Study On Synergistic Effect Of Pd-Cu Catalyst And Zero-Valent Iron On Nitrate Removal For Secondary Effluent Of Wastewater Treatment Plant

In this study,the sygergistic effect of zero-valent iron(Fe0)and Pd-Cu catalyst on nitrate removal was investigated to deal with the problem of high nitrate content in secondary effluent of municipal wastewater treatment plant.Firstly,the chemical reduction method using Fe0 as the reductant and the chemical catalytic reduction method with Fe0 and Pd catalyst were separately studied;and then the solution experiments and wastewater simulation tests were carried out to find out how to improve the chemical performance of the catalytic reduction of nitrate and analyze the regeneration and reuse of the catalyst;finally,the reaction mechanism and its kinetic model were further analyzed.The main results were listed as below:The chemical reduction method using Feoas the reductant could effectively remove nitrate,however,ammonium was the main by-product that needs another system to deal with.On the contrary,the catalytic chemical method with Fe0 and Pd/Al2O3 enabled to convert large parts of nitrate in waterbody into nitrogen gas(N2)under certain conditions,presenting better feasibility.Adding promoting ingredient-Cu to the catalyst could efficiently improve the catalytic performance.SiO2,?-Al2O3,kaolin,silica gel,diatomite,and graphene were investigated as the carrier of catalyst.Their catalytic efficiencies decreased in the following order:Pd-Cu/graphene>Pd-Cu/?-Al2O3>Pd-Cu/diatomite>Pd-Cu/silica gel>Pd-Cu/diatomite>Pd-Cu/SiO2.Among these catalysts,the nitrate removal and N2 selectivity for Pd-Cu/graphene were 80%and 75%,respectively.When the conditions were:20 mg/L of original nitrate concentration and 25 ?,the optimal parameters for these catalysts were listed as follows:.5 g/L of Fe0,3:1 of Pd:Cu,4.2 pH,4 g/L of catalyst(3 g/L for Pd-Cu/graphene),and 2 h of reaction time.The catalytic performances of these catalysts presented downward trends as the wastewater instead of the synthetic solution was used in the research.This phenomenon may result from the cations and anions exist in wastewater.The performances with different cations increased in the following order:K+<Na+<Ca2+<Mg2+<Al3+;for the anions,Cl-and SO42-had little effect on catalytic efficiency,while high content of HCO3-and PO43-may mildly refrain the chemical reaction.Furthermore,temperature,stirring intensity,DO concentration would greatly affect the catalytic performance.The study on regeneration and reuse of catalyst showed that the acidic wash with 2 mol/L HCl in 2 h enabled to effectively enhance the catalyst's activity and regenerate the catalyst to some extent.Additionally,study on reaction mechanism showed that Fe0 mainly serves as the electron provider in catalytic process,the active sites on Pd and Cu have been the places where the catalytic reaction proceeds.It's noteworthy that Pd plays the leading role and Cu acts as the promoting ingredient in the nitrate reduction.Furthermore,this process could be better explained by the first order kinetic model of Langmuir-Hinshelwood.The equation could be obtained as the catalyst Pd-Cu/graphene was used:y=247.1x+0.1398,R2=0.9975.The reaction velocity constant k was 7.153 and the adsorbtion constant K was 0.000566.