Comparison of the adsorption performance of coconut husk and palm kernel shells biochars for the removal of toxic metals from mining wastewater

• The biochar made from coconut husk and palm kernel shells adsorbs As, Cd, Cr, and Ni. • The adsorption efficiency of coconut husk biochar for As was ≥ 94.00 %, ≥15.49 % for Cd, ≥ 42.70 % for Cr and ≥ 53.11 % for Ni at 45 °C for 60 min. • The PSO suggested that the As, Cd, Cr, and Ni adsorption on coconut husk biochar and palm kernel shell biochar occurs due to chemical processes. • Coconut husk and palm kernel shells that are locally accessible, affordable, and ecologically friendly to support large-scale wastewater treatment operations. Toxic metal in mining wastewater is a significant concern for the environment and human health, prompting improved efforts to develop sustainable and efficient removal methods. This study characterised and compared the performance of coconut husk and palm kernel shells biochars in adsorbing toxic metals from mining wastewater. The biochars were produced through pyrolysis and characterised using Fourier transform infrared spectroscopy. Batch sorption experiments of toxic metals with biochars were conducted to investigate the effects of adsorbent dose, residence times and varied temperatures. The adsorption efficiency for the coconut husk biochar for arsenic ranged from 92.50 % to 97.50 %, 10.79 % to 24.88 % for cadmium, 46.79 % to 57.71 % for chromium and 52.78 % to 59.55 % for nickel at 30 °C for 30 min. The adsorption efficiency for palm kernel shell biochar ranged from 90.00 % to 97.50 % for As, 15.49 % to 48.35 % for Cd, 56.34 % to 91.81 % for Cr, and 34.18 % to 60.15 % for Ni at a temperature of 30 °C for 30 min. The coconut husk and palm kernel shells biochars showed high adsorption effectiveness, which can be attributed to their lignocellulose content, surface area, organic functional groups, and mineral content. Adsorption followed the Langmuir isotherm and pseudo-second order adsorption kinetics for the toxic metals, indicating monolayer chemisorption as the main mechanism. The coconut husk and palm kernel shell biochars are effective adsorbents for treating mining wastewater, with coconut husk biochar showing superior performance due to its higher surface reactivity. There is a need to create guidelines for selecting suitable materials for particular contaminants.