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Simultaneous crystallization of phosphate and potassium as magnesium potassium phosphate using bubble column reactor with draught tube
Journal of Environmental Chemical Engineering 1 (2013) 1154–1158
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Simultaneous crystallization of phosphate and potassium as magnesium potassium phosphate using bubble column reactor with draught tube Yamaguchi Satoshi a, Ohura Seichiro a, Harada Hiroyuki b,*, Akagi kotaro b, Yoshiharu Mitoma b, Kawakita Hidetaka a, Biplob K. Biswas c a b c
Department of Applied Chemistry, Saga University, Honjo, Saga 840-8502, Japan Department of Life and Environmental Science, Prefectural Hiroshima University, 562 Nanatsuka, Shobara Hiroshima 727-0023, Japan Department of Applied Chemistry and Chemical Engineering, Jessore Science and Technology University, Shadhinota Shorok, Jessore 7408 Bangladesh
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 March 2013 Received in revised form 25 August 2013 Accepted 27 August 2013
Livestock water drainage after secondary treatment contains potassium and phosphorus is influenced by the efficiency of the processing method. In this study, we examined the effect of ammonium ion concentration on the simultaneous recovery of coexisting potassium and phosphorus. The ammonium nitrogen, potassium, and phosphorus concentrations were 0.3–8.3 mM, 0.0–25.6 mM, and 7.6 mM, respectively. Magnesium chloride at a concentration of 3 mM was added to a synthetic wastewater bubble column with a draught tube, and the pH was maintained at around 10 using a pH controller. The hydraulic retention time was 2.3 h. When the ammonium nitrogen concentration was less than 1.1 mM, the decreases in the molar ratios of magnesium, potassium, and phosphorus in the liquid almost corresponded to the production of magnesium potassium phosphate (MPP). Also, MPP was confirmed to be the main component of crystals obtained at low ammonia nitrogen concentrations, using various analytical methods. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: Phosphorus Potassium Wastewater crystallization Recovery Bubble column
Introduction Phosphorus (P) and potassium (K) are widely used as chemical fertilizers, pesticides, insecticides, raw materials, and food additives. In particular, the demand for P and K for use in fertilizers will increase in the future because increased food production will be required as the global population increases. The future depletion of P and K resources is a cause for concern. The secondary effluent from livestock wastewater still contains large amounts of phosphorus (5.5 mM) and potassium ions (63.9 mM) [1]. This causes red tides when treated water is released into public water areas [1–3]. It is necessary to recover P and K ions not only to prevent environmental problems, but also to avoid loss of waste resources. Biological and coagulation methods are widely used for P removal [4]. These methods are used for the treatment of water with P concentrations corresponding to influent sewage concentrations. Coagulation–sedimentation methods have the disadvantage of producing large volumes of sludge [5]. Adsorption methods are promising for P removal from low-phosphate concentration wastewater [6]. In the last decade, many studies of P removal have been reported, and many adsorbents have been used, e.g.,
* Corresponding author. Tel.: +81 0724741758; fax: +81 0724741758. E-mail address: [email protected] (H. Hiroyuki). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.08.032
lanthanum-hydroxide-doped activated carbon [7], lanthanumoxide-loaded activated carbon [8], aluminum hydroxide [9], aluminum iron oxide [10], iron oxide [11], and silica [12]. In recent years, the development of adsorbents with high P-adsorption capacities has been pursued for non-toxic and environmentally friendly methods of P removal [13]. Adsorbed P was reported to be easily eluted with alkaline solutions [14]. P recovery from municipal and livestock wastewaters by recrystallization has been studied with a view to recycling. Various recrystallization methods from P-containing wastewaters, e.g., as MgNH4PO46H2O (MAP) [15] and hydroxyapatite [Ca5(PO4)3OH, HAP] [2,16], have already been reported. The solubility product of magnesium potassium phosphate (MPP) is larger than those of MAP and HAP, but MPP formation is a suitable method for treating secondary effluents containing high concentrations of potassium and phosphorus, such as livestock wastewater. Crystallization of MPP from secondary treated water has been studied by few researchers. When the solution pH is greater than 10, Mg reacts with hydroxide ions, selectively precipitating Mg(OH)2, so MPP is not crystallized. To prevent the precipitation of Mg(OH)2 at high pH, pH control is important to suppress generation of Mg(OH)2. Stirring with an impeller [17–19] or a gas [20,21] has been used in crystallization reactors. A bubble column with a draught tube was used in this study, because the shear stress and adhesion of particles in the bubble column are lower
[(Fig._1)TD$IG]
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than those produced in mechanical stirring. Fluid is circulated by the pressure difference caused by air fed into the tube. Previous studies have reported the simultaneous recovery of phosphorus and potassium by adding magnesium in urine or synthetic urine [17,18,22]. However, in the presence of N, MAP was generated more selectively than MPP. The coprecipitation of MPP and magnesium sodium phosphate (MgNaPO47H2O; MSP) also occurred when Na+ was present in the mixed solution [22], making the recovery of highly pure MPP difficult. N and Na can influence the MPP particle purity, but the impact of sodium ions is small at pH 10. The concentration of N in secondary treated water is influenced by the efficiency of the processing method. Understanding the influence of ammonium ions on particles and liquids is therefore important for the simultaneous recovery of potassium and phosphorus by the MPP method. Experimental Materials Sodium dihydrogen phosphate and magnesium chloride were obtained from the Kanto Chemical Co. Inc. (Tokyo/Japan), potassium chloride and ammonium chloride were obtained from SigmaAldrich. Sodium hydroxide was obtained from Wako Pure Chemical Industries Ltd. All chemicals and reagents were of analytical grade and used without further purification. Recovery of phosphate using bubble column
Fig. 1. Experimental set-up.
