ZnCl2-modified eucalyptus bark biochar as adsorbent: preparation, characterization and its application in adsorption of Cr(VI) from aqueous solutions

Biochar is a byproduct of biomass pyrolysis that has been identified as an effective and novel bio-adsorbent for contaminant removal from wastewater. A pyrolysis-chemical activation process was used to convert eucalyptus tree bark to activated biochar, which was then characterized using various techniques (BET, SEM, FTIR, EDX, XRD and pHpzc) and used as a bio-adsorbent to remove hexavalent chromium (Cr(VI)) from aqueous solution. The effects of adsorption process conditions (adsorbate concentration, contact time, adsorbent amount and pH) on Cr(VI) removal efficiency were investigated. The best experimental conditions for the maximum equilibrium uptake of Cr(VI) by activated eucalyptus bark biochar (AEBB) were 10 mg/L, 140 min, 6.0 and 0.3 g/L for initial Cr(VI) concentration, contact time, pH and adsorbent amount, respectively. The AEBB's outstanding performance was aided by its high surface area, well-developed porous structure and dominance of active surface functional groups, as validated by BET, SEM and FTIR. The Freundlich model fitted the equilibrium data the best. The pseudo-second-order model appeared to better predict the kinetics of Cr(VI) adsorption onto AEBB, indicating the dominance of chemisorption.