Influence of sintering temperature on orthophosphate and pyrophosphate removal behaviors of red mud granular adsorbents (RMGA)

Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 1–7

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Influence of sintering temperature on orthophosphate and pyrophosphate removal behaviors of red mud granular adsorbents (RMGA) Yaqin Zhao a , Qinyan Yue a,∗ , Qian Li a , Qiuju Li a , Baoyu Gao a , Shuxin Han a,b , Hui Yu a a Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, No. 27 Shanda South Road, Jinan 250100, Shandong, China b Shandong Analysis and Test Center, Shandong Academy of Sciences, No. 19 Keyuan Road, Jinan 250014, Shandong, China

a r t i c l e

i n f o

Article history: Received 27 August 2011 Accepted 12 November 2011 Available online 23 November 2011 Keywords: Red mud granular adsorbents Sintering temperature Orthophosphate removal Pyrophosphate removal Competitive adsorption

a b s t r a c t In this research, red mud granular adsorbents (RMGA) were manufactured under different sintering temperatures (ST), using red mud as the main raw material. These RMGA were applied to remove orthophosphate and pyrophosphate from aqueous solution, and the different phosphate removal behaviors were investigated comparatively. The increase of ST could lead to surface area increasing and effective components decreasing on RMGA simultaneously, and the combined effect on RMGA for phosphates removal varied with the initial pH (pHi ) in solution, because pH influenced the stability of RMGA structure to some extent. The competitive interaction of adsorption and precipitation implied the mechanism for phosphates removal in this research: the strong relationship between orthophosphate removal capacity and Ca2+ concentration in solution indicated that precipitation affected orthophosphate removal greatly, while adsorption was the dominant reaction for pyrophosphate removal. When orthophosphate and pyrophosphate coexisted in solutions, precipitation of orthophosphate was weakened due to complexation of metallic ions, and pyrophosphate was comparatively easily adsorbed because of the advantage on electrostatic attraction. The mechanism of phosphates adsorption on RMGA can be described as electrostatic attraction followed by ligand exchange, and some effective sites on RMGA could capture orthophosphate or pyrophosphate selectively. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Phosphorus (P) is a necessary nutrient for organisms in most ecosystems, and it is often presented as phosphates of low concentrations in water bodies, including organic phosphate, inorganic phosphate and polyphosphate (particulate P) [1]. Excessive supply of phosphates discharged into rivers and lakes is the major cause of eutrophication [2]. Heretofore, many techniques have been employed for phosphate removal in wastewater treatment, which are mainly based on biological and physicochemical technology. Biological phosphate removal is used less, because bioreactors are sensitive to wastewater composition and they only performs well under certain aerobic, anaerobic and anoxic conditions [3]. Chemical coagulation–precipitation system is the most effective and wellestablished technique to remove phosphate up till now, but the cost is high due to application of calcium, aluminium and iron salts [4]. In addition, the recovery of P from the produced inorganic solids is very difficult and complicates the sludge treatment [5]. While for adsorption process, since the operation is much easier and less

∗ Corresponding author. Tel.: +86 531 88365258; fax: +86 531 88364513. E-mail address: [email protected] (Q. Yue). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.11.013

sludge is produced, it has been considered as a more applicable method for the removal of phosphate from wastewater [6]. Compared with some modified natural adsorbents such as bentonite [7] and montmorillonite [8], the utilization of industrial wastes or by-products for phosphate removal has attracted more attention in economy concern [9]. In recent years, many researchers have made some progress in this field, and the materials used by them include alum sludge [10], fly ash [3], furnace slag [11], iron oxide tailings [1] and red mud [12], etc. Among these materials, red mud (RM), as the waste tailing generated after alumina producing process, is a kind of highly alkaline slurry with 15–30% solids [13]. Currently, quantities of RM are accumulated due to the lack of appropriate treatment methods, which causes large amount of farmland occupancy and surrounding environment pollution at the same time [14]. Since RM is well recognized as adsorbent, the application of RM in phosphates removal is a good approach for treating waste with waste. Up to now, the usage of activated powdered RM has been investigated commonly and some disadvantages have been revealed, including the high cost of chemical injecting, the hardness on dealing with the wastewater output during activation as well as the difficulty on the recovery and regeneration of adsorbents after application [15]. Therefore, based on the cementitious characteristic of RM [16], the object of making it into granular materials, which is economical,

