Effects of Porous Medium Geochemical Heterogeneity and Divalent Cations on the Transport of Humic Acid

In this work, the effects of the chemical heterogeneity on the transport of a commercial Humic Acid (HA), in the presence and in the absence of divalent cations, through two porous media, were investigated. The two porous media used were, a natural quartz sand (NQS) having heterogeneous chemical surface, and a pure quartz sand (PQS) having homogenous chemical surface and bearing negative charges. The data indicate HA adsorbed amount, from water onto the PQS and NQS samples, were found to be function of the nature of both, the divalent cation (Zn2+, Co2+ and Ni2+), and the porous medium. Thus, in the case of PQS, the HA retention was found to increase in the following order: Zn2+ > Co2+ > Ni2+, i.e., by increasing the divalent cation affinity towards the HA. However, in the case of NQS, the adsorbed HA amount was found to increase following the opposite order: Ni2+ > Co2+ > Zn2+, i.e., by decreasing the divalent cation affinity towards the HA. Such difference in behavior of the two porous media was found to result from the presence of chemical, and/or electrical charge, heterogeneities on the NQS surface. It is shown that whatever the porous medium, the adsorption of HA from water onto the solid surface is controlled by the electrostatic interactions.