Working group 1: Chemical speciation and bioavailability

The total metal concentration, the conditions during digestion such as pH and redox potential, and the kinetics of reduction, precipitation, complexation, and adsorption are expected to play a key role influencing the chemical speciation of micronutrients in liquid phase and biofilms. The increasing pH decreases solubility of metals in the matrix. The precipitation of metals by sulfide (S2-), carbonate (CO32-), and phosphate (PO43-), and their deposition in the bioreactor sludge plays an important role in the nutrient turnover of macro- and micronutrients. There is also probably a strong interaction of added Fe and the micronutrients in the matrix: micronutrients may react with the Fe-sulfide releasing Fe2+. The resulting Fe2+ may form precipitates as hydroxides (Fe(OH)2) or carbonates (FeCO3). Consequently, bioreactors have a considerable ability to sequester Fe2+-ions in the sludge. Simultaneously, non-alkali metals (e.g. Ca2+, Mg2+) form soluble ion pairs with a number of anions: HCO3, CO32-, OH, SO42-, S2-, Se2-.

Complexation reactions play an important role in bioreactors making a particular metal either more or less bioavailable. The level of soluble metals in the presence of CO32- and S2- may be increased by a factor of up to 104 by complexation, avoiding precipitation as carbonates or sulfides. Several authors describe a shift of micronutrients away from mobile forms toward more stable and less reactive and bioavailable forms during AD. However, in a growth chamber experiment, it has been found that AD does not reduce the plant bioavailability of micronutrients. The relevant processes driving the underlying effects are still not well understood. Furthermore, no data were found concerning the effects of a long-term digestate field application on the heavy metal accumulation in the soil and their bioavailability to plants (WG4).

WG1 will therefore focus on the development of new, direct and speciation-preserving analytical techniques applicable to study the interactions of TM within the AD environments, especially biofilms. The main challenge is to develop techniques for in situ use under dynamic conditions. The transport and immobilization phenomena still need to be understood regarding the physico-chemical heterogeneity of anaerobic biofilm and suspended stirred systems.

The analytical results (e.g. quantity of single species) can then be linked with specific microbial activity rates. Similarly, advanced chemical analyses will be adapted to be suitable for AD environments and further applied to determine organic and inorganic complexation. Additionally, information on the minerals and organic phases present in the biofilm matrix is required to better assess the impact of such phases on the spatial chemical heterogeneity encountered in biofilms at the micro-meter (i.e. cell) scale.

How are the chemical forms of TM related to bioavailability? In order to answer this question, the following aspects will therefore be discussed:

– Trace metals speciation in liquid and solid phases. At this stage recent analytical techniques still need to be developed or adapted (e.g. use of DGT (Diffusive Gradient in Thin film), ion selective electrode or DMT (Donnan Membrane Technique)) (Bartacek et al., 2008). In parallel a better characterization and quantification of the soluble Extracellular Polymeric Substances (EPS) and Dissolved Organic Matter (DOM) have to be developed due to the role of such organic macromolecules in TM complexation.

– Characterization of the organic (EPS) and mineral (more especially sulfur, phosphate and iron minerals) solid phase is therefore required in order to interpret the liquid phase speciation data. Molecular information regarding the mineral phase could be accessible via X-ray absorption spectroscopy. In addition, spatial EPS heterogeneity of the biofilm could be investigated by Confocal Laser Microscopy; however, sample preparation techniques should be developed.

– The link between the TM speciation in the liquid and solid phase, respectively, require the determination of trace metals leachability (i.e. a proxy for bioavailability?). At this stage a simple procedure still need to be developed despite that the acid volatile sulfide (AVS) extraction and semi-continuously extracted metals (SEM) methodological approaches gave promising results.