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eco-friendly and widely available in column techniques, has been put forward. In this paper, red mud granular adsorbents (RMGA) were produced with RM as the main raw material, while bentonite and starch were used as the simple additives instead of expensive chemicals. Since considerable amount of orthophosphate and pyrophosphate is contained in several kinds of wastewater, such as the effluents from farm lands and plating plants [17,18], RMGA were applied to remove these two phosphates from aqueous solutions. Because sintering temperature (ST) was an important parameter for producing RMGA [19], the characteristics of various RMGA manufactured under different ST for orthophosphate and pyrophosphate removal were investigated comparatively in this research. For the purpose of accumulating empiric data, the primary captive test was conducted in capped conical flasks. The pH effect on the performance of RMGA was studied and the mechanisms of RMGA for phosphates removal were investigated in details. 2. Material and methods 2.1. Raw materials In this study, RM was obtained from Shandong Aluminium Industry Corporation in Zibo, Shandong Province of China, and has been elutriated in tap water. After the impurities such as rocks and leaves were moved away, the sludge was filtrated in a Buchner funnel. Then, the solid was dried, comminuted and sieved through a 100 mesh screen. This obtained powdered RM was employed as the main raw material for manufacturing RMGA. Bentonite and starch were applied as the simple adminicular materials taking the place of cementing agent and aperture producer, respectively, for the reasons that bentonite can served as a kind of quality clay in ceramic industry [20] and starch can produce gas when being sintered. Similarly, they were also sieved through the 100 mesh screen. The chemical composition of processed RM and bentonite was determined by energy dispersive X-ray analysis using a PV9100 X-ray energy spectrometer that was connected with an S-520 scanning electron microscope. The results are presented in Table 1, similar with those shown in our previous research [19]. 2.2. Preparation of RMGA RM, bentonite and starch at the mass ratio of 85:10:5 were mixed evenly to produce RMGA. After an appropriate amount of pure water was injected in the mixed powdered materials, the mixture was stirred continuously until paste was formed. The paste was extruded through an aperture board and shreds with diameter of 1.5 mm were drawn out. Then, cylinder granulation of 1.5-mm-long was obtained by cutting the shred to particles. The raw granules were dried in a constant-temperature heating air-blowing dry box at 100 ◦ C for 20 min, preheated in a muffle furnace at 400 ◦ C for 15 min afterwards, and then sintered in a tubular furnace at the relevant temperature for 10 min. It had been proved that ST influenced the characteristics of RMGA sensitively in our previous study [19], so ST (increased progressively by 10 ◦ C from 950 ◦ C to 1100 ◦ C) was chosen as the variable parameter in preparing different RMGA herein. Finally, various kinds of RMGA were obtained after being naturally cooled down to room temperature.

Table 1 The composition of raw materials (wt.%). Composition

CaO

SiO2

Al2 O3

Fe2 O3

Na2 O

TiO2

MgO

K2 O

SO3

RM Bentonite

28.32 3.87

24.86 66.18

15.80 16.94

15.44 4.39

8.70 2.59

4.04 0.31

2.34 3.94

0.25 1.78

0.24 0.00

2.3. Characterization The specific surface area (SBET ) of RMGA was measured by the BET nitrogen gas sorption method using an ST-08A accelerated surface area and porosimetry. Some selected RMGA samples were pretreated by grinding and sieving through a 100 mesh screen for X-ray diffraction (XRD) analysis, which were obtained with a D/max-ra X-ray diffractometer using Cu K␣ radiation at 40 kV and 40 mA over the 2 range of 20–70◦ . 2.4. Reagents Orthophosphate and pyrophosphate solutions in this research were prepared by potassium dihydrogen phosphate (KH2 PO4 ) and sodium pyrophosphate (Na4 P2 O7 ) of guaranteed grade, respectively. The phosphates concentration in solution was determined with elemental P, which was surveyed according to the ascorbic acid method published in Monitoring and Analysis Methods of Water and Wastewater [21], using a 722E visible range spectrophotometer. The chemicals referred in this measuring method involved potassium peroxydisulfate (K2 S2 O8 ), sulphuric acid (H2 SO4 ), ascorbic acid (C6 H8 O6 ), ammonium molybdate tetrahydrate ((NH4 )6 Mo7 O24 ·4H2 O) and potassium antimonyl tartrate (K(SbO)C4 H4 O6 ·(1/2)H2 O), all of which were of analytical grade. The other chemicals, such as hydrochloric acid (HCl) and sodium hydroxide (NaOH), which were used to prepare pH modifying solutions. In addition, deionised water was applied as solvent for all the reagents. 2.5. Phosphates removal experiments The experiments were conducted by shaking 25 mL of 50 mg L−1 relevant phosphate solution (orthophosphate or pyrophosphate solution) with 0.1 ± 0.001 g RMGA (the adsorbents dosage was 4 g L−1 ) in the covered conical flasks for 4 h, using a stable temperature horizontal shaking bath that was regulated at aquatic temperature of 27 ± 1 ◦ C and stirring speed of 100 rpm. Initial pH (pHi ) in solutions was adjusted to the relevant value of 1.0–13.0 in different test with 1 mol L−1 HCl or NaOH solution. After adsorption operation, the final pH (pHf ) in solution was measured by a PHS25C acidometer, and the concentration of P in it was analyzed by the ascorbic acid method. The phosphate removal capacity (X) was evaluated by Eq. (1): X=

