Generative Mechanism Of Chlorinated Products Of PAHs In Textile Dyeing Sludge Degraded By Fenton Or Persulfate

According to the data of China Bureau of Statistics in 2018,the textile industry in China discharged about 1.90 billion tons of wastewater,ranking third in the industrial sector,and produced about 19 million tons of textile dyeing sludge(with a water content of 80%).The composition of textile dyeing sludge is complex,with more than 750 kinds of organic matter,such as polycyclic aromatic hydrocarbons(PAHs),aromatic amines,chlorobenzene,benzene,and dioxin.Textile dyeing sludge is difficult to be dewatered,so Fenton(Fe2+/H2O2)and other chemical conditioning are often used to break the extracellular polymers in the sludge,and then the sludge is dewatered and dried,and finally burned or landfilled.Source control of chlorinated organic substances and other key substances in the conditioning process of textile dyeing sludge is the key to avoid dioxin generation in the subsequent incomplete incineration process or secondary pollution in other disposal processes.Advanced oxidation processes(such as Fe2+/H2O2,Fenton-like,persulfate)have great potential to convert most organic pollutants into smaller compounds,but Cl-in textile dyeing sludge system can react with hydroxyl radical(·OH)or sulfate radical(SO4-)to form free chlorine(Cl2,HClO)and selective free radical reactive chlorine(·Cl,Cl2-,·ClO),which may lead to the formation of more chlorinated organic matters.In order to explore the potential hazards of Fe2+/H2O2,Fenton-like and Fe2+/peroxymonosulfate(Fe2+/PMS)systems in textile dyeing sludge treatment,this study used simulated sludge system to explore the action mechanism of free radical reactive chlorine on PAHs,and evaluated the efficiencies and potential risks of oxidation system in textile dyeing sludge treatment from various aspects.The main research results are as follows:(1)The main PAHs(phenanthrene,anthracene and fluoranthene)in textile dyeing sludge were taken as research objects,and the treatment of PAHs in simulated sludge by the Fe2+/H2O2 and Fe2+/PMS system was evaluated from the aspects of PAHs removal efficiency,element migration,Cl-influence and chlorination product formation.According to PAHs removal efficiency,the optimal dose of PMS in Fe2+/PMS system(?3 PAHs removal efficiency=64.66±2.82%)was 1/30 of the optimal dose of hydrogen peroxide in Fe2+/H2O2 system(?3 PAHs removal efficiency=78.63±0.38%).The content of elements(Fe,S,Ca)in simulated sludge was mainly affected by the degree of advanced oxidation and the amount of iron flocs produced.By studying PAHs removal,free chlorine formation,total organic chlorine content and degradation products in the Fe2+/H2O2 and Fe2+/PMS systems,it was found that ·Cl might have higher selective reaction to phenanthrene and fluoranthene,while Cl2-had the opposite.In the Fe2+/H2O2/Cl-system,PAHs were oxidized and degraded by·Cl and OH.In the Fe2+/PMS/Cl-system,PAHs and their products were chlorinated by free chlorine and·C1 to form six chlorinated products(trans-1,2-dichlorocyclopentane,2-chlorocyclopentanol,9-chlorophenanthrene,2-chloroanthracene,9,10-dichloroanthracene,3-chloro-fluoranthene).(2)The effects of Cl-concentration and pH on the removal of PAHs(phenanthrene,anthracene and fluoranthene)from simulated sludge by the Fe2+/H2O2/Cl-system were investigated.The steady-state concentrations of ·OH,·Cl and CL2·-([·OH]ss,[·Cl]ss,[Cl2·-]ss)in heterogeneous system were determined by tert-butyl alcohol combined with nitrobenzene and benzoic acid for the first time.The outcomes exhibited that increasing the Cl-content from 50 to 2000 mg/L(pH=3.0)or raising the pH from 3.0 to 5.0(1000 mg/L Cl-)caused[·OH]ss to continuously decrease and[Cl2-]ss and the concentration of