Influence Of Humic Acid On The Oxidation Of Phenols By Permanganate

To further investigate application of permanganate in drinking water treatment processes and environmental remediation, the effects of humic acids (HAs), as one of most common background matrices, on the phenol oxidation by permanganate were studied. Phenol oxidation by permanganate was enhanced by the presence of HA (1.0 mg L^-1) at pH 4-8 while slightly inhibited at pH 9-10. The effects of HA on phenol oxidation by permanganate were dependent on HA concentration and permanganate/phenol molar ratios.Due to heterogeneous HA structural features, the separation of HS into different nominal molecular weight (NMW) fractions to reduce their heterogeneity is a necessary step. The high NMW fractions of HA enhanced phenol oxidation by permanganate at pH 7 more significantly than the low fractions of HA. The apparent second-order rate constants of phenol oxidation by permanganate in the presence of HA correlated well with their specific ultraviolet absorption (SUVA) at 254 nm and specific violet absorption (SVA) at 465 or 665 nm of HAs. High positive correlation coefficients (R2>0.72) implied thatπ-electrons of HA strongly influenced reactivity of phenol towards permanganate, which agreed well with the information provided by Fluorescence spectroscopy. The FTIR analysis indicated that the HA fractions rich in aliphatic character, polysaccharide-like substances and the amount of carboxylate groups had less effects on phenol oxidation by permanganate.However, a limitation of the study was the use of commercial HA, which may not be appropriate as analogues of true soil or aqueous humic substances. Hence, the influence of HA extracted from different aquatic and soil sources was also investigated, including two types of commercial HAs, three types of soil HAs and one type of aqueous HA. The apparent second-order rate constants of oxidation of phenol and 2-chlorophenol by permanganate in the presence of HAs from different origins follow the order of commercial HA (Shanghai Chemical) > soil HAs>commercial HA (Fluka) > aqueous HA. The FTIR analysis indicated aqueous HA with high contents of carboxylate, aliphatic groups and polysaccharide-like substances and low amount of C=C moieties in aqueous HA resulted in no enhancement in the oxidation of phenol and 2-CP by permanganate under neutral conditions. Positive correlation between SVA/FRI and the apparent second-order reaction rates suggested that the reactions between permanganate and 2-CP/phenol were greatly affected by HA with large extent ofπ-electron, such as the great number of aromatic rings or long conjugated bonds in HA structure.