Adsorption of Cadmium onto Sand-Attapulgite Cutoff Wall Backfill Media

The adsorption of Cd by sand-attapulgite cutoff wall backfill media was investigated with batch experiments for different quantities of attapulgite in the mixture (30%, 40%, 60%, and 100%; dry weight). The adsorption capacity of the media for Cd increased with the increase of attapulgite content (A_p). The change of adsorbed amount (q_t) with time (t) exhibited two-stage behavior, with more than 80% of the final q_t attained in the first 30 min of the experiments. The Elovich equation was the most suitable for describing the adsorption kinetics of Cd by the sand-attapulgite media. The isotherm data were best fit by the Freundlich equation. Analysis of the results indicates that cation exchange was the major adsorption mechanism. The theoretical maximum adsorption capacities (q_m) of the sand-attapulgite media for Cd calculated by the Langmuir isotherm model are 6311 mg/kg (A_p = 30%), 6437 mg/kg (A_p = 40%), 6534 mg/kg (A_p = 60%), and 7034 mg/kg (A_p = 100%). The removal percentage (RP) and the distribution coefficient (K_d) of Cd decreased with the increase of the initial Cd concentration (C₀) in the solution. An empirical equation for predicting K_d in terms of A_p and C₀ was developed: log(K_d) = − 1.22log(C₀) + 0.71A_p + 4.17 (r² = 0.924). This equation can be used to estimate the distribution coefficient of Cd for sand-attapulgite backfill media. This will provide valuable parameters for the study of Cd transport in the sand-attapulgite cutoff wall media, and for the design of such systems for management of landfill leachate.