Appliation Of Electro-Catalytic Reduction-Oxidation Technology In Nitrophenol Wastewater Pretreatment

Nitrophenol wastewater is characterized by high toxicity and low biodegradability.It needs an effective pretreatment process to increase its biodegradability.Otherwise the biochemical system may breakdown easily.Zero-valent iron+Fenton+coagulation can effectively pretreat wastewater containing nitro aromatic compounds.However,this process consumes large amounts of reagents and is difficult to control the reaction conditions.What’s more,solid waste from the process pertains to hazardous solid waste.This kind of solid waste has high treatment cost and may produce secondary pollution.As a kind of green technology,electrochemical process has advantages of no chemical agents dosing,no secondary pollution,and easy automation.It has great prospects in the treatment of recalcitrant organics.However,there is less research and application of electrochemical reduction.The combination of electro-catalytic oxidation and electrochemical reduction of water treatment is also rarely reported.The efficiency of two-dimensional reactor is too low.To solve these problems,this paper attempts to adopt electro-catalytic reduction-oxidation as pretreatment process of nitrophenol wastewater,investigate the mechanism of o-nitrophenol(ONP)transformation and toxicity reduction.Apply the process in nitrophenol wastewater in pilot scale tests.In this paper,preferably anodic and cathodic fillers are chosen to develop a new three-dimensional electro-catalytic reduction-oxidation reactor.Compared with the conventional two-dimensional reactor,current efficiency and effect of new reactor are higher.The main conclusions of this paper are as follows:(1)The kinetics of ONP removal on cathode follows a pseudo-first-order reaction,and is controlled by diffusion rate.ONP removal efficiency and o-aminophenol(OAP)formation efficiency reach 100.0%and 79.3%at equilibrium,respectively.Orthogonal tests show that the main influence factor of ONP reduction is initial pH.Higher current density and longer reaction time result in lower current efficiency.Ti as cathode and Na2SO4 as electrolyte show better effect.The main intermediate products of reduction procedure are o-nitrosophenol,o-hydroxylamine phenol,and o-aminophenol.The final product is OAP.(2)Electro-reduction reinforces the following oxidation in ONP degradation,occurs in the removal and mineralization of ONP.OAP,the main product of ONP reduction,can be well degraded in electro-oxidation process.OAP removal efficiency and mineralization efficiency reach 95.2%and 83.4%at equilibrium,respectively.Main influence factor of OAP oxidation is electrolyte concentration.PbO2 as anode and NaCl as electrolyte show better effect.The main intermediate products of oxidation procedure are catechol,phenol,and p-benzoquinone,they are further oxidazed to small molecular acid and finally mineralized.The toxicity of the solution rises during reduction and then declines during oxidation.(3)Silica sand and activated carbon are suitable as cathodic and anodic filler in three-dimensional electro-catalytic reduction-oxidation reactor,respectively.Ti electrode and PbO2 electrode are chosen as cathode and anode in three-dimensional reactor.Compared with the two-dimensional reactor,three-dimensional reactor shows advantages in treating nitrobenzene wastewater.In terms of nitrobenzene removal efficiency,three-dimensional reactor is about 10%higher than two-dimensional reactor.Three-dimensional reactor is about 60%higher than two-dimensional reactor in nitrobenzene mineralization.(4)In the treatment of nitrophenol wastewater,continuous flow reactor is better than continuous stirred tank reactor.Ti as cathode is better than stainless steel.In pilot scale tests,improving the current density and prolonging the reaction time can increase the effect of pretreatment.Under optimal conditions,the removal efficiencies of UV254,SUVA,COD,and TOC reaches 66%,63%,46%,and 20%,respectively.It can also reduce the toxicity of the wastewater,by TU reduction rate of 82%.Electro-catalytic reduction-oxidation can remarkably improve the B/C of the wastewater,by 0.11 rising to 0.34.