The reverse osmosis (RO) method has been used to isolate the reference samples of natural organic matter (NOM) that are distributed by IHSS. The method is described in detail by Serkiz and Perdue (1990) and by Sun et al. (1995).
Reverse osmosis separates a feed solution consisting of water and aqueous solutes into a permeate solution (relatively pure water) and a retentate solution (somewhat higher solute concentrations than the feed solution). A general schematic diagram for using RO to concentrate solutes from a feed solution is given below.
The sample reservoir is filled initially with feed solution, which is produced by prefiltering the source water through one or more filters, the finest of which has a nominal pore size of 0.45 µm or less. As the feed solution is pumped to the RO system, the retentate solution is returned to the sample reservoir and the permeate solution is discarded. More feed solution is added either continuously or discontinuously to maintain a constant water level in the sample reservoir.
The concentrations of all solutes that are well rejected by the RO membranes gradually increase in the sample reservoir as more feed solution is processed. To avoid the buildup of polyvalent cations such as Ca(II), Al(III), and Fe(III), which leads to fouling of the RO membrane by precipitation of calcium carbonate, iron(III) hydroxide, etc., the feed solution is typically passed through a cation exchange resin in either the H+ or Na+ form. The use of H+ as the exchangeable cation causes pH to decrease gradually as a sample is processed, further preventing the formation of inorganic precipitates and also enabling the removal of HCO3– as CO2(g). Potentially negative effects are precipitation of less soluble organic acids and loss of low-molecular-weight organic acids.
The process is continued until a sufficient volume of feed solution has been processed, at which time the concentrated, partially desalted NOM is drained from the sample reservoir, the RO system, etc. After further desalting in the laboratory, most samples that are concentrated by this method may be freeze-dried. Samples containing a significant concentration of sulfate should be adjusted to neutral pH before they are freeze-dried to avoid reactions between NOM and concentrated sulfuric acid that forms during the freeze-drying process.
The reverse osmosis systems that have been used to isolate reference samples of NOM are manufactured by RealSoft Co. and can typically process filtered feed solutions at a rate of 4-5 L/min while achieving a 90% recovery of dissolved organic carbon (DOC) from the feed solution. To place these statistics in practical perspective, 1000 L of filtered freshwater having a DOC concentration of 5 mg/L can be processed using the RO method to obtain 4500 mg of DOC (approximately 9000 mg of NOM) in around four hours of operation.
Serkiz, S.M. and Perdue, E.M. (1990) Isolation of dissolved organic matter from Suwannee River using reverse osmosis. Water Res. 24, 911-916.
Sun, L., Perdue, E.M., and McCarthy, J.F. (1995) Using reverse osmosis to obtain organic matter from surface and ground waters. Water Res. 29, 1471-1477.