Chlorination Process Research Articles

Co-Extraction of Aluminum and Silicon and Kinetics Analysis in Carbochlorination Process of Low-Grade Bauxite.

Addressing the issue that the Bayer process is not suitable for low-grade bauxite, carbochlorination was proposed to recover aluminum and silicon from low-grade bauxite. This study focused on the behavior of aluminum and silicon during the carbochlorination process of low-grade bauxite. The impact of various process parameters on the chlorination efficiency was investigated, and the chlorination mechanism and kinetics of aluminum and silicon chlorination in bauxite were analyzed and discussed. Under optimal experimental conditions, the chlorination efficiency of Al2O3 and SiO2 reached 94.93% and 86.32%, respectively. The carbochlorination of aluminum and silicon in bauxite adhered to a shrinking, unreacted core model governed by gas diffusion within the product layer. This process can be bifurcated into two stages. Additionally, calculations were conducted to determine the apparent activation energy and reaction order of the chlorination processes involving Al2O3 and SiO2. Examining the chlorination mechanism revealed that the bauxite carbochlorination encompasses transformations among various minerals. Notably, the aluminum component prefers to participate in the carbothermal chlorination reaction over silicon.

Changes in combined toxicity of benzophenone-3 and humic acid on Daphnia magna and zebrafish during chlorination disinfection process

Conventional wastewater treatment methods cannot completely remove the ultraviolet (UV) filters or dissolved organic matter. The transformation characteristics of these substances during chlorination disinfection and the varying species-specific toxicities of their combinations remain unclear. Here, Daphnia magna and zebrafish were exposed to benzophenone-3 (BP-3) and humic acid (HA) before and after chlorination disinfection. The results from chemical indicators showed that chlorination treatment decreased UV254 values and changed the intensity of parallel factors in three-dimensional fluorescence. Based on chemical analysis, the chlorine concentration and chlorination time for the toxicity experiments were set at 5 mg/L and 6 h, respectively. Exposure to HA and BP-3 before and after chlorination decreased the heart rate (by 1.37–28.12 %) in both species. However, species-specific responses, including survival rate, swimming distance, and expression of genes related to neurodevelopment, growth, and oxidative stress, were induced by chlorination. Chlorination reduced the impact of HA exposure but worsened the effects of HA and BP-3 co-exposure on D. magna. However, in zebrafish, the toxic effects intensified in most of the exposure groups after chlorination. Correlation analysis showed that the parallel factors of three-dimensional fluorescence were correlated with toxic effects on zebrafish, whereas UV254 was more significantly correlated with toxic effects on D. magna. This study provides insights into the combined toxicity of UV filters and dissolved organic matter in different aquatic organisms during chlorination, which is useful for risk control and optimization of the chlorination process.

Asymmetric Ru-In atomic pairs promote highly active and stable acetylene hydrochlorination

Ru single-atom catalysts have great potential to replace toxic mercuric chloride in acetylene hydrochlorination. However, long-term catalytic stability remains a grand challenge due to the aggregation of Ru atoms caused by over-chlorination. Herein, we synthesize an asymmetric Ru-In atomic pair with vinyl chloride monomer yield (>99.5%) and stability (>600 h) at a gas hourly space velocity of 180 h−1, far surpassing those of the Ru single-atom counterparts. A combination of experimental and theoretical techniques reveals that there is a strong d-p orbital interaction between Ru and In atoms, which not only enables the selective adsorption of acetylene and hydrogen chloride at different atomic sites but also optimizes the electron configuration of Ru. As a result, the intrinsic energy barrier for vinyl chloride generation is lowered, and the thermodynamics of the chlorination process at the Ru site is switched from exothermal to endothermal due to the change of orbital couplings. This work provides a strategy to prevent the deactivation and depletion of active Ru centers during acetylene hydrochlorination.

