Evaluation of adsorption and coagulation characteristics of humic acids preceded by alternative advanced oxidation techniques

Humic substances mainly humic acids constitute the major fraction of natural organic matter in water supplies. Humics express diverse actions primarily related to the formation of potentially harmful disinfection by products (DBPs). Therefore, the removal of DBP precursors through numerous treatment techniques gains high importance. Besides the conventional treatment processes, the applications of advanced oxidation techniques are considered as effective tools for the elimination of humic acids (HAs) from natural waters. In the present study, both the adsorption and coagulation characteristics of humic acid samples are evaluated subsequent to pretreatment by photocatalytic oxidation and ozonation. The changes in preoxidized humic acid solutions are assessed in relation to the alterations induced in the UV-vis and fluorescence spectroscopic properties. The removal efficiency of color in terms of Color436 and aromaticity with respect to UV254 are presented relevant to each treatment step. Major key parameters such as the selection of coagulant types (alum and ferric chloride), coagulant aids (ionic and non-ionic polyelectrolyte) and optimum coagulant dose were comparatively discussed. The adsorption characteristics of powdered activated carbon (PAC) and granular activated carbon (GAC) were investigated.

In the present study, both the adsorption and coagulation characteristics of humic acid samples are evaluated subsequent to ozonation, photocatalysis and sequential oxidation systems. The changes in preoxidized humic acid solutions are assessed in relation to the alterations induced in the UV-vis spectroscopic properties. As supported by the low values of Color436, UV254, TOC and decrease of fluorescence intensity after the applied oxidation systems, the structural properties of humic acid significantly change considering the different reaction mechanisms of each treatment and affect both the adsorption and coagulation characteristics of the studied samples. An unconventional method of increased sorbent loadings against fixed sorbate concentrations were adopted throughout the adsorption experiments. This was because of the complex nature of the humic acid yielding unidentified products after each pretreatment disabling increased concentrations of sorbate for each batch. The isotherms exhibited increasing trends, for all the applied schemes and adsorbents, namely PAC and GAC. The regional patterns and the minor diversions from the generally defined C-type isotherm were because of the complexity of both the adsorbents and the humic acid. However, the oxidative regimes of photocatalysis and ozone also affected the isotherms. And most probably the non-selective oxidation pathway helped photocatalytically treated samples to emerge with a linear uniform profile for both PAC and GAC, amongst the other tested pretreatment methods. In the coagulation step, no significant fluctuations between different oxidation methods were observed for alum in terms of Color436. On the other hand, for ferric chloride the prominent difference was only less than 10%. Amongst all the treatment methods, ozonation shows the least removal efficiency for both coagulants. Between the two coagulants, alum coagulation was favored for each treatment step with removal efficiencies no less than 91%.