Thermal and chemical pretreatment of Ebenaeae Diospyros preusii seed for enhanced adsorptive removal of aqueous-bound Cr(VI)

Abstract

This paper described the thermal activation of Ebenaeae Diospyros preusii seed (EDP) as a cost effective biosorbent for Cr(VI) removal. SEM, FTIR, TGA, BET and XRD were used to characterise both the thermally modified EDP (TM-EDP) and unmodified EDP (UM-EDP) biosorbents obtained from ground Diospyros preusii seed. The obtained experimental data were modelled using Langmuir, Dubinin-Radushkevich, Temkin, Flory–Huggins and Freundlich isotherm to determine the process of removal of this ion from aqueous solution. Batch adsorption method was employed in the evaluation of the adsorbent’s adsorption performance. pH effect, initial Cr(VI) concentration, temperature and contact time influence on adsorption were also evaluated. Thermal modification of the EDP was found to increase the surface area from 471.665 m2/g to 771.738 m2/g. The FTIR showed the existence of C = O, -C-H, C-O-O- and –OH functional groups on both the functionalized and unmodified EDP. The characterization also showed that the thermally modified adsorbent is amorphous with rough surfaces which enhances adsorption of the Cr(VI). The thermally modified precursor showed a better adsorption capacity revealing the benefit of thermal modification. From the isotherm plots, Langmuir model best described the process of removal of the Cr(VI) from the aqueous solution with determination coefficient (R2) value of 0.991 which showed that Cr(VI) adsorption unto the adsorbent followed physisorption mechanism. From the kinetic studies, it was concluded that the pseudo-second order most appropriately explained the adsorption process indicating chemisorption. An exothermic, chemosorptive and non-spontaneous adsorption process was identified by thermodynamics investigations. This study therefore revealed the adsorption capacity of the thermally modified EDP seed to be 51.11% at pH 4.