Assessment of thermal treatment of trace element contaminated soil

Abstract: Many contaminated sites in Sweden are characterised by a complex contamination situation with a mixture of organic and inorganic contaminants. For example, wood preservation with creosote, pentachlorophenol and chromated copper arsenate (CCA) has caused this kind of co-contaminated sites. Generally, stepwise remediation has to be used for remediation of these sites with separation and destruction of the organic contaminants and concentration, separation and stabilization of the inorganic contaminants during separate treatment steps. Thermal treatment methods are suitable for the destruction of organics but can increase the mobility of some inorganic contaminants due to phase transitions in the soil minerals. These changes have to be considered during further handling of the soil. In this work, the effects of thermal treatment on the mobility of CCA and critical factors affecting the mobility were studied. Also, different types of thermal treatment methods suitable for remediation of wood preservatives are discussed. The study included laboratory tests and a literature study. A CCA- contaminated soil was separated in four particle size fractions and thermally treated at 800 °C. Batch leaching tests showed that the thermal treatment increased the leaching of As and Cr while the leaching of Cu decreased. A chemical sequential extraction test indicated that the enhanced As leaching probably depended on the reduction of available adsorption sites for trace elements in the soil, due to crystallisation of Fe oxides. The enhanced leaching of Cr could be explained by a minor increase of easily soluble Cr(VI), an increase of the pH, and the competition for sorption sites with other oxyanions. However, the amount of stable Cr species increased during the thermal treatment. The reduced leaching of Cu could be explained by the formation of stable Cu species. The volatility of trace elements is positively correlated to treatment time and temperature. Arsenic is relatively volatile while Cr and Cu are not, although chlorides in the soil enhance Cu volatility. Thermal treatment in a reducing atmosphere enhances the volatility of As. Thermal treatment techniques are divided in extraction/desorption techniques (100-800 °C) and incineration techniques (800-1400 °C). Because of the high energy demand, incineration is most beneficial for smaller quantities of soil. Thermal desorption techniques imply an after treatment of the volatilized organic contaminants: they are combusted in an afterburner, condensed, or collected in a filter. An effective air pollution control (APC) system is necessary in order to control the volatile emissions. In rotary dryer/kiln facilities particles < 0.075 mm are transferred to cyclones and bag filters causing pressure drops and build-up problems. For the treatment of trace element contaminated soil this fact could be beneficial, because the highly contaminated fine fraction of the soil can be handled separately subsequently. Although a low temperature is beneficial to control the volatility of trace elements, it is not always the best choice concerning the leaching, because some trace elements have their leaching maxima after thermal treatment at 200-400 °C. The concentration and mobility of trace elements in the treated soil is controlled by factors such as chemical speciation of the trace elements, interactions between trace elements and soil constituents and process parameters such as fuel, atmosphere, treatment temperature and time. Stabilizing amendments mixed in the soil before thermal treatment could probably reduce the mobility of trace elements and should be investigated further.