Treatment of Sulphate Rich Process Effluents using Coal Fly Ash

Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana

^* Corresponding author: ntulif@biust.ac.bw

Abstract

Coal fly ash obtained from a local power station was used as an adsorbent in the removal of sulphates in an aqueous solution. The coal fly ash was characterised using an x-ray fluorescence (XRF) which revealed that it was class C and sub-bituminous type fly ash, an x—ray diffractometer (XRD) which revealed the dominance of a large amorphous phase and a Fourier Transform Infrared Spectroscopy (FTIR) which showed the presence of both organic and inorganic functional groups. Adsorbent dosage, contact time, temperature and solution pH were varied and all the parameters showed a higher uptake of sulphate ions with their increase, except pH which showed a decreasing uptake. The pseudo second order model showed a better fit for the sorption data with lower error values than the pseudo first order model. The uptake of sulphate ions followed both the Langmuir and the Freundlich isotherm models with R_L values for the Langmuir within the range 0 < R_L < 1 and n-values for the Freundlich within the range 0 < n < 10 showing a favourable sorption for both models. All the thermodynamic parameters (ΔG°, ΔH° and ΔS°) were positive indicating a non – spontaneous, endothermic and a random solid – liquid interface. The process was shown to follow a unimolecular layer approach with chemical adsorption as the dominating mechanism. Generally, coal fly ash showed a higher affinity for sulphates leading to its effective removal.