The effect of NH4+ was investigated by changing the concentrations of ammonium (0.3–8.3 mM) and potassium ions (0.0– 25.6 mM). The experimental conditions are summarized in Table 1. Fig. 1 shows the experimental setup of the bubble column used in this study. The diameter, height, and volume of the bubble column were 350 mm, 600 mm, and 11.5 d m3, respectively. There was a draught tube structure inside the bubble column. Air was fed, using an air pump, from the bottom of the tower at a rate of 90 d m3/h. Crystals were collected from the bottom of the reactor after the experiments [23]. The bubble column was first filled with synthetic wastewater, and then potassium dihydrogen phosphate and magnesium chloride solutions were fed from the outer reactor to the inner tubes, in a molar flow ratio of 2.5:1; the flow rates of the phosphorus and magnesium solutions were set at 5.0 d m3/h and 0.6 d m3/h, respectively. The pH of the fluid in the tower was adjusted to 10 with 2.0 M NaOH solution, using a pH controller. After filtering the reactor solution, a white precipitate was obtained; it was desiccated at 40 8C for 24 h and sintered at 700 8C for 12 h to improve the crystal quality.
Since nitrate ions accumulate as a result of nitrification, also the effect of nitrate ion on the PPM generation was discussed. The initial concentrations in the reactor of phosphorus, potassium, and nitrate ions were 6.7, 25.6, and 8.3 mM, respectively, and pH control and Mg addition were the same as in the previous experiments. Analytical methods The concentrations of PO43 and NH4+ were determined using the molybdenum blue/ascorbic acid method and the indophenol method, with UV and visible spectroscopy (Jasco, V-630BIO), respectively. The concentrations of potassium and magnesium ions were measured using a polarized Zeeman atomic absorption spectrophotometer (Hitachi Z-2000). The white precipitate was dissolved in 3.0 M HCl for determination of the crystalline components. Scanning electron microscopy (SEM; JEOL, JCM5100) was used to observe the surface morphology of the crystals.
Table 1 Characteristics of treatment. Run no.
PO4-P (P) [mM]
K+ (K) [mM]
NH4-N (N) [mM]
Mg2+ (Mg) [mM]
1
Calculated concentrations in the reactor Effluent concentrations
7.6 4.1
25.5 1.5
8.5 3.5
3.0 0.0
2
Calculated concentrations in the reactor Effluent concentrations
7.7 4.4
25.5 2.1
4.3 4.3
3.0 0.0
3
Calculated concentrations in the reactor Effluent concentrations
7.6 3.9
24.1 3.7
1.1 0.4
3.1 0.0
4
Calculated concentrations in the reactor Effluent concentrations
7.6 3.7
22.5 3.3
8.5 0.2
3.0 0.0
5
Calculated concentrations in the reactor Effluent concentrations
7.6 4.0
0.0 0.0
8.9 3.6
3.0 0.0
[(Fig._2)TD$IG]
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The white precipitate was identified using X-ray diffraction (XRD; Rigaku). The morphology and chemical composition of the dried crystals were determined using SEM – energy-dispersive Xray spectrometry (EDX; Hitachi S3000) at an emission voltage of 15 kV. Results and discussion Effect of ammonium ions The potassium and phosphorus concentrations in the synthetic wastewater were based on the average data from the Saga Prefectural Livestock Research Institute (Japan). The concentrations of PO43-P (P) and K+ (K) used were therefore 250 ppm and 1000 ppm. Mg was added in an amount less than equimolar to P to avoid the formation of MSP and MAP. The initial concentrations, the concentrations after crystallization of the solution, and the precipitate yields are shown in Table 1. The P concentration decreased to the same level under all conditions, and the decrease in the amount of K increased, in contrast to the behavior for the reduction of ammonium nitrogen, NH4+-N (N). In experiments 1 and 2 in Table 1, the removal rate of N exceeded 50%, whereas in experiments 3 and 4, the removal rate was less than 50%. MAP was selectively crystallized over MPP in the presence of both N and K, because MAP is less soluble than MPP. Zhao et al. [24] and Tilley et al. [25] reported that K was not detected in the presence of N, and the solidification of P in the liquid competed with MPP and MAP formation; lowering the initial concentration of ammonia resulted in preferential production of MPP. The P removal rate was 50–57% when the hydraulic retention time (defined in Eq. (1)) was maintained at 2.3 h; this was much more consistent than the rate reported in the literature [22]. 3
3
HRT ¼ Total volume of reactor½d m =Influent flow rate½dm
1
h
(1) Characterization of precipitate The precipitate was dissolved to examine the constituent elements. The ratios of PO43 (P), Mg2+ (Mg), K+ (K), and NH4+ (N) in the precipitate are shown in Fig. 2. The concentration of P was normalized. In the presence of N, the solidification of P competed with MPP and MAP formation, as shown in Eqs. (2) and (3).
