- Email: [email protected]
Chemical modification of coal fly ash for the removal of phosphate from aqueous solution
Fuel 87 (2008) 2469–2476
Contents lists available at ScienceDirect
Fuel j o u r n a l h o m e p a g e : w w w . f u e l fi r s t . c o m
Chemical modification of coal fly ash for the removal of phosphate from aqueous solution P. Pengthamkeerati *, T. Satapanajaru, P. Chularuengoaksorn Environmental Technology Research Unit (EnviTech), Department of Environmental Science, Kasetsart University, Bangkok 10900, Thailand
a r t i c l e
i n f o
Article history: Received 11 October 2007 Received in revised form 10 March 2008 Accepted 13 March 2008 Available online 11 April 2008 Keywords: Zeolite Coal fly ash Chemical treatment Phosphate Immobilization
a b s t r a c t This study investigated the chemical modifications of coal fly ash treated with HCl and NaOH. Sorption behavior of phosphate from water solution on treated fly ash was examined. Results showed that the HCl-treated fly ash (TFA-HCl) had a greater specific surface area (SSA) than the NaOH-treated fly ash (TFA-NaOH) and untreated fly ash (FA). The XRF, XRD patterns, and SEM images revealed the decreased CaO content in the TFA-HCl and observed the presence of NaP1 and sodalite zeolites in the TFA-NaOH. The P sorption capacity of all studied fly ashes increased with increasing initial P concentration and mechanisms of P sorption were influenced by the equilibrium pH. Maximum phosphate immobilization capacity obtained from Langmuir model was in the following manner, TFA-NaOH > FA > TFA-HCl (57.14, 23.20, and 6.90 mg P g1, respectively). The decreased CaO content and acidic pH in the TFA-HCl were responsible for the lowest capacity of phosphate immobilization, because of unfavorable condition for calcium phosphate precipitation. In contrast, due to alkaline condition and relatively high calcium content, the precipitation of calcium phosphate was a key mechanism for phosphate removal in the FA and TFA-NaOH. The TFA-NaOH had a greatest phosphate immobilization, due to high CaO content and an increased SSA after the conversion of fly ash to zeolite. Both Langmuir and Freundlich models were good fitted for the TFA-NaOH, while was only Langmuir model for the FA and Freundlich model for the TFA-HCl. Results suggested that treating fly ash with alkaline solution was a promising way to enhance phosphate immobilization. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Increasing demand for energy throughout the world has led to an increase in the utilization of coal and, subsequently, in producing large quantities of fly ash as a waste product [1,2]. The Mae-Moh electric power station is one of Thailand’s largest electric power plants utilizing low quality lignite and providing a plenty of fly ash [3]. Despite a considerable portion of fly ash is used in relevant industry, such as construction or soil amendment, there is still a large portion that is directly disposed to the environment [4]. Such disposal is not economic and environmental sounds [5]. Alternative way to utilize fly ash is of interest and can be value enhancement to this material [6]. Since fly ash is enriched with SiO2 and Al2O3, this waste product can be transformed into zeolite-like crystalline materials as a result of chemical treatment [7,8]. Zeolite is porous material with large surface area and cavities of the basket-like frame [9,10].
* Corresponding author. Tel.: +66 2 942 8036; fax: +66 2 942 8715. E-mail address: [email protected] (P. Pengthamkeerati). 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.03.013
Hence, it has been used for various purposes, e.g., absorbent and ion exchanger in water and wastewater treatment [11,12]. Effluents from municipal, industrial plants, and animal farms are a major reason causing eutrophication in natural water bodies [13]. Typically, method for treating these effluents (especially from animal farm effluent) is by biological processes using anaerobic and aerobic ponds in a sequence [14]. However, its efficiency in nutrient removal is still in question. Adsorption is an attractive alternative in removing nutrients (especially phosphorus [P]) from the effluent by using porous materials, such as fly ash and zeolitelike substances [14,15]. This is because fly ash is enriched with oxides of aluminum, iron, and calcium. These oxides can strongly adsorb or precipitation phosphates [4]. Hence, fly ash and its fly ash-derived zeolite can be candidate materials for phosphate removal [4,12]. Although several studies had been reported the phosphate immobilization by fly ash, more information is required to gain a better understanding of the relationship between phosphate sorption properties of fly ashes and their chemical composition [13]. Therefore, this study investigates the physical and chemical changes of Thailand’s lignite fly ash with different chemical agents and their abilities to remove phosphate from an aqueous solution.
2470
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
2. Materials and methods 2.1. Modification of fly ash
Table 1 Chemical composition, surface area, and CEC of the untreated fly ash (FA), HCl-treated fly ash (TFA-HCl), and NaOH-treated fly ash (TFA-NaOH) Constituent
The coal fly ash used in this study was supplied by the Mae-Moh electric power station in the northern part of Thailand. The studied fly ash is a residue from lignite combustion using a pulverized coal firing with the furnace exit gas temperature of 1140 °C. The sample was oven-dried at 105 °C for 24 h before use. The fly ash was treated in a 2 M solution of NaOH or 1 M solution of HCl at a solution to fly ash ratio of 10:1 by weight. The mixture was then incubated at 100 °C for 24 h. At the end of the treatment, the mixture was filtered, washed thoroughly and oven-dried at 105 °C for 24 h. The final solid products were then subjected to physical and chemical analysis. 2.2. Physical and chemical measurements Chemical composition of the initial and treated fly ashes was analyzed by energy dispersive X-ray fluorescence (XRF) (Oxford ED2002 model). The fly ash samples were characterized by X-ray Diffraction (XRD) using Phillips X’Pert diffractometer or Bruker axs (D8 Advance) with Cu K radiation. The samples were scanned over the 5–50° 2h interval. Scanning electron microscopy (SEM) (JEOL JSM-5800LV model) was performed on the samples. Specific surface area (SSA) of the samples was also analyzed by Autosorb-1 analyzer using BET method. Cation exchange capacity (CEC) of the samples was measured according to Schollenberger [16], using neutral NH4OAC and 10% NaCl as saturating and replacing solutions, respectively. 2.3. Phosphate immobilization Batch experiment was carried out to measure the phosphate adsorption by the initial and treated fly ashes. The adsorbent (5 g) was added to 50 ml of synthetic phosphate solutions of varying concentration (100–15,000 mg P l1) prepared from KH2PO4. The mixture was shaken for various times (upto 24 h) at room temperature. After shaking, the mixture was filtered and the supernatant was measured for phosphate concentration by the molybdenum-blue ascorbic acid method with a UV–Vis spectrophotometer (GBC UV/VIS 918 model) [17] and the pH in equilibrium solution using pH meter (EcoScan pH6 model). To understand the immobilization behavior of P on fly ashes, the fly ash samples after performing P immobilization were oven-dried at 70 °C for XRD analysis. The effect of contact time was studied by using 1000 mg P l1, while other studies used 24 h of contact time. Langmuir and Freundlich isotherms were applied to determine the sorption capacity of the fly ashes. Analyses were performed in duplicate. 3. Results and discussion 3.1. Physical and chemical characteristics of treated fly ash Chemical composition of the samples showed in Table 1. The major components of all fly ashes were oxides of Si and Al, and various other oxides. The untreated fly ash (FA), NaOH-treated fly ash (TFA-NaOH) and HCl-treated fly ash (TFA-HCl) had similar compositions. But, a portion of calcium oxide (CaO) in the TFA-HCl decreased from 9.7% in the FA to 4.4% causing an increased portion of all other oxides, especially SiO2. The decreased CaO content in the TFA-HCl was due to the neutralization between HCl and CaO causing dissolution of CaO. XRD patterns of the samples were presented in Fig. 1. The XRD pattern of the FA showed the presences of quartz, mullite, iron
