Continuous Adsorption of a Cationic Dye on Surface Modified Rice Husk: Statistical Optimization and Dynamic Models

<p>The continuous adsorption of a cationic dye (Methylene Blue, MB) on surface-modified rice husk was investigated. First, rice husk was submitted to ultrasound-assisted, supercritical CO₂ and NaOH treatments. The adsorbents were characterized. Then, the continuous adsorption was optimized by response surface methodology (RSM), using raw rice husk as the adsorbent. Finally, under the optimal conditions, breakthrough curves were obtained using all adsorbents and the models were used to interpret these curves. The optimal bed performance was reached at a flow rate of 5 mL min⁻¹ and an initial MB concentration of 10 mg L⁻¹. Under these conditions, the breakthrough time was 109 min, the length of the mass transfer zone was 20.1 cm, and the maximum capacity of the column was 1.55 mg g⁻¹. All surface modifications were able to improve the rice husk characteristics in relation to the MB adsorption. Consequently, the bed performance was significantly improved when the surface-modified adsorbents were used. The breakthrough times were 109, 240, 155, and 385 min, respectively, when raw rice husk, UA–rice husk, SCO₂–rice husk, and NaOH–rice husk were used. The length of the mass transfer zone was 20.1, 7.9, 15.9, and 9.3 cm for raw rice husk, UA–rice husk, SCO₂–rice husk, and NaOH–rice husk, respectively. The dynamic models were able to fit the adsorption data and provided physically consistent parameters.</p

Microwave-activated carbons from tucumã (<i>Astrocaryum aculeatum</i>) seed for efficient removal of 2-nitrophenol from aqueous solutions

<p>Activated carbons (ACs) prepared from tucumã seed (<i>Astrocaryum aculeatum</i>) were used for 2-nitrophenol removal from aqueous solutions. The ACs were characterized by elemental analysis, FTIR, <i>N</i>₂ adsorption/desorption isotherms, TGA, hydrophobicity/hydrophilicity balance, and total of acidic and basic groups. The ACs showed to have hydrophilic surfaces and they presented high specific surface areas (up to 1318 m² g⁻¹). In batch optimization studies, maximum removal was obtained at pH 7, contact time of 30 min, adsorbent dosage 1.5 gL⁻¹ and temperature of 50°C. The general-order kinetic model and Liu isotherm model best fit the kinetic and equilibrium adsorption data with a maximum adsorption capacity of 1382 mg g⁻¹ at 50°C. Effect of temperature and thermodynamic studies revealed that the adsorption processes of 2-nitrophenol onto ACs are dependent on temperature and are exothermic and spontaneous, respectively. About the applicability of the ACs for treating simulated effluents, the tucumã seed-activated carbon showed an excellent outcome in the treatment of simulated effluents, evidencing its high efficiency for phenolic compound adsorption. Tucumã seed-ACs showed to be cost effective and highly efficient adsorbents for efficient removal of 2-nitrophenol from aqueous solutions.</p