Author: Salini Sasidharan
Sasidharan, Salini, 2016 Fate, Transport, and Retention of Viruses, Bacteria, and Nanoparticles in Saturated Porous Media, Flinders University, School of the Environment
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The main objective of this thesis was to gain a fundamental understanding of the mechanisms involved in the fate, transport and retention of colloidal particles such as viruses, bacteria, and nanoparticles in saturated porous media. Laboratory scale systematic column and batch experiments were conducted by employing sand or biochar as the saturated porous media. Bacteriophages (ΦX174, PRD1, and MS2) and Escherichia coli were used as surrogates for pathogenic viruses and bacteria, respectively. Negatively charged carboxyl-methylated latex nanoparticles (50 and 100 nm) were employed as model-engineered nanoparticles. The effects of water solution chemistries on colloid transport were investigated; specifically, pH (5.8–7.2), ionic strength (1–60 mM), ion type (Na+ and Ca2+), temperature (4 and 20 °C), and physical factors including flow velocity (0.1–20 m d-1), and solid grain surface physical (surface roughness) and chemical (metal oxides) heterogeneity. All the experiments were conducted in saturated packed columns to simulate the natural aquifer environment. Long-term colloid deposition experiments were conducted in order to determine the solid surface area that contributed to the attachment of colloids (Sf) at various physiochemical conditions. Colloid transport in saturated porous media was described by utilising a one-dimensional form of the advection-dispersion transport model that accounts for colloid interaction with the solid-water interfaces (SWI). Results clearly indicated that colloid retention in porous media and the value of Sf increased with decreasing colloidal size, colloid input concentration, pH of the electrolyte solution, pore water velocity, increasing ionic strength, and concentration of multivalent ions in the electrolyte solution. Simulation of the observed breakthrough concentrations (BTCs) using Hydrus-1D modelling showed that nanosized-colloids (nanoparticles and viruses) required a two-site kinetic model (site 1, which represents an initial delay due to the presence of highly favourable attachment sites, and site 2, which represents a sharp rise where blocking is present) to produce a good fit for the BTCs obtained from the column experiments. This was the first study to examine the effect of temperature on virus and nanoparticle attachment in a saturated packed column transport experiment. Systematic experiments were conducted and the BTCs were fitted using the Hydrus-1D model to determine the fitted parameters. Theoretical simulations and mathematical solutions were employed to quantify the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) interaction energy between a colloid and a heterogeneous (presence of surface roughness and positive charge) collector surfaces. This study established that the increase in attachment rate coefficient (katt) with an increase in temperature (4 to 20 °C) at moderate IS (10–30 mM) was a function of single-collector efficiency (η) and sticking efficiency (α), and, therefore, a function of Sf. The removal efficiency of various plant-based biochar materials for viruses has not previously been addressed. Batch experiments conducted with various types of biochar showed negligible attachment of viruses and bacteria to the biochar surface. Transport experiments conducted using biochar-amended sand columns showed enhanced transport of viruses and enhanced deposition of bacteria. However, elimination of the fine fraction of biochar (< 60 μm) particles in biochar-amended sand columns significantly reduced the bacteria retention. This study demonstrated that biochar plays a role in microbe (bacteria) retention via straining, by alteration of pore size distribution, and not via attachment. However, the straining mechanism does not result in virus removal due to their considerably smaller size. This research acknowledged that the colloid retention in saturated porous media and the value of Sf is determined by the coupled physio-chemical processes that strongly depend on colloid size, temperature, solution chemistry, and system hydrodynamics. Results from the actual column experiments and theoretical simulations have clearly shown that nanoscale chemical and physical heterogeneities on both collector and colloid surface determine the XDLVO interaction energy at the interfacial scale. Therefore, modelling colloid transport through saturated porous media and quantifying the interaction energy at interfacial scale will require non-traditional approaches to account for the aforementioned factors that are not addressed by classical DLVO calculations.
Keywords: Virus, Bacteriophages, MS2, PRD1, ФX174, E. coli Bacteria, Nanoparticle, Breakthrough Curves, Column Studies, Batch Studies, Retention, Release, Attachment, Detachment, Straining, Hydrodynamic, Solution Chemistry, Two-site Kinetic Model, Temperature, Surface Roughness, XDLVO Theory, Biochar, Porous Media.
Subject: Environmental Science thesis
Thesis type: Doctor of Philosophy
School: School of the Environment
Supervisor: Prof Peter G. Cook