A highly specific colorimetric fluorescent probe for rapid detection of hypobromous acid and its application in the environment

The highly reactive hypobromous acid (HOBr), which is generated after chlorination process of tap water, acts as a precursor of toxic brominated disinfection by-products (Br-DBPs) and further reacts with organic matter. In addition, HOBr produced from the oxidation of Br− during the degradation of pollutants by peroxymonosulfate (PMS, HSO5−) can be considered as the cause of the expedited degradation of pollutants. Therefore, it is particularly important to detect HOBr level in the water environment. Resazurin was selected as a fluorescent probe for selective recognition of HOBr in the water environment. The probe exhibited excellent spectral performance and showed high sensitivity to HOBr (LOD = 515 nM). This method has a relatively ideal recovery rate for HOBr detection in environmental water samples. Furthermore, the HOBr production during the chlorination disinfection process was simulated and the HOBr generated from this process was detected by the probe. Importantly, the process of HOBr recognition by the probe is accompanied by the change of color. Based on this, the relationship between the change of color B/G value and HOBr concentration was successfully constructed. The probe was loaded on the filter paper to make a test strip, which was utilized to the detection of HOBr. Collectively, this work provided a promising and powerful method for HOBr detection in the environment.

Formation of halonitromethanes from different nitrophenol compounds during UV/post-chlorination: Impact factors, DFT calculation, reaction mechanisms, and toxicity

As ubiquitous chemical substances in water bodies, nitrophenol compounds (NCs) can form chlorinated halonitromethanes (Cl-HNMs) in the chlorination process. This work chose six typical NCs to explore Cl-HNMs produced during the UV/post-chlorination process, and Cl-HNMs yields from these NCs followed the increasing order of 4-, 2-, 2-amino-3-, 2-methyl-3-, 3-, and 2-chloro-3-nitrophenol. The Cl-HNMs yields increased continually or increased firstly and declined with post-chlorination time. Increasing chlorine dosage favored Cl-HNMs formation, while excessive chlorine dosage decreased Cl-HNMs produced from 2- and 4-nitrophenol. Besides, appropriate UV radiation, acidic pH, and higher precursor concentrations facilitated Cl-HNMs formation. Then, the reaction mechanisms of Cl-HNMs generated from these different NCs were explored according to density functional theory calculation and identified transformation products (TPs), and the main reactions included chlorine substitution, benzoquinone compound formation, ring opening, and bond cleavage. Moreover, the Cl-HNMs generated from 2-chloro-3-nitrophenol were of the highest toxicity, and the six NCs and their TPs also presented ecotoxicity. Finally, two kinds of real waters were used to explore Cl-HNMs formation and toxicity, and they were significantly distinguishable compared to the phenomena observed in simulated waters. This work will give new insights into Cl-HNMs formation from different NCs in water disinfection processes and help better apply the UV/post-chlorination process to water treatments.

Removal of cyanide by peroxydisulfate activated by copper ions inherently present in the electroplating wastewater

Cyanide is widely used in electroplating processes to ensure the quality of metal plating. Alkaline chlorination process is commonly employed to remove cyanide from electroplating wastewater. However, several drawbacks exist, including the formation of toxic intermediates, high consumption of chemical reagents, and low removal efficiency of metal-cyanide complexes. In this study, persulfate advanced oxidation process was explored for the removal of cyanide from electroplating wastewater. The metal ions typically present in the electroplating wastewater, including Cu2+, Zn2+, Fe2+, Ag+ and Ni2+, may activate peroxydisulfate (PDS) to oxidize cyanide. Among these metal ions, cyanide can be most effectively removed in the presence of Cu2+ and PDS, likely due to the activation of PDS by Cu2+ and Cu(II)-cyanide complex. The results of lab studies indicated that 99 % removal of cyanide (4 mM) could be achieved in the presence of 10 mM PDS and 1 mM Cu2+ within 20 min. The results of radical scavenging experiments, radical probe tests and electron paramagnetic resonance (EPR) studies showed that sulfate radical (SO4•−) and hydroxyl radical (OH•) were primarily responsible for cyanide removal and the complexation of cyanide and Cu2+ significantly enhanced the formation of SO4•−. Cyanate and nitrite were the main by-products detected. For real electroplating wastewater, the addition of 40 mM PDS could almost completely remove cyanide (19.5 mM) in 20 min.

