Adsorption/desorption in a system consisting of humic acid, heavy metals, and clay minerals

Metal adsorption/desorption in a system consisting of humic acid, metal ion s, and clay minerals is described. Montmorillonite and purified humic acid were selected as a prototype materials for this study. At a constant ionic strength, the amount of humic acid adsorbed on montmorillonite decreases wh en pH is increased. A slight increase in humic acid adsorption on montmoril lonite is observed when there are bivalent metals present in the system. Th e metal adsorption on montmorillonite does not correlate to the amount of h umic acid adsorbed on montmorillonite. Montmorillonite with preadsorbed hum ic acid does not show a significant change in the capacity of adsorbed meta l ions. An increase in the ionic strength at a pH of 6.5 results in an incr ease in the adsorption of lead on montmotillonite in the presence of humic acid, while at a lower pH, the increase in ionic strength results in a decr ease in metal adsorption. The bridging of bivalent metal ions between montm orillonite and humic acid is proposed as the dominant adsorption mechanism. (C) 1999 Academic Press.