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Experiment Study on the Removal of Phosphorus in Eutrophic Water Bodies by the Utilization of Mineral Calcite
EARTH SCIENCE FRONTIERS Volume 15, Issue 4, July 2008 Online English edition of the Chinese language journal Cite this article as: Earth Science Frontiers, 2008, 15(4): 138–141.
RESEARCH PAPER
Experiment Study on the Removal of Phosphorus in Eutrophic Water Bodies by the Utilization of Mineral Calcite XU Hong1, , ZHANG Jing2, GAO Yiming1 1 School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China 2 Research Institute of Petroleum Exploration and Development, China National Petroleum Corporation, Beijing 100083, China
Abstract: For seeking a new method to solve the problem of eutrophication, we have made the experiments of removing phosphorus in eutropic water by use of mineral calcite. The results indicate that the mineral calcite can remove phosphorus from the solution, and that the initial phosphorus concentration may influence the efficiency of phosphorus removal. The dephosphorization rate is high when the initial phosphorus concentration is 5 mg/L, and phosphorus can be removed by 88.48%; the dephosphorization rate may reach 69.94% when the initial phosphorus concentration is 3 mg/L; at 1.2 mg/L initial concentration only 12.68% phosphorus can be removed. Increasing temperature can also raise the efficiency of phosphorus removal. The result of TEM shows that the Ca-P precipitation is not in crystalline state. Key Words: eutrophication; mineral; calcite; P concentration; P removal
1
Introduction
Since the late 1990s, 77% of the lakes and 30.8% of the reservoirs in China have been reported to be polluted with eutrophication[1]. In recent years, the problem has worsened. It is known that P concentration is one of the vital facts for eutrophication. Experiments have been conducted by researchers all over the world trying to solve this problem, for example, the crystal nucleus assisted crystallization process, putting mineral crystals as seeds into bearing-P solution to urge phosphorus precipitation thus decreasing the P concentration to prevent water system from eutrophication. The interaction during the crystal growth of crystallization of calcite and apatite have already been reported in some researches aboard[2–9]. Some researchers hold the view that when calcite and apatite are precipitated simultaneously, the precipitation of phosphate in the solution would restrain the precipitation of calcite and what’s more, the precipitated phosphate would enter the calcite crystal lattice. While other researchers believe that there is no evident phosphorus precipitation when phosphate concentration is <20 ȝmol·dm-3. When phosphate concentration increases, there will be
obviously calcium phosphate phase sedimentary on the surface of calcite and affects the crystallization of calcite[10]. L. J. Plant and W. A. House’s experiment suggests that when solution with Ca(HCO3)2, KH2PO4 and CO32í is alkaline, calcium phosphate will be precipitated during the process of calcite precipitation[11]. Dietfried Donnert and Manfred Saleker’s experiment indicates that calcite crystal seeds can induce the precipitation of calcium phosphate and P concentration decreased in the solution when P concentration of solution is above 20 mg/L[12]. However, some researchers hold the view that calcite crystal seeds have no effect on the crystallization and precipitation of calcium phosphate. The above mentioned experiments show that different experimental objects and experimental conditions result in different conclusions. This article mainly focuses on whether the existence of calcite in P-bearing water bodies can promote dephosphorization and whether phosphorus concentration of water bodies will affect dephosphorization.
2
Experiment of dephosphorization by calcite Experiment reagents and minerals: K2HPO4·3H2O, NaF,
Received date: 06-May-2008; Accepted date: 10-Jun-2008.
Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Grand Fundamental Research 973 Program of China (No. G1999045700). Copyright © 2008, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B.V. All rights reserved.