SiO2 Al2O3 CaO Fe2O3 MgO K2O Specific surface area (m2 g1) CEC (meq g1)
Weight (%) FA
TFA-HCl
TFA-NaOH
45.8 13.6 9.7 6.7 0.4 1.6 3.39 0.03
60.9 19.4 4.4 7.0 1.1 2.0 61.84 –
50.1 14.0 9.4 7.2 0.2 0.4 35.38 1.88
oxide, hematite, sulfur oxide, and CaO (Fig. 1a). The TFA-HCl had a similar XRD pattern to the FA, but the peaks of CaO and sulfur oxide were not clearly observed (Fig. 1b). After alkaline treatment, the amounts of quartz, mullite, iron oxide and CaO in the TFANaOH were decreased from the FA, as was observed in the reduction of these peak intensities (Fig. 1c). We also observed the appearance of zeolites, which were NaP1 (Na6Al6Si10O32.12H2O) and hydroxyl-sodalite (1.08 Na2O.Al2O3.1.62SiO2.1.8H2O). This observation indicated the conversion of fly ash into zeolite-like material under alkaline condition. Previous studies also reported a similar observation [8,18,19]. For the TFA-NaOH, the peak intensity of CaO was decreased from the FA (Fig. 1a and c), while the portion of CaO from the XRF analysis was 9.4%, which was almost the same as the FA (9.7%) (Table 1). This observation suggested that CaO partly dissolved in NaOH medium under heat treatment, but the dissolved calcium remained in the TFA-NaOH by changing to other forms of calcium. SEM images of the fly ash samples were shown in Fig. 2. SEM observation of the FA showed the presence of micro-particles in the shape of smooth balls (microspheres) (Fig. 2a and b). After treating with HCl, the ball-shaped particles of FA were partly transformed into agglomerations of undefined shape with no observation of crystal formation (Fig. 2c and d). NaOH solution affected the FA by deforming ball-like shape and changing smooth surface into various shapes of crystals, e.g., plates and rods (Fig. 2e and f). The particle surface of TFA-NaOH showed the formation of zeolites, as was confirmed by the XRD data of this study (Fig. 1c) and was also observed by other studies [8,11,20]. The specific surface area (SSA) of the chemically treated fly ash was greater than that of the untreated FA in the following manner: TFA-HCl > TFA-NaOH > FA (61.84, 35.38, and 3.39 m2 g1, respectively) (Table 1). The greater SSA of TFA-HCl may be attributed to (i) surface modification of the FA particles by acid solution, and (ii) an increase in a portion of micropore after CaO dissolution [20]. The greater SSA of TFA-NaOH was because of a transformation of fly ash into zeolite-like structure as supported by the XRD pattern (Fig. 1c). This observation was in agreement with the study by Sarbak and Kramer-Wachowiak [20], who reported an increased SSA of the fly ash treated with HCl and NaOH, compared to the untreated fly ash (105, 59, and 3 m2 g1, respectively). Penilla et al. [11] and Woolard et al. [18] and found that treating coal fly ash with NaOH increased SSA to 18.5 and <45 m2 g1, respectively. After NaOH treatment, the TFA-NaOH had a greater CEC value (1.88 meq g1) than the FA (0.03 meq g1) (Table 1). The greater CEC of TFA-NaOH was probably due to the formation of zeolites, especially NaP1 in this study (Fig. 1c). Juan et al. [7] also observed an increased CEC value of zeolitized fly ash (NaP1) from the original fly ash (from 0.02 to 1.9 meq g1).
2471
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
a
Intensity
SO
H
Q M
Fe
M H Ca Ca
Q
M
M H
SO
Fe
FA
Q
5
15
25
35
45
55
2Theta
b Q
Intensity
H M M
M
5
Fe
Q
15
M H
25
Q = M = SO = Fe = Ca = H = N = S =
Quartz Mullite Sulfur oxide Iron oxide (Fe 3 O4) Calcium oxide Hematite (Fe 2O3 ) NaP1 (zeolite) Sodalite (zeolite)
M H Fe Q
35
TFA-HCl
45
55
2Theta
c N
Intensity
N S
N N S
N
H Q M M Q
5
15
25
35
M S H
N
45
TFA-NaOH
55
2Theta Fig. 1. XRD patterns of (a) the untreated fly ash (FA), (b) the HCl-treated fly ash (TFA-HCl), and (c) the NaOH-treated fly ash (TFA-NaOH).
3.2. Phosphate immobilization 3.2.1. Effect of contact time The effect of contact time on phosphate immobilization by fly ash was presented in Fig. 3. The sorbed P on fly ash increased with an increase of contact time, in particular of the first 30 min. After 2 to 4 h of sorption, only FA and TFA-NaOH showed an additional sorption capacity of P, but the sorption rate of TFA-NaOH was
slower than that of the FA. A slower rate of P sorption by TFA-NaOH may be because the sorption seemed to be influenced by diffusion through rough surface and channels in the TFA-NaOH, which was different from the smooth surface of the FA (Fig. 2). After 24-h contact time, the P sorption on all studied fly ashes remained nearly constant. The sorption capacities of the FA and TFA-NaOH were almost the same and much greater than that of the TFA-HCl. Previous studies suggested that P immobilization on fly ash was attributed
2472
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
Fig. 2. SEM images of (a and b) the untreated fly ash, (c and d) the HCl-treated fly ash, and (e and f) the NaOH-treated fly ash.
to (i) binding of phosphate and calcium resulting in precipitation of calcium phosphate, and (ii) adsorption of phosphate on the surface of fly ash [13,15,21]. The lowest P sorption capacity by the TFA-HCl may be due to the decreased CaO after HCl treatment, resulting in low Ca2+ concentration for calcium phosphate precipitation. Khelifi et al. [6] also found that HCl-treated blast furnace slag had a lower phosphate sorption capacity than the untreated blast furnace slag. From this present study, the contact time of 24 h was chosen for further studies. 3.2.2. Effect of initial P concentration The relationships between the initial P concentration (C0) and P sorption or equilibrium pH of the studied fly ashes were shown in Fig. 4a and b. P sorption capacity of the FA sharply increased with an increase of P concentration until the C0 of 3,000 mg l1, while the equilibrium pH rapidly decreased from about 12 to 7.5. After this C0, the P sorption capacity remained nearly constant, while
the pH was slightly decreased from about 7.5 to 6. It is known that calcium phosphate precipitation is predominant mechanism in P immobilization at alkaline pH [2,13]. When pH decreased, the concentration of Ca2+ increased, which provided higher Ca2+ concentration for calcium phosphate precipitation [13,15]. Hence, P sorption capacity of the FA increased with increasing the equilibrium pH from about 12 to 7.5. However, with decreasing pH to acidic condition, calcium phosphate precipitation would be less favorable. At neutral and acidic pH, iron (Fe) and aluminum (Al) phases became positively charged and their adsorption capacity with phosphate through ligand exchange would increase [12,15,22]. For example, hematite (Fe2O3) had a point of zero charge (PZC) at the pH of 8.5–9.1 [4]. Below the PZC, hematite was protonated and had the ability to form complex with phosphate ions. According to these reasons, P sorption capacity of the FA remained nearly constant when the pH was in a range of 6– 7.5, indicating that both calcium phosphate precipitation and P
2473
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
Sorbed phosphate (mg P g-1)
12
10
8 FA TFA-HCl TFA-NaOH
6
4
2
0 0
5
10 15 Time (h)
20
25
Fig. 3. The effect of contact time on P immobilization by fly ashes. Experimental conditions: fly ash dose 10% w/v; initial phosphate concentration 1000 mg P l1.
