Characterization of natural and synthetic humic substances (melanoidins) by chemical composition and adsorption measurements

Melanoidins, condensation products of sugars and amino acids, are thought to represent a key link in the transformation of polysaccharides and proteins to humic material in the marine environment. We investigated adsorption behaviour of melanoidins prepared in equimolar solutions of glucose and amino acids of choice (glutamic acid, valine and lysine) and pseudomelanoidins which were prepared from glucose only. Melanoidins were prepared using different condensation times (2, 4, 16 and 32 days). Synthesized melanoidins were separated into different molecular mass fractions. Fractionation of melanoidins by sorption on the macroreticular resin XAD-8 separated melanoidins into hydrophobic neutral, hydrophobic acid and hydrophilic fractions. Adsorption of melanoidins and their different fractions was studied at a mercury electrode by directly measuring the change of the double layer capacitance caused by the adsorption of organic molecules on the electrode surface through phase sensitive alternating current voltammetry. The hydrophobic acid fraction of melanoidins accounted for most of the adsorption behaviour of melanoidins. Consequently, the higher molecular mass fraction of melanoidins (>10 kDa) exhibits a stronger adsorption in comparison to the lower molecular mass fraction (<3 kDa) of the same melanoidin. The good fit of adsorption data of melanoidins and pseudomelanoidins to the same adsorption isotherm supports the idea that melanoidins are comprised of a sugar derived backbone that is responsible for the adsorption behaviour of melanoidin, while the presence of nitrogen atoms is responsible for the complexation of copper ions. Adsorption characteristics and complexation ability of melanoidins and natural organic matter were similar. Our results suggest that in the process of humification, selective adsorption of condensation products on aqueous surfaces may lead to a progressive immobilization of certain fractions, i.e., it is probable that higher molecular mass components accumulate at aquatic surfaces, while lower mass components remain in solution. (C) 2009 Elsevier Ltd. All rights reserved.