Dietmar Rahner

Behavior of cadmium, lead and zinc at the sediment-water interface by electrochemically initiated processes

Abstract Nowadays, electrokinetic remediation is one of the popular and cheapest in situ remediation techniques for contaminated soils. This method uses a low-level electrical energy and is known for removal of heavy metals like, cadmium, chromium, copper, iron, lead, mercury, nickel, zinc and the metalloid arsenic from polluted and spiked soils. The driving force is the migration of those ions in the soil body by the applied electric field. In most cases the heavy metals are concentrated near the cathode depending on the actual pH profile and the solubility products of formed non-soluble compounds (oxides, oxihydroxides, carbonates etc.). Sometimes it is also possible to deposit heavy metals directly at the cathode surface (e.g. copper, cadmium). Often it is necessary to add an acid to prevent the precipitation during such an electrochemical process. Therefore, the mobilization and accumulation of cadmium, lead and zinc at the sediment–water interface with different positions and conditions of the electrode arrangement was studied. The tests were carried out with a natural heavy metal containing sediment from the river Weisse Elster of Germany. The results showed that they were mobilized by the effects of the anodic polarization and transported by migration from sediment into the water–sediment interface. By constructing a pH-barrier at the sediment–water interface, those metals were precipitated at the steep pH-gradient. The metals were accumulated at the sediment–water interface. In the opposite situation, where the cathode is at the sediment surface, the alkaline front penetrates into the sediment and a fixation of amphoteric metals at weak alkaline and a mobilization at strong alkaline conditions occurs. In our view, the electrode arrangement anode in the sediment–cathode in the water body with a relatively small distance between both electrodes to create a steep pH- and Eh-gradient is the best arrangement for the predicted mobilization and accumulation of those heavy metals. After this it should be possible to mobilize them in a relative short time and to remove the concentrated heavy metals from the solid phase by reversing the polarity of the electrodes. This will be a possibility to support the in-situ-cleaning of sediments by constructing a pH- and Eh-barrier to eliminate the heavy metals.

Passivity of lithium in organic solvents

Abstract A short overview concerning the nature of lithium ‘passivity’ and the use of in situ techniques in lithium research will be given in order to emphasize the important role of the properties of the phase-boundary metal /electrolyte. The electrochemical behaviour of lithium is strongly influenced by the formation of a surface layer due to the reduction of the solvent and of the electrolyte. A kinetic model for the layer formation at uncovered lithium surfaces will be suggested.

The effect of alkaline earth titanates on the rechargeability of manganese dioxide in alkaline electrolyte

Abstract Various alkaline earth titanates were tested as the additives for manganese dioxide electrodes in aqueous electrolyte ( 9 mol 1 KOH ) at room temperature. The influence of the additives on the discharge capacity of primary cells and especially on cycling behaviour of recha, geable alkaline batteries is discussed.

Intercalation Materials for Lithium Rechargeable Batteries

In this contribution an overview will be given about the intercalation materials both for the negative and positive electrode of lithium batteries in comparison with results of our own research. Besides lithium metal as a negative electrode, interest is focused on insertion materials based on aluminium alloys. In the case of the positive electrode metal-oxides, those based on manganese, nickel and cobalt are discussed.

Fe3O4 as part of the passive layer on iron

Abstract A short overview concerning the nature of the passive layer on iron will be given. It will be shown by the rotating split ring-disc technique that the experimental conditions, especially during the processes of Fe 3 O 4 reduction, play an important role for the establishment of a “passive” layer model. The current efficiencies of various Fe 3 O 4 reduction reactions will be discussed. During the electroreductive dissolution of iron oxides, clusters of Fe(II)oxy-hydroxides or hydroxylated Fe(II) ions leave the oxide surface.

Studies of Al-Al3Ni eutectic mixtures as insertion anodes in rechargeable lithium batteries

Abstract This contribution will give a short overview of aluminium-nickel eutectic mixture alloys as the anode materials in lithium secondary batteries. These compounds allow to create an alloy matrix of modified grain size with stabilizing properties toward ‘mechanical stressing’ during charge/discharge processes of lithium. Several electrochemical techniques have been used to investigate the electrochemical behaviour of these lithium-inserting materials.

Fixation of manganese and iron in freshwater sediments through electrochemically initiated processes I: Principles and laboratory studies

Abstract.Most existing in-lake methods used to minimize the remobilization of redox-sensitive compounds from sediments are costly and limited in their effectiveness. Therefore, new approaches for this problem are needed. We describe electrochemically initiated transformation processes of iron and manganese, resulting in their immobilization in the sediment. This process could possibly be applied to prevent remobilization of phosphate from lake sediments. With electrodes positioned 3 cm in the sediment and 7 cm above the sediment-water interface, an electrochemically controlled redox- and pH-”barrier” was constructed. By means of laboratory experiments in columns filled with sediment and water from a reservoir, we demonstrated the possibility to fix and concentrate Fe and Mn under specific conditions in the sediment, or if required, to mobilize and drain them in concentrated solution. The behaviors of these metals were studied by simultaneous measurements of their concentrations, pH-, Eh-values, and O2-concentration at the interface and in the water with and without current influence. The results show that soluble species can be transported and concentrated in an electric field. Iron and manganese can be prevented from being released from the sediments by sustainable in-situ fixation.

Elimination of heavy metals from process waters of the bioleaching process by electrolysis

2 ConclusionsThe investigations into the membrane electrolysis cell show that electrochemical metal separation from bioleaching process waters can represent a practical alternative for metal separation by alcalization, Coal and platinized titanium material exhibit good anodic resistance at the current densities tested. By contrast, high-grade steel and to some extent lead anodes were dissolved and are hence unsuitable for this purpose. However, for practical application, suitable ways are required to discharge the precipitates containing heavy metals deposited on the electrodes from the electrolysis cell and to prevent membrane clogging. Regarding the main components zinc, manganese, and nickel, the combination electrodes proved to be suitable for eliminating the heavy metals from the aqueous phase.Another way of treating diluted process waters containing sulphuric acidic and heavy metals is to concentrate the sulphuric acid in the anode region and to precipitate the heavy metals in the cathode region. The sulphuric acid recovered could then be returned to the leaching process, hence avoiding wastewater.

Reactive interaction between surfaces of lithium metal and electrolyte components

Abstract A brief overview concerning the nature of lithium “passivity” and the use of in-situ techniques in lithium research will be given in order to emphasize the important role of the properties of the phase boundary metal/electrolyte. The electrochemical behaviour of lithium is strongly influenced by the formation of a surface layer due to the reduction of the solvent and of the electrolyte. A model for layer formation at uncovered lithium surfaces will be suggested.