Distribution of different molecular weight fractions of dissolved organic matters and their complexation with Cu and Pb in the sediment from Dianchi Lake, China

The methods of equilibrium membrane dialysis were employed to determine the molecular weight distribution of dissolved organic matters in the sediment of the Dianchi Lake. Ion selective electrode (ISE) and spectroscopy technology were applied to analyze spectral characteristics of dissolved organic matter (DOM) with different molecular weights and the interaction with Cu and Pb, as well as explore the effect of molecular weight on the interaction between heavy metals and DOM. Results indicated that Caohai DOM was dominated with the molecular weight between 3.5 and 5.0 ku, while Waihai DOM was dominated with the molecular weight between 0.5 and 2.0 ku. Overall, the molecular weights of DOM for Caohai were higher than that in Waihai. The UV absorption ratio values (A3/A4) of the various molecular weight DOM, characterizing the humification degree, aromatic structure, ranged from 1.57 to 6.00, and the degree of humification and aromaticity of DOM increased with the molecular weights increased. The ISE results and 3DEEM spectral characteristics showed that Cu was mainly combined with DOM fraction with molecular weights from 2.0 to 3.5 ku; Pb tended to be associated with low molecular weight (≤0.5 ku) DOMs. Complexing capacity and distribution coefficients (Kd) were used to estimate the capability of Cu and Pb associated with DOM between the dissolved and the associated phases. The results indicated that, the combined Cu was maximal in the DOM with molecular weight of 2.0-3.5 ku, 717.65 and 340.27 mg/g in Caohai and Waihai, respectively; and the combined Pb was maximal in lower molecular weight DOM (≤0.5 ku), 2988.84 and 5073.45 mg/g in Caohai and Waihai, respectively.