(Ci − Cf )V m

(1)

where X signifies the amount of P removed by per unit mass of RMGA (mg g−1 ); Ci and Cf are the initial and final concentrations of elemental P in solution (mg L−1 ), respectively; V is the volume of phosphate solution (L); and m indicates the weight of dry RMGA (g). 3. Results and discussion 3.1. Characteristics of RMGA samples 3.1.1. XRD analysis of RMGA The crystalline phases of some RMGA samples, which were sintered at 950 ◦ C, 1000 ◦ C, 1050 ◦ C and 1100 ◦ C, are illustrated in Fig. 1. As can be seen, the chemical components in RMGA changed with the variation of ST. According to the research done by Sglavo et al. [22], reactions occurred in RMGA with ST increasing were considered as Eqs. (2)–(6). It could be found that ␥-Al2 O3 , CaO and Fe2 O3 in RMGA decreased as ST increased, due to the transformation and combination reactions. Since ␥-Al2 O3 , CaO and Fe2 O3 were effective components for phosphates removal [12,23], RMGA sintered

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Table 2 SBET of various RMGA manufactured at different ST. ST (◦ C)

960

980

1000

1020

1040

1060

1080

1100

SBET (m2 g−1 )

7.70

8.95

10.91

12.67

13.68

14.90

15.30

16.65

3.1.2. SBET of RMGA The specific surface areas (SBET ) of various RMGA are listed in Table 2. It could be found that SBET of RMGA increased with the increase of ST in the tested area, which was caused by the gasproducing reaction (a total reaction of Eqs. (4) and (5)) shown as Eq. (7). This reaction occurred gradually when temperature rose and made apertures spread on RMGA: 2Fe2 O3 + C → 4FeO + CO2

(7)

Thus, the increase of ST could lead to two evidently opposite effects on the performance of RMGA for phosphates removal: one was the decrease of effective components, which would bring about a negative effect; the other was the increase of surface area, which would provide more contact sites and enhance the adsorption probability. 3.2. Influence of pHi on phosphates removal behaviors In order to find the optional pHi for the removal of orthophosphate and pyrophosphate, pHi in orthophosphate or pyrophosphate solution was adjusted to different value between 1.0 and 13.0. RMGA sintered at 1000 ◦ C was applied in this experiment, and the influence of pHi on the removal behaviors of two kinds of phosphorus is presented in Fig. 2. It was found that both the removal capacities of orthophosphate and pyrophosphate increased gently with the increase of pHi firstly, and then tended to decrease when pHi was higher than 5.0. The reason for the existence of peak value was considered as follows: when pH was below 2.0, P mainly existed in the form of phosphoric acid (H3 PO4 or H4 P2 O7 ), which was not very active in chemical adsorption. When pH rose, the removal capacities tended larger, because the amount of phosphates radical such as H2 PO4 − , HPO4 2− , H2 P2 O7 2− and HP2 O7 3− increased. While, when pHi was above 7.0, less metallic ions (like Ca2+ and Fe3+ ), which were responsible to the precipitation of phosphates, would be released from RMGA into solutions (this would be discussed further in Section 3.3.3). At the same time, a stronger competition adsorption between

