Adsorption characteristics of phenolic and amino organic compounds on nano-structured silicas functionalized with phenyl groups
Graphical abstract
Introduction
There have been growing concerns over the potential health threats by endocrine disrupting chemicals (EDCs) which may interfere with human hormones. The exposure to EDCs, which can occur through the contact with soil, water, or air contaminated with chemicals, is one of the most important topics in recent environmental studies [1], [2], [3]. Bisphenol A (BPA), one of EDCs, is an organic compound used to produce polycarbonate plastics and epoxy resins. These chemical products containing BPA are widely used as containers of food and drink, dental sealants, and baby bottles [4], [5], [6], [7], [8], [9]. Low concentrations of BPA have been identified in wastewater, river water, effluents after treatment of landfill leachate, and even in finished-water samples because BPA is not completely removed during the conventional water and wastewater treatment processes [10], [11], [12], [13]. This can lead to the presence of BPA in aquatic environment and drinking water.
It is necessary to reduce the amount of BPA in water, wastewater, and drinking water treatment plants to the levels less than the concentrations required to prevent adverse health effects. Among the several alternatives for the removal of EDCs, adsorption technology has been recognized as an effective and economic method [2], [14], [15]. Many researches on the BPA removal from aqueous solutions have been carried out using adsorbents such as mineral, carbon materials, and zeolites [16], [17], [18], [19]. However, most of adsorption processes does not seem to remove BPA selectively, and thus high concentrations of another organic pollutant can be primarily captured, saturating adsorption sites.
In recent years, mesoporous silicas, which have the possibility of modifying surface functional groups and the unique structural and surface chemistry, have been intensively studied for the adsorption of pollutants in water [20], [21], [22], [23], [24], [25], [26]. In particular, high adsorption selectivity for some EDCs was obtained via simple grafting hydrophobic functional groups onto the surface of mesoporous silica. The molecular form of organic pollutants as well as the functional groups on the surface of the mesoporous silicas was the key factor in the selective adsorption of EDCs on the mesoporous adsorbents [27], [28], [29], [30]. In the previous work [31], we reported that organic–inorganic hybrid mesoporous silicas incorporated with phenyl groups showed large adsorption capacity and higher adsorption selectivity for BPA against phenol than activated carbon (AC) from aqueous solutions. The high adsorption selectivity may be ascribed to the hydrophobic and hydrophilic surface groups of the hybrid adsorbents interacting with two benzene rings and two hydroxyl groups of BPA, respectively [31].
In this work, we prepared phenyl-functionalized mesoporous silicas (Ph-MSs) with different compositions and thus different surface hydrophobicities via co-condensation method. Adsorption characteristics of BPA from aqueous solutions were investigated using the prepared materials. In addition, the selective adsorption behavior of Ph-MS was discussed using the equilibrium adsorption isotherms of the organic compounds including phenol, aniline, p-t-butylphenol, and p-t-butylaniline, which had the similar molecular structures with BPA.
Section snippets
Reagents
Silica sources, tetraethoxysilane (TEOS, 99%) and phenyltriethoxysilane (PhTES, 99%), were obtained from Aldrich. Hexadecyltrimethylammonium bromide (CTAB, 99%, Sigma), sodium hydroxide (NaOH, 98%, Junsei), and ethanol (99.8%, Ducksan) were used to synthesize adsorbents. Bisphenol A (BPA, 99%, Aldrich), phenol (99%, Aldrich), p-t-butylphenol (99%, Aldrich), p-t-butylaniline (99%, Aldrich) and aniline (99.5%, Aldrich) were used as adsorbates. All reagents were used as received without further
Characterizations
XRD patterns of MCM-41 and template-extracted Ph-MSs, and unit cell parameter (ahex) and d100 spacing values of the prepared materials were shown in Fig. 1 and Table 1. XRD data of the prepared materials except Ph-MS30 exhibited a major intense (1 0 0) reflection and a few additional weak peaks, indicating good structural order of the synthesized mesoporous structures. Structural order of Ph-MSs decreased with the increase of molar ratio of initial phenyl moieties. Decrease of the peak
Conclusion
Phenyl-functionalized mesoporous silicas (Ph-MSs) with different molar densities of functional groups were successfully synthesized by co-condensation method, and used as adsorbents for BPA removal from aqueous solutions. Ph-MSs with relatively large initial phenyl moieties had higher adsorption affinities for BPA of low concentrations in aqueous solutions, while they had relatively low mesostructural order, small surface area, and small adsorption capacity for BPA. Among the prepared
Acknowledgments
This research was supported by a Grant (R-01-2007-000-20690-0) and Basic Science Research Program (2013R1A1A2009723) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.
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