Behaviour of trace elements during the naturel evaporation of sea water: case of solar salt works of Sfax saline (S.E. of Tunisia)

Abstract

We have carried out a geochemical study on the behaviour of certain trace elements during the evaporative concentration of free brines (salinity from 41 to 400 ‰) of the solar salt works of Sfax saline (S.E of Tunisia). The elements concerned by this survey are Zinc, Cadmium, Manganese, Molybdenum, Lead, Copper, Aluminium, Iron and Barium. Adequate analytical techniques, adapted to this environment of high salinity, have been used to follow the evolution of the concentration of concerned elements in brines subjected to the evaporation. The results obtained have been presented in function of concentration factor calculated on the basis of the Lithium content in seawater (Coast of Sfax) and those in the brines of salt works. During the evaporation process, the concentration of trace elements in brines was affected by the evaporation phenomenon for the same reason as the major elements. Nevertheless, their concentrations remain very weak and don't reach their saturation doorsteps. The participation of these elements in the mineral phases can take place only by co-precipitation with saline paragenesis. It is the case of Zinc that can precipitate with Sulphates Salts, the Cadmium, Barium and Copper with Carbonate and Gypsum, the Manganese and the Molybdenum with Potassium Salts; others like Iron and Aluminium, are characterized by a very complex behaviour and are subjected, therefore to effects others than those of the evaporation and the co–precipitation. We can mention the activity of the biological system and adsorption phenomena, in particular, on the organic and mineral particles. They are frequent in ponds where brines are not yet very concentrated; allowing an important biological productivity. Contents in these elements are then variable and very dependent on the growth and the physiological state of the organic matter. These micro-organisms use some metals in their metabolic activities, and notably those that act like vitamin factors. The analysis of sediment and algae sampled from the first ponds of the saline, shows that they are capable as well to fix some important quantities of trace elements. The liberation of these elements by deterioration of organic matter, provoke the important fluctuations of their contents in free brines. In basins where the biologic activity is very limited because of the increase of brines salinity, the evolution of the concentration of trace elements translate the only effect of the evaporation, counterbalanced by adsorption phenomenon and the co-precipitation with the salts deposit.