A comparative study on the photocatalytic degradation of humic substances of various origins

The photocatalytic removal of model humic and fulvic acids of different origins (terrestrial and aquatic) was investigated using TiO2 Degussa P-25 as the photocatalyst. The results were presented comparatively based on pseudofirst-order and the Langmuir–Hinshelwood kinetic models. The reaction rates calculated for the UV-vis parameters follow the general trend: IHSS soil humic acid > Aldrich humic acid > IHSS humic acid > Roth humic acid > IHSS fulvic acid. However, the rate constants show differences depending on the UV-vis absorption parameters. The evaluation of the Langmuir–Hinshelwood kinetic model reveals that, in all cases, Roth humic acid has the lowest Langmuir–Hinshelwood rate, rate constant and adsorption coefficient, and the ordering of the rates follows the same trend with that of pseudo-first order. Furthermore, data related to the molecular size distribution profiles of raw and treated humic acids suggest that the photocatalytic degradation occurs irrespective of the molecular size fractions.

On the basis of their diverse chemical and physical properties such as molecular weight, molecular size, elemental composition and source of origin, substantial differences are observed in photocatalytic removal efficiencies of humic and fulvic acids. The pseudo-first-order rate constants calculated for Color436 of each of the humic substances show a decreasing trend as follows: AHA > IHSS SHA > IHSS FA > IHSS HA > RHA. On the other hand, the order of the rate constants of aromaticity removal (UV254) is as follows: IHSS SHA > AHA > IHSS HA > IHSS FA > RHA. The rate constants calculated for TOC removal exhibit a different trend: IHSS SHA > IHSS FA > AHA > RHA > IHSS HA. Higher removal rates have been achieved with UV254 values compared to that of Color436 values. All of the studied HA exhibit higher pseudo-firstorder removal rates compared to that for FA. The ordering of the L-H rates is similar to that of first-order rates; however, the calculated L-H rates for the HA are approximately half the value of the pseudo-first-order rates for all of the studied UV vis parameters. Comparison of the molecular size distribution profiles of the photocatalytically treated HA with those of raw untreated samples reveals the confirmation of the formation of lower molecular size fractions exhibiting low SUVA254 values.