Characterizing chromium(VI) removal mechanism by raw leaf powder of local ‘Gapis’ (Saraca thaipingensis) tree for use as biosorbent

This paper documents the first ever characterization of non-enzymatic reduction and uptake of hexavalent chromium [Cr(VI)] from aqueous solution by raw dry leaf powder of ‘Gapis’ (S. thaipingensis), a tropical tree indigenous to Peninsular Malaysia and parts of South East Asia. In this study, the concentration of Cr(VI) reduced, Cr adsorbed, and total Cr removed by the leaf powder were compared to describe Cr(VI) removal mechanism. Chromium adsorption was of higher magnitude (28.86-74.37%) as opposed to Cr(VI) reduction to Cr(III), (19.49-25.63%), and significant (p < 0.05) at 1-60 mg/L Cr(VI) in contributing to overall Cr(VI) removal. Chromium(VI) removal decreased from 100% at 1 mg/L Cr(VI) to 95.83% (10 mg/L), and to 48.35% as Cr(VI) concentration increased to 100 mg/L, due to active binding sites saturation. Chromium uptake plot fitted the Freundlich isotherm model (Kf = 6.69 mg/g; n=3.17; R2 = 0.9912; qm = 28.60 mg/g) suggesting multilayer adsorption with interactions between adsorbed Cr on leaf powder surface and Cr ions in the solution. Scanning electron microscopy coupled energy dispersive X-ray (SEM-EDX) and X-ray diffraction (XRD) observations attested that non-enzymatic reduction of Cr(VI) to Cr(III) was possible with non-living biomass material. Hexavalent Cr was found adsorbed as chromium(VI) trioxide (CrO3) and calcium chromate (CaCrO4), while the reduced Cr(III) was present both in the solution and immobilized within the leaf powder in the form of chromium(III) oxide (Cr2O3) and chromium(III) phosphate (CrPO4). An electrostatic adsorption coupled reduction complexation process with precipitation is suggested as the probable removal mechanism. The findings are significant for future development of low cost leaf-based biosorbent materials for environmental remediation locally.