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Liquid-Liquid-Extraction of copper from ammoniacal solution with cyclohexyl substituted ß-diketones
(1993)
A series of new heteroleptic MN2S2 transition metal complexes with M = Cu2+ for EPR measurements and as diamagnetic hosts Ni2+, Zn2+, and Pd2+ were synthesized and characterized. The ligands are N2 = 4, 4'-bis(tert-butyl)-2, 2'-bipyridine (tBu2bpy) and S2 =1, 2-dithiooxalate, (dto), 1, 2-dithiosquarate, (dtsq), maleonitrile-1, 2-dithiolate, or 1, 2-dicyanoethene-1, 2-dithiolate, (mnt). The CuII complexes were studied by EPR in solution and as powders, diamagnetically diluted in the isostructural planar [NiII(tBu2bpy)(S2)] or[PdII(tBu2bpy)(S2)] as well as in tetrahedrally coordinated[ZnII(tBu2bpy)(S2)] host structures to put steric stress on the coordination geometry of the central CuN2S2 unit. The spin density contributions for different geometries calculated from experimental parameters are compared with the electronic situation in the frontier orbital, namely in the semi-occupied molecular orbital (SOMO) of the copper complex, derived from quantum chemical calculations on different levels (EHT and DFT). One of the hosts, [NiII(tBu2bpy)(mnt)], is characterized by X-ray structure analysis to prove the coordination geometry. The complex crystallizes in a square-planar coordination mode in the monoclinic space group P21/a with Z = 4 and the unit cell parameters a = 10.4508(10) angstrom, b = 18.266(2) angstrom, c = 12.6566(12) angstrom, beta = 112.095(7)degrees. Oxidation and reductions potentials of one of the host complexes, [Ni(tBu2bpy)(mnt)], were obtained by cyclovoltammetric measurements.
Based on a study of the German Ministry of Research and Technology in 1990 every year about 2.5 t of heavy metals are released per hectare of landfill by aqueous leaching. This leachate contains approximately 2000 t of heavy metals per year. Their decontaminations in accordance with the legal requirements represent an enormous potential for easing the burden on the environment. On the other hand, this potential opens new possibilities in the production and recycling of selected compounds. The composition of a given model solution corresponds to a landfill leachate resulting from a municipal dump leachate produced by an average German city. Actually, in most cases, a decontamination of such solutions occurs by the transfer of the problematic cations into insoluble compounds usually done by precipitation. The result of the procedure is an unspecific separation of all liquid components - and the "cycle of waste" (landfill- leaching-deposition) begins anew, certainly on a higher level. The objective of our work is the simultaneous separation of heavy metals from alkaline earth metals and iron in order to obtain a leachate which we can lead back to the landfill. The reactive extraction as a separation process offers the possibility of a selective separation of cations that is the separation of toxic - from unproblematic components and includes also the possibility of electrolysis or further winning processes to obtain the wished metals. For the realisation of extraction processes, several commercial extractants and technical equipments are available. Apart from iron, LIX 54 could be used advantageously for an extraction process - unfortunately without any considerable extraction of cadmium. But it is favourable to separate non toxic alkaline earth metals from problematic heavy metals. Such a complex task as the separation of cations from natural solutions cannot be solved easily in a one-step-extraction process. Better results should be obtained by the combination of different procedures, e.g. extraction, ion-exchange and precipitation.
Toxic heavy metals are extracted simultaneously by reactive liquid-liquid-extraction from a model landfill leachate. This is taken as an example to generate an unproblematic waste water. Alkylphosphoric acids (commercial D2EHPA and D2EHTPA), ß-diketones, 4-Acyl-5-pyrazolones and a novel commercial alkyl-methyl-phosphonic acid are used as extractants. By means of the functions E%=f (pH, t) and lg D=f (pH, cL) some thermodynamic parameters of the extraction are determined. In the case of the alkylthiophosphoric acid the thiophilic cations are extracted advantageously in contrast to chromium, magnesium or calcium. There is no significant separation between the heavy metals and the alkaline earth metals. The complexes are extracted with the composition ML(HL)n. According to their ligand force, the presence of complexing agents in the aqueous phase (for example cyanide, tartrate, ammonia or chloride) hinders the extraction. The influence of humic acid is comparable to that of tartaric acid. Because of the formation of less soluble compounds, sulphate ions make the extraction of calcium more difficult. The extraction data points out that aromatic or chloric solvents are not advantageous in contrast to the non toxic aliphatic hydrocarbons. With the novel aminomethylphosphonic acid (ironex, BASF) the extraction of iron, indium and gallium is possible at a pH > 1.5 as ML2(HL) and ML(HL)2 resp. Copper and zinc were extracted quantitatively as 1:2 complexes only in a small pH-range between 4 to 6. Generally, the best results were found for the ß-diketones (LIX 54 included) and 4-acyl-5-pyrazolones. In these cases the toxic heavy metals could be separated quantitatively. Only 6% of calcium and 23% of magnesium are coextracted by the ligands. The extraction process can further be optimized by synergistic effects.