH (BDD(•OH)). This behavior demonstrated the toxic risk could be limited by choosing an adequate anode. The evolutions of active chlorine, Cl−, ClO3−, ClO4−, NH4+ and NO3− were determined to clarify the behavior of electrodes in EF process. Nine aromatic intermediates and eleven volatile intermediates were detected by liquid and/or gas chromatography mass spectrometry respectively, meanwhile four carboxylic acids (oxalic, oxamic, formic and fumaric acids) were quantified as final byproducts, allowing the proposal of a plausible pathway of CFPZ mineralization in Cl−-containing medium.

Comparison of homogeneous and heterogeneous electrochemical advanced oxidation processes for treatment of textile industry wastewater

This study aimed at understanding the influence of the generation of oxidants in a heterogeneous way at boron-doped diamond (BDD) anode (anodic oxidation (AO)) or homogeneously in the bulk (electro-Fenton (EF)) during treatment of a textile industry wastewater. Both processes achieved high TOC removal. A yield of 95 % was obtained by combining EF with BDD anode during 6 h of treatment. The EF process was found to be faster and more efficient for discoloration of the effluent, whereas AO was more effective to limit the formation of degradation by-products in the bulk. An advantage of AO was to treat this alkaline effluent without any pH adjustment. Operating these processes under current limitation allowed optimizing energy consumption in both cases. However, using BDD anode led to the formation of very high concentration of ClO3-/ClO4- from Cl- oxidation (even at low current density), which appears as a key challenge for treatment of such effluent by AO. By comparison, EF with Pt anode strongly reduced the formation of ClO3-/ClO4-. Operating EF at low current density even maintained these concentrations below 0.5 % of the initial Cl- concentration. A trade-off should be considered between TOC removal and formation of toxic chlorinated by-products.

Minimizing toxic chlorinated byproducts during electrochemical oxidation of Ni-EDTA: Importance of active chlorine-triggered Fe(II) transition to Fe(IV)

The formation of chlorinated byproducts represents a significant threat to the quality of the effluent treated using electrochemical advanced oxidation processes (EAOPs), thus spurring investigation into alleviating their production. This study presents a new strategy to minimize the release of chlorinated intermediates during the electrochemical oxidation of Ni-EDTA by establishing a dual mixed metal oxide (MMO)/Fe anode system. The results indicate that the dual-anode system achieved a substantially higher rate (0.141 min−1) of Ni-EDTA destruction and accordingly allowed a more pronounced removal of aqueous Ni (from 39.85 to 0.63 mg L−1) after alkaline precipitation, compared with its single MMO anode (0.017 min−1 of Ni-EDTA removal, with 14.38 mg L−1 Ni remaining) and single Fe anode (insignificant Ni-EDTA removal, with 38.37 mg L−1 Ni remaining) counterparts. Compared to reactive chlorine species (RCS) produced from the single MMO anode system, Fe(IV) was in situ generated from the dual-anode system and was predominantly responsible for the attenuation of chlorinated byproducts and thus the decrease in the acute toxicity of the treated solution (evaluated using luminescent bacteria). The Fe(IV)-dominated dual-anode system also exhibited superior performance in removing multiple pollutants (including organic ligands, Ni, and phosphite) in the real electroless plating effluent. The findings suggest that the strategy for Fe(II) transition to Fe(IV) by active chlorine paves a new avenue for yielding less chlorinated products with lower toxicity when EAOPs are used to treat chloride-containing organic wastewater.

High Degradation of Trichloroethylene in Water by Nanostructured MeNPs@CALB Biohybrid Catalysts

In this study, a methodology was developed for the rapid degradation of trichloroethylene (TCE) and 1,1-dichloroethylene (1,1-DCE) in distilled water and room temperature without the production of toxic chlorinated by-products. This process was carried out using bionanohybrids of different metals (Pd, Fe, Cu and Zn) obtained by enzyme–metal coordination called MeNPs@CALB, which present different metal species and nanoparticle sizes. The Cu2O@CALB biohybrid, which contained Cu2O nanoparticles, showed excellent catalytic performance in TCE degradation by removing 95% (&gt;125 ppm) in 10 min using 1.5 g/L of catalyst. On the other hand, in the degradation reaction of 1,1-DCE, Cu2O@CALB eliminated 94% (93 ppm) in 1 min. Cu2O@CALB exhibited excellent stability and recyclability under sustainable conditions, maintaining its effectiveness in more than 90% for three cycles.

