Synthesis and two-step KOH based activation of porous biochar of wheat straw and waste tire for adsorptive exclusion of chromium (VI) from aqueous solution; thermodynamic and regeneration study

Abstract

Biochar derived from Wheat Straw (WS), Waste Tire (WT), and their combination of 1:1 (WS/WT) was processed through different chemical activation methods i.e., wet (molar solution of KOH) and dry processes (physical mixing of biochar and KOH). The synthesized adsorbents were characterized by SEM-EDS, FTIR, BET, XRD and Point of Zero charge (PZC). Efficient adsorbent was selected based on the results of characterization and removal efficiency for chromium (VI). Among the synthesized adsorbents, the wheat straw-based biochar, activated by 1:3 proportion of KOH in dry process (WSKOHdry 1:3) showed maximum removal, possessed well developed porous structure and the largest specific surface area (529.48 m2/g). The adsorbent WSKOHdry 1:3 achieved maximum removal efficiency of Cr (VI) at optimum conditions i.e., pH 2, contact time 120 min, adsorbent dosage 2 g/L and temperature 25 °C. The kinetic study indicates that the removal of Cr (VI) by WSKOHdry 1:3 followed pseudo-second order kinetic model and Elovich kinetic model. Moreover, the Freundlich Isotherm model presented better fitting of the experimental data of Cr (VI) adsorption. The maximum Langmuir adsorption capacity (qmax) of WSKOHdry 1:3 at 25 °C was 96.082 mg/g. The thermodynamic analyses revealed that Cr (VI) uptake is endothermic (+ΔH) and spontaneous (−ΔG) in nature. Furthermore, electrostatic attraction, complexation, ion exchange, and reduction to chromium (III) were involved in the removal of Cr (VI) by WSKOHdry 1:3. Hence, the results proposed that WSKOHdry 1:3 exhibited great potential for Cr (VI) adsorption.