F.A. Bezza, E.M.N. Chirwa
University of Pretoria,
Keywords: silica stabilized zero-valent iron (NZVI) , Arsenic (III), Adsorption, silica coating , remediation
Summary:Rapid industrialization and urbanization have caused contamination of the environment by heavy metals. Heavy metal contamination is extremely harmful to both public health and aquatic life, they are bioaccumulative, environmentally persistent and hazardous even when present at very low concentrations. Arsenic (As) is one of such hazardous heavy metals that is released to the environment in large quantities every year. In terrestrial environment, the inorganic forms of As (such as trivalent arsenite, As(III) and pentavalent arsenate, As(V)) are more prevalent and hazardous than the organic forms in general. Arsenic is confirmed carcinogen and affects virtually all organ systems including the cardiovascular, renal, gastro-intestinal, and respiratory systems. Recently, Zero-valent iron nanoparticles (nZVI) with a higher adsorption capacity have been found to be the most effective adsorbent for removing heavy metals from aqueous solutions in comparison to a variety of adsorbents. The extremely small size of nZVIs, typically in the range 1 to 100nm, generates a large surface to volume ratio, providing them with an enhanced surface reactivity and more available adsorption sites in comparison with micro-sized. However, nZVI particles have strong tendency to aggregate to form larger clusters or chains in the micrometer scale, which subsequently leads to a significant and rapid loss in reactivity and adsorption capacity. In order to be effective, nZVI particles need to be in the form of stable dispersion in water so that they can be delivered to water-saturated porous material in the contaminated areas . Among several techniques of stable nanoparticles synthesis, nZVI particle synthesis using silica coating has been gaining familiarity for an environmentally friendly and colloidal stable nZVI synthesis. In the current study colloidal dispersed and size controlled Zero-valent iron nanoparticles were synthesized using the Stober approach. Transmission Electron Microscopy images analysis of iron nanoparticles synthesized in the presence of silica coating revealed that 85% of the silica coated nZVI were within the range of 80–100nm. The results of the adsorption study demonstrated that silica coated nanoparticles adsorption efficiency increased with increasing adsorbent dosage. When the nZVI dosage was increased from 1 to 4.0 g/L, the percent As(III) removal increased from 66.4% at 1g/L to ∼99.9% at ≥ 3g/L nZVI dosages. This is due to the greater surface area and availability of more adsorption sites at higher dosages of the adsorbent. Although the As(III) removal percentage increased with increasing nZVI dosage, the metal uptake decreased with increasing nZVI loading at higher nZVI dosages due to saturation. The experimental data were fitted to Langmuir and Freundlich adsorption models, and it was found that the Langmuir isotherm fitted the data better than the Freundlich isotherm with maximum adsorption capacity of 21.72mg/g of adsorbent. Investigation of effect of solution pH on As (III) adsorption revealed a tremendous increase from ~38% at pH3 to approximately 96.5% at pH4, and kept slightly increasing to 98.5% at pH8. The study demonstrates the potential significance of silica coated zero-valent iron nanoparticles for remediation of Arsenic (III) contaminated wastewater.