Thermodynamic Analysis of Chloride Corrosion in Steel for Energy System Applications in Fe-O-Cl-Na Environments

The assumptions of contemporary energy policies are increasing the share of renewable energy sources. Biomass combustion is developing as an alternative to fossil fuels. However, it faces challenges such as limited corrosion resistance of steel boiler components due to chloride compounds in flue gases and fly ash. This paper provides a comprehensive thermodynamic analysis of chloride-induced corrosion in steel in the Fe-O-Cl-Na environment, focusing on the influence of steam concentration in the gas phase. The study was performed by using the general thermodynamic rules, the thermodynamic properties of the pure components involved in the reaction, and the properties of the solutions formed in the liquid and gas phases. The study also examined the impact of alkali metal chlorides, particularly NaCl, on the formation of NaFeO2 in the passive oxide scale layer Fe3O4/Fe2O3. Furthermore, it investigated the condensation of NaCl vapour formation of low-melting eutectic mixtures in deposits and the resulting consequences on the corrosion process. The role of HCl in the chlorination and oxidation process of steel in melted ash deposits was also discussed. The presented thermodynamic analysis was compared with assumptions of an “active oxidation” model. This study can be a valuable resource for experimental research planning and a guide for preventing corrosion in industrial settings.

Maximizing Chlorobenzene Product Yield by Modifying The Benzene Chlorination Process

Chlorobenzene is extensively used in the manufacture of phenol, aniline, and DDT; as a solvent for paints; and as a heat transfer medium. It is also occasionally used in the dry-cleaning industry. With the many uses of chlorobenzene, its production must have high efficiency both in terms of energy and mass in order to obtain maximum profits. In this paper, we will explain how to maximize chlorobenzene product yield by modifying the benzene chlorination process. The process modification was carried out by adding one distillation column unit and one mixer unit. Meanwhile, to carry out sensitivity analysis, case study tools on chemical engineering software were used. Based on process modifications, an increase in chlorobenzene yield was obtained from 83% to 98%. The results of the case study indicate that the higher the benzene pressure entering the reactor, the lower the yield of the liquid product exiting the reactor. Meanwhile, the higher the ratio of benzene mass flow to chlorine gas mass flow, the higher the yield of the liquid product exiting the reactor. Copyright © 2024 by Authors, Published by Universitas Diponegoro and BCREC Publishing Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

In Situ Preparation of Chlorine-Regenerable Antimicrobial Polymer Molecular Sieve Membranes.

Microbial contamination has profoundly impacted human health, and the effective eradication of widespread microbial issues is essential for addressing serious hygiene concerns. Taking polystyrene (PS) membrane as an example, we herein developed report a robust strategy for the in situ preparation of chlorine-regenerable antimicrobial polymer molecular sieve membranes through combining post-crosslinking and nucleophilic substitution reaction. The cross-linking PS membranes underwent a reaction with 5,5-dimethylhydantoin (DMH), leading to the formation of polymeric N-halamine precursors (PS-DMH). These hydantoinyl groups within PS-DMH were then efficiently converted into biocidal N-halamine structures (PS-DMH-Cl) via a simple chlorination process. ATR-FTIR and XPS spectra were recorded to confirm the chemical composition of the as-prepared PS-DMH-Cl membranes. SEM analyses revealed that the chlorinated PS-DMH-Cl membranes displayed a rough surface with a multitude of humps. The effect of chlorination temperature and time on the oxidative chlorine content in the PS-DMH-Cl membranes was systematically studied. The antimicrobial assays demonstrated that the PS-DMH-Cl membranes could achieve a 6-log inactivation of E. coli and S. aureus within just 4 min of contact time. Additionally, the resulting PS-DMH-Cl membranes exhibited excellent stability and regenerability of the oxidative chlorine content.

