Effects of Attapulgite and Sepiolite Adsorbing Cadmium in Aqueous Solution

Effects of attapulgite and sepiolite adsorbing Cd²⁺ in aqueous solutions relative to initial concentration, solid-liquid ratio, exposure duration, and pH were investigated through a static batch experiment under a set NaNO₃ ionic strength (0.01 mol·L^-1). Results show that, under the set NaNO₃ ionic strength, Cd²⁺ adsorption of sepiolite and attapulgite of high grade and attapulgite of low grade is positively related to initial concentration of Cd²⁺ in the solution, but negatively to solid-liquid ratio of the solution. Fitting with the Langmuir isothermal adsorption equation reveals an order of high grade attapulgite (33.67 mg·g^-1) >high grade sepiolite (25.55 mg·g^-1) >low grade attapulgite(11.52 mg·g^-1) >low grade sepiolite (5.24 mg·g^-1) in terms of theoretical saturated Cd²⁺adsorption; Cd²⁺ adsorption on sepiolite and attapulgite is strongly influenced by pH of the solution, and peaks when pH is 2-4; Cd²⁺ adsorption on attapulgite is relatively stable and reaches equilibrium basically in 3 h; in solutions 0.01 mol·L^-1 NaNO₃ in ionic strength, 625 mg·L^-1 in initial Cd²⁺ concentration, 15 g·L^-1 in clay mineral addition amount and 2-4 in pH, the minerals display an order of high grade sepiolite >high grade attapulgite >low grade attapulgite >low grade sepiolite in terms of Cd removal efficiency.