Hydrous iron(III)-tin(IV) binary mixed oxide: Arsenic adsorption behaviour from aqueous solution

Groundwater in the Bengal delta plain is primarily in an anoxic environment and As(III)/As(total) ratios reported are in the range of 0.60 to 0.90. Most of the studied adsorbents showed greater adsorption affinity for less toxic As(V) than for more toxic As(III). The synthesis and physicochemical characterizations of iron(III)-tin(IV) binary mixed oxide composite with systematic arsenic adsorption behaviour are reported here. The parameters studied are the effect of pH, adsorption capacity with varying arsenic loads, contact time for kinetics and isotherm modelling by batch method. In the pH range of 3.0 to 9.0, the adsorption of As(III) is found to be nearly the same while that of As(V) decreases with increasing pH for the studied arsenic concentrations (5.0 to 20.0 mg L-1). The adsorption data were analyzed using various kinetic and isotherm model equations. The rate of adsorption of As(III) was faster than that of As(V) at equilibrium pH similar to 7.0 and temperature 30(+/- 2)degrees C. The adsorption kinetic data for arsenic fitted best with the Ho's pseudo-second order [R-2 similar to 0.99-1.00 for As(III) and similar to 0.97-0.98 for As(V)] and parabolic diffusion (R-2 similar to 0.98-0.99) equations. The present kinetic data fitted the power function and simple Elovich kinetic equations well (R-2 = similar to 0.97-1.00), except the data obtained for As(III) from the 20.0 mg 1:1 solution. The analyses of adsorption data for As(III) and As(V), respectively, at equilibrium PH 6.8(+/- 0.1) and 7.3(+/- 0.1) with three different two-parameter isotherm model equations show the fitting order: Langmuir > Freundlich > Temkin. The monolayer adsorption capacity (theta(0), mg g(-1)) values evaluated from the Langmuir isotherm for As(III) and As(V) are 43.86 and 27.55, respectively.