Modified Biochars For Heavy Metal And Organic Pollutant Removal: Performance And Mechanism

Biomass waste was a sort of renewable,wide source,and low cost carbon source.Biochar derived from biomass pyrolysis exhibited great potential in adsorption,catalysis and electrocatalysis process during wastewater treatment.However,the original biochar manifests low surface area,poor pore structure,unideal surface functionilites and unfunctional inorganic compositions,which results in poor performance in the above applications.This work realized the modifications of biochar by pore structure trailing,surface functionalization and inorganic compounds loading and improved their performance in heavy metal ions adsorption,persulfate activation for pollutant degradation,and electrocatalytic oxygen reduction to hydrogen peroxide.The influences of the modification procedures on the biochar structure were characterized,the mechanism of the performance improvement were excluded,the ineractions between the pollutant,oxidant and modified biochar were analyzed.The influence factors of biochar based water treatment technologies in real applications were also considered.This work provided new insights and theoretical supports for biochar modification and their application in pollutant removal in wastewater.At first,the KMn O₄ modification method was adopted to increase the adsorption quantity for Pb(?)and Cd(?)and stabilize the magnetization of magnetic biochar.The modified magnetic biochar manifested nearly 7 times of the unmodified ones in Pb(?)and Cd(?)adsorption with 0.7mmol/g for both.The successful loading of manganese oxides,the inceased content of surface functional groups all contributed to the adsorption performance improvement.The adsorption performance of modified magnetic biochar would drop at low p Hs,which couldn't be influenced by ionic strength and dissolved organic matters.The adsorption capacities of modified magnetic biochar could maintain over 50%and 87%after four cycles for Pb(?)and Cd(?),respectively.Then,the CO₂ activation method was adopted to improve the catalytic performance in persulfate based advanced oxidation process for phenolic pollutant degradation.After the CO₂modification,the surface area increased to 2185 m²/g with a uniform microporous structure,the surface oxygen content decreased with 9.6%,the ration of C-O/C=O increased from 0.13 to 0.56.The best catalytic performance of modified biochar came at the temperature of 950°C,with an oxidation ability of 0.5 mol/mol oxidant/h/g,which outperformed in biochar based materials and is similar with nanocarbons.The singlet oxygen generation and electrons transfer pathway were both verified through various methods to exclude the working mechanism of biochar/persulfate system.The system would be influenced by HCO₃^-and HPO₄^2-.The chlorophenols would be degraded fast in such system which followed the order:CP>DCP>TCP?phenol.The para-chlorine substituent was the first to degrade to form quinone like compounds.The high temperature pyrolysis(700°C)would recover the catalytic performance of 94%.What's more,by adding Zn Cl₂ during carbonization,the persulfate activation performance of biochar from low temperature carbonization was also improved.The biochar/persulfate system was efficenct for antibiotics degradation.The hirerarchical porous structure was successfully controlled by adjust Zn Cl₂ dosage.As the Zn Cl₂blending ratio increased,the defect degree and the Zeta potential of biochars would increased,while the persistent free radicals of biochar would decreased.The best catalytic performance of modified biochars came at the blending ration of 3due to the highest ketone groups content.The linear relationship analysis also confirmed the important role of defect degree,the mesopore volume and Zeta potential in persulfate adsorption quantity on biochars.Singlet oxygen is verified as the primary reactive oxygen species,which attack the-NH₂ group aligned to the benzene ring or-SO₂ group in SMX molecule.The biochar catalysts are filled in hydrogel beads and packed in a flow-through packed-bed column which yields a high removal efficiency of over 86%for 8 h without declined in efficiency.Finally,the best electrocatalytic oxygen reduction to hydrogen peroxide performance of biochars were obtained by optimizing the carbonization temperature.The biochar based electro-fenton system was also fabricated for degradation of various organic pollutant,such as phenolic compounds,antibiotics and dyes.The Zn Cl₂ activation method brought hierarchical porous structure with micropores and mesopores to the biochars with a high surface area of 2000 m²/g.As the carbonization temperature increased from 400 to 700°C,the surface oxygen functionities distribution was trailed with a decreased ration of C-O/C=O,the surface defect degree and graphitization degree increased.The linear relationship analysis showed that the selectivity of hydrogen peroxide was controlled by the ratio of C-O/C=O,while the electrocatalytic oxygen reduction performance was more influenced by defect and graphitization degree.Zn BC-550 was the best cathode material with a high ORR activity without compromise in H₂O₂ selectivity;a high production rate of H₂O₂(796.1 mg/g/h)was attained at-0.25V vs RHE at p H of 1,which outperformed through amorphous carbon materials.The electrochemical stability of the modified biochar could maintain over 8 h and the degradation ability could maintain over 81%after 10 runs.This work provided therotical insights and technical support for biochar modification and their application in adsorption,catalysis and electrocatalysis during wastewater purification.