Influence of biodegradable plastics on the generation of disinfection byproducts in the chlorination process

Biodegradable plastics (BPs) have seen a continuous increase in annual production and application due to their environmentally sustainable characteristics. However, research on the formation of disinfection byproducts (DBPs) from biodegradable microplastics (BMPs) during chlorination is limited, and the effects of aqueous solution chemistry on this process have yet to be explored. Therefore, two biodegradable microplastics, polylactic acid (PLA) and polybutylene adipate terephthalate (PBAT), were investigated in this study to examine the changes in their physicochemical properties before and after chlorination, and the formation of DBPs under different environmental conditions. The results showed that PLA was more chlorine-responsive, and generated more DBPs. The pH converted some of the intermediates into more stable DBPs by affecting the concentration of HClO and base-catalyzed reactions, whereas ionic strength slightly reduced DBP concentration by ion adsorption and promoting the aggregation of BMPs. Finally, since PLA has a slightly greater volume of mesopores and micropores compared to PBAT, it may more effectively adsorb DBP precursors beyond natural organic matter (NOM), such as some anthropogenic pollutants, thus potentially decreasing the formation of chlorinated DBPs in surface water. This research explored the potentiality for DBP formation by BMPs under different water quality conditions during the disinfection process, which is useful for assessing the environmental hazards of BMPs.

The diverse roles of bromide in the degradation of tinidazole during UV-LED/chlorine process at different wavelengths: Kinetics, radical contribution, and degradation pathway

Bromide (Br–) is widely present in aquatic environment and plays important roles in pollutant degradation during advanced oxidation processes. This work systematically investigated the effect of Br– on the degradation of tinidazole (TNZ) during ultraviolet light-emitting diode (UV-LED)/chlorine process at different wavelengths (265 and 365 nm). The observed pseudo-first-order constant (k') for the TNZ degradation during UV-LED (365 nm)/chlorine process in the presence of 0.4 mM Br– (0.1125 min−1) was 1.56 times that during UV-LED (365 nm)/chlorine process in the absence of Br–. However, the k' value during UV-LED (265 nm)/chlorine process was reduced by 49.=7 % in the presence of 0.4 mM Br–. At 0–0.4 mM Br−, elevated levels of hydroxyl radicals (HO·) and active bromine species promoted TNZ degradation during UV-LED (365 nm)/chlorine/Br– process, whereas reduced HO· concentration was the main reason for the inhibition of TNZ degradation during UV-LED (265 nm)/chlorine/Br– process. With increasing solution pH from 5.8 to 7.8, the k' value of both processes decreased in the presence of Br–. Meanwhile, the k' value increased with increasing chlorine dosage and decreased with increasing initial TNZ concentration during both processes in the presence of Br–. The presence of Cl− and HCO3– presented negligible effects on the degradation of TNZ. According to the density functional theory calculation and detected intermediates, the degradation pathway of TNZ during both processes in the presence of Br– was proposed. The toxicity evaluation demonstrated that some of the degradation products of TNZ presented lower predicted toxicity values than TNZ. Overall, this work provides a thorough insight into the role of Br– in TNZ degradation during UV-LED/chlorine process at different wavelengths.

In-situ oxidation of ammonia nitrogen to N2 in low temperature groundwater through Cl• − mediated peroxymonosulfate – Fe3O4 / chlorine process

The conventional ammonia in-situ treatment technology fails to achieve complete NH4+-N degradation, while the high concentration of native iron in groundwater further complicates its removal process, potentially leading to adverse environmental consequences. This study investigates the introduction of peroxymonosulfate (PMS) into groundwater containing high concentrations Fe2+ (Fe3O4 as a ferrous source), which can act as persistent catalysts, and Cl-, causing the degradation of ammonia through chlorine free radical-based advanced oxidation processes (PMS- Fe3O4/Cl-), and evaluates the advantages and limitations of employing the PMS- Fe3O4/Cl- system for ammonia treatment. The rapid reaction between SO4•- and Cl- resulted in the formation of Cl• as primary products. Cl• initiates a cascade of subsequent reactions with pH-dependent secondary active products. Under optimal conditions, the ammonia removal efficiency reached 97.44 % using the PMS- Fe3O4/Cl- system within 120 min of operation, which can be attributed to the selective oxidation of ammonia by chlorine reactive species. And this finding was substantiated by observations from experiments involving free radical quenching and EPR spectra coupled with DMPO. The HCO3– and humic acid (HA) in water can significantly affect the removal of ammonia, owing to their capability of quenching SO4•- and Cl•. Meanwhile, the levels of generated toxic chlorates in our system remain within acceptable limits. The PMS- Fe3O4/Cl- system exhibited outstanding transport capacity and stability in simulated columns, demonstrating excellent ammonia oxidation efficiency in natural groundwater, thereby confirming its promising practical potential. All experiments were executed at a temperature of 12 ℃, thereby verifying the remarkable efficacy of the PMS- Fe3O4/Cl- system in removing ammonia even under microtherm conditions.

