Dissolved organic matter apparent molecular weight distribution and number-average apparent molecular weight by batch ultrafiltration

Knowledge of dissolved organic matter (DOM) size distribution is of tremendous practical significance in both natural and engineered aquatic and terrestrial systems. This study was conducted to demonstrate the use batch ultrafiltration data in conjunction with a two-parameter nonlinear permeation coefficient model (PCM) [Logan, B. E.; Jian, Q. J. Environ. Eng. (Reston, Va.) 1990, 116, 1046-1062] to determine distributed and number-average apparent molecular weight (AMW(N)) information for DOM in environmental samples. Batch ultrafiltration data was collected by filtering a natural water and its hydrophobic and hydrophilic acid, base, and neutral DOM subfractions through Amicon YC/YM series membranes. The actual proportion of DOM sized to pass each membrane (C-r0*) was determined using the PCM. The Weibell cumulative distribution function was fit to log-transformed batch ultrafiltration data (log C-r0*) using nonlinear least-squares regression. Best-fit values for Weibell scale (alpha) and shape (beta) parameters were calculated for each log-transformed ultrafiltration data set, from which DOM AMW(N) values were found in the range of 510-3910 Da. R-2 values for all nonlinear regressions were >0.95, indicating that the Weibell distribution adequately described log C-r0* data. Batch ultrafiltration data that has been adjusted for membrane rejection using the PCM may be used to characterize environmental samples that display generally unimodal DOM size distribution.