[(Fig._3)TD$IG] 2þ þ Kþ þ HPO4 2 þ 6H2 O ! MgKPO4 6H2 O Mg
(2)
1.2 Mg
Relative Reduction ratio
1156
N
1
K 0.8
0.6
0.4
0.2
0 N:8.3mM/K:25.6mM
N:4.4mM/K:25.6mM
N:1.1mM/K:25.6mM
N:0.3mM/K:25.6mM
N:8.3mM/K:0mM
Fig. 2. The relative decreasing amounts of the magnesium, potassium when based on the reduction of phosphorus.
Mg2þ þ NH4 þ þ HPO4 2 þ 6H2 O ! MgNH4 PO4 6H2 O
(3)
The solubility products (pKsp) of MAP and MPP were reported to be 12.60–13.36 [26] and 10.62 [27], respectively. MAP has a smaller pKsp than MPP, so MAP is produced selectively when potassium ions and ammonium ions coexist [28]. The decrease in the amount of K is higher at lower initial N concentrations. Only 0.03 mM N moved to the solid phase when the initial ammonium concentration was 0.3 mM. The morphology of the white precipitate was observed using SEM. SEM images of the white precipitate are shown in Fig. 3. MAP and MPP had typical needle-like orthorhombic structures, and no differences were observed under different conditions. Elongated crystal particles crumbled as a result of impingement of particles, resulting in the growth of flat crystals. Fig. 4 shows the XRD patterns, which were used to identify the white precipitate. In Fig. 4(a) (without sintering), four peaks, at 158, 218, 308, and 348, are found. These four peaks well match those reported by Sugiyama et al. [29], indicating MAP formation. The MAP peaks disappeared after sintering and a new peak was generated at 318. MAP decomposed slowly to magnesium pyrophosphate (Mg2P2O7) at room temperature, losing NH4+. This new peak well matched that of Mg2P2O7 reported by Zhang et al. [30]. For N = 0.3 mM and K = 25.6 mM, the MAP peaks disappeared and five peaks emerged, at 138, 218, 278, 378, and 428. These five peaks were assigned to MPP, as reported by Aramendia et al. [31]; the MPP crystallinity increased and the MAP peaks disappeared as a result of decomposition to Mg2P2O7. Impurity elements and the theoretical constituent elements were confirmed using EDX measurements of the precipitate surface. The EDX of the white
Fig. 3. Analysis via SEM of white precipitate (a) [NH4+] = 8.3 mM, [K+] = 25.6 mM (b) [NH4+] = 4.4 mM, [K+] = 25.6 mM (c) [NH4+] = 1.1 mM, [K+] = 25.6 mM (d) [NH4+] = 0.3 mM, [K+] = 25.6 mM (e) [NH4+] = 8.3 mM, [K+] = 0.0 mM.
[(Fig._4)TD$IG]
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Fig. 4. X-ray diffraction pattern of white precipitate (a) sintering before (b) after sintering.
[(Fig._5)TD$IG]
[(Fig._6)TD$IG] Fig. 5. Analysis via EDX of white precipitate ([NH
+ 4 ]
= 0.3 mM, [K+] = 25.6 mM).
precipitate are shown in Figs. 5 and 6. The five peaks at 0.50 keV, 1.20 keV, 2.00 keV, 3.25 keV, and 3.60 keV were oxygen, magnesium, phosphorus, potassium, and elemental potassium, respectively. Carbon was also detected. Sodium was not found at 1.10 keV, indicating that Na+ was not present in the MPP. The coprecipitation of MPP and MgNaPO47H2O (MSP) in the presence of Na+ has been reported [22]. In this study, the pH was adjusted to 10.0 and the Na concentration was 0.1 mM, so the influence of Na was small. The N peak was not found at 0.40 keV because the amount of nitrogen in the precipitate was small. At an initial ammonium concentration of 0.3 mM, the elemental ratio obtained by EDX agreed well with that obtained using a dissolved liquid, so a homogeneous precipitate of MPP in the crystal was obtained. The molar ratios of Mg, K, and P in MPP, based on EDX analysis, were 0.9:1:1, almost corresponding to the theoretical molar ratios; the MPP yield was 36.0%.
Fig. 6. Mapping analysis using EDX ([NH4+] = 0.3 mM, [K+] = 25.6 mM).
[(Fig._7)TD$IG]
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Run.1(BLANK)
Relative Reduction ratio
Run.2(8.3mM Nitrate ion presence condition
1.2 0.9 0.6 0.3 0
Δ K/P
Δ Mg/P
Fig. 7. The relative decreasing amounts of the ammonia and potassium when based on the reduction of phosphorus.
