Transformation of metoprolol in UV/PDS process: Role and mechanisms of degradation and polymerization.

Advanced oxidation processes for the treatment of organic pollutants in wastewater suffer from difficulties in mineralization, potential risks of dissolved residues, and high oxidant consumption. In this study, radical-initiated polymerization is dominated in an UV/peroxydisulfate (PDS) process to eliminate organic pollutant of pharmaceutical metoprolol (MTP). Compared with an ideal degradation-based UV/PDS process, the present process can save four fifths of PDS consumption at the same dissolved organic carbon removal of 47.3%. Simultaneously, organic carbon can be recovered from aqueous solution by separating solid polymers at a ratio of 50% of the initial chemical oxygen demand. The chemical structure of products was analyzed to infer the transformation pathways of MTP. Unlike previous studies on simple organic pollutants that the polymerization can occur independently, the polymerization of MTP is dependent on the partial degradation of MTP, and the main monomer in polymerization is a dominant degradation product (4-(2-methoxyethyl)-phenol, denoted as DP151). The separated solid polymers are formed by repeated oxidation and coupling of DP151 or its derivatives through a series of intermediate oligomers. This proof-of-concept study demonstrates the advantage of polymerization-dominated mechanism on dealing with large organic molecules with complex structures, as well as the potential of UV/PDS process for simultaneous organic pollution reduction and organic carbon recovery from aqueous solution.

Catalytic ozonation of reverse osmosis concentrate from coking wastewater reuse by surface oxidation over Mn-Ce/γ-Al2O3: Effluent organic matter transformation and its catalytic mechanism.

Degradation of organics in high-salinity wastewater is beneficial to meeting the requirement of zero liquid discharge for coking wastewater treatment. Creating efficient and stable performance catalysts for high-salinity wastewater treatment is vital in catalytic ozonation process. Compared with ozonation alone, Mn and Ce co-doped γ-Al2O3 could remarkably enhance activities of catalytic ozonation for chemical oxygen demand (COD) removal (38.9%) of brine derived from a two-stage reverse osmosis treatment. Experimental and theoretical calculation results indicate that introducing Mn could increase the active points of catalyst surface, and introducing Ce could optimize d-band electronic structures and promote the electron transport capacity, enhancing HO• bound to the catalyst surface ([HO•]ads) generation. [HO•]ads plays key roles for degrading the intermediates and transfer them into low molecular weight organics, and further decrease COD, molecular weights and number of organics in reverse osmosis concentrate. Under the same reaction conditions, the presence of Mn/γ-Al2O3 catalyst can reduce ΔO3/ΔCOD by at least 37.6% compared to ozonation alone. Furthermore, Mn-Ce/γ-Al2O3 catalytic ozonation can reduce the ΔO3/ΔCOD from 2.6 of Mn/γ-Al2O3 catalytic ozonation to 0.9 in the case of achieving similar COD removal. Catalytic ozonation has the potential to treat reverse osmosis concentrate derived from bio-treated coking wastewater reclamation.

Insight into micropollutant abatement during ultraviolet light-emitting diode combined electrochemical process: Reaction mechanism, contributions of reactive species and degradation routes.

Electrochemical process coupling with ultraviolet light-emitting diode for micropollutant abatement was evaluated in the treatment of wastewater containing Cl-. Four representative micropollutants, atrazine, primidone, ibuprofen and carbamazepine, were selected as target compounds. The impacts of operating conditions and water matrix on micropollutant degradation were investigated. Fluorescence excitation-emission matrix spectroscopy spectra and high performance size exclusion chromatography were employed to characterize the transformation of effluent organic matter in treatment. The degradation efficiencies of atrazine, primidone, ibuprofen and carbamazepine are 83.6 %, 80.6 %, 68.7 % and 99.8 % after 15 min treatment, respectively. The increment of current, Cl- concentration and ultraviolet irradiance promote the micropollutant degradation. However, the presence of bicarbonate and humic acid inhibit micropollutant degradation. The mechanism of micropollutant abatement was elaborated based on reactive species contributions, density functional theory calculation and degradation routes. Free radicals (HO•, Cl•, ClO• and Cl2•-) could be generated by chlorine photolysis and subsequent propagation reactions. The concentrations of HO• and Cl• are 1.14 × 10-13 M and 2.0 × 10-14 M in optimal condition, respectively, and the total contributions of HO• and Cl• for the degradation of atrazine, primidone, ibuprofen and carbamazepine are 24 %, 48 %, 70 % and 43 %, respectively. The degradation routes of four micropollutants are elucidated based on intermediate identification, Fukui function and frontier orbital theory. Micropollutants can be effectively degraded in actual wastewater effluent, and the small molecule compound proportion increases during effluent organic matter evolution. Compared with photolysis and electrolysis, the coupling of the two processes has potential for energy saving in micropollutant degradation, which shed light on the prospects of ultraviolet light-emitting diode coupling with electrochemical process for effluent treatment.

