Efficiency And Mechanism On The Removal Of Organic Pollutants In Water By Advanced Oxidation Systems Enhanced With Biochar

Permanganate（KMn O₄）and ferrate,as two typical oxidants containing high-valence transition metals,have been widely employed to remove organic micropollutants in water.However,due to their selective oxidation property and secondary pollution with excessive dosage,traditional water treatment technologies with KMn O₄ and ferrate are necessary to be improved.Biochar is a kind of porous carbonmaterial.Due to its rich porous structure,abundant functional groups,and low cost,it has been used as a potential catalyst in advanced oxidation processes.This study investigated the effect of biochar on KMn O₄ and ferrate oxidation to degrade organic micropollutants.The reactive oxidative species involved in the KMn O₄/biochar and ferrate/biochar systems were respectively identified.After reaction with KMn O₄ or ferrate,the resulted Mn-loaded biochar and Fe-loaded biochar were investigated in catalytic ozonation of organic micropollutants.This would be a promising way to improve the reusability of biochar.It was found that biochar significantly enhanced the KMn O₄ oxidation of organic micropollutants.Sulfamethoxazole（SMX）,which is hardly oxidized by KMn O₄,was selected as the target organic compound to explore the effect of biochar on KMn O₄oxidation.Results demonstrated that with addition of biochar,97%of SMX was degraded by KMn O₄,and 58%of total organic carbon（TOC）was removed.Several influencing factors on SMX oxidation kinetics by KMn O₄/biochar system were evaluated,including biochar concentration,KMn O₄ concentration,reuse times of biochar and different water background.The inhibitory effect of complexing agent and ultraviolet-visible spectroscopy suggested that highly oxidative intermediate Mn species were generated by the interaction of KMn O₄ and biochar.After reaction with KMn O₄,it was found that oxidative groups on the biochar surface was increased,and surface area and micropore structure of biochar were increased.4-nitrophenol（4-NP）was selected as the representative of phenolic pollutants,and the oxidation efficiency of 4-NP by KMn O4/biochar was investigated.Results showed that KMn O4/biochar could effectively degrade 4-NP and reduce the biological toxicityof 4-NP and its oxidation products.By establishing the linear free ener g y relationship between KMn O₄/biochar system and phenolic compounds,it reflected the relationship between degradation rate of substituted phenols with substituents in this system.Oxidation products of triclosan（TCS）by KMn O₄,KMn O₄/biochar and Mn（III）systems were compared.It was found that Mn（III）was not responsible for the oxidation of TCS and formation of 2,4-DCP in the KMn O₄/biochar system.Further,we used methyl phenyl sulfoxide（PMSO）as the target compound and detected the yield of methyl phenyl sulfone（PMSO₂）.It was found that biochar accelerated the degradation kinetic of PMSO by KMn O₄ but had little effect on the yield of PMSO₂.Therefore,it were highly-reactive intermediates Mn（VI）/Mn（V）that played the dominant role in the KMn O₄/biochar system.Similar to permanganate,ferrate is also a high-valence transition metal oxidant.Based on the catalytic effect of biochar on KMn O₄,we further explored its effect on ferrate oxidation of organic micropollutants.With addition of biochar,oxidation rates of5 selected organic pollutants by ferrate were increased by 3～14 times,and TOC removal ratios were increased by 2.4～8 times.Through radical quenching experiment,electron paramagnetic resonance analysis,and probe compound（sulfoxide）oxidation experiment,it demonstrated that biochar could promote ferrate to decay into intermediate iron species（i.e.Fe（IV）/Fe（V））,and Fe（IV）/Fe（V）had a high oxidizing activity to degrade organic pollutants.When ferrate/biochar was applied in the treatment of authentic actual,the removal ratio of TOC was increased from 8.7%to 31.6%as the concentration of ferrate and biochar increasing from 20μM and 5 mg/L to 80μM and20 mg/L,and the generation of disinfection by-products（DBPs）was reduced by 9.2%～23.9%.After reaction with ferrate,reductive groups on biochar surface decreased,and the surface area and pore volume of biochar were expanded.Reuse experiment of biochar in ferrate system showed that the catalytic performance of biochar increased with the increase of reuse times,which may be attributed to the catalytic activity of iron oxides attached to its surface.Therefore,we speculated that Mn-loaded（Mn Ox/biochar）recovered from KMn O₄ system and Fe-loaded biochar（Fe Ox/biochar）recovered from ferrate system may have some new catalytic properties due to the loading of manganese oxides or iron oxides,and can be used to activate ozone to degrade organic micro-pollutants.In this study,O₃ alone（2.5mg/L）eliminated 48%of atrazine（ATZ,5μM）within 30 min at p H 7.0.In comparison,the addition of Mn Ox/biochar and Fe Ox/biochar（20 mg/L）could increased the ozonation efficiency of ATZ to 83%and 100%,respectively.Quenching experiment and electron paramagnetic resonance analysis confirmed that hydroxyl radical（·OH）was the dominant oxidant.Lewis acid sites on Mn Ox/biochar and Fe Ox/biochar were active sites to adsorb ozone and promote the decomposition of ozone into reactive oxygen species.Moreover,redox pairs of metal oxides on Mn Ox/biochar and Fe Ox/biochar also played important roles in ozone decomposition.Degradation products of ATZ by Mn Ox/biochar catalyzed ozonation and Fe Ox/biochar catalyzed ozonation were identified,and possible degradation pathways were proposed.Compared with unactivated ozonation,it was also found that these two catalytic systems had better dechlorination effects on ATZ and could significantly reduce the biological toxicity of ATZ and its oxidation products.