The biodegradation of iron-citrate complexes depends on the structure of the complex formed between the metal and citric acid. Ferric iron formed a bidentate complex with citric acid, [Fe(III) (OH)(2) cit] involving two carboxylic acid groups, and was degraded at the rate of 86 muM h. In contrast, ferrous iron formed a tridentate complex with citric acid, [Fe(II) cit], involving two carboxylic acid groups and the hydroxyl group, and was resistant to biodegradation. However, oxidation and hydrolysis of the ferrous iron resulted in the formation of a tridentate ferric-citrate complex, [Fe(III)OH cit], which was further hydrolyzed to a bidentate complex, [Fe(III)(OH)(2) cit], that was readily degraded. The rate of degradation of the ferrous-citrate complex depended on the rate of its conversion to the more hydrolyzed form of the ferric-citrate complex. Bacteria accelerated the conversion much more than did chemical oxidation and hydrolysis.