Role of smectites and A1-substituted goethites in the catalytic condensation of arginine and glucose

The polyphenol theory of humic-substance formation has been studied extensively; however, an alternative theory, that humic substances are formed through the condensation of amino acids and reducing sugars (Maillard reaction), has not been explored to the same extent. The general objectives of this study were to determine whether smectites and goethites catalyze the abiotic polymerization of arginine and glucose to form humic-like compounds. The effects of smectite type, saturating cation, and the degree of Al substitution in goethites on the polymerization reaction were also studied. Four cation-saturated smectites and four Al-substituted goethites were incubated abiotically with solutions containing a mixture of arginine + glucose for 21 days at 37°C. After the incubations, total C recovered ranged from 80.6 to 123.8% and from 100.5 to 105.1% for the smectite and goethite systems, respectively. At the end of the incubations, 21.4–50.3% of the added C and 16.5–90% of the added N were sorbed on the various smectites, and 6.2–9.0% of the added C and 2.3–4.6% of added N was sorbed on the goethites in a form that could not be desorbed by washing with 100 mM CaCl2. X-ray diffraction analysis indicated that some of the sorbed C was intercalated in the smectites and FTIR analysis provided evidence of new absorption bands at 1650 and 1668 cm−1, which are consistent with Maillard reaction products. Thus, it is concluded that smectites catalyze the condensation of arginine and glucose to form humic-like products. Goethites, however, have little or no ability to catalyze this reaction.