Abstract
A leading challenge in drinking water treatment is to remove small-sized viruses from the water in a simple and efficient manner. Multi-walled carbon nanotubes (MWCNT) are new generation adsorbents with previously demonstrated potential as filter media to improve virus removal. This study therefore aimed to evaluate the field applicability of MWCNT-filters for virus removal in water containing natural organic matter (NOM) as co-solute to viruses, using batch equilibrium experiments. Contrary to previous studies, our results showed with MS2 bacteriophages single-solute systems that the affinity of MWCNT for MS2 was low, since after 3 h of equilibration only 4 log10 reduction value (LRV) of MS2 (20 mL at an initial concentration of 106 PFU MS2/mL) were reached. Single solute experiments with Suwannee river NOM (SRNOM) performed with environmentally-relevant concentrations showed MWCNT surface saturation at initial SRNOM concentrations between 10 and 15 mgC/L, for water pH between 5.2 and 8.7. These results suggested that at NOM:virus ratios found in natural waters, the NOM would competitively suppress virus adsorption onto MWCNT, even at low NOM concentrations. We confirmed this expectation with SRNOM-MS2 co-solute experiments, which showed an exponential decrease of the MS2 LRV by MWCNT with an increase in the initial SRNOM concentration. More interestingly, we showed that pre-equilibrating MWCNT with a SRNOM solution at a concentration as low as 0.4 mgC/L resulted in a LRV decrease of 3 for MS2, due to the formation of a negatively charged SRNOM adlayer on the MWCNT surface. Complementary batch experiments with natural NOM-containing waters and competition experiments with SRNOM in the presence of CaCl2 confirmed that the presence of NOM in waters challenges virus removal by MWCNT-filters, irrespective of the concentration and type of NOM and also in the presence of Ca2+. We therefore conclude that MWCNT-filters produced with commercially available pristine MWCNT cannot be considered as a viable technology for drinking water virus removal.
Highlights
Viruses are responsible for a large share of the global burden of respiratory and diarrheal infectious diseases (Kotloff et al, 2012) and can be responsible for major outbreaks
A multiple linear regression was calculated to evaluate both effects of multi-walled carbon nanotubes (MWCNT) mass and pH on MS2 log10 removal value (LRV)
MS2 LRV was significantly influenced by MWCNT mass, but neither pH 7.7 nor pH 8.7 were statistically significant relative to pH 5.2
Summary
Viruses are responsible for a large share of the global burden of respiratory and diarrheal infectious diseases (Kotloff et al, 2012) and can be responsible for major outbreaks. Carbon nanotubes (CNT) are broadly considered promising materials for future water treatment applications, including filtration and/or adsorption. Brady-Estevez et al (2010b) demonstrated that a filter composed of a mixture of single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) reached a virus log removal value (LRV) of 6 for MS2, PRD1 and T4 bacteriophages, at a flux through the filter of 160 L/m2/h. MS2 removal by a MWCNT filter was between 5 and 8 LRV at low pressure and LRV was higher than what was achieved using a SWCNT filter (Brady-Estevez et al, 2010a, 2010c). Numerous virus-sorbent interaction forces are expected to drive MS2 adsorption onto MWCNT. The past work supports the potential of MWCNT filters to complement existing water purification technologies that are already used in low and middle-income countries for households or water kiosks drinking water treatment
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