Abatement of Aromatic Contaminants from Wastewater by a Heat/Persulfate Process Based on a Polymerization Mechanism.

A novel approach to the abatement of pollutants consisting of their conversion to separable solid polymers is explored by a heat/persulfate (PDS) process for the treatment of high-temperature wastewaters. During this process, a simultaneous decontamination and carbon recovery can be achieved with minimal use of PDS, which is significantly different from conventional degradation processes. The feasibility of this process is demonstrated by eight kinds of typical organic pollutants and by a real coking wastewater. For the treatment of the selected pollutants, 30.2-91.9% DOC abatement was achieved with 24.8-91.2% carbon recovery; meanwhile, only 5.2-47.0% of PDS was consumed compared to a conventional degradation process. For the treatment of a real coking wastewater, 71.0% DOC abatement was achieved with 66.0% carbon recovery. With phenol as a representative compound, our polymerization-based heat/PDS process is applicable in a wide pH range (3.5-9.0) with a carbon recovery of >87%. Both SO4•- and HO• can be initiators for polymerization, with different contribution ratios under various conditions. Phenol monomers are semioxidized to form phenolic radicals, which are polymerized via chain transfer or chain growth processes to form separable solid phenol polymers, benzenediol polymers, and cross-linked polymers.

Organics abatement and recovery from wastewater by a polymerization-based electrochemically assisted persulfate process: Promotion effect of chloride ion and its mechanism.

Ubiquitous chloride ion (Cl-) in wastewaters usually inhibits the degradation of organic contaminants and generates numerous toxic chlorinated products in conventional degradation-based advanced oxidation processes (AOPs). Herein, a more Cl- tolerant polymerization-based electrochemical AOP for organic contaminants abatement and simultaneous organic resource recovery was demonstrated with eight typical organic contaminants and two real industrial wastewaters for the first time. This process can significantly promote dissolved organic carbon (DOC) abatement in the presence of Cl-, differing greatly from conventional degradation-based processes. Compared to sulfate radical (SO4•-) (or hydroxyl radical (HO•)), dichloride radical (Cl2•-) derived from Cl- has moderate reactivity towards most contaminants, which facilitates the organics polymerization as it ensures the formation of polymerizable organic radicals while inhibiting their excessive degradation. Thus, high DOC abatement (over 75 %) and high organic resource recovery ratio (48-79 % separable organic-polymer yield) can be achieved for most contaminants. Both soluble chlorinated compounds and solid chlorinated polymers are formed in the presence of Cl-. The chlorinated products (e.g. chlorophenols) can be polymerized as new monomers, thus the concentration of dissolved organic chlorinated products is much lower than that in conventional degradation-based process. The tolerance of the present process to Cl- is tested in real coking wastewaters, and exceeding 60 % of the abated chemical oxygen demand (COD) is obtained in the form of recoverable organic-polymers.

Treatment of contaminants by a cathode/FeIII/peroxydisulfate process: Formation of suspended solid organic-polymers.

Treatment of highly contaminated wastewaters containing refractory or toxic organic contaminants (e.g. industrial wastewaters) is becoming a global challenge. Most technologies focus on efficient degradation of organic contaminants. Here we improve the cathode/FeIII/peroxydisulfate (PDS) technology by turning down the current density and develop an innovative mechanism for organic contaminants abatement, namely polymerization rather than degradation, which allows simultaneous contaminants removal and resource recovery from wastewater. This polymerization leads to organic-particles (suspended solid organic-polymers) formation in bulk solution, which is demonstrated by eight kinds of representative organic contaminants. Taking phenol as a representative, 83% of PDS is saved compared to degradation process, with 87.2% of DOC removal. The formed suspended solid organic-polymers occupy 59.2% of COD of the original organics in solution, and can be easily separated from aqueous solution by sedimentation or filtration. The separated organic-polymers are a series of polymers coupled by phenolic monomers, as confirmed by FTIR and ESI-MS analyzes. The energy contained in the recovered organic polymers (4.76 × 10-5 kWh for 100 mL of 1 mM phenol solution in this study) can fully compensate the consumed electrical energy (2.8 × 10-5 kWh) in the treatment process. A representative polymerization model for this process is established, in which the SO4•- and HO• generated from PDS activation initiate the polymerization and improve the polymerization degree by the production of oligomer intermediates. A practical coking wastewater treatment is carried out to verify the research results and get positive feedback, with 56.0% of DOC abatement and the suspended solid organic-polymers accounts for 42.5% of the total COD in the raw wastewater. The energy consumption (47 kWh/kg COD, including electricity and PDS cost) is lower than the values in previous reports. This study provides a novel method for industrial wastewater treatment based on polymerization mechanism, which is expected to recover resources while removing pollutants with low consumption.

