Activated Alumina for Air Dryer
Fine activated alumina (FAA) acting as an adsorbent for phosphate was synthesized from an industrial sodium aluminate solution based on phase evolution from Al(OH) 3 and NH 4 Al(OH) 2 CO 3 . This material was obtained in the form of γ-Al 2 O 3 with an open mesoporous structure and a specific surface area of 648.02 m 2 g −1 . The phosphate adsorption capacity of the FAA gradually increased with increases in phosphate concentration or contact time. The maximum adsorption capacity was 261.66 mg g −1 when phosphate was present as H 2 PO 4 − at a pH of 5.0. A removal efficiency of over 96% was achieved in a 50 mg L −1 phosphate solution. The adsorption of phosphate anions could be explained using non-linear Langmuir or Freundlich isotherm models and a pseudo-second-order kinetic model. Tetra-coordinate AlO 4 sites acting as Lewis acids resulted in some chemisorption, while (O) n Al(OH) 2 + (n = 4, 5, 6) Brønsted acid groups generated by the protonation of AlO 4 or AlO 6 sites in the FAA led to physisorption. Analyses of aluminum-oxygen coordination units using Fourier transform infrared and X-ray photoelectron spectroscopy demonstrated that physisorption was predominant. Minimal chemisorption was also verified by the significant desorption rate observed in dilute NaOH solutions and the high performance of the regenerated FAA. The high specific surface area, many open mesopores and numerous highly active tetra-coordinate AlO 4 sites on the FAA all synergistically contributed to its exceptional adsorption capacity.
Wastewater containing high concentrations of phosphate as a pollutant can be generated as a result of papermaking, the use of phosphorus-based fertilizers and the surface treatment of metals. This is problematic because excessive amounts of phosphate in aquatic systems lead to serious water pollution effects, such as eutrophication and algae bloom.1 Traditionally, phosphate has been removed from wastewater by chemical precipitation, crystallization, ion exchange, electrostatic techniques, hydrobiological processes and adsorption.2–4 Among these, adsorption methods have been widely adopted because of their operational simplicity, and excellent treatment efficient, and the adsorbents can be regenerated in some cases. The adsorbents used to date include natural minerals, industrial slag and synthetic materials. However, the use of natural minerals such as bentonite, attapulgite and kaolin or industrial slag (such as red mud) are limited by the low adsorption capacity of these substances (8–56 mg g−1), the large amount of sludge they generate and the potential for secondary pollution. In addition, synthetic inorganic adsorbents (including iron oxide, titanium oxide, cerium oxide, water talc, and calcined layered materials) are difficult to prepare and are also not readily regenerated. Activated alumina, serving as a replacement for α-Al2O3, is considered a promising inorganic adsorbent because it provides numerous active sites for highly efficient phosphate adsorption and is also inexpensive, stable and environmentally-friendly.5–8 As a consequence of these attributes, the use of activated alumina has been widely studied. Even so, conventional fine activated alumina (FAA) with a medium particle size of d(50) > 1 μm has been found to exhibit a low adsorption capacity (30.2 mg g−1) as a result of its minimal specific surface area (less than 300 m2 g−1).9 Therefore, both nano-Al(OH)3 and nano-AlOOH have been used as precursor to synthesize nano-alumina as a means of increasing the specific surface area, leading to an improved values from 300 to 600 m2 g−1.10 The phosphate adsorption capacity of this nanoscale material was determined to be 31.1–102 mg g−1 when applied to solutions containing phosphate concentrations ranging from 50 to 1400 mg L−1. However, the preparation of the nano-alumina from aluminum-bearing salts (Al2(SO4)3 Al(NO3)3, AlCl3) and aluminum alkoxides (Al(OR)3) as raw materials is difficult because the resulting nanoparticles tend to undergo significant aggregation and because a considerable amount of saline wastewater is produced in conjunction with the use of alkaline reagents. By contrast, gibbsite or boehmite precipitated from industrial sodium aluminate solution generated in alumina refineries is remarkably inexpensive and has a minimal negative environmental impact because this process allows the sodium aluminate solution to be recycled and does not generate wastewater.11 This method therefore represents a green, economical approach to synthesizing FAA having a high specific surface area (>300 m2 g−1) as an alternative to nano-alumina.
To date, various isotherm and kinetic models have been employed to assess phosphate removal mechanisms. Specifically, the Langmuir, Freundlich, Tempkin, and Dubinin–Radushkevich isotherm models have all been applied to assess the adsorption of phosphate anions on alumina surfaces.12–14 In addition, pseudo-first-order, pseudo-second-order and Elovich kinetic models have also been adopted.15 Simulations of the distribution of various phosphate and Al3+ species in solution at different pH values have indicated that AlPO4 might be formed on alumina surfaces.3,16–18 Furthermore, Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and zeta potential measurements have all been used to study the interactions between phosphate anions and alumina as well as to examine physisorption or chemisorption processes.19 Nevertheless, the interactions of phosphate anions with FAA have not determined.
The adsorption properties of activated alumina are primarily the result of active sites on the material. Compared with α-Al2O3, which is inert and almost completely composed of hexa-coordinated AlO6, various other aluminum–oxygen coordination units (AlOx where x = 4 or 5) may occur in the activated alumina, leading to catalytic activity.20 These observations may assist in determining the mechanism by which this material removes phosphate from wastewater and may also help to optimize the process. Therefore, in addition to distribution of Al–O units, the interactions between phosphate anions and AlO4, AlO5 and AlO6 units in activated alumina are expected to affect adsorption properties.
The present work evaluated the phosphate removal performance of FAA and explored the associated mechanism. FAA having a high specific surface area was prepared by phase evolution from gibbsite and characterized with regard to its particle size distribution (PSD) and using X-ray diffraction (XRD), N2 adsorption–desorption isotherms analyses and scanning electron microscopy (SEM). The phosphate adsorption isotherm and kinetics of this material were then studied based on batch experiments while the removal mechanism was investigated using zeta potential measurements, FTIR spectroscopy and XPS. The resulting data were used to determine the AlO4 and AlO6 distribution in the FAA before and after phosphate adsorption as a means of evaluating the adsorption mechanism. The data from this work offer an improved understanding of the adsorption properties of the activated alumina and could lead to optimization of phosphate removal from wastewater with this material.