Removal Of Tetracycline In Water Using Peroxodisulfate Activated By Biochars And N-Doped Biochar From Paper Mill Sludge

The large consumption of tetracycline（TC）had polluted the water environment in China.TC is difficult to degrade naturally in the environment and remains for a long time,which will induce the production of resistant microorganisms and resistant genes,and eventually will pose a huge threat to human health.Advanced oxidation technology based on persulfate（PS）is a potential method for efficient and rapid degradation of TC residues in water because of its strong oxidation activity.The choice of catalyst is the key in persulfate oxidation technology.Biochar is the best choice for non-metallic catalysts because of its wide source,green environment protection,large specific surface area and rich surface functional group.In view of the pollution status of TC,the paper mill sludge was used as raw material to prepare biochars with the idea of“using waste to treat waste”,and then the catalysts will be used for activating persulfate to remove tetracycline（TC）from water.The paper mill sludge biochars（SBC）were prepared at different pyrolysis temperatures,and its chemical composition,surface morphology and functional groups were characterized.On the basis of exploring the adsorption characteristics of SBC for TC,the catalytic characteristics,catalytic mechanism and the effects of experimental conditions on the removal of TC were studied;the stability of the catalysts were evaluated by reuse experiments.In order to further improve the catalytic properties,the biochar was modified by doping nitrogen atoms.Urea was selected as the additional nitrogen source to study the effect of nitrogen on the adsorption performance and catalytic activity of biochar.The main research contents are as follows:（1）SBC300,SBC500 and SBC700 were prepared by oxygen limited pyrolysis at 300,500 and 700℃respectively.With the increased of pyrolysis temperature,the specific surface area of SBC increased,but the species and quantities of oxygen-containing functional groups decreased.With the increased of pyrolysis temperature,the adsorption performances of SBC were better.Further study found that the adsorption process of SBC to TC were mainly chemical adsorption,including strong electrostatic interaction or ion exchange interaction.The correlation coefficient R²of Langmuir model was higher than Freundlich equation and Temkin equation,which was indicated that the adsorption of TC by SBC were monolayer uniform adsorption,the fitting results showed that the saturated adsorption capacity of TC by SBC300,SBC500 and SBC700 were 112.9,122.0 and 125.2 mg/g,respectively.The thermodynamic study showed that the adsorption process were spontaneous and endothermic.SBC could effectively activate PDS to degrade TC,among these,SBC700/PDS system could achieve 90.2%removal rate in 10 h,followed by SBC500 with85.7%,and SBC300 had the worst catalytic activity with 78.9%Through the free radical quenching experiments,it was found that although SO₄^-·and·OH participated in the oxidation degradation process of TC,they were not the dominant factors.The addition of L-histidine significantly inhibited the oxidative degradation of TC in SBC/PDS systems,which proved that singlet oxygen（¹O₂）played an important role in the mixed systems.Electrochemical test showed that there were electron transfer process in SBC/PDS systems.The oxidative degradation of TC in SBC/PDS systems were mainly realized by non-radical pathway.The poor catalytic performance of SBC prepared at low temperature was due to its small specific surface area and poor electron transport ability.The increased of SBC and PDS could improve the removal efficiency of TC in different degrees;the temperature had little effects on the removal efficiency.The effects of p H value was obvious,and acid condition could promote the oxidative degradation process;alkaline environment wouldl inhibit the oxidative degradation process.The results of reusability test showed that SBC had good stabilities.（2）N-doped biochar NSBC500 was prepared by using paper sludge as raw material and urea as additional nitrogen source.The results of elemental composition and X-ray photoelectron spectroscopy showed that nitrogen was successfully introduced into NSBC500.The contents of pyridine nitrogen,pyrrole nitrogen,graphite nitrogen and nitrogen oxides were 37.0%,50.3%,7.3%and 5.4%,respectively.After 6 h adsorption reaction,the removal rate of TC in solution of NSBC500 could reach 66.2%.The adsorption process of NSBC500for TC was mainly chemical adsorption,and the adsorption rate was affected by solution concentration,molecular size and other factors.In addition,the correlation coefficient R²of Freundlich model was higher than that of other equations,which indicated that the adsorption process was multi-layer heterogeneous adsorption.the thermodynamic results showed that the adsorption process was spontaneous and endothermic.NSBC500 could effectively activate PDS to degrade TC,and the removal rate was 91.2%in about 14 h.The results of free radical quenching experiments showed that SO₄^-·and·OH were produced in NSBC500/PDS system,but they were not the main active species.The introduction of L-histidine greatly inhibited the oxidative degradation of TC,indicated that ¹O₂was one of the main ways of oxidative degradation of TC.The electrochemical results showed that the electron transfer process was another non-radical pathway for the oxidative degradation of TC in NSBC500/PDS system.Therefore,the oxidative degradation of TC in NSBC500/PDS system was mainly realized through non-radical pathway.When the p H value of solution was 3,the concentration of NSBC500 was 1.0 g/L,and the molar ratio of PDS to TC was 100:1,the removal rate of TC was the highest,up to 95.2%.Nsbc500 also has good stability.