| Oil water is a common wastewater with complex elements and a wide range of industries such as leather, steel, textile, food, petrochemical and oil production. Deep bed filtration is technically and economically one of the most effective tertiary treatment procedure of oil water, suits especially for oil particles smaller than 10 μm and the lower concentration. For deep bed filtration process, a hydrophobic filter medium has a better removal than a hydrophilic one. To improve the hydrophobicity of filter and its oil removal performance, quartz sand filters were modified by using three different coupling agents(DN101, KH550 and DL411) respectively. Since the coupling agent has two different functional groups, and one of them can be chemically bounded to the filter surface, as a result, the other organic group is grafted to the filter surface. Thus several hydrophobic filters can be obtained.Hydrophobic filters MQW-Ti, MQW-Si and MQW-Al were obtained by modifying quartz sand filter using DN101, KH550 and DL411 as the modifying agents respectively by wet method, MQD-Ti was obtained by modifying quartz sand filter using DN101 as the modifying agent by dry method, and MQD-Ti Si was obtained by modifying quartz sand using a compound modifying agent consisting of DN101 and KH550 by dry method. The optimal modifying conditions of the five hydrophobic filters were determined by studying the effects of modification condition on the contact angle between water and filter surface respectively. The results indicate that the contact angle were affected by the coupling agent dosage, react temperature and react time, etc. Coupling agent dosage is most effective in modification of quartz sand filter. After modification, the contact angles of MQW-Ti, MQW-Si, MQW-Al, MQD-Ti, MQD-Ti Si are increased form 40.1° to a range of 80.6°~89.0°. As a result, the hydrophobicity of quartz sand filter is significantly enhanced.Surface physical morphologies and chemical compositions of quartz sand filter before and after modification were characterized by using Scanning Electron Microscope(SEM), X-ray Photoelectron Spectroscopy(XPS), Fourier Transform Infra-Red Spectroscopy(FTIR) and Specific Surface Area System. SEM shows that a thin layer of coupling agent is grafted on the surface of quartz sand filter after surface modified. MQW-Ti, MQW-Si, MQW-Al surfaces possesses a comparatively flatter surface, while MQD-Ti and MQD-Ti Si show more extremely rough nature than UQS, as a result, the surface roughness is decreased by the wet method of modification and increased by the dry method of modification. XPS and FTIR demonstrat the C—H chains and C—C chains of coupling agents are successfully grafted to the surfaces of quartz sand filter through Si—O—Ti, Si—O—P, Si—O—Si and Si—O—Al bonds.Wettability of quartz sand filter before and after surface modification were studied. Theoretically, the effects of equivalent hydraulic radius rd can be eliminated by static weight method for testing the wettability. When rs=0.6mm, =50°, the equilibration time is 130 s, which is agree with experimental data. Therefore it can be concluded that the method is feasible and the experimental data are credible. For the six filters, Cyclohexane is total wetting liquid, i.e., the contact angles equal zero, while the contact angles of water are greater than 0°. As a result, the theoretical calculation formula of was derived and used to quantitatively compare the wettabilities of the above-mentioned six filers:Wettabilities of different filters depend on the functional groups on the filers surfaces. The value order of six filters is MQW-Ti > MQW-Si > MQD-Ti > MQD-Ti Si > MQW-Al > UQS. This order is explained as follows. The —Ti—OH groups of the alcoholysis products of DN101 react with Si—OH of the quartz sand filter, therefore, —C17H35 and —C8H17 groups are grafted to quartz sand filter surface. As a result, the polar component of the surface free energy of MQW-Ti and MQD-Ti surfaced is decreased, thus the is increased. Similarly, the —NH2(CH2)3 group of KH550 was grafted to the quartz sand filter surface, but the of MQW-Si is lower than MQW-Ti and MQD-Ti because the polarity of —NH2(CH2)3 is less than that of long chain alkyl. The of MQW-Al is the lowest of all due to the lowest polarity of —OCOR’. Compared with wet method, the dry method need less modifier and with least values.1stopt software was used to estimate the apolar and polar components of the surface free energy by curve fitting. Values R2 are greater than 0.99 which confirm the method is feasible. The surface free energy polar and apolar components of UQS, MQW-Ti, MQW-Si、MQW-Al, MQD-Ti and MQD-Ti Si are 2.4 m J/m2, 2.9 m J/m2, 4.2 m J/m2, 3.4 m J/m2, 3.9 m J/m2 respectively and 28.9 m J/m2, 29.0 m J/m2, 32.3 m J/m2, 29.7 m J/m2, 30.9 m J/m2 respectively, experimentally verifying that the surface free energy is greatly decreased by the surface modification.Filtration and backwashing process were operated in a lab simulated small deep media bed. According to the modified trajectory model, the collector’s contact efficiency of oil wastewater is minimum when the oil particle diameter is 1.3 μm. Surface modification can’t change the collector’s contact efficiencies of filters but can increase the attachment efficiencies from 0.48 of UQS to 0.94、0.89、0.55、0.86 and 0.57, which corresponding to MQW-Ti, MQW-Si、MQW-Al, MQD-Ti and MQD-Ti Si, respectively. Deeper filter depth and lower filtration velocity results in greater removal efficiencies. Based on extended DLVO theory, when oil particles locate at the secondary minimum interaction energy state, the minus values of are greater with less surface free energy acidic and basic components or greater,and the expressions are:Originating from detachment theory, extended DLVO theory and v OCG theory, the semi-empirical formula for calculation of minimum diameter of oil particles detached from filters is obtained as follows:Calculated by the formula as mentioned above, the minimum diameter of oil particle are 0.82μm, 1.31μm, 1.29μm, 1.24μm, 1.27μm and 1.26μm for UQS, MQW-Ti, MQW-Si、MQW-Al, MQD-Ti and MQD-Ti Si, respectively. When filtration velocity is 4m/h, it is verified that the oil remove efficiency is improved by modification because the oil particles become more difficult to be detached.The backwashing efficiency of UQS, MQD-Ti and MQD-Ti Si are 87.6%, 78.4% and 82.8%, respectively, which were also measured by experimental method. During the backwashing process, the minimum diameter of oil particles detached from filters surfaces are 0.248 μm, 0.385 μm and 0.381 μm. Due to the fact that there is little oil particles smaller than 0.4μm, surface free energy of filters surface have less effect on backwashing than filtration process. |
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