Highly porous biochars from different biomasses as potential adsorbents for chromium removal: optimization by response surface methodology

This work focused on preparing biochars from different biomasses: orange, pomegranate, and watermelon peels, as promising materials for the removal of Cr (VI) removal. Chemical activation was performed using the phosphoric acid at the ratio 2:1 (wt%) between the biomasses and the acid, followed by nitrogen pyrolysis. The physicochemical properties of the activated carbons were characterized via pH of zero charge, Fourier transform infrared spectroscopy, elemental analysis, surface area, nitrogen adsorption–desorption isotherms, particle size distribution, and scanning electron microscopy-energy-dispersive X-ray spectroscopy. Changing the precursor type had significant effect on the porous structure of the produced activated carbon. Pomegranate-based biochar exhibited the highest specific area exceeding 2000 m2 g−1 with a total pore of 1.2 cm3 g−1. Doehlert experimental design was employed to evaluate the influence of adsorption process factors (adsorbent amount, pH, and temperature) on Cr (VI) removal by the three biochars. All three models present good correlation coefficient R2 greater than 0.975. The optimum conditions were at pH 2.54, temperature of 36.83 °C and an amount of 9 mg for orange biochar. For both biochars derived from pomegranate and watermelon, the optimal conditions were at pH 1.5, temperature of 22 °C and an amount of 10 mg. The kinetic results showed good correlation with pseudo-first-order model for orange and watermelon biochars, while the pseudo-second-order fits the data for pomegranate biochar. Maximum Langmuir adsorption capacities of hexavalent chromium were found to be 53.38 mg g−1, 73.01 mg g−1, 64.21 mg g−1, for orange, pomegranate, and watermelon biochars, respectively.