Efficiency And Mechanism Of Electrochemical Activation Of Persulfate For Degradation Of Typical Organic Contaminants

Industrial production released a large amount of organic contaminants into water.In addition to the normal organic contaminants,antibiotics,pharmaceuticals,personal care products and industrial chemicals were also detected in water,which presented a serious threat to the safety of drinking water and water recycling.Persulfate-based advanced oxidation technologies could effectively remove the recalcitrant organic contaminants from water.This study developed a novel method for activation of persulfate,i.e.,electrochemical activation of persulfate at anode.The efficiency of electrochemical activation of persulfate using titanium plated with platinum(Ti/Pt),boron-doped diamond(BDD)and polytetrafluorothylene(PTFE)-carbon anodes for degradation of typical organic contaminants was investigated.Identification of the reactive species was conducted by radical quenching(methanol and tert-butanol)and degradation of radical probe compounds(atrazine(ATZ)and nitrobenzene(NB)).Then,this study proposed the mechanism of electrochemical activation of persulfate at anode and verified the proposed mechanism by electrochemical characterization including linear sweep voltammetry,cyclic voltammetry and chronoamperometry.Effect of sodium peroxydisulfate(PDS)concentration,current density and water matrices including chloride ion(Cl-),bicarbonate ion(HCO3-),phosphate ion(PO43-)and humic acid(HA)on the degradation of organic contaminants was investigated.The transformation products and degradation pathway of typical organic contaminant in electrochemical activation of persulfate using three types of electrode were investigated.Firstly,this study investigated the feasibility of electrochemical activation of PDS using Ti/Pt anode.The experimental results indicated that electrochemical activation of PDS using Ti/Pt anode could remarkably enhance the degradation of typical organic contaminants including carbamazepine(CBZ),sulfamethoxazole(SMX),propranolol(PPL)and benzoic acid(BA).The degradation of organic contaminants followed pesudofirst-order kinetic.The degradation rate of organic contaminants was increased to about two times of that in electrolysis process.Addition of excess methanol or tert-butanol produced negligible effect on the degradation of organic contaminant.In addition,electrochemical activation of PDS using Ti/Pt anode could not enhance the degradation of radical probes ATZ and NB,suggesting that electrochemical activation of PDS using Ti/Pt anode belonged to the novel nonradical oxidation system.The results of linear sweep voltammetry and chronoamperometry indicated that there was no remarkable electron transfer between PDS and electrode surface.A special transition state structure between the adsorbed PDS molecule and electrode surface might be formed upon applying current.The stable PDS molecule was converted to its activated state(PDS*).PDS* had high reactivity towards special organic contaminants,playing as the nonradical oxidation.Solution p H had negligible effect on the degradation of organic contaminant.Increasing PDS concentration or current density could enhance the degradation of organic contaminant.Addition of Cl-significantly promoted the degradation of organic contaminant,while addition of HCO3-,PO43-and HA inhibited the degradation of organic contaminant.Enhanced degradation of organic contaminants in electrochemcial activation of PDS using BDD anode was also achieved,which exhibited higher efficiency than electrochemical activation of PDS using Ti/Pt anode.The experimental results of radical quenching and degradation of radical probe indicated that nonradical oxidation and hydroxyl radical(HO·)oxidation jointly contributed to the degradation of organic contaminants.Electrochemical activation of potassium peroxymonosulfate(PMS)produced more nonradical and HO· than electrochemical activation of PDS.The relative contribution of HO· in electrochemical activation of PDS and PMS was 82.21% and 90.27%.Thus,HO· was the main reactive species.The result of cyclic voltammetry suggested that addition of PDS or PMS increased the oxygen evolution potential of BDD anode from 1.94 V to 1.98 V and 2.09 V.Simultaneously,addition of PDS or PMS reduced the oxygen evolution rate of BDD anode from 0.50 mg L-1 min-1 to 0.43 mg L-1 min-1 and 0.32 mg L-1 min-1.PDS* not only acted as the nonradical oxidation,but also increased the oxygen evolution potential and inhibited the oxygen evolution,thus to enhance the production of HO·.Similar to electrochemical activation of PDS using Ti/Pt anode,addition of Cl-enhanced the degradation of organic contaminant,while addition of HCO3-,PO43-and HA produced inhibitory effect on the degradation of organic contaminant.In order to reduce the cost of electrode material,four types of PTFE-carbon electrodes including multi-walled carbon nanotube(MWCNT),graphite(GR),black carbon(BC),granulat activated carbon(GAC)were prepared and characterized.Then,the performance of electrochemical activation of PDS using four PTFE-carbon anodes for degradation of organic contaminant was investigated.Electrochemical activation of PDS using carbon anode significantly enhanced the degradation of typical organic contaminant,which produced higher degradation rate of contaminant than electrochemical activation of PDS using Ti/Pt and BDD anodes.Differed from Ti/Pt and BDD anodes,the degradation of organic contaminant in electrochemical activation of PDS using carbon anode showed dual kinetics: an initial induction stage followed by a quick decay stage.Radical quenching and degradation of radical probes suggested that electrochemical activation of PDS using carbon anode produced nonradical,HO· and sulfate radical(SO4·-).The electrochemically activated PDS molecule(PDS*)acted as the nonradical oxidation.Its further decomposition and transformation produced HO· and SO4·-.Increase of PDS concentration or current density enhanced the removal of organic contaminant.Addition of Cl-,HCO3-,PO43-and HA generated inhibitory effect on the degradation of organic contaminant.After 50 hours of long-term electrolysis,the performance of electrochemical activation of PDS using carbon anode for degradation of organic contaminant decreased.Evenso,the removal efficiency of contaminant still remained above 65% within 5 min.Identification of the transformation products of typical organic contaminant CBZ in electrochemical activation of PDS using three types of electrode was performed.It was difficult to distinguish between nonradical oxidation and radical oxidation from the transformation products of CBZ.The main transformation pathway of CBZ was hydroxylation,intramolecular cyclization and hydrolysis of urea group(-NHCONH2).Reactive species was liable to attack at the olefinic double bond in the heterocyclic ring of CBZ.