Adsorption of Cr(VI), Pb(II) Ions and Methylene Blue Dye from Aqueous Solution using Pristine and Modified Coral Limestone

Abstract

This work reports the feasibility of using pristine and chemically modified coral limestones by acid and base. Their potential adsorptive capabilities is probed by treatment of toxic Cr(VI), Pb(II) ions and methylene blue in aqueous solution under different experimental parameters by batch method. Parameters such as agitation time, concentration, temperature and pH were varied to understand the sorption behaviour of the adsorbents in each case. The adsorbents were characterized by SEM, XRD and FTIR. Morphological analysis by SEM micrographs show that the surface of all adsorbents was irregular in nature. XRD spectra confirmed the orthorhombic structure of aragonite in the pristine coral limestones (PCL), acid modified coral limestones (ACL) and base modified coral limestones (BCL). FTIR results affirmed the presence of (CO3 2-) and (-C=O) groups of the carbonate ions and Ca-O attachment to the surface of PCL and removal of CaCO3 characteristic peaks in ACL and BCL. However, in the modified adsorbents shifting of Ca-O peaks occurred. The recorded maximum adsorption capacities of PCL, ACL and BCL for Cr(VI) ions were 69.42, 65.04, 64.88 mg/g, Pb(II) ions 39.36, 74.11, 78.34 mg/g and methylene blue 37.24, 46.28, 46.39 mg/g, respectively. The uptake of Pb(II), Cr(VI) ions, methylene blue onto PCL fitted Freundlich model. Also the uptake of Cr(VI) ions and methylene blue onto ACL and BCL fitted Freundlich isotherm. However, uptake of Pb(II) ions onto both ACL and BCL fitted Langmuir isotherm. The data revealed that the adsorption of Pb(II) ions onto PCL and ACL and methylene blue dye onto PCL was exothermic. Whilst the adsorption of Cr(VI) ions onto PCL, ACL and BCL and methylene blue dye onto ACL and BCL were endothermic in nature, hence increasing the temperature would enhance the uptake of Pb(II) ions onto BCL, Cr(VI) ions onto ACL and BCL and methylene blue onto ACL and BCL. The obtained (ΔGº) values at all studied temperatures for the adsorption of Pb(II), Cr(VI) ions and methylene blue onto PLC, ACL and BCL indicated a spontaneous process.