Effect of dissolved calcium on the removal of bacteriophage PRD1 during soil passage: The role of double-layer interactions Gholamreza Sadeghi a, b , Thilo Behrends b , Jack F. Schijven b, c, ⁎, S. Majid Hassanizadeh b a Department of Environmental Health Engineering, Zanjan University of Medical Sciences, Zanjan, Iran b Department of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The Netherlands c Expert Centre for Methodology and Information Services, National Institute of Public Health and the Environment (RIVM), P.O. Box 1, 3720 BA Bilthoven, The Netherlands article info abstract Article history: Received 25 November 2011 Received in revised form 19 September 2012 Accepted 8 October 2012 Available online 23 October 2012 The objective of this work was to investigate and obtain quantitative relations for the effects of Ca 2+ 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 CaCl 2, NaCl and NaHCO 3 to de-ionized water. Values of pH and ionic strength were fixed at 7 and 10 mM, 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 Ca 2+ was developed. Numerical solutions of the Poisson–Boltzmann equation were obtained to evaluate the effect of Ca 2+ 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 Ca 2+ 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 Ca 2+ to a Ca 2+ -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. © 2012 Elsevier B.V. All rights reserved. Keywords: Colloids Solute transport Bacteriophage PRD1 Calcium 1. Introduction Groundwater is the main source of drinking water for the world's population. It usually is of excellent microbial quality and often directly underfoot at low capital cost. Nevertheless, it may become contaminated with pathogenic microorganisms, especially viruses, and that may pose a public health risk. Therefore, it is extremely important to be able to describe and quantify all processes which affect virus transport through soil and aquifers. The two most significant processes controlling virus mobility in the subsurface environment are virus attach- ment and inactivation (Schijven and Hassanizadeh, 2000). Based on earlier studies, many factors have been identified that impact these processes, such as ionic strength (IS) and pH of the ground water (Fontes et al., 1991; Sadeghi et al., 2011) and organic content of the ground water (Foppen et al., 2006; Pieper et al., 1997; Zhuang and Jin, 2003a), aquifer substrate grain size (Fontes et al., 1991) and soil water content (Jin et al., Journal of Contaminant Hydrology 144 (2013) 78–87 ⁎ Corresponding author at: Expert Centre for Methodology and Information Services, National Institute of Public Health and the Environment (RIVM), P.O. Box 1, The Netherlands. 0169-7722/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jconhyd.2012.10.006 Contents lists available at SciVerse ScienceDirect Journal of Contaminant Hydrology journal homepage: www.elsevier.com/locate/jconhyd