Effective blockage of chloride ion quenching and chlorinated by-product generation in photocatalytic wastewater treatment

Photocatalytic degradation of pollutants in high salinity wastewater usually shows extremely low activities and produces highly toxic by-products, often related to the presence of excess chloride ion (Cl−). Herein, we report for the first time that involvement of Cl− (quenching active species and generating chlorinated by-products) could be effectively blocked during photocatalytic processes. Based on a comprehensive investigation of its mechanism, we found that Cl− could quench superoxide radicals (O2−) through a cyclic indirect quenching model with holes (h+) and hydroxyl radicals (OH) quenching as "initiators". Thus, scavenging h+ and OH could successfully block the chain reactions between Cl− and O2−, and photocatalytic degradation of methyl orange (a refractory dye, with O2− as dominant attacking species) could be enhanced by nearly 50 times, even when Cl− content was up to 10 wt%. More importantly, both HPLC-MS analyses and DFT calculation validated that, by blocking its quenching effect, Cl− could be successfully excluded from the pollutant degradation processes, thus preventing the generation of toxic chlorinated by-products. This work provides new insights into control of chlorinated by-products and proposes feasible strategies to extend photocatalytic technology in high salinity wastewater.

Improved chlorine tolerance of a polyvinyl pyrrolidone-polysulfone membrane enabled by carboxylated carbon nanotubes

Chemical cleaning of membranes may be limited by the tolerance of some polymeric membranes to chlorine. In this work we show that modification of a polyvinyl pyrrolidone-polysulfone (PVP-PSF) membrane with carboxylated carbon nanotubes (CNTs) leads to greater chlorine tolerance of the membrane along with smoothing the surface roughness and improving some membrane properties including permeability, hydrophilicity and antifouling ability. In comparison with PVP-PSF membrane, incorporating carboxylated CNTs reduced the release of organic matter from the membrane and caused less decrease in hydrophilicity, permeability and BSA rejection. After chlorine treatment of a conventional PVP-PSF membrane, an infrared absorbance peak around 1670 cm−1, representing the vibration of the amide bond in the PVP, decreased significantly and two smaller peaks at around 1725 and 1775 cm−1 were observed. The percentage of C−Cl (2p) in the PVP-PSF membrane was about 2.8 times greater than that of the carboxylated CNT-composite PVP-PSF (CNT-PVP-PSF) membrane after chlorine exposure, suggesting greater chlorine tolerance of CNT-PVP-PSF membrane. In comparison with PVP-PSF membrane, the formation of toxic chlorinated byproducts during chlorination of organic matter released from CNT-PVP-PSF membrane was much lower. The improvement of tolerance to chlorine is attributed to stabilization of the polymer through hydrogen bonding between polymer and carboxylated CNTs.

Can electrochemical oxidation techniques really decontaminate saline dyes wastewater?

Formation of toxic chlorinated organic byproducts is of great concern when selecting electrochemical oxidation (EO) as decontamination technology for saline dye wastewater, but still not verified. To test the applicability of EO, methyl orange (MO) was used as a model dye for anodic contact glow discharge electrolysis (CGDE) and conventional electrolysis (CE) in the presence of chloride. The degradation kinetics and organic intermediates were analyzed. In the presence of chloride, the rates of dye degradation were significantly increased as CGDE and CE were applied. CE resulted in higher mineralization efficiency than CGDE which needs much energy input. Several refractory chlorinated aromatic and even aliphatic compounds were identified during MO degradation, as well as the other anthraquinone dye, alizarin red S (AR). Therefore, the issues of toxic chlorinated byproducts and energy cost should be preferentially evaluated prior to the selection of EO technologies.

Photocatalytic degradation and chlorination of azo dye in saline wastewater: Kinetics and AOX formation

Chloride ion as one of the major salt components in dyestuff wastewaters can greatly affect the homogenous OH radical-based advanced oxidation processes (AOPs), but little information is available now for heterogeneous AOPs like UV/TiO2 under similar conditions. Here the effects of chloride on kinetics and reaction intermediates of dye degradation in heterogeneous photocatalysis were examined. The dye degradation efficiency in UV/TiO2 process was investigated as a function of a wide range of salinity and pH. The chloride ion was found to have a dual effect on both the dye decoloration and mineralization in UV/TiO2 system due to different mechanisms involved. Higher Cl− concentration would inhibit dye degradation, especially in acidic medium. AOX (halogenated organic compounds adsorbable on activated carbon) generated increased with the increasing content of chloride ion present in the solution. Several toxic chlorinated byproducts were firstly identified during the dye degradation in UV/TiO2 process using GC–MS method and possible decomposition pathways were proposed. These findings may have significant technical implications for optimizing the photochemical technologies in salt-rich wastewater treatment.