Fig. 7 shows the effect of nitrate. Experiments were performed using the solutions which contain 8.3 mM nitrate ion and no nitrate ion. The amounts of decreases in potassium and magnesium are almost coincided with the decrease in phosphorus in 8.3 mM nitrate ion solution. Approximately the same ratios were obtained, regardless of the presence of nitrate ions, so it was concluded that the effect of nitrate ions on MPP generation is small. Conclusions The simultaneous recovery of phosphorus and potassium by MPP crystallization from synthetic secondary livestock effluent in a bubble column with a draught tube was investigated. When 39% Mg was added relative to the stoichiometric amount of phosphorus, the MPP yield was 36% for N concentrations below 1.1 mM. The crystals were confirmed to be MPP, using various analytical methods. The produced precipitate had a needle-like orthorhombic structure. The effects of nitrate ions and sodium ions on MPP generation were small. It is believed that this method could be used for simultaneous recovery of potassium and phosphorus from livestock wastewater. References [1] T. Tanaka, N. Koike, T. Sto, T. Arai, N. Taira, Recovery of phosphate from Livestock Wastewater by Electrolysis, Journal of Japan Society on Water Environment 32 (2) (2009) 79–85. , http://dx.doi.org/10.2965/jswe.32.79. [2] X. Chen, H. Kong, D. Wu, X. Wang, Y. Lin, Phosphate removal and recovery through crystallization of hydroxyapatite using xonotlite as seed crystal, Journal of Environmental Sciences 21 (2009) 575–580. , http://dx.doi.org/10.1016/S10010742(08)62310-4. [3] A. Kuroda, N. Takiguchi, J. Kato, H. Ohtake, Development of technologies to save phosphorus resources in response to phosphate crisis, Journal of Environmental Biotechnology 4 (2) (2005) 87–94. [4] K. Ogata, S. Morisada, Y. Oinuma, Y. Seida, Y. Nakano, Preparation of adsorbent for phosphate recovery from aqueous solutions based on condensed tannin gel, Journal of Hazardous Materials 192 (2011) 698–703. , http://dx.doi.org/ 10.1016/j.jhazmat.2011.05.073. [5] S.H. Lee, K.H. Yeon, H. Park, S.H. Lee, Y.M. Park, M. Iwamoto, Zirconium mesostructures immobilized in calcium alginate for phosphate removal, Korean Journal of Chemical Engineering 25 (2008) 1040–1046. , http://dx.doi.org/10.1007/ s11814-008-0170-7. [6] Y. Bashan, L.E. de-Bashan, Recent advances in removing phosphorus from wastewater and its future use as fertilizer, Water Research 38 (2004) 4222–4246. , http://dx.doi.org/10.1016/j.watres.2004.07.014. [7] L. Zhang, Q. Zhou, J. Liu, N. Chang, L. Wan, J. Chen, Phosphate adsorption on lanthanum hydroxide-doped activated carbon fiber, Chemical Engineering Journal 185/186 (2012) 160–167.
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Gao, Nutrients removal and recovery by crystallization of magnesium ammonium phosphate from synthetic swine wastewater, Chemosphere 69 (2007) 319–324. , http://dx.doi.org/10.1016/j.chemosphere.2007.06.001 (PMid: 17619051). [16] H. Jang, S.H. Kang, Phosphorus removal using cow bone in hydroxyapatite crystallization, Water Research 36 (2002) 1324–13300. , http://dx.doi.org/ 10.1016/S0043-1354(01)00329-3. [17] K. Xu, C. Wanga, H. Liu, Y. Qian, Simultaneous removal of phosphorus and potassium from synthetic urine through the precipitation of magnesium potassium phosphate hexahydrate, Chemosphere 84 (2011) 207–212. , http:// dx.doi.org/10.1016/j.chemosphere.2011.04.057 (PMid: 21596418). [18] J.A. Wilsenach, C.A.H. Schuurbiers, M.C.M. van Loosdrecht, Phosphate and potassium recovery from source separated urine through struvite precipitation, Water Research 41 (2007) 458–466. , http://dx.doi.org/10.1016/j.watres.2006.10.014. [19] E.H. Kim, S.B. Yiu, H.C. Jung, E.J. Lee, Hydroxyapatite crystallization from a hightly concentrated phosphate solution using powdered converter slag as a seed material, Journal of Hazardous Materials B136 (2006) 690–697. , http://dx.doi.org/ 10.1016/j.jhazmat.2005.12.051 (PMid: 16504382). [20] M.Y. Chisti, B. Halard, M. Moo-Young, Liquid circulation in airlift reactors, Chemical Engineering Science 43 (3) (1988) 451–457. , http://dx.doi.org/ 10.1016/0009-2509(88)87005-2. [21] T. Hano, M. Matsumoto, K. Kuribayashi, Y. Hatate, Biological nitrogen removal in a bubble column with a draught tube, Chemical Engineering Science 47 (1992) 3737–3744. , http://dx.doi.org/10.1016/0009-2509(92)85092-P. [22] K. Xu, C. Wang, X. Wang, Y. Qian, Laboratory experiments on simultaneous removal of K and P from synthetic and real urine for nutrient recycle by crystallization of magnesium–potassium–phosphate–hexahydrate in a draft tube and baffle reactor, Chemosphere 88 (2012) 219–223. , http://dx.doi.org/10.1016/ j.chemosphere.2012.02.061 (PMid: 22445958). [23] H. Shirage, Recovery of phosphate using MAP method, Journal of Environmental Biotechnology 4 (2) (2005) 109–115. [24] Q. Zhao, Z. Liu, W. Li, W. Qiu, J. Wang, Phosphorus