Fixation of Heavy Metals in Clay and Sludge under Elevated Temperatures and Pressures

Abstract

The retention of heavy metal by clay minerals has been thoroughly studied. The smectic types, in particular montmorilkmite. have a large capacity of cation exchange, a property which can be related to their structure. The experiments in this work were carried out with bentonite, a clay with a high content of montmorillonite. Its capacity for absorption of different metals was shown to decrease in the order Cu > Zn > Ni > Cd

Laboratory work was carried out with equipment simulating a continuous pipe autoclave. Reaction time was typically 30 min and the temperature could be varied up to279°C. After the treatment, leaching tests were performed at pH 5 and 3. It was shown that the fixation increased with temperature and with the amount of added Na₂CO₃ or NaHCO₃, the effect of the former being slightly greater than the latter. The order of retention was Ni > Cu > Zn > Cd. Only 1.2% of nickel but 35% of cadmium are leached under the same conditions (H₂SO₄ at pH 3.0-3.1). This probably reflects the adaptation of the ion to the silicate lattice. By substituting Ca(OH)₂ for Na₂CO₃, the ability to retain heavy metals became neutralized during the autoclave treatment. This is again ascribed to the thorough conversion of the montmorillonite structure by reaction with Ca(OH)₂

Many soils contain smectic clay minerals in amounts sufficient to ensure fixation by direct autoclaving. In other cases such as industrial waste or municipal sludge, it is necessary to add an adequate clay. Cadmium as one of the most hazardous pollutants is not satisfactorily fixed by silicate lattices. In such a case, a more promising approach is transforming the pollutant into sulphide which can be done hydrothermally in the autoclave by adding a small amount of sulphur under anoxic conditions. The case for disposing sulphide-bound metals to the environment under oxidizing conditions presents problems. The problem is that the ubiquitous thiobacilli will start to convert the sulphides into more or less soluble sulphates when the oxygen pressure is sufficient. Therefore, dumping sites must be chosen with special reference to long term anoxic conditions.