74
Removal of Heavy Metals From Aqueous By Thephosphate Dihydrate Dicalcium.
J. Hmimou
1, E.H. Rifi
1, A. Lebkiri
1, L.Chafki
1, M. Ebn Touhami
2, M. Galai
2, Z. Hatim
31Laboratory Organic Synthesis and Process d`Extraction, Faculty of Sciences, University Ibn Tofail, Kenitra, Morocco.
2Laboratory Electrochemistry Corrosion and Environment, Faculty of Sciences, Ibn Tofail University, Kenitra, Morocco
3Laboratory Electrochemistry and Surface Treatment, Faculty of Sciences, El Jadida, Morocco
* Corresponding author. E-mail: [email protected]; Tel: (+212661466396) Received 17 Oct 2014, Revised 23 Oct 2014, Accepted 11 Dec 2014
Abstract: The objective of this work is the use of dicalcium phosphate dihydrate CaHPO4, 2H2O (SAP) for the decontamination of aqueous solutions charged in divalent metal ions Cu2+, Zn2+ and Cd2+. After realization of a reliable experimental protocol giving reproducible results, the metals of aqueous solutions are analyzed by flame atomic absorption spectroscopy. The Mass balance before and after extraction makes possible the determination of the rate of metal retention R of the apatitic support. The obtained results show that the R values are closely related to the concentration of metal and the imposed pH in the supernatant solution. Indeed, at low concentrations of Cu2+ ions (10-4M) the R value is close to 0.08. The increase of the pH favours the fixation of the Cu2+ ions on SAP (R passes from 0.058 to 0.085, respectively for pH= 3.5 and 6.5). The kinetic study shows that the mechanism of fixation of the Cu2+ ions by SAP is controlled by a cation exchange: Ca2+ (SAP) /Cu2+ (Solution). The application of SAP to the treatment of metal mixtures of ions leads to a selective extraction: Cd > Zn > Cu.
Keywords: Copper, Zinc, Cadmium, extraction liquid-solid, apatite, dicalcium phosphate dihydrate.
1 .Introduction
Metal pollution takes magnitudes increasingly worrisome. The discharge of waste containing heavy metals in rivers led to the deterioration of their quality and transfer metal ions to humans through the food chain.
Among these, we find the micro Cd, Hg, Pb, Al, As, Cr,Cu, Ni, Zn, ... from industrial waste, urban activities and fertilizers.To remedy this problem, specific and effective techniques have been employed for the removal of toxic metals from dilute aqueous solutions, which may be industrial effluent or contaminated water. Most commonly used are:Liquid-liquid extraction [1-4], Liquid -gel extraction [5-12], The liquid- solid extraction [13-17], Membrane separation [18-21], Ion exchange [22-25], The precipitation,flotation...
Among the different methods of separation of metal ions, the solid phase extraction remains one of the most used methods because to the many advantages it offers:
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Simplicity of implementation.
Low energy demand.
Simple Regeneration of extracting medium.
Possibility of return of the species extracted into a new aqueous phase with large concentration factors.
Choice of unrestricted solid phase. In fact, many natural or synthetic media are used (apatites, zeolites, resins, silicates ...).In recent years , the use of apatite materials such as fixing, agents has grown considerably , particularly because of their strong ability to bind a large number of metal ions ( Pb2 + , Ni2+ , Cu2+ , Cd2+ , Zn2+ , Sr2+ , Cr3+ , As5+ ... ) when they are placed in contact with aqueous solutions of [26-29].
Hydroxyapatite is among the absorbing heavy metals studied most recently, as evidenced by the numerous publications on this topic [30-38].The present work is to study the liquid- solid extraction of metal cations Cu2+, Cd2+, Zn2+, chloride medium, a new phosphate medium, namely, dicalcium phosphate dihydrate CaHPO4,2H2O (SAP). Our goal is to develop an experimental protocol to determine:
The pH of the aqueous solution and the concentration of the metal giving better retention of these metal cations by SAP.
The time at which equilibrium with the SAP charged metal aqueous solution.
