Decontamination Of Aqueous Nitrogen Group Pollutants By Electroactive Modified Carbon Nanotube Filter:Performance And Mechanism

In the past decades,the detrimental effects of excessive nitrogen group elements（such as phosphate,arsenite and antimonite）on water environment and ecological toxicity have attracted extensive attention.It is,therefore,imperative to seek well-directed strategies to eliminate nitrogen group elements from water environment.This paper innovatively designed two kinds of electroactive modified carbon nanotube（CNT）filters,which realized the efficient removal（or recovery）of typical nitrogen compounds based on the auxiliary electric field.The specific research contents are as follows:（1）The excessive phosphorus in water bodies has been identified to be the major cause of eutrophication.Meanwhile,as a scare resource,phosphorus needs to be recycled in waste streams.Therefore,electrochemically mediated precipitation of phosphate and co-existing calcium ions（the main component of phosphate rock）hold vast potential in removing and recovering phosphorus.Herein,an electrochemical CNT filter functionalized with calcium carbonate（CaCO₃）was developed.The concept was based on loaded CaCO₃ particles that facilitated calcium phosphate precipitation through buffering H⁺produced from water oxidation at the anode and releasing Ca²⁺,thereby a high p H micro-environment was established in the vicinity of cathode.The results showed that CaCO₃-loaded CNT filter could achieve 84.1%phosphate recovery in 3 h under applied voltage of 3 V and flow rate of 1.5 m L/min.This value was 71.9%higher than that for conventional batch reactor（12.2%）,which should result from the co-strengthening contribution of auxiliary electric field and convective mass transport.Moreover,the system showed good phosphate recovery performance over 2-20 mg/L simulated wastewater and actual domestic sewage.（2）The co-presence of arsenic（As）and antimony（Sb）in water bodies has been commonly reported.The toxicity of As and Sb varies with different speciation.For example,As（Ⅲ）/Sb（Ⅲ）is much more toxic than As（Ⅴ）/Sb（Ⅴ）.Herein,we designed a dual-functional electrochemical filter toward detoxification of highly toxic As（Ⅲ）/Sb（Ⅲ）to low toxic As（Ⅴ）/Sb（Ⅴ）based on valence regulation strategy,followed by the sequestration of As（Ⅴ）/Sb（Ⅴ）on the nanoscale goethite.Results showed that 97.9%As（Ⅲ）and 91.9%Sb（Ⅲ）transformation and 86.4%As_total and 70.1%Sb_totalremoval efficiency can be obtained over 2 h continuous filtration under optimized conditions.The As_total removal kinetics and efficiency enhanced with flow rate and applied voltage(e.g.,the As_totalremoval efficiency increased from 62.9%at 0 V to 86.4%at 2.5 V).XPS and TEM analysis confirmed that As/Sb was mainly adsorbed by goethite.Moreover,the proposed technology works effectively across a wide p H range.Only negligible inhibition was observed in the presence of nitrate,chloride,and carbonate.Exhausted hybrid filters can be effectively regenerated by using chemical wash with Na OH solution.The two electroactive modified CNT filters proposed in this study are expected to provide practical solutions and ideas for solving the problem of excessive nitrogen group compounds in wastewater.