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Iron and manganese are ubiquitous in the natural environment. FeII-FeIII layered double hydroxide, commonly called green rust (GR), and MnIII-MnIV birnessite (Bir) are also well known to be reactive solid compounds. Therefore, studying the chemical interactions between Fe and Mn species could contribute to understanding the interactions between their respective biogeochemical cycles. Moreover, ferromanganese solid compounds are potentially interesting materials for water treatment. Here, a {Fe(OH)2, FeIIaq} mixture was oxidized by Bir in sulphated aqueous media in the presence or absence of dissolved O2. In oxic conditions for an initial FeII/OH&minus, ratio of 0.6, a single GR phase was obtained in a first step, the oxidation kinetics being faster than without Bir. In a second step, GR was oxidised into various final products, mainly in a spinel structure. A partial substitution of Fe by Mn species was suspected in both GR and the spinel. In anoxic condition, GR was also observed but other by-products were concomitantly formed. All the oxidation products were characterized by XRD, XPS, and M&ouml, ssbauer spectroscopy. Hence, oxidation of FeII species by Bir can be considered as a new chemical pathway for producing ferromanganese spinels. Furthermore, these results suggest that Bir may participate in the formation of GR minerals.

International audience; Divalent iron and zinc cations can be removed from water by electrocoagulation (EC) with aluminium electrodes in a discontinuous system: the effect of hydrocarbonate HCO3-ion often present in liquid waste and in groundwater on the EC process has been investigated. For the two ions, the presence of hydrocarbonate strongly limits the pH variations by its buffering properties and reduces the rates of Al dissolution by corrosion. Removal of the two cations was then shown to require longer treatment times and larger amounts of dissolved aluminium. Whereas the local pH gradients near the electrode surface with HCO3-free water was previously shown to allow local formation of stable Zn and Fe hydroxides, which actively contribute to their elimination, the presence of hydrocarbonate nearly suppresses this positive phenomenon, resulting in far less efficient EC treatment. Whereas removal of zinc cations from carbonated water can be considered as their simple adsorption on the Al flocs, Fe 2+ ions are oxidized to Fe(OH)3 by air oxidation after their adsorption. Use of an overall adsorption model allowed quantitative comparison of the EC treatments, with very different adsorption parameters for the two metal studied.

Electrocoagulation (EC) is an electrochemical technique for removal of various pollutants – in particular metal cations – from waste or groundwaters. In comparison with other metal cations, iron can be under divalent and trivalent forms, with different physicochemical properties, so various physicochemical phenomena are to occur for its removal. For the case of Fe2+ ions removal from a synthetic aqueous solution by EC with aluminium electrodes, we investigated both occurrence and significance of the various phenomena encountered e.g. formation of divalent iron hydroxide, adsorption of Fe species on Al(III) flocs formed and oxidation phenomena. Designed experiments have been conducted with an electrochemical cell at various current densities below 10 mA cm−2 and pH, and depending on the nature of the gaseous atmosphere, or in an electroless stirred vessel. Analysis of the data showed that, in addition to direct Fe2+ adsorption on generated solid aluminium hydroxide, Fe(II) hydroxide formed near the cathode surface, precipitates jointly on the Al flocs. Fe(II) oxidation by air oxygen in the liquid phase in EC runs was predicted to be little significant, whereas solid Fe(OH)2 is oxidized more efficiently to Fe(OH)3. However, Fe(II) oxidations improve only little its removal from the water treated by electrocoagulation, as shown by EC tests in anoxic conditions.

During the production of hydrocarbons from subterranean reservoirs, scaling with calcium carbonate and barium sulfate causes flux decline and dangerous problems in production facilities. This work is intended to study the effect of calcium ions on the precipitation of barium sulfate (barite); then, the effect of the formed barite on calcium carbonate crystallization. The conductometric and pH methods were used to follow the progress of the precipitation reaction in aqueous medium. The obtained precipitates were characterized by FTIR, RAMAN, SEM, and XRD. It was shown that Ca2+ in the reaction media does not affect the microstructure of barite even for higher calcium–barium molar ratio. It influences the precipitation kinetics and the solubility of barite by the formation of CaSO4° ion pairing as a predominant role of complex formation (CaSO4) and the increase of the ionic strength. In Ca(HCO3)2-BaSO4-NaCl aqueous system, experiments have showed that added or formed barite in the reaction media accelerates calcite precipitation. No effect on the microstructure of heterogeneous formed calcite which remain calcite shape. However the presence of carbonate ions affects slightly the microstructure of barite.