The amount of SAP that allows to purify a 100 ml solution of Cu2+ions, Zn2+ and Cd2+ to 10-4 M.
Monitoring of pH changes and the concentration of Cu2+ ions (aqueous phase) and Ca2+(released by SAP) in the supernatant solution, versus time allows access to the extraction mechanism.
Extraction from an equimolar mixture of divalent metals used to study the selectivity of this method.
2 .Experimental part
2.1 .Material used
dicalcium phosphate dihydrate CaHPO4, 2H2O (SAP ) used as extractant matrix of Cu2+ ions, Cd2+ and Zn2+, was synthesized in the Laboratory of Electrochemistry and Surface Treatment of the Faculty of Sciences of El Jadida ( Morocco ). It is in the form of white powder. This ligand has several advantages over other products conventionally used for extraction of metals:
It's inexpensive and easy to prepare
Its implementation is easy extraction
It has a high chemical stability in a wide pH range
The amount of powder put into contact with the metal solution is selected to have an excess of ligand relative to the metal (SAP mole / mole metal = 5).
2.2. Preparation of the feeding phase
The reagents used in this work were all of analytical grade. The stock solutions at 1000 mg / l were prepared by dissolving 2.1181, 2.0841 and 1.6310g of chloride Copper, Zinc and Cadmium, respectively, in 1000 ml of distilled water. The solutions of lower concentrations are obtained by dilution; pH is adjusted with hydrochloric acid 0.2 N.
2.3 .Experimental protocol
The extraction experiments were performed in a cell of double-walled glass thermoregulated at (25 ± 0.2) ° C and equal to about 150 ml capacity. The cell is provided with a thermometer to control a combined glass electrode and a magnetic stirrer. 6.9 mg of powder are brought into contact with 100 ml of an aqueous
76 solution containing the metal to be extracted having a concentration 10-4 M. The mixture is stirred until the extraction equilibrium. Each sample of SAP was introduced into a previously prepared bag made of filter paper dimensions 2cm×3cm closed by a yarn which also keeps it suspended above the stir bar. This technique has two advantages:
Avoid crushing apatite grains by the magnetic bar during the agitation system which has the effect of increasing the exchange surface between the solid phase and the metal solution and therefore a good reproducibility of results.
Take samples,at many times, in the metal solution, easily and smoothly drive the apatite grains in the samples to be assayed.
The time t = 0 corresponds to the use of apatite in contact with the metal supporting aqueous phase.
If the apatite is brought directly into contact with the metal, solution is carried out by stirring the above, using a Heidolph RZR 2000 engine. When the extraction equilibrium is reached, the metal loaded carrier is separated from the supernatant solution by filtration.
2.4. Instrumentation
The concentrations of the aqueous solutions of copper, zinc and cadmium, before and after treatment with SAP, are determined by spectrometric assay flame atomic absorption. The spectrometer used is the type Unicam 929 AA spectrometer. Calibration of the spectrometer was carried out using standard solutions for each metal. The calibration range is spread between 0.2 and 2 mg / l.
The extraction efficiency (E) of metal ions by SAP is given by the following relationship:
𝐄 =C0 − Ce
C0 × 100
C0: represents the initial concentration of metal in the aqueous solution.
Ce: Represents the metal concentration at equilibrium of extraction, in the supernatant solution.
The retention rate of metal ion by SAP is defined by the following ratio:
𝐑 =Moles of fixed metal SAP Moles
3. Results and Discussion
3.1. Dicalcium phosphate dihydrate synthesis
Synthesis powder dicalcium phosphate dihydrate (brushite or) CaHPO4,2H2O was conducted using a new protocol developed in the laboratory. This preparation process is simple and provides powders brushite composition and controlled morphology.The synthesis is carried out by precipitation in aqueous phase in a mixed solution of calcium chloride to a solution of orthophosphoric acid with an atomic ratio Ca/P = 1. The precipitate obtained at room temperature and at pH = 4.3, is then washed and dried at 60 ° C overnight.The product purity was verified by infrared absorption spectrometry and X-ray diffraction the product is crystallized in the form of grains of which the surface area is 25 m2 / g, and average size equal to (22.7 ± 0.3) microns, as determined by laser granulometry.