removal and recovery from human urine with MAP crystallization, Journal of Environmental Sciences 28 (2007) 2223–2229 (in Chinese). [25] E. Tilley, J. Atwater, D. Mavinic, Effects of storage on phosphorus recovery from urine, Journal of Environmental Technology 29 (2008) 807–816. , http:// dx.doi.org/10.1080/09593330801987145 (PMid: 18697522). [26] M. Ronteltap, M. Maurer, W. Gujer, Struvite precipitation thermodynamics in source-separated urine, Water Research 41 (2007) 977–984. , http://dx.doi.org/ 10.1016/j.watres.2006.11.046 (PMid: 17258264). [27] A.W. Taylor, E.L. Gurney, A.W. Frazier, Solubility products of magnesium ammonium and magnesium potassium phosphates, Transactions of the Faraday Society 59 (1963) 1580–1584. , http://dx.doi.org/10.1039/tf9635901580. [28] A. Gunay, D. Karadag, I. Tosun, M. Ozturk, Use of magnesit as a magnesium source for ammonium removal from leachate, Journal of Hazardous Materials 156 (2008) 619–623. , http://dx.doi.org/10.1016/j.jhazmat.2007.12.067 (PMid: 18243541). [29] S. Sugiyama, M. Fujii, K. Fukuta, K. Seyama, K.I. Sotowa, N. Shigemoto, Preparation of alkaline earth phosphates with sol containing sodium alginate and sodium diphosphate, Journal of Colloid and Interface Science 295 (2006) 141–147. , http://dx.doi.org/10.1016/j.jcis.2005.08.014 (PMid: 16154579). [30] S. Zhang, Y. Huang, H.J. 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Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Simultaneous crystallization of phosphate and potassium as magnesium potassium phosphate using bubble column reactor with draught tube Yamaguchi Satoshi a, Ohura Seichiro a, Harada Hiroyuki b,*, Akagi kotaro b, Yoshiharu Mitoma b, Kawakita Hidetaka a, Biplob K. Biswas c a b c
Department of Applied Chemistry, Saga University, Honjo, Saga 840-8502, Japan Department of Life and Environmental Science, Prefectural Hiroshima University, 562 Nanatsuka, Shobara Hiroshima 727-0023, Japan Department of Applied Chemistry and Chemical Engineering, Jessore Science and Technology University, Shadhinota Shorok, Jessore 7408 Bangladesh
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 March 2013 Received in revised form 25 August 2013 Accepted 27 August 2013
Livestock water drainage after secondary treatment contains potassium and phosphorus is influenced by the efficiency of the processing method. In this study, we examined the effect of ammonium ion concentration on the simultaneous recovery of coexisting potassium and phosphorus. The ammonium nitrogen, potassium, and phosphorus concentrations were 0.3–8.3 mM, 0.0–25.6 mM, and 7.6 mM, respectively. Magnesium chloride at a concentration of 3 mM was added to a synthetic wastewater bubble column with a draught tube, and the pH was maintained at around 10 using a pH controller. The hydraulic retention time was 2.3 h. When the ammonium nitrogen concentration was less than 1.1 mM, the decreases in the molar ratios of magnesium, potassium, and phosphorus in the liquid almost corresponded to the production of magnesium potassium phosphate (MPP). Also, MPP was confirmed to be the main component of crystals obtained at low ammonia nitrogen concentrations, using various analytical methods. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: Phosphorus Potassium Wastewater crystallization Recovery Bubble column
Introduction Phosphorus (P) and potassium (K) are widely used as chemical fertilizers, pesticides, insecticides, raw materials, and food additives. In particular, the demand for P and K for use in fertilizers will increase in the future because increased food production will be required as the global population increases. The future depletion of P and K resources is a cause for concern. The secondary effluent from livestock wastewater still contains large amounts of phosphorus (5.5 mM) and potassium ions (63.9 mM) [1]. This causes red tides when treated water is released into public water areas [1–3]. It is necessary to recover P and K ions not only to prevent environmental problems, but also to avoid loss of waste resources. Biological and coagulation methods are widely used for P removal [4]. These methods are used for the treatment of water with P concentrations corresponding to influent sewage concentrations. Coagulation–sedimentation methods have the disadvantage of producing large volumes of sludge [5]. Adsorption methods are promising for P removal from low-phosphate concentration wastewater [6]. In the last decade, many studies of P removal have been reported, and many adsorbents have been used, e.g.,
* Corresponding author. Tel.: +81 0724741758; fax: +81 0724741758. E-mail address: [email protected] (H. Hiroyuki). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.08.032
