Adsorption of Cu2+ and Cd2+ on sierozem compositely modified by zwitterionic and anionic modifier

This study aimed to investigate the adsorption of heavy metal ions onto zwitterionic and anionic modified sierozem, dodecyl dimethyl betaine (zwitterionic modifier, BS) and sodium dodecyl sulfate (anionic modifier, SDS), which were used to simultaneously modify sierozems. The adsorption characteristics of Cu2+ and Cd2+ on each composite modified sierozem were studied by batch method, and the effect of temperature, pH, and ionic strength on adsorption were compared. The following results were obtained: (1) the adsorption isotherms of Cu2+ and Cd2+ in different modified sierozems followed the Langmuir model. The adsorption capacities of Cu2+ and Cd2+ were in the order of BS+SDS-modified > BS-modified > unmodified sierozem. With the increase of modified proportion of SDS, the amount of Cu2+ and Cd2+ adsorbed onto BS+SDS-modified sierozem increased. Cu2+ has a higher adsorption amount on the same modified sierozem than Cd2+. (2) The increase of temperature inhibited Cu2+ and Cd2+ adsorption in BS+SDS modified sierozems but promoted Cu2+ and Cd2+ adsorption in BS-modified and unmodified sierozems. Low ionic strength and high pH promoted the Cu2+ and Cd2+ adsorption in all tested modified sierozems. (3) The adsorption of Cu2+ and Cd2+ onto different sierozem samples showed a spontaneous, entropy increasing, and endothermic process on CK and BS-modified sierozem and spontaneous, entropy-increasing, and exothermic process on BS+SDS-modified sierozem. The spontaneity and entropy of Cu2+ adsorption were higher than those of Cd2+.