Adsorption of Cr(VI) using Natural Iron Oxyhydroxides

Abstract

Hexavalent chromium [Cr(VI)] is one of the common toxic ions caused due to both natural and anthropogenic processes. It is easily soluble in water and highly mobile in an aquatic environment. A long exposure to Cr(VI) may cause many disorders on human health such as sickness, increase the risk of lung cancer, nausea and may also damage the small capillaries in the kidneys and intestine. Thus, according to the World Health Organization and US Environmental Protection Agency, its maximum permissible limit for potable water and inland surface water are 0.05 mg/L and 0.1 mg/L, respectively.
Therefore, in this thesis, an attempt is made to evaluate the removal of Cr(VI) from aqueous solution by using natural Fe-oxyhydroxides, namely, goethite, calcined goethite, schwertmannite, 2-line ferrihydrite and 6-line ferrihydrite as adsorbents. Natural Fe-oxyhydroxides are identified using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, N2 adsorption/desorption isotherm and high resolution transmission electron microscope (HRTEM). Then adsorption batch experiments are conducted under different concentration of Cr(VI), time of contact, pH of the solution, temperature and dose of adsorbents. It is found that Langmuir isotherm fits the adsorption data of Cr(VI) for all the adsorbents. The Langmuir adsorption capacity is found to be maximum for 6-line ferrihydrite followed by 2-line ferrihydrite, schwertmannite, calcined goethite at 300 °C and goethite. The variation in the adsorption capacity is mostly attributed to the differences in the surface area. The surface area of 2-line ferrihydrite, schwertmannite, calcined goethite at 300 °C and goethite is 259, 164, 78, 126 and 7 m2/g respectively. The N2 adsorption/desorption isotherms used for determination of BET surface area indicate that adsorbents belong to a type-IV category which is an indicative of the presence of mesoporosity. The hysteresis loop can be classified into H3-type which indicates that the adsorbents consist of slit-shaped pores and the pores are interconnected. Though schwertmannite has less surface area than that of calcined goethite at 300 °C but the former has higher removal capacity. It might be due to its tunnel shaped structure. This indicates that the natural Fe-oxyhydroxides can act as an effective and promising adsorbent for removal of Cr(VI) ions from aqueous solution. Except schwertmannite, removal of Cr(VI) decreases with increase in pH whereas for schwertmannite, removal increases up to pH 9 and then decreases. Removal increases with increase in contact time and decreases with increase in initial concentration of Cr(VI).
Based on thermodynamic study, the negative change in Gibbs free energy indicates that adsorption process is spontaneous and feasible and endothermic in nature. The values of ΔH˚ are positive for all adsorbents which indicate adsorption is endothermic in nature. Positive value of ΔS˚ for all adsorbents also reflect increase in randomness at the solid–solution interface during the fixation of Cr(VI) on their active sites. The pseudo-second order model is more feasible than pseudo-first order model for all adsorbents which indicate that chemisorption is the dominant process. The intraparticle diffusion model indicates more than one mechanism controls the adsorption process. The boundary layer thickness is found to be very less.