Keywords: Calcium phosphate; Collector; Flotation; Oleate; Phosphate; Wastewater
3. Results and discussion
Experimental residual phosphate concentrations as compared with model
predictions are shown in Fig. 1. Molar ratio, [Ca2+]/[HPO42-], of 1:1 resulted in residual
phosphate concentration of 35 mg/l at pH 10, and decreased to 30 mg/l as pH became
higher. Higher removal efficiency of phosphate was found at molar ratio of 2:1, and
residual phosphate of < 3 mg/l was obtained at molar ratio of 3:1 at pH > 9.0. It is in
agreement with literature that calcium phosphate precipitation is almost complete for pH
> 9.8 [14]. It was found that residual phosphate concentrations at pH > 6 were all higher
than model predictions regardless of molar ratio. Chemical equilibrium between calcium
and phosphate at molar ratio of 2:1 or 3:1, as predicted by PHREEQC, shows that HAP
starts to form at pH > 5.0, and HAP is the dominant species for pH > 6.5. However, it has
been indicated that precipitation reaction of HAP is very slow, and a number of species,
such as amorphous calcium phosphate (ACP), octacalcium phosphate (OCP) and brushite,
act as precursors to the precipitation of HAP; With time these species will be transformed
to HAP [15, 16]. The dominant precipitates formed, depending upon pH, are in fact
brushite, monetite, and ACP, while predicted equilibrium solubility is much lower since it
is based on HAP [15]. This is the reason for the differences between experimental results
and model predictions of residual phosphate in lower pH range. It is confirmed in the
XRD analysis (Fig. 2) in which monetite, ACP, and HAP are found in calcium phosphate
precipitates formed at molar ratio of 3;1 and at pH 10.0.
Effects of collector type and concentration on the removal of phosphate and calcium
phosphate precipitates are illustrated in Figs. 3A and 3b. For initial phosphate
concentration of 100mg/l, pH of 10.0, and [Ca2+]/[HPO42-] of 3, it was found that residual
phosphate concentrations were all below 0.5 mg/l, while the removal efficiency of
suspended solid increased with SDS concentration and reached 98 % at SDS
concentration of 50 mg/l. It increased as SDS concentration increased further. When SOl
was used, it was found that higher concentration (100 mg/l) was needed to obtain 97 %
removal of suspended solid, and that was the maximum efficiency regardless of SOl
dosage. Data not shown here indicated that neither cationic collector, cetyltrimethyl
ammonium bromide (CTAB), nor nonionic collectors, Triton X-100 and Brij 35, could
induce effective flotation removal of suspended solid. This is in agreement with literature
that anionic collectors are extensively used for phosphate flotation [6 - 10].
Zeta potentials of precipitates as affected by collector concentration are shown in Figs.
4A and 4B. The surface of calcium phosphate precipitates were positively charged at pH
10.0, probably because of the presence of excess Ca2+. The positive zeta potential
decreased slightly with increasing SDS concentration, and became negative only at SDS
concentration of 250 mg/l. The decrease was caused by the charge neutralization as SDS
was adsorbed on solid surfaces [17]. It is probable for Ca2+ to react with SDS and form
Ca(DS)2 precipitate whose solubility product is 10-9.7. Since the remaining Ca2+
concentration is 1.55 x 10-3 M, Ca(DS)2 will form as SDS concentration exceeds 100
mg/l/. Fig. 4B shows that zeta potential abruptly decreased in the presence of SOl, and
charge reversal was found at low concentration of SOl. It can be explained by the
adsorption of SOl. Monolayer chemisorption is predominant and occurs by the reaction of
carboxylate head group with surface calcium sites of apatite surface when oleate
concentration is low [6]. The adsorption of SOl on apatite could become bilayer, and
surface calcium oleate precipitation will occur under high adsorption density; and result in
significant decrease in zeta potential [18]. Flotation of hydrophilic apatite requires the
adsorption of surfactant to render the mineral surface hydrophobic, and flotation reaction
is controlled by the formation and adsorption of calcium oleate in the interfacial region
[11].
