Performance And Mechanism Of The Oxidation Of Typical Organic Pollutants By Nanoscale Hydrous Manganese Dioxide

Manganese is one of the most abundant elements in the nature. Its oxide not onlyparticipates in the migration and transformation of organic and inorganic compounds inthe environment，but also plays an important role on water purification. In watertreatment processes, manganese dioxide is usually in situ generated by thepermanganate oxidation, which is significant to the oxidation. Furthermore, it alsoshows an excellent performance on the removal of pollutants. Sulfonamides andbromophenols frequently occur in natural waters whose reactive functional groupstoward manganese dioxide are aromatic amine and phenolic hydroxyl group,respectively. In this paper, nanoscale hydrous manganese dioxide (nHMO) is generatedby the reduction of permanganate by thiosulfate to mimic the in situ generated one, andthe oxidative or catalytic behaviors are investigated.The particle size of nHMO is distributed between24.4-91.2nm, and its reactivitykeeps almost the same for40days. The preparation concentration, temperature andmixing approach can affect the particle size of nHMO, and thus change its reactivity.The average size of nHMO formed by mixing two reactants instantaneously is18.41nmsmaller than by titrating thiosulfate into permanganate. The removal efficiency of SMZat pH7.3by the former one is higher than that by the latter one by28%. Both lowpreparation concentration and high temperature facilitate the small particle size and highoxidative reactivity of nHMO.Sulfamethazine (SMZ), sulfamethizole (SMZO) and sulfamethoxazole (SMX) areemployed to study the oxidative reactivity of nHMO. The reactivity of nHMO towardthree compounds is in the order of SMZ> SMX> SMZO. The inhibition of manganeseion on the oxidation by nHMO is more significant than other common divalent ions, sothat the removal of SMZ decreases by22%in the presence of5μM manganese ion atpH6.0. Both phosphate in high concentration and humic acid inhibit the oxidationprocess, but phosphate in low concentration has little effect on it. Oxidative kinetics ofthree compounds deviates from the pseudo first-order kinetic curve, with a plateau inthe later stage. Two kinetic parameters Srand k, which represent the number of reactivesites of nHMO and the reaction rate constant based on the reactive sites, respectively,are introduced to fit the whole oxidation curve of SMZ. Increasing the initial concentration of SMZ, the value of Srincreases, while the value of k decreases. Both thevalues of Srand k increase when increasing the initial concentration of nHMO, butdecrease when increasing the solution pH.Three bromophenols are selected to study the dehalogenation during the reactionbetween nHMO and halophenols. Both the oxidative reactivity and debrominationcapability are in the order of4-bromophenol (4-BrP)>2-bromophenol (2-BrP)>3-bromophenol (3-BrP). The pseudo first-order rate constants are almost the same fordifferent initial concentration of4-BrP, which indicates the rate-determining step of thereaction between nHMO and4-bromophenol (4-BrP) is electron transfer of surfacecomplex. The debromination number (DN) decreases while pH increases, whichindicates there are still other pathways in addition to the mechanism of oxidativecoupling during the oxidation of bromophenols by nHMO. The reaction rate anddebromination capability decrease under nigrogen atomosphere, and the inhibition at pH5.0is larger than pH4.0. During the oxidation of4-BrP, the particle size of nHMOkeeps increasing. Both high initial concentration of the reactants and low pH facilitatethe particle growth. The concentration of manganese ion released to the solution is verylow compared with the removal of bromophenols, which indicates that most ofmanganese ion adsorbs on to the surface of manganese dioxide.In view of the relationship between the reactivity and particle size of nHMO, theaggregation of nHMO during the oxidation has been studied. The critical coagulationconcentration of manganese ion is relatively low for aggregation of nHMO, which is inthe order of magnitude of10-5M. Higher pH leads to the higher aggregation rate andlower critical coagulation concentration. Aggregation rate induced by the oxidation oforganic compounds depends on its oxidation rate. The increase of pH decreasesoxidation rate of organic compounds, and hence decreases the aggregation rate ofnHMO. Since oxidative aggregation strongly affects the oxidation efficiency,permanganate is employed to effectively maintain the particle size of nHMO andalleviate the aggregation. When oxidizing bisphenol A (BPA) by nHMO and tracepermanganate together, the pseudo first-order rate constant is the same with that at theinitial phase of the oxidation by nHMO, which indicates that the important role ofpermanganate is to oxidize the manganese ion and keep the particle size of nHMOconstant. When oxidizing4-nitrophenol (4-NP) by the same system, the removal of4-NP is improved due to the participation of permanganate in the electron transfer of the surface complex. In this case, nHMO exhibits the catalytic capability towardspermanganate.Because of the catalytic effect of manganese oxide on permanganate oxidation,carbamazepine (CMZ) with double bond and phenols like bisphenol A (BPA), phenol(Phen),2-chlorophenol (2-CP),2,4-dichlorophenol (2,4-DCP) and2,4,6-trichlorophenol(2,4,6-TCP) are chosen to study the role of nHMO and other reactive manganeseintermediates during the simultaneous oxidation of two pollutants by permanganate.Under weakly acidic conditions, CMZ can accelerate the oxidation of Phen while theoxidation of itself keeps unchanged. Phenols can enhance their oxidation mutually, andthe oxidation of the pollutant with a smaller reaction constant is accelerated more. Thecatalytic effect of nHMO is the main reason for the phenomena under weakly acidicconditions. Under weakly alkaline conditions, the coexistence of CMZ and Phen haslittle effect on the oxidation of each other, while the coexsitence of phenols exhibitsdifferent that the oxidation of phenolic pollutant with smaller rate constant is inhibitedbut almost unchanged for the other. The competitive kinetics shows that two phenolicpollutants may compete for a new reactive intermediate in the oxidation rather thanpermanganate and manganese dioxide. Since the mechanism of the oxidation of CMZ isdifferent with that of Phen, CMZ and Phen do not compete for the same reactiveintermediate so that oxidation of Phen is not inhibited.