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Non-applicability of the term Biodegradation

For inorganic metal salts the concept of biodegradation is not applicable in general to inorganic metal salts (OECD 2001 Series on Testing and Assessment number 27) such as the iron salt category. Accordingly testing according to standard protocols for the assessment of biodegradation has been waived.

Important abiotic processes, which are involved in the degradation/transformation processes of iron salts in the environment are covered in the IUCLID 5 Endpoint summaries 5.1.2 Hydolysis, 5.1.3 Phototransformation in water, and 5.4 Transport and distribution, which correspond to the chapters Hydrolysis, Phototransformation in water, and 4.2 Environmental distribution in the CSR (Chemical Safety Report) and Sections 2.2.3, 2.2.2, and 2.2.4 of the SIAR respectively. As discussed in the section on Biodegradation in water: screening tests (below), rapid removal of iron from the water column is considered as iron forms metal hydroxides that are rapidly removed from the water column at various pH values. With time, these hydroxides either polymerise to form larger insoluble stable complexes or they are trapped and buried in sediments

The role of biota in iron speciation

Biota can play a role in the fate of metals in the environment as they may influence the speciation. Available data on biodegradation have been reviewed and discussed in peer-reviewed published SIAR for iron salts (OECD 2007, Section 2.2.5). The discussion is adopted and reproduced here.

Since iron is an essential element, it is also subjected to biological activity and control. Organometallic complexes play an important role in biota as haemoglobin contains iron. Due to their essentiality iron chemistry in biota controlled and readily biodegradation for the recoupment of the iron applies. Bacterial transformations of iron are extensively studied and basically well-known processes. Iron is abundant in the environment from natural mineral sources and iron transformations and the whole iron cycle in the environment is a combination of abiotic and biological processes.

Iron reducing bacteria

Two papers demonstrate the importance of natural bacteria according to OECD (2007). Chen et al (2003) studied ferric iron reduction kinetics and capacity by three fractionated NOM subcomponents in the presence or absence of metal reducing bacteria (Shewanella putrefaciens), strain CN32. which are generally associated with aquatic or marine environments. Results indicate that natural organic matter (NOM) was able to reduce ferric iron abiotically; the reduction was pH-dependent and varied greatly with different fractions of NOM. The polyphenolic-rich fraction (NOM-PP) exhibited the highest reactivity and oxidation capacity at a low pH (<4) as compared with the carbohydrate-rich fraction (NOM-CH) and a soil humic acid (soil HA) in reducing ferric iron. However, at a pH >4, soil HA showed a relatively high oxidation capacity. In the presence of the bacteria, all NOM fractions were found to enhance the microbial reduction of ferric iron under anaerobic, neutral pH conditions.

A further level of complication was demonstrated in a study by Straub et al (2001). Numerous ferric iron-reducing bacteria have been isolated from a great diversity of anoxic environments, including sediments, soils, deep terrestrial subsurfaces and hot springs. In contrast, only few ferrous iron-oxidizing bacteria are known so far. At neutral pH, iron minerals are barely soluble, and the mechanisms of electron transfer to or from iron minerals are still only poorly understood. In natural habitats, humic substances may act as electron carriers for ferric iron-reducing bacteria. Anaerobic ferrous iron-oxidizing phototrophic bacteria, on the other hand, appear to excrete complexing agents to prevent precipitation of ferric iron oxides at their cell surfaces. This finding is an illustration of the extent of adaptation of the environment to iron ions.

Interaction of microbial biota with the humic redox system

OECD (2007) discusses that in natural systems, bacteria interact with the humic redox system. Kappler et al (2004) suggest that microbial reduction of humic acids and subsequent chemical reduction of poorly soluble iron(III) minerals by the reduced humic acids represents an important path of electron flow in anoxic natural environments such as freshwater sediments as well as soil. Natural organic matter (NOM) can act as an electron acceptor for microbial respiration by iron-reducing bacteria.

  • Chen J, Gu B, Royer RA, Burgos WD (2003). The roles of natural organic matter in chemical and microbial reduction of ferric iron, Science of the Total Environment 307(1-3):167-78
  • Kappler A, Benz M, Schink B, Brune A (2004). Electron shuttling via humic acids in microbial iron(III) reduction in a freshwater sediment, FEMS Microbiology Ecology 47(1):85-92.
  • OECD Organisation for Economic Co-operation and Development (2007). SIDS Initial Assessment Report for SIAM 24. OECD, Paris, France, 17-20 April 2007. 138 p
  • Straub KL, Benz M, Schink B (2001). Iron metabolism in anoxic environments at near neutral pH, FEMS Microbiology Ecology 34(3):181-6.