Fig. 1. XRD patterns of RMGA sintered at 950 ◦ C (a), 1000 ◦ C (b), 1050 ◦ C (c) and 1100 ◦ C (d) (the raw materials – RM: bentonite: starch was 85:10:5 by weight).

at higher temperatures might not perform so good comparing with those sintered at lower temperatures. However, thanks to the formation of Ca3 Al2 O6 and Ca3 Fe2 Si3 O12 , the strength of RMGA was enhanced and the structure of RMGA was more stable when ST tended higher. 3CaO + Al2 O3 → Ca3 Al2 O6 ␥-Al2 O3 → ␣-Al2 O3

(above900 ◦ C)

(above1000 ◦ C)

(3)

C + CO2 → 2CO (above1100 ◦ C) Fe2 O3 + CO → 2FeO + CO2

(4)

(1000–1100 ◦ C)

3CaO + Fe2 O3 + 3SiO2 → Ca3 Fe2 Si3 O12

(2)

(above1100 ◦ C)

(5) (6)

Fig. 2. Influence of pHi on orthophosphate and pyrophosphate removal for RMGA sintered at 1000 ◦ C.

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OH− and phosphates might occur [24], thus the adsorption capacities decreased quickly. In addition, when pH increased, the negative increase of zeta potential of RMGA (proved in our previous study [19]) also contributed to the decrease of anions adsorption in most cases [25]. It could be noted that lager removal capacities of orthophosphate and pyrophosphate both occurred in pHi range of 1.0–6.0. However, our former research showed that RMGA could be eroded in solutions with pH value below 2.0 [26]. Therefore, pHi between 3.0 and 6.0 was considered appropriate, and the influence of pHi on orthophosphate and pyrophosphate removal was discussed further for different RMGA sintered at different temperatures. 3.3. Comparison of orthophosphate and pyrophosphate removal by various RMGA 3.3.1. Removal capacities of orthophosphate and pyrophosphate Fig. 3 shows the removal capacities of orthophosphate and pyrophosphate for various RMGA under different pHi . When pHi in solutions was between 3.00 and 6.00, RMGA sintered at lower temperatures performed relatively better on both orthophosphate and pyrophosphate removal. The result was mainly caused by the fact that effective components for phosphates removal decreased with the increase of ST, which agreed with the XRD analysis of RMGA. Moreover, for RMGA sintered below 1000 ◦ C, the removal capacities of orthophosphate were approximately larger than those of pyrophosphate; while for RMGA sintered above 1000 ◦ C, the situation was different. The performance of RMGA on pyrophosphate removal was a little better than that on orthophosphate when RMGA were sintered under higher temperatures, and peak values were found on the curves of pyrophosphate removal capacity.

In consideration of the two opposite effects on RMGA caused by increasing ST (see Section 3.1.2), it was speculated that the effect of surface area increasing was more positive for pyrophosphate removal than for orthophosphate removal. However, the negative effect of reducing effective components tended much stronger with the increase of ST than the positive effect of increasing area. Thus, both the removal capacities of orthophosphate and pyrophosphate decreased rapidly when ST was higher than a certain degree. In addition, it also can be seen that the performance of a certain RMGA on orthophosphate removal was essentially similar under different pHi , but the performance of it on pyrophosphate removal changed comparatively greatly with pHi variation. When pHi was adjusted at 5.00, the removal capacities of pyrophosphate were largest compared with those at pHi of 3.00, 4.00 and 6.00, and the optimal ST (when RMGA sintered at this temperature were applied, the largest removal capacity could be obtained) for RMGA at pHi of 5.00 was highest (1030 ◦ C, in Fig. 3(c)), too. The phenomenon above suggested that a maximum utilization of effective sites for pyrophosphate removal could be obtained at pHi of 5.00. Because the increase of ST led to surface area increasing and effective components decreasing simultaneously, it was supposed that the quantity of effective sites on the surface of RMGA, which is suitable for pyrophosphate adsorption, was maximized at ST around 1030 ◦ C.

3.3.2. The pH variation during phosphates removal The removal capacities of orthophosphate and pyrophosphate were also considered to be related with some other factors, such as the variation of pH during the adsorption process and the concentrations of some metallic ions released into solutions.

Fig. 3. The removal capacities of orthophosphate and pyrophosphate for various RMGA at pHi = 3.00 (a), pHi = 4.00 (b), pHi = 5.00 (c) and pHi = 6.00 (d).

