Photochemical Synergistic Removal Of Hexavalent Chromium And Bisphenol A In Water

Heavy metals and organic contaminants are two kinds of primary pollutants in water.The main focus of attention was paid to develop effective methods to remove single pollutant in previous studies.However,the two kinds of contaminants usually coexist in water,so the combined water pollution caused by them has drawn increasing attention in recent years.Hexavalent chromium（Cr（VI））and bisphenol A（BPA）are the typical toxic heavy metal ion and endocrine disruptor chemical,respectively.Meanwhile,they are also two important risk substances in water and often selected as model pollutants to study the synergistic removal of heavy metals and organic pollutants.By now,the more effectively synergistic removal methods of Cr（VI）and BPA include adsorption,photocatalysis and photo-electrocatalysis.Nevertheless,the fabrication of absorbents,photocatalysts and photoelectrode materials is complicated and costly.Moreover,it is hard to recover them and thus easy to cause secondary pollution.Therefore,it is very important to develop green and highly effective methods/technologies for synergistic removal of Cr（VI）and BPA from water.In this paper,the synergistic removal of Cr（VI）and BPA in water under the irradiation of 185nm vacuum ultraviolet light（VUV）or in 254 nm ultraviolet light/persulfate（UV/PS）process were investigated.The effects of various factors on the synergistic removal were explored.According to the analysis of activate species and intermediates,the possible synergistic removal mechanism of Cr（VI）and BPA were proposed.The main results obtained are as follow:1.Study on the synergistic removal of Cr（VI）and BPA under the irradiation of VUV:（1）It was difficult to remove Cr（VI）under the irradiation of VUV,while after the addition of BPA,not only BPA was degraded effectively but also the removal of Cr（VI）was obviously promoted.（2）With the increase of the ratio of initial concentration of BPA and the initial concentration of Cr（VI）（[BPA]₀/[Cr（VI）]₀）,the degradation of BPA was inhibited,while too high or too low[BPA]₀/[Cr（VI）]₀is harmful to the removal of Cr（VI）.Moreover,the removal of Cr（VI）decreased apparently with the increase of solution p H,while it was beneficial to BPA degradation under the condition of strong acid or strong alkaline.The existence of Cl^-,SO₄^2-,HCO₃^-and NO₃^-had a negative impact on BPA degradation and Cr（VI）removal.（3）15 by-products of BPA were identified by LC-MS and GC-MS.The results of quenching experiments showed that hydrated electron(e^-_aq)was the predominant active species for the reduction of Cr（VI）and hydroxyl radical（·OH）was mainly responsible for the degradation of BPA.Under the coaction of the irradiation of VUV and·OH,BPA was gradually decomposed into small molecule compounds via bond broken,hydroxylation,oxidation and ring opening reactions.At the same time,Cr（VI）was reduced to Cr（III）.2.Study on the synergistic removal of Cr（VI）and BPA in UV/PS process:（1）In UV/PS process,it was inefficiently to remove Cr（VI）alone,while after mixed with BPA,it was found that BPA was effectively degraded and the removal of Cr（VI）was also significantly promoted.（2）The degradation of BPA was inhibited with the increasing of the ratio of initial concentration of BPA and the initial concentration of Cr（VI）（[BPA]₀/[Cr（VI）]₀）,while too high or too low[BPA]₀/[Cr（VI）]₀ratio was adverse to the removal of Cr（VI）.When the initial concentration of PS gradually increased from 0.1 mmol·L^-1 to 1.0 mmol·L^-1,the synergistic removal of Cr（VI）and BPA were all promoted.However,the removal of Cr（VI）was inhibited when the initial concentration of PS further increased to 1.5 mmol·L^-1.The effect of the initial solution p H on the synergistic removal of Cr（VI）and BPA was insignificant.In addition,the existence of Cl^-,SO₄^2-,HCO₃^-and NO₃^-had slightly impact on the degradation of BPA,but the presence of Cl^-,HCO₃^-and NO₃^-obviously inhibited the removal of Cr（VI）.（3）16 intermediates were detected by LC-MS.Both sulfate radical(SO₄^-·)and·OH were the dominant active radicals for BPA degradation.They made BPA gradually decomposed into small molecule compounds through bond broken,hydroxylation,hydrolysis,oxidation and ring opening.Meanwhile,Cr（VI）was transferred in to Cr（III）by some reducing substances produced during the degradation of BPA.