Effect of Ozonation and Hydroxyl Peroxide Oxidation on the Structure of Humic Acids and their Removal. Introduction.
Oxidation on the Structure of Humic Acids
and their Removal
Pre-oxidation is a common practice nowadays for the removal of offensive smell and odour and control of DBPs precursors as organic pollution of source water has become a serious problem. Ozone and hydroxyl peroxide are the oxidants widely applied. One topic drawing attention is the effect of oxidation on the structure of natural organic matter which is the main DBPs precursor. In this study, the authors applied a pyrolysis-GC-MS technique to analyze the main functional groups before and after oxidation by ozone and/or hydroxyl peroxide. The effect of pre-oxidation was evaluated from the results of organic removal by oxidation itself and after subsequent coagulation and/or GAC adsorption.
Results & Discussion
TOC and UV254 removal
Effect of oxidation on the structure of HA
Figure 2 shows pre-oxidation by ozone, H2O2 and H2O2 with catalysts can more or less result in direct removal of organic matter. H2O2 can bring about certain removal of UV254 especially at higher doses but its effect of TOC removal is not apparent. As a low dosage of catalyst is applied, significant improvement can be achieved in the removal of both UV254 and TOC by H2O2, and O3 shows the best catalysing effect. It is also noticeable that no matter which oxidant is applied, the removal of UV254 is much higher than that of TOC. The result implies that an important function of the oxidants applied in this study is to alter the structure of the organic molecules rather than to decompose them into inorganic matter.
Figure 1 shows some of the pyrolysis-GC-MS analysis results for raw water, ozonated water and oxidized water by H2O2 with O3 as catalyst. It is seen from this figure that the dominant functional groups detected from the raw water are cyclic hydrocarbons and aromatics, such as benzoaldehyde, cyclohexanone, benzenetricarboxylic acid, benzophenone etc, while after oxidation by ozone or H2O2 with ozone, many of these cyclic hydrocarbons are no longer in existence and many chain hydrocarbons and aliphatic acids with oxygenated groups such as carboxyl, hydroxyl, ketone, ether, ester etc. are newly formed. Apparently the organic functional groups undergo a significant change in their structure, mostly from cyclic to chain hydrocarbons and/or from aromatics to aliphatics.
Figure 2 Direct Removal of TOC and UV254 by Different Oxidant
Effect of pre-oxidation on coagulation and GAC adsorption
Figure 3 shows the effect of pre-oxidation on the overall removal of TOC as coagulation (aluminium sulphate as coagulant and at pH 5.0) or GAC adsorption is subsequently applied. Pre-oxidation can apparently improve the TOC removal, especially under the condition of pre-oxidation by ozone or H2O2 with ozone. Aliphatic groups such as carboxyl and hydroxyl groups are believed to be more readily to react with alum to form Al-humic complexes which result in better coagulation. Because pre-oxidation brings about an increase in the portion of aliphatic groups, the HA molecules in the solution become more coagulable with aluminium coagulant. For GAC adsorption, the molecular weight of the organic matter is an important factor. Larger organic matter can be broken into smaller ones by pre-oxidation. This makes the organic matter more favourable to be adsorbed by GAC.
Figure 3 Effect of pre-oxidation on coagulation and GAC adsorption
Figure 1 Pyrolysis –GC-MS Analysis Results
Both ozone and H2O2 with a small dose of zone as catalyst can bring about a decrease of cyclic hydrocarbons and aromatics and an increase of chain hydrocarbons and aliphatic acids. This is accompanied by a remarkable decrease of UV254 though an effective TOC removal may not be achieved. H2O2 itself is not effective for pre-oxidation unless certain chemicals, such as ozone, are used as catalysts. Pre-oxidation can improve the efficiency of overall organic removal by subsequent coagulation and/or GAC adsorption.