Chromium removal from aqueous solution using bimetallic Bi⁰/Cu⁰-based nanocomposite biochar

Chromium (Cr), due to its greater contamination in aquifers and distinct eco-toxic impacts, is of greater environmental concern. This study aimed to synthesize nanocomposites of almond shells biochar (BC) with zerovalent bismuth and/or copper (Bi⁰/BC, Cu⁰/BC, and Bi⁰-Cu⁰/BC) for the removal of Cr from aqueous solution. The synthesized nanocomposites were investigated using various characterization techniques such as XRD, FTIR spectroscopy, SEM, and EDX. The Cr removal potential by the nanocomposites was explored under different Cr concentrations (25-100 mg/L), adsorbent doses (0.5-2.0 g/L), solution pH (2-8), and contact time (10-160 min). The above-mentioned advanced techniques verified successful formation of Bi⁰/Cu⁰ and their composite with BC. The synthesized nanocomposites were highly effective in the removal of Cr. The Bi⁰-Cu⁰/BC nano-biocomposites showed higher Cr removal efficiency (92%) compared to Cu⁰/BC (85%), Bi⁰/BC (76%), and BC (67%). The prepared nanocomposites led to effective Cr removal at lower Cr concentrations (25 mg/L) and acidic pH (4.0). The Cr solubility changes with pH, resulting in different degrees of Cr removal by Bi⁰-Cu⁰/BC, with Cr(VI) being more soluble and easier to adsorb at low pH levels and Cr(III) being less soluble and more difficult to adsorb at high pH levels. The experimental Cr adsorption well fitted with the Freundlich adsorption isotherm model (R² > 0.99) and pseudo-second-order kinetic model. Among the prepared nanocomposites, the Bi⁰-Cu⁰/BC showed greater stability and reusability. It was established that the as-synthesized Bi⁰-Cu⁰/BC nano-biocomposite showed excellent adsorption potential for practical Cr removal from contaminated water.