Alkaline treatment of seashells and its use for the removal of Cu(II) from aqueous solution

Abstract

Natural aragonite, NA, (in the form of crushed seashells) has a limited capability to remove Cu(II) from an aqueous solution at ambient temperatures (removal = 22.5% at 25°C). At higher temperature, removal increases (removal = 83.5% at 50°C). Ion exchange is proposed as the mechanism of removal in this case. However, the treatment of NA by 4% NaOH- solution produced Na-modified aragonite, NA-Na, which has higher capability for Cu(II) removal at ambient temperatures (removal = 76% at 25°C). This is practically preferred. X-ray fluorescence of Na-modified aragonite showed presence of Na element on its surface with about 1% which suggests inclusion of Na in the NA lattice structure and this is in agreement with the results of the X-ray diffraction measurements. It is suggested that the modification occurs due to the replacement of some calcium ions by sodium ions over the outer layers of NA lattice, (i.e. facial and sub-facial isomorphous-substitution). This substitution causes deficiency in positive charge which is compensated facially by Ca⁺⁺OH^- moieties. It is believed that these moieties are responsible for the micro-precipitation of Cu(II) upon removal process. Cu(II) removal by batch method (at pH value of 6.5) was carried out to assess NA-Na removing performance in terms of capacity (the Langmuir and Freundlich isotherm models) and rate (the pseudo-first and second order models). The Langmuir model gives Cu-Adsorption capacity of 108.7 mg/g at 257°C. For kinetic study, the pseudo-second-order model is the one that best represents the Cu(II) removal by NA-Na. This suggests that removal is one-step chemisorption, reversible and initial concentration-dependent process.