XU Hong et al. / Earth Science Frontiers, 2008, 15(4): 138–141
CaCl2, calcite (# 160–200). 2.1
Effectiveness of dephosphorization by calcite
To know whether calcite can help with dephosphorization, the experiment was done on six groups of P-bearing solution with the same initial phosphorus concentration at 5.26 mg/L. The composition and concentration of the P-bearing solutions are shown in Table 1. Each group is same in concentration but different in additional materials. Results from all groups (Table 1 and Fig.1) were compared to find that P concentration in group 1-3, 1-4, 1-5 and 1-6 had declined significantly with calcite crystal added as seed. P concentration could drop to below 2.7 mg/L with the lowest value at 0.247 mg/L (group 1-6). However, there is no change in P concentration of solution in the group 1-1, 2-2 with no presence of calcite in the solution. It is obvious that the presence of calcite crystal seed is beneficial for P precipitation. Therefore, it can be concluded that calcite help with reducing the P concentration in water bodies. The experiment by Dietfried Donnert and Manfred Saleker show that calcite can induce the precipitation of high P concentration (20 mg/L). In the above-mentioned experiment, initial phosphorus concentration is only 5.26 mg/L, far below that in Dietfried Donnert and Manfred Saleker’s experiment. This indicates that calcite can still dephosphorize low P concentration solution. 2.2 Effects of calcite dephosphorization in different initial P concentration To further indicate the relationship between the effects of Table 1 Composition and concentration of P-bearing solution Group
K2HPO4·3H2O
NaF
CaCl2
calcite dephosphorization and the P concentration in P-bearing solution, experiments with different initial P concentration in water bodies were conducted. The initial P concentrations were set at 5 mg/L, 3 mg/L and 1.2 mg/L (Table 2) respectively. Table 2 and Figure 2 show the changes in six days and cumulative attenuation amount of phosphorus solution after six days. The experiment shows that calcite can decrease the P concentration in a solution at different degrees. Attenuation rate of P concentration in solutions differs with the initial P concentration. They are 88.48%, 69.94%, 12.68% when the initial P concentration of solution are 5 mg/L, 3 mg/L and 1.2 mg/L respectively. The higher initial P concentration is in the solution, the more effective dephosphorization is. There is no obvious dephosphorization of P-bearing solution when initial P concentration is 1.2 mg/L (Fig. 3, Table 3). 2.3 Effect of temperature on the dephosphorization by calcite When the above experiments were conducted at room temperature with the concentration of P-bearing solution at 1.2 mg/L, there is no clear effect of dephosphorization by calcite. To strengthen the effect of temperature on the experiments, the initial P concentration of solution was set lower at 0.590 mg/L, and then proper amounts of calcite, CaCl2 and NaF were Table 2 Concentrations of components in different experiment groups of solution Group
Calcite/mg
K2HPO4·3H2O/mg
NaF/mg
CaCl2/mg
2-1 (P 5 mg/L)
46.95
2.76
36.21
2-2 (P 3 mg/L)
28.17
1.66
21.73
1000
2-3 (P 1.2 mg/L)
9.39
0.55
7.24
1000
(#160–200) 1000
Calcite
(P 5.26 mg/L)
(F 0.55 mg/L) (Ca 7.24 mg/L) (1 g, #160–200)
Table 3 Variations of P concentration in different experiment
1-1
ƾ
ƾ
groups of solution (mg/L)
1-2
ƾ
1-3
ƾ
1-4
ƾ
1-5
ƾ
1-6
ƾ
ƾ
ƾ
Sequence No. ƾ
ƾ ƾ
ƾ ƾ
ƾ
ƾ
ƾ
ƾ: material composition added in the solution.
2-1 (5 mg/L)
2-2 (3 mg/L)
1 (4 hours)
4.724
2.748
1.199
2 (4 days)
0.777
1.195
1.031
3 (5 days)
0.631
0.935
1.085
4 (6 days) Attenuation rate of P concentration (%)
0.544
0.826
Fig. 2 Fig. 1
Variation of P concentration in different experiment groups
88.48
69.94
2-3 (1.2 mg/L)
1.047 12.68
Variations of P concentration in different experiment groups of solution
XU Hong et al. / Earth Science Frontiers, 2008, 15(4): 138–141
Fig. 3
Variations of P concentration in solution at 80ć
added proportionately. The solution was heated continuously at 80ć and samples were fetched every half an hour 8 times (Figure 3. Serial number 1–8). Four hours later, P concentration of the solution declined from 0.590 mg/L to 0.0108 mg/L below the critical value of water eutrophication. The effect of dephosphorization is better than that at room temperature, which means that heating can strengthen P precipitation. From the above data, conclusion can be made that calcite crystal nucleus can help to lower P concentration in solution, but not in low P concentration (1.2 mg/L) solutions. The effect of dephosphorization strengthens with corresponding amount of Ca and F existing in the solution.
