Biodegradation of synthetic organo-metallic complexes of iron and aluminium with selected metal to carbon ratios

Abstract

Synthetic organo-ferric and organo-aluminous complexes with various metal :C ratios were prepared and incubated for 44 days in an Eutrochrept A 1 horizon. Citric acid was used as a model of a natural soil acid. Fungal melanins synthesized by Epicoccum purpurescens were separated in fulvic acid-like and humic acid-like polymers and used as models for natural transformed soil organic matter. Under certain conditions, the biodegradation of such metal-organic complexes was slower than that of the free forms of the corresponding organic compounds. The intensity of this protective effect depended mainly on the metal to complexing functional groups molar ratio (M/CG) of the complexes and was also related to their aqueous solubility. For some organics, the protective effect was observed at ratios exceeding the saturation of their complexing sites, and sometimes needed a metal content exceeding more than twice their complexing capacity. In such cases, the formation of a flocculated metallic hydroxide trapping and wrapping the organic molecules is suggested to account better for the observed protective effect than chemical binding per se. As the solubility of the “humic” acids decreased faster than that of the “fulvic” ones when their metallic charge increased, the biodegradability of the former decreased more rapidly than that of the latter by metal complexing. For the same reasons, citric acid was more protected by complexing Al than Fe. The protective effect of both aluminium and iron against the biodegradation of all types of organic matter was high and of a comparable intensity at high M/CG molar ratios. These data are interpreted in terms of pedological processes.