Roles of surface interaction on Cryptosporidium parvum oocyst transport in subsurface environment
The influence of Cryptosporidium (C.) parvum oocyst and collector surface properties on transport of C. parvum oocysts and microspheres was studied with a radial stagnation point flow (RSPF) cell and 2-dimensional (2-D) micromodels. The quartz or natural organic matter coated quartz surfaces were used as collector in the RSPF cell. The cylindrical silica collectors or silica collectors coated with 0, 10, 20, 50 and, 100% Fe2O3 patches were firstly fabricated in micromodels. Real time transport of oocyst in RSPF cell and micromodel in electrolyte solutions with systematically varied composition was monitored. Surface polarity and chemical heterogeneity of oocysts surface were characterized by liquid chromatography/nanoelectrospray ionization tandem mass spectrometry (LC-MS/MS), microbial adhesion to hydrocarbon test (MATH), contact angle, and Fourier transform Infrared Spectroscopic (FT-IR) imaging. The most important fundings of this research are the followings: 1) this research developed a method to precisely design complicated yet well-controlled collector properties of porous media and showed the importance of spatial distribution of charged heterogeneous patches on collectors; 2) the results illustrated that the composition and conformation of oocyst surface macromolecules determined oocyst transport by controlling the surface interactions and complexations between oocyst and quartz, NOM, Fe2O3 and charged heterogeneous mineral surfaces. Especially, this research firstly revealed that the electrophoretic softness of oocyst surface macromolecules was essential to van der Waals interaction and steric repulsion between oocyst and quartz surfaces. RSPF experiments illustrated that steric repulsion caused by oocyst surface macromolecules prevented oocyst deposition at higher ionic strength where the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory predicted maximum deposition. Charge heterogeneity was responsible for oocyst deposition on natural organic matter (NOM) surfaces and at lower ionic strength where the DLVO theory predicted zero deposition. To study the role of oocyst surface macromolecules, deposition of untreated oocysts and oocysts modified by proteinase K in RSPF cell were compared. The proteinase K modification resulted in 10 times higher electrophoretic softness and 100 times lower van der Waals interaction of proteinase K modified oocysts than untreated oocysts. Though oocysts modified by proteinase K were less negatively charged, the change of composition and conformation of oocyst surface macromolecules led to significantly low deposition rate compared to that of untreated oocysts. The results indicated that oocyst surface macromolecules determined oocyst deposition mechanisms by controlling van der Waals interaction and steric repulsion. In addition, oocysts were directly observed to be entrapped in the DLVO secondary minimum energy well. The results illustrated that attachment in secondary minimum energy well was reversible and oocysts were either resuspended due to hydrodynamic forces or transferred to more stable primary minimum energy well. Distribution of oocyst attachment on collectors within a micromodel was correlated with local flow rate. Only a small portion of oocysts attached on collectors in micromodels were released when pumping in oocysts free electrolyte solution with low ionic strength. The results confirmed the previous observation that oocysts entrapped in secondary minimum energy well would either be transferred to primary minimum energy well or be resuspended. Significant numbers of attached oocysts were released when pumping in oocysts free electrolyte solution with pH 11. Since positively charged functional groups will be deprotonated at pH 11, the release of oocysts was contributed to surface charge heterogeneity. An attempt to characterize oocyst surface chemical heterogeneity was made using FT-IR imaging. FT-IR spectra varied with location on a layer of oocysts. Though the resolution of this technique is not high enough to quantify oocyst surface chemical heterogeneity so far, great efforts are made to improve its resolution. Recently, the resolution of FT-IR imaging is improved from 1.6 µm to 1.0 µm. Therefore, FT-IR imaging showed promising future in characterizing surface chemical heterogeneity. Each silica collector in the micromodel was periodically coated with Fe2O3 patches to create charged heterogeneous collectors. Oocyst average single collector removal efficiency (η) on Fe2O3 patches was 2-3 times higher than on collector coated with 100% Fe2O3, which implied the importance of periodic electrostatic interactions caused by alternating charged heterogeneous patches. In addition, oocyst attached more on Fe2O3 surface at pH 8.1, where DLVO predicted energy barrier, than at pH 5.8, where DLVO predicted no energy barrier. This indicated different complexations formed between oocyst carboxylate groups and Fe metal centers on Fe2O3 surfaces at different pH conditions.
Cryptosporidium parvum oocysts, transport, porous media, micromodel, charge heterogeneity
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign