Chemical and spectroscopic characterization of marine dissolved organic matter isolated using coupled reverse osmosis–electrodialysis

Abstract

The coupled reverse osmosis-electrodialysis (RO/ED) method was used to isolate dissolved organic matter (DOM) from 16 seawater samples. The average yield of organic carbon was 75 ± 12%, which is consistently greater than the yields of organic carbon that have been commonly achieved using XAD resins, C 18 adsorbents, and cross-flow ultrafiltration. UV-visible absorbance spectra and molar C/N ratios of isolated samples were consistent with the corresponding properties of DOM in the original seawater samples, indicating that DOM samples can be isolated using the coupled RO/ED method without any bias for/against these two properties. Five of the samples were desalted sufficiently that reliable measurements of their 13 C and 1 H NMR spectra and their Fourier transform ion cyclotron resonance (FTICR) mass spectra could be obtained. The 13 C and 1 H NMR spectra of RO/ED samples differed distinctly from those of samples that have been isolated in much lower yields by other methods. In particular, RO/ED samples contained a relatively lower proportion of carbohydrate carbon and a relatively greater proportion of alkyl carbon than samples that have been isolated using cross-flow ultrafiltration. From the FTICR mass spectra of RO/ED samples, samples from the open ocean contained a much lower proportion of unsaturated compounds and a much higher proportion of fatty acids than coastal samples.

Compositions and constituents of freshwater dissolved organic matter isolated by reverse osmosis

“…However, relatively constant CF values (26.7 ± 9.4, n = 47) were previously applied in the DOM isolation to minimize the biases derived from the RO process (e.g. Koprivnjak et al, 2006Koprivnjak et al, , 2009Ouellet et al, 2008). Here a wide range of CF (17-221) was adopted to test the effect of the CF in DOC yields of the RO system.…”

“…The former contained much higher alkyl and alkoxyl C, and lower aromatic and carboxyl C (33.7%, 29.1%, 10.2%, and 7.7%, respectively) than the latter (20.1%, 24.7%, 14.7%, and 12.5%, respectively) ( Table 2). It was noted that the RO DOM included the LMW and HMW DOM, but the ultrafiltration only isolated HMW DOM (Benner et al, 1992;Koprivnjak et al, 2009). Thus, the above comparison provided an insight into the functional groups of LMW DOM by difference.…”

“…To avoid the inconsistency caused by different isolation approaches, coastal and marine DOM samples separated by RO (Koprivnjak et al, 2009;Mao et al, 2012) were selected to compare with our freshwater DOM samples ( Fig. 4B; Table 2).…”

“…13 C nuclear magnetic resonance ( 13 C NMR) spectroscopy has been widely applied to characterize DOM (e.g. Abdulla et al, 2013;Koprivnjak et al, 2009;Mao et al, 2012;Sannigrahi et al, 2005). Most of the previous work has focused on specific fractions of freshwater DOM, such as dissolved humic substances (XAD resin), and high molecular weight (HMW) DOM (>1000 Da, tangential-flow ultrafiltration) (see references in Perdue and Ritchie, 2003).…”

“…However, to the best of our knowledge, this technique has not been used for characterizing RO DOM from freshwater. Little information is available on solid-state 13 C NMR of RO DOM, which was only investigated in the marine environment (Koprivnjak et al, 2009;Mao et al, 2012). In the present study, a large range of CFs was adopted for water samples to evaluate its role in DOM sorption and yield on a RO system.…”