Effective removal of Cr(VI) ions from the aqueous solution by agro-waste-based biochar: an exploration of batch and column studies

In the present study, the thermal conversion of Arachis hypogaea (groundnut) husk into biochar and its subsequent application in the adsorptive removal of Cr(VI) from an aqueous solution has been studied. The SEM–EDX and LIBS analyses showed that the adsorbent material incorporates both magnesium (Mg) and chromium (Cr) on its surface. The FTIR spectra revealed the involvement of several functional groups in the Cr(VI) adsorption, including –OH, Mg–O–Mg, –CH, –NH, and –COO. Specific surface area (SSA) is determined using BET (Brunauer–Emmett–Teller) analysis and the SSA of adsorbent material was 35.83 m² g⁻¹. Data from batch adsorption experiments demonstrated that magnesium-supported biochar increased the binding capacity of the adsorbent to the Cr(VI) ions. Adsorption experiment data at equilibrium conditions showed that the Langmuir model provided a best suited to the nMgO@GHBC than the Freundlich and Temkin models with a maximum adsorption capacity of 93.86 mg g⁻¹. The kinetic-second-order model with the highest R² value of 0.993 and the lowest values for root mean square error and chi-square (χ²) can reasonably explain the hexavalent chromium adsorption data. This study suggested that the mechanism of Cr(VI) adsorption onto both adsorbents might be chemisorption or ion exchange processes as adsorption data effectively fitted the PSO model. The outcomes of thermodynamic parameters like ΔG° indicated that Cr(VI) adsorption on pGHBC and nMgO@GHBC was spontaneous. Fixed-bed-column studies were conducted at different experimental conditions, such as inlet flow rate and bed height. The outcomes of the experimental data showed that the adsorption capacity of adsorbent for Cr(VI) adsorption in a fixed bed continuous flow was 50 mg g⁻¹. The adsorbent was recycled ten times using NaOH, HNO₃, H₂SO₄, and EDTA. The continuous adsorption and desorption of Cr ions for up to 10 cycles showed the reusability of spent biochar. Graphical Abstract: (Figure presented.)