Along with chlorinated hydrocarbons and mineral oils, heavy metals are one of the most common pollution in contaminated sites. In-situ immobilisation is a way for a successful remediation of heavy metals in soils. Co-contaminations with semi-volatile and volatile organic substances are often treated with thermal processes such as Thermal Conductive Heating. Heavy metals are still present in the treated soil. The temperature load influences the chemical and mineralogical composition as well as the structure of the soil. There is also a change in the pH value, water balance and microbial activity. Due to this influence, heavy metals can be mobilised again, which, counteracts the remediation success and has not yet been researched in detail.
This work deals with the temperature effect on the change in mobility of specific heavy metals in five different soil samples that had been subjected to thermal treatment at 105 °C, 300 °C and 500 °C. The samples were then subjected to pH-dependent elution tests (EN 14429) at room temperature and the effect of the previous treatment was investigated. The maximum leachable concentration of an element in an experiment was used as the available concentration for modelling the mobility-controlling mechanisms to the simulation programme LeachXSTM with the ORCHESTRA database. This is either the solubility equilibrium of a mineral phase containing heavy metals or the adsorption equilibrium at a mineral surface. Elemental distribution maps from microprobe investigations enabled a comparison of results by showing the mineralogical bonding form of the heavy metals.
The pH-dependent leaching tests show an amphoteric behaviour for Cu, Pb, Cd and Zn, i.e. a minimum mobilisation in the middle pH range. The minimum shifts with increasing treatment temperature to higher pH values and lower eluate concentrations, i.e. immobilisation occurs. The modelling shows that adsorption processes are the exclusive release-controlling mechanism for Cd and a major release-controlling mechanism for Cu, Pb and Zn. In addition, according to the modelling, oxides and hydroxides occur for the latter elements in the alkaline range, whose lower solubility in thermally treated samples could be a cause of immobilisation. The microprobe investigations show Pb in the form of phosphates, silicates and sulphates, which, however, according to the model have a higher solubility, so that adsorption and precipitation of secondary phases subsequently occur. Chromium shows a partly different behaviour from the other heavy metals. Although a more amphoteric behaviour can be observed in untreated and slightly heated samples, the release is constantly high at a treatment of 300 °C and more, thus a mobilisation occurs due to the thermal treatment. The mobility of chromium is controlled exclusively by adsorption processes. Since the equilibrium between Cr(III) and Cr(VI) shifts with increasing temperature towards Cr(VI), which is present as an anion, the higher mobility may be due to this oxidation reaction. The microprobe investigations show the binding of chromium in chromite, FeCr2O4, which, according to modelling, should then partially dissolve before the chromium is fixed again by adsorption.