THERMODYNAMICS OF HEAVY METALS (Hg2+, Cd2+, Fe2+AND Ni2+) ADSORPTION USING BOILER FLY ASH

Abstract

The effect of temperature on the adsorption of heavy metals, Hg²⁺, Cd²⁺, Fe²⁺ and Ni²⁺ using unactivated boiler fly ash (an agricultural waste) was investigated. The results showed that Hg²⁺ and Cd²⁺ ions were mostly affected by increase in temperature because their sorption rate decreased at high temperature. At the temperature of 273 K and 383 K, the amount of Hg²⁺ adsorbed (mg/g) by the unactivated boiler fly ash was 49.890 mg/g and 49.653 mg/g respectively. For Cd²⁺ ion, the amount adsorbed were 49.671 mg/g and 48.962 mg/g at temperatures of 273 K and 383 K respectively. The sorption of Fe²⁺ and Ni²⁺ fluctuated as temperature was increased. However; more of Fe²⁺ was removed from its aqueous solution followed by Ni²⁺ ion. The enthalpy of adsorption of Hg²⁺ and Fe²⁺ were found to be endothermic while that of Cd²⁺ and Ni²⁺ ions were exothermic. More so, ΔS^θ for all the metal ions were greater than zero (> 0) and ∆G^o were all negative which showed that the adsorption process was spontaneous, irreversible and feasible. Furthermore, lower temperatures favoured the removal of Hg²⁺ and Ni²⁺ ions from their solutions. Therefore, effect of temperature is a very important parameter to be investigated for proper decontamination of metal ions from wastewater.

Key words: Adsorption, Boiler fly ash, Heavy metals, Temperature, Unactivated.