a
70
Sorbed P (mg Pg-1)
60 50 40 30 20 FA TFA-HCl TFA-NaOH
10 0 0
3000
6000
9000
12000
15000
C0 (mg l-1)
b
14
For the TFA-NaOH, the P sorption capacity continuously increased with increasing C0, while the equilibrium pH was decreased, but more gradually and less than that of the FA, from the pH of about 12–8. Among the studied fly ashes, the TFA-NaOH had a greatest P sorption capacity. In term of pH, the greater P sorption capacity of the TFA-NaOH than the FA might be because the pH of 8–12 was in an alkaline condition and in a range that was more favorable for calcium phosphate precipitation than the FA. Only for the TFA-HCl, the P sorption and equilibrium pH slightly increased with C0. The pH increased from 4.3 to 4.9 in a range of the studied P concentrations, but was still in an acidic condition. P adsorption onto mineral surfaces of Fe and Al was regulated by the electrostatic attractions of the phosphate ions and mineral surface [4]. At acidic pH, the condition was more favorable for phosphate adsorption onto the surfaces of Fe and Al phases than for calcium phosphate precipitation [15]. Based on this information, adsorption may be a key mechanism for P immobilization by the TFA-HCl. 3.2.3. Sorption isotherm Sorption isotherms of the studied fly ashes were shown in Fig. 5. The obtained isotherms of the studied fly ashes were in various shapes as determined by a previous study [23]. The sorption isotherm of the FA increased and then remained constant (H-type), indicating very strong interaction between adsorbate and adsorbent. The sorption isotherm of the TFA-NaOH was classified into the L-type or Langmuir isotherm. The L-type isotherm also suggested a strong interaction between adsorbate and adsorbent, but relatively less than the H-type isotherm. For the TFA-HCl, the sorption isotherm was in the S-type, suggesting cooperative adsorption and favoring the clustering of adsorbate at the surface. These data supported the finding in the effect of contact time on phosphate immobilization that the precipitation and adsorption were key processes in phosphate immobilization by the studied fly ashes. For the FA and TFA-NaOH, calcium phosphate precipitation seemed to be a significant mechanism for P immobilization rather than the adsorption. But the TFA-HCl immobilized phosphate mainly by adsorption. In addition, the greatest P sorption capacity of the TFA-NaOH suggested that this treated fly ash could further adsorb phosphate much greater than the FA and TFA-HCl, due to its higher calcium content, greater surface area, and changes
10
70
8
60
6
50
5 4
pH
3
4 2 0 0
3000
6000
9000
12000
15000
C0 (mg l-1) Fig. 4. The relationships between the initial P concentration (C0) and P sorption (a) or equilibrium pH (b) of the fly ash (FA), HCl-treated fly ash (TFA-HCl), and NaOHtreated fly ash (TFA-NaOH). Experimental conditions: fly ash dose 10% w/v; contact time 24 h.
adsorption on the surface of Fe and Al phases played a role in phosphate removal.
Sorbed P (mg Pg-1)
2
40
1 0
500
1000
1500
0 2000
Sorbed P (mg P g-1)
12
Ce (mg l-1)
30 20 FA TFA-HCl TFA-NaOH
10 0 0
2000
4000
6000
8000
10000
Ce (mg l-1) Fig. 5. P sorption isotherms of the fly ash (FA), HCl-treated fly ash (TFA-HCl), and NaOH-treated fly ash (TFA-NaOH). Experimental conditions: fly ash dose 10% w/v; contact time 24 h. Ce is equilibrium concentration of P (mg l1). Inset figure shows an enlarge P sorption isotherm of the TFA-HCl.
2474
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
in fly-ash properties during the conversion of fly ash to zeolite. Despite zeolites are aluminosilicates and can probably be considered to have an insignificant capability to adsorbed P [4,12], the conversion of fly ash to zeolite had a positive effect on P sorption capacity due to the following reasons. The formation of rough surface structure and an increased surface area after the NaOH treatment favored the adsorption and precipitation reaction of phosphate [15]. Chen et al. [15] and Wu et al. [21] also suggested that the increase in phosphate immobilization capacity after zeolite synthesis process was mainly attributed to the increase in specific surface area and the increase in the contents of the dissociated Fe2O3. Chen et al. [15] also observed that Fe played an increasingly important role in immobilizing phosphate after zeolite synthesis. Despite the effect of exchangeable Ca2+ on P sorption was not well investigated in this study, in calcium saturated condition, a substantially increased CEC of the zeolitized fly ash could have an advantage on the availability of exchangeable Ca2+ on the negatively charged zeolite surface [21]. This exchangeable Ca2+ may enhance phosphate immobilization through the precipitation of calcium phosphate. Sorption data were fitted to the Langmuir and Freundlich models. Langmuir equation : C e =qe ¼ 1=ðQ 0 K L Þ þ ð1=Q 0 ÞC e 1
ð1Þ
1
where qe (mg g ) and Ce (mg l ) are the amounts of adsorbed dye per unit weigh of adsorbent and equilibrium dye concentration in solution, respectively. Q0 (mg g1) is a maximum adsorption capacity of the dye (forming a monolayer) per unit weight of adsorbent. KL is a constant related to the affinity of the binding sites (l mg1). Freundlich equation : ln qe ¼ ln K f þ ð1=nÞ ln C e
immobilization. Previous study suggested that phosphate immobilization by fly ash was governed by Ca contents of CaO and CaSO4 and Fe content [13]. The untreated fly ash used in this study gave a similar Q0 to medium-Ca fly ashes (e.g., 8.32 and 10.94% CaO) with a sorption capacity for phosphate of 24.82 and 28.00 mg P g1, respectively [15]. After NaOH treatment, these medium-Ca fly ashes also had an increased Q0 (34.68–35.69 mg P l1), as was also observed in the TFA-NaOH of this study [6]. The Kf from the Freunlich isotherm suggested that the sorption capacity of TFA-NaOH (1.93 mg g1) was greater than that of TFAHCl (0.08 mg g1). The decreased Kf of the TFA-HCl suggested that decreasing CaO content of the fly ash by acid treatment had a negative effect on phosphate immobilization capacity. Khelifi et al. [6] observed that HCl-treated blast furnace slag had a lower phosphate removal with the Kf of 0.021 mg g1, when compared to the untreated blast furnace slag (7.6 mg g1). In addition, the n values of the TFA-NaOH and TFA-HCl were 2.79 and 1.84, respectively, indicating favorable adsorption (1 < n < 10). 3.2.4. XRD patterns of P-sorbed fly ashes XRD analysis was performed to determine forms of P sorbed on the studied fly ashes using the initial P concentration of 15,000 mg l1. After P sorption, XRD patterns of the FA and TFA-NaOH revealed the formation of calcium phosphate (Fig. 7), but not for the TFA-HCl (data not shown). The calcium phosphate precipitated
400
ð2Þ Ce / qe (g l-1)
300
200
100 FA ; Ce/qe = 0.0431 Ce+ 0.8403 TFA-HCl ; Ce/qe = 0.1449 Ce + 151.01 TFA-NaOH ; Ce/qe = 0.0175 Ce + 14.383
0 0
2000
4000
6000
8000
10000
Ce (mg l-1) 5 FA ; ln qe = 0.1812 ln Ce + 1.7949 TFA-HCl ; ln qe = 0.5421 ln Ce - 2.5748 TFA-NaOH ; ln qe = 0.3590ln Ce + 0.6576
4 3
lnqe
where Kf and n are constants indicating adsorption capacity and intensity, respectively. The Freundlich adsorption constant, n, should be in a range of 1–10 for beneficial adsorption. Table 2 showed the sorption parameters and regression coefficients (r2) obtained from the linear regression equation between the values of Ce/qe and Ce for the Langmuir isotherm and between ln qe and ln Ce for the Freundlich isotherm (Fig. 6a and b). The obtained data of the FA was best fit to the Langmuir isotherm with r2 of 0.99, while was the Freundlich isotherm for the TFA-HCl (r2 = 0.90). The TFA-NaOH data was good fit to both Langmuir and Freundlich isotherms (r2 = 0.92 and 0.97, respectively). Khelifi et al. [6] also observed the experimental data of zeolite and acidtreated blast furnace slag were good fit to the Freundlich isotherm. The Q0 from the Langmuir isotherm indicated that the sorption capacity of phosphate decreased in the following manner, TFANaOH (57.14 mg g1) > FA (23.20 mg g1) > TFA-HCl (6.90 mg g1). The greater Q0 of the TFA-NaOH than the FA was because NaOH activation caused an increase of surface area and transformed fly ash into zeolites with a relatively higher sorption capacity, which may favor the P adsorption and precipitation reaction. Comparison of Q0 between the FA (or TFA-NaOH) and TFA-HCl suggested that the amount of CaO was significant to the degree of phosphate
2 1 0
Table 2 The Q0 and KL values for the Langmuir isotherm, the Kf and n values for the Freundlich isotherm and the regression coefficients of equations
-1 -6
Fly ash
Langmuir r2
Kf
n
r2
23.20 6.90
5.1 10 9.6 104
0.99 0.67
6.02 0.08
5.52 1.84
0.75 0.90
57.14
1.2 103
0.92
1.93
2.79
0.97
KL (l mg1)
-4
-2
0
2
4
6
8
10
ln Ce
2
Q0 (mg g1) Untreated fly ash (FA) HCl-treated fly ash (TFA-HCl) NaOH-treated fly ash (TFA-NaOH)
Freundlich
Fig. 6. Langmuir (a) and Freundlich (b) sorption isotherms of P on the fly ash (FA), HCl-treated fly ash (TFA-HCl), and NaOH-treated fly ash (TFA-NaOH). Experimental conditions: fly ash dose 10% w/v; contact time 24 h. Ce and qe are amount of sorbed P (mg P g1) and equilibrium concentration of P (mg l1), respectively. Points: experimental data, lines: predicted linear regression models using Langmuir and Freundlich models.
