Synthesis of L-Arabinose-based Coordination Polymers for the Removal of Lead from Aqueous Solutions

Abstract

L–Arabinose-based coordination polymers were synthesized by the oxidation of L–arabinose using warm HNO3 to form arabinaric acid and allowing the resulting arabinaric acid to react with the different metal ions such as zinc(II), lead(II), copper(II), iron(III), and chromium(III). Only zinc(II) and lead(II) ions produced crystalline coordination polymers suitable for analysis while copper(II) produced a gummy solid. Preliminary characterization of the coordination polymers was based on their solubility, melting points, UV–Visible spectra, and infrared spectra. The coordination polymers were insoluble in common organic solvents and in acidic aqueous solutions but soluble in moderately basic aqueous solutions (pH 12). The zinc(II) and lead(II) coordination polymers have melting points above 300°C while the copper(II) polymer melts from 218–225°C. UV–Visible spectra showed that these polymers have maximum absorption in the ultraviolet region. Zinc(II), lead(II), and copper(II) arabinarate coordination polymers had maximum absorbance at λmax = 217 nm, λmax = 240 nm, and λmax = 220 nm, respectively. The infrared spectra of the coordination polymers indicated the association of the arabinarate ligand with the metal ions. Introduction and incorporation of molecular species such as azobenzene and molecular iodine to the coordination polymers were also investigated. Infrared spectra obtained from the azobenzene inclusion experiments showed additional infrared absorption bands below 1000 cm –1 implying absorption of azobenzene within the polymers. Iodometric titrations also confirmed the inclusion of iodine in the coordination polymers. The potential of the coordination polymers to incorporate lead(II) ions in aqueous solutions was also investigated using column elution of different sample concentrations of lead(II). Atomic Absorption Spectrometric analysis illustrated significant lead reduction of up to 80.70 ppm of lead(II) on the eluted samples. Introduction Rationale of the Study Rapid industrialization in the modern world today causes severe pollution in the environment. This may be due to toxic, inorganic, or non-biodegradable materials released to the environment as waste products during industrial processes. Heavy metals, like lead, are among the most harmful of the environmental elemental pollutants. Lead contamination from excessive exposure has been found to cause several health hazards and long term detrimental effects. Prominent conventional processes for the removal of lead from aqueous mixtures, such as sludge separation, chemical precipitation, and electrochemical treatments are relatively expensive and can cause secondary pollutants (Manahan, 1993 as cited in Acabal et al., 2000). Due to the problems of pollution, “green” industrial productions of compounds and their derivatives based on renewable starting materials and biodegradable products are developed (Kiely, 2001). Carbohydrates are biodegradable, widely available, and generally economical. Hence, their study for chemical and industrial applications would be very sensible. Because of their ease in functionalization forming sugar derivatives, they could serve as valuable building blocks for potentially useful polymers (Abrahams et al.,