Effect of liquid and supercritical carbon dioxide treatments on the leaching performance of a cement-stabilised waste form

Cement-based stabilisation processes are frequently used for immobilising wastes containing a variety of toxic compounds. In order to improve and extend cement-based stabilisation processes, research has been carried out to examine the role of carbonation (i.e. the interaction of cement-based materials with atmospheric CO2) on the leaching of inorganic contaminants from cementitious waste forms. Because supercritical CO2 has a high diffusivity and a density over 2 orders of magnitude higher than gaseous CO2, carbonation reactions in cementitious materials can be strongly accelerated by using supercritical CO2 as the carbonating atmosphere, rather than using CO2 gas under ambient pressure. In this paper, therefore, the effect of subcritical (liquid) and supercritical CO2 treatments on the leaching behaviour of a cementitious waste form is presented. Different CO2 pressure and temperature conditions (100 bar<p<250 bar, 20 °C<T<50 °C) were applied. Two different leaching tests were performed to study the effect of the CO2 treatments on the leaching of different elements (As, Ca, Cd, Cr, Cu, K, Mo, Na, Ni, Sb, Pb, V, Zn, chloride, fluoride and sulphate). The results indicated that liquid and supercritical carbonation of cement-immobilised slags indeed affected the leaching of the different contaminants investigated, but the different CO2 treatments did not affect the leaching in the same manner all the time. It was found that the leachability (i.e. the leaching under normal environmental conditions) of the elements Cr, Cu, Mo, Sb, chloride and sulphate did not decrease (and even increased considerably under some conditions) upon treatment of the cement-immobilised slags with liquid and scCO2. For the elements Pb, Ca, K, Na and fluoride, the effect of a liquid and scCO2 treatment on their leachability appeared to be more beneficial for the environmental quality of the cement-immobilised slag material, since (for most contact times) these treatments resulted in a decrease of the leaching of these elements. In addition, it was found that the availability (i.e. leaching under extreme environmental conditions) of the element V increased considerably upon treatment of the cement-immobilised slags with liquid and scCO2. For the elements Cd, Pb, Zn, Na and Sb, the effect of a liquid and scCO2 treatment on their availability appeared to be more beneficial, since (for most contact times) these treatments resulted in a decrease of the availability of these elements. For the elements Ca, K, Mo, chloride and sulphate, the effect of liquid and scCO2 treatments on their availability appeared to be negligible. For some other contaminants (such as As, Cr, Cu, Ni and fluoride), the effect on the availability was strongly dependent on the type of CO2 treatment. In general, however, there was no clear relation between the extent of carbonation and the leachability or availability of the different contaminants. Further research should indicate if this new recycling technology will be justifiable, both in technical and economical terms, to reduce waste arisings.