Use of agro-industrial wastes for the removal of chromium(VI) from contaminated water: an electrochemical exploration

Abstract

Bioadsorption with agro-industrial wastes is a viable technique for the removal of heavy metals from wastewater. This work assessed the capacity as removal agent chromium(Cr)(VI) of walnut, coffee, and moringa shells. Three equilibrium experiments were carried out, the effect of Cr(VI) concentration, changes in pH solution, and desorption degree; the Cr(VI) content was determined by UV–vis spectroscopy; and an electrochemical analysis was performed. The biomaterials showed an adsorption capacity from highest to lowest as follows: walnut > coffee > moringa, recording up to 31 mg/g. At slightly acidic pH the materials presented their highest adsorption capacity with 98% of Cr(VI) retained; and, the lowest desorption of Cr(VI) was recorded with CaCl2, while the highest Cr concentration in moringa was desorbed with HCl, while NaOH had its maximum desorption with coffee and walnut shells. Electrochemically, the three biomaterials presented a high potential to be used as biosorbents because they present the Cr(VI)/Cr(III) redox couple, although coffee presented the highest efficiency, followed by walnut and moringa shells.