| Electroless plating process is widely applied to manufacturing industry due to its high efficient,energy saving,environmental protection,and convenient.In order to ensure and maintain the stability and sealing of the plating solution,various complexing agents,brighteners and buffer solutions are added during the electroless plating process.However,those novel complexing agents are able to readily chelate with heavy metals in the bath to form highly soluble and refractory organic-metals complexes,especially organic-nickel(Ni)complexes.Therefore,the treatment of electroless nickel plating effluents is becoming difficult.Nowadays,there were still low-concentration and high-steady organic-metal complexes in electroless nickel plating effluents after conventional physicochemical treatment,leading to the subsequent general technical problems that is difficult to biochemically degrade and reaching drainage standard for the wastewater.For the advanced treatment of organic-metals complexes,the traditional process mainly adopts oxidation of Fenton combined with chemical precipitation to achieve the purpose of removal of heavy metals.Nevertheless,a large amount of sludge,secondary pollution and un-stability of the effluent quality would be happened during above process.Meanwhile,national and local Ministry of Environmental Protection have become more stringent on the discharge of electroplating wastewater pollutants and contaminant.Among them,the Special Discharge Limit of Water Pollutants(Table 3)based on the Electroplating Water Pollutants Discharge Standard(GB21900-2008)stipulates that the discharge limitation of total nickel(TNi)and total copper(TCu)concentration of the treated effluent is stable below 0.1 mg L-1and 0.3 mg L-1 respectively,and the emission limitation of Chemical Oxygen Demand(COD)is less than 50 mg L-1 in recent years.Obviously,traditional physical and chemical methods are unsatisfied with the latest national emission requirements.Therefore,the electroless nickel plating effluents after the conventional physicochemical treatment was adopted to our research object,and integrated treatment process of catalytic ozonation assisted with heavy metal chelation were developed,which was applied to advanced treatment of organic-metal complexes and various organic contaminant in electroless nickel plating effluent.Moreover,the operating parameters and optimal conditions for catalytic ozonation and heavy metal chelation were optimized,and the mechanism of catalytic ozoantion and decomplexation of organic-metal complexes was discussed in depth.Meanwhile,the expanded and pilot-scale experimental equipment was designed and constructed.Further,in order to theoretically and actually evaluate the efficiency,feasibility and stability of the integrated process for advanced removal of heavy metals and degradation of organic contaminant,long-term pilot-scale and expanded continuous operation experiments were conducted and evaluation system was established.Finally,it provides theoretical basis,guiding function and useful reference for large-scale applications and engineering practice of advanced treatment of electroless nickel plating effluents.(1)The integrated process of heterogeneous catalytic ozonation assisted with heavy metal chelation system(O3/Fe OOH@Al2O3–MDCR)was established with catalyst Fe OOH@Al2O3(SAO3II)and the heavy metal trapping agent,magnetic dithiocarbamate chelating resin(MDCR).The feasibility of above integrated process to treat organic-Ni complexes and TNiof the electroless nickel plating effluents was investigated,and the optimal parameters of the operational process were determined.Moreover,the mechanism of catalytic ozonation by Fe OOH@Al2O3 was further and deeply explored.Subsequently,the results showed that the removal of TNi could reached to 95.6%,and the concentration TNi of effluent was less than 0.1mg L-1.Addition of catalyst Fe OOH@Al2O3 can shorten the oxidation time by at least 25%in O3/Fe OOH@Al2O3–MDCR,compared with single O3–MDCR system.The generation of the strong oxidizing radicals was due to the transformation of O3catalyzed by Fe and its oxides of the catalyst Fe OOH@Al2O3,accelerating the oxidation efficiency of the reaction system.In addition,it could be educed that process of catalytic ozonation was the foundation and predominant role of the whole reaction system,and was preliminary demonstrate that organic-Ni complexes were not completely mineralized and release Ni(II)through the catalytic ozonation reaction.They were gradually decomposed from a highly steady macromolecular state to the volatile micromolecular state by radicals and O3 during the catalytic ozonation process.Then MDCR chelated with Ni from a low molecular state to form an insoluble and stable chelate via competitive coordination,and achieve the purpose of removal of TNi.