The Study Of Surface Properties And Oil-Removal Efficiency Of Water Treatment Filter Media

Surface physicochemical characteristics of water and wastewater treatment filter media play an important role in oil removal during oil-bearing water treatment when using deep bed filtration process. This important role is becoming universally concerned in research field at home and abroad in recent years. Therefore, in order to get a better understanding of the mechanisms affecting oil partcle attachment which helps a lot in selecting and designing of more efficient filter meia for a wide range of industrial applications, the surface properties such as wettabilities, free energy components and Zeta potentials of several commonly-used granular filter media with hard texture, i.e. walnut shell, anthracite, magnetite, manganese sand, zeolite and quartz sand, were investigated in this thesis. At the same time, a laboratory simulated small scale deep bed filtration process was also constructed to treat oil-in-water emulsions.(1) Surface physical morphologies and chemical compositions of these filter media were characterized using Scanning Electron Microscope (SEM), Specific Surface Area System (BET), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infra-Red Spectroscopy (FTIR). The unaided visual inspections and SEM images indicate that all the filter media particles are irregular in shape, being angularly rectangular or rhombic. SEM micrographs at 1000 time magnification of samples show the regular morphological and homogenous characterization of the walnut shell and anthracite partical surfaces, and irregular surface micro structure and heterogenous nature of the magnetite dotted with crystal particulates and crystal plates. At the same time, the morphologies of manganese sand, zeolite and quartz sand particals are uneven, disorder and heterogenous as the magnetite, of which the zeolite presents notablely larger pore volume and specific surface area. The analysis results of EDS, XPS and FTIR spectra tell us that walnut shell and anthracite surfaces are principally organic functional groups constituted of carbon, hydrogen and oxygen elements, making them an apolar and lower energy chemical surface. Magnetite surface is mostly made up of iron and oxygen elements, which combine into Fe₃O₄ species. Magnetite possesses a higher surface energy because the chemical bonds of Fe₃O₄ on the surface are not saturated. Manganese sand has MnO₂ and Mn₂O₃ on surface, and zeolite and quartz sand contain Si-O bonds and multivalent coordination compounds, which make them a polar and hydrophilic surface characterization.(2) Considering the irregular and uneven nature of filter media and based on Washburn's equation, a lipophilic hydrophilic ratio (LHR) concept was defined for the first time and used to compare quantitatively the wettabilities of the above-mentioned filters:The calculated LHR values of walnut shell, anthracite, magnetite, manganese sand, quartz sand and zeolite with a size range of 0.45-0.9mm at 20℃were 76.4, 2.41, 1.06, 0.74, 0.65 and 0.64 respectively, which means walnut shell and anthracite are lipophilic, magnetite is not only lipophilic but also hydrophilic, while manganese sand, zeolite and quartz sand are hydrophilic. These results of LHR values are in accordance with the differences of filter's surface physicochemical propoties. Namely, walnut shell and anthracite surfaces are apolar and have lower energy, which makes them wetting easily only to apolar oil(cyclohexane) with lower surface tension. Magnetite possess a higher surface energy, therefore, not only apolar oil but also polar water with higher surface tension are wettable favorably. Manganese sand, zeolite and quartz sand, however, are wetted better with polar water than apolar oil because of their polar surfaces.Oginating from van Oss-Chaudhury-Good (VCG) theory and valuating the physical data of water and cyclohexane at 20℃, the relationship between LHR and surface free energy components of filter media was deduced as follows:This equation theoretically discloses that filter's LHR value is attributed to their surface free energy acidic componentγ_s⁺ and basic oneγ_s^- Also, surface free energy components of the filters were estimated through porous capillary penetration technique, using Washburn's equation and VCG theory with n-hexane, a-bromonaphthalene, formamide and water as probe liquids in this paper. The estimated surface free energy apolar and polar components of walnut shell, anthracite, manganese sand, quartz sand and zeolite were 38.8mJ·m^-2, 38.8mJ·m^-2, 38.1mJ·m^-2, 37.7mJ·m^-2, 38.2mJ·m^-2 and 0.37mJ·m^-2, 0.73mJ·m^-2, 6.79mJ·m^-2, 8.66mJ·m^-2, 9.42mJ·m^-2, experimentally verifying that the wettabilities of filter media are correlated to their surface free energy polar components (γ_s⁺andγ_s^-), and the latter could be ascribed to the differences in surface physicochemistry which is consistent with the characterized results in Section 4.(3) Zeta potentials of filter particles were also measured on the basis of Helmholtz-Smoluchowskiequation. Theζvalues of walnut shell, anthracite, magnetite, manganese sand, quartz sand and zeolite in fluid media of distilled water and 0.1mol·L^-1 KC1 solution at 20℃were -17mV, -17mV, -11mV, -34mV, -64mV, -11mV and -5.1mV, -4.2mV, -1.4mV, -0.11mV, -0.19mV, -0.15mV, respectively. These results make clear that concentration of electrolyte solution will bring significant effect on the Zeta potentials on filter media surfaces.(4) During 10 hours of filtration process running in a lab simulated small scale deep media bed, the oil removal efficiencisηand backwash efficienciesη_r of walnut shell, anthracite, magnetite, manganese sand, quartz sand and zeolite packed media were 96%, 80%, 72%, 64%, 60%, 78% and 16%, 30%, 37%, 73%, 83, 69%. Compared these efficiencies with LHR values, the following relations were fitted which illustrate that the better the filter's lipophilicity (the greater of the LHR) is, the easier the removal of oil from wastewater and the harder the backwash of it from packed media, and vice versa:(5) Calculated by XDLVO theory, the Electrostatic interaction energy changing between filter grain, oil and water were all about zero, i.e.â–³G₁₃₂^EL≈0; the Lifshitz-van der Waals energy changingâ–³G₁₃₂^LW for walnut shell, anthracite, manganese sand, quartz sand and zeolite were -7.52×10^-20J, -7.52×10^-20J, -7.25×10^-20J, -7.09×10^-20J, -7.29×10^-20J; the Lewis acidic-basic energy changingâ–³G₁₃₂^AB for them were -4.31×10^-18J, -3.99×10^-18J, -3.39×10^-18J, -2.93×10^-18J, -2.87×10^-18J; and the total interaction energy changingâ–³G₁₃₂^TOT for them were -4.38×10^-18J, -4.07×10^-18J, -3.46×10^-18J, -2.99×10^-18J, -2.94×10^-18J, respectively. Allâ–³G₁₃₂^TOT between filter grains and oil/water were less than zero, indicating the attachment of oil particulates on media surfaces is spontaneous and these spontaneous trends of filter media are identical to their orders of LHR values and oil removal efficiencies. Furthermore,â–³G₁₃₂^AB ofsystem is the major donor toâ–³G₁₃₂^TOT, once again proving that the effects of LHR andγ_s^AB of filter media on oil removal are identical in thermodynamic essence.