2475
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
a
CaP1
CaP1
Intensity
CaP1
Q
H CaP1 CaP1 M H
M
M H
Fe FA
Q M
5
15
25
35
45
55
2Theta
b
Intensity
Q M Fe H S
= = = = =
Quartz Mullite Iron oxide (Fe 3O4) Hematite (Fe 2O3) Sodalite (zeolite)
CaP1 = CaPO3OH.2H2O CaP2 = NaCaPO4
CaP2 Q S M
CaP2 H
M H S
TFA-NaOH
Q S
5
CaP2
15
25
35
45
55
2Theta Fig. 7. XRD patterns of (a) the untreated fly ash (FA) and (b) NaOH-treated fly ash (TFA-NaOH) after performing P sorption using an initial P concentration of 15,000 mg l1.
in the forms of calcium phosphate hydroxide hydrate (CaPO3OH.2H2O) for the FA and sodium calcium phosphate (NaCaPO4) for the TFA-NaOH. These forms of calcium phosphate may be promoted in the neutral to mildly alkaline conditions. These findings confirmed that P removal by the FA and TFA-NaOH was due to calcium phosphate precipitation. But for the TFA-HCl, P removal may be attributed to adsorption.
phosphate immobilization capacity (57.14 mg P g1) than the untreated (23.20 mg P g1) and HCl-treated fly ash (6.90 mg P g1), respectively. Sorption isotherm of the untreated fly ash was fitted to Langmuir model, while was Freundlich model for the HCl-treated fly ash. For the NaOH-treated fly ash, both models were good fitted. Overall, the alkaline modification of the fly ash enhanced a capacity of phosphate immobilization and the treated fly ash can be used as a potential adsorbent for phosphate wastewater.
4. Conclusions Acknowledgements Chemical treatment of coal fly ash observed in this study influenced changes in property of treated fly ashes. Sample treated with HCl had the greatest specific surface area due to the dissolution of CaO after acid treatment, and had a similar XRD pattern with the untreated fly ash. NaOH treatment also increased surface area and CEC of the initial fly ash due to particle modification from smooth surface to plate- and rod-shape crystals, which was indicated by the XRD analysis as NaP1 and sodalite zeolites. Results from the sorption study provide a better understanding of the influence of chemical treatment of fly ash on phosphate immobilization behavior. The CaO content in the fly ash and pH at equilibrium solution play an important role in phosphate removal by precipitating calcium phosphate and adsorption. Due to high CaO content and an increased surface area after transformation into zeolites, the fly ash treated with NaOH had a greater
We wish to acknowledge the funding by the Faculty of Science, Kasetsart University (Science, Research Fund, ScRF) and Thai Research Fund for supporting this research. Thanks also to Orawan Singchan, Saowalak Sirijareontanapun, and Siriwan Geawchingduang, for her technical assistance. References [1] Hui KS, Chao CYH. Effects of step-change of synthesis temperature on synthesis of zeolie 4A from coal fly ash. Microporous Mesoporous Mater 2005;88:145–51. [2] Yan J, Kirk DW, Jia CQ, Liu X. Sorption of aqueous phosphorus onto bituminous and lignitous coal ashes. J Hazard Mater 2007;148:395–401. [3] Chareonpanich M, Namto T, Kongkachuichay P, Limtrakul J. Synthesis of ZSM-5 zeolite from lignite fly ash and rice husk ash. Fuel Process Technol 2004;85:1623–34.
2476
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
[4] Grubb DG, Guimaraes MS, Valencia R. Phosphate immobilization using an acidic type F fly ash. J Hazard Mater 2000;76:217–36. [5] Iyer RS, Scott JA. Power station fly ash – a review of value-added utilization outside of the construction industry. Resource Conserv Recycle 2001;31:217–28. [6] Khelifi O, Kozuki Y, Murakami H, Kurata K, Nishioka M. Nutrients adsorption from seawater by new porous carrier made from zeolited fly ash and slag. Mar Pollut Bull 2002;45:311–5. [7] Juan R, Hernandez S, Querol X, Andres JM, Moreno N. Zeolite material synthesized from fly ash: use as cationic exchanger. J Chem Technol Biotechnol 2002;77:299–304. [8] Moutsatsou A, Stamatakis E, Hatzizotzia K, Protonotarios V. The utilization of Ca-rich and Ca–Si-rich fly ashes in zeolites production. Fuel 2006;85:657–63. [9] Notario del Pino JS, Arteaga Padron IJ, Gonzalez Martin MM, Garcia Hernandez JE. Phosphorus and potassium release from phillipsite-based slow-release fertilizers. J Controlled Release 1995;34:25–9. [10] Pickering HW, Menzies NW, Hunter MN. Zeolite/rock phosphate – a novel slow release phosphorus fertilizer for potted plant production. Sci Horticulturae 2002;94:333–43. [11] Penilla PR, Bustos AG, Elizalde SG. Immobilization of Cs, Cd, Pb and Cr by synthetic zeolites from Spanish low-calcium coal fly ash. Fuel 2006;85:823–32. [12] Wu D, Zhang B, Li C, Zhang Z, Kong H. Simultaneous removal of ammonium and phosphate by zeolite synthesized from fly ash as influenced by salt treatment. J Colloid Interface Sci 2006;304:300–6. [13] Chen J, Kong H, Wu D, Chen X, Zhang D, Sun Z. Phosphate immobilization from aqueous solution by fly ashes in relation to their composition. J Hazard Mater 2007;139:293–300.
[14] Bolan NS, Wong L, Andriano DC. Nutrient removal from farm effluents. Bioresource Technol 2004;94:251–60. [15] Chen J, Kong H, Wu D, Hu Z, Wang Z, Wang Y. Removal of phosphate from aqueous solution by zeolite synthesized from fly ash. J Colloid Interface Sci 2006;300:491–7. [16] Schollenberger CJ, Simen RH. Determination of exchange capacity and exchangeable bases in soil-ammonium acetate method. Soil Sci 1954;59:13–24. [17] American Public Health Association (APHA). Standards for examination of water and wastewater. Washington DC, 1995. [18] Woolard CD, Petrus K, van der Horst M. The use of a modified fly ash as an adsorbent for lead. Water SA 2000;26:531–6. [19] Querol X, Moreno N, Umana JC, Alastuey A, Hernandez E, Lopez-Soler A, et al. Synthesis of zeolites from coal fly ash: an overview. Int J Coal Geol 2002;50:413–23. [20] Sarbak Z, Kramer-Wachowiak M. Porous structure of waste fly ashes and their chemical modifications. Powder Technol 2002;123:53–8. [21] Wu D, Sui Y, Chen X, He S, Wang X, Kong H. Changes of mineralogical– chemical composition, cation exchange capacity, and phosphate immobilization during the hydrothermal conversion process of coal fly ash into zeolite. Fuel 2007. doi:10.1016/j.fuel.2007.10.028. [22] Zhang B, Wu D, Wang C, He S, Zhang Z, Kong H. Simultaneous removal of ammonium and phosphate by zeolite synthesized from coal fly ash as influenced by acid treatment. J Environ Sci 2007:540–5. [23] McBride MB. Environmental chemistry of soils. New York: Oxford University Press; 1994.