Effects of ionic strength on the removal of phosphate and calcium phosphate
precipitates are illustrated in Figs. 5A and 5B. Residual phosphate concentration
increased gradually as sodium nitrate (NaNO3) concentration increased, due to the
decreased activity of phosphate ion. It is noted that the flotation removal of calcium
phosphate precipitates decreased significantly with increasing ionic strength when SDS
(50 mg/l) was used as collector. Removal of suspended solid decreased to 70 % at 0.05 M
of NaNO3, and it was completely inhibited at 0.1 M of NaNO3. Possible explanation is
that SDS is adsorbed on solid surface via electrostatic interactions, which is “screened”
and becomes weaker as ionic strength increases [19]. The competition between nitrate and
phosphate for adsorption sites might exacerbate the condition. However, flotation of
calcium precipitates was not affected by ionic strength when SOl was the collector (Fig.
5B). It is noted that excess Ca2+ in the solution will increase adsorption of oleate collector
onto apatite, and precipitates on the mineral surface [6]. The predominant chemisorption
of oleate renders it not significantly affected by ionic strength [18]. In addition,
precipitated calcium oleate is hydrophobic and its effect is not affected by ionic strength
either [6]. The hindered flotation of calcium phosphate precipitates using SDS as collector
under 0.05 M of NaNO3 can be overcome by increasing SDS concentration or combined
use of SDS and SOl (Table 1). Removal efficiency of suspended solid was improved from
70 % to 91 % when SDS concentration was doubled to 100 mg/l, and it became > 99 % as
SDS concentration increased to > 150 mg/l. On the other hand, removal efficiency of
suspended solid increased to 90 % when 5 mg/l of SOl was added to the suspension, and
it reached over 99 % at SOl concentration of 10 mg/l. This may be due to the synergistic
advantages of using surfactant mixtures [6].
Since mixed acids are commonly used in etching unit in both semiconductor and
optoelectronic industries, anion interference was studied by using 0.05 M of NaNO3, 0.05
M of NaCl, and 0.0125 M of Na2SO4 as background electrolyte of identical ionic strength,
respectively. It was found that flotation removal of calcium precipitates, using SDS as
collector, decreased from 70 % to 55 % when NaNO3 was replaced by NaCl, and to 36 %
in the case by Na2SO4 (Table 2). This is in agreement with literature that anion with
higher valency has stronger effect on flotation efficiency [20]. It is because both SO42- and
Cl- could potentially be specifically adsorbed onto solid surfaces and interferes with the
adsorption of SDS. Again, flotation was not affected by anions when SOl was the
collector. It could be reasoned that the chemisorption of SOl onto calcium phosphate
precipitates was not affected by anions.
Application of precipitation flotation in treating industrial wastewater with high
concentration of phosphate seemed feasible from the study. Very low residual phosphate
concentration could be obtained as long as molar ratio and pH are properly controlled.
The separation of calcium phosphate precipitates could be induced by anionic collectors,
even under situations of high ionic strength or presence of interfering anions. Advantages
of the process include reusable sludge, flexible operation, and small footprint. However,
some issues need to be examined in further study. For example, to minimize excess Ca2+
in reacting with SOl, and to investigate the selective separation of PO43- and F-, a common
constituent in semiconductor and optoelectronic wastewater.
4. Conclusions
The current study demonstrated that calcium chloride was effective for removal of
phosphate from water. When molar ratio, [Ca2+]/[HPO42-], was 3:1, residual phosphate
concentration of < 3 mg/l was obtained at pH > 9.0. Though affected by molar ration and
pH, major precipitates are found to be monetite, ACP, and HAP. Anionic collectors, SDS
and SOl, were effective in flotation removal of calcium phosphate precipitates from water.
It was proposed that SDS was adsorbed onto solid surfaces through electrostatic
interactions and were significantly affected by increased ionic strength and anion
interferences. The chemisorption of SOl onto solid surfaces made the flotation efficiency
not much affected by changes in ionic strength and presence of anions, such as sulfate and
chloride.
References
[1] Metcalf and Eddy, Wastewater Engineering: Treatment and Reuse, 4th Ed., McGraw-Hill Co., New York, (2003), pp. 186.
[2] M.F. Chang and J.C. Liu, Precipitation removal of fluoride from semiconductor wastewater, J. Environ. Eng. 133(4), (2007), pp. 419-425.
