Removal capacity and adsorption mechanism of heat-treated palygorskite clay for methylene blue
Highlights
► Heat treatment method can improve adsorption capacity of palygorskite for dye. ► Change of CEC basically accorded with that of adsorption capacity. ► Cation exchange is a main mechanism in the adsorption process. ► A decisive influence of the leaching of divalent ions on adsorption.
Introduction
The increasing use of dye organic compounds endangering the environment encourages search for more efficient adsorbents. Clay minerals have great potential as inexpensive and environmental friendly sorbents due to their large quantities, chemical and mechanical stability, high surface area and structural properties. There is an upsurge of interest in recent years to utilize clay minerals to remedy dye wastewater [1], [2]. The mechanism of interaction between clay mineral and dye molecule in liquid phase is more complex than that of traditional materials considering its unique structure, and is the research hotspot of nano-mineralogy, geology and environmental science [3], [4].
Palygorskite (also known as attapulgite) is one of silicate mineral with specific layer chain structure and widely exists in palygorskite clay ore deposit. Unlike most of the other clay minerals, palygorskite has a unique fibrous or rod-like microstructure due to inversion of oxygen atoms on the edge of silicon-oxygen tetrahedral layers, which results in discontinuous arrangement of aluminum-oxide octahedral layers. Thus, a lot of nano-tunnels parallel to the rod axis (3.7 Å × 6.4 Å) form [5]. The structural characteristics of palygorskite give its large surface area, which is of benefit for applying it as the adsorbent material for the removal of pollutants such as heavy metal ions [6], [7], [8], [9], [10], organic compounds [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] and inorganic anions [22], [23] in wastewaters.
Palygorskite is a fibrous magnesium aluminum silicate clay mineral having the ideal formula [Si8(Mg,Al,Fe)5O20(OH)2(OH2)4]·4H2O [24], [25], where H2O, (OH2) and (OH) represent zeolitic water, coordinated water and structural water, respectively (Fig. 1). The octahedral sites are principally occupied by Mg(II) cations, with some replacement principally by Al(III) or Fe(III) cations. Heat treatment is a common method to improve the adsorption capacity of palygorskite [10], [22], [23]. Different kinds of water located in intracrystalline tunnels can be selectively removed by calcining at different temperature, which correspondingly changes the pore structure and surface properties [26], [27], [28]. So, heat treatment has an influence on the specific surface area of palygorskite, which is a very important indicator to judge adsorption capacity of adsorbent [10]. To date, there are many reports about the study on the dye removal using palygorskite clay, which were mainly focused on natural [11], [12], [13], acid and/or base activated [14], [15], and surface modified samples [16], [17], [18], [21], but little information concerning the effect of heat treatment on removal capacity of palygorskite clay for dye can be seen.
It is therefore the objective of this study to investigate the removal capacity of a typical cationic dye, methylene blue, from aqueous solutions on natural and heat-treated palygorskite samples. Based on the discussion on the variations of cation exchange capacity, pH before and after adsorption, and metal ions content in the supernatant, the adsorption mechanism under acidic, neutral and basic conditions was elucidated.
Section snippets
Materials and reagents
The palygorskite clay (supplied by Anhui Mingguang Rare Minerals Ltd. Co., China) was milled into a size of 320-mesh. A stock solution of dye, prepared by dissolving methylene blue in deionized water, was taken as the adsorptive solution. Other agents used, such as HCl, NaOH, were all of analytical grade and all solutions were prepared with deionized water.
Preparation of heat-treated palygorskite
Heat-treated palygorskite samples were obtained according to the report before [10]. Specifically, 10.0 g palygorskite micro-powder was
XRD analysis
X-ray diffraction analysis is an important method for identifying mineral species. Before determining adsorption capacity of heat-treated palygorskite, natural and heat-treated palygorskite samples were characterized using X-ray diffractometer and the results were displayed in Fig. 2. For natural sample, there are strong reflection at 10.5 Å (1 1 0) and moderate reflections at 6.49 Å (2 0 0) and 5.42 Å (1 3 0), which indicate existence of palygorskite [17], [22]. According to XRD data, there are also
Conclusions
- (1)
Heat treatment method can effectively improve the removal capacity of palygorskite clay for methylene blue. With increasing the temperature, the adsorption capacity of palygorskite clay firstly increases and reaches the maximum at 700 °C, and then decreases with further increasing the temperature.
- (2)
Cation exchange capacity of the adsorbent increases at the beginning and then decreases with increasing the calcined temperature, and has the maximum also at 700 °C. The change of CEC basically is in
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No. 20903070). The authors are grateful to Dr. Wenbo Wang for providing BET testing of the samples. The authors also appreciate the anonymous reviewers for their insightful comments and suggestions.
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