3 Discussion Scientists have conducted dephosphorization experiments with calcite seeds. Experimental results of Dietfried Donnert and Manfred Saleker show that dephosphorization by calcite is effective when phosphorus concentration is above 10 mg/L. Experiments done by Li Zuyin, Lü Jialong[13]on P-fixing characteristics of calcium carbonate shows that dephosphorization by calcite is related to the initial phosphorus concentration of the solution. Phosphorus removal rate is 36.4%–16.3% when phosphorus concentration is between 2.5–10 mg/L and the phosphorus removal rate is 84%–93.8% when the initial phosphorus concentration is between 20–500 mg/L. Presently, it is well known that eutrophication will break out when the soluble P concentration in water body is 0.01–0.025 mg/L (the critical value of eutrophication) with proper flow velocity, temperature and sunshine in water body. So the experiments in this article were conducted at initial P concentration 5 mg/L, 3 mg/L and 1.2 mg/L. The effectivity of dephosporization is well pronounced with initial P concentration 3 mg/L, P removal rate 69.94%, which is obviously lower than that of the experiments by Dietfried Donnert and Li Zuyin. Calcite can remove phosphorus at initial P concentration 1.2 mg/L, and the result of P removal is not considerably under ambient temperature. But under high temperature after dephosphorization by calcite, the soluble P is only 0.0108 mg/L, lower than the critical value of eutrophiacation. That is because high temperature
accelerates ions and cations motion and calcium phosphate precipitation. The experiments above suggest that calcite is an effective factor in dephosphorization. Still some researchers have different views on how calcite helps with the precipitation of phosphorus and whether calcite act as crystal seed during the phosphorus precipitation. Whether calcite act as crystal seed depends on whether phosphorus is attached to the surface of calcite crystal or enters into the calcite crystal lattice. To figure out this problem, deposits of the dephosphorization were analyzed. The analysis was done through transmission electron microscope. First, the nondisposal precipitate of dephosphorization was analyzed with no P-bearing material found. This may result from the low phosphorus content energy spectrum detection line. Then an experiment on precipitate called ultrasonic vibrating was done with the result observed through transmission electron microscope after natural drying of oscillating turbidity liquor. The shapes of the sedimentary particles under the transmission electron microscope were mainly in two types: angular and spherical. The angular one has the diffraction spot which is regarded as crystalline. Its energy spectrum has spectral peak of Ca but no phosphorus was found. Characteristics of diffraction spot should be the fine-grained calcite. In the spherical one, no clear diffraction spot can be found and materials seen are mainly non-crystalline. Through energy spectrum both Ca and P can be found, which means that the sediment is phosphate. The analysis under transmission electron microscope showed that phosphorus precipitated are non-crystalline at room temperature and cannot be precipitated on the surface of calcite and Ca-P materials. In other words, calcite can promote the sedimentary of phosphorus but did not act as crystal seed. There is no evidence for the growth of phosphate on calcite crystal and the phase of apatite is also non-crystalline.
4 Conclusions From our experiments, we can see that calcite has the effect of dephosphorization but phosphorus is not simply attached to the surface of calcite crystal as homogeneous sedimentation. Phosphorus did not enter into the calcite crystal lattice. Series of dephosphorization experiments suggests: (1) Mineral calcite can promote dephosphorization. (2) Initial phosphorus concentration influence P removal. Dephosphorization rate is high at initial P concentration of 5 mg/L, and P can be removed by 88.48%; The dephosphorization rate can reach 69.94% when the phosphorus concentration is 3 mg/L; at 1.2 mg/L initial concentration only 12.68% P could be removed. (3) High temperature increases dephosphorization rate. Phosphorus-bearing solution with initial concentration of 1.2
XU Hong et al. / Earth Science Frontiers, 2008, 15(4): 138–141
mg/L can be dephosphorized by 12.68% at room temperature, but phosphorus concentration can be decreased to 0.0108 mg/L at high temperature, below the critical nutrition-rich level. (4) Ca-P materials precipitated at room temperature are non-crystalline, probably as Ca-P colloids, which are not adhered on the surface of calcite. It is worth pointing out that calcite crystal is used in the experiment. But the crystallization of calcite and apatite is not discussed here; thus no further study is done on the relationship between the crystallization of calcite and the sedimentary of apatite.
hypereutrophic Halfmoon Lake, Alberta. Lake and Reservoir Management, 1994, 8: 131–142. [5]
Dittrich M, Dittrich T, Sieber I, et al. A balance analysis of elimination of phosphorus by artificial calcite precipitation in stratified hardwater lake. Water Research, 1997, 31: 237–248.
