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TECHNICAL REPORTS

Ground Water Quality

Modifying the Surface Charge of Pathogen-Sized Microspheres for Studying Pathogen Transport in Groundwater

^a Institute of Environmental Science & Research Ltd., P.O. Box 29181, Christchurch, New Zealand
^b Chemistry Dep., Univ. of Otago, PO Box 56, Dunedin, New Zealand
^c Dep. of Civil, Environmental, and Architectural Engineering, Univ. of Colorado, Boulder, CO

^* Corresponding author (liping.pang{at}esr.cri.nz).

Received for publication October 19, 2008. Consuming pathogen-contaminated groundwater has caused many waterborne disease worldwide. Microspheres are often used as pathogen surrogates because they can be made similar to pathogens in terms of their sizes, buoyant densities, and shapes. Laboratory studies have, however, shown that the surface charges of microspheres are very different from those of pathogens of comparable sizes, and that their attenuation and transport behaviors differ significantly to those of the pathogens mimicked. Thus, for microspheres to be better surrogates, their surface charges need to be modified. We have demonstrated that the surface charge of a microorganism can be closely mimicked by microspheres covalently coated with a protein that has a similar pH_PZC to the microorganism. Using MS2 bacteriophage to test our concept, 20 nm carboxylated microspheres were covalently coated with casein. Zeta potentials as a function of pH were determined for purified MS2, casein, and uncoated and coated microspheres. The uncoated microspheres were significantly more negatively charged than MS2. The coated microspheres displayed zeta potentials and a pH_PZC value similar to MS2. The modified surface charge on the microspheres was stable for at least 4 mo. Using the concept developed from this study, surrogates for many specific pathogens of concern can be developed, and the results can be corrected with pathogen die-off determined independently in the laboratory. Protein-coated microspheres could provide a new and alternative approach to investigate pathogen transport in groundwater. Future research is required to validate the surrogates' resemblances to pathogens in terms of their attenuation and transport behaviors in groundwater.

Abbreviations: pH_PZC, the point of zero charge (the pH at which the net surface charge of a particle is zero • above this pH, particles are negatively charged • below this pH, particles are positively charged) • , zeta potential (quantifying the magnitude of the electrical charge at the double layer). Zeta potential is strictly not surface charge but an approximation of the surface potential, and surface potential can be related to surface charge by assuming a model of the surface (e.g., a Guoy-Chapman double layer model). For convenience and simplicity, we loosely refer zeta potential to surface charge in this paper • VLP, virus-like particle