Adsorption optimization and modeling of Hg2+ ions from aqueous solutions using response surface methodology by SNPs-CS bionanocomposite produced from rice husk agro-industrial waste as a novel environmentally-friendly bionanoadsorbent

In the present research, extraction of silica (SiO2) from rice husk (RH) was optimized and silica nanoparticles (SNPs) was produced using it and functionalized by chitosan (CS) functional groups to obtain CS functionalized SNPs (SNPs-CS) bionanocomposite for the first time. The physical and chemical characteristics of the produced materials were examined using structural analyses. The results of structural analyses confirmed the fine structure of the produced materials. The SNPs-CS bionanocomposite was applied to effectively remove Hg2+ ions from aqueous solutions as an environmentally-friendly bionanoadsorbent and optimization and modeling of the adsorption conditions was explored using designed experiments by Design-Expert software with central composite design (CCD) and response surface methodology (RSM). Optimum adsorption conditions were obtained as solution pH of 6, SNPs-CS dosage of 0.1 g L-1 and Hg2+ ions concentration of 100 mg L-1 by removal efficiency of 85% and desirability function of 0.876. The results of adsorption kinetic showed a better fit of the pseudo-second-order model with experimental data, indicating the chemisorption of the adsorption process. The better fit of the Langmuir model with experimental data was confirmed by the results of adsorption isotherms, demonstrating monolayer adsorption on the homogeneous surface. The adsorption thermodynamic results illustrated the exothermicity and spontaneity of the adsorption reaction. The results of SNPs-CS recovery depicted its excellent recovery ability of removal efficiency with more than 90% after five consecutive adsorption and desorption cycles, which proved high potential of the produced bionanocomposite for industrial applications.

Biocompatible Fe3O4@SiO2-NH2 nanocomposite as a green nanofiller embedded in PES-nanofiltration membrane matrix for salts, heavy metal ion and dye removal: Long-term operation and reusability tests

High purity amorphous silica (95.55%) was simply extracted from rice husk and used as a shell for Fe₃O₄ magnetic nanoparticles. The obtained eco-friendly nanomaterial was functionalized by (3-Aminopropyl)trimethoxysilane and characterized by SEM, FT-IR, XRD, VSM, TEM and zeta potential analyses. The synthesized Fe₃O₄@SiO₂-NH₂ nanocomposite was embedded into the polyethersulfone membranes with different concentrations via phase inversion method. The effects of Fe₃O₄@SiO₂-NH₂ on various properties of the prepared nanofiltration membranes including membrane morphology, hydrophilicity, porosity, and mechanical stability were investigated using SEM image, water contact angle test, porosity measurement and mechanical property analysis, respectively. The performance of Fe₃O₄@SiO₂-NH₂ nanocomposite modified membranes was evaluated by measuring the pure water flux, salts rejection, Cd(II) ions removal, MR dye retention, and antifouling property. The results showed that the pure water flux was significantly increased in Fe₃O₄@SiO₂-NH₂ modified membranes due to the presence of hydrophilic functional groups on the Fe₃O₄@SiO₂-NH₂ surface. In addition, the significant enhancement in efficiency of modified membranes for removal of Cd(II) ions and MR dye was observed due to the adsorption properties of Fe₃O₄@SiO₂-NH₂ nanocomposite. Among the modified membranes with different concentrations of Fe₃O₄@SiO₂-NH₂, the 0.5 wt% concentration of nanocomposite showed the highest efficiency for the removal of Cd(II) ions (93%) and MR dye (97%). Membrane reusability study indicated a slightly decrease (≈7%) in Cd(II) ions removal after five continuous cycles. Membrane long-term filtration study showed a little reduction in permeate flux with unchanged MR dye retention during long-term filtration, which confirmed the stability of modified membrane.

Synthesis of magMCM-41 with rice husk silica as cadmium sorbent from aqueous solutions: parameters' optimization by response surface methodology

The magnetic mesoporous silica of magMCM-41 with large surface area (695 m² g^-1) and high magnetization (10.79 emu g^-1) was synthesized using extracted amorphous silica from rice husk. The synthesized materials were applied for adsorption of Cd(II) ions from aqueous solution in batch operation systems. A highly selective adsorbent was obtained by grafting 3-aminopropyltrimethoxysilane on the pores of the magMCM-41 in which the adsorption capacity of Cd(II) ions increased from 41.8 to 86 mg g^-1, under the same conditions. A total of 20 sets of experiments were planned by the central composite design under response surface methodology. The effects of three independent variables pH, initial Cd(II) ion concentration and sorbent dosage were investigated on the adsorption capacity (q_e) and removal efficiency (R) of cadmium. The best responses for Cd(II) adsorption capacity and removal efficiency were 493.21 mg g^-1 and 60.25%, respectively, which was achieved at pH of 5.05, sorbent dosage of 0.1 g L^-1 and Cd(II) concentration of 150 mg L^-1. Additionally, the obtained value for desirability was equal to 0.807. The theoretical isotherm models were applied to describe the adsorption process that the Langmuir model provides the best correlation of the equilibrium data. The kinetics study revealed that data from the experiments fitted well to the pseudo-second-order equation than the pseudo-first-order equation. The thermodynamic parameters revealed that the adsorption process strongly depended on temperature and indicated the exothermic behavior and spontaneous nature of the adsorption.