Cd complexation with mercapto-functionalized attapulgite (MATP): Adsorption and DFT study

Mercapto-functionalized attapulgite (MATP) could efficiently adsorb Cd from aqueous solution, achieving adsorption equilibrium within 30 min with the maximum adsorption capacity of 120 mg g(-1) at pH 6.7, 7.89-fold higher than that of ATP. Density functional theory (DFT) calculations revealed that the formed S-Cd-S and Cd-S bonds on MATP had lower adsorption energy (-3.75 and -2.84 eV) and higher bond order (1.14 and 0.39) than the corresponding values of Cd-O (-1.02 eV and 0.26) on ATP, which resulted in higher adsorption affinity of MATP toward Cd than ATP. The adsorption capacity increased by 7 times with the increasing of pH from 2.0 to 6.7. The positive-charged surface and protonation of MATP inhibited its adsorption for Cd at strong acid condition (pH < 2.4). However, complexation reactions of the mercapto groups on MATP with Cd at weak acidic and neutral conditions significantly facilitated its adsorption capacity. Mulliken net charge analysis showed that the terminal mercapto groups (-SH) rather than the disulfide (S-S) on MATP were the main active adsorption sites for Cd, which acted as the electron donors and transferred electrons to Cd form the surface complexes. Surface complexation models (SCM) modeling showed that the bidentate complexes predominated across the pH range of 2-7 due to their higher stable configurations than the monodentate. MATP was proved to be an efficient adsorbent for the removal of Cd from water bodies.