Working group 4: Fate of TM in environment

Metal species are formed exclusively in the extremely reducing, organic and metal rich conditions during AD, either as microbially formed inorganic-organic precipitates (so called biominerals) or as organically complexed species. After AD, TM are released into the environment (i.e. soils and composts) via different routes of entry (i.e. suspended / dissolved in effluents or in solid as excess biomass). Once outside the reducing conditions of AD fermenters, they face oxidizing conditions that may favour their transformation to other thermodynamically stable species. WG4 will use the tools and knowledge gained in WG1 to study the fate (redox-stability, sorption, mobility, and bioavailability) of these TM species in different relevant environmental compartments, such as fresh waters, sediments and soils. As special focus will be given to biominerals that occur as result to different dosing strategies (WG1, WG3), since they may bear a considerably different environmental fate in contrast to chemically synthetized analogues. For instance, biominerals of zinc sulfide (regularly formed in AD), were shown to have a unique environmental distribution due to extracellular protein modifications on the mineral surface – a result of the microbial origin (Moreau et al., 2007). Due to their altered physico-chemical properties, biominerals can thus not be appropriately assessed by standard speciation and transport modelling or risk-assessment methodologies. WG4 will study:

TM speciation together with WG1 (i.e. oxidative dissolution, phase transformations, complex stability), distribution, and eventually bioavailability to model organisms using the latest state-of-the-art methodologies. For speciation, a special focus will be given to direct, species specific methods in solid (i.e. XFAS) and liquid phase [e.g. (LC)-ICP-MS/AFS; high-resolution MS] (WG1).

Biominerals distribution will be studied in model systems (water – sediment/soil) using pristine (i.e. freshly prepared in anaerobic conditions) and deteriorated (i.e. oxidatively degraded) state. Bioavailability will be studied both in model in vivo (e.g. Daphnia, Danio rerio) and in vitro (cell lines) systems.

Direct information on metal speciation in changing redox conditions and data on bioavailability and toxicity may thus serve as input to state-of-the-art methods for risk assessment of heavy metals in the environment for policy makers. For instance, oxidative dissolution data may be included in the biotic ligand models (BLM) as a second-tier risk assessment tool, recently approved by the Scientific Committee on Health and Environmental Risks (2010).