lanthanum-hydroxide-doped activated carbon [7], lanthanumoxide-loaded activated carbon [8], aluminum hydroxide [9], aluminum iron oxide [10], iron oxide [11], and silica [12]. In recent years, the development of adsorbents with high P-adsorption capacities has been pursued for non-toxic and environmentally friendly methods of P removal [13]. Adsorbed P was reported to be easily eluted with alkaline solutions [14]. P recovery from municipal and livestock wastewaters by recrystallization has been studied with a view to recycling. Various recrystallization methods from P-containing wastewaters, e.g., as MgNH4PO46H2O (MAP) [15] and hydroxyapatite [Ca5(PO4)3OH, HAP] [2,16], have already been reported. The solubility product of magnesium potassium phosphate (MPP) is larger than those of MAP and HAP, but MPP formation is a suitable method for treating secondary effluents containing high concentrations of potassium and phosphorus, such as livestock wastewater. Crystallization of MPP from secondary treated water has been studied by few researchers. When the solution pH is greater than 10, Mg reacts with hydroxide ions, selectively precipitating Mg(OH)2, so MPP is not crystallized. To prevent the precipitation of Mg(OH)2 at high pH, pH control is important to suppress generation of Mg(OH)2. Stirring with an impeller [17–19] or a gas [20,21] has been used in crystallization reactors. A bubble column with a draught tube was used in this study, because the shear stress and adhesion of particles in the bubble column are lower
[(Fig._1)TD$IG]
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than those produced in mechanical stirring. Fluid is circulated by the pressure difference caused by air fed into the tube. Previous studies have reported the simultaneous recovery of phosphorus and potassium by adding magnesium in urine or synthetic urine [17,18,22]. However, in the presence of N, MAP was generated more selectively than MPP. The coprecipitation of MPP and magnesium sodium phosphate (MgNaPO47H2O; MSP) also occurred when Na+ was present in the mixed solution [22], making the recovery of highly pure MPP difficult. N and Na can influence the MPP particle purity, but the impact of sodium ions is small at pH 10. The concentration of N in secondary treated water is influenced by the efficiency of the processing method. Understanding the influence of ammonium ions on particles and liquids is therefore important for the simultaneous recovery of potassium and phosphorus by the MPP method. Experimental Materials Sodium dihydrogen phosphate and magnesium chloride were obtained from the Kanto Chemical Co. Inc. (Tokyo/Japan), potassium chloride and ammonium chloride were obtained from SigmaAldrich. Sodium hydroxide was obtained from Wako Pure Chemical Industries Ltd. All chemicals and reagents were of analytical grade and used without further purification. Recovery of phosphate using bubble column
Fig. 1. Experimental set-up.
The effect of NH4+ was investigated by changing the concentrations of ammonium (0.3–8.3 mM) and potassium ions (0.0– 25.6 mM). The experimental conditions are summarized in Table 1. Fig. 1 shows the experimental setup of the bubble column used in this study. The diameter, height, and volume of the bubble column were 350 mm, 600 mm, and 11.5 d m3, respectively. There was a draught tube structure inside the bubble column. Air was fed, using an air pump, from the bottom of the tower at a rate of 90 d m3/h. Crystals were collected from the bottom of the reactor after the experiments [23]. The bubble column was first filled with synthetic wastewater, and then potassium dihydrogen phosphate and magnesium chloride solutions were fed from the outer reactor to the inner tubes, in a molar flow ratio of 2.5:1; the flow rates of the phosphorus and magnesium solutions were set at 5.0 d m3/h and 0.6 d m3/h, respectively. The pH of the fluid in the tower was adjusted to 10 with 2.0 M NaOH solution, using a pH controller. After filtering the reactor solution, a white precipitate was obtained; it was desiccated at 40 8C for 24 h and sintered at 700 8C for 12 h to improve the crystal quality.
Since nitrate ions accumulate as a result of nitrification, also the effect of nitrate ion on the PPM generation was discussed. The initial concentrations in the reactor of phosphorus, potassium, and nitrate ions were 6.7, 25.6, and 8.3 mM, respectively, and pH control and Mg addition were the same as in the previous experiments. Analytical methods The concentrations of PO43 and NH4+ were determined using the molybdenum blue/ascorbic acid method and the indophenol method, with UV and visible spectroscopy (Jasco, V-630BIO), respectively. The concentrations of potassium and magnesium ions were measured using a polarized Zeeman atomic absorption spectrophotometer (Hitachi Z-2000). The white precipitate was dissolved in 3.0 M HCl for determination of the crystalline components. Scanning electron microscopy (SEM; JEOL, JCM5100) was used to observe the surface morphology of the crystals.
Table 1 Characteristics of treatment. Run no.
