Application of response surface methodology for optimization of Pb and Cr adsorption on iron-coated limestone

Abstract

Daily discharging of excessive wastewater containing heavy metals into receiving water causes environmental problems such as eutrophication, corrosion and fouling. This paper presents the results of research carried out for Lead and Chromium adsorption on iron-coated limestone (ICL) as low cost adsorbent using response surface methodology (RSM) for optimization. Characteristics of the adsorbent studied were including pH, adsorbent particle size, bulk density, iodine value and surface area of 8.40, ≤63 μm Ø, 1.623 g/cm³, 27.3 mg/g, 1031 m²/g respectively. Scanning electron microscopy (SEM) of the adsorbent at 500 X magnification revealed a more porous nature on the surface and an irregular texture. X-ray fluorescence (XRF) analysis also showed appreciable increase in % weight of Fe₂O₃, Cl, MgO, SiO₂ and SO₃ due to the addition of FeCl₃.6H₂O for coating. The factors considered for optimization were initial concentration (mg/L), adsorbent mass (g), contact time (mins), and the response obtained was the removal efficiency (%). The analysis of variance (ANOVA) was used to develop the model equations for both Pb and Cr removal efficiency. The result revealed that optimum conditions for the adsorption of Pb on iron-coated limestone are; initial concentration of 6.02 mg/L, adsorbent mass of 0.37 g and contact time of 33.84 minutes, this produced a removal efficiency of 95.38%, Cr has optimal conditions of initial concentration at 56.14 mg/L, adsorbent mass of 0.73 g and contact time of 157.14 minutes with a removal efficiency of 91.03%. The optimum adsorbent doses for both Pb and Cr showed that the adsorbent had more adsorption site and was more utilized at lower adsorbent dose.