A novel treatment of biogas digestate waste for biochar production and its adsorption of methylene blue and malachite green in a binary system

Abstract This study presents a novel treatment of biogas digestate to produce biochar at processing temperatures of 500–700 °C. The resulting biochar was applied for the simultaneous removal of methylene blue (MB) and malachite green (MG) in a binary system. Characterization included metal element analysis, nitrogen adsorption/desorption isotherms, energy‐dispersive X‐ray spectroscopy with elemental mapping, scanning electron microscopy, and X‐ray diffraction. The biochar exhibited a high surface area with a porous structure ranging from 1 to 20 μm in diameter. Due to spectral overlap between the dyes, a principal component multivariate regression‐assisted spectrophotometric method was developed for their simultaneous determination in aqueous solution. Equilibrium data for the single‐component systems were evaluated using the Langmuir, Freundlich, and Sips isotherm models, with the Langmuir model providing the best fit. For the two‐component system, adsorption data were analyzed using the extended Langmuir (EL) model, the P‐factor model, and three combination models based on ideal adsorbed solution theory (IAST): IAST‐L (derived from the Langmuir isotherm), IAST‐F (from the Freundlich isotherm), and IAST‐S (from the Sips isotherm). The results showed that the simultaneous adsorption of MB and MG onto biogas digestate biochar (BDB) at 600 °C was best described by the IAST‐L and IAST‐F models. The maximum adsorption capacities in the single/binary systems, based on the Langmuir and IAST‐L models, were approximately 0.136/0.103 for MG and 0.32/0.24 for MB at 303 K. A thermodynamic analysis was also conducted. The desorption of MG and MB by BDB at 600 °C (BDB‐600) with methanol solvent exceeded 86% and 90% in the first adsorption/desorption cycle and did not decrease significantly after three cycles. The results of this research open up a new recycling route for the disposal of biogas digestate waste and its application as a sustainable alternative for the simultaneous removal of dyes.