[6]
Danen-Louwerse
H
J,
Lijklema
L,
Coenraats
M.
Coprecipitation of phosphate with calcium carbonate in Lake Veluwe. Water Research, 1995, 29: 1781–1785. [7]
Kleiner J. Coprecipitation of phosphate with calcite in lake water: a laboratory experiment modeling phosphorus removal with calcite in Lake Constance. Water Research, 1988, 22: 1259–1265.
[8]
Acknowledgements
Koschel R, Benndorf J, Proft G, et al. Calcite precipitation as a natural
control
mechanism
of
eutrophication.
Archives
Hydrobiology, 1983, 98: 131–139.
The authors thank Professor Huang Huaiceng, Chinese Academy of Geological Sciences, for the support and guidance in the experiment.
[9]
Murphy T P, Hall K J, Yesaki I. Coprecipitation of phosphate with calcite in a natural eutrophic lake. Liminol Oceanogr, 1983, 28: 58–69.
[10] Stumm W, Morgan J J. Aquatic Chemistry, an Introduction
References
Emphasizing Chemical Equilibria in Nature Water. New York: Wiley-Interscience, 1970: 558.
[1]
Ma J A, Li H Q. Preliminary discussion on eutrophication status of lakes, reservoirs and rivers in China and overseas.
inorganic phosphate. Colloids and Surfaces A: Physicochemical
Resources and Environment in the Yangtze Basin, 2002, 11:
and Engineering Aspects, 2002, 203: 143–153.
575–577. [2]
[11] Plant L J, House W A. Precipitation of calcite in the presence of
Okazaki M. Fluoridated apatite synthesized using a multi-step fluoride supply system. Biomaterials, 1999, 20: 1303–1307.
[12] Dietfried D, Manfred S. Elimination of phosphorus from municipal and industrial waste water. Water Science Technology, 1999, 40: 195–202.
[3]
Eggers E. Full-scale experiences with phosphate crystallization in a crystalactor. Water Science Technology, 1986, 23: 819–824.
absorption with calcium carbonate and clayey substance. Soil,
[4]
Babin J, Prepas E E, Murpgy T P, et al. Impact of lime on
1995, 27: 304–310.
sediment phosphorus release in hard water lakes: the case of
[13] Li Z Y, Lü J L. The study of the feature on phosphate
RESEARCH PAPER
Experiment Study on the Removal of Phosphorus in Eutrophic Water Bodies by the Utilization of Mineral Calcite XU Hong1, , ZHANG Jing2, GAO Yiming1 1 School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China 2 Research Institute of Petroleum Exploration and Development, China National Petroleum Corporation, Beijing 100083, China
Abstract: For seeking a new method to solve the problem of eutrophication, we have made the experiments of removing phosphorus in eutropic water by use of mineral calcite. The results indicate that the mineral calcite can remove phosphorus from the solution, and that the initial phosphorus concentration may influence the efficiency of phosphorus removal. The dephosphorization rate is high when the initial phosphorus concentration is 5 mg/L, and phosphorus can be removed by 88.48%; the dephosphorization rate may reach 69.94% when the initial phosphorus concentration is 3 mg/L; at 1.2 mg/L initial concentration only 12.68% phosphorus can be removed. Increasing temperature can also raise the efficiency of phosphorus removal. The result of TEM shows that the Ca-P precipitation is not in crystalline state. Key Words: eutrophication; mineral; calcite; P concentration; P removal
1
Introduction
Since the late 1990s, 77% of the lakes and 30.8% of the reservoirs in China have been reported to be polluted with eutrophication[1]. In recent years, the problem has worsened. It is known that P concentration is one of the vital facts for eutrophication. Experiments have been conducted by researchers all over the world trying to solve this problem, for example, the crystal nucleus assisted crystallization process, putting mineral crystals as seeds into bearing-P solution to urge phosphorus precipitation thus decreasing the P concentration to prevent water system from eutrophication. The interaction during the crystal growth of crystallization of calcite and apatite have already been reported in some researches aboard[2–9]. Some researchers hold the view that when calcite and apatite are precipitated simultaneously, the precipitation of phosphate in the solution would restrain the precipitation of calcite and what’s more, the precipitated phosphate would enter the calcite crystal lattice. While other researchers believe that there is no evident phosphorus precipitation when phosphate concentration is <20 ȝmol·dm-3. When phosphate concentration increases, there will be
obviously calcium phosphate phase sedimentary on the surface of calcite and affects the crystallization of calcite[10]. L. J. Plant and W. A. House’s experiment suggests that when solution with Ca(HCO3)2, KH2PO4 and CO32í is alkaline, calcium phosphate will be precipitated during the process of calcite precipitation[11]. Dietfried Donnert and Manfred Saleker’s experiment indicates that calcite crystal seeds can induce the precipitation of calcium phosphate and P concentration decreased in the solution when P concentration of solution is above 20 mg/L[12]. However, some researchers hold the view that calcite crystal seeds have no effect on the crystallization and precipitation of calcium phosphate. The above mentioned experiments show that different experimental objects and experimental conditions result in different conclusions. This article mainly focuses on whether the existence of calcite in P-bearing water bodies can promote dephosphorization and whether phosphorus concentration of water bodies will affect dephosphorization.