(2)In order to explore the feasibility of advanced removal of organic-metal complexes from the electroless nickel plating effluents,the expanded and pilot-scale experimental equipment and facilities was designed and construct,and further assemble into a continuous pilot-scale system(CPSS).Similarly,synchronous bi-directional gas-liquid flow pattern was firstly proposed and adopted for the treatment of electroless nickel plating effluents,and the optimal operational parameters of CPSS were systematically explored.Furthermore,continuous experiments of CPSS were conducted more than 300 days under the optimal operational conditions.The results demonstrate that catalytic and oxidation efficiency of catalytic ozonation reaction system can be effectively improved in synchronous bi-directional flow pattern.The removal efficiency of TNi and TCu both exceeded 95%,and the TNi and TCuconcentrations of effluent could be stabilized below 0.1 mg L-1 and 0.3 mg L-1,respectively,during the continuous experiments of CPSS for 300 days,meeting the discharge standards stipulated in China.At present,the treatment flowrate(Q)of electroless nickel plating effluents could exceed 3.0 t d-1in our pilot-scale experimental equipment.(3)According to the field investigations and experimental demonstration,ethylenediamine tetraacetic acid-Ni(EDTA-Ni)and citrate-Cu(CA-Cu)were conformed to be the predominant organic-Ni and-Cu complexes in electroless nickel plating effluent,respectively.In addition,10 intermediates were detected,and the potential degradation pathways of EDTA-Ni and CA-Cu were established based on the identified degradation intermediates.According to the degradation characteristics of organic-Ni and-Cu complexes,as well as the changes of p H during the continuous experiments of CPSS,it was inferred that the change of p H(?p H)before and after treatment might be used as a criterion to estimate the concentrations of TNi in the final effluent.These findings provide valuable insights and guiding significance for adjustment of the process parameter during the process,as well as the removal of organic-Ni and-Cu complexes.(4)In order to further enhance the utilization efficiency of ozone,a Mn Fe2O4-C@Al2O3catalyst with a core-shell structure was synthesized and the system of Mn Fe2O4-C@Al2O3/O3-MDCR was established.Moreover,the optimal parameters of the system and the catalytic mechanism of synthesized Mn Fe2O4-C@Al2O3 was systematically studied.Results show that the introduction of Mn Fe2O4-C@Al2O3 significantly increased the TNi removal efficiency(99.3%),and the effluent concentration of TNi is stably meeting the discharge standard.Compared with O3/Fe OOH@Al2O3–MDCR and other systems,Mn Fe2O4-C@Al2O3/O3-MDCR system possesses the highest reaction rate constant(k=0.7461 min-1)and need to add the lowest dosage of catalyst.Moreover,the O3-utilization efficiency for removal of TNi(?TNi/?O3),COD(?COD/?O3)and TOC(?TOC/?O3)remained at a highest level in Mn Fe2O4-C@Al2O3/O3-MDCR system.In addition,Analysis of catalytic mechanism and characterization demonstrated that surface hydroxyl groups,oxygen vacancy,metals(and its oxides),the carbon surface and its functional groups,can all potentially serve as catalytic active sites,with ~1O2,·OH and·O2-considered to the predominant reactive oxygen species.Therefore,it could be observed that Mn Fe2O4-C@Al2O3 exhibit the excellent catalytic performance and excellent stability,which could be applied in the long-term experiments of continuous operation and practical application.In addition,the toxicity of electroless nickel plating effluents before and after treatment was investigated and the toxicity evaluation methond was preliminarily established.The results showed that the toxicity of electroless nickel plating effluents was significantly weakened and biodegradability was markedly improved during the integrated process of Mn Fe2O4-C@Al2O3/O3-MDCR treatment. |
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