A binary oxide-biochar composite for adsorption of arsenic from aqueous solutions: Combined microwave pyrolysis and electrochemical modification

Abstract

Biochar composites are promising and cost-effective materials that can be applied in wastewater treatment. In the current study, a novel biochar composite impregnated with Zn and Al oxides (AlZnBC) was synthesized utilizing a non-conventional microwave pyrolysis combined with an electrochemical modification. The temperature distribution inside the sample was modeled using COMSOL Multiphysics (USA) to identify the level of temperature non-uniformity, and a pathway was proposed for the deposition of Al(OH)3 on the composite surface. A temperature gradient of 399 to 741 °C from the reactor surface to the center of the sample was found after 9 min of microwave irradiation. Maximum arsenite and arsenate adsorption capacities of 10,728 and 11,786 µg/g were attained by AlZnBC which were markedly higher than Zn oxide impregnated biochar capacities (ZnBC) of 6,030 µg/g and 7,783 µg/g, respectively. Studying the effect of co-existing ions selenite (SeO32−), selenate (SeO42−), phosphate (PO43−), nitrate (NO3−), and chloride (Cl−) on the biochar arsenic adsorption capacities indicated an interfering effect of the SeO32− and PO43− ions with inner-sphere complexation proposed as the main adsorption mechanism. X-ray absorption near edge structure spectroscopy (XANES) demonstrated that about 24% of the added arsenite was oxidized to arsenate, while no reduction was detected in arsenate adsorption experiments. Overall, both the ZnBC and AlZnBC exhibit promising potential for use in treating arsenic contaminated waters.