Contents lists available at ScienceDirect
Fuel j o u r n a l h o m e p a g e : w w w . f u e l fi r s t . c o m
Chemical modification of coal fly ash for the removal of phosphate from aqueous solution P. Pengthamkeerati *, T. Satapanajaru, P. Chularuengoaksorn Environmental Technology Research Unit (EnviTech), Department of Environmental Science, Kasetsart University, Bangkok 10900, Thailand
a r t i c l e
i n f o
Article history: Received 11 October 2007 Received in revised form 10 March 2008 Accepted 13 March 2008 Available online 11 April 2008 Keywords: Zeolite Coal fly ash Chemical treatment Phosphate Immobilization
a b s t r a c t This study investigated the chemical modifications of coal fly ash treated with HCl and NaOH. Sorption behavior of phosphate from water solution on treated fly ash was examined. Results showed that the HCl-treated fly ash (TFA-HCl) had a greater specific surface area (SSA) than the NaOH-treated fly ash (TFA-NaOH) and untreated fly ash (FA). The XRF, XRD patterns, and SEM images revealed the decreased CaO content in the TFA-HCl and observed the presence of NaP1 and sodalite zeolites in the TFA-NaOH. The P sorption capacity of all studied fly ashes increased with increasing initial P concentration and mechanisms of P sorption were influenced by the equilibrium pH. Maximum phosphate immobilization capacity obtained from Langmuir model was in the following manner, TFA-NaOH > FA > TFA-HCl (57.14, 23.20, and 6.90 mg P g1, respectively). The decreased CaO content and acidic pH in the TFA-HCl were responsible for the lowest capacity of phosphate immobilization, because of unfavorable condition for calcium phosphate precipitation. In contrast, due to alkaline condition and relatively high calcium content, the precipitation of calcium phosphate was a key mechanism for phosphate removal in the FA and TFA-NaOH. The TFA-NaOH had a greatest phosphate immobilization, due to high CaO content and an increased SSA after the conversion of fly ash to zeolite. Both Langmuir and Freundlich models were good fitted for the TFA-NaOH, while was only Langmuir model for the FA and Freundlich model for the TFA-HCl. Results suggested that treating fly ash with alkaline solution was a promising way to enhance phosphate immobilization. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Increasing demand for energy throughout the world has led to an increase in the utilization of coal and, subsequently, in producing large quantities of fly ash as a waste product [1,2]. The Mae-Moh electric power station is one of Thailand’s largest electric power plants utilizing low quality lignite and providing a plenty of fly ash [3]. Despite a considerable portion of fly ash is used in relevant industry, such as construction or soil amendment, there is still a large portion that is directly disposed to the environment [4]. Such disposal is not economic and environmental sounds [5]. Alternative way to utilize fly ash is of interest and can be value enhancement to this material [6]. Since fly ash is enriched with SiO2 and Al2O3, this waste product can be transformed into zeolite-like crystalline materials as a result of chemical treatment [7,8]. Zeolite is porous material with large surface area and cavities of the basket-like frame [9,10].
* Corresponding author. Tel.: +66 2 942 8036; fax: +66 2 942 8715. E-mail address: [email protected] (P. Pengthamkeerati). 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.03.013
Hence, it has been used for various purposes, e.g., absorbent and ion exchanger in water and wastewater treatment [11,12]. Effluents from municipal, industrial plants, and animal farms are a major reason causing eutrophication in natural water bodies [13]. Typically, method for treating these effluents (especially from animal farm effluent) is by biological processes using anaerobic and aerobic ponds in a sequence [14]. However, its efficiency in nutrient removal is still in question. Adsorption is an attractive alternative in removing nutrients (especially phosphorus [P]) from the effluent by using porous materials, such as fly ash and zeolitelike substances [14,15]. This is because fly ash is enriched with oxides of aluminum, iron, and calcium. These oxides can strongly adsorb or precipitation phosphates [4]. Hence, fly ash and its fly ash-derived zeolite can be candidate materials for phosphate removal [4,12]. Although several studies had been reported the phosphate immobilization by fly ash, more information is required to gain a better understanding of the relationship between phosphate sorption properties of fly ashes and their chemical composition [13]. Therefore, this study investigates the physical and chemical changes of Thailand’s lignite fly ash with different chemical agents and their abilities to remove phosphate from an aqueous solution.
2470
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
2. Materials and methods 2.1. Modification of fly ash
Table 1 Chemical composition, surface area, and CEC of the untreated fly ash (FA), HCl-treated fly ash (TFA-HCl), and NaOH-treated fly ash (TFA-NaOH) Constituent
The coal fly ash used in this study was supplied by the Mae-Moh electric power station in the northern part of Thailand. The studied fly ash is a residue from lignite combustion using a pulverized coal firing with the furnace exit gas temperature of 1140 °C. The sample was oven-dried at 105 °C for 24 h before use. The fly ash was treated in a 2 M solution of NaOH or 1 M solution of HCl at a solution to fly ash ratio of 10:1 by weight. The mixture was then incubated at 100 °C for 24 h. At the end of the treatment, the mixture was filtered, washed thoroughly and oven-dried at 105 °C for 24 h. The final solid products were then subjected to physical and chemical analysis. 2.2. Physical and chemical measurements Chemical composition of the initial and treated fly ashes was analyzed by energy dispersive X-ray fluorescence (XRF) (Oxford ED2002 model). The fly ash samples were characterized by X-ray Diffraction (XRD) using Phillips X’Pert diffractometer or Bruker axs (D8 Advance) with Cu K radiation. The samples were scanned over the 5–50° 2h interval. Scanning electron microscopy (SEM) (JEOL JSM-5800LV model) was performed on the samples. Specific surface area (SSA) of the samples was also analyzed by Autosorb-1 analyzer using BET method. Cation exchange capacity (CEC) of the samples was measured according to Schollenberger [16], using neutral NH4OAC and 10% NaCl as saturating and replacing solutions, respectively. 2.3. Phosphate immobilization Batch experiment was carried out to measure the phosphate adsorption by the initial and treated fly ashes. The adsorbent (5 g) was added to 50 ml of synthetic phosphate solutions of varying concentration (100–15,000 mg P l1) prepared from KH2PO4. The mixture was shaken for various times (upto 24 h) at room temperature. After shaking, the mixture was filtered and the supernatant was measured for phosphate concentration by the molybdenum-blue ascorbic acid method with a UV–Vis spectrophotometer (GBC UV/VIS 918 model) [17] and the pH in equilibrium solution using pH meter (EcoScan pH6 model). To understand the immobilization behavior of P on fly ashes, the fly ash samples after performing P immobilization were oven-dried at 70 °C for XRD analysis. The effect of contact time was studied by using 1000 mg P l1, while other studies used 24 h of contact time. Langmuir and Freundlich isotherms were applied to determine the sorption capacity of the fly ashes. Analyses were performed in duplicate. 3. Results and discussion 3.1. Physical and chemical characteristics of treated fly ash Chemical composition of the samples showed in Table 1. The major components of all fly ashes were oxides of Si and Al, and various other oxides. The untreated fly ash (FA), NaOH-treated fly ash (TFA-NaOH) and HCl-treated fly ash (TFA-HCl) had similar compositions. But, a portion of calcium oxide (CaO) in the TFA-HCl decreased from 9.7% in the FA to 4.4% causing an increased portion of all other oxides, especially SiO2. The decreased CaO content in the TFA-HCl was due to the neutralization between HCl and CaO causing dissolution of CaO. XRD patterns of the samples were presented in Fig. 1. The XRD pattern of the FA showed the presences of quartz, mullite, iron
