Role of Divalent Cations on Deposition of Cryptosporidium parvum Oocysts on Natural Organic Matter Surfaces

A Radial Stagnation Point Flow (RSPF) system coupled with a microscope was used to study deposition of Cryptosporidium parvum oocysts on quartz and Suwannee River Natural Organic Matter (SRNOM)-coated surfaces in solutions with different Ca2+ or Mg2+ concentrations. Both untreated and proteinase K-treated oocysts were used. Deposition of oocysts on a SRNOM surface in Ca2+ solution was higher than in Mg2+ solution, even though the energy barriers calculated from Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for Ca2+ solution were higher than for Mg2+ solution. On the other hand, the attachment of oocysts on a quartz surface was the same in both Ca2+ and Mg2+ solution and in qualitative agreement with the DLVO energy profiles. Inductive coupled plasma (ICP) was employed to measure the free divalent cation concentration in solutions containing oocysts. ICP data showed more Ca2+ bound to oocyst surface than Mg2+. Moreover, proteinase K treatment of oocysts led to a significant decrease in deposition rate due to less binding of Ca2+ to the surface of the treated oocysts as shown by the ICP data. The deposition and ICP results suggested that inner-sphere complexation of Ca2+ with carboxylate groups on both SRNOM and oocyst surfaces enhanced deposition of oocysts on a SRNOM surface.