Characterization of the Submarine Disposal of a Bayer Effluent (gardanne Alumina Plant, Southern France): II. Chemical Composition of the Clarified Effluent and Mineralogical Composition of the Concretions Formed by Its Discharge in the Mediterranean Sea

The Gardanne alumina plant (Marseille region, Southeast France) has disposed its residues as a slurry (red muds) in the Mediterranean Sea up to 2015 when new regulations allowed the sole discharge of a clarified effluent. This reduced the quantity of suspended material from ~300 g/L to less than 10 mg/L. The detailed chemical composition of this clarified effluent has been determined by ICP-OES, ionic chromatography, 1H NMR spectroscopy and carbon analysis. It is a Na–Al–OH aqueous solution at pH 12.5 having a solute load of 1.5 g/L and an organic carbon content of about 100 mg/L, all other elements being in minor or trace amounts. A X-ray diffraction and SEM study showed that the submarine concretions forming at the outfall upon mixing the effluent with seawater are predominantly composed of a Mg–Al–OH double layer hydroxide of the hydrotalcite supergroup that contains Ca and S (wermlandite subgroup), along with minor calcite, bayerite, and rarely brucite and akaganeite (an iron hydroxide). The investigation of the stability of standard hydrotalcite in the marine environment through solubility calculations showed that hydrotalcite minerals are not stable in normal seawater and as such are prone to dissolve and release their components to the environment.