Transport and retention of humic acid through natural quartz sand: Influence of the ionic strength and the nature of divalent cation

In order to elucidate the environmental problems arising from the mobility enhancement of metallic contaminants due to the presence of humic acid (HA) in water, transport and retention experiments of HA, in the presence and in the absence of divalent ions Me2+ (Ca2+, Cu2+ and Pb2+), through natural quartz sand (NQS) porous medium, were carried out. The breakthrough curve and the HA adsorbed amount were measured in each experiment, indicating colloid retention was highly dependent on the suspension ionic strength, and the nature of divalent cation present in the aqueous phase. The data show that in the absence of divalent cations, the adsorbed HA amount at solid–water interface was found to increase with increasing the ionic strength, whereas the flow rate does not affect the retained HA amount on quartz sand. Further, in the presence of divalent cations, the adsorbed HA amount was found to be function, not only of the ionic strength, but also to depend on the divalent cation affinity toward the HA macromolecule. Thus, cations having lower affinity for HA, lead to highest adsorbed HA, whereas cations having higher affinity for HA, give the lowest adsorbed HA on the NQS surface. It is shown that the adsorption of HA, or its complex form with divalent cations (HA–Me2+), on the mineral–water interface, is an irreversible process which is limited by the blocking effect.