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, ≤63μm Ø, 1.623g/cm3, 27.3mg/g, 1031m2/g respectively. Scanning electron microscopy (SEM) of the adsorbent at 500X 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 6.02mg/L, adsorbent mass of 0.37g and contact time of 33.84minutes, this produced a removal efficiency of 95.38%, Cr has optimal conditions of initial concentration at 56.14mg/L, adsorbent mass of 0.73g and contact time of 157.14minutes 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.

Key words: Response surface methodology (RSM), adsorption, iron-coated limestone (ICL), surface area, SEM, removal efficiency.