3.2. Extraction Kinetics
The figure 1 represents the simultaneous variations of the concentration of copper and the pH of the aqueous solution in function of the contact time. The study was performed by performing sampling with time in the
77 supernatant solution. Each sample was diluted so that its concentration does not exceed the calibration range of the spectrometer used for the determination of metal ions. The curves obtained show a first solution of the rapid loss of metal at the beginning of the experiment. Then, a step corresponding to the extraction equilibrium is reached after 45 min when the extraction yield is about 30%. At the same time, we see an increase in pH, from 5.5 to 5.9. Increasing the pH shows that the phosphate carrier is balanced with the solution laden with metal consuming H+protons. There is a competition between Cu2+ ions and protons H+ to bind to the phosphate carrier. The same phenomena are observed in the solid - liquid extraction of the Zn2+
ions by hydroxyapatite and tricalcium phosphate (39). We followed atthe same time the evolution of the concentration of Ca2+ ions released by the SAP, in the aqueous solution in function of time. The curve obtained is shown in the same figure. It is noted that the amount of calcium increases SAP dropped during the extraction. From 100 min SAP contact with the copper solution, the concentration of Ca2+ is stationary.
The amount of Ca2+ ions released by SAP is equal to 9.10-5 moles. The mass balance, prepared taking into account the initial composition of the solution in Ca2+, shows that the number of moles of Cu2+ and H+ set by SAP is almost equal to the number of moles of Ca2+ released by the calcium phosphate support, which is in agreement with an extraction by a cation exchange mechanism: Cu2+ cations and the protons H+ in the aqueous phase are exchanged against the Ca2+ apatite network, according to the simplified reaction:
SAP − Ca2+ ←→ SAP − Cu2+ + Ca2+
0 40 80 120 160
0,4 0,6 0,8 1,0
Cu2+
pH Ca2+
Temps(min) [M2+]10-4 M
5,5 5,6 5,7 5,8 5,9
pH
Figure 1:Kinetics of copper extraction by SAP (M2+ = Cu2+ or Ca2+), Solution: Power Vaq = 100 ml, [Cu 2+]o,aq = 10-4 M, pHo= 5.5, T = 25 ° C, Support SAP apatite: mSAP = 6.9 mg
The Figure 2 shows the kinetic curves, plotted in the same experimental conditions, in the case of extraction of ions Zn2+ and Cd2+ SAP compared to that obtained with Cu2+ ions. It appears from the experimental data that the pH of the aqueous solution increases during the extraction, which confirms our previous findings.
The pHf is 6.35 (Zn) and 6.7 (Cd) higher than that recorded with Cu (5.9). The extraction reaction of Zn2+
ions is slow compared to ions Cu2+ and Cd2+. In fact, 45 minutes of contact are provided so that SAP equilibrium solutions charged with Cu and Cd, so the Zn with the contact time exceeds 75 min. The (35 %) yield of extraction of Zn and Cd is slightly higher than Cu.
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0 20 40 60 80 100 120 140 160 180
0,65 0,70 0,75 0,80 0,85 0,90 0,95 1,00
[Zn2+] PHf(Zn) [Cd2+] PHf(Cd) [Cu2+] PHf(Cu)
Temps (mn) [M2+ ].10-4 M
5,4 5,6 5,8 6,0 6,2 6,4 6,6 6,8
PH
Figure 2: Changes in the simultaneous concentration of divalent ions M2+and pH as a function of contact time: M2+ = Cu 2+, Cd 2+ or Zn2+, Solution: Vaq = 100 ml, [M2+]o,aq = 10-4 M, pHo = 5.5, T = 25 ° C, SAP:
mSAP = 6.9 mg
3.3. Effect of pH
To determine the pH that achieves the best rate fixing ions Cu2+, Cd2+ or Zn2+ on the phosphate carrier, the evolution of metal retention (R) as a function of the concentration of hydrogen ions was studied. The initial concentrations were set at 6.35, 11.24 and 6.54 mg / l, respectively for Cu, Zn and Cd, so that their molar concentration in the aqueous phase is the same (the 10-4mol.L-1). Figure 3 shows that the optimum rate of fixation was achieved from pH= 6 it was noted that the following values of R: 0.092 (Zn), 0.088 (Cd) and 0.078 (Cu).
