Removal of hexavalent chromium and pentavalent arsenic from aqueous solutions by adsorption on polyethylenimine-modified silica nanoparticles.

Chromium and arsenic are commonly found in water and wastewater as hexavalent chromium, Cr(VI), and inorganic arsenic species, such as pentavalent arsenic, As(V). In aqueous media, both Cr(VI) and As(V) exist predominantly in the form of oxy-anions. In our study, we prepared a polyethylenimine-silica composite material (SiO₂-PEI) as an adsorbent to study the adsorption capacity for chromate and arsenate ions. Polyethylenimine (PEI) can effectively bind negatively charged species through electrostatic interactions. The parameters that were evaluated, regarding the adsorption capacity were the effect of pH, the effect of the initial concentration of Cr(VI) and As(V), and the presence of other anions. Also, we examined the effect of Cr(VI) and As(V) on each other by studying the simultaneous removal of chromate and arsenate ions. Isotherm, kinetic, and thermodynamic analyses on the experimental data obtained were conducted. The results showed that the pH of the solution is affecting the charge density of PEI, and at pH 4, the lower value that was examined, the SiO₂-PEI exhibited higher adsorption capacity. The presence of other anions, such as phosphate, nitrate, and sulfate ions, had an adverse effect on the adsorption capacity of the SiO₂-PEI material for both chromate and arsenate ions. The adsorption capacity of arsenate was more affected by the presence of other anions compared to the chromate, and the higher impact was observed by the sulfate ions at pH 4, on the contrary for the chromate ions the higher impact was observed by the sulfate ions at pH 7 and by the phosphate ions at pH 4. Concerning the effect of temperature, the adsorption is an exothermic process and it was more favorable at lower environmental temperature. The study of simultaneous removal of Cr(VI) and As(V) declares the material's ability to accommodate both types of ions without requiring separate treatment steps. Addressing the simultaneous removal is essential for reducing the complexity of water treatment systems, lowering operational costs, and improving the safety of treated water.