Adsorption of Hexavalent Chromium onto Wood-Based Adsorbent: Kinetic and Isotherm Study

The valorization of biomass waste into functional adsorbent materials represents a sustainable approach for water treatment. Hydrochar exhibits promising adsorption properties, due to its porosity and active functional groups on the surface. In this study, the potential of hydrochar for hexavalent chromium removal from water was investigated. A mixture of beech and oak sawdust was converted into hydrochar (HC) through hydrothermal treatment in a reactor. The reaction was performed at 200°C and 20 bar for 165 min using a biomass-to-water ratio of 1:10. Batch adsorption experiments were conducted at pH 2 using 0.1 g of HC and 100 mL of Cr(VI) solution. Kinetic studies were carried out using a 50 mg/l Cr(VI) solution under continuous magnetic stirring during different time intervals (5-2880 min). For isotherm studies, Cr(VI) concentrations ranged from 5 to 300 mg/L during 24 h. The mixture was filtered and residual concentrations were determined by a spectrophotometric method using diphenylcarbazide. Non-linear regression was used to calculate the adsorption parameters and models that best fit the experimental data. The adsorption experiment data were processed using pseudo-first-order, pseudo-second-order, and Elovich models. Best fit is shown by the Elovich model (R²=0.9871), indicating that adsorption is heterogeneous with energetically different adsorption sites. Adsorption on HC is best described by the Temkin isotherm compared to the Langmuir and the Freundlcih model. In addition, the three-parameter Sips model, which represents a combination of Langmuir and the Freundlich isotherm models, was also used to analyze process. The obtained value of the parameter n_s (0.362) confirms that the adsorption system does not ideally follow the Langmuir model (n_s≠1). The highest removal efficiency after 24 h (83.9%) was achieved at an initial Cr(VI) concentration of 50 mg/l. The results indicate that HC has the potential to be used as a biosorbent for the removal of Cr(VI) ions.