SOLVENT EXTRACTION WITH ORGANOPHOSPHONIC MONO-ACIDIC ESTERS

Mono-acidic liquid ion exchangers (extractants) containing the phosphonyl functional group arc reviewed.     

THE THERMODYNAMICS OF EXTRACTION OF SOME LANTHANIDE AND OTHER IONS BY DINONYLNAPHTHALENESULPHONIC ACID

The thermodynamics of extraction of some lanthanide ions and bismuth, aluminium, calcium and zinc ions from perchloric acid into solutions of dinonylnaphthalenesulphonic acid have been studied. Extraction of the trivalent ions is dominated by the entropy of complexing. Electrostriction of large complex micelles by the complexed ion is postulated in order to account for the entropy effect. In the case of divalent ions, the enthalpy of dehydration of the ion is more important. A strategy for improving the separation factors is proposed.     

SOLVENT EXTRACTION WITH SULPHONIC ACIDS.

Advances in solvent extraction of metal ions with sulphonic acids is reviewed for the period 1973 to the present. Physical aspects of micelle formation and water uptake by the extractant are dealt with as well as applications to specific systems. The synergism of mixtures, notably those containing crown ethers, is reviewed.     

THE EFFECT OF TEMPERATURE ON THE SYNERGISTIC EXTRACTION OF THORIUM( IV) AND URANIUM( VI) BY 2-ETHYLHEXYL PHENYLPHOSPHONIC ACID AND MICELLES OF DINONYL NAPHTHALENE SULPHONIC ACID

ABSTRACT

The influence of temperature in the range 15 to 75° C on the extraction of thorium( IV) and uranium( VI) from nitric acid solutions into ligroine solutions of 2-ethylhexyl phenylphosphonic acid ( HEHφ P) micellar dinonyl naphthalene sulphonic acid ( HDNNS) extractants and mixture of the two extractants is reported. From the variation of the distribution ratio with temperature, the enthalpy changes associated with the extraction were determined, and an attempt was made to estimate the free energy and entropy changes associated with the extraction process. The results indicate that the extraction of thorium is favored by both entropy change and enthalpy change when the solvent is HEHφ P and the mixture of HEHOP and HDNNS, but entropy controlled when HDNNS is the extractant. On the other hand, the extraction of uranium by HEHφ P is enthalpy controlled but favored by entropy change when the solvent is HDNNS or a mixture of the two ligands. Temperature did not affect the extraction stoichiometry.     

THE THERMODYNAMICS OF EXTRACTION OF SOME LANTHANIDE AND OTHER IONS BY 2-ETHYLHEXYLHYDROGEN-P-PHENYLPHOSPHONATE.

The thermodynamics of extraction of some lanthanide ions and bismuth, aluminium, calcium and zinc ions from perchloric acid into solutions of 2-ethylhexylhydrogen-p-phenylphosphonate have been studied. The extractions have been compared with those that employ dinonylnaphthalenesulphonic acid and factors that are responsible for the greater selectivity of the phosphonate have been elucidated. Charge density of the cation has a major influence on the mechanism of the reaction and in turn upon the thermodynamic parameters.     

Paleolimnological evidence of the effects of recent cultural eutrophication during the last 200 years in Lake Malawi,East Africa

To assess the long-term water quality changes in Lake Malawi, paleolimnological reconstructions of four radiometrically dated sediment cores collected in 1997/98 along a longitudinal transect of the lake were based on preserved diatom assemblages, as well as stratigraphies of organic carbon, total nitrogen, total phosphorus and biogenic silica concentrations. Population growth, deforestation and intensive agriculture, especially in the southern catchments, have accelerated soil erosion causing rivers to transport greater sediment and nutrient loads into Lake Malawi. Southern cores contain evidence of nutrient enrichment starting around 1940, as indicated by increased silica, carbon and nitrogen influxes. By 1980, increased rates of sedimentation, phosphorus influx, diatom influx, and relative abundances of eutrophic diatom taxa are attributable to accelerated nutrient enrichment accompanying soil erosion. The succession of diatoms in southern Lake Malawi begins with dominance by Aulacoseira nyassensis and Fragilaria africana, followed by a shift in 1980 towards Stephanodiscus, Cyclostephanos and Nitzschia, diatom taxa that have reduced silica requirements. Paleoecological results indicate that patterns of diatom assemblage change are not uniform lake-wide. Evidence of eutrophication extends to central Lake Malawi, but is not observable in the paleo-record from the deeper northern basin. The recent cultural eutrophication of the southern lake has impacted the biogeochemical cycling of silica, the available silica to phosphorus ratios, and the diatom communities of a large portion of Lake Malawi. These results provide a warning that eutrophication of this great lake is underway and will continue unless changes are made to current land use practices within the lake's catchment.