Mechanism Of Rice Husk Derived Biochar Performing In Reducing Cr(Ⅵ) And Oxidizing Organic Pollutants

In the present study,detoxifying the stubborn system of Cr（Ⅵ）coexisting with organic pollutants was conducted using biochar via mediating its redox components.Concurrently,the principle of biochar reducing Cr（Ⅵ）and oxidizing organic pollutants was illustrated.Firstly,it was explored the properties of biochar altering with pyrolysis temperatures.Increasing pyrolysis temperature accomplished great contents of C and Si on biochar,along with the decline in N,O,and H on biochar.As the charring temperature was lifted,the ratio of C/H,as the indicator of biochar aromatization,was raised,meanwhile declined in the ratio of（N+O）/C,reflecting the polar intensity and amount of oxygen-containing functional groups on biochar.Furthermore,a decreased trend with lifting charring temperature was obtained for-OH,-CH₂/CH₃,and-COOH/COO^-via FTIR and Beohm titration.Besides,biochar under high pyrolysis temperature possessed great specific surface area and developed pore structure.Secondly,three kinds of biochar were used to decontaminate Cr（Ⅵ）.It was found that acidic conditions benefited the Cr（Ⅵ）removal.Biochar with high charring temperature functioned excellently on Cr（Ⅵ）removal.Especially,10 g/L R550,preparation from rice husk charring under 550℃,could decontaminate 84.4%of Cr（Ⅵ）（50 mg/L）.The addition of NO₃^-,SO₄^2-,PO₄^3-at a high concentration of 10 m M just alleviated the Cr（Ⅵ）removal efficiency less than 10%,demonstrating the slight role of adsorption.Meanwhile,the observation about Cr transformation upon eliminating the reaction indicated that most of the disappeared Cr（Ⅵ）was transformed to Cr（Ⅲ）precipitation on biochar.Thus,Cr（Ⅵ）removal was mainly caused by the reduction rather than the sorption.The change of functional groups of biochar upon the Cr（Ⅵ）removal indicated both hydroquinone functional groups（phenolic hydroxyl）and semi-quinone persistent radicals on biochar contributed to the Cr（Ⅵ）reduction.Thirdly,the potential of biochar in oxidizing organic pollutants and its aging process was investigated.It was found that 10 g/L of biochar could decontaminate 78.7%of benzoic acid within 360 min under pH 3,and evaluating to 95.2%as ultraviolet（UV）presented.The experiments about quenching free radicals and purging N₂ confirmed the contribution of O₂,O₂^·-,and·OH to benzoic acid degradation.The experiments of capturing reactive oxygen species demonstrated the production of·OH and H₂O₂.Besides,UV boosted the production of·OH and ¹O₂.Upon degrading benzoic acid,the change of functional groups demonstrated that the conversion of phenolic hydroxyl groups to semi-quinone free radicals,C=O,and carboxyl groups on biochar,which triggered the O₂ activation and the serial production of O₂^·-,H₂O₂ and·OH.The importation of UV facilitated the electron transfer between biochar and O₂,thus facilitating the production of·OH and degradation of benzoic acid.Meanwhile,quinone-like structures（C=O）on biochar boosted the generation of ¹O₂ via energy transfer among biochar,UV,and O₂,which favored the degradation of benzoic acid.Oxidization treatment using gradient concentrations of H₂O₂was employed to accelerate the aging process of biochar.As the aging marched,the biochar,with the elimination of reductive functional groups,possessed sluggishly in·OH production from O₂ activation and the radical degradation of organic pollutants.Fourthly,the application of biochar in the concurrent decontamination of Cr（Ⅵ）and organic pollutants was explored.It was found that 10 g/L of R550 acted effectively on the synchronous decontamination of 1 m M benzoic acid and Cr（Ⅵ）（0～1 m M）.Meanwhile,Cr（Ⅵ）affected the degradation efficiency of benzoic acid via the dual roles of biochar.As the Cr（Ⅵ）concentration raised from 0 to 0.2 m M,the reaction rate constant of removing benzoic acid within 7 h increased from 0.2575 to 0.3432 h^-1,and further declined to 0.0793 h^-1 with the Cr（Ⅵ）concentration evaluating to 1 m M.Free radicals quenching experiments indicated that benzoic acid was mainly degraded by·OH from R550 activating O₂ with low content of Cr（Ⅵ）coexisted;additionally,low content of benzoic acid was oxidized by Cr（Ⅵ）via the mediation of biochar.Increasing Cr（Ⅵ）content gradually inhibited the radical degradation of benzoic acid but compensating for the non-radical oxidization of benzoic acid.The presence of benzoic acid retarded the sorption of Cr（Ⅵ）by biochar,but could reduce Cr（Ⅵ）via the mediation of biochar,finally resulting in a slight change in Cr（Ⅵ）reduction.The defective sites on biochar were confirmed as active sites to promote the reaction between Cr（Ⅵ）and organic pollutants.Furthermore,organic pollutants with the stronger capacity of donating electron were more likely to react with Cr（Ⅵ）in the presence of biochar.Fifthly,the cooperation of biochar and sulfite was utilized in the concurrent decontamination of Cr（Ⅵ）and organic pollutants at neutral conditions.In the system containing 0.2 m M Cr（Ⅵ）,1 g/L of R550,and 0.04 m M p-nitrophenol under neutral conditions,the addition of 5 m M sulfite had the degradation reaction rate constant of p-nitrophenol within 30 min increase to 0.0272 min^-1 from 0.0036 min^-1 in the solo S（IV）-Cr（Ⅵ）system and 0.0017 min^-1 in the R550 system.The experiments quenching free radicals indicated that both SO₄^·-and·OH contributed to the degradation of p-nitrophenol in the system of R550 coexisting with S（IV）-Cr（Ⅵ）,which was confirmed by the detection of the stable complexes of DMPO and free radicals by Electron Paramagnetic Resonance.By analyzing the change of biochar functional groups upon completing the reaction,the carboxyl on biochar triggered these enhancements.As a Lewis acidic site,the carboxyl on biochar enhanced the reactivity of Cr（Ⅵ）via coordination,which did not only facilitate the performance of Cr（Ⅵ）on sulfite activation but also improved the conversion efficiency of SO₃^2-to SO₄^·-via alleviating the unnecessary transformation of SO₃^2-to SO₄^2-.