superoxide anions(HO2·/O2·-)to continuously increase.When the pH was 3.0 and the Cl-concentration was 1000 mg/L,[·Cl]ss had a maximum value of 9.27x10-14 M.Combined with the results of PAHs removal,radical quenching and product analysis,it was confirmed that·Cl in the Fe2+/H2O2/Cl-system could promote the oxidative degradation of phenantnrene without forming chlorinated products,but phenanthrene was degraded to form more.toxic 2,4-di-tert-butylphenol under the catalysis of FeCl3.In addition,HO2·/O2·-was helpful in removing anthracene and fluoranthene.Under the environment of high Cl-content(?1000 mg/L),PAHs could be removed more effectively by using HO2·/O2·-.(3)Treatment of actual textile dyeing sludge by the Fe2+/H2O2 and Fenton-like system was evaluated from the aspects of Cl-influence,8 PAHs removal,organic carbon removal and migration,and organic halogen removal and formation.The results showed that the organic halogen in textile dyeing sludge was mainly organic chlorine(AOCl),and accumulated in solid phase(0.30 mg/g AOCl).In the Fe2+/H2O2 system with 500 mg/L Cl-,strong acid and weak oxidation ability,the removal of phenanthrene was promoted due to the action of ·Cl,and AOCl in the solid phase of textile dyeing sludge increased'to 0.55 mg/g.According to infrared spectrum analysis,it was found that·Cl could chlorinate aromatic and aliphatic organic compounds(except PAHs)in the sludge solid phase at the same time,and eventually led to the accumulation of aromatic chlorides in the sludge solid phase.Strengthening the oxidation ability of Fe2+/H2O2 system increased the formation of organic chlorine in textile dyeing sludge liquid phase and solid phase.In weak acid environment,the oxidation and desorption of HO2·/O2·-promoted the removal and migration of PAHs and organic carbon in the solid phase of textile dyeing sludge,and reduced the formation of total organic chlorine.The Fenton-like system(zero-valent iron/ethylenediaminetetraacetic acid/air:ZEA)dominated by non-·OH could realize the complete mineralization removal of PAHs,organic carbon and organic chlorine instead of migration.(4)Six kinds of simulated sludge were prepared with different flocculants and inorganic salts,and the effects of inorganic salts(silicate,sulfate,phosphate,hydroxide and iron salt)on the adsorption of PAHs by sludge and the removal of 16 PAHs by ultrasonic combined with ZEA(US/ZEA)system were studied by using the simulated system.The results showed that the simulated sludge solid phase containing inorganic salts had larger surface area,which was beneficial to the adsorption of PAHs.Inorganic salts could make more low-ring PAHs adsorbed on the surface of simulated sludge solid phase,which was beneficial to the degradation of low-ring PAHs by the US/ZEA system.The removal of PAHs increased by 15.37%and 11.19%,respectively,due to the existence of silicate and phosphate.Silicate and phosphate promoted the formation of OH.The reaction of Mg(OH)2 with Fe3+-(ethylenediaminetetraacetic acid)chelate([Fe?(EDTA)]),the reaction of iron salt with EDTA,and the corrosion inhibition of SO42-on zero-valent iron inhibited the formation of OH in the US/ZEA system.Compared with Fe2+/PMS system,Fe2+/H2O2 was friendlier to degrade PAHs in chlorine-containing sludge.However,Fe2+/H2O2 conditioning would lead to the accumulation of iron,sulfur and aromatic chlorides on the solid phase of textile dyeing sludge,which had certain environmental risks.Fenton-like system dominated by non-·OH and SO4·-can be more friendly to conditioning textile dyeing sludge.This paper provides a scientific basis for the subsequent utilization of free radical reactive chlorine and HO2·/O2·-,the selection of sludge conditioning methods and the overall environmental risk reduction of sludge treatment.