[3] H. Ratnaweena, H. Odegaard and J. Fettig, Coagulation with prepolymerized aluminum salts and their influence on particle and phosphorus removal, Water Sci. Tech.
26 (5-6, (1992), pp. 1229-1237.
[4] K. Urano and H. Tackikawa, Development for removal and recovery of phosphorus from wastewater by a new adsorbent. 1. Preparation method and adsorption capability of a new adsorbent, Ind. Eng. Chem. Res. 30 (8), (1991), pp. 1893-1896.
.
[5] T. Moutin, J. Y. Gal, H. E. Halouani, B. Picot, and J. Bontoux, Decrease of phosphate concentration in a high rate pond by precipitation of calcium phosphate: theoretical and experimental results, Water Res. 26 (11), (1992), pp. 1445-1450.
[6] H. Sis and S. Chander, Adsorption and contact angle of single and binary mixtures of surfactants on apatite, Min. Eng. 16 (9), (2003), pp. 839-848.
[7] K. H. Rao, B. M. Antti and E. Forssberg, Mechanism of fatty acid adsorption in salt-type mineral flotation, Int. J. Min. Eng. 28 (1-2), (1989), pp. 59-79.
[8] B. M. Moudgil and D. Ince, Effects of sodium chloride on flotation of dolomite from apatite, Min. Metallur. Proc. 8 (3), (1991), pp. 139-143.
[9] A. J. Rodrigues and P. R. G. Brandao, The influence of crystal chemistry properties on the flotability of apatites with sodium oleate, Min. Eng. 6 (6), (1993), pp. 643-653.
.
[10]. C. Maltesh, P. Somasundaran and G. A Gruber, Fundamentals of oleic acid adsorption on phosphate flotation feed during anionic conditioning, Min. Metallur. Proc.
13 (4), (1996), pp. 156-160.
[11] M. C. Fuerstenau and K. N. Han, Metal-surfactant precipitation and adsorption in froth flotation, J. Colloid Interface Sci. 256 (1), (2002), pp. 175-182.
[12] C.J. Huang and J.C. Liu, Precipitate flotation of fluoride-containing wastewater from semiconductor manufacturer, Water Res. 33 (16), (1999), pp. 3403-3412.
[13] D. L. Parkhurst, C. A. J. Appelo, User’s Guide to PHREEQC (Version 2) – a
computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, Water Resource Investigation Report, Denver CO: U.S.
Geological Survey, (1999), pp 99-4259.
[14] C. A. Gray and A. P. Schwab, Phosphorus-fixing ability of high pH, high calcium, coal-combustion, waste materials, Water Air Soil Pollut. 69(3-4), (1993), pp. 309-320.
[15] E. Abbona, H. E. Lundager Madsen and R. Boistelle, The initial phases of calcium and magnesium phosphate precipitated from solutions of high to medium concentrations, J. Crystal Growth 74 (3), (1989), pp 581-590.
[16] E. V. Musvoto, M. C. Wentzel, G. A. Ekama, Integrated chemical-physical process Modeling - II. Simulation aeration treatment for anaerobic digester supernatant, Water Res.
34 (6), (2000), pp. 1868-1880.
[17] A. Martinez-Luevanos, A. Uribe-Salas, and A. Lopez-Valdivieso, Mechanism of adsorption of sodium dodecylsulfonate on celestite and calcite, Min. Eng. 12 (8), (1999), pp. 919-936.
[18] K. H. Rao and K. S. E. Forssberg, Mechanism of fatty acid adsorption in salt-type mineral flotation, Min. Eng. 4 (7-11), (1991), pp. 879-890.
[19] H. G. M. van de Steeg, M. A. C. Stuart, A. de Keizer, and B. H. Bijsterbosch, Polyelectrolyte adsorption: a subtle balance of forces, Langmuir 8(10), (1992), pp. 2538-2546.
[20] S. J. Choi and Y. H. Choi, Removal of Direct Red from aqueous solution by foam separation techniques of ion and adsorbing colloid flotation, Sep. Sci. Technol. 31 (15), (1996), pp. 2105-2116.
List of Table
Table 1 The improvement of hindered flotation of suspended solid under 0.05 M of
NaNO3.
