Effects of humic acid on phthalate adsorption to vermiculite

This study investigated the adsorption of phthalates to vermiculite and the effects of natural organic matter (NOM) on the adsorption process using Di-(2-ethylhexyl) phthalate (DEHP) as a representative phthalate and humic acid (HA) as a model NOM. Environment conditions, such as pH and temperature, were also investigated in order to elucidate the adsorption mechanism. The results indicated that DEHP molecules were adsorbed on the surface of vermiculite in a monolayer and that the acting force was mainly due to hydrophobic interactions. The presence of HA in the system consistently inhibited the adsorption of DEHP to vermiculite by the competition for adsorption sites and the co-transport which enhanced the suspension of DEHP; however, an excess of both HA (2000 mu/g) and acidity weakened the competitiveness of HA. The process of DEHP adsorption to vermiculite was strongly pH-dependent; adsorption increased dramatically as the pH decreased, but extreme acidity (pH 3) inhibited adsorption. The adsorption process was exothermic and spontaneous. (C) 2013 Elsevier B.V. All rights reserved.