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 (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.