Cetyltrimethylammonium bromide (CTAB) functionalization of sodium silicate from rice husks ash for Naphthol Green B and Congo Red adsorption

The present work investigates the potential use of sodium silicate generated from rice husk ash and functionalized with cetyltrimethylammonium bromide (CTAB), for adsorption of Congo Red (CR) and Naphthol Green B (NGB) from aqueous solutions. The adsorption capacity of raw rice husk, calcinated rice husk (673, 873, 1073, and 1273 K), and functionalized sodium silicate were investigated. They were characterized by SEM, FTIR, TGA–DSC, and zeta potential analysis. The results showed that sodium silicate adsorption capacity and the functionalization effect were maximum at 873 K. The optimum adsorption conditions for CR and NGB onto Na₂SiO₃-CTAB adsorbent are as follows: contact time 30 min, pH 6, adsorbent dosage of 0.1 g, temperature 303 K, and initial dye concentration of 50 mg/L and 150 mg/L for NGB and CR, respectively. The maximum adsorption capacities for CR and NGB were recorded at 73.04 and 86.59 mg/g at 303 K, respectively. The pseudo-first-order kinetic model described well the adsorption of these dyes. However, Freundlich and Liu isotherm models described well the adsorption of NGB and CR, respectively. According to thermodynamic parameters, these adsorptions were endothermic and involved physical processes. Desorption capacities of 96.21% (for CR) and 92.21% (for NGB) were reached using 50% acetone solution. A maximum of four adsorption/desorption cycles was carried out. Thus, it can be concluded that sodium silicate modified with CTAB after calcination at 873 K is a potential low-cost adsorbent and renewable sorbent for the removal of dyes from industrial wastewater.