Synthesis and characterization of promising biochars for hexavalent chromium removal: application of response surface methodology approach

Hexavalent chromium removal was evaluated through adsorption on two biochars, chemically modified with zinc chloride at different impregnation ratios. The prepared materials were characterized via elemental analysis, surface area, particle size distribution, pH of zero charge, Fourier transform infrared spectroscopy and scanning electronic microscopy analysis. The biochar prepared at 1:1 impregnation ratio developed higher microporosity (65%) and a higher specific area (1293 m2 g−1). Three-factor model was investigated to evaluate the removal efficiency of each biochar, according to the Doehlert design. The experimental results revealed that pH is the most significant factor and negatively affects the removal yield. Optimal adsorption conditions were obtained at pH equal to 1.77 and 1.83, adsorbent amount of 11 mg and at 31 °C, leading to hexavalent chromium removal yield of 70% and 95%, for biochars with 1:0.5 and 1:1 impregnation ratios, respectively. Maximum Langmuir adsorption capacities of hexavalent chromium were found to be 57.72 and 177.64 mg g−1, for biochars with impregnation ratios 1:0.5 and 1:1, respectively.