The Role of Ferrate (VI) in the Pretreatment of Algal Cells and Algal Organic Matters: A Review

Algal blooms are caused by excessive levels of nitrogen, phosphorus, and other plant nutrients in water. Algae and algal organic matter (AOM) pose a great threat to the quality of drinking water. This manuscript offers a systematic review of algal removal by ferrate (Fe(VI)) oxidation, including the conditions for the removal of different algae by Fe(VI) and the factors affecting the removal efficiency. On this basis, the oxidation and coagulation mechanisms of algae removal by Fe(VI) are discussed. Then, the review introduces the process combining Fe(VI) pre-oxidation with aluminum sulfate action. The addition of aluminum sulfate can further enhance the coagulation effect and reduce the formation of disinfection byproducts (DBPs) in the subsequent chlorination process by effectively removing AOM, which is recognized as a precursor of DBPs. In addition, recent studies on the combined application of Fe(VI) and Fe(II) are also reviewed. In a reasonable dose range, the synergistic effect of Fe(VI) and Fe(II) can significantly improve the removal of algae and algal toxins. Finally, this review provides a comprehensive evaluation of the applicability of Fe(VI) in removing algal material, offers guidance for the harmless treatment of algae with Fe(VI), and identifies future research questions.

Optimizing removal of antiretroviral drugs from tertiary wastewater using chlorination and AI-based prediction with response surface methodology

Chemical and pharmaceutical chemicals found in water sources create substantial risks to human health and the environment. The presence of pharmaceutical contaminants in water can cause antibiotic resistance development, toxicity to aquatic organisms, and endocrine disruption. Hence, the elimination of chemicals and other contaminants from wastewater prior to its release is a burgeoning concern in the domains of engineering and science. The use of treatment technologies in wastewater treatment plants can remove pharmaceutical contaminants through the oxidation process. However, many traditional wastewater treatment plants lack the advanced monitoring tools required to detect low concentrations of pharmaceuticals. Without the ability to detect these compounds, it's challenging to treat them effectively. The goal of this study was to use Response Surface Methodology (RSM) and Artificial Neural Networks (ANN) algorithms to model and improve how Nevirapine and Efavirenz break down in different chlorination conditions. The RSM analysis revealed statistically significant models (F-values: Nevirapine, pH-t: 108.15, T-t: 76.55, ICC-t: 110.84), indicating a strong correlation between operational parameters (pH, temperature, and initial chlorine concentration) and degradation behavior. The ANN model accurately predicted the degradation of both Nevirapine and Efavirenz under various chlorination conditions, as confirmed by analyzing actual-predicted graphs, residual plots, and Mean Squared Error (MSE) values. The ANN model using ICC-t achieved the highest MOD value of 31.31 % for Nevirapine. The ANN model based on ICC-t yielded a maximum MOD value of 16.06 % for Efavirenz. These findings provide valuable insights into optimizing chlorination processes for better removal of these pharmaceutical contaminants from water.

A Study on the Production of Anhydrous Neodymium Chloride through the Chlorination Reaction of Neodymium Oxide and Ammonium Chloride

The chlorination mechanism of neodymium oxide for the production of anhydrous neodymium chloride was analyzed based on the reaction temperature and reaction ratio of ammonium chloride, considering the suppression of the generation of NdOCl, an intermediate product of the reaction process. The results were obtained by distinguishing the shape of the produced NdCl3 (powder and bulk) and the setup of the chlorination equipment, reflecting its sensitivity to moisture and oxygen. The powdered form of NdCl3 produced at 400 °C and under argon gas flow was identified as NdCl3·6(H2O), while the bulk form of NdCl3 produced by melting at 760 °C after a chlorination process consisted of anhydrous NdCl3 and NdCl3∙n(H2O). The powdered NdCl3 produced in an argon gas environment with a controlled level of oxygen (below 16.05 ppm) and moisture (below 0.01 ppm) content was identified as single-phase anhydrous NdCl3 and showed the highest chlorination conversion rate of 98.65%. The addition of overstoichiometric ratios of NH4Cl in the chlorination process decreased the total amount of impurities (N, H, and O) in the NdCl3 product and increased the conversion rate of NdCl3.

