Adsorption of hexavalent chromium from wastewater using magnetic biochar derived from peanut hulls

In this study, the utilization of peanut hulls as a precursor for the preparation of magnetic biochar through pyrolysis was investigated. To enhance the magnetic and adsorption properties of the biochar, the peanut hulls biomass was modified using ferric chloride hexahydrate and magnesium chloride hexahydrate. Response surface methodology was employed to evaluate the influence of biomass metal concentration, pyrolysis temperature, pyrolysis period and flow of nitrogen on the yield and Cr (VI) adsorption efficiency of the synthesized biochar. A 17-run experimental matrix was generated using Optimal Design to investigate the interactions among four input parameters. The results led to the development of a quadratic model, which demonstrated a high degree of predictability in accordance with the experimental data. Analysis of variance (ANOVA) confirmed that the models for yield and Cr (VI) adsorption efficiency were highly significant (p < 0.05), with coefficients of determination (R²) values of 0.891 and 0.988, respectively. The optimal synthesis conditions for producing biochar with superior physicochemical properties were identified as a pyrolysis temperature of 300 °C, a pyrolysis duration of 2 h, a metal-to-biomass ratio of 0.5, and a constant flow of nitrogen. A desirability of 85% was achieved through numerical optimization, corresponding to a yield of 63% and complete Cr (VI) removal. Further optimization of Cr (VI) adsorption efficiency, considering the effects of pH (3–12), adsorbent loading (1–15 g/L), and initial Cr (VI) concentration (5–20 mg/L), was performed using a 19-run experimental matrix. ANOVA for Cr (VI) adsorption efficiency model revealed high significance (p < 0.05) with an R² value of 0.916.

The magnetic biochar demonstrated a remarkable adsorption efficiency of 98% under the experimental conditions of solution pH 3, adsorbent dosage of 5 g/L, and an initial Cr (VI) concentration of 20 mg/L. The desirability of 100% was obtained by a numerical optimization method representing Cr (VI) removal of 98%. The adsorption behaviour was adequately described by the Freundlich isotherm model, suggesting multilayer adsorption, with a maximum adsorption capacity of 12 mg/g. Biochar also proved to have strong magnetic properties which enhanced solid-liquid separation post adsorption experiments.