Adsorptive capacity of activated carbon derived from rice husks for azo dye removal from solution

This study aimed at developing low-cost adsorbents from waste rice husk which is a nuisance to farmers through sulfuric (H2SO4) and phosphoric (H3PO4) acid treatment. The raw rice husk (RRH) was treated with 85% H3PO4 and H2SO4 and calcined at 500 °C in a muffle furnace. The effects of initial methyl orange (MO) concentrations, adsorbent dosage and contact time on the dye removal process was studies. The adsorbents were characterized by Fourier transform infrared spectroscopy (FITR), Energy Dispersive X-ray (EDX) and scanning electron microscopy (SEM). The infrared spectral data revealed the emergence of a new peak at 1097. 27 cm−1 in the phosphoric acid modified RH (MH3PO4RH) along with shifts and reductions in the intensities of functional groups. The adsorption peaks at 3377. 12, 3336. 73 and 3449. 99 cm−1 for the RH indicates the existence of hydrogen-bonded OH stretching. In RH, the C–H Stretching vibration at 2923. 00, 2844. 20 and 2925. 57 cm−1 respectively revealed the alkane functional group being present. The peaks around 2143. 06 – 2378. 09 cm−1 indicate the presence of C≡C stretching while the peaks around 1671. 63 – 1741. 96 cm−1 correspond to C = O stretching that may be attributed to the hemicelluloses. The surface morphology changed, as seen in SEM and EDX photos, with rougher cavities and a higher proportion of carbon content, which may have an impact on the adsorptive qualities. The surface area of RRH, 0. 32 m2/g increased by 14. 5 fold in M H₃ P O₄ RH and sulphoric acid modified RH (M H₂ S O₄ RH) by 22 fold. Additionally, after being treated with acids, the rice husk’s surface structure changed from being non-porous to porous. The percentage adsorption of MO was discovered to increase with the contact time increase, initial MO concentration and dosage. For MH3PO4RH, the Langmuir isotherm was the most suitable to explain the adsorption process, while the Freundlich isotherm was the most suitable for MH2SO4RH. . The Langmuir maximum monolayer adsorption capacity were 166. 7 (M H₂ S O₄ RH) and 312. 5 mg/g (M H₃ P O₄ RH). The M H₂ S O₄ RH kinetics data were fitted well to the pseudo-second order adsorption model with a higher coefficient of correlation R2 = 0. 999. An endothermic, spontaneous and positive enthalpy and entropy change were observed, indicating the affinity of MO molecules to the adsorbent surface.