Sepiolite and sepiolite-bound humic acid interactions in alkaline media and the mechanism of the formation of sepiolite-humic acid complexes

Abstract

The surface-controlled reactions and interfaces of clays and/or clay minerals are strongly determined by the adsorption of dissolved humic substances which have surface active characters. Consequently, this may lead to an improved understanding of organo-clay complexes for remediation purposes of polluted areas. The interactions between sepiolite and sepiolite-bound humic acid (HA), and its influence on the mechanism of the formation of sepiolite-humic acid complexes were examined by characterizing the structure of sepiolite through UV/Vis spectroscopy, field emission scanning electron microscopy (SEM+FEG), specific surface area, pore size and volume, and Fourier transform infrared spectroscopy (FTIR). The UV–vis spectral results of the raw sepiolite showed that there is no correlation between UV254 and the amount of humic substance released from sepiolite is dependent on increasing extraction time. The low values of humification index of the sepiolite confirmed the presence of HA with higher, and similar molecular weight, particle size, humification degree and aromaticity. Hence, it can be concluded that HA groups are presumably associated with the large quantity of silanol groups located on the sepiolite external surface preferentially through H-bonds and/or surface complexation interactions between the functional groups in HA and dissolved metal cations (Mn+). This finding is in good agreement with results from FTIR and SEM-FEG investigations. EDAX analysis of the HA-extracted sepiolite illustrated a strong decrease in Mg2+, Ca2+ and K+ content indicating that electrically charged sites on HA may form metal complexes of high stability through chelation.