2
Experiment of dephosphorization by calcite Experiment reagents and minerals: K2HPO4·3H2O, NaF,
Received date: 06-May-2008; Accepted date: 10-Jun-2008.
Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Grand Fundamental Research 973 Program of China (No. G1999045700). Copyright © 2008, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B.V. All rights reserved.
XU Hong et al. / Earth Science Frontiers, 2008, 15(4): 138–141
CaCl2, calcite (# 160–200). 2.1
Effectiveness of dephosphorization by calcite
To know whether calcite can help with dephosphorization, the experiment was done on six groups of P-bearing solution with the same initial phosphorus concentration at 5.26 mg/L. The composition and concentration of the P-bearing solutions are shown in Table 1. Each group is same in concentration but different in additional materials. Results from all groups (Table 1 and Fig.1) were compared to find that P concentration in group 1-3, 1-4, 1-5 and 1-6 had declined significantly with calcite crystal added as seed. P concentration could drop to below 2.7 mg/L with the lowest value at 0.247 mg/L (group 1-6). However, there is no change in P concentration of solution in the group 1-1, 2-2 with no presence of calcite in the solution. It is obvious that the presence of calcite crystal seed is beneficial for P precipitation. Therefore, it can be concluded that calcite help with reducing the P concentration in water bodies. The experiment by Dietfried Donnert and Manfred Saleker show that calcite can induce the precipitation of high P concentration (20 mg/L). In the above-mentioned experiment, initial phosphorus concentration is only 5.26 mg/L, far below that in Dietfried Donnert and Manfred Saleker’s experiment. This indicates that calcite can still dephosphorize low P concentration solution. 2.2 Effects of calcite dephosphorization in different initial P concentration To further indicate the relationship between the effects of Table 1 Composition and concentration of P-bearing solution Group
K2HPO4·3H2O
NaF
CaCl2
calcite dephosphorization and the P concentration in P-bearing solution, experiments with different initial P concentration in water bodies were conducted. The initial P concentrations were set at 5 mg/L, 3 mg/L and 1.2 mg/L (Table 2) respectively. Table 2 and Figure 2 show the changes in six days and cumulative attenuation amount of phosphorus solution after six days. The experiment shows that calcite can decrease the P concentration in a solution at different degrees. Attenuation rate of P concentration in solutions differs with the initial P concentration. They are 88.48%, 69.94%, 12.68% when the initial P concentration of solution are 5 mg/L, 3 mg/L and 1.2 mg/L respectively. The higher initial P concentration is in the solution, the more effective dephosphorization is. There is no obvious dephosphorization of P-bearing solution when initial P concentration is 1.2 mg/L (Fig. 3, Table 3). 2.3 Effect of temperature on the dephosphorization by calcite When the above experiments were conducted at room temperature with the concentration of P-bearing solution at 1.2 mg/L, there is no clear effect of dephosphorization by calcite. To strengthen the effect of temperature on the experiments, the initial P concentration of solution was set lower at 0.590 mg/L, and then proper amounts of calcite, CaCl2 and NaF were Table 2 Concentrations of components in different experiment groups of solution Group
Calcite/mg
K2HPO4·3H2O/mg
NaF/mg
CaCl2/mg
2-1 (P 5 mg/L)
46.95
2.76
36.21
2-2 (P 3 mg/L)
28.17
1.66
21.73
1000
2-3 (P 1.2 mg/L)
9.39
0.55
7.24
1000
(#160–200) 1000
Calcite
(P 5.26 mg/L)
(F 0.55 mg/L) (Ca 7.24 mg/L) (1 g, #160–200)
Table 3 Variations of P concentration in different experiment
1-1
ƾ
ƾ
groups of solution (mg/L)
1-2
ƾ
1-3
ƾ
1-4
ƾ
1-5
ƾ
1-6
ƾ
ƾ
ƾ
Sequence No. ƾ
ƾ ƾ
ƾ ƾ
ƾ
ƾ
ƾ
ƾ: material composition added in the solution.