Determination methods for steady-state concentrations of HO• and SO4•- in electrochemical advanced oxidation processes.

Competitive kinetics and scavenging assay are commonly used for radical quantification. However, the accuracy of the two methods has been challenged in electrochemical advanced oxidation processes (EAOPs) since the strong reactivity of electrode against organic indicators may disrupt the quantitative relationship between indicator consumption and radical concentration. The present study focused on screening suitable indicators and developing suitable methods for determining the steady-state concentrations of SO4•- and HO• ([SO4•-]ss and [HO•]ss) in several EAOPs for water treatment based on competitive kinetics and scavenging assay. The applicability of the modified methods and available indicators were investigated through experimental and kinetic analysis. In anode alone process, the competitive kinetics was more appropriate than scavenging assay and benzoic acid (BA) met the basic requirement of being a competitor to determine the [HO•]ss. In cathode alone process, BA was more resistant to interfering factors than other competitors (ibuprofen, atrazine and nitrobenzene) and its reaction rate involved only the radical oxidation even when the reaction conditions varied over a wide range. Therefore, the [HO•]ss could be obtained by the competitive kinetic equation of BA when HO• existed alone. When HO• coexisted with SO4•-, a two-step method combining scavenging assay and competitive kinetics was proposed to measure [SO4•-]ss and [HO•]ss, in which tert-butyl alcohol and BA were added as scavenger and competitor, respectively. Furthermore, the reliability of each approach was verified by the experimental results and kinetic analysis.

Comparison of inert and non-inert cathode in cathode/Fe3+/Peroxymonosulfate processes on iohexol degradation.

To investigate the effect of cathode materials on organics degradation in a cathode/Fe3+/PMS process, different cathode materials (platinum, copper and iron) were selected and their performances were compared with iohexol as target organics. The optimal conditions were found to be different for different cathode/Fe3+/PMS processes. With a relatively high cathodic current input (2.0 mA/cm2), similar results were found for all the three cathode/Fe3+/PMS processes. With a small cathodic current input (not higher than 1.0 mA/cm2), the iohexol removal followed the order of Fe-cathode/Fe3+/PMS > Cu-cathode/Fe3+/PMS > Pt-cathode/Fe3+/PMS, due to the corrosion of Cu-cathode and Fe-cathode and the more serious corrosion of Fe-cathode than Cu-cathode. The corrosion of non-inert cathode materials (Cu-cathode and Fe-cathode) meant that these cathodes not only transmitted electrons but also participated in aqueous reactions, which complicated the mechanisms of cathode/Fe3+/PMS processes. The radical identification experiments indicated that SO4- was more important than OH for iohexol degradation in Cu-cathode/Fe3+/PMS process, while OH played a major role in Pt-cathode/Fe3+/PMS and Fe-cathode/Fe3+/PMS processes. The different reaction mechanisms resulted in different iohexol transformation pathways in cathode/Fe3+/PMS processes with different cathode materials.

Performance of Cu-cathode/Fe3+/peroxymonosulfate process on iohexol degradation.

Copper was used as a non-inert cathode material in a Cathode/Fe3+/peroxymonosulfate(PMS) system, and the performance of this novel Cu-cathode/Fe3+/PMS system was tested with a typical iodinated X-ray contrast media (iohexol) as target organics. The reaction mechanisms and the iohexol degradation pathways were investigated. The operational conditions of Cu-cathode/Fe3+/PMS process on iohexol degradation were optimized to be 1.0 mM Fe3+ dosage, 3.0 mM PMS dosage and 0.50 mA/cm2 of current input. The much lower current applied in the present study than previous reports would help to save energy and be more economical. Compared with typical inert cathode (Pt-cathode), the Cu-cathode/Fe3+/PMS process has better performance on both iohexol removal and deiodination, due to that Cu-cathode participated in Fe2+ regeneration and PMS activation via surface Cu°-Cu+(s)-Cu2+-Cu° redox cycle. Fe2+ could be produced via reactions between Fe3+ and Cu/Cu+(s) as well as cathodic reduction of Fe3+. SO4- was generated from PMS activation by Fe2+, Cu/Cu+(s) and cathodic reduction. OH was also generated in this process but SO4- played a dominant role in iohexol degradation. The intermediate products of iohexol and its transformation pathways were complex due to the varied reaction mechanisms involving both oxidation and reduction in Cu-cathode/Fe3+/PMS process.