Molecular size distributions of dissolved organic matter

Discrete molecular size distributions of dissolved organic matter (DOM) in natural waters and wastewaters are usually determined using an array of ultrafiltration membranes in stirred cells. However, many researchers neglect membrane rejection, resulting in an underestimation of DOM in low molecular weight size classes. In this paper, we develop a simple model, based on a permeation coefficient, that can be used to improve the accuracy of size distributions. Since permeation coefficients are a function of membrane material and water composition, these coefficients must be determined for every water sample. The utility of the model is demonstrated by comparing unadjusted and adjusted size distributions of dissolved organic carbon (DOC) in ground waters and municipal wastewaters. For example, when membrane rejection is neglected, 41% of the DOC in Biscayne Aquifer ground water has an apparent molecular weight greater than 5,000 amu. Size distributions determined using the permeation coefficient model, however, indicate only 16% of the DOC is greater than 5,000 amu.