2-1 (5 mg/L)
2-2 (3 mg/L)
1 (4 hours)
4.724
2.748
1.199
2 (4 days)
0.777
1.195
1.031
3 (5 days)
0.631
0.935
1.085
4 (6 days) Attenuation rate of P concentration (%)
0.544
0.826
Fig. 2 Fig. 1
Variation of P concentration in different experiment groups
88.48
69.94
2-3 (1.2 mg/L)
1.047 12.68
Variations of P concentration in different experiment groups of solution
XU Hong et al. / Earth Science Frontiers, 2008, 15(4): 138–141
Fig. 3
Variations of P concentration in solution at 80ć
added proportionately. The solution was heated continuously at 80ć and samples were fetched every half an hour 8 times (Figure 3. Serial number 1–8). Four hours later, P concentration of the solution declined from 0.590 mg/L to 0.0108 mg/L below the critical value of water eutrophication. The effect of dephosphorization is better than that at room temperature, which means that heating can strengthen P precipitation. From the above data, conclusion can be made that calcite crystal nucleus can help to lower P concentration in solution, but not in low P concentration (1.2 mg/L) solutions. The effect of dephosphorization strengthens with corresponding amount of Ca and F existing in the solution.
3 Discussion Scientists have conducted dephosphorization experiments with calcite seeds. Experimental results of Dietfried Donnert and Manfred Saleker show that dephosphorization by calcite is effective when phosphorus concentration is above 10 mg/L. Experiments done by Li Zuyin, Lü Jialong[13]on P-fixing characteristics of calcium carbonate shows that dephosphorization by calcite is related to the initial phosphorus concentration of the solution. Phosphorus removal rate is 36.4%–16.3% when phosphorus concentration is between 2.5–10 mg/L and the phosphorus removal rate is 84%–93.8% when the initial phosphorus concentration is between 20–500 mg/L. Presently, it is well known that eutrophication will break out when the soluble P concentration in water body is 0.01–0.025 mg/L (the critical value of eutrophication) with proper flow velocity, temperature and sunshine in water body. So the experiments in this article were conducted at initial P concentration 5 mg/L, 3 mg/L and 1.2 mg/L. The effectivity of dephosporization is well pronounced with initial P concentration 3 mg/L, P removal rate 69.94%, which is obviously lower than that of the experiments by Dietfried Donnert and Li Zuyin. Calcite can remove phosphorus at initial P concentration 1.2 mg/L, and the result of P removal is not considerably under ambient temperature. But under high temperature after dephosphorization by calcite, the soluble P is only 0.0108 mg/L, lower than the critical value of eutrophiacation. That is because high temperature
accelerates ions and cations motion and calcium phosphate precipitation. The experiments above suggest that calcite is an effective factor in dephosphorization. Still some researchers have different views on how calcite helps with the precipitation of phosphorus and whether calcite act as crystal seed during the phosphorus precipitation. Whether calcite act as crystal seed depends on whether phosphorus is attached to the surface of calcite crystal or enters into the calcite crystal lattice. To figure out this problem, deposits of the dephosphorization were analyzed. The analysis was done through transmission electron microscope. First, the nondisposal precipitate of dephosphorization was analyzed with no P-bearing material found. This may result from the low phosphorus content energy spectrum detection line. Then an experiment on precipitate called ultrasonic vibrating was done with the result observed through transmission electron microscope after natural drying of oscillating turbidity liquor. The shapes of the sedimentary particles under the transmission electron microscope were mainly in two types: angular and spherical. The angular one has the diffraction spot which is regarded as crystalline. Its energy spectrum has spectral peak of Ca but no phosphorus was found. Characteristics of diffraction spot should be the fine-grained calcite. In the spherical one, no clear diffraction spot can be found and materials seen are mainly non-crystalline. Through energy spectrum both Ca and P can be found, which means that the sediment is phosphate. The analysis under transmission electron microscope showed that phosphorus precipitated are non-crystalline at room temperature and cannot be precipitated on the surface of calcite and Ca-P materials. In other words, calcite can promote the sedimentary of phosphorus but did not act as crystal seed. There is no evidence for the growth of phosphate on calcite crystal and the phase of apatite is also non-crystalline.
