Effects of sorbent addition on the transport of inorganic and organic chemicals in soil-bentonite cutoff wall containment barriers.

Abstract

Diffusion can be a significant mechanism for chemical transport in low-permeability soil barriers used for waste containment. The objective of this research was to identify practical sorbents for inclusion in soil-bentonite cutoff wall barriers to retard such transport, and to assess the impact of sorbent addition on contaminant diffusion and barrier hydraulic conductivity. Sorption isotherms were obtained for three organoclays and a natural shale (Ohio Shale) using trichloroethylene (TCE), 1,2,4-trichlorobenzene, and methyl isobutyl ketone. The organoclays included cetyl pyridinium bentonite (CPC-bent), hexadecyltrimethyl ammonium bentonite (HDTMA-bent), and trimethylphenyl ammonium bentonite (TMPA-bent). A diffusion cell/permeameter was used to examine the effects of sorbent addition on bromide and TCE diffusion through soil barriers containing 0% or 4% bentonite, and on barrier hydraulic conductivity $(K\sb{c}).$ HDTMA-bent and CPC-bent (long alkyl chain organoclays) exhibited relatively low solute uptake and linear sorption isotherms, results indicative of an absorption-dominated process. Ohio Shale (moderate solute uptake and nonlinear isotherms) and TMPA-bent (a short chain organoclay; high solute uptake and some nonlinear isotherms) conversely appeared to function as adsorbents. The amount of sorption was related to sorbate hydrophobicity. Addition of 5% organoclay and up to 20% Ohio Shale to soil-bentonite barriers did not affect the diffusion coefficients for bromide or TCE. Retardation of TCE diffusive transport was related to barrier sorption capacity. For soil with 4% bentonite, addition of Ohio Shale or TMPA-bent had either no effect or slightly increased $K\sb{c}.$ In contrast, $K\sb{c}$ for the same soil increased significantly (an order of magnitude) with addition of 5% HDTMA-bent or 10% CPC-bent. Effects of sorbent addition on contaminant breakthrough are primarily a function of the sorption capacity of the material, resulting changes in barrier $K\sb{c},$ applied hydraulic gradient, and the amount of contaminant present in the system. Under certain conditions sorbent addition may adversely affect contaminant breakthrough by increasing a barrier's hydraulic conductivity. Including economic considerations, shale appeared to be the most practical of the sorbents examined for large scale applications such as addition to soil-bentonite cutoff walls.