Characterization of marine organic matters and heavy metals with respect to desalination with RO and NF membranes

The transmission and toxicity of metals and metalloids were initially investigated with various nanofiltration (NF) and seawater reverse osmosis (SWRO) membranes, under oxic condition, using wastewater effluent and seawater samples. Various metals and metalloids were categorized into three groups according to their properties, such as specific gravity and concentration level, as well as others. The levels of various transition metals and metalloids included in the wastewater effluent and seawater samples were within ng/L–μg/L range; the concentrations of almost all compounds were substantially reduced to ng/L or a few μg/L levels after membrane filtrations, as measured by ICP-MS. Toxicity measurements were performed using bioassays, including both the Microtox and daphnia methods, for raw and membrane treated samples of wastewater effluent and seawater. The results indicated that (1) the SWRO membrane exhibited very good removal efficiencies for all metals and metalloids tested, (2) most metals and metalloids tested were efficiently removed by the tight NF membranes employed; however, some metals and metalloids included in seawater were not efficiently removed by the relatively loose NF membrane, (3) based on the Microtox measurements, only the seawater sample treated using the relatively loose NF membrane provided a relatively high toxic unit close to 1.0; otherwise, all other samples (both raw and membrane treated samples) exhibited relatively low toxicities.

The key goal of this work was to provide initial investigation for the removal trends of 12 different metals and metalloids from seawater, using RO and NF membranes for both wastewater reclamation and desalination. Therefore, robust conclusions or verification of specific mechanisms for membrane efficiencies cannot be given. However, general observations with regard to the transmission of metals and metalloids through membranes, and the corresponding toxicity changes, may be provided. From this works, the followings were observed: (1) the SWRO membrane tested exhibited greater performance in terms of metals and metalloids removals than the NF membranes tested; of the NF membranes, the two tight NF membranes, as expected, were proved better than the loose NF membrane, (2) there was no distinct difference in removal trends between the three metals and metalloids groups, but the effective removal of aluminum and antimony from seawater were observed to be difficult using both the tight and loose NF membranes, and (3) whether the results of both (1) and (2) are related to toxicity measurements results is uncertain, but the sample treated using the loose NF membrane provided the highest toxic unit value; however, this was still believed to be low.