Enhanced Removal of Methylene Blue Dye by Sustainable Biochar Derived from Rice Straw Digestate

Abstract

The bioconversion of biomass-based feedstocks to carbonaceous adsorbents is considered a cost-effective and environmentally friendly route for pollutant removal from wastewater. This study focused on preparing biochar from the pyrolysis of digestate of anaerobically processed rice straw, namely digested rice straw biochar (DRSB). The DRSB material was employed as an adsorbent to eliminate methylene blue (MB) from aqueous solutions. The adsorption mechanism was illustrated regarding DRSB characterization via Scanning Electron Microscopy (SEM) related surface morphology and Fourier Transform Infrared Spectroscopy (FTIR) associated surface functional groups. The effect of adsorption factors (solution pH, initial dye concentration, contact time, and adsorbent dosage) on the MB removal efficiency was investigated. The optimum adsorption pH value was 7, achieving MB removal efficiency of 92.98% using adsorbent dosage = 6 g/L and initial MB concentration = 20 mg/L, within 60 min. The experimental data fitted well onto the Langmuir (R2= 0.94) and Freundlich (R2= 0.99) adsorption isotherm models. The Langmuir maximum adsorption capacity (Qm) was estimated as 18.90 mg/g and the Freundlich intensity parameter (1/n) was derived as 0.58, indicating the favorability of MB adsorption onto DRSB. Adsorption kinetics were also tested and explained using pseudo-first-order and pseudo-second-order models, suggesting the contribution of both chemisorption and physisorption mechanisms for MB uptake. The synthesis of DRSB revealed a feasible economic adsorbent with a total cost of 0.3022 US$/kg. This study depicted that the utilization of digested residues resulting from the anaerobic digestion of agricultural wastes for preparing biochar adsorbent would be considered for the real application of dye-laden textile wastewater treatment.