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

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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.The effect of parameters influencing the process such as the crown ether concentration in the membrane, types of the organic diluent, acid concentration in the feed phase and time is investigated and discussed. Four replicate experiments show that an average of 95.6 (±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 (±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.