Alternative treatments to improve the potential of rice husk as adsorbent for methylene blue

Alternative treatments, such as, NaOH, ultrasound assisted (UA) and supercritical CO₂ (SCO₂), were performed to improve the potential of rice husk as adsorbent to remove methylene blue (MB) from aqueous media. All the treatments improved the surface characteristics of rice husk, exposing its organic fraction and/or providing more adsorption sites. The Langmuir and Hill models were able to explain the MB adsorption for all adsorbents in all studied temperatures. The experimental and modeled parameters demonstrated that the MB adsorption was favored by the temperature increase and by the use of NaOH-rice husk. The maximum adsorption capacities for the MB solutions (ranging from 10 to 100 mg L^-1), estimated from the Langmuir model at 328 K, were in the following order: NaOH rice-husk (65.0 mg g^-1) > UA-rice husk (58.7 mg g^-1) > SCO₂-rice husk (56.4 mg g^-1) > raw rice husk (52.2 mg g^-1). The adsorption was a spontaneous, favorable and endothermic process. In general, this work demonstrated that NaOH, UA and SCO₂ treatments are alternatives to improve the potential of rice husk as adsorbent.