Measurement Technology of Dry Etching Chlorinated Byproducts and Improvement Strategy in Thin Film Transistor Liquid Crystal Display Production

PP-29-018 Background/Aims: The advanced TFT-LCD processes, dry etching of thin film layers is increasingly preferred instead of conventional wet etching processes for new process. The previous studies are focus on greenhouse gases removal efficiency by scrubber, and indicate potential fluorinated or chlorinated compounds of the material being etched. The environment, safety, and health difficult challenges are lack of information on how the chemicals and materials are used and what process byproducts are formed, additional needs are characterizing and controlling plasma etches gas emissions and byproducts. Methods: The thermal processing unit consists of an oxide combustion chamber, a wet scrubber, a vacuum pump, and temperature/pressure control system, integrated in an enclosed unit. Determination of dry etching gas and byproducts using an online extractive Fourier transform infrared spectroscopy and canister sampling/gas chromatography-mass spectrometry method. A portable photo ionization detector used to detect total volatile organic compounds after scrubber. We found sulfur hexafluoride etching gas and 3 byproducts, including sulfur dioxide, ethylene, and hydrogen chloride by FTIR. There were 14 unknown and 6 byproducts before TPU, and 13 byproducts after TPU by Gas Chromatograph-Mass Spectrometer. Results: We found that the scrubber has high destruction removal efficiency for dry etching unknown byproducts at 1000°C. The temperature of partial combustion reactor processing equipment less than 750°C, toxic chlorinated byproducts would be generated after TPU. In this study, we found 34 chlorinated byproducts at abnormal local scrubber. The portable photoionization detector screen method is able to find abnormal scrubber, the concentration of total volatile organic compounds more than normal scrubber after TPU. Conclusion: We have established the dry etching chlorinated byproducts measurement technology and improvement strategy for local scrubber. When the local scrubber was not well maintained, toxic chlorinated byproducts would be generated. The screen method is able to screen out and modify the abnormal local scrubber for improvement.

Effects of potassium alkalis and sodium alkalis on the dechlorination of o-chlorophenol in supercritical water

Effects of potassium alkalis and sodium alkalis on the dechlorination of o-chlorophenol ( o-CP) in supercritical water (SCW) were studied in this paper under the conditions of 450 °C and 25 MPa. Experimental results indicated that the dechlorination of o-CP can be accelerated significantly by all alkalis investigated. The dechlorination of o-CP proceeded mainly via two pathways: hydrodechlorination and hydrolysis. Both of the two pathways can be promoted by alkalis, and the dechlorination of o-CP can be accelerated by both the cations and hydroxide ion dissociated from alkalis. The overall dechlorination of o-CP can be accelerated by cations via promoting the hydrodechlorination pathway, while, hydroxide ion via promoting the hydrolysis pathway. In addition, the hydrodechlorination can be accelerated faster by sodium alkalis than that by potassium ones, while, the hydrolysis can be promoted faster by potassium alkalis. This difference may be caused by the different charge density between potassium ion and sodium ion, and the different solubility and dissociation constant between potassium alkalis and sodium alkalis in SCW. Dechlorination of o-CP with addition of alkalis prior to supercritical water oxidation (SCWO) process not only can avoid the reactor corrosion caused by the generated hydrochloric acid in direct SCWO of o-CP, but also can reduce the formation of toxic chlorinated byproducts compared with direct SCWO process or SCWO of o-CP with addition of alkali.

Production of high quality water for reuse purposes: the West Basin experience

Water reuse is becoming an essential factor for developing sound water and environment managaement policies, The main challenge for the operators of water reuse facilities is to increase the cost-efficiency and reliability of water reuse systems. To achieve this objective, an R&amp;D program has been implemented at the West Basin Recycling Plant in California to study the removal of trace organics by advanced treatment (UV disinfection, membrane filtration). The pilot studies demonstrated the efficiency of two UV systems for final disinfection and in particular its high performance for the MF/RO effluent at low UV doses. UV appears as a cost competitive alternative to chlorination; it prevents the generation of toxic chlorinated by-products and reduces the salt content. It was demonstrated that microfiltration, as a replacement of conventional lime pretreatment for reverse osmosis, also offers numerous advantages. Despite the improved removal of trace organics, the membrane pretreatment before RO leads to a saving of 45% of life costs.