Ultrafiltration separation of aquatic natural organic matter: chemical probes for quality assurance.

Characterization of molecular size of natural organic matter (NOM) is a valuable tool when assessing its effect on the performance of water treatment systems as well as its geochemical origin. Size fractionation can be accomplished by ultrafiltration (UF). Unfortunately, membrane manufacturing generates a range of pore sizes. Many membrane manufacturers use molecular weight cutoff (MWCO) metric based on a 90% retention of given solute after specified duration of filtration. The objective of this study was to characterize the ability of different commercially available UF membranes to separate different size fractions of NOM. The UF membranes characterized were YM (regenerated cellulose, negatively charged) and PB (polyethersulfone, negatively charged) product lines by Millipore. The probes used to represent the size, shape and charge of NOM were polymers (polyethylene glycols (PEGs), dextrans, polystyrene sulfonates (PSSs)), dyes (bromocresol green, congo red, methyl red, methyl orange) and biological molecules (vitamin B-12 and bacitracin). The results show that MWCO definition does not hold for membranes of 5kDa and 10kDa pore openings using most polymers and dyes. The MWCO definition holds for 1kDa membrane for all tested probes. Under natural water conditions PSSs assume random coil configurations that are nearly identical to Suwannee fulvic acid. The results show that PSS agrees with stated MWCOs. The study demonstrates that ultrafiltration is not a simple mechanical sieving process, but that charges on the membrane and the constituent play a significant role in the rejection process. Effective probe size was increased seven- to fourteen-fold by charge interactions between the negative probes and negatively charged membrane. Uncharged molecules larger than specified MWCOs are able to pass through pores (PEGs), while small charged molecules (dyes) do not pass. For probes with low or neutral charges, shape becomes an important factor, with globular being favored over linear structure. Thus, MWCOs cannot be trusted for purposes of NOM size characterization. The study recommends the use of YM 1K, PB 5K and YM 10kDa membranes for comparative-only NOM size ultrafiltration characterization within the 1-10kDa size range.

The effect of surface pore size on the differentiation of rat bone marrow cells: morphological observations and expression of bone related protein mRNA.

The purpose of this study was to investigate the behavior of rat bone marrow cells (RBM) growing on surfaces with different pore sizes. RBM behavior on Millipore filters (MF-Millipore membrane filter) made from cellulose mixed esters with 5 different pore surfaces (0.45 microm, 1.2 microm, 3.0 microm, 5.0 microm and 8.0 microm) were compared in terms of morphological changes on the different pore sizes. Furthermore, the expressions of osteopontin and osteocalcin mRNAs were investigated. On the 1.2 microm and 3.0 microm pore surfaces, RBM attached to the substrate well, but cells on the 5.0 microm and 8.0 microm pore surfaces invaded deeply into the pores. Higher levels of both osteopontin and osteocalcin mRNA expression were always observed in cells cultured on the 1.2 microm filter. These results suggest that the 1.2 microm Millipore filter pore size is the most suitable for inducing RBM to differentiate into an osteoblastic phenotype among these surfaces and is probably related to production of the ECM but not to the phenomenon of cell spreading.