Heavy metals in municipal solid waste deposits

Extensive use of heavy metals in modern society influences routes followed by fluxes on the surface of the Earth. The changed flow paths may be harmful for the balance of biological systems at different levels, micro-organisms, human beings and whole ecosystems, since the toxicity of heavy metals is determined by their concentrations and chemical forms. The fluxes of cadmium in Sweden and especially anthropogenic fluxes were analysed. It was found that a large amount of cadmium has been accumulated in the community and therefore also in municipal landfills since the 1940s. These amounts, although concentrated to confined locations, are comparable to those that can be found in the first few centimetres of all soils. Nevertheless, the rates of which heavy metals are emitted from municipal landfill sites are found to be very low when the amounts accumulated are taken into account. It would, at the present emission rates, take thousands of years to release half the amount of heavy metals stored. The emissions from modern landfills to neighbouring environments are even smaller because the leachate produced in active municipal landfills is collected and treated on site or in a sewage plant. Despite the low mobility of heavy metals (Zn, Cu, Pb, Cr, Ni and Cd) in municipal landfills, it was found that extensive transformations of the binding forms of heavy metal take place within the waste mass during the degradation of the waste. These changes appear to be closely related to the development of early diagenetic solid phases, i.e. new secondary solid phases formed in the waste. The heavy metals often constitute a minor part of these phases and the bindings include several forms such as adsorption, complexation, coprecipitation, precipitation, etc. It was also found that the associations between heavy metals and solid phases are dominated by several binding forms to one specific substrate rather than bindings to various solid phases. The mobility of iron and manganese seems to increase during the processes involved in waste degradation due to the solution of oxide/hydroxide phases, while the heavy metals appear to become less mobile due to their binding to organic compounds and sulphides. However, one exception in this case may be nickel. Another aspect of the transformation of heavy metals is the accumulation of pools of heavy metals which can become susceptible to environmental changes, such as oxidation or acidification. However, the risks of increased mobilisation caused by lower pH values seem to be limited since municipal solid waste has a large buffer capacity