SiO2 Al2O3 CaO Fe2O3 MgO K2O Specific surface area (m2 g1) CEC (meq g1)
Weight (%) FA
TFA-HCl
TFA-NaOH
45.8 13.6 9.7 6.7 0.4 1.6 3.39 0.03
60.9 19.4 4.4 7.0 1.1 2.0 61.84 –
50.1 14.0 9.4 7.2 0.2 0.4 35.38 1.88
oxide, hematite, sulfur oxide, and CaO (Fig. 1a). The TFA-HCl had a similar XRD pattern to the FA, but the peaks of CaO and sulfur oxide were not clearly observed (Fig. 1b). After alkaline treatment, the amounts of quartz, mullite, iron oxide and CaO in the TFANaOH were decreased from the FA, as was observed in the reduction of these peak intensities (Fig. 1c). We also observed the appearance of zeolites, which were NaP1 (Na6Al6Si10O32.12H2O) and hydroxyl-sodalite (1.08 Na2O.Al2O3.1.62SiO2.1.8H2O). This observation indicated the conversion of fly ash into zeolite-like material under alkaline condition. Previous studies also reported a similar observation [8,18,19]. For the TFA-NaOH, the peak intensity of CaO was decreased from the FA (Fig. 1a and c), while the portion of CaO from the XRF analysis was 9.4%, which was almost the same as the FA (9.7%) (Table 1). This observation suggested that CaO partly dissolved in NaOH medium under heat treatment, but the dissolved calcium remained in the TFA-NaOH by changing to other forms of calcium. SEM images of the fly ash samples were shown in Fig. 2. SEM observation of the FA showed the presence of micro-particles in the shape of smooth balls (microspheres) (Fig. 2a and b). After treating with HCl, the ball-shaped particles of FA were partly transformed into agglomerations of undefined shape with no observation of crystal formation (Fig. 2c and d). NaOH solution affected the FA by deforming ball-like shape and changing smooth surface into various shapes of crystals, e.g., plates and rods (Fig. 2e and f). The particle surface of TFA-NaOH showed the formation of zeolites, as was confirmed by the XRD data of this study (Fig. 1c) and was also observed by other studies [8,11,20]. The specific surface area (SSA) of the chemically treated fly ash was greater than that of the untreated FA in the following manner: TFA-HCl > TFA-NaOH > FA (61.84, 35.38, and 3.39 m2 g1, respectively) (Table 1). The greater SSA of TFA-HCl may be attributed to (i) surface modification of the FA particles by acid solution, and (ii) an increase in a portion of micropore after CaO dissolution [20]. The greater SSA of TFA-NaOH was because of a transformation of fly ash into zeolite-like structure as supported by the XRD pattern (Fig. 1c). This observation was in agreement with the study by Sarbak and Kramer-Wachowiak [20], who reported an increased SSA of the fly ash treated with HCl and NaOH, compared to the untreated fly ash (105, 59, and 3 m2 g1, respectively). Penilla et al. [11] and Woolard et al. [18] and found that treating coal fly ash with NaOH increased SSA to 18.5 and <45 m2 g1, respectively. After NaOH treatment, the TFA-NaOH had a greater CEC value (1.88 meq g1) than the FA (0.03 meq g1) (Table 1). The greater CEC of TFA-NaOH was probably due to the formation of zeolites, especially NaP1 in this study (Fig. 1c). Juan et al. [7] also observed an increased CEC value of zeolitized fly ash (NaP1) from the original fly ash (from 0.02 to 1.9 meq g1).
2471
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
a
Intensity
SO
H
Q M
Fe
M H Ca Ca
Q
M
M H
SO
Fe
FA
Q
5
15
25
35
45
55
2Theta
b Q
Intensity
H M M
M
5
Fe
Q
15
M H
25
Q = M = SO = Fe = Ca = H = N = S =
Quartz Mullite Sulfur oxide Iron oxide (Fe 3 O4) Calcium oxide Hematite (Fe 2O3 ) NaP1 (zeolite) Sodalite (zeolite)
M H Fe Q
35
TFA-HCl
45
55
2Theta
c N
Intensity
N S
N N S
N
H Q M M Q
5
15
25
35
M S H
N
45
TFA-NaOH
55
2Theta Fig. 1. XRD patterns of (a) the untreated fly ash (FA), (b) the HCl-treated fly ash (TFA-HCl), and (c) the NaOH-treated fly ash (TFA-NaOH).
3.2. Phosphate immobilization 3.2.1. Effect of contact time The effect of contact time on phosphate immobilization by fly ash was presented in Fig. 3. The sorbed P on fly ash increased with an increase of contact time, in particular of the first 30 min. After 2 to 4 h of sorption, only FA and TFA-NaOH showed an additional sorption capacity of P, but the sorption rate of TFA-NaOH was
slower than that of the FA. A slower rate of P sorption by TFA-NaOH may be because the sorption seemed to be influenced by diffusion through rough surface and channels in the TFA-NaOH, which was different from the smooth surface of the FA (Fig. 2). After 24-h contact time, the P sorption on all studied fly ashes remained nearly constant. The sorption capacities of the FA and TFA-NaOH were almost the same and much greater than that of the TFA-HCl. Previous studies suggested that P immobilization on fly ash was attributed
2472
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
Fig. 2. SEM images of (a and b) the untreated fly ash, (c and d) the HCl-treated fly ash, and (e and f) the NaOH-treated fly ash.
to (i) binding of phosphate and calcium resulting in precipitation of calcium phosphate, and (ii) adsorption of phosphate on the surface of fly ash [13,15,21]. The lowest P sorption capacity by the TFA-HCl may be due to the decreased CaO after HCl treatment, resulting in low Ca2+ concentration for calcium phosphate precipitation. Khelifi et al. [6] also found that HCl-treated blast furnace slag had a lower phosphate sorption capacity than the untreated blast furnace slag. From this present study, the contact time of 24 h was chosen for further studies. 3.2.2. Effect of initial P concentration The relationships between the initial P concentration (C0) and P sorption or equilibrium pH of the studied fly ashes were shown in Fig. 4a and b. P sorption capacity of the FA sharply increased with an increase of P concentration until the C0 of 3,000 mg l1, while the equilibrium pH rapidly decreased from about 12 to 7.5. After this C0, the P sorption capacity remained nearly constant, while
the pH was slightly decreased from about 7.5 to 6. It is known that calcium phosphate precipitation is predominant mechanism in P immobilization at alkaline pH [2,13]. When pH decreased, the concentration of Ca2+ increased, which provided higher Ca2+ concentration for calcium phosphate precipitation [13,15]. Hence, P sorption capacity of the FA increased with increasing the equilibrium pH from about 12 to 7.5. However, with decreasing pH to acidic condition, calcium phosphate precipitation would be less favorable. At neutral and acidic pH, iron (Fe) and aluminum (Al) phases became positively charged and their adsorption capacity with phosphate through ligand exchange would increase [12,15,22]. For example, hematite (Fe2O3) had a point of zero charge (PZC) at the pH of 8.5–9.1 [4]. Below the PZC, hematite was protonated and had the ability to form complex with phosphate ions. According to these reasons, P sorption capacity of the FA remained nearly constant when the pH was in a range of 6– 7.5, indicating that both calcium phosphate precipitation and P
2473
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
Sorbed phosphate (mg P g-1)
12
10
8 FA TFA-HCl TFA-NaOH
6
4
2
0 0
5
10 15 Time (h)
20
25
Fig. 3. The effect of contact time on P immobilization by fly ashes. Experimental conditions: fly ash dose 10% w/v; initial phosphate concentration 1000 mg P l1.