Y. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 1–7

The pHf in solutions for various RMGA after the adsorption process of orthophosphate and pyrophosphate are shown in Fig. 4. It can be noticed that pH in solutions all rose after the experiment, because the phosphates adsorption was a process followed by hydroxyl ions releasing and some metallic oxides (such as CaO and Na2 O) in RMGA could also be dissolved in solutions [27,28]. Furthermore, when RMGA manufactured at lower ST was applied for phosphates removal, the pHf in solution was comparatively higher. This implied that RMGA sintered at lower temperatures contained more compositions leading to pH rising, including CaO, Na2 O and the effective components for phosphates adsorption (␥-Al2 O3 and Fe2 O3 ). It was also obvious that larger amount of CaO and Na2 O in RMGA might be dissolved out when a lower pHi was applied. In consideration of the pH ranging from 3.00 to 11.05 in this experiment (pHi was between 3.00 and 6.00, and pHf was between 5.97 and 11.05 as shown in Fig. 4), orthophosphate forms were determined mainly as H2 PO4 − and HPO4 2− and pyrophosphate forms were H2 P2 O7 2− , HP2 O7 3− and P2 O7 4− . According to the mechanisms of RM for phosphate adsorption that explained by some researchers [23,29], the adsorption reactions in our research were described as ligand exchange in Eqs. (8)–(12), where S represents the effective site on the surface of RMGA. Since the pH in solution was rising throughout the adsorption process, the adsorp-

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tion reactions presented in different phosphates forms might occur in turn. S-OH + H2 PO4 − →

S-H2 PO4 + OH−

(2.12 < pH < 7.20)

(8)

2 S-OH + HPO4 2− → ( S)2 -HPO4 + 2OH− (7.20 < pH < 12.36)

(9)

2 S-OH + H2 P2 O7 2− → ( S)2 -H2 P2 O7 + 2OH− (2.36 < pH < 6.60)

(10)

3 S-OH + HP2 O7 3− → ( S)3 -HP2 O7 + 3OH− (6.60 < pH < 9.25)

(11)

4 S-OH + P2 O7 4− → ( S)4 -P2 O7 + 4OH− (pH > 9.25)

(12)

3.3.3. Relationship between phosphates removal behaviors and Ca2+ /Fe3+ concentrations Since some metallic oxides in RMGA gradually dissolved in solution during phosphate removal and the pHf in solution was relatively higher, precipitation might occur during adsorption process. For the reason that phosphates salts of calcium and iron were commonly insoluble under high pH, Ca2+ and Fe3+ (Fe2+ in solution was easily oxidized under higher pH, so Fe3+ was applied to represent the total iron ion herein) concentrations in orthophosphate and pyrophosphate solutions with different pHi were measured after phosphates removal process. The corresponding results for several RMGA samples are presented in Table 3, which were determined by the atomic absorption spectrometry with a TAS-990 atomic absorption spectrophotometer. The variation of Ca2+ and Fe3+ concentrations was quite complicated. It was assumed that Ca2+ (or Fe3+ ) contained in solution could be calculated by the quantity of CaO (or Fe2 O3 and FeO) dissolved minus the amount of Ca2+ (or Fe3+ ) precipitated. Also, the dissolution of CaO (or Fe2 O3 and FeO) accompanied with the rising of pH Table 3 The Ca2+ and Fe3+ concentrations in orthophosphate or pyrophosphate solutions at different pHi for several RMGA samples after phosphates removal process (mg L−1 ). Ion

Phosphates solution

Ca2+ Orthophosphate

Pyrophosphate

Fe3+ Orthophosphate

Pyrophosphate Fig. 4. The pHf in solutions for various RMGA after orthophosphate removal (a) and pyrophosphate removal (b) at different pHi .

