Study of Cd 2 + , Al 3 + , and − 24 SO Ions Influence on Struvite Precipitation from Synthetic Water by Dissolved CO 2 Degasification Technique

The effect of cadmium, aluminum, and sulphate ions on struvite precipitation kinetics and morphology of solids obtained was investigated in this study. Thus, these ions were introduced as additives in solution where struvite precipitation was achieved. The main results obtained showed that the Cd2+ ions cause the probable co-formation, with struvite, of an amorphous phase observed from Cd2+ concentration of 10 mg/L; the addition of aluminum ions in the solution does not affect the nature of the crystalline phase, identified as struvite. However, these ions cause the formation of agglomerates of larger and larger as the concentration rises. Moreover, their increasing improved significantly the yield of phosphates removal; although the sulfate ions have no effect on the nature of the precipitated phase identified as struvite, independently of molar ratio studied, they affect the thermal decomposition of struvite. This latter is done in a single step by increasing the concentration of these ions for the 2 3 4 4 SO PO − − molar ratio of 8.


Synthetic Water Preparation
Synthetic water used in all experiments in this work was prepared by mixing the respective aqueous solutions of MgCl  The study of the effect of cadmium, aluminum and sulfate ions was carried out using solutions prepared from CdCl 2 •H 2 O, AlCl 3 •6H 2 O, and Na 2 SO 4 solids.Cadmium ions were adding in synthetic solution at a concentration varying from 0 to 100 mg/L; aluminum from 0 to 6 mg/L.Sulfate ions were introduced at

SO PO
− − molar ratio between 0 and 8.It should be noted that all these experiments are conducted at 25˚C, an airflow rate of 40 L•min −1 , an initial solution pH of 6.5 in a PVC cell by the CO 2 degasification technique [7].

Effect of Cd 2+
X-rays diffractograms of solids obtained after addition of Cd 2+ , with concentrations ranging from 0 to 100 mg•L −1 , showed, in addition to precipitation of struvite, probable occurrence of an amorphous phase affecting the resolution peaks of struvite from a cadmium concentration of 10 mg•L −1 (Figure 1).This phase is more pronounced when the concentration of Cd 2+ increased.This finding is also supported by the decrease of the characteristics of struvite needles (Figure 2(a) & Figure 2(b)).It is unidentified, but it is probably consisting of Cd, Ca and P as shown elemental analysis (EDAX) of the sample (Figure 2(a') & Figure 2(b')).This result is in agreement with the study of Ronteltap et al., (2007) [16].
The temporal evolution of the pH of the solution, the time course of 3 4 PO − concentration in the solution for different Cd 2+ concentration are shown in Figure 3 and Figure 4.The precipitation pH and induction times are practically constant.Thus, we deduced that the addition of Cd 2+ did not affect these two parameters.Furthermore, the yield of 35.5% phosphates removal for Cd 2+ concentration equal to 10 mg/L (Table 1) was obtained.Beyond this concentration, the yield decreases.This result is probably related to the formation of the amorphous phase.Indeed, Figure 2(b') showed that the solid phase obtained contains less magnesium than that of Figure 2(a').Consequently, given struvite equimolarity, the compound obtained in the presence of 100 mg/L of Cd 2+     contains less phosphate than that obtained in the presence of 1 mg/L of Cd 2+ .These remarks may explain the decrease in phosphate removal ratio observed when the Cd 2+ concentration increased.

Effect of Al 3+
The addition of aluminum ions in the solution did not affect the nature of the crystalline phase, identified to struvite.However, influence on peak intensities was observed (Figure 5).
In addition, theses ions led to the formation of agglomerates of larger and larger amounts when its concentration increased (Figure 6).These results are in agreement with those of Le Corre et al. (2007) [17] according to which the Al 3+ ions are considered as coagulants.On the other hand, the addition of aluminum ions generally led to a reduction of the induction time which changed from 35 to 21 min, when its concentration varied from 0 to 6 mg/L.The precipitation pH varied between 8.57 and 8.98 in the studied interval of Al 3+ concentration (Figure 7 and Table 2).It is also noted that the phosphate removal yield was, significantly, improved by the addition of these ions, as the latter passed from 32 to 67% when the concentration of Al 3+ ions varied from 0 to 6 mg/L (Figure 8 and Table 2).

