| The development of industry and modern agriculture has brought about the discharge of heavy metals like mercury, carbofuran, pirimicarb, carbaryl, acetochlor and butachlor etc. and pesticides into environmental water, causing serious water pollution. These inorganic and organic pollutants have been affecting human health and damaging the environment, which has become a worldwide concern. Therefore, it is necessary to take some measures to deal with water containing these pollutants.Among numerous water treatment methods, adsorption method has been adopted by many researchers owing to its high efficiency, easy operation and environmentally friendly mode; and it is still the hotspot to develop new adsorbent and utilize it for water treatment. Carbon nanotubes (CNTs) and mesoporous silica are two absorbent materials of great application foreground, and pertinent literature has reported the researches using them to remove contaminants in the water. CNTs have the following advantages:large specific surface area, good mechanical properties and good thermal stability; however, they also have weaknesses, especially the poor adsorption efficiency of water pollutants resulted from their bad dispersibility in water. Mesoporous silica is of commendable dispersibility performance in aqueous solution because of the silicon hydroxyl in its surface. Cladding carbon nanotubes with mesoporous silica in the surface can largely improve the hydrophilia property of carbon nanotubes; the preparative material combined with the porosity of mesoporous silica should be more suitable for the adsorption and removal of pollutants in water than original carbon nanotubes, and compared with mesoporous silica of general micron size, it has a faster adsorption mass transfer rate. Because the complexing ability of silicon hydroxyl with heavy metal ion is not strong enough, it is expected to obtain better adsorption effect by further functional modification of the coated mesoporous silica.Within the existing methods of water treatment, advanced oxidation processes (AOPs) are a kind of method with high efficiency and application prospect. Its specialty is to produce certain oxidation species in situ, such as hydroxyl radical (HO·), superoxide anion (O2·-)* holes (h+) and ozonide anion free radical (O3·-), etc., among which HO·is the most powerful oxidant in the water body to break C-C, C-H, C-N, C-O, O-H and N-H bonds of organics, and therefore, it can oxidize most of the organic pollutants and part of inorganic pollutants to eventually degrade them into CO2, H2O and inorganic ions (i.e. mineralization). There are plenty of methods of AOPs, such as ozonization and UV light photolysis (O3/UV), UV light catalysis (TiO2/UV), ozonization and UV light catalysis (O3/TiO2/UV), Fenton process (Fenton), photo Fenton (UV/Fenton), and wet air oxidation, etc. Among all these AOPs methods, it is particularly efficient to adopt the three processes of TiO2/UV, O3/UV and O3/TiO2/UV, which are therefore widely used to remove various kinds of organic pollutants in water environment (such as dyes, drugs, phenolic wastewater, pesticides, etc.), especially the process of O3/TiO2/UV.This paper prepared the adsorption materials:MWCNTs coated with mesoporous silica (MWCNTs@mSiO2). On this basis, this paper studied the adsorption behavior of MWCNTs@mSiO2 on acetochlor. Furthermore, with TiO2 film fixed on the slide as catalyst, the TiO2/UV process was utilized to study photocatalytic degradation and mineralization of acetochlor and butachlor. In addition, this paper adopted three different AOPs systems of TiO2/UV, O3/UV and TiO2/UV/O3 to study the degradation and mineralization of carbaryl, carbofuran and pirimicarb (TiO2 film was prepared with sol-gel and fixed on the slide). This paper also prepared the adsorption materials:3-mercaptopropyltriethoxysilane (MPTS) modified mesoporous silica based on multiwalled carbon nanotubes (MWCNTs@mSiO2-MPTS). On this basis, this paper attempted to study the adsorption behavior of MWCNTs@mSiO2-MPTS on heavy metal Hg2+. The details are as follows:(1) With MWCNTs as substrate and cetyl trimethyl ammonium bromide (CTAB) as template agent, MWCNTs@mSiO2 was successfully prepared through the hydrolysis of tetraethoxysilane (TEOS) under the condition of ammonia alkali, and synthetic materials were characterized and confirmed through Fourier-transform infrared (FT-IR) spectra, transmission electron microscope (TEM) and N2 adsorption-desorption tests. Compared with original MWCNTs, water dispersibility of MWCNTs@mSiO2 was greatly improved. With MWCNTs@mSiO2 as adsorbent for adsorption research of acetochlor in water, the study mainly investigated the effects of the solution pH, adsorption time and initial concentration of acetochlor on the adsorption capacity. Employing pseudo-first-order kinetic model and pseudo-second-order kinetic model to simulate the data of adsorption