Adsorptive interaction of bisphenol A with mesoporous titanosilicate/reduced graphene oxide nanocomposite materials: FT-IR and Raman analyses
© Nguyen-Huy et al.; licensee Springer. 2014
Received: 7 August 2014
Accepted: 29 August 2014
Published: 3 September 2014
Nanocomposite materials containing graphene oxide have attracted tremendous interest as catalysts and adsorbents for water purification. In this study, mesoporous titanosilicate/reduced graphene oxide composite materials with different Ti contents were employed as adsorbents for removing bisphenol A (BPA) from water systems. The adsorptive interaction between BPA and adsorption sites on the composite materials was investigated by Fourier transform infrared (FT-IR) and Raman spectroscopy. Adsorption capacities of BPA at equilibrium, q e (mg/g), decreased with increasing Ti contents, proportional to the surface area of the composite materials. FT-IR observations for fresh and spent adsorbents indicated that BPA adsorbed onto the composite materials by the electrostatic interaction between OH functional groups contained in BPA and on the adsorbents. The electrostatic adsorption sites on the adsorbents were categorized into three hydroxyl groups: Si-OH, Ti-OH, and graphene-OH. In Raman spectra, the intensity ratios of D to G band were decreased after the adsorption of BPA, implying adsorptive interaction of benzene rings of BPA with the sp2 hybrid structure of the reduced graphene oxide.
Endocrine-disrupting chemicals (EDCs) are substances that mimic natural hormones in the endocrine system causing adverse effects on humans and wildlife [1, 2]. EDCs are considered to be exogenous agents that interfere with the synthesis, secretion, transport, binding, action, and elimination of natural hormones in the body responsible for the maintenance of homeostasis, reproduction, development, and behavior. Exposure to EDCs could have a substantial effect on the body, disrupting bodily functions and processes. Bisphenol A (BPA), an EDC, is a cause of considerable social and scientific concern. It is heavily used as a monomer in the synthesis of epoxy resins and polycarbonate plastics  and is considered to be a significant pollutant since its weak estrogen-like effect is harmful to organisms [3–5]. Various diseases (including carcinogenesis) may result from exposure to BPA. In recent years, BPA has been detected in industrial wastewater, groundwater, surface water, and drinking water [6–9].
Various technologies have been attempted to remove BPA from water systems, such as adsorption [10–12], biological treatment , and photodegradation technology . Among these methods, adsorption is a superior and promising method for removing low-concentration contaminants from water systems in terms of cost, ease of operation, and lack of harmful secondary products. The removal of pollutants by the adsorption process depends on the inherent physicochemical properties of the pollutants and adsorbents.
Graphene, a single sp2-hybridized carbon layer arranged in a honeycomb structure, is currently one of the most exciting new materials due to its large surface area, excellent electronic and mechanical properties, and good thermal conductivity [15–17]. The remarkable properties of graphene allow it to be applied in many research fields, including adsorbents. Mesoporous silicate has been used for environmental applications because of its large surface area and interconnected channels [18, 19]. Ti has been incorporated into the framework of mesoporous silicate to significantly improve its hydrothermal stability, adsorbability, and photocatalytic performance [18–21]. Recently, the combination of graphene or reduced graphene oxide nanosheets and TiO2 particles has been shown to be a promising candidate for both adsorption and photocatalysis in water systems owing to the improved accessibility to pollutants and efficient facilitation of charge carrier separation [22–25].
In this work, mesoporous titanosilicate/reduced graphene oxide composite materials were synthesized as a function of Ti content, and their adsorptive characteristic for BPA in an aqueous solution was investigated through FT-IR and Raman spectroscopy for the first time. FT-IR analysis showed that the hydroxyl groups on the composite materials interacted with BPA to produce hydrogen bonding and the hydroxyl groups in the mesoporous titanosilicate could be divided into two different types - Si-OH and Ti-OH. In addition, Raman spectra gave evidence that the sp2 hybrid structure of graphene oxide interacted with the benzene rings of BPA. This work may provide new insights into adsorptive interactions between the adsorption sites of inorganic/organic composite materials and organic contaminants.
