Solvent extraction by dialkyl-substituted diphosphonic acids in a depolymerizing diluent. I. Alkaline earth cations

In this work, the extraction of Ca2+, S2+ and Ba2+ by P,P ' -di(2-ethylhexyl) methylene- (H2DEH[MDP]), ethylene(H2DEH[EDP]), and butylene- (H2DEH[BuDP]) diphosphonic acids, has been investigated in the monomerizing diluent 1-decanol. In all cases, the slope of the acid dependencies at low aqueous acidities equals -2, indicating that two protons are displaced from the extractant molecules, regardless of the type of complexes formed in the organic phase. Extraction of metal nitrates through a solvation mechanism, however, has also been identified. The extractant dependency data exhibit variable slopes, consistent with the simultaneous formation of complexes having 1:1 (MA) and 1:2 (MA.H(2)A or M(HA)(2)) metal to ligand stoichiometry (where A denotes the anion of the extractants). The latter species becomes predominant at higher extractant concentrations. Based on the selectivity sequences observed for the alkaline earth cations with the di(2-ethylhexyl) diphosphonic acids, probable structures have been assigned to the metal-extractant complexes. Comparison of the extraction data in 1-decanol with those obtained previously in o-xylene shows that 1-decanol strongly suppresses metal extraction by the di(2-ethylhexyl) diphosphonic acids. This is consistent with strong hydrogen-bonding of the monomeric extractant molecules with the diluent.