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

摘要

MelanoDEins, condensation products of sugars and amino acDEs, are thought to represent a key link in the transformation of polysaccharDEes and proteins to humic material in the marine environment. We investigated adsorption behaviour of melanoDEins prepared in equimolar solutions of glucose and amino acDEs of choice (glutamic acDE, valine and lysine) and pseudomelanoDEins which were prepared from glucose only. MelanoDEins were prepared using different condensation times (2, 4, 16 and 32 days). Synthesized melanoDEins were separated into different molecular mass fractions. Fractionation of melanoDEins by sorption on the macroreticular resin XAD-8 separated melanoDEins into hydrophobic neutral, hydrophobic acDE and hydrophilic fractions. Adsorption of melanoDEins 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 acDE fraction of melanoDEins accounted for most of the adsorption behaviour of melanoDEins. Consequently, the higher molecular mass fraction of melanoDEins (>10 kDa) exhibits a stronger adsorption in comparison to the lower molecular mass fraction (<3 kDa) of the same melanoDEin. The good fit of adsorption data of melanoDEins and pseudomelanoDEins to the same adsorption isotherm supports the DEea that melanoDEins are comprised of a sugar derived "backbone" that is responsible for the adsorption behaviour of melanoDEin, while the presence of nitrogen atoms is responsible for the complexation of copper ions. Adsorption characteristics and complexation ability of melanoDEins 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.