capacity with time-varying and adopting Langmuir and Freundlich adsorption isotherm models to simulate the data of adsorption capacity with the changes of initial concentration of acetochlor, the results show that under the condition of solution pH=2.0 and 4.0, the adsorption capacity of acetochlor by MWCNTs@mSiO2 is relatively close to each other, but as the solution pH continuing to rise from 4.0, the corresponding adsorption capacity to some extent declines; therefore, pH=4.0 is taken as optimized experimental condition through overall consideration. Kinetic researches show that, the adsorption of acetochlor by MWCNTs@mSiO2 reaches its balance in about 150 min, and the adsorption process conforms to pseudo-second-order kinetic equation. The analysis of adsorption isotherm shows that, the adsorption process of acetochlor by MWCNTs@mSiO2 can be better described with Freundlich adsorption isotherm model. After five-time cycle use of MWCNTs@mSiO2, its adsorption capacity of acetochlor did not significantly decline, and all the results show that, for the removal of acetochlor in water, MWCNTs@mSiO2 is an effective adsorbent.(2) The degradation and mineralization of acetochlor and butachlor were studied with TiO2/UV system (TiO2 film was prepared with sol-gel and fixed on the slide), and the experiment operating parameters’(pH value, TiO2 dosage and additive anion) effects on photocatalytic degradation of acetochlor and butachlor were investigated. The results show that, photocatalytic degradation and mineralization of acetochlor and butachlor conforms to pseudo-first-order kinetic model; when pH=9, DR and k value of photocatalytic degradation of acetochlor and butachlor reach the maximum; moreover, the optimum dosage of TiO2 for photocatalytic degradation of acetochlor and butachlor is respectively 150 mg and 200 mg, and DR and k value have a slight increase but really slow if the dosage is more than the optimum dosage; the additive Cl-, CO32- and SO42- have significant inhibiting effects on the photocatalytic degradation of acetochlor and butachlor, and the inhibiting effects of three anions are close to each other for acetochlor while there are obvious differences among the three anions’ inhibiting effects for butachlor that the inhibiting effects of Cl" is the biggest, successively followed by CO32- and SO42-. Under the optimal experiment conditions, the DR value of acetochlor after 80 minutes of photocatalysis is 96.1% and k value is 0.04185 min-1; the DR value of butachlor after 90 minutes of photocatalysis is 95.5% and k value is 0.03741 min-1; the MR value of acetochlor after 180 minutes of photocatalysis is 86.4% and K value is 0.00938 min-1; the MR value of butachlor after 180 minutes of photocatalysis is 81.3% and K value is 0.00752 min-1. Obviously, it can be seen from the results that, acetochlor is easier in photocatalytic degradation and mineralization than butachlor. Compared with suspension system of TiO2, immobilized TiO2 photocatalysis technology facilitates the catalyst recycling and simplifies the post-processing steps, which can be effectively used in treatment of water sample polluted by acetochlor and butachlor.(3) This study adopted three different kinds of advanced oxidation processes (AOPs) of TiO2/UV, O3/UV, and TiO2/UV/O3 to study the degradation and mineralization of three carbamate pesticides of carbaryl, carbofuran and pirimicarb (TiO2 film was prepared with sol-gel and fixed on the slide), and investigated the operating parameters (pH value and flow rate of O3) of the experiment. The experimental results show that, the degradation and mineralization of three carbamate pesticides conforms to pseudo-first-order kinetic model. Under the conditions of pH=7 and flow rate of O3 as 0.54 g/h, the degradation rate (DR) and reaction rate constant (k) all reach their maximum when TiO2/UV/O3 process is employed to degrade three carbamate pesticides; therefore, TiO2/UV/O3 process is of the highest efficiency. The pirimicarb in the three carbamate pesticides is easier to degrade than carbofuran and carbaryl, while carbaryl is the hardest to degrade. Under the circumstances of pH=4.0 and 7.0, the k value of the three carbamate pesticides in the degradation of TiO2/UV/O3 process is bigger than the sum of those of TiO2/UV and O3/UV processes, that is to say, there exists synergistic effect; when pH=9, there exists the loss of synergism of the three carbamate pesticides in the degradation of TiO2/UV/O3 system. The flow rate of O3 plays a very important role to degrade the three carbamate pesticides using O3/UV and TiO2/UV/O3 processes. As the flow rate of O3 increases from 0.32 g/h to 0.54 g/h, DR and k of the three carbamate pesticides increase accordingly. When pH=7 and the flow rate of O3 is 0.54 g/h, the mineralization rate (MR) to degrade the three carbamate pesticides with TiO2/UV/O3 process reaches the maximum, and the mineralization rate constant K under this situation is higher than the sum of those of TiO2/UV and O3/UV processes, that is to say, there exists synergistic effect. In the degradation of TiO2/UV/O3 process, the DR and k of carbaryl, carbofuran and pirimicarb respectively are 96.2% and 0.05118 min-1 (t=60 min),97.7% and 0.07031 min-1 (t=50 min),98.5% and 0.13103 min-1 (t=30 min). And in the degradation of TiO2/UV/O3 process, the MR and K of carbaryl, carbofuran and pirimicarb respectively are 82.7% and 0.01087 min-1,87.8% and 0.01324 mim-1,92.2% and 0.01568 min-1. Hence, TiO2/UV/O3 can be utilized for processing wastewater polluted by carbaryl, carbofuran and pirimicarb as a very effective and promising method.