Removal of SiO2 nanoparticles from industry wastewaters and subsurface waters by ultrafiltration: Investigation of process efficiency, deposit properties and fouling mechanism

Abstract

The novel properties of nanomaterials offer great promise of new technological breakthroughs. Consequently, their use in industry is expanding, and with it the risk of their being released in industrial effluents, and later in surface waters. This raises the problem of potential environmental hazards that are as yet unknown. The work presented in this paper evaluates the feasibility of membrane ultrafiltration for the removal of SiO2 nanoparticles (78nm mean hydrodynamic diameter) in suspension at 0.14wt% with the objective of treating them as pollutant compounds in industrial effluents. Regenerated cellulose membranes with two MWCOs of 10 and 100kDa were implemented. The influence of the membrane MWCO, the TMP and the stirring rate during filtration were examined. Stirred filtrations were performed at increasing TMP so as to characterise flux behaviour and retention rates. The feasibility of membrane ultrafiltration for this application was proved since the results of the 10kDa membrane showed the highest expectable retention rates (>99.6%) given the analytical methods used, and steady permeability results [30–40Lh−1m−2bar−1] independently of TMP and filtration course. Moreover, performance was shown to be sustainable as fouling was mostly reversible (>90%), being removable by simple membrane surface rinsing with ultrapure water. This was confirmed by the reconstruction of fouling mechanisms for both membranes. Unstirred and constant TMP 40, 80 and 200kPa filtration experiments were run to study fouling phenomena and the filtration cake characteristics (compressibility and porosity). Two distinct fouling mechanisms were highlighted: the 100kDa membrane underwent internal and sometimes irreversible fouling in the form of pore clogging before cake filtration, while the 10kDa membrane was characterised by filtration cake only. This work proposes solutions for the predicament of nanoparticle release into the environment and opens up perspectives for efficient wastewater treatment in the context of their industrial development.