3,5 4,0 4,5 5,0 5,5 6,0 6,5
0,05 0,06 0,07 0,08 0,09 0,10
R(moles M2+ /moles SAP)
pH0
Zn Cd Cu
Figure3:Influence of pH on the aqueous solution imposed on the retention of ionsCu 2+, Cd 2+ and Zn 2+ by SAP Vaq = 100 ml, [M2+] o,aq = 10-4 M, T = 25 °, mSAP = 6.9 mg
3.4. Effect of the concentration of metal
To determine the maximum quantity of metal absorbed by dicalcium phosphate dihydrate, the saturation of the solid support SAP by Cu2+, Cd2+ and Zn2+ have been studied systematically by dipping the same amount
79 of SAP (6.9 mg ) in baths concentrations from 0 to 25.10-4 mol.L-1 . The contact time was set at 90 min. The pH of each solution was adjusted to 5.5. Saturation curves are plotted in Figure 4. It appears clear that at low concentrations of Cu2+ ions, Zn2+ and Cd2+ (C< 12.10-4 mol.L-1) the amount of metal ions retained by the phosphate carrier increases rapidly when their initial tenures in the aqueous solution increases. The performance of maximum absorption of Cu2+, Zn2+ and Cd2+ were achieved at 0.82, 0.67 and 0.74 moles of selected metal / mole SAP , respectively, using the following initial concentrations : 23.6 10-4 mol.L-1 ( Cu ), 10.7 10-4 mol.L-1 (Zn,Cd). Furthermore, the effect of the concentration of Cd2+ ions on the removal efficiency by supports based anhydrous tricalcium phosphate and hydroxyapatite has been studied (39).
Saturation ratios, obtained under the same experimental conditions, are 0.89 and 0.97, respectively. These reports are slightly higher than those achieved in the present study. As against, the amount of Zn2+ ions fixed to the saturation of the two supports is much lower. The saturation rate is 0.23 for the anhydrous tricalcium phosphate and 0.27 for hydroxyapatite [39].
0 5 10 15 20 25
0,0 0,2 0,4 0,6 0,8
R(moles M2+ /moles SAP)
[M2+]. 10-4 M
[Zn2+] [Cd2+] [Cu2+]
Figure 4: Effect of initial concentration of Cu2+ ions, Zn2+ and Cd2+on the retention rate by SAPVaq = 100 ml, pH0 = 5.5, T = 25 ° mSAP= 6.9 mg
3.5. Effect of the mass of the ligand
The objective of this study is to test the effectiveness of this method in the dilute decontamination containing Cu2+ ions aqueous solutions, Cd2+, or Zn2+, and therefore its application in sewage loaded with heavy metals.The treatment solutions of same characteristics (V = 100 ml , [ M2+] = 10-4 mol.L -1 , pH = 5 ) , loaded with Cu2+ , Cd2+ , or Zn2+ , was carried out by immersing the quantities of SAP mass increasing (6.9 , 12, 20, 30, 45, 60, 75mg ) in each solution.The experimental data gathered in Table 1 allow us to draw the following conclusions:
Increasing the pH with increasing dose of SAP which stabilizes at 7.4 (Cd), 6.93 (Zn) and 6.9 (Cu) from 45 to 60 mg of SAP, respectively for Cd and Zn,Cu.
Very fast increase of the efficiency of adsorption of Cu2+ions, Zn2+ and Cd2+, with an increase of the mass of ligand of from 6.9 to 12 mg which is already extracted over 80% of each metal. The maximum removal efficiency of 99% was obtained with the use of 60 mg of calcium phosphate powder. Increasing the extraction efficiency can be attributed to the fact that, with an increase of the amount of powder, the surface of contact with the metal laden solution becomes important.
