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.