Role of soil organic matter for immobilisation of metals : treatment of leachate from MSWI bottom ashes

In this thesis, the focus is laid on the ability of natural organic matter (OM) to serve as a metal stabilising agent. The metal contaminants investigated arise from the MSWI bottom ash leachate. Besides the high content of metals, elevated pH and salinity are characteristic for ash leachate that, in turn, can alter the functionality of OM. Batch and column leaching tests were used to study the retention capacity of substrates with different amount of OM. Also, field observations were made of the influence of ash leachate on soil and plants. In this case, ash leachate was generated under field conditions from an experimental road built on municipal solid waste incineration (MSWI) bottom ashes. It has been shown that copper, chromium, and lead retention is proportional to OM content of the substrates. Zinc retention showed to have the least dependence on OM. The metals were not leached in proportion to dissolved OM. Most probably several mechanisms were responsible for the retention of metals: (i) high concentration of Ca in ash leachate could lead to the formation of Ca-dissolved organic carbon (DOC) complexes that have the ability to precipitate some metals (ionic strength effect) and prevent metal transport; (ii) high solution pH could favour hydroxide formation and counteract the metal complexation with dissolved OM, as well as (iii) surface adsorption could contribute to metal retention. Despite the good metal retention capacity of OM, a continuous metal load will occupy binding sites of OM and therefore reduce its retention capacity. Metal retention capacity of rich in OM substrate could be improved by additional soil amendments and vegetation. Tolerant plant species that are capable to grow at high soil metal concentrations and immobilize pollutants within the root zone have a potential to be used for phytostabilisation of metal contaminated soil. Such plants are also associated with a low risk of the translocation of contaminants from soil through plant roots to shoots, i.e. from one media to another. Immobilization is not a technology for the removal of contaminants from soil but for the stabilization (inactivation) of potentially toxic metals. The aim of soil remediation is to reduce the contaminant exposure and spread. Then the reduced leaching, bioavailability, as well as ecotoxicity of metals as a result of phytostabilisation might be a proper solution. Development of suitable soil and amendment mixtures capable to retain broad range of metals and interaction of plants with stabilised matrix are the questions to be answered in the future research.Godkänd; 2003; 20070109 (mlk