Magnetic rice-straw-derived biochar for adsorptive removal of Hg(II) from aqueous solution: Optimization using response surface methodology

Magnetic rice-straw-derived biochar (MRBC) composites were synthesized by binding magnetite to the surface of the biochar for the highly efficient removal of Hg(II) from an aqueous solution. The maximum adsorption capacity of the adsorbent was 85.8 mg/g, which is much higher than those of other previously reported adsorbents for Hg(II) removal. The experimental data exhibited a good fit with a pseudo-second-order kinetic model, and the adsorption behavior followed the Langmuir isotherm. A thermodynamic study indicated that Hg(II) adsorption was spontaneous and endothermic. The individual effects and interactions of three key parameters such as initial Hg(II) concentration, adsorbent dosage, and pH were investigated by the Box–Behnken design of response surface methodology. The maximum removal efficiency of Hg(II) was 94.91% under an optimal condition of pH 3.9, an initial Hg(II) concentration of 14.3 mg/L, and an adsorbent dosage of 4.5 g/L. In the adsorption–desorption investigation, the MRBC was reusable for five adsorption–desorption cycles. The synthesized MRBC is a simple and inexpensive absorbent that can be readily isolated from a solution using a magnet after Hg(II) remediation.