4 Conclusions From our experiments, we can see that calcite has the effect of dephosphorization but phosphorus is not simply attached to the surface of calcite crystal as homogeneous sedimentation. Phosphorus did not enter into the calcite crystal lattice. Series of dephosphorization experiments suggests: (1) Mineral calcite can promote dephosphorization. (2) Initial phosphorus concentration influence P removal. Dephosphorization rate is high at initial P concentration of 5 mg/L, and P can be removed by 88.48%; The dephosphorization rate can reach 69.94% when the phosphorus concentration is 3 mg/L; at 1.2 mg/L initial concentration only 12.68% P could be removed. (3) High temperature increases dephosphorization rate. Phosphorus-bearing solution with initial concentration of 1.2
XU Hong et al. / Earth Science Frontiers, 2008, 15(4): 138–141
mg/L can be dephosphorized by 12.68% at room temperature, but phosphorus concentration can be decreased to 0.0108 mg/L at high temperature, below the critical nutrition-rich level. (4) Ca-P materials precipitated at room temperature are non-crystalline, probably as Ca-P colloids, which are not adhered on the surface of calcite. It is worth pointing out that calcite crystal is used in the experiment. But the crystallization of calcite and apatite is not discussed here; thus no further study is done on the relationship between the crystallization of calcite and the sedimentary of apatite.
hypereutrophic Halfmoon Lake, Alberta. Lake and Reservoir Management, 1994, 8: 131–142. [5]
Dittrich M, Dittrich T, Sieber I, et al. A balance analysis of elimination of phosphorus by artificial calcite precipitation in stratified hardwater lake. Water Research, 1997, 31: 237–248.
[6]
Danen-Louwerse
H
J,
Lijklema
L,
Coenraats
M.
Coprecipitation of phosphate with calcium carbonate in Lake Veluwe. Water Research, 1995, 29: 1781–1785. [7]
Kleiner J. Coprecipitation of phosphate with calcite in lake water: a laboratory experiment modeling phosphorus removal with calcite in Lake Constance. Water Research, 1988, 22: 1259–1265.
[8]
Acknowledgements
Koschel R, Benndorf J, Proft G, et al. Calcite precipitation as a natural
control
mechanism
of
eutrophication.
Archives
Hydrobiology, 1983, 98: 131–139.
The authors thank Professor Huang Huaiceng, Chinese Academy of Geological Sciences, for the support and guidance in the experiment.
[9]
Murphy T P, Hall K J, Yesaki I. Coprecipitation of phosphate with calcite in a natural eutrophic lake. Liminol Oceanogr, 1983, 28: 58–69.
[10] Stumm W, Morgan J J. Aquatic Chemistry, an Introduction
References
Emphasizing Chemical Equilibria in Nature Water. New York: Wiley-Interscience, 1970: 558.
[1]
Ma J A, Li H Q. Preliminary discussion on eutrophication status of lakes, reservoirs and rivers in China and overseas.
inorganic phosphate. Colloids and Surfaces A: Physicochemical
Resources and Environment in the Yangtze Basin, 2002, 11:
and Engineering Aspects, 2002, 203: 143–153.
575–577. [2]
[11] Plant L J, House W A. Precipitation of calcite in the presence of
Okazaki M. Fluoridated apatite synthesized using a multi-step fluoride supply system. Biomaterials, 1999, 20: 1303–1307.
[12] Dietfried D, Manfred S. Elimination of phosphorus from municipal and industrial waste water. Water Science Technology, 1999, 40: 195–202.
[3]
Eggers E. Full-scale experiences with phosphate crystallization in a crystalactor. Water Science Technology, 1986, 23: 819–824.
absorption with calcium carbonate and clayey substance. Soil,
[4]
Babin J, Prepas E E, Murpgy T P, et al. Impact of lime on
1995, 27: 304–310.
sediment phosphorus release in hard water lakes: the case of
[13] Li Z Y, Lü J L. The study of the feature on phosphate