Removal Efficiency And Mechanism Of Tetracycline By Coupling Modified Activated Carbon And Heterogeneous Fenton-like Oxidation Processes

Powdered activated carbon(PAC)and the conventional homogeneous Fenton process(CFP)have been widely used in the field of wastewater treatment due to their high adsorption capacity and strong oxidation performance,respectively.However,the difficulties in separation and regeneration of PAC and massive catalyst loss and iron sludge production of CFP have restricted their application.The typical chemical co-precipitation method was adopted for the modification of PAC,the optimized modified activated carbon(C@FONC)was coupled with heterogeneous Fenton-like process(C@FONC/H2O2)to combine their advantages and to avoid similar limitations of PAC and CFP.The treatment efficiency and mechanism of C@FONC/H2O2 system were investigated.Based on precipitants and Fe3O4/PAC mass ratios effect,the effect of preparation conditions on C@FONC structure and treatment efficiency were investigated.The results showed that,Na OH as the precipitant can improve the treatment efficiency of C@FONC/H2O2 system;The iron leaching of C@FONC/H2O2 system showed a good linear positive correlation with the proportion of Fe3O4 in C@FONC(y=3.2469x-0.2117,R2=0.97).High adsorption capacity and strong oxidation performance of C@FONC/H2O2 system were achieved with the Fe3O4/PAC mass ratio of 1/1.The results of BET,SEM-EDS,TG-DTG,VSM,XRD,Raman,FTIR,and XPS revealed that the pore size distributions of C@FONC were mainly concentrated in 3.87 nm,which demonstrated the existence of mesoporous structure accounted for 89.47%.Fe3O4 was distributed on the surface,pores,and channels of C@FONC with an average particle size of 20-50 nm.The results showed,the existence of Fe3O4 could improve the adsorption capacities by increasing the adsorption sites,the best adsorption capacity of C@FONC for tetracycline(TC)was observed(140.2 mg/g at 300 min)in the p H value of 3.Alkaline-earth cations(e.g.,Mg2+ and Ca2+)inhibited TC adsorption on C@FONC;Alkali-metal cations(e.g.,Na+ and K+),Cl-,and NO3-had little impact on adsorption process;Coexisting SO42-and HPO42-anions could slightly enhance the adsorption capacity of TC on C@FONC.The adsorption process could be described by Freundlich isotherm and Elovich kinetic models.The adsorption process was spontaneous endothermic reaction,which was mainly carried out via chemical sorption mechanism,the positive ?H value was 63.15 k J·mol-1.The studies of the effect of natural organic matter demonstrated that C @FONC could maintain satisfactory TC adsorption capacity in various complex environmental water.After five cycles,the TC adsorption capacity still reached 52.7 mg/g.Under the conditions of an initial TC concentration of 150 ppm,p H 3.0,C@FONC dosage of 0.5 g/L,and H2O2 dosage of 5 m M,an optimal TC removal rate of 97.9% was obtained,the corresponding TOC removal rate and H2O2 consumption rate were 52.7% and 81.4%,respectively.The orders of factors influencing TC removal and iron leaching were “HRT>C@FONC>H2O2>p H” and “p H>C@FONC>HRT>H2O2”,respectively.Optimal operation parameters were obtained by response surface model(p H 3.0,C@FONC loading=0.49 g/L,H2O2=5.1 m M).With HRT of 3 h,the TC removal efficiency reached 85.8% after 9 cycles of use.Moreover,the C@FONC/H2O2 system could degrade TC efficiently over a wide range of p H(2-7).The C@FONC/H2O2 system achieved favorable effect for the mineralization of actual TC wastewater,the concentration of COD of the treated wastewater was below 50 mg/L.The C@FONC/H2O2 system could degrade TC efficiently over a wider range of p H than CFP,the treatment efficiency was close to PAC.Compared with that of the other heterogeneous catalysts,the system exhibited the highest TC degradation rate(5.5×10-3 mmol.g-1.min-1).The results demonstrated,TC degradation in the C@FONC/H2O2 system followed pseudo-first-order kinetics,and the TC removal efficiency reached 97% in 120 min(TC0=150 ppm).The activation energy was calculated as 24.9 k J.mol-1,implying that the apparent reaction rate was dominated by the rate of intrinsic chemical reactions on the oxide surface.O-containing functional groups on the carbon matrix changed significantly during the reaction,C-OH(285.4 e V)disappeared,C-O-C(286.4 e V)appeared,and O-C=O shifted,stable Fe 2p state were obtained.The electron spin resonance(ESR)analysis and free radical quenching experiments indicated that · OH was the primary ROS in the TC degradation process,in which HO2· was also involved.The enhancement factor(R=1.25)revealed a synergistic effect between PAC adsorption and heterogeneous Fenton-like processes.In total 20 intermediates were detected by UPLC-MS/MS,suggesting that TC was mainly decomposed via hydroxylation,dehydroxylation,demethylation,and amide bond cleavage.The intermediate toxicities during TC degradation were predicted via QSAR analysis calculated by the ECOSAR program.