The present project proposes the potential use of coupled precipitation/dissolution processes for metal (Me) removal from polluted soils and waste waters by biogenic aragonitic (CaCO3) shell surfaces according to the following overall reaction:
Historically, metal elimination through limestone filter beds has made use of inorganic calcite to precipitate heavy metals. Our recent results indicate that calcite surfaces are covered almost immediately by otavite (CdCO3) precipitation. Subsequently, the dissolution/precipitation process comes to a standstill. This causes a very low efficiency and high costs for waste disposal. In contrast to inorganic calcite, the proposed biogenic aragonite shells consist of aggregates of microcrystalline aragonites with a very high reactive surface area. The aragonite crystals act as nucleation sites for a rather isolated growth of cadmium carbonate crystals leading to cadmium fixation that is more than 100 times higher than that for inorganic calcite. Preliminary results from ongoing experiments show that an analogous behaviour may be observed for the reaction of lead (Pb) and zinc (Zn) with aragonite shell surfaces. Moreover, such highly efficient shells are provided as a waste product in many countries and thus may out compete many other mineral components used for metal elimination. Accordingly, such process has the potential of being used in three different areas of water treatment: a) use of shells as a cheap and effective geologic barrier for contaminated ground or surface waters, b) use as a material in filter beds for selective cleaning of waste water with the potential of partial metal recovery and c) use as seed crystals during the elimination of metals through precipitation with soda (Na2CO3).
We propose to extend our previous metal elimination study with the aim of designing a cost-effective elimination system for various types of metal rich waste waters. Detailed mineralogical and chemical information will be used together with the know-how of state-of the art engineering waste water treatment to design a) a small scale metal elimination reactor, b) develop a packed column system and c) setup a flow system that reproduces typical groundwater flux rates. Access to a wide selection of relevant industrial and other waste waters within the Zero Emission Retrofitting Method for Existing Galvanizing Plants (ZERMEG) program and a number of leachate fractions from selected waste storage sites will allow to select appropriate waters. These will allow to to study, identify, and document key parameters for optimization and adjustment during the process both with regards to legally effective environmental regulations for rejection and cost-effectiveness.