| Among industrial wastewater recycling technologies, reverse osmosis (RO) is considered as the most promising option because of the efficiency in the removal of foulants and low energy consumption. However, the foulants in the wastewater easily deposited onto the surface of the membrane, which induces the membrane fouling. Membrane fouling results in the decline in water quantity and quality, the increase in the operating cost and the reduction of membrane lifetime. Based on the efficiency of the chemical cleaning, membrane fouling can be divided into reversible and irreversible fouling. It is of practical importance to understand the causes of irreversible fouling. Focusing on the problem discussed above, the mechanism of the organic fouling and regeneration was studied in this thesis.Octanoic acid was selected as model organic compound. The mechanism of octanoic acid inreversible fouling RO membrane was studied according to the attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and the extended Derjaguin Landau Verwey Overbeek (extended DLVO) approach. The experiment results show that the inreversible adsorption amount is related to the irreversible fouling and the Lewis acid-base interfacial free energy is used to predict the fouling trend. Fouling was more significant at solution chemistries that resulted in larger adsorption amount, namely, lower pH, higher ionic strength, in absence of Ca2+and Mg2+, and lower temperature. The flux decline decreased at the ionic strength50mM due to the diverse ion effect. The rate and extent of flux decline decreased with higher Ca2+concentration due to the decreasing organic compounds hydrophobicity. In the presence of magnesium, fouling behavior decreased slightly after the addition of magnesium ion, whereas, flux decline barely changed when Mg+concentration was increased gradually. Octanoic acid could chelate with Ca2+and Mg2+, and Ca(C8H15O2)2and Mg(C8H15O2)42-were formed respectively. Compared with the octanoic acid molecular layer, the layer of the octanoic acid complexes was more hydrophilic. At the low temperature, the mass transfer coefficient of the membrane decresed, which favor the membrane fouling.Methylene blue (MB) and rhodamine B (RB) were selected as model organic foulants. Three kinds of RO membrane (CPA2, ESPA2and PROC10) were used. The calculated amount of the irreversible sorption was related to the irreversible flux decline. Characterized by means of atomic force microscope (AFM), contact angle and Zeta potential, the permeate flux tends to be more decreasing as membrane surface becomes rougher, more hydrophobic and negative charge. The fouling has been delineated, comprising of the Langmuir-Hinshelwood (L-H) kinetics model followed by the cake formation, in sequence. The characteristic fouling kinetics was accounted by L-H kinetics model for initial fouling, with the fouling rate constant k=0.0556?ms·min-1and k=0.0181?m·s-1·min-1for MB and RB fouling RO membrane CPA2, respectively. The value of k can predict the fouling trend. The adsorption dominated in the initial of the fouling, and the subsequent fouling was attributed to the growth of a dye cake. Adsorption of MB on PROC10membrane obeyed Langmuir isotherm. Dynamic light scattering data suggested that the aggregate size and size distribution are strongly influenced by the RB concentration. At the high concentration, the RB molecules assemble as micelle-like structures. In the presence of Ca2+and Mg2+, the extent of flux decline was related to the competition model. A remarkable correlation was obtained between the quantified irreversible sorption and irreversible flux decline under the solution chemistries investigated.The aromatic compounds were selected as the model organic foulants. The study was about the effect of force between organic compounds and membrane surface on membrane fouling. The experiment results show that the irreversible fouling in the long time is found to strongly depend on hydrogen bond formation. The initial fouling rate is dominated by electrostatic force and hydrophobic force, and the electrostatic force is stronger than the hydrophobic force. The correlation between the membrane fouling and Keeson force, as well as Debye force, is not significant.Fouling behaviors of RO membrane for petrochemical secondary effluent reclamation were studied. Based on the chemical cleaning (NaOH solution, pH=11.0), the flux decline was discriminated to concentration polarization, irreversible fouling and reversible fouling. The transition points were studied. The experiment results show that the flux decline due to fouling was the dominant flux decline mechanism. The associated fouling theory has been delineated for a crossflow membrane filtration, comprising of the Langmuir-Hinshelwood (L-H) kinetics model followed by the cake formation, in sequence. The fouling layer was more significant at conditions that resulted in the faster adsorption of the organics on the membrane surface, namely, lower pH, higher temperature, higher transmembrane pressure (TMP) and lower shear rate. With the aid of micro-analysis of the fouling layer, the residual organic foulants was responsible for the evolution of the irreversible fouling. Analyzed by FTIR spectra and gas chromatograph-mass spectrometer (GC-MS), the aromatics were detected to be the predominant organic foulants found on the membrane surface. The metal cations, such as Fe and Mg, contributed the irreversible fouling to some extent. The adsorption/cake formation model is successful for prediction of flux during petrochemical secondary effluent reclamation.The regeneration of fouled membrane used in the steel wastewater reclamation was studied. The fouled membranes were characterized by means of FTIR, SEM-EDX, XPS analyses and theoretical calculation. Organic fouling was found to be the dominant fouling mechanism. The amino sugar, the aromatics and the aliphatic acid were detected to be the predominant organic foulants. The hydroxides and oxides of metals and clay were detected as the dominant inorganic composition of fouling layer. The hydroxides and oxides of metals included Al2O3, SiO2, Fe2O3, ?-FeOOH, Fe(OH)3. Firstly, the oxidant in alkaline solution was developed to regenerate reverse osmosis membranes. Micro-analyzed with the means of FTIR, SEM, and AFM, as well as mechanical properties, the membrane integrity was not affected by the oxidant in the optimized cleaning conditions (pH=12.0). The performances of cleaned membranes were slightly improved, including permeate flux and rejection. L-H kinetic expression based on fouling resistance decline was developed to delineate cleaning behaviors. Analysis indicated the concept of a cleaning energy barrier and a cleaning rate constant with an Arrhenius'temperature dependence. The NaCIO cleaning method was able to overcome a higher activation energy barrier. For achieving higher cleaning efficiency, reagent combination and cleaning method were improved. In the addition of metal complexing agent, the cleaning efficiency of the oxidant in alkaline solution was improved. The cleaning efficiency was also influenced by the cleaning order. Cleaning with metal complexing agent followed by alkaline oxidant solution can achieve higher efficiency than the combination of both agents. When the cleaning agents were added in the opposite order (alkaline oxidant?metal complexing agent), the cleaning efficiency was observed lower than that observed when using the combination of both agents. |
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