(4) With MPTS as functional reagent, MWCNTs@mSiO2 was modified and the MWCNTs@mSiO2-MPTS was successfully prepared. FT-IR, TEM and N2 adsorption-desorption tests were adopted to characterize the synthetic materials. With MWCNTs@mSiO2-MPTS as adsorbent, this paper carried out detailed study of its performance to adsorb Hg2+ in the water, operating parameters (pH, adsorption time, initial concentration and adsorption temperature) and the regeneration behavior of adsorbent. The adsorption equilibrium data obtained in the study was compared with adsorption isotherm models between Langmuir and Freundlich, and at the same time, dynamics and thermodynamics in the process of adsorption were also studied. The results show that, with the solution pH value increased from 2.0 to 4.0, the adsorption capacity of Hg2+increases accordingly; when pH value is higher than 4.0, the adsorption capacity of Hg2+ almost remains the same, so pH=6.0 is finally taken as optimized experimental conditions. Under the conditions of initial concentration of Hg2+ as 100.0 mg/L, the solution volume of 40 ml, and temperature of 30℃, the adsorption rate of Hg2+ by MWCNTs@mSiO2-MPTS was fast in the first 30 minutes, and then became slow gradually and reached a balance in about 50 minutes; therefore, the adsorption time in follow-up experiments was chosen as 1 hour. In addition, Hg2+ equilibrium adsorption quantity Qe increases with the increase of initial concentration of Hg2+(concentration range of 20-250 mg/L) and the rise of adsorption temperature (temperature range of 20-40℃), and the adsorption data is more in line with the Langmuir model according to which the maximum adsorption capacity is 349.65 mg/g under 40℃, significantly higher than that of MWCNTs and MWCNTs@mSiO2. Kinetic analysis shows that, the adsorption process adheres to pseudo-second-order kinetic equation; thermodynamic parameters calculation shows that, the adsorption process is endothermic and spontaneous. After regeneration cycle of MWCNTs@mSiO2-MPTS for five times, its adsorption capacity of Hg2+ has no distinct change. All the results show that, MWCNTs@mSiO2-MPTS is an effective adsorbent to remove Hg2+ in the water.MWCNTs@SiO2 prepared in this paper shows good adsorption performance to acetochlor, and further obtained MWCNTs@mSiO2-MPTS an efficient adsorbent for Hg2+ in aqueous solution. In the process of preparing these two materials, CTAB is taken as pore-forming agent. Different pore-forming agents obviously lead to different material apertures, and thereby affect the specific surface area of the material as well as the modification of the functional groups, which ultimately affects the adsorption capacity on the target object. Therefore, on the basis of existing research in the paper, macromolecular template agent (such as triblock copolymer poly (ethylene glycol)-poly (propylene glycol)-poly (ethylene glycol)) can be utilized to prepare mesoporous silica of large aperture or double template agent (such as diblock copolymer poly(ethylene oxide)-poly(methyl methacrylate) and long-chain alkyl trimethyl ammonium bromide) can be utilized to prepare mesoporous silica materials with dual aperture structure, and the prepared materials can be processed with various functional modification and be tested the effects of pesticides and metal ions’removal in water. In water treatment technologies, immobilized TiO2/UV process can effectively dispose water samples containing acetochlor and butachlor to further improve the efficiency of photocatalysis; and supported TiO2 can be tested as catalyst while there are plenty of carriers for supported catalyst, like activated carbon fibers, clay, carbon nanotubes and mesoporous silica, etc., and TiO2 even can be tried to load on MWCNTs@mSiO2 to study its capability on the degradation and mineralization of pesticides under TiO2/UV process with prepared catalyst.It is a good choice to degrade carbaryl, carbofuran and pirimicarb in aqueous solution with TiO2/UV/O3 process. In order to accelerate the degradation and complete mineralization of pesticides, it can be tried to combine TiO2/UV/O3 with biochemical treatment. |
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