Adsorption characteristics of Cd²⁺ and Zn²⁺ by extracellular polymeric substances with varied constituents from activated sludge

Three types of extracellular polymeric substances (EPS), EPS1, EPS2 and EP3, with protein/sugar content ratio of 2.5:1, 7:1 and 9:1, respectively, were investigated as sorbent for adsorbing Cd²⁺ and Zn²⁺ from wastewater. The experimental results showed that the adsorption efficiency of EPS toward Cd²⁺ and Zn²⁺ were dependent on the composition of EPS. The adsorptive capacities of EPS1, EPS2, EPS3 to Cd²⁺ were 19.5 mg/g, 27 mg/g, 17 mg/g and to Zn²⁺ 40.5 mg/g, 47.5 mg/g, 37 mg/g, respectively. Kinetic study showed that EPS1 has the highest reaction rate, while EPS2 has the highest adsorption capacity. All of the adsorption processes reached equilibrium within 1 h following both the Freundlich and Langmuir isotherms. The fitted parameters indicated that varied reactions may occur in EPS-metal adsorption process and EPS as adsorbent has much higher adsorption stabilization, adsorption ability and appetency to Zn²⁺ than to Cd²⁺. The adsorption capacity of EPS toward Cd²⁺ and Zn²⁺ increased with the sugar content, which suggests sugar is of great importance in the adsorption.