A Study on the Removal of Chromium(VI) Oxanions from Acid Solutions by Using Oxonium Ion-Crown Ether Complexes as Mobile Carrier Agents

Abstract

<em>Crown ethers 12-crown-4 (12C4), benzo-15-crown-5 (B15C5), 18-crown-6 (18C6), nitrobenzo-18-crown-6 (NB18C6) dibenzo-18-crown-6 (DB18C6) and dicyclohexyl-18-crown-6 (DC18C6), dissolved in dichloromethane, are able to form oxonium-ion complexes in contact with aqueous sulfuric and hydrochloric acid solutions. This ability allows the transfer of Cr(VI) oxanions from the acid media, through an organic liquid membrane, into a distilled water receiving phase. Among the studied crown ethers, DC18C6 shows the highest transport efficiency.</em><em>The effect of parameters </em><em>influencing the process such as the crown ether </em><em>concentration in the membrane, types of the organic diluent, acid concentration in the feed phase and time is investigated and discussed.</em><em> Four replicate experiments show that an average of 95.6 (</em><em>±</em><em>1.3)% of the initial chromium in the source phase (10^-3 M Cr(VI), 6 M H₂SO₄, 10 ml) is extracted into the distilled water(10 ml) through the membrane (DC18C6 0.05 M in CH₂Cl₂, 20 ml) after 8 h. The corresponding result, under similar conditions, in the presence of hydrochloric acid solution (4 M) was found to be 95.0 (</em><em>±</em><em>1.2)%. The selectivity of the process was assessed by performing the competitive transport experiments on a solution containing Cr(VI), Ni(II), Co(II), Pb(II), Mn(II), Zn(II) and Fe(III) ions. The method was used for the recovery of chromium from two real samples provided by chromium-electroplating industries.</em>