Removal of Chromium (VI) Ions from Polluted Water Using Silica Supported Zero-Valent Iron (nZVI) Particles

F.A. Bezza, E.M.N. Chirwa
University of Pretoria,
South Africa

Keywords: heavy metal, silica coated zerovalent iron particles, adsorption, chromium (VI), waste water

Summary:

Heavy metals are common pollutants of water and soil posing risk for the ecosystems, due to their ubiquity, toxicity at a trace level, bioaccumulation and persistence in the environment (Jia, et al., 2018). Hexavalent chromium, Cr(VI) is among these heavy metals which is highly toxic, carcinogenic and mutagenic to living organisms, placed on top of the priority list of toxic pollutants by the USEPA . The USEPA has mandated a maximum acceptable Cr(VI) concentration of 50 μg/L in potable water (Pan et al., 2016). Among different widely used methods, adsorption is considered as a superior to other methods to remove Cr(VI), because of its simplicity, ability for regeneration, cost-effectiveness, and enabling large-scale applications (Pan et al., 2016; Pakade et al., 2019). Recently, nanoscale zero-valent iron (nZVI) has demonstrated excellent removal capabilities towards various organic and inorganic pollutants mainly due to their large surface area and high reactivity. However due to the extremely large area-to-volume ratio, magnetic attraction, and high surface energy and reactivity, ZVI nanoparticles tend to aggregate rapidly into micro scale flocs ( Pakade et al., 2019). Among several techniques of stable nanoparticles synthesis, nZVI particle synthesis using silica coating has been recently 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 (Li et al., 2012). Scanning Electron Microscopy (SEM) images analysis of iron nanoparticles synthesized in the presence and absence of silica coating revealed that 85% of the silica coated nZVI were within the range of 80–100nm. In contrast, the bare nanoparticles synthesized in the absence of silica coating have a larger dendritic floc- like morphology with particles sizes ranging between 500 and 1000 nm. Batch adsorption experiments were carried out to study the effect of nZVI on removal of lead from 10 mg/L Cr(VI) contaminated water at pH ranges from pH 3 to 9 at different dosages. 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 0.5 to 4.0 g/L, the percent Cr(VI) removal increased from 55.8 % at 0.5 g/L to ∼99.99 % at 4 g/L nZVI dosages. This is due to the greater surface area and availability of more adsorption sites at higher dosages of the adsorbent. The removal of Cr(VI) decreased with the increase in the initial pH values from 3.0 to 8.0, and nearly 99.9 % of Cr(VI) is removed at pH 3. With decreasing pH (pH< 3.0), there is a possibility of formation of negatively charged chromium oxide species while the surface of the adsorbent becomes highly protonated favoring the uptake of Cr(VI). The experimental data were fitted to Langmuir and Freundlich adsorption models, and both the models fitted the data well with maximum adsorption capacity of 31.68 mg/g of adsorbent. The study demonstrates the potential significance of silica coated nZVI particles for remediation of Cr(VI) contaminated wastewater.