Mimicking virus removal and transport in aquifer media using surface-modified silica nanoparticles
Thesis DisciplineBiological Sciences
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Contamination of drinking water sources, such as groundwater, by pathogens (protozoa, bacteria and viruses) is of major concern globally. Due to their small size, mobility and high infectivity, enteric viruses have been a focus of groundwater research. However, the behaviour of enteric viruses in aquifer media is still poorly understood, which is partially attributable to the lack of reliable surrogates for these viruses. In the study reported in this thesis, a new type of surrogate was characterised and validated for its use in studying virus fate and transport in groundwater. The surrogates developed were composed of 70 nm carboxylated silica nanoparticles, labelled with dsDNA tags for sensitive detection, and coated with selected proteins to mimic the physico-chemical characteristics (size, charge, density) of two enteric viruses, human rotavirus and adenovirus, frequently found in faecal-contaminated groundwater. The selected enteric viruses and a commonly used virus surrogate, the MS2 bacteriophage, were purified and characterised in terms of size, surface charge, hydrophobicity and aggregation. For validation, the characteristics, the adsorption, degradation and transport of the surface-modified nanoparticles and the viruses were investigated in laboratory studies and compared. The characterisation of the viruses and particles revealed that the modified silica nanoparticles resemble the size and negative surface charge of the rotavirus and adenovirus. In general, the nanoparticles were found to be less hydrophobic than the enteric viruses, thus presumably less interactive with hydrophobic media. In contrast, the MS2 bacteriophage was smaller in size than the enteric viruses studied and considerably more hydrophobic implying stronger interactions with hydrophobic media. The surface-modified nanoparticles were found to be more stable and remained more monodispersed over time than the purified enteric viruses. In laboratory studies using simulated groundwater, the DNA-labelled nanoparticles were more stable over time than the rotavirus, the adenovirus or a plasmid DNA on its own. Interestingly, the study revealed that rotavirus was more persistent than the adenovirus over time in terms of degradation and aggregation, however, day light considerably enhanced rotavirus degradation. The adsorption studies revealed strong interactions between the enteric viruses and natural aquifer media (gravel and sand), whereas most of the surface-modified nanoparticles adsorbed weakly to these media. Only the casein-coated nanoparticles adsorbed strongly to the sand. The MS2 adsorbed to the gravel strongly, but weakly to the sand implying different interactions. The studies on virus and nanoparticle adsorption to hydrophobic-coated and non-modified Ottawa sand supported the results of characterisation. Column studies investigating the transport of the viruses and the nanoparticles in gravel and sand showed that even though gravel had high adsorption capacity in the adsorption tests, all viruses and nanoparticles travelled though the gravel columns with little retention, probably due to insufficient interaction time. This highlights the vulnerability of gravel aquifers to virus contamination. Experiments using sand columns showed great differences in the transport of the particles. Results suggested that the recovery of the DNA-labelled nanoparticles was similar to the recovery of the adenovirus, however, their transport pattern was different. The glycoprotein-, the protein A- and the AMBP-coated nanoparticles mimicked the transport pattern and low recovery of the rotavirus. In contrast, the streptavidin- and casein-coated nanoparticles were not recovered, emphasising the great importance of surface structure in particle transport. The results of this study demonstrated the usefulness of protein-coated silica nanoparticles as virus surrogates in groundwater studies. Surface-modified nanoparticles are able to mimic the surface characteristics of viruses. The glycoprotein-, protein A- and AMBP-coated particles were found to be suitable surrogates for rotavirus, whereas the DNA-labelled nanoparticles resembled adenovirus behaviour in hydrophilic media. Using particles with different material, size and protein-coating other pathogens can be modelled as well. Furthermore, these particles are expected to besafe to humans and the environment, thus can be used in a great variety of experiments in environmental research.