Regular Paper Production of Granulated Boehmite by Compression and Its Adsorption of Phosphate in a Single-Solution System

In this study, powdered boehmite (AlO(OH)) was compressed to obtain a granulated form without using a binder. The granulated product was assessed via SEM imagery, TG-DTA, specific surface area, surface pH, and hydroxyl groups. The SEM images showed that powdered BE could be successfully granulated by compression without binders. TG-DTA showed that dehydration of adhesion and bound water occurred at 96.0 and 394.6◦C, respectively. The amount of phosphate adsorbed onto granulated boehmite increased with temperature, indicating a chemisorption mechanism. Moreover, equilibrium adsorption was reached within 20-24 h. Adsorption kinetics data was fitted to the pseudo-second-order model. [DOI: 10.1380/ejssnt.2012.518]


I. INTRODUCTION
The presence of trace amounts of phosphorus (exceeding about 1 mg/L) in treated wastewater can stimulate algal growth (eutrophication), since phosphorus is usually considered a rate-limiting factor for algal growth [1].Many physicochemical methods have been studied for removal of phosphate.Adsorption is an important method, and is usually applied in the removal of phosphate in an aqueous solution system [2].Other methods investigated to remove phosphate from an aqueous solution include surfactant-modified zeolite and Ca-zaolite, fast pyrolysis of biomass waste, and innovative modified bentonites [2][3][4].We previously reported that aluminum compounds (aluminum oxyhydroxide, aluminum hydroxide, and aluminum hydroxide gel) exhibited the ability to adsorb phosphate [5][6][7].Moreover, there was strong selectivity for phosphate adsorption onto aluminum compounds.
However, the particle diameter of previously reported adsorbents was very small, which limited practical application.If using a column with adsorbent (small particles), plugging occurred, which indicated that the particle size of the adsorbent is too small.Therefore, it is very important to reliably produce adsorbents with a larger particle diameter.We previously reported a method of granulating aluminum compound that utilized a binder [8][9][10].These reports showed both the method's merits and demerits for removal of phosphate.One advantage is that the method produces adsorbent particles with a larger diameter.However, there is concern that the binder from the granulated adsorbent may be released into the water body, causing pollution.Therefore, a method of granulation is required that does not require a binder, but which still produces particles of a useful size.There are no previous reports on the use of boehmite in a granulation method without binder.
This study produced adsorbent without the use of a binder, and investigated its properties and adsorption of phosphate.
Boehmite (AlO(OH)), hereafter abbreviated as BE, was purchased as adsorbent (Tomita Pharmaceutical Co., Ltd.).Granulated boehmite (hereafter G-BE) was prepared as follows: The noncrystalline aluminum hydroxide BE powder was placed in an aqueous medium; this was heat-treated at 90-100 • C for 1-3 h, to obtain aluminum oxyhydroxide, which was then spray-dried.The BE powder was compressed for granulation without binders.Its main chemical composition was Al 2 O 3 , with less than 0.1% Cl − and less than 0.8% SO 2− 4 .Loss of mass upon drying, pH, mean particle size, and pore volume were 7.8%, 8.2, 100 µm, and 0.402 mL/g, respectively [9].Scanning electron microscopy (SEM) was carried out (JSM-5500LV; JEOL, Japan), and thermogravimetricdifferential thermal analysis (TG-DTA) was carried out using a simultaneous DTA/TGA analyzer (DTG-60AH; Shimadzu).A specific surface analyzer (NOVA4200e; Yuasa Ionic, Japan) was used to measure specific surface area.The number of surface hydroxyl groups was measured using the method of fluoride ion adsorption [11].The pH of the solution was measured using a digital pH meter (Mettler-Toledo International Inc., Japan).The phosphate solution was prepared with potassium dihydrogen phosphate (Wako Pure Chemical Ind.).

