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

Adsorption of Pb(II) and Cd(II) on Fe(II) and Fe(III) silicate precipitation tubes (Fe^II SPT and Fe^III SPT) in aqueous medium has been studied and reported. Fe^III SPT is a better adsorbent than Fe^II SPT 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 Fe^II SPT and Fe^III SPT and Cd(II) adsorption on Fe^II SPT may be better represented by Freundlich isotherm while Cd(II) adsorption on Fe^III SPT 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 Fe^II SPT and Fe^III SPT are identified with very high Pb(II) and Cd(II) loading capacities.