Mechanism of Cr(VI) uptake onto sagwan sawdust derived biochar and statistical optimization via response surface methodology

The present study elaborates the use of sagwan sawdust biochar as an efficient adsorbent for Cr(VI) abatement. Effect of experimental variables like solution pH (2-10), initial Cr(VI) concentration (30-100 mg/L), temperature (30-40 degrees C), adsorbent dose/Cr(VI) concentration (16.67-200), and contact time (10-80 min) was studied through batch operations. The optimization and interaction study for process parameters was done by employing response surface methodology through Box-Behnken design. The optimized condition for the maximum adsorption was pH 2.07, temperature 30.72 degrees C, and dose/Cr(VI) concentration 160.93. The surface characteristics of the adsorbent were evaluated via pH(zpc), FTIR, BET surface area, SEM-EDS, XRD, and XPS analysis. Functional groups like O-H, C-O, C-H, and C=O were responsible for the adsorption process, and change in the oxidation state of Cr was affirmed by XPS survey. The Cr(VI) adsorption was governed by second-order kinetics and the Langmuir isotherm with Q(0)of 9.62 mg/g. The thermodynamic study revealed the process to be exothermic and spontaneous. Mass transfer studies confirmed film diffusion to be favorable in the adsorption process. Adsorption occurred via electrostatic and physical attraction, reduction, and complexation. Therefore, the results suggest sagwan sawdust biochar is satisfactory and effective in removing Cr(VI) from aqueous solution.