Electrochemical Technology For Simultaneous Removal Of Nitrogen & Phosphorous And Elimination Of Residual Active Chlorine

The use and discharge of nitrogenous and phosphorous compounds have caused serious water pollution.As a green and efficient technology for wastewater treatment and water remediation,electrochemical method has received more and more attention and application.For instance,ammonia nitrogen can be effectively removed by electrochemical oxidation method,and phosphorus-containing wastewater can also be treated by electrocoagulation.Aluminum is one of the most common sacrificial anodes in electrocoagulation processes.At present,many studies have reported that the phenomenon of"super-Faraday"dissolution of aluminum occurred in the electro-flocculation process with the dual-aluminum electrode system,that is,the total amount of released aluminum exceeds the theoretical dissolution amount of the anode calculated according to Faraday’s law.This should be attributed to the chemical corrosion of the aluminum cathode.This article systematically studied the dissolution behavior of aluminum cathodes,and established a new system coupled aluminum and chlorine-evolution electrodes for simultaneous removal of nitrogen and phosphorus in the wastewater.After the treatment in this system,an excess free chlorine may be analyzed in the effluent.Therefore,we also studied the failure mechanism and regeneration of activated carbon in the removal processes of free chlorine.The main conclusions of this article are as follows:(1)The release amounts of aluminum cathode and aluminum anode,electrode surface morphology and electrochemical behavior were compared.Electrolysis experiment results showed that a similar aluminum release amount could be get between anode and cathode at the current density of 2 m A/cm~2 after 6 h reaction.In addition,compared with the passivation of the aluminum anode,the aluminum cathode had a"depassivation"behavior due to the chemical corrosion caused by the local alkalinity.As the current density increased,the dissolution rate of aluminum first increased and then stabilized.This may be because when the current density increased to a certain value,the generation and diffusion of OH~-reached equilibrium,making the local pH remained relatively stable,so the dissolution rate of aluminum tended to be stable.In order to further determine the local pH near the aluminum cathode,the local pH was determined by studying the dissolution rate of aluminum under alkaline conditions with different pHs,and then the relationship between pH and dissolution rate was established to roughly determine the cathode local pH.Also,the influence of various factors on the dissolution rate of aluminum were studied.The study showed that higher temperature could get a faster dissolution rate of aluminum;pH and electrolyte had little effect on the dissolution of aluminum cathode.(2)Based on the dissolution behavior of the aluminum cathode,by coupling it with the DSA anode,the electro-flocculation and electro-oxidation processes can be realized in a single electrolysis system,thereby constructing an electrochemical system for simultaneous removal of nitrogen and phosphorus.Due to the aluminum dissolution process in the cathode,the aluminum species in the solution play a buffering role,keeping the solution pH at a weak alkalinity,greatly improving the removal efficiency of ammonia nitrogen in the anode process.By increasing the applied current density,the cumulative concentration of free chlorine in the solution can be significantly increased,thereby improving the removal efficiency of ammonia nitrogen in the system.The cathode dissolution of aluminum will also be enhanced by higher current density,so the removal efficiency of phosphorus will also be enhanced.Throughout the process,the concentration of by-products such as nitrate and nitrite were very low,which proved that the conversion selectivity of ammonia nitrogen to nitrogen was high.(3)When the above electrochemical oxidation method was used to remove ammonia nitrogen,a higher concentration of free chlorine could be found in the effluent.Generally,the activated carbon method is used for the removal of free chlorine in water.Acidic pH conditions,more activated carbon dosage and smaller activated carbon size could bring better free chlorine removal.And the activated carbon before and after the reaction was analyzed by comparing the surface structure and physicochemical properties.it is found that the failure of activated carbon is mainly due to the change in pore structure and specific surface area caused by the consumption of its surface reducing functional groups and the oxidative destruction of the surface structure.Thermal regeneration of the failed activated carbon under different atmospheres could restore the free chlorine removal ability,and the ammonia atmosphere was the best.Also,the activated carbon was subjected to continuous flow column experiments to confirm that it could operate effectively for a long time.