B. Adsorption of phosphate
The amount of phosphate adsorbed was measured as follows: 0.05 g of G-BE was added to 50 mL of phosphate solution at different initial solution volumes (1-50 mg/L).The suspension was shaken at 25 • C (or 15 • C) for 24 h at 100 rpm, and was subsequently filtrated using a 0.45µm membrane filter.The concentration of phosphate was measured using a portable colorimeter (DR/890; Hach, USA).The amount of phosphate adsorbed onto the G-BE was calculated using the concentrations before and after adsorption.The effect of contact time was examined for e-Journal of Surface Science and Nanotechnology Volume 10 (2012)   phosphate adsorption onto G-BE (contact time: 1, 3, 6, 9, 13, 16, 20, and 24 h).

III. RESULTS AND DISCUSSION
SEM imagery and TG-DTA of G-BE are shown in Fig. 1, which shows that the powdered BE could be successfully granulated by compression without binders.The G-BE showed a spherical form.Moreover, the particle diameter of the G-BE was greater than that of the original powdered form.Dehydration of adhesion and bound water occurred at 96.0 and 394.6 • C, respectively.Specific surface area, surface pH, amount of hydroxyl groups of G-BE, and saturated amount of phosphate adsorbed were 305.0 m 2 /g, 7.4, 0.70 mmol/g, and 57.8 mg/g, respectively.Saturated amount of phosphate adsorbed onto G-BE showed similar trends by comparing to conventional materials [5,12].Adsorption isotherms of phosphate onto G-BE are shown in Fig. 2. Adsorption increased with temperature, indicating a chemisorption mechanism of phosphate adsorption onto G-BE.The Langmuir and Freundlich equations expressed in Eqs. ( 1) and ( 2) were used to model these adsorption isotherm data [13,14]: where C e (mg/L) and q e (mg/g) are the equilibrium adsorbate concentrations in the aqueous and solid phases.
Here, q m (mg/g) is the maximum adsorption capacity, and b is the Langmuir adsorption equilibrium constant; K f is the Freundlich equilibrium constant, indicative of adsorption capacity; and 1/n is the Freundlich adsorption constant, the reciprocal of which is indicative of adsorption intensity.
The fitted constants for the Langmuir and Freundlich isotherm models, along with regression correlation coefficients, are summarized in Table I.The correlation coefficient values obtained for the Freundlich and Langmuir isotherms were both greater than 0.937, indicating a very good mathematical fit by both models.Similar results were reported for the adsorption of phosphate by alkaline fly ash [15].
The effect of contact time for phosphate adsorption is shown in Fig. 3; equilibrium adsorption was reached within 20-24 h.The adsorption kinetics data of phosphate were determined by testing pseudo-first-order (3) and pseudo-second-order (4) kinetic models.Better agreement was achieved for the pseudo-second-order equation [16]: ln(q e − q t ) = ln q e − k 1 t, (3) t/q t = 1/(k 2 q e ) 2 + t/q e , ( where q e and q t are the amounts of phosphate adsorbed at equilibrium and at time t (mg/g), respectively; k 1 is the equilibrium rate constant of pseudo-first-order kinetics (1/h); and k 2 is the equilibrium rate constant of pseudo-second-order kinetics (g/mg h).The constants k 1 and k 2 were 0.273 1/h and 0.017 g/mg h, respectively.Moreover, the correlation coefficients r of pseudo-first-order and pseudo-second-order models were 0.875 and 0.926, respectively.The good agreement of the data with the pseudo-second-order kinetics model (r: 0.926) suggested that the chemisorption process could be a rate-limiting step [17].

IV. CONCLUSIONS
G-BE was prepared for phosphate adsorption.Specific surface area, surface pH, amount of hydroxyl groups of G-BE, and saturated amount of phosphate adsorbed were 305.0 m 2 /g, 7.4, 0.70 mmol/g, and 57.8 mg/g, respectively.Adsorption isotherms of phosphate adsorbed onto G-BE were fitted to both the Freundlich (0.947-0.990) and Langmuir (0.937-0.969) equations.Equilibrium adsorption was reached within 20-24 h.Adsorption kinetics data were in agreement with the pseudo-second-order model, which suggested that the chemisorption process could be a rate-limiting step.

Fig. 3
Fig. 3 Adsorption rate of phosphate onto G-BE FIG.3: Adsorption rate of phosphate onto G-BE.

TABLE I :
Constants of the Freundlich and Langmuir equation for phosphate adsorption.