Areca nut husk biochar prepared through a low-cost kiln for the adsorptive elimination of nickel and manganese from water: a farmer-friendly approach for agro-residue management

The application of biochar, a thermochemically converted blackish biomass, is considered an economical and efficient strategy for the remediation of nickel (Ni²⁺) and manganese (Mn²⁺) ions from watery matrix to mitigate environmental contamination and safeguard public health. A wide variety of techniques, ranging from laboratory to industrial scale, have been developed to produce biochar. However, in rural areas of developing countries, biomass is typically used for cooking without being charred. The strategies for producing biochar on a small scale for farmers have not advanced significantly. In this current study, a scraped 200-L oil drum is modified to fabricate a drum kiln setup for producing biochar from areca nut husk (AH), a locally available agro-residue of Northeast India. The prepared areca nut husk biochar (AHB) was further evaluated for adsorptive removal of Ni²⁺ and Mn²⁺ from water. AHB reaches the equilibrium time at 150 min and 180 min for Ni²⁺ and Mn²⁺, respectively. The experimental data has shown a good fit for the pseudo-second-order kinetic model and Freundlich isotherm model for both metals, revealing a chemisorption-driven monolayer adsorption mechanism. The presence of oxygen-containing functional groups on the AHB surface required for chemisorption was confirmed by different spectroscopy methods. AHB could be regarded as an effective adsorbent for the rapid elimination of heavy metals from water, and the biochar production strategy using a drum kiln has been demonstrated to be the simplest and most effective method for achieving self-sustainability.