Electrochemical Reduction Of Nitrate With Pd-Cu/Activated Carbon Fiber And Charcoal Cathodes

Nitrate pollution problems are widespread in surface water,groundwater,aquaculture water and reverse osmosis concentrated water,which threaten human health and destroy the ecological balance.Among nitrate removal methods,microbial method is not suitable for the treatment of wastewater with high salt and low carbon content.Highly concentrated reject water produced by reverse osmosis,ion exchange,and electrodialysis treatment is still a tricky problem.However,nitrate can be completely removed from water via electrochemical reduction nitrate to nitrogen or ammonia.The electrode materials play a key role in electrochemical reduction of nitrate.In this work,Pd-Cu loaded activated carbon fiber felt and charcoal electrodes were prepared to electroreduce nitrate from water efficiently.The main research contents and conclusions are as follows:In this research,activated carbon fiber felt was used as substrate,and Pd-Cu was loaded on activated carbon fiber felt electrode by dipping method.Effects of Pd-Cu ratio,current,initial NO₃^--N concentration,pH and electrolyte on nitrate removal were investigated.The results exhibited that the Pd-Cu ratio of 3:1 and reaction current of 80 m A were the optimal nitrate removal conditions.Nitrate could be completely removed.The selectivity of ammonia in product reached 86%,and the selectivity of nitrite was only 0.43%Pd-Cu/ACF electrodes were not suitable for nitrate reduction under pH 3.0 or 11.0 and Cl^-electrolyte conditions.In order to decrease the cost of electrodes,precious metals should be avoided to use in electrode materials.Non-metallic carbon materials have the advantages of low price,high activity,and strong electronic structure adjustment,which are potential cathodes for the electroreduction of nitrate.The charcoal cathodes were fabricated by carbonization method.It displayed moderate sp³ C structure and oxygen-containing functional groups,which provided more active sites for the reduction reaction.Effects of voltage,pH,initial NO₃^--N concentration,Cl^-electrolyte and its concentration on nitrate removal were discussed.The results showed that charcoal cathodes were suitable for neutral or slightly acidic conditions and charcoal cathode exhibited high nitrate removal rate（91.2%）,outstanding selectivity（98.5%）,and the maximum ammonia yield was 0.57 mmol L^-1h^-1cm^-2.Nitrate could not be removed due to Cl O^-produced by oxidation of Cl^-when 0.05 mol L^-1Na Cl was used as electrolyte.When the concentration of Cl^-decreased to 0.01 mol L^-1,the nitrate removal rate restored to 50%.It was proved that charcoal electrodes possessed excellent stability after five repeated experiments.The heteroatom doping introduced defects in the carbon materials,which could change surface properties and electron cloud arrangement of carbon materials,thereby more active sites were provided.Phosphorus-doped charcoal cathodes（P-C）were prepared by the impregnation-carbonization method.The XPS characterization confirmed that the phosphorus element was successfully doped into charcoal.Compared with charcoal,the ability of P-C cathodes were significantly improved,and ammonia selectivity increased from 66%to 86%.The optimized reaction voltage was 4.2V and nitrate removal rate increased from 32%to 65%when phosphorus doping amount increased from 1.02%to 2.13%.Three repeated experiments proved that the P-C had good stability.The Pd-Cu/ACF,charcoal and P-C cathodes can effectively remove nitrate from water,which proved that carbon-based materials have potential to electrochemical reduce nitrate for practical applications.