The pHi in solution

3.00 4.00 5.00 6.00 3.00 4.00 5.00 6.00 3.00 4.00 5.00 6.00 3.00 4.00 5.00 6.00

ST of RMGA applied in adsorption process 950 ◦ C

1000 ◦ C

1050 ◦ C

1100 ◦ C

4.181 2.033 1.078 1.610 5.075 5.476 5.582 5.710 0 0 0 0 0.054 0 0 0

5.515 1.456 0.942 1.403 5.343 5.546 5.416 4.688 0 0 0 0 0.013 0 0 0

6.589 3.699 2.327 2.558 5.620 5.531 5.204 5.399 0 0 0 0 0.693 0.030 0 0

7.258 5.696 4.325 4.737 6.216 5.857 5.676 5.628 0 0 0 0 2.018 0.621 0.360 0

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and the precipitation of Ca2+ (or Fe3+ ) in solutions, and equilibrium among them would be obtained at the end of phosphate removal process. Therefore, the situation should be differentiated for different RMGA: (1) when RMGA of lower ST were applied, because of the relatively unstable structure, more CaO and Fe2 O3 in RMGA was dissolved, higher pHf was turn up, more precipitation reaction took place and a quicker equilibrium in solution would be obtained; (2) if the applied RMGA was sintered at higher temperature, because the chemical structure of it was more stable, CaO, Fe2 O3 and FeO (FeO existed in RMGA with higher ST) in RMGA were difficult to be dissolved in solution and smaller amount of phosphates would be precipitated, especially under a higher pHi condition. Because Fe3+ was more insoluble in solution and the composition of Fe was fixed relatively stable in RMGA [26], the concentrations of Fe3+ in solutions were all extremely low. As shown in Fig. 4, under the same experiment condition, orthophosphate solution was more acidic than pyrophosphate solution after phosphate removal process, but less Ca2+ /Fe3+ was present in orthophosphate solution (in Table 3), which seemed disagree with the dissolution regulation. Because Ca2+ and Fe3+ were much easier to be precipitated with orthophosphate than pyrophosphate, it was presumed that precipitation with orthophosphate consumed large amount of Ca2+ and Fe3+ in solution. Thus, precipitation occupied a certain proportion in orthophosphate removal, and the deposits attached on RMGA restrained further adsorption. The adsorption reaction which could stimulate pH rising was weaker consequently, and this could explain why pHf in orthophosphate solution was comparatively lower. In addition, it was noticed that the lowest Ca2+ concentration in Table 3 occurred at ST of 1000 ◦ C and pHi of 5.00, suggesting that proportion of precipitation in orthophosphate removal was maximized under this condition. For pyrophosphate solution, Ca2+ concentration increased with the decrease of pHf (Fig. 4(b)) generally, and Fe3+ existed only when pHf was low. As calcium pyrophosphates and iron pyrophosphates were essentially soluble, dissolution of CaO, Fe2 O3 and FeO from RMGA was considered insignificant for pyrophosphate removal in this research, but affected the variation of pH; and the mechanism of pyrophosphate removal was mainly adsorption on RMGA.

3.4. The competitive adsorption of orthophosphate and pyrophosphate on RMGA In order to explore the performance of RMGA for orthophosphate and pyrophosphate removal when they coexisted in solution, an investigation about competitive adsorption was executed. As presented in Fig. 3, larger removal capacities of both orthophosphate and pyrophosphate could be obtained at pHi of 4.00 for RMGA that were sintered at temperatures around 1000 ◦ C. Thus, the experiment was carried out in the mixed solution (the ratio of orthophosphate to pyrophosphate was 1:1 estimated by the amount of substance, and the total P concentration was also 50 mg L−1 ) at pHi of 4.00, using RMGA manufactured at ST of 960 ◦ C, 980 ◦ C, 1000 ◦ C, 1020 ◦ C and 1040 ◦ C. The removal capacities of orthophosphate, pyrophosphate and total phosphate for those RMGA are illustrated in Fig. 5. Ca2+ were measured after adsorption process, with concentrations ranging from 3.01 to 3.30 mg L−1 , and no Fe3+ was found in the solutions. Since the pHf in solution was around 10 (shown in Fig. 5), which was similar to that of pyrophosphate in Fig. 4(b), and Ca2+ and Fe3+ concentrations were lower than the corresponding values in pure pyrophosphate solution (in Table 3), it was implied that a moderate amount of precipitation might occurred. It can be learnt from Fig. 5 that the removal capacity of orthophosphate was smaller than that of pyrophosphate for the

Fig. 5. The removal capacities of orthophosphate, pyrophosphate and total phosphate for several RMGA in the competitive adsorption experiment at pHi of 4.00.

