MgO-impregnated niger straw derived activated carbon: a low-cost adsorbent for efficient removal of hexavalent chromium from aqueous solution and real tannery wastewater

Abstract The discharge of untreated industrial effluents containing toxic pollutants like heavy metals, dyes, and other chemicals raised serious concern due to the significant risk it poses to the human health and ecosystem. These days, chromium is one of the heavy metal contaminants arising from industrial settings and causes major problems for the environment and human health. This study investigates batch adsorption of hexavalent chromium, Cr (VI), from aqueous solutions using activated carbon derived from Niger straw (Guizotia abyssinica) and impregnated with MgO. To improve the percentage removal efficiency, the Guizotia abyssinica straw was chemically activated using phosphoric acid (50%) and further modified using thermal pyrolysis at 500 °C. The MgO-impregnated activated carbon was prepared via wet impregnation with a Mg (NO₃)₂·6H₂O solution to enhance the performance of the activated carbon in removing hexavalent chromium, Cr(VI), from tannery wastewater. The effect of parameters affecting the adsorption process, such as initial chromium (VI) concentration, pH, adsorbent dosage, agitation speed, and contact time, was examined. The result showed that MgO-impregnated activated carbon was determined to have a hexavalent chromium removal efficiency of 91.96% from real tannery wastewater, which is far better than the removal efficiency using bare activated carbon, 86.04%. XRD, TGA, and FTIR were used to characterize the produced adsorbent. The synthesized MgO-nanoparticle/activated carbon composite was determined to have an average particle size of 61.08 nm. The MgO/AC composite was determined to have a Cr(VI) removal efficiency of 99.57% from aqueous solution at optimum working conditions of pH 2, initial Cr (VI) concentration of 5 mg/L, adsorbent dose of 0.8 mg/L, contact time of 40 min, and agitation speed of 200 rpm. Linear pseudo-second-order kinetics (R 2 = 0.99991) and the Freundlich isotherm model (R 2 = 0.9873) were seen to be the best models describing Cr (VI) adsorption on the MgO/AC composite. On the other hand, nonlinear fitting of the experimental data demonstrated pseudo-second-order (R 2 = 0.99265) and Langmuir isotherm (R 2 = 0.9409) to better describe the adsorption process. Overall, the MgO/AC composite exhibited excellent adsorption performance for hexavalent chromium, demonstrating its potential as an efficient, stable, and low-cost adsorbent for water treatment. The study further recommends the use of Guizotia abyssinica straw waste as an abundant and economical precursor for activated carbon production, providing a sustainable alternative to more expensive materials for Cr (VI) removal.