Studies on Pb(II) and Cd(II) adsorption by Fe(II) and Fe(III) silicate precipitation tubes in aqueous medium

Abstract

Adsorption of Pb(II) and Cd(II) on Fe(II) and Fe(III) silicate precipitation tubes (FeIISPT and FeIIISPT) in aqueous medium has been studied and reported. FeIIISPT is a better adsorbent than FeIISPT with reference to Pb(II) and Cd(II). Adsorption varies nonlinearly with initial concentration and adsorbent dose. Experimental adsorption data are modeled with Langmuir and Freundlich isotherms and isotherm parameters are obtained through regression of linearized isotherm equations as well as through direct and robust optimization of parent equations. Robust optimization always yields better fits as opposed to linear regression in terms of Sum Square Error (SSE) and goodness of the fit (R2) between experimental and optimized model data. Pb(II) adsorption on both FeIISPT and FeIIISPT and Cd(II) adsorption on FeIISPT may be better represented by Freundlich isotherm while Cd(II) adsorption on FeIIISPT is more amenable to Langmuir form. Modifications to traditional Langmuir and Freundlich isotherms are proposed in which initial adsorbate concentration is introduced into the original form. The fit of experimental data into the modified Langmuir form is remarkable. Both FeIISPT and FeIIISPT are identified with very high Pb(II) and Cd(II) loading capacities.