The Photodegradation Mechanism Of Rhodamine B Mediated By Rice Straw Biochars

Biochar is a carbon-rich particle derived from biomass after thermal pyrolysis under an under oxygen-limited conditions.It has attracted increasing attentions in contaminant removal because of its large specific surface area,rich variety and abundant content of surface functional groups.Besides,the detected environmentally persistent free radicals(EPFRs)signal on biochar particles prepared at 300 to 700°C can also contributed to the removal of organic contaminants.However,what kind of mechanism driven the superiority performance of biochar is still debatable,especially for the organic contaminants.Recently,the intermediate products were detected in the biocha-contaminant system frequently.This phenomenon indicated the co-existence of adsorption and degradation in the removal organics.Compared with the adsorption process,the degradation process is more complicated since containing a variety of chemical conversion process,in addition,there are no unified conclusion of mechanisms for the organic contaminant degradation by biochars in the previous published literatures.Furthermore,the uncertainty of the degradation pathway will inevitably bring some difficulty for risk evaluation of organic and biochar application.In the degradation of organics by biochar,the role of EPFRs in biochar has initially attracted the attention of researchers.EPFRs can not only directly oxidize organic contaminants,but also induce the generation of hydroxyl radicals(·OH)in the liquid phase and degrade organics indirectly.However,on obvious correlation was observed between the degradation of organics and the signal intensity of EPFRs.We thus speculated that the degradation of organics mediated by biochar may drive by other pathways besides EPFRs.Therefore,the biochar-rhodamine B(RhB)system was built to investigate the degradation mechanism as well as the role of surface characteristics of biochar.The main content is divided into the following points:(1)The biochars were produced from a common agricultural waste called rice straw under different pyrolysis temperatues of 200??500?and 1000?,and then the degradation mechanism on RhB involving reactive oxygen species(ROS)produced by the interaction of dissolved organic matter(DOM)of the biochar prepared at 200?with EPFRs under photocatalytic conditions was investigated.The biochar-RhB system firstly reacted under dark conditions for 72 h,and then the resulting solution was devided into two groups as controls.One part continued to react under dark conditions for 72 h,and the other part reacted under light conditions for 72 h.The effect of light on the adsorption-degradation of biochar on RhB was explored by cmparing the removal rate of RhB and the amount of adsorption-degradation under the two conditions after the reaction.In addition,the hydroxyl quencher-tert-butanol(TBA)was added to the above experimental system to eliminate the contribution of·OH to the degradation of RhB.The degradation mechanism was pointed to the process that superoxide anion radical(O₂·^-)generated by the DOM and EPFRs under light induction promoted the degradation of RhB.(2)In order to further clarify the mechanism of biochar particle mediating photodegradation of RhB in biochar-RhB system and distinguish the effect of dissolved organic matter(DOM)in the degradation mechanism.In the follow-up experiments,raw materials and biochar particles prepared at 200??500??1000?were simply washed to remove the soluble organic components on the surface,and the biochars were labeled as washed and non washed particles to compare the difference of RhB degradation.The results showed that compared with the dark reaction conditions,the addition of UV light increased the degradation amount of RhB by 3 times,and the highest degradation rate of biochar on RhB was up to 10times by water washing treatment.The highest increase of photodegradation was observed at 1000?(no obvious EPFRs signal)biochar particle,which directly excluded the main role of EPFRs in the degradation of biochar-RhB system.The biochar supernatant refill experiment inhibited the photodegradation degree of RhB,which indicated that DOM did not significantly promote the photodegradation of RhB.Therefore,the mechanism of RhB photodegradation was mainly related to solid particles.However,the reaction results of RhB with single graphite structure and solid particles after demineralization showed that pure graphite and inorganic minerals would not degrade RhB.Moreover,HNO₃modified biochar particles inhibit the degradation of RhB.Combined with XPS and FTIR analysis,we believe that quinoid C=O in the oxygen-containing structure of biochar may play an important role in the photodegradation of RhB.(3)Previous experiments showed that the oxygen-containing structure of biochar played an important role in the photodegradation of RhB.In order to further distinguish and quantify the role of oxygen-containing functional groups in the degradation,the oxidants HNO₃,H₂O₂and strong reductant KBH₄were used to pretreat the biochar at four temperatures.Batch adsorption and degradation experiments were carried out under UV irradiation,and the pH value in the reaction process was adjusted to explore the effect of oxygen-containing functional groups on the degradation at different temperatures.The results showed the effect on RhB degradation by biochar prepared at high temperature and low temperature with the same modification methodis opposite.The degradation of RhB by low-temperature biochar(protoplasm,200°C)decreased with the increase of pH while the degradation of RhB by high-temperature biochar(500,1000°C)increased with the increase of pH,which indicated that the mechanism of RhB degradation of biochar prepared at high temperature and low temperature may be different.The degradation of RhB by low-temperature biochar accelerated the degradation of RhB mainly through the combination of oxygen-containing free radicals and H⁺.The degradation of high-temperature biochar was not only related to the oxygen-containing structure,but its graphitized structure also contributed to the degradation of RhB.After characterization and analysis of biochar at four temperatures before and after modification,it is found that the active site of low temperature biochar is phenolic hydroxyl group,which is more conducive to the degradation of RhB under acidic conditions,while the reactive activity of high temperature biochar(preparation temperature is higher than 500?)is more conducive to the degradation of RhB under neutral or alkaline conditions.Based on controling reaction conditions(gas atmosphere,light and pH value) and comparing the difference results of the adsorption and degradation of RhB by the biochar prepared at different temperatures,this study explored the role of DOM and oxygen-containing structures on the surface(such as Phenolic and Quinoid)except EPFRs in the degradation process.The results show that the role of EPFRs and DOM in the degradation of RhB was limited and the degradation of RhB was closely related to the introduction of biochar particles.In addition,the oxygen-containing structures of phenolic-OH and quinoid C=O played an important role in the degradation of contaminants and the activity of phenolic-OH and quinoid C=O is affected by pH value.Under acidic and neutral conditions,it was more conducive for low-temperature biochar to adsorb and degrade pollutants,while alkaline conditions were more conducive to high-temperature biochar to adsorb and degrade pollutants.