Table 2 Flotation of suspended solid as affected by anions
Table 1 The improvement of hindered flotation of suspended solid under 0.05 M of NaNO3
Collector Removal of suspended solid (%)
SDS (50 mg/l) 70
SDS (100 mg/l) 91
SDS (150 mg/l) 99
SDS (50 mg/l) + SOl (5mg/l) 90 SDS (50 mg/l) + SOl (10mg/l) > 99 SDS (50 mg/l) + SOl (15mg/l) > 99
Table 2 Flotation of suspended solid as affected by anions Salt type Removal (%) of SS with 50
mg/l of SDS
Removal (%) of SS with 100 mg/l of SOl
0.05 M NaNO3 70 88
0.05 M NaCl 55 89
0.0125 M Na2SO4 36 90
List of Figures
Fig. 1 Chemical species of calcium and phosphate as affected by molar ratio and pH.
Fig. 2 Crystal structure of calcium phosphate precipitates formed at pH of 10.0, and
[Ca2+]/[HPO42-] of 3, as analyzed by X-ray diffraction.
Fig. 3A Flotation of calcium phosphate precipitates and residual phosphate as affected by
SDS concentration.
Fig. 3B Flotation of calcium phosphate precipitates and residual phosphate as affected by
SOl concentration.
Fig. 4A Zeta potential of calcium phosphate precipitates at pH of 10.0 as affected by SDS
concentration.
Fig. 4B Zeta potential of calcium phosphate precipitates at pH of 10.0 as affected by SOl
concentration.
Fig. 5A Effect of ionic strength on flotation of calcium phosphate precipitates and residual
phosphate when SDS was the collector
Fig. 5B Effect of ionic strength on flotation of calcium phosphate precipitates and residual
phosphate when SOl was the collector
Fig. 1
3 4 5 6 7 8 9 10 11 12
0 10 20 30 40 50 60 70 80 90 100
I = 0.005 M
[HPO42-] = 100 mg/L Temp = 25 oC
Residual soluble phosphate concentration (mg/L)
pH
Theoritical prediction : ([Ca]:[HPO42-]) = 1:1 ([Ca]:[HPO42-]) = 2:1 ([Ca]:[HPO42-]) = 3:1 Experimental result
([Ca]:[HPO42-]) = 1:1 ([Ca]:[HPO42-]) = 2:1 ([Ca]:[HPO42-]) = 3:1
Fig. 2
10 20 30 40 50 60 70 80 90
0 50 100 150 200 250 300 350 400 450 500
ＣＣ
Ｃ Ｃ Ｃ
B
B B Ａ B
Ａ Ａ
Ａ
A：CaHPO4 B：Ca5(PO4)3(OH) C：Ca3(PO4)2．xH2O
In ten sity (co u n ts )
2 θ
Fig.3 A
Removal of suspended solid (%)
0 N2 Flowrate= 100 ml/min Flotation Time=10 min
Residual phsophate concentration (mg/l)
Fig.3 B
Removal of suspended solid (%)
0 N2 Flowrate= 100 ml/min Flotation Time=10 min
Residual phsophate concentration (mg/l)
Fig. 4A
0 25 50 75 100 125 150 175 200 225 250
-50 -40 -30 -20 -10 0 10 20 30 40 50
T=21.3℃ pH=10.0± 0.1 [HPO42-]=100 mg/l [Ca2+]/[HPO42-]=3
Z eta p o ten tia l (m V )
[SDS] (mg/l)
Fig. 4B
0 50 100 150 200 250 300
-50 -40 -30 -20 -10 0 10 20 30 40 50
T=26.4℃
pH=10.0± 0.1 [HPO42-]=100 mg/l [Ca2+]/[HPO42-]=3
Z eta p o ten tial (m V )
[NaOl] (mg/l)
Fig. 5A
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0 N2 Flowrate= 100 ml/min Flotation Time= 10min [SDS]=50 mg/l
Ionic strength (M)
Removal of suspended solid (%)
0
Residual phsophate concentration (mg/l)
Fig. 5B
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0 N2 Flowrate= 100 ml/min Flotation Time= 10min [NaOl]= 100 mg/l NaNO3
Ionic strength (M)
Removal of suspended solid (%)
-1
Residual phsophate concentration (mg/l)