Effect of dissolved calcium on the removal of bacteriophage PRD1 during soil passage: The role of double-layer interactions

The objective of this work was to investigate and obtain quantitative relations for the effects of Ca2+ concentration on virus removal in saturated soil and to compare the experimental findings with predictions of the DLVO theory. In order to do so, a systematic study was performed with a range of calcium concentrations corresponding to natural field conditions. Experiments were conducted in a 50-cm column with clean quartz sand under saturated conditions. Inflow solutions were prepared by adding CaCl2, NaCl and NaHCO3 to de-ionized water. Values of pH and ionic strength were fixed at 7 and 10mM, respectively. Bacteriophage PRD1 was used as a conservative model virus for virus removal. The samples were assayed using the plaque forming technique. Attachment, detachment and inactivation rate coefficients were determined from fitting breakthrough curves. Attachment rate coefficients were found to increase with increasing calcium concentration. Results were used to calculate sticking efficiency, for which an empirical formula as a function of Ca2+ was developed. Numerical solutions of the Poisson–Boltzmann equation were obtained to evaluate the effect of Ca2+ on the double-layer interactions between quartz and PRD1. Based on these results, the DLVO interaction energies were calculated. It turned out that the experimental findings cannot be explained with the distance profiles of the DLVO interaction. The discrepancy between theory and experiment can be attributed to underestimation of the van der Waals interactions, chemisorption of Ca2+ onto the surfaces, or by factors affecting the double-layer interactions, which are not included in the Poisson–Boltzmann equation.When abruptly changing from inflow solution containing Ca2+ to a Ca2+-free solution, pronounced mobilization of viruses was observed. This indicates virus removal is not irreversible and that chemical perturbations of the groundwater can cause a burst of released viruses.