Biobased lipophilic chelating agents and their applications in metals recovery

• Main Author: Asemave, Kaana
• Other Authors: Clark, James. H. ; Hunt, Andrew. J. ; Farmer, Thomas. J.
• Published: University of York 2016
• Subjects: 541
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.694159

A bioderived lipophilic β-diketone (14,16-hentriacontanedione, Htd) was isolated from wheat straw wax. Keto-enol tautomerisation studies revealed that Htd was readily enolised in cyclohexane-D12, toluene-D8 and CDCl3, thus demonstrating its suitability as a metal chelator when dissolved in nonpolar solvents. This work demonstrated for the first time that Htd in liquid-liquid biphasic condition was effective in the removal of Ni(II), Co(II), Cu(II), Pb(II), Cr(III) and Fe(III) comparable to dibenzoylmethane. Htd exhibited an excellent capacity for the rapid extraction of metal ions from aqueous solutions especially Ni(II) better than dibenzoylmethane. Htd was modified with methyl acrylate and itaconate using a KF/alumina mediated Michael addition reaction followed by further hydrolysis, with hope of enhancing it metal chelating ability. The resulting modified Htd were characterised and tested in metal extraction. The carboxylate acids of the modified Htd were found to be more effective in the extraction of Cu(II) and Pb(II) than the unmodified Htd. It was found that the counter ion of the metal being chelated had a profound effect on the efficiency of extraction. These biobased compounds extracted metals ions from Cl- and OAc- media better than from SO42- and NO3-. The unmodified Htd and carboxylate esters of modified Htd were efficiently recycled and re-used for metals extraction. Attempts were made to make the purification of the Htd from wheat straw wax a greener process, as this was initially conducted with petroleum ether and cuprous acetate, in yields of 18.00wt%. Alternative solvents to petroleum ether with better environmental and safety credentials; cyclohexane, cyclopentyl methyl ether, p-cymene and 2,2,5,5-tetramethylTHF were tested for this purpose. The resulting isolated yields of the Htd were 6.00wt%, 7.80wt%, 8.20wt% and 21.40wt% for the cyclohexane, cyclopentyl methyl ether, p-cymene and 2,2,5,5-tetramethylTHF respectively. In order to avoid the use of both organic solvents and cuprous acetate, supercritical carbon dioxide (scCO2) fractionation of the wheat straw wax for Htd were performed. The scCO2 fractionation of the wheat straw wax using silica gel adsorbent at 300 bar, 313 K was found to give somewhat better selectivity for Htd than alumina and celite adsorbents. Although this method demonstrated some promise it was not as effective as the system utilising 2,2,5,5-tetramethylTHF and cuprous acetate, in which both the solvent and copper could be recovered for reuse.