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Application Of Al - Modified SBA - 15 Mesoporous Material In Arsenic Adsorption And Removal Mechanism

Posted on:2016-04-03Degree:DoctorType:Dissertation
Country:ChinaCandidate:S F HeFull Text:PDF
GTID:1101330470969483Subject:Metallurgical physical chemistry
Abstract/Summary:PDF Full Text Request
Arsenic contamination in natural water is a worldwide problem, and has become a major problem of catastrophic proportions. The considerable attentions have been focused on adsorption owing to cost effectiveness, safety, ease of operation, high efficiency, and absence of by-products. Therefore, the removal of arsenic species from contaminated waters using adsorption process has been extensive. Because of its appealing textural properties, including large surface area, adjustable pore diameter, thicker pore walls and appreciable hydrothermal stability, SBA-15 is suitable to be used as adsorbent/adsorbent substrate in remediation of water. However, pure siliceous SBA-15 materials have only silanol groups on their surface, which is electrically neutral, chemical inertness and of low acidity, and thus SBA-15 is always adsorption non-active. Extensive efforts have been devoted to functionalize surface of SBA-15 to further extend its applications in adsorptionConsidering that aluminum oxides are among the most excellent candidates for the removal of arsenic from aqueous solution, aluminum was used to decorate the SBA-15 material in this dissertation. The behaviors and characteristics of alumina functionalized highly-ordered mesoporous SBA-15 (Al-SBA-15) for arsenic(Ⅴ) removal were investigated in detail. Also, the effects of calcinations and the loading of aluminum on the properties and adsorption performances of Al-SBA-15 were investigated carefully. Based on these, the adsorption mechanisms of arsenic(Ⅴ) on Al-SBA-15 were proposed. Finally, the corresponding results were summarized and listed as follows:1. It is not the SiO2 that acts as the active adsorption sites for arsenic(Ⅴ) removal, but the highly-dispersed aluminum species. SBA-15 works as a support to provide more active adsorption sites by helping achive the highly-dispersed aluminum species, and its uniformly accessible pores facilitate the transportation of arsenic(Ⅴ) species. The incorporation of aluminum created a lot of different aluminum species, including AlO6, AlO5, AlO4 and AlO3, over the Al-SBA-15. These aluminum species result in the formation of terminal Al-OH groups and different acid sites, which are responsible for the active adsorption sites for arsenic(Ⅴ) removal.2. The effects of calcination temperature on the properties of Al-SBA-15, and their relationship with adsorption performance were investigated. It was found that the temperature of 400℃ is the optimal temperature for calcinations of Al-SBA-15. The calcination plays a vital role in the preparation Al-SBA-15 adsorbent, which degraded the precursor to form the oxides of aluminum over Al-SBA-15. Calcination at high temperature can cause the following two effects: (ⅰ) leads to the decrease of surface area and pore volume of A1SBA-15, and also the deterioration of the pore ordering; (ⅱ) Enhanced the dehydroxylation and dehydration of Al-OH groups to transform the oxides of aluminum with the lower coordination of aluminum, leading to the decrease in the amount of acid sites. In addition to the decrease of surface area and pore volume of Al-SBA-15, the deterioration of the pore ordering, and the decrease in the amount of adsorption active sites, including terminal Al-OH groups and acid sites, are attributed to the calcinations at high temperature, finally resulting in the reduction of As(V) adsorption capacity of Al-SBA-15. For Al-SBA-15 calcined at 700℃, the adsorption of As(Ⅴ) was controlled by intraparticle diffusion becaused of serious deterioration of the pore ordering by calcinations at 700 ℃3. The effects of aluminum loading on the properties of Al-SBA-15, and their relationship with adsorption performance were also investigated. The aluminum of 10% is recognized as the optimal loading for Al-SBA-15 adsorbents. Since the aluminum species are responsible for the active adsorption centers for the removal of As(Ⅴ), the content of aluminum is essential for As(Ⅴ) adsorption on Al-SBA-15. Actually, the available adsorption centers for As(Ⅴ) removal increase with Al loading, which is contributed to the increase in As(Ⅴ) removal. However, with the further increasing of aluminum loading beyond the optimal value, a slight decrease in the removal percentage of As(Ⅴ) is detected, which is due to the facts that, the overlap of terminal Al-OH groups, caused by excess aluminum, may dehydrate with-OH groups close to it, leading to the transformation of aluminum from high coordination to low coordination, and the decrease in the amount of terminal Al-OH groups and acid sites. Therefore, the removal percentage of As(Ⅴ) decreased with Al loading. Additionally, the decrease in surface area and pore volume of Al-SBA-15, together with the deterioration of the pore ordering, which were also caused by increasing Al loading, is also contributed to the reduction of As(Ⅴ) removal.4. Coupling the outstanding properties of SBA-15 with excellent arsenic adsorption performance of alumina, Al10-SBA-15400, which prepared at the optimal condition (calciantion temperature:400℃, loading of aluminum:10%), shows mesoporous structure, large specific surface area, and high alumina dispersion, and so it is an effective adsorbent for removing As(Ⅴ) from water in the wide pH range of 2.0 to 8.2. The maximum adsorption capacity for arsenic(Ⅴ) is as high as 197.7mg/gAl. More importantly, Al10-SBA-15400 can remove As(Ⅴ) from water containing arsenate of ≤2.235mg/L to reach levels in accordance with the regulations for drinking water purposes (<10μg/L).5. The adsorption process for As(Ⅴ) removal on Al10-SBA-15400 is a feasible, spontaneous and endothermic process, with the boundary layer (film) diffusion as well as intraparticle diffusion both contributing to the rate-controlling steps. And the adsorption kinetics and isotherm data were better fitted by a pseudo-second-order model and a Langmuir model, respectively. The effects of coexisting anions on the removal of As(Ⅴ) suggested that the removal percentage of As(Ⅴ) was slightly decreased by the presence of SO42- and NO3-, while the presence of PO43-leaded to a sharp decline in removal effectivities.6. Combined arsenate species definition with pH change results, the arsenate adsorption mechanisms were proposed as follows.1) For AlO6 and AlO5:H3ASO4 is adsorbed by unprotonated surface hydroxyl groups via hydrogen bond, while H2ASO4- and HASO42- anions are removed by the acidic centers and the protonated hydroxyl groups via electrostatic interaction, and also by exchanging with OH- adsorbed over the alkalized acidic.2) For AlO4: H2ASO4- and HASO42- anions are absorbed by Bronsted acidic sites via electrostatic interaction and ion exchange with OH-.3) For pure SBA-15:H3ASO4 is adsorbed by electrically neutral silanol groups via hydrogen bond at initial pH of 2.0, while the adsorption of arsenate is restrained under near-neutral and alkaline conditions, due to electrostatic repulsion by negatively charged surface of SB A-15.
Keywords/Search Tags:removal of As(Ⅴ), alumina functionalized SBA-15(Al_x-SBA-15), mesoporous material, adsorption, calcination temperature, adsorption mechanisms
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