Uncertainty in organic matter analysis for seawater reverse osmosis (SWRO) desalination

Organic matter dissolved in seawater should be analyzed carefully since organic fouling is one of the main reasons for low performances of seawater reverse osmosis (SWRO) processes. Because of various ionic constitutions with high concentrations in seawater, the seawater organic matter analysis is difficult to handle with. Dissolved organic carbon (DOC) as a quantitative and general parameter and molecular weight distribution (MWD) as a qualitative and, more specific one for the same model organic matter, were measured with various ionic concentrations and constitutions. As a result, the wet oxidation method was found unreliable for measuring DOC in seawater. The ions inhibiting wet chemical oxidation are not only chloride but also other ions that are found in seawater. It was also found that MWD patterns for the same organic matter were variable along with different ionic concentrations. In conclusion, the uncertainty in seawater organic analysis exists because of various ionic inhibition effects, and it may induce wrong analysis results.

Three different DOC analysis approaches, including the combustion method, sparging– combustion method, and wet oxidation method, were tested for the same standard DOC materials with different ionic concentrations and constitutions. As a result, the wet oxidation method was not reliable when the samples were measured in terms of ion species with relatively high concen- trations of high contents of ion species while other two combustion-based methods showed stable and reliable performances. In contrast to common knowledge that chloride ions inhibit the wet oxidation reaction by persulfate, this study found that ions other than the chloride ion, for example, sodium and potassium, could be the source of the inaccuracy in measuring DOC. It was also found that sample ionic concentrations affected the MWD patterns because of the effect of ionic strength on the interfacial interaction between organic molecules and media. In conclusion, the uncertainty in seawater organic analysis exists because of various ionic inhibition effects, and it may induce incorrect analysis results. It is therefore suggested to use combustion methods to measure seawater DOC and to consider that HPLC–SEC data for seawater organic matter are subjected to eluant and sample ionic concentrations.