Fe₃O₄and Fe₃O₄/bentonite were prepared by chemical co-precipitation method. They were characterized by X-ray powder diffraction (XRD), Fourier infrared spectroscopy (FTIR), and transmission electron microscope (TEM). Adsorption of cobalt(II) on the bentonite, Fe₃O₄, and Fe₃O₄/bentonite nanocomposite was studied. The results indicated that the metal oxides mainly occurred in the form of spinel structure of Fe₃O₄and the presence of Fe₃O₄significantly affect the surface area and pore structure of the bentonite. The specific surface area (Brunauer–Emmett–Teller (BET) method) of bentonite, Fe₃O₄, and Fe₃O₄/bentonite were determined to be 34.44, 98.44, and 140.5 m² g⁻¹, respectively. TEM image of Fe₃O₄/bentonite shows the particle diameter at 10 nm. The maximum adsorption capacity of cobalt(II) by Fe₃O₄/bentonite nanocomposite was determined to be 18.76 mg g⁻¹. The adsorption strongly depends on pH, where the removal efficiency increases as the pH turns to alkaline range (pH 9). The results suggest that higher adsorption capacity of composite than bentonite is attributed to the presence of Fe₃O₄. The adsorption process follows pseudo-second-order kinetics. The equilibrium data was analyzed by Langmuir model showing high correlation coefficient. The thermodynamic study of adsorption process showed that the adsorption of Co(II) onto Fe₃O₄/bentonite was carried out spontaneously.

A novel cerium-loaded cellulose nanocomposite bead (CCNB) is synthesized and tested for fluoride adsorption. The optimization of the process under the cooperative influence of different experimental variables was made employing response surface methodology (RSM). It is found from fractional factorial design (FFD) that among the different experimental variables, only adsorbent dose, temperature, and pH are significant. At the optimum condition (adsorbent dose 1 g L⁻¹, temperature 313 K, pH 3.0), a maximum fluoride adsorption of 94 % was observed for an initial fluoride concentration of 2.5 mg L⁻¹. A quadratic polynomial model equation based on central composite design (CCD) was built to predict the extent of adsorption. The result of the analysis of variance (ANOVA) shows high coefficients of determination (correlation coefficient; R² = 0.9772, adjusted R² = 0.9545, and adequate precision = 18.1045) and low probability value (Prob > F, 0.001) which signifies the validity of the model. The equilibrium adsorption data conformed to the Tempkin isotherm, having higher R²and lower SE value, among the Langmuir, Freundlich, and Tempkin equations at different temperatures. The adsorption data was found to fit well the second-order rate equation with film diffusion governing the overall rate. The activation energy value was calculated to be 16.74 kJ mol⁻¹. Fluoride can be eluted from fluoride-loaded CCNB using alkali. CCNB can be reused at least for five successive operations.

Preparation of novel nanocomposite particles (NCPs) with high visible-light-driven photocatalytic activity and possessing recovery potential after advanced oxidation process (AOP) is much desired. In this study, pure anatase phase titania (TiO₂) nanoparticles (NPs) as well as three types of NCPs including nitrogen-doped titania (TiO₂-N), titania-coated magnetic silica (Fe₃O₄ cluster@SiO₂@TiO₂ (FST)), and a novel magnetically recoverable TiO₂ nanocomposite photocatalyst containing nitrogen element (Fe₃O₄ cluster@SiO₂@TiO₂-N (FST-N)) were successfully synthesized via a sol–gel process. The photocatalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FE-SEM) with an energy-dispersive X-ray (EDX) spectroscopy analysis, X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), and vibrating sample magnetometer (VSM). The photocatalytic activity of as-prepared samples was further investigated and compared with each other by degradation of phenol, as a model for the organic pollutants, in deionized (DI) water under visible light irradiation. The TiO₂-N (55 ± 1.5 %) and FST-N (46 ± 1.5 %) samples exhibited efficient photocatalytic activity in terms of phenol degradation under visible light irradiation, while undoped samples were almost inactive under same operating conditions. Moreover, the effects of key operational parameters, the optimum sample calcination temperature, and reusability of FST-N NCPs were evaluated. Under optimum conditions (calcination temperature of 400 °C and near-neutral reaction medium), the obtained results revealed efficient degradation of phenol for FST-N NCPs under visible light irradiation (46 ± 1.5 %), high yield magnetic separation and efficient reusability of FST-N NCPs (88.88 % of its initial value) over 10 times reuse.

Graphene oxide (GO) and nano-sized zero-valent iron-reduced graphene oxide (nZVI-rGO) composite were prepared. The GO and nZVI-rGO composite were characterized by transmission electron microscopy (TEM), Fourier transform infrared (FTIR), energy-dispersive spectroscopy (EDS), and Raman spectroscopy. The size of nZVI was about 6 nm as observed by TEM. The system of nZVI-rGO and persulfate (PS) was used for the degradation of trichloroethylene (TCE) in water, and showed 26.5 % more efficiency as compared to nZVI/PS system. The different parameters were studied to determine the efficiency of nZVI-rGO to activate the PS system for the TCE degradation. By increasing the PS amount, TCE removal was also improved while no obvious effect was observed by varying the catalyst loading. Degradation was decreased as the TCE initial concentration was increased from 20 to 100 mg/L. Moreover, when initial solution pH was increased, efficiency deteriorated to 80 %. Bicarbonate showed more negative effect on TCE removal among the solution matrix. To better understand the effects of radical species in the system, the scavenger tests were performed. The •SO₄ ⁻ and •O₂ ⁻ were predominant species responsible for TCE removal. The nZVI-rGO-activated PS process shows potential applications in remediation of highly toxic organic contaminants such as TCE present in the groundwater. Graphical abstract Persulfate activated by reduced graphene oxide and nano-sized zero-valent iron composite can be used for efficient degradation of trichloroethylene (TCE) in water.