SO
To determine the effect of sulfate ions on struvite precipitation, increasing amounts of this ion were added in the solution so that the

SO PO
− − molar ratios varied from 0 to 8. The solids obtained, analyzed by X-ray diffraction, for these molar ratios, is identified to struvite (Figure 9).Just, the peak intensities were changed when this molar ratio increased.Moreover, the SEM pictures, and the elemental analysis showed the same grain morphology (Figure 10(b)) and the presence of the same elements of struvite (Figure 10(a') & Figure 10(b')).
On the other hand, the precipitation pH of the compound ranged from 8.6 to 8.9 in this range of the molar ratio studied (Figure 11 and Table 3).Moreover, the best phosphate recovery efficiency is obtained for

SO PO
− − molar ratio = 2 (about 42%) (Figure 12 and Table 3).In addition, the induction time remained almost constant (about 20 min) in the interval [2] [4] molar ratio (Figure 12 and Table 3).This parameter passed to 43 min when the molar ratio became equal to 8.This result is in agreement with the study of Kabdasli et al. (2006) [18] who found an increase in the induction time of struvite with the concentration of 2 4 SO − .Increasing the amount of sulfate in the solution affected also the thermal decomposition of struvite.Indeed, figure 13, showed the presence of two types of evolution of the solid obtained.The first is observed at      MgNH Then: This result is an agreement with the previous studies [19] [20].By increasing

Conclusions
After this study, we can conclude as follow:   -The Cd 2+ ions cause the probable co-formation, with struvite, of an amorphous phase observed from Cd 2+ concentration of 10 mg/L.-The addition of aluminum ions (Al 3+ ) in the solution does not affect the nature of the crystalline phase, identified to struvite.However, these ions cause the formation of agglomerates of larger and larger as the aluminum concentration rises.Moreover, their increasing improves significantly the yield of phosphate removal.
Although the sulfate ions ( ) SO − have no effect on the nature of the precipitated phase, identified to struvite, irrespective of molar ratio studied, however, they affect the thermal decomposition of struvite.This latter is done, in a single step, by increasing the concentration of these ions for the molar ratio of 8.

Figure 1 .
Figure 1.Superposition of X-rays diffractograms of precipitates obtained for different Cd 2+ concentration.

Figure 3 .
Figure 3. Temporal evolution of solution pH for different Cd 2+ concentration.

Figure 4 .
Figure 4. Temporal evolution of solution phosphate concentration for different Cd 2+ concentration.

Figure 5 .
Figure 5. Superposition of X-rays diffractograms of solids obtained for different Al 3+ concentration.

Figure 7 .
Figure 7. Temporal evolution of solution pH for different Al 3+ concentration.

Figure 8 .
Figure 8. Temporal evolution of solution phosphate concentration for different Al 3+ .
mulitransformation by a gradual elimination of ammonia and water by the following equations:

Figure 9 .
Figure 9. Superposition of X-rays diffractograms of solids obtained for different

2 4 SOmolar reached 8 .
− concentration, there is a decrease in the temperature of the first step, reaching about 110˚C which became the only decomposition step of struvite by removing simultaneously 6 molecules of water with These observations showed that the decomposition of struvite, in one step, is facilitated by increasing its amount for

Figure 11 .
Figure 11.Temporal evolution of solution pH for different

Figure 12 .
Figure 12.Temporal evolution of solution phosphate concentration for different

Figure 13 .
Figure 13.Heat flux in function of temperature for different 2 •6H 2 O and NH 4 H 2 PO 4 in desired proportions in a calcium carbonate solution.This solution was previously prepared by dissolving calcium carbonate solids in a flow of CO 2 .Reagents MgCl 2 •6H 2 O (purity > 99%), NH 4 H 2 PO 4 (purity > 99%) and CaCO 3 (purity: 99%) were respectively supplied by Fluka, Sigma Aldrich and Merck.

Table 1 .
Induction time, Precipitation pH, and Posphate removal ratio for different Cd 2+ concentration.

Table 2 .
Induction time, precipitation pH, and phosphate removal ratio for different Al 3+ concentration.

Table 3 .
Induction time, precipitation pH, and phosphate removal ratio for different