PO4-P (P) [mM]
K+ (K) [mM]
NH4-N (N) [mM]
Mg2+ (Mg) [mM]
1
Calculated concentrations in the reactor Effluent concentrations
7.6 4.1
25.5 1.5
8.5 3.5
3.0 0.0
2
Calculated concentrations in the reactor Effluent concentrations
7.7 4.4
25.5 2.1
4.3 4.3
3.0 0.0
3
Calculated concentrations in the reactor Effluent concentrations
7.6 3.9
24.1 3.7
1.1 0.4
3.1 0.0
4
Calculated concentrations in the reactor Effluent concentrations
7.6 3.7
22.5 3.3
8.5 0.2
3.0 0.0
5
Calculated concentrations in the reactor Effluent concentrations
7.6 4.0
0.0 0.0
8.9 3.6
3.0 0.0
[(Fig._2)TD$IG]
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The white precipitate was identified using X-ray diffraction (XRD; Rigaku). The morphology and chemical composition of the dried crystals were determined using SEM – energy-dispersive Xray spectrometry (EDX; Hitachi S3000) at an emission voltage of 15 kV. Results and discussion Effect of ammonium ions The potassium and phosphorus concentrations in the synthetic wastewater were based on the average data from the Saga Prefectural Livestock Research Institute (Japan). The concentrations of PO43-P (P) and K+ (K) used were therefore 250 ppm and 1000 ppm. Mg was added in an amount less than equimolar to P to avoid the formation of MSP and MAP. The initial concentrations, the concentrations after crystallization of the solution, and the precipitate yields are shown in Table 1. The P concentration decreased to the same level under all conditions, and the decrease in the amount of K increased, in contrast to the behavior for the reduction of ammonium nitrogen, NH4+-N (N). In experiments 1 and 2 in Table 1, the removal rate of N exceeded 50%, whereas in experiments 3 and 4, the removal rate was less than 50%. MAP was selectively crystallized over MPP in the presence of both N and K, because MAP is less soluble than MPP. Zhao et al. [24] and Tilley et al. [25] reported that K was not detected in the presence of N, and the solidification of P in the liquid competed with MPP and MAP formation; lowering the initial concentration of ammonia resulted in preferential production of MPP. The P removal rate was 50–57% when the hydraulic retention time (defined in Eq. (1)) was maintained at 2.3 h; this was much more consistent than the rate reported in the literature [22]. 3
3
HRT ¼ Total volume of reactor½d m =Influent flow rate½dm
1
h
(1) Characterization of precipitate The precipitate was dissolved to examine the constituent elements. The ratios of PO43 (P), Mg2+ (Mg), K+ (K), and NH4+ (N) in the precipitate are shown in Fig. 2. The concentration of P was normalized. In the presence of N, the solidification of P competed with MPP and MAP formation, as shown in Eqs. (2) and (3).
[(Fig._3)TD$IG] 2þ þ Kþ þ HPO4 2 þ 6H2 O ! MgKPO4 6H2 O Mg
(2)
1.2 Mg
Relative Reduction ratio
1156
N
1
K 0.8
0.6
0.4
0.2
0 N:8.3mM/K:25.6mM
N:4.4mM/K:25.6mM
N:1.1mM/K:25.6mM
N:0.3mM/K:25.6mM
N:8.3mM/K:0mM
Fig. 2. The relative decreasing amounts of the magnesium, potassium when based on the reduction of phosphorus.
Mg2þ þ NH4 þ þ HPO4 2 þ 6H2 O ! MgNH4 PO4 6H2 O
(3)
The solubility products (pKsp) of MAP and MPP were reported to be 12.60–13.36 [26] and 10.62 [27], respectively. MAP has a smaller pKsp than MPP, so MAP is produced selectively when potassium ions and ammonium ions coexist [28]. The decrease in the amount of K is higher at lower initial N concentrations. Only 0.03 mM N moved to the solid phase when the initial ammonium concentration was 0.3 mM. The morphology of the white precipitate was observed using SEM. SEM images of the white precipitate are shown in Fig. 3. MAP and MPP had typical needle-like orthorhombic structures, and no differences were observed under different conditions. Elongated crystal particles crumbled as a result of impingement of particles, resulting in the growth of flat crystals. Fig. 4 shows the XRD patterns, which were used to identify the white precipitate. In Fig. 4(a) (without sintering), four peaks, at 158, 218, 308, and 348, are found. These four peaks well match those reported by Sugiyama et al. [29], indicating MAP formation. The MAP peaks disappeared after sintering and a new peak was generated at 318. MAP decomposed slowly to magnesium pyrophosphate (Mg2P2O7) at room temperature, losing NH4+. This new peak well matched that of Mg2P2O7 reported by Zhang et al. [30]. For N = 0.3 mM and K = 25.6 mM, the MAP peaks disappeared and five peaks emerged, at 138, 218, 278, 378, and 428. These five peaks were assigned to MPP, as reported by Aramendia et al. [31]; the MPP crystallinity increased and the MAP peaks disappeared as a result of decomposition to Mg2P2O7. Impurity elements and the theoretical constituent elements were confirmed using EDX measurements of the precipitate surface. The EDX of the white
Fig. 3. Analysis via SEM of white precipitate (a) [NH4+] = 8.3 mM, [K+] = 25.6 mM (b) [NH4+] = 4.4 mM, [K+] = 25.6 mM (c) [NH4+] = 1.1 mM, [K+] = 25.6 mM (d) [NH4+] = 0.3 mM, [K+] = 25.6 mM (e) [NH4+] = 8.3 mM, [K+] = 0.0 mM.
