Mineral micropollutants in waste waters and drinking water are essentially composed of heavy metal salts. Their elimination, when present in trace quantities, is discussed in this paper.
The techniques, which water treatment facilities can use, necessitate preclarification in open basins (efficiency varying between 20 and 90%), elimination by filtration, with efficiencies depending largely on preoxidation but particularly on coagulation-precipitation and related techniques. Our investigations, as well as certain literature data, demonstrate the predominating importance of contact between water and sludges in decanters-clarificators. Thus, Jar tests often yield only a poorly representative result of the elimination of heavy metals by coagulation-flocculation-decantation. As to the use of aluminium sulphate, for example, the elimination rates mentioned relating to the elimination rate or % removal in laboratory experiments, in pilot tests with sludge beds and in treatment plants respectively are as follows: Fe: 100, 95, 100; Mn: 4, 38, 97; Cu: 91, 83, 85; Cr: 29, 8, 83; Cd: 21, 29, 96; Ni: 0, 20, 50; Co: 0, 16, 51; Pb; -, 52, 61. It should be noted that treatment in plants is much more elaborate (oxidation by chlorine dioxide, application of activated silica and a one hour contact time with the sludge bed) than in pilot experiments (15 minutes contact time and a sludge bed without activated carbon).
The oxidation state of metals plays an important role in techniques for their elimination by the usual methods such as coagulation-flocculation-decantation as well as in coprecipitation phenomena of Fe and Mn for example.
The method of alkaline precipitation, in which the pH value needed is obtained by addition of lime, results in elimination rates over 80% and generally far over 90%. This method uses at the same time direct precipitation and coprecipitation in a calcium carbonate/magnesium oxide matrix.
Ion exchange gives highly variable elimination results according to the ion present and its concentration. In practice, strong cation exchange resin are generally applied.
In treatment plants, adsorption on carbon allows an overall yield of 50-100% removal of the metal concerned, provided that preoxidation and sufficient contact times are allowed. The experimental distinction of the specific effect due to adsorption is difficult, but according to our studies, adsorption occurs to a small extent only in the majority of cases.
30-50% of the remaining traces are eliminated by infiltration of purified waters underground, where a levelling of concentrations also occurs.
In addition to the investigations presented, studies still being carried out demonstrate a sufficient stabilization of heavy metals immobilized with lime in order to allow them to be dumped in the environment without risk. This method seems confirmed as a method of choice in treatment of waste waters loaded with heavy metals.
Washing Batch Test of Contaminated Sediment: The Case of Augusta Bay (SR, Italy)