Evaluation of shale and organoclays as sorbent additives for low-permeability soil containment barriers

A natural share and four synthetic organoclays were compared as potential s orbent additives to containment barriers at hazardous waste sites. Trimethy lphenyl ammonium bentonite (TMPA-bent) was shown in batch experiments to ha ve the greatest sorption capacities for 1,2,4-trichlorobenzene, trichloroet hylene, and methyl isobutyl ketone, followed by the share and a commercial organoclay. Sorption capacities were lowest for hexadecyltrimethyl ammonium bentonite (HDTMA-bent) and hexadecyl pyridinium bentonite (HDP-bent). Oper ative sorption mechanisms for the organoclays depended on the size of the o rganic modifier, i.e., uptake by the TMPA-bent occurred via adsorption onto mineral surfaces, while that for the HDTMA-bent and HDP-bent took place by absorption into organic phases formed by their long hydrocarbon tails. The share was found to be by far the most cost-effective sorbent, an important factor for large scale applications. Solids concentration effects (i.e., h igher apparent sorption capacities at lower experimental sorbent concentrat ions) were exhibited by HDTMA-bent and HDP-bent. This can be attributed to aggregation of sorbent particles as a result of interactions among their hy drocarbon chains. Solids effects were observed to decline and eventually di sappear as sorbent concentrations were increased. Such effects must be cons idered in applying batch sorption results to flow-through systems.