Measurement of size and number of suspended and dissolved nanoparticles in water for evaluation of colloidal fouling in RO membranes

We developed a real-time aerosolization method combined with membrane filtration technique to determine size and number of dissolved and suspended colloid nanoparticles in water. Feed water before and after being treated by MF, UF or NF membranes was in-situ aerosolized and dried, and size and number of particles were determined in real time. Humic acid, CaCO3, and NaCl nanoparticles with varying solute concentrations, and standard nanoparticles with known size and number, were used to evaluate performance of the current method. We also measured size and number of colloidal nanoparticles in wastewater effluent, river water, and seawater. We found that there exists a significant amount of nanoparticles remaining even after UF and NF filtration, suggesting that dissolved nanoparticles that survived by UF and NF filtration may affect the fouling in RO membranes in the desalination process. The dissolved nanoparticles are defined here as those that are originally not in particle phase in feed water but that are able to precipitate near the membranes by crystallization or concentration polarization in the form of particle phase. Separating of dissolved and suspended nanoparticles in water was achieved with the current method, leading to determine their relative contribution in the feed water. The fraction of dissolved nanoparticles in seawater that survived through NF filtration in total particle count is 66% while that in river water is 44%. TEM/EDS analysis verified that a significant amount of dissolved nanoparticles survived through UF and NF filtration, and that distinct types of dissolved nanoparticles exist in river water and seawater. Dissolved nanoparticles in seawater were classified into four types: carbon-rich spherical particles, carbon-mineral rich (Ca-O-C-Mg-S-Cl-Si) irregular particles, carbonmineral rich spherical particles (C-Cu-Ca-O-Si-Mg), and Na-containing cubic particles.

We developed the new aerosolization method combined with the membrane filtration technique to determine size and number of suspended and dissolved nanoparticles of 20–600 nm in feed water. Data for humic acid, CaCO3, and NaCl nanoparticles with varying solute concentrations, and standard nanoparticles with a known size and number showed that the method can be used to quantify size and number of nanoparticles in water. Measurements of size and number of particles in several types of natural waters (seawater, waste water, and river water) were performed. Data showed that seawater had the highest particle number concentration (~1.0×107 particle/ cm3) with the highest GMD of 129 nm while the Yeongsan River water had the smallest concentration (1.7×105 particle/cm3) with the smallest GMD of 84 nm in the particle size range of 20–600 nm. The estimated fraction of dissolved nanoparticles in seawater that survived through NF filtration is 66% while that in river water is 44%. We found that a significant amount of nanoparticles remained even after NF filtration, suggesting that dissolved nanoparticles that survived through pre-filtration processes should affect the fouling in RO membrane in the desalination process. TEM/EDS analysis also showed that a significant amount of dissolved nanoparticles survived through UF and NF filtration and that distinct types of dissolved nanoparticles exist in river water and seawater.