a
70
Sorbed P (mg Pg-1)
60 50 40 30 20 FA TFA-HCl TFA-NaOH
10 0 0
3000
6000
9000
12000
15000
C0 (mg l-1)
b
14
For the TFA-NaOH, the P sorption capacity continuously increased with increasing C0, while the equilibrium pH was decreased, but more gradually and less than that of the FA, from the pH of about 12–8. Among the studied fly ashes, the TFA-NaOH had a greatest P sorption capacity. In term of pH, the greater P sorption capacity of the TFA-NaOH than the FA might be because the pH of 8–12 was in an alkaline condition and in a range that was more favorable for calcium phosphate precipitation than the FA. Only for the TFA-HCl, the P sorption and equilibrium pH slightly increased with C0. The pH increased from 4.3 to 4.9 in a range of the studied P concentrations, but was still in an acidic condition. P adsorption onto mineral surfaces of Fe and Al was regulated by the electrostatic attractions of the phosphate ions and mineral surface [4]. At acidic pH, the condition was more favorable for phosphate adsorption onto the surfaces of Fe and Al phases than for calcium phosphate precipitation [15]. Based on this information, adsorption may be a key mechanism for P immobilization by the TFA-HCl. 3.2.3. Sorption isotherm Sorption isotherms of the studied fly ashes were shown in Fig. 5. The obtained isotherms of the studied fly ashes were in various shapes as determined by a previous study [23]. The sorption isotherm of the FA increased and then remained constant (H-type), indicating very strong interaction between adsorbate and adsorbent. The sorption isotherm of the TFA-NaOH was classified into the L-type or Langmuir isotherm. The L-type isotherm also suggested a strong interaction between adsorbate and adsorbent, but relatively less than the H-type isotherm. For the TFA-HCl, the sorption isotherm was in the S-type, suggesting cooperative adsorption and favoring the clustering of adsorbate at the surface. These data supported the finding in the effect of contact time on phosphate immobilization that the precipitation and adsorption were key processes in phosphate immobilization by the studied fly ashes. For the FA and TFA-NaOH, calcium phosphate precipitation seemed to be a significant mechanism for P immobilization rather than the adsorption. But the TFA-HCl immobilized phosphate mainly by adsorption. In addition, the greatest P sorption capacity of the TFA-NaOH suggested that this treated fly ash could further adsorb phosphate much greater than the FA and TFA-HCl, due to its higher calcium content, greater surface area, and changes
10
70
8
60
6
50
5 4
pH
3
4 2 0 0
3000
6000
9000
12000
15000
C0 (mg l-1) Fig. 4. The relationships between the initial P concentration (C0) and P sorption (a) or equilibrium pH (b) of the fly ash (FA), HCl-treated fly ash (TFA-HCl), and NaOHtreated fly ash (TFA-NaOH). Experimental conditions: fly ash dose 10% w/v; contact time 24 h.
adsorption on the surface of Fe and Al phases played a role in phosphate removal.
Sorbed P (mg Pg-1)
2
40
1 0
500
1000
1500
0 2000
Sorbed P (mg P g-1)
12
Ce (mg l-1)
30 20 FA TFA-HCl TFA-NaOH
10 0 0
2000
4000
6000
8000
10000
Ce (mg l-1) Fig. 5. P sorption isotherms of the fly ash (FA), HCl-treated fly ash (TFA-HCl), and NaOH-treated fly ash (TFA-NaOH). Experimental conditions: fly ash dose 10% w/v; contact time 24 h. Ce is equilibrium concentration of P (mg l1). Inset figure shows an enlarge P sorption isotherm of the TFA-HCl.
2474
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
in fly-ash properties during the conversion of fly ash to zeolite. Despite zeolites are aluminosilicates and can probably be considered to have an insignificant capability to adsorbed P [4,12], the conversion of fly ash to zeolite had a positive effect on P sorption capacity due to the following reasons. The formation of rough surface structure and an increased surface area after the NaOH treatment favored the adsorption and precipitation reaction of phosphate [15]. Chen et al. [15] and Wu et al. [21] also suggested that the increase in phosphate immobilization capacity after zeolite synthesis process was mainly attributed to the increase in specific surface area and the increase in the contents of the dissociated Fe2O3. Chen et al. [15] also observed that Fe played an increasingly important role in immobilizing phosphate after zeolite synthesis. Despite the effect of exchangeable Ca2+ on P sorption was not well investigated in this study, in calcium saturated condition, a substantially increased CEC of the zeolitized fly ash could have an advantage on the availability of exchangeable Ca2+ on the negatively charged zeolite surface [21]. This exchangeable Ca2+ may enhance phosphate immobilization through the precipitation of calcium phosphate. Sorption data were fitted to the Langmuir and Freundlich models. Langmuir equation : C e =qe ¼ 1=ðQ 0 K L Þ þ ð1=Q 0 ÞC e 1
ð1Þ
1
where qe (mg g ) and Ce (mg l ) are the amounts of adsorbed dye per unit weigh of adsorbent and equilibrium dye concentration in solution, respectively. Q0 (mg g1) is a maximum adsorption capacity of the dye (forming a monolayer) per unit weight of adsorbent. KL is a constant related to the affinity of the binding sites (l mg1). Freundlich equation : ln qe ¼ ln K f þ ð1=nÞ ln C e
immobilization. Previous study suggested that phosphate immobilization by fly ash was governed by Ca contents of CaO and CaSO4 and Fe content [13]. The untreated fly ash used in this study gave a similar Q0 to medium-Ca fly ashes (e.g., 8.32 and 10.94% CaO) with a sorption capacity for phosphate of 24.82 and 28.00 mg P g1, respectively [15]. After NaOH treatment, these medium-Ca fly ashes also had an increased Q0 (34.68–35.69 mg P l1), as was also observed in the TFA-NaOH of this study [6]. The Kf from the Freunlich isotherm suggested that the sorption capacity of TFA-NaOH (1.93 mg g1) was greater than that of TFAHCl (0.08 mg g1). The decreased Kf of the TFA-HCl suggested that decreasing CaO content of the fly ash by acid treatment had a negative effect on phosphate immobilization capacity. Khelifi et al. [6] observed that HCl-treated blast furnace slag had a lower phosphate removal with the Kf of 0.021 mg g1, when compared to the untreated blast furnace slag (7.6 mg g1). In addition, the n values of the TFA-NaOH and TFA-HCl were 2.79 and 1.84, respectively, indicating favorable adsorption (1 < n < 10). 3.2.4. XRD patterns of P-sorbed fly ashes XRD analysis was performed to determine forms of P sorbed on the studied fly ashes using the initial P concentration of 15,000 mg l1. After P sorption, XRD patterns of the FA and TFA-NaOH revealed the formation of calcium phosphate (Fig. 7), but not for the TFA-HCl (data not shown). The calcium phosphate precipitated
400
ð2Þ Ce / qe (g l-1)
300
200
100 FA ; Ce/qe = 0.0431 Ce+ 0.8403 TFA-HCl ; Ce/qe = 0.1449 Ce + 151.01 TFA-NaOH ; Ce/qe = 0.0175 Ce + 14.383
0 0
2000
4000
6000
8000
10000
Ce (mg l-1) 5 FA ; ln qe = 0.1812 ln Ce + 1.7949 TFA-HCl ; ln qe = 0.5421 ln Ce - 2.5748 TFA-NaOH ; ln qe = 0.3590ln Ce + 0.6576
4 3
lnqe
where Kf and n are constants indicating adsorption capacity and intensity, respectively. The Freundlich adsorption constant, n, should be in a range of 1–10 for beneficial adsorption. Table 2 showed the sorption parameters and regression coefficients (r2) obtained from the linear regression equation between the values of Ce/qe and Ce for the Langmuir isotherm and between ln qe and ln Ce for the Freundlich isotherm (Fig. 6a and b). The obtained data of the FA was best fit to the Langmuir isotherm with r2 of 0.99, while was the Freundlich isotherm for the TFA-HCl (r2 = 0.90). The TFA-NaOH data was good fit to both Langmuir and Freundlich isotherms (r2 = 0.92 and 0.97, respectively). Khelifi et al. [6] also observed the experimental data of zeolite and acidtreated blast furnace slag were good fit to the Freundlich isotherm. The Q0 from the Langmuir isotherm indicated that the sorption capacity of phosphate decreased in the following manner, TFANaOH (57.14 mg g1) > FA (23.20 mg g1) > TFA-HCl (6.90 mg g1). The greater Q0 of the TFA-NaOH than the FA was because NaOH activation caused an increase of surface area and transformed fly ash into zeolites with a relatively higher sorption capacity, which may favor the P adsorption and precipitation reaction. Comparison of Q0 between the FA (or TFA-NaOH) and TFA-HCl suggested that the amount of CaO was significant to the degree of phosphate
2 1 0
Table 2 The Q0 and KL values for the Langmuir isotherm, the Kf and n values for the Freundlich isotherm and the regression coefficients of equations
-1 -6
Fly ash
Langmuir r2
Kf
n
r2
23.20 6.90
5.1 10 9.6 104
0.99 0.67
6.02 0.08
5.52 1.84
0.75 0.90
57.14
1.2 103
0.92
1.93
2.79
0.97
KL (l mg1)
-4
-2
0
2
4
6
8
10
ln Ce
2
Q0 (mg g1) Untreated fly ash (FA) HCl-treated fly ash (TFA-HCl) NaOH-treated fly ash (TFA-NaOH)
Freundlich
Fig. 6. Langmuir (a) and Freundlich (b) sorption isotherms of P on the fly ash (FA), HCl-treated fly ash (TFA-HCl), and NaOH-treated fly ash (TFA-NaOH). Experimental conditions: fly ash dose 10% w/v; contact time 24 h. Ce and qe are amount of sorbed P (mg P g1) and equilibrium concentration of P (mg l1), respectively. Points: experimental data, lines: predicted linear regression models using Langmuir and Freundlich models.
