Mechanism Investigation Of The Influence Of Humic Acid On Phenol Oxidation By Permanganate

Humic acid (HA) is widespread in natural water and significantly influence theprocess of water treatment. The role of humic acid in phenols oxidation bypermanganate over the pH range of5.0-9.0was systematically examined in this study.HA could enhance phenols’ oxidation by permanganate under slight acidic andneutral conditions with greater enhancement at lower pH. The UV analysis revealed thatthe presence of HA improved the generation of manganese dioxide.The oxidation capacity of manganese dioxide was highly depended by pH valueswith high oxidation activation under low pH. Phenol absorbed by manganese dioxidemakes up a small part of phenol removed by manganese dioxide. Catalysis ofmanganese dioxide was not found when permanganate and manganese dioxide werecoexisting to degradate phenols.The HA pre-oxidized by ozone improved phenol oxidation by permanganate at pH5.0-6.0to a less extent compared to the pristine HA. HA could only enhance theoxidation of phenols, oxidizable by preformed manganese dioxide, by permanganateand there was a good correlation between HA-induced improvement in the oxidationrates of phenols by permanganate and those by preformed colloidal manganese dioxideat pH5.0.Comparing the influence of Na2S2O3and HA on phenol’s oxidation bypermanganate further confirmed that the continuous generation of fresh manganesedioxide and stabilizing the manganese dioxide formed in-situ by HA were crucial forthe HA-induced enhancement in phenols’ oxidation by permanganate at≤p H7.0.Manganese dioxide precipitation formed when magnetic stirring was offered throughoutthe experiment in the presence of sodium thiosulfate, and the precipitation holdsignificant effects on the properties of freshly formed manganese dioxide. The influenceof HA on phenols’ oxidation by permanganate at pH5.0was strongly dependent on theposition of chlorine substituents, following the sequence of Cl at ortho position>> Cl atpara position> Cl at meta position.A model was established based on the mechanism proposed above which couldwell simulated the experimental data. The high slope on the correlation of phenol oxidation by freshly in situ formed manganese dioxide and performed manganesedioxide suggest that freshly formed manganese own much higher activity. The differentconditions of manganese dioxide in presence/absence of phosphate showed thatphosphate influence phenol oxidation by influencing the properties of manganesedioxide.At pH8.0-9.0the overall degradation rates of phenols by permanganate wasslightly depressed by the presence of HA, which may be ascribed to consumption ofpermanganate by HA and the poor oxidation ability of in-situ generated manganesedioxide under this condition. The interaction between HA and undissociated phenolscontributed to the acceleration of their oxidation by permanganate in the initial stage atpH9.0. A model was established on initial stage of phenol degradation by permanganatein the presence of HA and showed that combined phenol with HA was more easilyoxidized by permanganate.