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Electron Spin Resonance Investigation of Mn²⁺ Complexation in Natural and Synthetic Organics¹

ABSTRACT

Mechanisms of Mn²⁺ bonding in soil organics and pure organic compounds were compared by using electron spin resonance (ESR) spectroscopy. The observation that Mn²⁺ in lower-than-cubic symmetry produces a very broad signal was used to quantitatively determine the degree of metal complexation by organics. As the pH was raised, multiligand molecules such as carboxymethylcellulose, citrate, and succinate formed complexes with Mn²⁺ as shown by a decrease of ESR signal intensity. However, monocarboxylic acids such as benzoate and salicylate do not readily form complexes with Mn²⁺. A fulvic acid extracted from acid soil possessed a very limited number of strong complexation sites which became available to Mn²⁺ only at pH values > 6.0. Polycarboxylic acids, including natural organics (peat, humic acid) and synthetic polymers (polymethacrylate, polygalacturonic acid) bond Mn²⁺ electrostatically as the hydrated ion at low pH, but complex increasing amounts of Mn²⁺ as the pH is raised. A comparison of the ESR characteristics of Mn²⁺ bound to soluble carboxylic acids, polymeric carboxylic acids, and strong acid compounds such as sulfonate resin is used to characterize the bonding behavior of Mn²⁺ in soil organics. The cooperative effort of several surface functional groups appears to be necessary to cause Mn²⁺ to form an inner-sphere complex with organic solids. Higher temperatures generally favor the complexation of Mn²⁺.

¹ Contribution from the Dep. of Agronomy, Cornell University, Ithaca, NY 14853. Research supported by NSF Grant no. EAR-7923290.

² Associated Professor of Soil Chemistry.

Received for publication June 22, 1982. Accepted for publication July 19, 1982.