Removal of Cd(Ⅱ) from dilute aqueous solutions by complexation–ultrafiltration using rotating disk membrane and the shear stability of PAA–Cd complex

Removal of cadmium(Ⅱ) ions from dilute aqueous solutions by complexation–ultrafiltration using rotating disk membrane was investigated. Polyacrylic acid sodium(PAAS) was used as complexation agent, as key factors of complexation, pH and the mass ratio of PAAS to Cd~(2+)(P/M) were studied, and the optimum complexation–ultrafiltration conditions were obtained. The effects of rotating speed(n) on the stability of PAA–Cd complex was studied with two kinds of rotating disk, disk Ⅰ(without vane) and disk Ⅱ(with six rectangular vanes) at a certain range of rotating speed. Both of the rejection could reach 99.7% when n was lower than 2370 r·min~(-1) and 1320 r·min~(-1), for disk I and disk Ⅱ, respectively. However, when rotating speed exceeds a certain value,the critical rotating speed(n_c), the rejection of Cd(Ⅱ) decreases greatly. The distribution of form of cadmium on the membrane was established by the membrane partition model, and the critical shear rate(γ_c), the smallest shear rate at which the PAA–Cd complex begins to dissociate, was calculated based on the membrane partition model and mass balance. The critical shear rates(γ_c) of PAA–Cd complex were 5.9 × 10~4 s~(-1), 1.01 × 10~5 s~(-1),and 1.31 × 10~5 s~(-1) at pH = 5.0, 5.5, and 6.0, respectively. In addition, the regeneration of PAAS was achieved by shear induced dissociation and ultrafiltration.