tested RMGA, and several reasons were speculated as follows: firstly, a part of Ca2+ in the mixed solution could combine with pyrophosphate due to the complexation reaction [30], thus the amount of orthophosphate precipitated by Ca2+ was reduced; moreover, the pyrophosphate form (H2 P2 O7 2− , HP2 O7 3− or P2 O7 4− ) was more negatively charged than that of orthophosphate (H2 PO4 − or HPO4 2− ) at equal pH level, so pyrophosphate could be attracted on RMGA more quickly. Therefore, more effective sites on the surface of RMGA were occupied by pyrophosphate, and relatively less orthophosphate could be adsorbed further. While, the removal capacity of orthophosphate approached gradually to that of pyrophosphate with the increase of ST in the tested range. This phenomenon indicated that the effects above, which brought about the two kinds of phosphates differing in the competitiveness of adsorption on RMGA, might be weakened when ST of RMGA was raised. As the zeta potential of RMGA tended more negative with the increase of ST [19], the difference between the electric attractions of orthophosphate and pyrophosphate on RMGA was smaller for RMGA sintered at higher temperatures. Consequently, the mechanism of phosphates adsorption on RMGA was assumed as Fig. 6, which involved two steps: first, the electric attraction determined the adsorption order of orthophosphate and pyrophosphate; second, the ligand exchange reaction, which released hydroxyl ions, determined the adsorbed phosphate kind and the final removal capacity.

Fig. 6. The mechanism of orthophosphate and pyrophosphate removal by RMGA.

Y. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 394 (2012) 1–7

In addition, RMGA was considered more applicative in the mixed phosphates solution, since the removal capacities of total phosphate herein (7.1–8.0 mg g−1 ) were larger than the corresponding ones of pure orthophosphate or pyrophosphate in the former experiment. This phenomenon implied that some effective sites on RMGA were exclusively available for orthophosphate or pyrophosphate removal specifically. It was also noticed that the removal capacity of total phosphate increased slowly with the increase of ST at first and then decreased gently, and a peak value was obtained when RMGA sintered at 1000 ◦ C were used. According to the similar tendency of pHf , the variation of pH in the mixed solution was thought to be depended mainly on adsorption reaction. Moreover, it was supposed that the amount of total effective sites for orthophosphate and pyrophosphate adsorption was maximized on the surface of RMGA at ST of 1000 ◦ C, due to the interaction of the two opposite effects caused by the increasing ST.

4. Conclusions Various RMGA manufactured under different ST were applied for orthophosphate and pyrophosphate removal. It was concluded that the increasing of ST could result in surface area increasing, effective components decreasing and structure stability enhancing on RMGA simultaneously. These actions jointly affected the behaviors of RMGA, and the results showed that RMGA sintered below 1030 ◦ C performed well for phosphates removal. Because pH also influenced the stability of metallic oxides structure in RMGA to some extent, the combined effect of ST on RMGA varied with pHi in solution and the proper pHi for phosphates removal was around 4.00. Since some metallic oxides in RMGA could be dissolved into solutions and these metallic ions (Ca2+ and Fe3+ ) could be precipitated with phosphates, the mechanism for phosphates removal in this research was competitive interaction of adsorption and precipitation: pyrophosphate removed from aqueous solutions was mainly due to adsorbing on the effective sites on RMGA; while precipitation of phosphate with calcium and iron salts played an important role in orthophosphate removal, and the amount of it was depended on the pHi in solution and the stability of metallic oxides in RMGA samples, especially for RMGA manufactured at lower ST and applied in solution at lower pH. In the competitive adsorption between orthophosphate and pyrophosphate, pyrophosphate was comparatively easy to be adsorbed on RMGA, and the precipitation of orthophosphate was weakened due to the complexation of metallic ions with pyrophosphate. The mechanism of phosphates adsorption on RMGA was speculated as a two-step action, which was electrostatic attraction followed by ligand exchange reaction. The adsorption capacities of total phosphate were larger than the corresponding ones of pure orthophosphate or pyrophosphate, implying that some specific effective sites on RMGA were selectively suitable for orthophosphate or pyrophosphate adsorption, and 1000 ◦ C was considered as the optimal ST for RMGA producing. Acknowledgements This research was supported by Research Fund for the Doctoral Program of Higher Education of China 20100131110005, Jinan Excellent Young Scientists Program 20090215 and Shandong University Graduate Innovation Fund 11440070613208, Shandong Province, China.

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