Preparation of composite materials
All chemicals were obtained from Sigma-Aldrich Korea (Yongin, Kyunggi, South Korea) and were used as received without any purification. In a typical procedure, graphene oxide (GO) was synthesized from expanded graphite (grade 1721, Asbury Carbons, Asbury, NJ, USA) by a modified Hummers method . For the preparation of mesoporous titanosilicate/reduced graphene oxide composite materials, an aqueous solution containing cetyltrimethyl ammonium bromide (CTAB) and NaOH was mixed until it became homogeneous; then, the GO dispersion was added (approximately 10 wt%). The mixture was sonicated for 2 h at room temperature and then magnetically stirred at 313 K for 2 h. A mixture of tetraethyl orthosilicate and titanium isopropoxide (various Ti:Si molar ratios were used for different compositions) was added drop-wise. The whole solution was stirred at 313 K for 12 h and then hydrothermally treated at 353 K for 10 h. Solid products were recovered by washing with warm ethanol and drying at 353 K overnight. The as-synthesized powders were finally heat-treated at 823 K for 4 h under flowing Ar (15 ml/min). The calcined materials were denoted as ‘MTSG-i’ for mesoporous titanosilicate/graphene composite materials, where ‘i’ is the Ti content (1, 5, 10, and 20 wt%).
Fourier transform infrared (FT-IR) spectra were obtained on a Nicolet 380 FT-IR spectrometer (Thermo Electron Co., Waltham, MA, USA) using the KBr pellet technique. Raman spectra were collected using a DRX Raman microscope (Thermo Fisher Scientific, Waltham, MA, USA) with a 633-nm laser excitation. N2 sorption measurements were conducted on a Micromeritics ASAP 2020 instrument (Micromeritics, Norcross, GA, USA). The samples were degassed under a vacuum at 240°C for 5 h prior to automatic analyzer analysis at −196.15°C. The Brunauer-Emmett-Teller (BET) calculation method was applied to determine the specific surface area.
where C 0 and C e are the initial and equilibrium liquid-phase concentrations of BPA (mg/l), V is the volume of the solution (l), and W is the weight of the dry composite materials used (g).
After adsorption tests, spent adsorbents were then filtered using a 0.22-μm GV filter, dried inside a hood, and wrapped in aluminum foil. Dried spent adsorbents were used for Raman and FT-IR analyses for observing the adsorption interaction between BPA and the adsorbents.
Results and discussion
Adsorption amount at equilibrium ( q e ) and coefficient of the pseudo-second-order model for BPA adsorption
Experimental qe,exp(mg/gads) a
Adsorption efficiency (%) b
k(gads/mg · min) c
q e (mg/gads) c
R 2 value c
Surface area (m2/g) d
Adsorptive interaction of BPA with the composite materials
We prepared mesoporous titanosilicate/reduced graphene oxide composite materials with different Ti contents and used them as adsorbents for removing BPA from a water system. Adsorption amounts acquired from adsorption kinetics are inversely proportional to the Ti content since the surface areas of the composite materials decreased with increasing Ti content. In this study, the adsorptive interaction of BPA with the adsorbents was observed by FT-IR and Raman analyses. From the observation, we reached the conclusion that BPA is adsorbed onto the composite materials through two different interactions: (i) electrostatic interactions between the hydroxyl groups of BPA and mesoporous titanosilicate or reduced graphene oxide and (ii) π-π interactions between the benzene rings of BPA and the sp2 domains of reduced graphene oxide.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2010-0008810) and by Business for Cooperative R&D between Industry, Academy, and Research Institute funded by the Korea Small and Medium Business Administration in 2013 (Grant No. C0113499).
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