Full Length ArticleInsight into highly efficient removal of cadmium and methylene blue by eco-friendly magnesium silicate-hydrothermal carbon composite
Graphical abstract
Introduction
Heavy metals and azo dyes, common pollutants in freshwater and wastewater, have detrimental impacts on living organisms and ecosystem. Due to the improper storage and disposal of wastewater, large amounts of heavy metals and dyes are discharged from the textile, rubber, plastic and electroplate industries [1], [2]. The cadmium (Cd(II)) with non-biodegradable and toxic properties is classified as category I carcinogen (human carcinogen) by the International Agency for Research on Cancer (IARC) [3]. Methylene blue (MB) poses a great potential threat to living organisms because of its carcinogenic and mutagenic actions [4]. Under this circumstance, highly efficient techniques are required to remove the excess Cd(II) and MB from wastewater.
To date, various techniques have been employed to eliminate Cd(II) ion or MB pollution, including chemical precipitation, electrochemical treatment, solvent extraction, complexation, biological operations, adsorption, membrane separation and photocatalysis [5], [6], [7], [8], [9], [10]. Among these methods, adsorption has many advantages including high efficiency, low cost and insensitivity to toxic substances together with easy operation. Various materials have been widely adopted for water decontamination, such as activated carbon, bio-char [11], [12], multi-walled carbon nanotubes [13], graphene oxide [7] and clays [14], [15].
Natural magnesium silicate minerals, including serpentine, attapulgite and sepiolite, are not only major components of soil governing pollutants transport and immobilization, but also become available and extensively used adsorbents [16]. The previous report showed that pure magnesium silicates (MS) had excellent adsorption capacities of 276, 362, 424, and 929 mg/g for MB, Rhodamine B, Pb(II) and UO2(II), respectively [17]. Owing to unique structure, high surface area and ion exchange capacity, MS has been served as one of the potential absorbents for water purification [18]. Unfortunately, the adsorption capability of MS is still unable to satisfy the requirement for large-scale application.
In order to improve the performance of MS in wastewater treatment, a lot of investigations mainly focus on fabricating MS with various morphology such as hollow sphere [19], core-shell [20], ordered mesoporous [21] and nanotube [22]. However, MS modified by carbon related materials to increase the number of surface oxygen-containing functional groups is seldom studied. Hydrothermal carbon (HC), possessing numerous oxygen-containing groups on its surface, can be synthesized from the hydrothermal carbonization (HTC) of biomass. Meanwhile, these surface groups are favorable to improve the hydrophilicity, dispersion and the adsorption capacity of materials. It has been reported that serving as an ideal carrier, clay can be functionalized by carbon layer through the dehydration and carbonization of glucose [23]. For instance, Chen et al. has synthesized attapulgite clay@carbon with adsorption capacity of 177 mg/g for Cr(VI), while Cr(VI) is barely adsorbed by attapulgite clay [24].
The addition of acrylic acid during the HTC process leads to higher degree of carboxylic groups and favorable adsorption performance of HC [25]. Moreover, carbon materials activated by alkali could create the active pore structure, increase the specific surface area as well as adsorption affinity [26]. Herein, a novel hydrothermal carbon modified magnesium silicate composite was facilely prepared via one-step HTC and used to remove heavy metal and dye. The structural characteristics of as-obtained materials were evaluated via various characterized techniques. To further quantitatively investigate the adsorption performance of MS-C, the effects of time, initial concentration, temperature, ion strength, and pH on the adsorption properties were carried out in details. Moreover, the adsorption kinetics and isothermals were subsequently investigated. Finally, the performance of MS-C on the removal of Cd(II) and MB from various real water samples water were explored.
Section snippets
Materials
Magnesium sulfate (MgSO4·7H2O, A.R. grade), sodium silicate (Na2SiO3·9H2O, A.R. grade), Glucose (A.R. grade), Acrylic acid (A.R. grade) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All other reagents and solvents were of analytical reagent grade and used as received from commercial suppliers.
Synthesis of magnesium silicate
The pure magnesium silicate was fabricated by a hydrothermal method. In brief, 15 mL 0.02 M MgSO4 solution was added dropwise to 20 mL 0.015 M Na2SiO3 aqueous solution under
Characterization of MS-C composite
The morphology of pure C, pure MS, and MS-C, measured by SEM and TEM, are shown in Fig. 1(a–d). As displayed in Fig. 1b, MS sample presents puff-porous and aggregated morphology, consisting of dozens of irregular block-based particles. From Fig. 1c and d, it has revealed that a large number of smooth carbon sphere with the diameter of 0.5–1 μm grown onto the surface of MS, providing clear support for the formation of MS-C. The energy dispersive spectrometer (EDS) mapping of MS-C (Fig. 1d)
Conclusions
A carbon modified MS composite was prepared, characterized, and applied to remove Cd(II) and MB. According to FT-IR spectra and XPS analyses, there could be the COSi covalent bonding between HC and MS in the composite. Batch adsorption studies showed that the surface negative charged adsorbent had high affinity for both Cd(II) and MB. The Cd(II) and MB adsorption isotherm both fitted with Langmuir model. The maximum adsorption capacity was 108 mg/g for Cd(II) and 418 mg/g for MB. The main
Acknowledgments
The authors gratefully acknowledge the financial support provided by the Foundation for the Key research and development project of Hunan Province, China (No. 2016SK2015), Innovative Research Groups of the National Natural Science Foundation of China (No. 51521006), National Natural Science Foundation of China (No. 71431006).
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