Performance And Mechanisms Of Pollutants Removal From Water Using As-prepared Sludge Biochar

Sludge is seriously harmful to the environment due to the presence of a large amount of heavy metals,organic pollutants and various pathogens.Sludge treatment has become a research hotspot in recent years.Due to the poor dewatering performance and transportation difficulties of the anaerobic digestion sludge,zero-valent iron activated persulfate(PDS)is used to improve sludge dewaterability.However,the sludge residue,whether activated sludge,anaerobic digestion sludge or dewatered sludge,still poses hazard to the environment.Therefore,in this study,the sludge residue is utilized to produce waste activated sludge biochar(WASBC),anaerobic digestion sludge biochar(ADSBC)and dewatered sludge biochar(Fe-ADSBC)by thermochemical conversion process,and the changes in surface structure,physical and chemical characteristics of biochar is explored.In addition,their adsorption capacity for pollutants,as well as the performance of activated PDS to degrade organic matter and the removal mechanism is analyzed in detail.It was found that when the pyrolysis temperature of the sludge is in the range of 400 ? to 1000 ?,the ADSBC prepared at a relatively low temperature of 600 ? has more organic functional groups,and a better heavy metals adsorption effect,while the ADSBC and Fe-ADSBC prepared at a relatively high temperature possess higher graphity and more Fe in multiple forms,and have better performance in activating PDS to degrade organics.The main research contents and results are as follows:The properties of WASBC and ADSBC derived from waste activated sludge and anaerobic digestion sludge under pyrolysis temperature varying from 400 ? to 800 ? are investigated.The surface structure and physicochemical properties of materials are analyzed via various characterization methods,and the adsorption performance of WASBC and ADSBC for various heavy metals are studied.The experimental results showed that with the increase of pyrolysis temperature,the graphitization degree,stability and oxidation resistance of sludge biochar materials were improved.Among WASBC and ADSBC at different pyrolysis temperatures,the biochar(ADSBC 600)produced at 600 ? has the best adsorption effect for a variety of heavy metals,among which Pb2+ shows the highest removal rate.The Pb2+ adsorption kinetics and isotherm for ADSBC 600 can be well described using the pseudo second-order model and Langmuir isotherm,respectively.Analysis of the biochar characteristics before and after metal treatment suggests that electrostatic attraction,surface complexation and ion exchange are the main Pb2+ removal mechanisms.Biochar catalysts are produced via pyrolysis of anaerobic digestion sludge as effective PDS activators.The biochar(ADSBC 1000)produced at 1000 ? shows great stability and low biotoxicity due to the limited metals leaching and eliminated persistent free radicals,as well as dissolved organic matter.In addition,ADSBC possesses large specific surface areas,a high degree of graphitization and good conductivity,which can be used for effectively oxidizing various pollutants including dyes,estrogens and sulfonamides with PDS in broad p H and temperature ranges.It is showed that PDS activated by ADSBC pyrolyzed at 1000 ? can completely remove sulfathiazole(STZ)in 90 min,while ADSBC pyrolyzed at 400 ? can only remove 20.25% of STZ.The mechanism of the ADSBCs/PDS system is critically discussed via selectively radical screening tests,solvent exchange,selectivity to pollutants and electrochemical analysis.These results reveal that the organics are decomposed by a nonradical pathway via electron transfer rather than free radicals or singlet oxygen.A peroxydisulfate/zero-valent iron(PDS-ZVI)system is proposed to destroy proteins in soluble extracellular polymeric substances and loosely bound EPS in anaerobic digested sludge to improve the sludge dewaterability.Moreover,anaerobic digestion sludge derived biochars supported via iron oxides(Fe-ADSBC)are generated by dewatering and pyrolysis.Intriguingly,the iron species are discovered to gradually transform from Fe3O4 to Fe O with the increasing pyrolysis temperatures from 600 oC to 1000 oC.The removal rates of sulfamethazine(SMT)in PDS activated by Fe-ADSBC prepared at 600 and 800 oC were only 33.3 and 61.78% within 90 minutes,while Fe-ADSBC prepared at 1000 oC could completely remove SMT within 60 minutes.The in situ radical scavenging and capturing tests revealed that the principal reactive oxygen species in Fe-ADSBC/PDS system experienced a variation from hydroxyl radical into sulfate radical at higher pyrolysis temperature(1000 oC).In addition,the carbonaceous ADSBC can promote the catalytic activity of iron oxides by synergistically facilitating the adsorption of reactants and charge transfer through C-O-Fe bonds at the interfaces.This study enables insight into the properties and catalytic performance of Fe-ADSBC,meanwhile unveils the mechanism,reaction pathways,and environmental impacts of the ultimate transformation products from SMT degradation in the Fe-ADSBC/PDS system.Therefore,the preparation of waste sludge as biochar material not only can effectively treat the sludge,but also can be used as a low-cost and environmentally friendly material for the removal of pollutants,providing a promising way for the recycling of sludge waste.