Double-Silicate Derived Hybrid Foams for High-Capacity Adsorption of Textile Dye Effluent: Statistical Optimization and Adsorption Studies

In this work, a lightweight inorganic–organic hybrid foam adsorbent is processed out of economically cheap “double-silicate” precursors employing natural bentonite and water glass through a facile cross-linking and polymerization technique. Poly­(vinyl alcohol) was used to provide structural strength to the three-dimensional framework. The foam adsorbent possessed an apparent density of 0.083 g cm^–3, indicating its internetworks and exposed surface area for the uptake of dyes. The foam was systematically studied for the treatment of textile dye effluent. Multivariate optimization process was carried out using response surface methodology. The Box–Behnken model was used for the design of experiments and to study the interplay between the variables. Batch adsorption and continuous column adsorption studies were carried out at respective levels of initial concentration (200–1000 μM), adsorbent dose (1–10 g L^–1), and contact time (0–120 min). The results revealed that the hybrid silicate foam exhibits adsorption capacity as high as 99.9, 98.9, and 98.2% for 200, 600, and 1000 μM concentrations of methylene blue in 120 min, respectively, and 100% adsorption for 200, 600, and 1000 μM concentrations of crystal violet in 120 min for 10 g L^–1 of adsorbent. Adsorption equilibrium data fitted well to Langmuir isotherm, and the kinetics followed second-order kinetic model. Synthetic industrial effluent with 1000 μM dye concentration was also prepared and studied with continuous column for determining the working capacity of the adsorbent, and the results are presented. The silicate hybrid foam is a cheap adsorbent that does not produce any secondary waste and can be repeatedly used making it attractive for dye industries.