Study On Up-grading Effluent Of Biochemical Treatment Coking Wastewater Based On Sulfate Radical Oxidation Combined Iron Flocculation

Coking wastewater is still held lower B/C after the biochemical process,that the COD_cr,cyanide and volatile phenol can not up to national emission standards.Nitrogen heterocyclic compounds(NHCs),which have significant biological toxicity and carcinogenicity.Due to the molecular structure of NHCs contain an N atom with a strong"electron^-withdrawing effect",the molecular space steric hindrance increased,and showed significant biological toxicity.The biological degradation process of NHCs was more than 72 h,known as a typical refractory organic pollutant in coking wastewater.Among the technologies,the advanced oxidation process(AOPs)is one of the most effective methods to remove such organic pollutants.Sulfate radical(SO₄^?-)has the advantage of high oxidation potential(E⁰=2.6?3.1e V)and a wide p H range.Additionally,Fe³⁺ion,which was the reaction product of Fe²⁺and Fe⁰(activator),has three positive charges and significant flocculation effect.This paper was focused on the sulfate radical oxidation coupling Fe³⁺flocculation up^-grading treatment process for BECW.Firstly,quinoline(Qu),as a representative of NHCs,was deeply studied on its removal efficiency and the reaction mechanism by chemical reaction kinetics and the quantum chemical method.Secondly,the actual BECW was analysis of the characteristics of water quality parameters change the solution appearance changes,kinds of organic matter and the target pollutant removal efficiency by SO₄^?-oxidation process.Finally,the efficiency of SO₄^?-oxidation coupled Fe³⁺flocculation for up-grading treatment of BECW was analyzed through flocculation particle size,Zeta potential,sludge production,and flocculation kinetics.Results showed that Fe²⁺/PS and ZVI/PS system is the fastest activation system and the maximum yield system of SO₄^?-,respectively.The optimal mole ratio is Fe²⁺:PS:Qu=48:24:1,and ZVI:PS:Qu=24:24:1.Optimize the way of adding activator can effectively improve the yield of SO₄^?-,and the Qu removal rate can be improved from 44.69%to 60.87%by incrementally adding Fe²⁺within 50 min.Moreover,in initial state addition,due to a small amount of Fe²⁺ions in the ZVI/PS system can accelerate the corrosion of ZVI and produce"active iron",the removal rate of Qu was increased from 85.8%to 92.9%.By the study of the physicochemical conditions of the oxidation reaction,the degradation rate decreased as a function of initial aqueous p H and increased as a function of temperature.When the temperature was lower than 40?,Fe²⁺and heat were synergistic.When the temperature was higher than 55?,the activation effect of Fe²⁺gradually decreases and becomes the quenching agent of SO₄^?-.The reaction mechanism of the degradation of Qu by SO₄^?-can be explained by molecular orbital theory and Lewis acid^-base theory.In acidic solution,Qu was first protonized,and the interband gap energy between the highest occupied orbital(HOMO)and the lowest unoccupied orbital(LUMO)in molecules was reduced,and it was more easily oxidized by free radicals of the single occupied molecular orbital(SOMO).Based on the Hirshfeld charge was calculated with Guassian 09 software,the each individual atom of Qu,and the primary attack position were analyzed with the Gaussian 09 software and density functional theory,then,the broken ring position of Qu was speculated.The 5 C and 3 C position of the benzene ring of Qu are verified to be the priority attack positionwhen Qu was oxidized by SO₄^?-,and organic acid and organic nitrile were formed,that could reduce the biological toxicity.In this reaction,Qu is a typical Lewis base and SO₄^?-is a Lewis acid,which conforms to the Lewis acid^-base theory.Fe³⁺ions with flocculating effect can be produced electrostatic forces to precipitate kaolin colloid solution.Experiments on the degradation of actual BECW by Fe²⁺and ZVI co^-activation system showed that HCO₃^-,CO₃^2-,CN^-,SCN^-ions,and other substances that can participate in free radical chain reactions and competed for the active point position of SO₄^?-,resulting in the reduction of removal rate of target pollutants.The initial p H was adjusted to 4.5to remove some affected ions,and the removal rate of total organic carbon(TOC)was increased from 56.29%to 59.08%.The removal rates of cyanide,volatile phenol,suspension and chromaticity were 99%,62.1%,66.1%,and 91.04%,respectively.The intermediate products of BECW oxidized by SO₄^?-were determined by GC/MS.Results demonstrated that the special ultraviolet absorption parameter(SUVA)was reduced during the reaction,indicating that the aromatic and unsaturated bonds organic compounds in the solution were removed in large quantities.This study revealed the pathway and mechanism of the oxidation of Qu and actual BECW by SO₄^?-,and constructed a completely new process for up^-grading treatment of BECW.