Secondary risks induced by polyethylene microplastics during the disinfection processes of chlorination, UV irradiation, and ozone treatment

ABSTRACT The presence of microplastics in surface waters poses potential hazards to both organisms and human health. Traditional water treatment processes have proven to be inadequate in effectively removing microplastics. Besides, previous studies have often utilized disinfectant dosages that surpass those commonly employed in drinking water treatment plants (DWTPs), and the biotoxicity changes induced by microplastics after disinfection remain largely unexplored. To address these knowledge gaps, our study aimed to investigate the impact of practical disinfectant dosages on the leaching of organic compounds from microplastics and the formation of disinfection byproducts (DBPs). Furthermore, the resulting acute and genetic toxicity was evaluated through bioassays. Our findings revealed an elevated release of dissolved organic matter (DOM) derived from microplastics. Additionally, both acute and genetic toxicity increased following various disinfection processes. Specifically, chlorination exhibited the highest level of toxicity in short-term disinfection, while the pretreatment of ozonation followed by residual chlorination demonstrated the highest toxicity in the long-term disinfection, which was correlated with the concentration of CHCl3 formed. These findings will advance our understanding of the potential risks associated with microplastics during various disinfection processes, with the ultimate goal of safeguarding water supplies.

Removal of benzophenone-4 via chlorination and advanced oxidation processes in aqueous solution: kinetics, by-product identification, and mechanistic studies.

Benzophenone-4 (BP-4) is one of the UV filters widely used in personal care products (PCPs). BP-4 has been identified as an emergent contaminant detected in influent and effluent of wastewater treatment plants (WWTPs) at high concentrations showing that conventional treatment is unable to remove it, subsequently, the presence of BP-4 in surface water is inevitable. In this study, we focus on the degradation of this compound by chlorine, and we report the efficiency of its removal from water by applying two advanced oxidation processes UV/TiO2 and UV/H2O2 aiming to achieve a superior mineralization result. The study was performed in purified water (pH = 6.5, temperature = 25 °C) with an initial concentration of BP-4 similar to that detected in WWTPs (10 mg/L). The results showed that 76% of BP-4 was degraded after 80 min of reaction with chlorine leading to the formation of one by-product persistent in the solution. The oxidation by UV/TiO2 and UV/H2O2 led to a total removal of BP-4 and their generated by-products after 50 and 10 min of reactions, respectively. The kinetic study showed that BP-4 degradation by UV/H2O2 and UV/TiO2 followed pseudo-first-order reaction kinetics and the apparent rate constants (kapp) were determined to be 0.48 min-1 and 0.08 min-1, respectively. The degradation of BP-4 by chlorine followed first-order reaction kinetics with kapp = 0.02 min-1. The identification of by-product structures was performed using liquid chromatography with electrospray ionization and tandem mass spectrometry (MS/MS. The fragmentation of BP-4 and by-product ions at different collision energies allowed to propose the pathways of degradation and to predict the toxicity using a silico toxicity program which confirmed a higher toxicity of all generated by-products.

Natural Disinfection-like Process Unveiled in Soil Microenvironments by Enzyme-Catalyzed Chlorination.

Substantial natural chlorination processes are a growing concern in diverse terrestrial ecosystems, occurring through abiotic redox reactions or biological enzymatic reactions. Among these, exoenzymatically mediated chlorination is suggested to be an important pathway for producing organochlorines and converting chloride ions (Cl-) to reactive chlorine species (RCS) in the presence of reactive oxygen species like hydrogen peroxide (H2O2). However, the role of natural enzymatic chlorination in antibacterial activity occurring in soil microenvironments remains unexplored. Here, we conceptualized that heme-containing chloroperoxidase (CPO)-catalyzed chlorination functions as a naturally occurring disinfection process in soils. Combining antimicrobial experiments and microfluidic chip-based fluorescence imaging, we showed that the enzymatic chlorination process exhibited significantly enhanced antibacterial activity against Escherichia coli and Bacillus subtilis compared to H2O2. This enhancement was primarily attributed to in situ-formed RCS. Based on semiquantitative imaging of RCS distribution using a fluorescence probe, the effective distance of this antibacterial effect was estimated to be approximately 2 mm. Ultrahigh-resolution mass spectrometry analysis showed over 97% similarity between chlorine-containing formulas from CPO-catalyzed chlorination and abiotic chlorination (by sodium hypochlorite) of model dissolved organic matter, indicating a natural source of disinfection byproduct analogues. Our findings unveil a novel natural disinfection process in soils mediated by indigenous enzymes, which effectively links chlorine-carbon interactions and reactive species dynamics.

