S. O. Lai

An evaluation of direct measurement techniques for mercury dry deposition

In this project, several surrogate surfaces designed to directly measure Hg dry deposition were investigated. Static water surrogate surfaces (SWSS) containing deionized (DI), acidified water, or salt solutions, and a knife-edge surrogate surface (KSS) using quartz fiber filters (QFF), KCl-coated QFF and gold-coated QFF were evaluated as a means to directly measure mercury (Hg) dry deposition. The SWSS was hypothesized to collect deposited elemental mercury (Hg⁰), reactive gaseous/oxidized mercury (RGM), and mercury associated with particulate matter (Hg(p)) while the QFF, KCl-coated QFF, and gold-coated QFF on the KSS were hypothesized to collect Hg(p), RGM+Hg(p), and Hg⁰+RGM+Hg(p), respectively. The Hg flux measured by the DI water was significantly smaller than that captured by the acidified water, probably because Hg⁰ was oxidized to Hg²+ which stabilized the deposited Hg and decreased mass transfer resistance. Acidified BrCl, which efficiently oxidizes Hg⁰, captured significantly more Hg than other solutions. However, of all collection media, gold-coated QFFs captured 6 to 100 times greater Hg mass than the other surfaces, probably because there is no surface resistance for Hg⁰ deposition to gold surfaces. In addition, the Hg⁰ concentration is usually 100-1000 times higher than RGM and Hg(p). For all other media, co-located samples were not significantly different, and the combination of daytime plus nighttime results were comparable to 24-h samples, implying that Hg⁰, RGM and Hg(p) were not released after they deposited nor did the surfaces reach equilibrium with the atmosphere. Based on measured Hg ambient air concentrations and fluxes, dry deposition velocities of RGM and Hg⁰ to DI water and other surfaces were 5.6±5.4 and 0.005-0.68 cm s⁻¹ in this study, respectively. These results suggest surrogate surfaces can be used to measure Hg dry deposition; however, extrapolating the results to natural surface can be challenging.

A study of Computational Fluid Dynamics on membrane module in membrane distillation

Membrane distillation is one of the recently interest rising membrane separation processes used for the separation of seawater and saline wastewater, and etc. Membrane distillation has the advantage of adopting the low grade waste energy and/or renewable energy such as solar and geothermal energy due to the nature of thermal driven process and low temperature range. Computational Fluid Dynamics (CFD) is a numerical simulation tool that is able to perform the calculation in order to investigate and simulate the performance of the processes that involve fluid, heat and mass transfer. In this study, a direct contact membrane distillation (DCMD) experiment will be studied using hollow fiber membrane module. A three dimensional (3D) CFD simulation will be examined for its viability in the investigation of the DCMD. Furthermore, various CFD multiphase models will be studied for its suitability in predicting heat and mass behavior within the membrane.

Preparation, characterisation and performance of polyvinylidene fluoride membrane for sodium chloride rejection in direct contact membrane distillation

Abstract In this study, two types of hydrophobic polyvinylidene fluoride hollow fibre membranes were fabricated by the wet/dry phase inversion method with N,N-dimethylacetamide and N-1-methyl- 2-pyrrolidone as solvents, respectively. The effects of solvent on the membrane characteristics were investigated based on the results of gravitational method and scanning electron microscope. The gravitational test revealed that the membrane porosity and pore size could be improved with the use of N-1-methyl-2-pyrrolidone solvent. The scanning electron microscope images of membrane cross-section demonstrated a similar finger-like layer structure in both membranes, extending from inner to outer surface. Lastly, the performance test of sodium chloride removal by direct contact membrane distillation indicated that the polyvinylidene fluoride membrane with N-1-methyl-2-pyrrolidone solvent had a better performance in relative to that with N,N-dimethylacetamide solvent. This finding implied that the polyvinylidene fluoride membrane with N-1-methyl-2-pyrrolidone as solvent had the greater potential to be utilised in water and wastewater treatment.