[(Fig._4)TD$IG]
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Fig. 4. X-ray diffraction pattern of white precipitate (a) sintering before (b) after sintering.
[(Fig._5)TD$IG]
[(Fig._6)TD$IG] Fig. 5. Analysis via EDX of white precipitate ([NH
+ 4 ]
= 0.3 mM, [K+] = 25.6 mM).
precipitate are shown in Figs. 5 and 6. The five peaks at 0.50 keV, 1.20 keV, 2.00 keV, 3.25 keV, and 3.60 keV were oxygen, magnesium, phosphorus, potassium, and elemental potassium, respectively. Carbon was also detected. Sodium was not found at 1.10 keV, indicating that Na+ was not present in the MPP. The coprecipitation of MPP and MgNaPO47H2O (MSP) in the presence of Na+ has been reported [22]. In this study, the pH was adjusted to 10.0 and the Na concentration was 0.1 mM, so the influence of Na was small. The N peak was not found at 0.40 keV because the amount of nitrogen in the precipitate was small. At an initial ammonium concentration of 0.3 mM, the elemental ratio obtained by EDX agreed well with that obtained using a dissolved liquid, so a homogeneous precipitate of MPP in the crystal was obtained. The molar ratios of Mg, K, and P in MPP, based on EDX analysis, were 0.9:1:1, almost corresponding to the theoretical molar ratios; the MPP yield was 36.0%.
Fig. 6. Mapping analysis using EDX ([NH4+] = 0.3 mM, [K+] = 25.6 mM).
[(Fig._7)TD$IG]
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Run.1(BLANK)
Relative Reduction ratio
Run.2(8.3mM Nitrate ion presence condition
1.2 0.9 0.6 0.3 0
Δ K/P
Δ Mg/P
Fig. 7. The relative decreasing amounts of the ammonia and potassium when based on the reduction of phosphorus.
Fig. 7 shows the effect of nitrate. Experiments were performed using the solutions which contain 8.3 mM nitrate ion and no nitrate ion. The amounts of decreases in potassium and magnesium are almost coincided with the decrease in phosphorus in 8.3 mM nitrate ion solution. Approximately the same ratios were obtained, regardless of the presence of nitrate ions, so it was concluded that the effect of nitrate ions on MPP generation is small. Conclusions The simultaneous recovery of phosphorus and potassium by MPP crystallization from synthetic secondary livestock effluent in a bubble column with a draught tube was investigated. When 39% Mg was added relative to the stoichiometric amount of phosphorus, the MPP yield was 36% for N concentrations below 1.1 mM. The crystals were confirmed to be MPP, using various analytical methods. The produced precipitate had a needle-like orthorhombic structure. The effects of nitrate ions and sodium ions on MPP generation were small. It is believed that this method could be used for simultaneous recovery of potassium and phosphorus from livestock wastewater. References [1] T. Tanaka, N. Koike, T. Sto, T. Arai, N. Taira, Recovery of phosphate from Livestock Wastewater by Electrolysis, Journal of Japan Society on Water Environment 32 (2) (2009) 79–85. , http://dx.doi.org/10.2965/jswe.32.79. [2] X. Chen, H. Kong, D. Wu, X. Wang, Y. Lin, Phosphate removal and recovery through crystallization of hydroxyapatite using xonotlite as seed crystal, Journal of Environmental Sciences 21 (2009) 575–580. , http://dx.doi.org/10.1016/S10010742(08)62310-4. [3] A. Kuroda, N. Takiguchi, J. Kato, H. Ohtake, Development of technologies to save phosphorus resources in response to phosphate crisis, Journal of Environmental Biotechnology 4 (2) (2005) 87–94. [4] K. Ogata, S. Morisada, Y. Oinuma, Y. Seida, Y. Nakano, Preparation of adsorbent for phosphate recovery from aqueous solutions based on condensed tannin gel, Journal of Hazardous Materials 192 (2011) 698–703. , http://dx.doi.org/ 10.1016/j.jhazmat.2011.05.073. [5] S.H. Lee, K.H. Yeon, H. Park, S.H. Lee, Y.M. Park, M. Iwamoto, Zirconium mesostructures immobilized in calcium alginate for phosphate removal, Korean Journal of Chemical Engineering 25 (2008) 1040–1046. , http://dx.doi.org/10.1007/ s11814-008-0170-7. [6] Y. Bashan, L.E. de-Bashan, Recent advances in removing phosphorus from wastewater and its future use as fertilizer, Water Research 38 (2004) 4222–4246. , http://dx.doi.org/10.1016/j.watres.2004.07.014. [7] L. Zhang, Q. Zhou, J. Liu, N. Chang, L. Wan, J. Chen, Phosphate adsorption on lanthanum hydroxide-doped activated carbon fiber, Chemical Engineering Journal 185/186 (2012) 160–167.
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