Evaluation of the selective adsorption of silica-sand/anionized-starch composite for removal of dyes and Cupper(II) from their aqueous mixtures.

A silica-sand/anionized-starch composite (CMS-SS) was prepared simply. CMS-SS was used as an efficient adsorbent for removal of cationic dyes [methyl blue (MB) and crystal violet (CV)] and metal ions [cupper(II), Cu(II)] from water in respective single and binary systems. Compared with the anionized-starch without silica sand, CMS-SS shows evidently improved adsorption capacities, i.e. approximately 653.31 ± 27.30, 1246.40 ± 34.10, and 383.08 ± 13.50 mg·g, for MB, CV, and Cu(II), respectively, ascribed to the additional carboxyl groups. The isotherms and kinetics study indicated that the Langmuir model and the pseudo-second-order model were more suitable. The adsorption process is thus a homogeneous monolayer chemisorption. The adsorptions of these three pollutants are spontaneous and exothermal processes driven by increasing entropy. The adsorption behaviors of CMS-SS have high pH dependence, and electrostatic attraction play an important role in adsorption. Dyes showed higher affinity to CMS-SS than metal ions causing a preferential adsorption of dye over Cu(II) in their aqueous mixture. This adsorbent after saturated adsorption could be rapidly separated from water due to its enlarged density after embedded silica sand; moreover, those rapidly recovered adsorbents were tried to use as new adsorbents for removal of an anionic dye from water due to the complete changes in their surface structures after saturated adsorption.