Electroplating wastewater treatment and resource recovery via a hybrid process of stepwise alkalization, Fenton, and chlorination

Electroplating wastewater rich in metal ions and acids has dual attributes of real environmental pollutant and potential resource carrier. Targeting resource recovery and up-to-standard discharge from electroplating wastewater treatment in this study, a hybrid process of stepwise alkalization, Fenton, and chlorination was proposed to treat real electroplating wastewater from Guangzhou, China, where pH was 1.1 and Ag, Fe, Ni, Cu, Zn, Cr, P, F, chemical oxygen demand (COD), NH3-N was 624.9, 11266.9, 914.7, 1169.6, 13.8, 1971, 8750.8, 7433.1, 382.3, 451.3 mg/L, respectively. Pollutants removal from liquid phase and precipitates composition were systematically investigated during stepwise alkalization via pH control, showing that first adding NaOH to pH of 3 produced high-purity (98.4 %) Fe(PO3)3 precipitates suitable for high value-added resource recovery, second adding Ca(OH)2 to pH of 9 achieved high pollutants removal (especially F and P) and cost savings, and centrifugation with 2000 g and 10 min was appropriate for precipitates separation. Aiming at COD and chelated/complexed Ni removal from treated electroplating wastewater by stepwise alkalization, the optimal parameters for Fenton process in this study were determined as the initial pH of 3, mass ratio of H2O2 to COD of 2.0, molar ratio of Fe2+ to H2O2 of 1.11, and reaction time of 2 h, under which all pollutants except NH3-N met the discharge standards in Guangdong Province, China (DB 44/1597-2015), where the maximum concentration of Ag, Fe, Ni, Cu, Zn, Cr, P, F, COD, NH3-N was set as 0.1, 2, 0.5, 0.5, 1, 0.5, 1, 10, 80, 15 mg/L, respectively. Further chlorination treatment via adding NaClO for NH3-N removal showed that the optimal molar ratio of NaClO to NH3-N was 2.89 and the optimal reaction time was 90 min in this study, under which NH3-N removal followed first-order reaction kinetics with rate constant of 0.01831 min−1, NH3-N decreased to 12.6 mg/L satisfying the discharge standard, and COD was simultaneously reduced to 26.7 mg/L. These findings demonstrate the proposed hybrid process of stepwise alkalization, Fenton, and chlorination is promising for electroplating wastewater treatment and resource recovery.

Persistent free radicals of activated carbon induce highly toxic by-products formation during chlorination

Both activated carbon (AC) and chlorine are commonly used in water treatment processes and thus the exposure of chlorine to AC surface is unavoidable under some circumstances. Therefore, evaluating the effects of AC, particularly the persistent free radicals (PFRs) of AC on the chlorination by-products is critical for drinking water safety. Here, through comparison of the chlorination process of different ACs with similar pore structure but different PFR intensities, it was found that AC with more PFRs generated more unknown by-products with lower relative cell viability (85%). The Fourier transform ion cyclotron resonance- mass spectrometry results showed that the proportions of one-chlorine and nitrogen containing by-products generated by AC with higher PFR intensity were 19.6% and 16.9%, respectively, which were higher than those generated by AC with lower PFR intensity. Furthermore, the polycyclic aromatic compounds and polyphenols with high toxicity were only found in the chlorination products of AC with high PFR intensity. The PFRs of AC could promote the chlorine decay and induce the generation of superoxide radical and chlorine radical, and thus led to the formation of more toxic by-products. This study provides new insights into the risks of water chlorination process and by-product formation in the presence of AC.