Utilization of torrefied rice husk and deoiled cashew nut shell cake biomass waste for removal of hazardous Reactive Violet 5 dye

Abstract

Torrefied biomass is a renewable, sustainable, carbon–neutral fuel that is replacing coal in many areas. This organic source has increased surface area and porosity, making it an effective adsorbent. Only a very few works have been reported in the literature on exploring the use of torrefied biomass and its ash as inexpensive adsorbents for the removal of dyes. In this work, rice husk and deoiled cashew nut shell cake in its torrefied and ash forms have been employed as adsorbents for Reactive Violet 5 dye removal. Thus, four adsorbents were synthesized and characterized by BET, SEM, XRD, and FTIR. The optimum parameters for maximum dye removal efficiency were 0.8 g/L adsorbent dosage, pH 2.0, and 40-min contact time for batch experiments using 100 mL of 10 ppm dye solution. The maximum adsorption capacity of torrefied rice husk, torrefied deoiled cashew nut shell cake, torrefied rice husk ash, and torrefied deoiled cashew nut shell cake ash were 108.58 mg/g, 88.38 mg/g, 68.0 mg/g, and 29.97 mg/g, respectively. Torrefied rice husk exhibited the best fit with the Freundlich isotherm, whereas other adsorbents fitted best with the Langmuir isotherm. The non-linear pseudo-second-order model gave the best fit. The intraparticle diffusion model showed that adsorption involves multiple diffusion stages. The thermodynamic parameters indicated that the process was spontaneous, feasible, and exothermic. These eco-friendly materials can be efficiently used in dye removal, thus paving the way for torrefied biomass or its ash to be used as low-cost adsorbents for large-scale wastewater treatment. © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2025.