Study On Deep Mineralization Of Cyanide-containing Wastewater Based On Multi Field Synergetic Electrochemistry

Cyanides（CN^-）are widely used in numerous industrial processes such as mining,electroplating,coking,etc.,due to their characteristic properties for complexing metals.As a result,cyanides are often present as complexations with metals in the corresponding wastewater.Although the toxicity of cyanocomplexes is relatively lower than that of free cyanides,these cyanocomplexes can be dissociated under specific severe conditions（such as strong acid,UV radiation,or high temperature）,releasing highly toxic hydrogen cyanide（HCN）and causing potential damage to the ecosystem.At present,there are still many problems during the treatment process of containing-cyanide wastewater.In addition to the high cost and low treatment performance,CN^-cannot be efficiently mineralized into nitrogen（N₂）or nitrate（NO₃^-）.To inhibit the evolution of highly toxic HCN gas,the treatment system must be maintained in a strongly alkaline medium.The oxidation product cyanate（CNO^-）cannot be hydrolyzed under alkaline conditions,which makes the mineralization of containing-cyanide wastewater very difficult.To achieve the efficient mineralization of CN^-,relevant research is conducted as follows:1)A novel Electro-Fenton system was constructed,in which the free cyanides released from ferricyanide photolysis can be efficiently mineralized by the synergy of^·OH and O₂^·-.The complex bonds in ferricyanide（100 m L,0.25 m M）were completely broken within 80 min under ultraviolet radiation,releasing free cyanides.Subsequently,in combination with the heterogeneous Electro-Fenton（EF）process,^·OH and O₂^·-were simultaneously generated and 92.9%of free cyanides were transformed into NO₃^-within 120 min.No low-toxic CNO^-intermediates were accumulated during the EF process.A new conversion mechanism was proposed that CN^-was activated into electron-deficient cyanide radical（^·CN）by^·OH,and then the^·CN intermediates reacted with O₂^·-via nucleophilic addition to quickly form NO₃^-,preventing the formation of CNO^-and promoting the mineralization of cyanide.Furthermore,this new strategy was used to treat the actual cyanide residue eluent,and75%of CN^-in the actual cyanide residue eluent was efficiently mineralized into NO₃^-.The EF system has excellent application prospects for practical cyanide-containing wastewater.2)A novel electrocatalytic（EC）system regulating the enrichment direction of CN^-and utilizing the synergy of anode and cathode was constructed.Theoretical calculation and static-concentration distribution confirmed that CN^-was chemically adsorbed on the Co₃O₄cathode with the input of the potential,getting rid of the electrostatic attraction of the anode.As a result,CN^-was preferentially hydrogenated to CH₃NH₂and NH₃by H*at the Co₃O₄cathode.Subsequently,CH₃NH₂and NH₃were further converted into N₂by free chlorine derived from the DSA anode.The EC system shows outstanding feasibility for the mineralization of the practical containing-cyanide wastewater.It provides a new idea for the highly efficient and selective mineralization of CN^-into N₂in the electrocatalytic system.3)By introducing sulfur（S）into the Co₃O₄cathode,the hydrogen evolution performance of the cathode was changed,and the conversion of CN^-to N₂in the above EC system was promoted.Structural characterization confirmed that S mainly grafted with the terminal oxygen in Co₃O₄,but not bonded with Co.With the introduction of S,the hydrophobicity of the Co₃O₄-S cathode was improved,which inhibited the generation of excess H*and prevented the self-quenching of H*to generate H₂.As a result,the utilization of H*was improved.When the S source is 0.2m M(Co₃O₄-S_0.2),1.35 m M of CN^-could be completely mineralized into N₂within 2h,which was much superior to the Co₃O₄cathode.In addition,benefitting from moderate hydrogen evolution performance,the Co₃O₄-S_0.2cathode has better stability and performance for the mineralization of cyanide-containing wastewater than the Co₃O₄cathode.4)A novel system of electrocatalysis coupled with ultraviolet-based advanced oxidation processes（UV/EC/PS）was constructed,exhibiting extraordinarily high activity and selectivity for ferricyanides mineralization.Sufficient active-chlorine species(Cl O^·/Cl₂^·-)were generated under the synergistic effects of electrocatalysis and advanced oxidation processes and they are high-selective for cyanide mineralization.Concurrently,induced by the interconversion between active species,the p H value in the UV/EC/PS system dynamically decreased from 11.50 to 3.30 at a rate of 1.1×10^-2min^-1,immensely accelerating the hydrolysis of cyanate intermediates.Persulfate（PS）played a vital role in the formation of Cl O^·/Cl₂^·-and the self-decrease of p H value in the UV/EC/PS system.Under the synergy of Cl O^·/Cl₂^·-and self-decreased p H value,0.25 m M of ferricyanide could be thoroughly mineralized into nitrogen within 80 min and no HCN evolved.Furthermore,the UV/EC/PS system shows excellent feasibility for the mineralization of practical cyanide residue eluent.