performance of a disc tube (DT) methodology, originally designed for reverse
osmosis (RO) in desalination, and applied here for ultrafiltration (UF) and
nanofiltration (NF) of dissolved organic matter (DOM) in a set of fresh (0ppt), brackish (10ppt) and saline (30ppt) waters at low (1 - 2mg/L),
medium (5 - 6mg/L) and
high (10-12mg/L) dissolved organic carbon (DOC) content
is presented. The DT module allows for time efficient processing of large volumes
of sample and can be operated with RO, NF and UF membranes. We examined the
performance of a NF membrane with nominal molecular weight cut-off (MWCO) of
500 Da and UF membranes with MWCO of 1 kDa and 10 kDa. Throughout the
experiments we monitored DOM in terms of DOC, UV-absorption coefficient ratios
(E2/E3) and the specific UV-absorbance at 254 nm (SUVA254).Detailed protocols for operating the disc tube
modules are proposed. The membranes can be efficiently cleaned to provide low
carbon blanks (<0.2mg/L). Calibration
confirmed separation of high and low molecular weight standards into the
retentate and permeate fractions, respectively. DOC mass balance of
fractionated DOM samples showed good recoveries (123% ±32% at 500 Da, 95% ±12% at 1 kDa and 99% ±11% at 10 kDa) (n
= 9). The actual retention rates for 500 Da and 1 kDA were close to those
reported by the manufacturer (~200 Da and ~3 kDa, respectively). However, the
manufacturer’s rated 10 kDa membrane actually retained only
DOM >35 kDa. Salt addition (up to 30ppt) caused a significant reduction in the
amount of high molecular weight (HMW) compounds isolated in the 1 and 10 kDa
retentates, due to coiling of macromolecules with increasing ionic strength. These findings underline the importance of
stringent operating protocols and the salinity dependence of HMW retention.
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