2475
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
a
CaP1
CaP1
Intensity
CaP1
Q
H CaP1 CaP1 M H
M
M H
Fe FA
Q M
5
15
25
35
45
55
2Theta
b
Intensity
Q M Fe H S
= = = = =
Quartz Mullite Iron oxide (Fe 3O4) Hematite (Fe 2O3) Sodalite (zeolite)
CaP1 = CaPO3OH.2H2O CaP2 = NaCaPO4
CaP2 Q S M
CaP2 H
M H S
TFA-NaOH
Q S
5
CaP2
15
25
35
45
55
2Theta Fig. 7. XRD patterns of (a) the untreated fly ash (FA) and (b) NaOH-treated fly ash (TFA-NaOH) after performing P sorption using an initial P concentration of 15,000 mg l1.
in the forms of calcium phosphate hydroxide hydrate (CaPO3OH.2H2O) for the FA and sodium calcium phosphate (NaCaPO4) for the TFA-NaOH. These forms of calcium phosphate may be promoted in the neutral to mildly alkaline conditions. These findings confirmed that P removal by the FA and TFA-NaOH was due to calcium phosphate precipitation. But for the TFA-HCl, P removal may be attributed to adsorption.
phosphate immobilization capacity (57.14 mg P g1) than the untreated (23.20 mg P g1) and HCl-treated fly ash (6.90 mg P g1), respectively. Sorption isotherm of the untreated fly ash was fitted to Langmuir model, while was Freundlich model for the HCl-treated fly ash. For the NaOH-treated fly ash, both models were good fitted. Overall, the alkaline modification of the fly ash enhanced a capacity of phosphate immobilization and the treated fly ash can be used as a potential adsorbent for phosphate wastewater.
4. Conclusions Acknowledgements Chemical treatment of coal fly ash observed in this study influenced changes in property of treated fly ashes. Sample treated with HCl had the greatest specific surface area due to the dissolution of CaO after acid treatment, and had a similar XRD pattern with the untreated fly ash. NaOH treatment also increased surface area and CEC of the initial fly ash due to particle modification from smooth surface to plate- and rod-shape crystals, which was indicated by the XRD analysis as NaP1 and sodalite zeolites. Results from the sorption study provide a better understanding of the influence of chemical treatment of fly ash on phosphate immobilization behavior. The CaO content in the fly ash and pH at equilibrium solution play an important role in phosphate removal by precipitating calcium phosphate and adsorption. Due to high CaO content and an increased surface area after transformation into zeolites, the fly ash treated with NaOH had a greater
We wish to acknowledge the funding by the Faculty of Science, Kasetsart University (Science, Research Fund, ScRF) and Thai Research Fund for supporting this research. Thanks also to Orawan Singchan, Saowalak Sirijareontanapun, and Siriwan Geawchingduang, for her technical assistance. References [1] Hui KS, Chao CYH. Effects of step-change of synthesis temperature on synthesis of zeolie 4A from coal fly ash. Microporous Mesoporous Mater 2005;88:145–51. [2] Yan J, Kirk DW, Jia CQ, Liu X. Sorption of aqueous phosphorus onto bituminous and lignitous coal ashes. J Hazard Mater 2007;148:395–401. [3] Chareonpanich M, Namto T, Kongkachuichay P, Limtrakul J. Synthesis of ZSM-5 zeolite from lignite fly ash and rice husk ash. Fuel Process Technol 2004;85:1623–34.
2476
P. Pengthamkeerati et al. / Fuel 87 (2008) 2469–2476
[4] Grubb DG, Guimaraes MS, Valencia R. Phosphate immobilization using an acidic type F fly ash. J Hazard Mater 2000;76:217–36. [5] Iyer RS, Scott JA. Power station fly ash – a review of value-added utilization outside of the construction industry. Resource Conserv Recycle 2001;31:217–28. [6] Khelifi O, Kozuki Y, Murakami H, Kurata K, Nishioka M. Nutrients adsorption from seawater by new porous carrier made from zeolited fly ash and slag. Mar Pollut Bull 2002;45:311–5. [7] Juan R, Hernandez S, Querol X, Andres JM, Moreno N. Zeolite material synthesized from fly ash: use as cationic exchanger. J Chem Technol Biotechnol 2002;77:299–304. [8] Moutsatsou A, Stamatakis E, Hatzizotzia K, Protonotarios V. The utilization of Ca-rich and Ca–Si-rich fly ashes in zeolites production. Fuel 2006;85:657–63. [9] Notario del Pino JS, Arteaga Padron IJ, Gonzalez Martin MM, Garcia Hernandez JE. Phosphorus and potassium release from phillipsite-based slow-release fertilizers. J Controlled Release 1995;34:25–9. [10] Pickering HW, Menzies NW, Hunter MN. Zeolite/rock phosphate – a novel slow release phosphorus fertilizer for potted plant production. Sci Horticulturae 2002;94:333–43. [11] Penilla PR, Bustos AG, Elizalde SG. Immobilization of Cs, Cd, Pb and Cr by synthetic zeolites from Spanish low-calcium coal fly ash. Fuel 2006;85:823–32. [12] Wu D, Zhang B, Li C, Zhang Z, Kong H. Simultaneous removal of ammonium and phosphate by zeolite synthesized from fly ash as influenced by salt treatment. J Colloid Interface Sci 2006;304:300–6. [13] Chen J, Kong H, Wu D, Chen X, Zhang D, Sun Z. Phosphate immobilization from aqueous solution by fly ashes in relation to their composition. J Hazard Mater 2007;139:293–300.
[14] Bolan NS, Wong L, Andriano DC. Nutrient removal from farm effluents. Bioresource Technol 2004;94:251–60. [15] Chen J, Kong H, Wu D, Hu Z, Wang Z, Wang Y. Removal of phosphate from aqueous solution by zeolite synthesized from fly ash. J Colloid Interface Sci 2006;300:491–7. [16] Schollenberger CJ, Simen RH. Determination of exchange capacity and exchangeable bases in soil-ammonium acetate method. Soil Sci 1954;59:13–24. [17] American Public Health Association (APHA). Standards for examination of water and wastewater. Washington DC, 1995. [18] Woolard CD, Petrus K, van der Horst M. The use of a modified fly ash as an adsorbent for lead. Water SA 2000;26:531–6. [19] Querol X, Moreno N, Umana JC, Alastuey A, Hernandez E, Lopez-Soler A, et al. Synthesis of zeolites from coal fly ash: an overview. Int J Coal Geol 2002;50:413–23. [20] Sarbak Z, Kramer-Wachowiak M. Porous structure of waste fly ashes and their chemical modifications. Powder Technol 2002;123:53–8. [21] Wu D, Sui Y, Chen X, He S, Wang X, Kong H. Changes of mineralogical– chemical composition, cation exchange capacity, and phosphate immobilization during the hydrothermal conversion process of coal fly ash into zeolite. Fuel 2007. doi:10.1016/j.fuel.2007.10.028. [22] Zhang B, Wu D, Wang C, He S, Zhang Z, Kong H. Simultaneous removal of ammonium and phosphate by zeolite synthesized from coal fly ash as influenced by acid treatment. J Environ Sci 2007:540–5. [23] McBride MB. Environmental chemistry of soils. New York: Oxford University Press; 1994.