Polypyrrole–polyaniline copolymer coated green rice husk ash as an effective adsorbent for the removal of hexavalent chromium from contaminated water

Abstract

The acute toxicity and mobility of hexavalent chromium [Cr(VI)] in water and wastewater pose a severe risk to the environment and human health. In view of this, the present work is focused on the fabrication and characterization of polypyrrole–polyaniline coated rice husk ash (termed as PPY–PANI@RHA) by in situ polymerization and its application as an adsorbent in the removal of Cr(VI) from the aqueous solution. Our findings have shown a Cr(VI) removal rate of ∼98% at room temperature (303 K) under optimum conditions with an adsorbent dose of 0.8 g L⁻¹, an adsorbate concentration of 50 mg L⁻¹, a solution pH of ∼2, and a contact time of 300 min. The adsorption of Cr(VI) followed the Elovich kinetics and is better described using the Freundlich isotherm with a maximum adsorption capacity of 769.15 mg g⁻¹. The co-existing ion study result shows that the Cr(VI) removal efficiency is slightly decreased in the presence of high concentrations of Na⁺, Mg²⁺, Ca²⁺, Cl⁻, HCO₃⁻, NO₃⁻, SO₄²⁻ and PO₄³⁻ ions. Thermodynamic investigations have shown that the adsorption phenomenon of Cr(VI) on PPY–PANI@RHA is endothermic, feasible, and spontaneous in nature. Additionally, the reusability study of the spent PPY–PANI@RHA adsorbent indicated its high removal efficiency for several times. Finally, PPY–PANI@RHA was also successfully used to remove Cr(VI) ions from the highly polluted raw tannery wastewater sample. Our findings clearly demonstrated that PPY–PANI-coated rice hush ash could be an alternative as an effective bio adsorbent in the removal of Cr(VI) for wastewater treatment and benefit the industries in the future.