80 Table 1: Depletion of metal aqueous solutions of Zn, Cu and Cdby means of a solid support -based phosphatedicalcium phosphate dihydrate (SAP)
Mass (mg)
Cd Zn Cu
[Cd]e(ppm) pHe E% [Zn]e
(ppm) pHe E% [Cu]e(ppm)
pHe E%
0 9,47 5,5 - 6,5 5,5 - 6,35 5,5 -
6,9 7,28 6,70 35,2 4,26 6,35 34,5 4,36 5,90 31,3
12 4,44 6,85 53,1 1,15 6,45 82,3 1,12 6,38 82,4
20 1,61 7,05 83 1,13 - 82,6 1,08 6,45 83,0
30 0,48 7,10 94,9 0,99 6,59 84,8 0,92 6,52 85,5
45 0,23 7,40 97,6 0,48 6,80 92,7 0,41 6,78 93,5
60 0,15 7,40 98,4 0,14 6,93 97,8 0,05 6,90 99,2
75 0,13 7,40 98,6 0,13 6,93 98 0,04 6,90 99,4
3.6. Selectivity
The mixture we treated is an equimolar solution (10-4 M and pH = 5.5) of zinc, cadmium and copper.The experiment is performed as follows: A quantity of SAP mass equal to 50 mg is introduced in a volume of 100 ml of metal solution. After extraction equilibrium, the mixture was filtered and the metal cations contained in the filtrate were measured. The extractions of metals by these SAP yields are given in Table 2.
It is noted that the selectivity of this method follows the following order:
Cd > Zn > Cu
This result is different from that obtained under the same experimental conditions with hydroxyapatite and tricalcium phosphate [34] where the order of selectivity is established:
Cd > Cu > Zn > Co
Table 2: Treatment of a mixture of divalent metal (Zn,Cd and Cu) by (SAP)
Métal (M2+) Cd Zn Cu
[M2+]i (ppm) 11,24 6,53 6.35
[M2+]f (ppm) 2.09 1.35 2.32
R (%) 81 79 63
4. Conclusion
The adsorption of the divalent metal in aqueous diluted solution of dicalcium phosphate dihydrate, CaHPO4, 2H2O (SAP) was studied. The main results are as follows:
The study of the evolution of the copper concentration of cadmium and zinc as a function of time shows that the binding kinetics on the studied medium is rapid equilibrium is reached in about 30 minutes stirringsystem.
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pH measurements during the experiment show that it increases with time , the calcium released by ( SAP) during extraction also increases. It appears that dicalcium phosphate dihydrate is balanced with the supernatant solution by consuming the protons H+ and Cu2+ ions and dropping Ca2+cations.
The binding capacity of copper and cadmium from zinc by the carrier (C = number of moles of copper attached / number of moles of substrate) depends on parameters related to the metal solution (metal concentration, pH, temperature, volume of the solution ...).
Changes in copper -binding capacity of cadmium and zinc by the support function of pH were also studied. The results obtained show that it increases with the pH of thesupernatant solution. A bearing is obtained at pH >5.5, where Cmax is in the order of:
Cmax (Copper): 0.085 Cmax (Cadmium): 0.095 Cmax (Zinc): 0.09
The saturation of the amount of (SAP) of copper and cadmium zinc is carried out by dipping it in solutions of increasing concentrations.
Changes in C as a function of [M2+] show that the support is based on a saturated
solution of the metal to 120 ppm which is equal to Cmax
Cmax (Copper): 0.82 Cmax (Cadmium): 0.67 Cmax (Zinc): 0.74
The total depletion of a solution of copper and cadmium from zinc, takes place byonly using an amount of 75 mg for the dicalcium phosphate dihydrate. With a yield of:
R (Copper): 99% R (Cadmium): 89% R (Zinc): 89%
These results show that this type of support may be used for the detoxification of waste water.
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