Removal of trace Cd2+ using continuous multistage ion foam fractionation. Part III—Effect of salt addition

In this study, a multistage foam fractionation column with 5 bubble-cap trays was used to remove cadmium (Cd) at a low concentration (mg/L) with sodium dodecyl sulfate (SDS) used to generate foam. The effects of anions (NO3−, Cl−, and SO42−) and cations (Na+, K+, Mg2+, and Ca2+) were investigated by addition of sodium salts and nitrate salts, respectively. The addition of each studied cation was found to increase the residual factor of Cd (Cd concentration in effluent/Cd concentration in feed) due to the effect of the competitive adsorption of added counterions with Cd2+ at the air–water interface of foam at a relatively high concentration of cation whereas, for all studied sodium salts, the residual factor of Cd decreased slightly with increasing counterion concentration at low concentrations. However, the added anions had a little effect. The effect of added cations and anions on the reduction of Cd removal are in the following order: Ca2+ > Mg2+ ≫K+ > Na+ and SO42− ≈ Cl− > NO3−, respectively. The SDS recovery was found to increase significantly with increasing added salt concentration because the repulsion force among the head groups of SDS is greatly reduced by the co-adsorption of the counterions from added salt, leading to increasing SDS adsorption at the air–water interface of foam. The added salts of divalent cations were found to affect both Cd and SDS separation performance more than those of monovalent cations.