Effect Mechanisms Of PH On The Advanced Oxidative Degradation Of Typical Aromatic Acids And Bases In Water

Aromatic contaminants(ACs)have attracted special attention due to their high toxicity,persistence,bioaccumulation and low biodegradability,which can disrupt ecological balance and endanger animal and human health.Advanced oxidation processes(AOPs)are promising treatment processes for the removal of organic pollutants,which can effectively degrade ACs in water.UV/H2O2,UV/chlorine and Solar/chlorine processes degrade organic pollutants by generating various free radical species(such as HO·,Cl·,CIO·,Br·,BrO· and etc.).The pH is one of the most important environmental parameters affecting the removal efficiency of pollutants in AOPs.The pH can not only change the dissociation configurations of aromatic acids and bases(AABs)and affect their chemical activities,but also change the concentrations of different radical species and their relative distributions.However,experimental studies often only focus on the change of free radical concentration,and ignore the differences in activities between different dissociated forms.Therefore.it is of great significance to study the degradation reactions of different dissociated forms initiated by different free radicals.Quantum chemical calculations can explore the differences of the reactivities,degradation mechanisms and kinetic properties between different dissociated forms at the molecular level,which is complementary and instructive to experiments.In this paper,dozens of typical AABs are selected as research objects,including syringol,syringaldehyde,p-aminobenzoic acid,phydroxybenzoic acid,p-aminophenol,phenol,aniline,benzoic acid,chlorophenols and halogenated pyridinols.The pH-dependent degradation of AABs in UV/H2O2,UV/chlorine and Solar/chlorine processes are studied by multiple computational chemical simulation methods.Special emphases are laid on the effects of pH on the reaction mechanisms,reaction kinetics,radical contributions and degradation efficiency.The toxicity prediction and risk assessment in the reaction processes are also carried out.Some important results are obtained as follows:1.Effects of pH on degradation of syringol and syringaldehyde by HO·Syringol and syringaldehyde are firstly selected as research objects for the study of the effects of pH on their hydroxylation degradation.Results show that pH possesses significant effects on hydroxylation reaction mechanisms and kinetics.The hydroxylation reaction mechanisms of neutral molecules(HSy and HSya)and anions(Sy-and Sya-)in syringol and syringaldehyde include H-atom abstraction(HAA),radical adduct formation(RAF)and single electron transfer(SET).The energy barriers(△G≠,kcal mol-1)of most reaction pathways in Syand Sya-are obviously lower compared with that in HSy and HSya.As the pH increases from 1 to 14(disregarding the dissociation of HO’),the second-order rate constants(k,M-1 s-1)of syringol and syringaldehyde with HO· in water increase from 5.43 × 1010 and 3.70 × 1010 M-1 s-1 to 9.87 × 1010 and 1.14 × 1011 M-1 s-1,respectively.And the branching ratios of HAA reaction pathways(ΓHAA)increase from 24%and 50%to 62%and 53%,respectively.These indicate that increasing pH improves the reactivity of syringol and syringaldehyde towards HO· and the relative importance of HAA reaction pathways.2.Effects of pH on degradation of five aromatic acids and bases by HO·In order to comprehensively consider the effects of pH on second-order rate constants and HO· concentration([HO·]),five aromatic acids and bases(AABs)are selected to study their hydroxylation reaction mechanisms and kinetics in UV/H2O2 process.Results show that the reactivity of HO·towards the 12 different dissociated forms is positively correlated with electron-donating effect of substituents,which contributes to the positively pH-dependent reactivity of HO· towards AABs.Considering the dissociation of HO·,the k values increase as the pH values increase from about 1 to 10,but they decrease as the pH values increase from about 11 to 14.However,the best pH values for degradation are not around 10～1 1 because the[HO·]decreases continuously with the increasing pH values from 3 to 9.5.Combining the factors of k and[HO·],the best degradation pH values are 5.5-7.5 for p-hydroxybenzoic acid,p-aminophenol,aniline and benzoic acid,3.0～7.5 for phenol and 5.5～7.5 for mixed pollutants of these aromatic acids/bases in UV/H2O2 process.Altogether,this work provides comprehensive understanding of the effects of pH on HO·-initiated degradation processes of AABs.3.Effects of pH on degradation of six aromatic acids and bases by ClO· and BrO·In UV/chlorine and Solar/chlorine processes,ClO· and BrO· as two newly discovered free radicals have attracted extensive attention.In this work,six AABs are selected to investigate the pH-dependent degradation mechanisms and kinetics by ClO· and BrO·.Among the 16 dissociation species,the structures with electron-rich rings possess stronger reactivities to ClO·and BrO· than those with electron-poor rings,which is similar to the result of HO·.However,ClO· and BrO· are considered to be more selective and pH-sensitive than HO· when reacting with AABs based on the k values.Compared with acidic pH,the basic pH could improve the pseudo-first-order rate constants of most AABs in both systems.As pH increases from 6 to 8,the contribution percentages of ClO· for the removal of AABs(except for benzoic acid)rise rapidly in both systems.The pH-dependent contributions of various radicals are attributed to structure-dependent reactivities of compounds and pH-dependent concentrations of various radicals.This work is necessary for enhancing the understanding of the pH-dependent contributions of individual radicals during the removal of dissociable organic pollutants in UV/chlorine and Solar/chlorine systems.4.Effects of pH on degradation of chlorophenols and halogenated pyridinols by HO·In this work,sixteen typical halogenated aromatic DBPs(five chlorophenols and eleven halogenated pyridinols)are selected as the research objects to study their pH-dependent reaction regularity in UV/H2O2 process.For the reactions between HO’ with 37 different dissociated forms,RAF is the main reaction pathway,and the reactivities decrease with the increase of the halogenated number.Compared with chlorophenols,N substitution reduces the reactivities;compared with chloropyridines,Br substitution possesses little effect on the reactivities.The k values increase with the increase of pH from 0 to 10,and decrease with the increase of pH from 10 to 14.Compared with phenol,the greater of the chlorinated number in chlorophenols,the greater the pH sensitivity of the k values are;compared with chlorophenols,N substitution further enhances the pH sensitivity of the k values.As the pH increases from 2 to 10.5,the degradation efficiency increases at first and then decreases.With the increase of the halogenated number,the degradation efficiency range increases,the pH sensitivity increases,the optimal degradation efficiency increases and the optimal degradation pH decreases.The ecotoxicities and bioaccumulative effects of most hydroxylated products are lower than their parental halogenated disinfection by-products.It is shown that the UV/H2O2 process can effectively degrade halogenated disinfection by-products and reduce their toxicities to the ecosystem.But there are still a few more toxic products,such as hydroquinones,which should be further oxidized and degradated.