Open Journal of Inorganic Chemistry
Vol.05 No.03(2015), Article ID:57826,10 pages
10.4236/ojic.2015.53006

Study of Cd2+, Al3+, and Ions Influence on Struvite Precipitation from Synthetic Water by Dissolved CO2 Degasification Technique

Hassidou Saidou1,2,3*, Atef Korchef2,3, Sami Ben Moussa2, Mohamed Ben Amor2

1Departement of Chemistry, Faculty of Sciences and Techniques, Dan Dicko Dan Koulodo University of Maradi, Maradi, Niger

2Laboratory of Traitment of Natural Water, Center of Research and Technology of Water, Technopark of Borj Cedria, Tunisia

3Laboratory of Valorization of Useful Materials, National Center of Research in Material Sciences, Technopark of Borj Cedria, Tunisia

Email: *saidouhassidou@gmail.com, *saidouhassidou@yahoo.fr

Copyright © 2015 by authors and Scientific Research Publishing Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).

http://creativecommons.org/licenses/by/4.0/

Received 4 April 2015; accepted 2015

ABSTRACT

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 molar ratio of 8.

Keywords:

CO2 Degasification Technique, Struvite Precipitation, Cd2+, Al3+,

1. Introduction

Struvite (MgNH4PO4×6H2O) is a sparingly soluble salt which can be used as a valuable fertilizer in agriculture [1] . It can also contribute to scaling phenomenon as it can precipitate with other hard salts in wastewater treatment equipments plants [2] -[4] . Many parameters are susceptible to control the precipitation of this salt: supersaturation ratio [4] , concentration of major ions [5] , temperature [6] , pH [7] , nature of materials [4] , airflow rate [7] and foreign ions [8] [9] . Several techniques have been used to precipitate struvite: stirring [9] , aeration [10] - [12] , seeding [13] , CO2 degasification technique [4] [7] . During struvite precipitation, a relatively important ratio of phosphate can be recovered according to conditions and technique used. Indeed, Suzuki et al., (2005) [12] have obtained 65%; Ali and Schneider, (2006): 70% [14] , Battistoni et al., (2002): 80% [10] , Saidou et al., (2009a): 92% [7] , Jaffer et al., (2002): 97% [11] , and Guadie et al., (2014): 98% [15] . The study aims to investigate the effect of some foreign ions on struvite precipitation. Thus, Cd2+, Al3+, and were added, separately, in solution where struvite might precipitate, and the effect of each ion on kinetics and morphology of struvite was studied using CO2 degasification technique reported previously [4] [7] .

2. Materials and Methods

2.1. Synthetic Water Preparation

Synthetic water used in all experiments in this work was prepared by mixing the respective aqueous solutions of MgCl2∙6H2O and NH4H2PO4 in desired proportions in a calcium carbonate solution. This solution was previously prepared by dissolving calcium carbonate solids in a flow of CO2. Reagents MgCl2∙6H2O (purity > 99%), NH4H2PO4 (purity > 99%) and CaCO3 (purity: 99%) were respectively supplied by Fluka, Sigma Aldrich and Merck.

The study of the effect of cadmium, aluminum and sulfate ions was carried out using solutions prepared from CdCl2∙H2O, AlCl3∙6H2O, and Na2SO4 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 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 CO2 degasification technique [7] .

2.2. Analyzes

The phosphate concentration in the solution was determined by the colorimetric method using the reagent vanadomolybdic UV-visible spectrophotometer (model: HACH DR/4000). The solids obtained were analyzed using different physicochemical techniques: X-rays diffraction (XRD: X-PERT PRO model), scanning electron microscopy (SEM: Phillips model), infrared (IR: PerkinElmer model), laser particle size (model: Malvern Mastersizer 2000), differential scanning calorimetry (DSC: Mettler Toledo model).

3. Results and Discussion

3.1. Effect of Cd2+

X-rays diffractograms of solids obtained after addition of Cd2+, 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 Cd2+ 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 concentration in the solution for different Cd2+ 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 Cd2+ did not affect these two parameters. Furthermore, the yield of 35.5% phosphates removal for Cd2+ 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 Cd2+

Figure 1. Superposition of X-rays diffractograms of precipitates obtained for different Cd2+ concentration.

Figure 2. SEM pictures (a) (b) and EDAX spectra ((a') (b')) of solids obtained for different Cd2+ concentration: ((a) (a')): 1 mg Cd2+/L; ((b) (b')): 100 mg Cd2+/L.

Figure 3. Temporal evolution of solution pH for different Cd2+ concentration.

Figure 4. Temporal evolution of solution phosphate concentration for different Cd2+ concentration.

Table 1. Induction time, Precipitation pH, and Posphate removal ratio for different Cd2+ concentration.

contains less phosphate than that obtained in the presence of 1 mg/L of Cd2+. These remarks may explain the decrease in phosphate removal ratio observed when the Cd2+ concentration increased.

3.2. Effect of Al3+

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 Al3+ ions are considered as coagulants.

Figure 5. Superposition of X-rays diffractograms of solids obtained for different Al3+ concentration.

(a)(b) (c)

Figure 6. SEM pictures of solids obtained for different Al3+ concentration: (a) 0.5 mg/L; (b) 2 mg/L; (c) 6 mg/L.

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 Al3+ 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 Al3+ ions varied from 0 to 6 mg/L (Figure 8 and Table 2).

Figure 7. Temporal evolution of solution pH for different Al3+ concentration.

Figure 8. Temporal evolution of solution phosphate concentration for different Al3+.

3.3. Effect of

To determine the effect of sulfate ions on struvite precipitation, increasing amounts of this ion were added in the solution so that the 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 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. 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 molar ratio of 2 where it passed from a monotransformation () to mulitransformation by a gradual elimination of ammonia and water by the following equations:

Table 2. Induction time, precipitation pH, and phosphate removal ratio for different Al3+ concentration.

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

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

Figure 10. SEM pictures and EDAX spectra of solids obtained: ((a) (a')) = 2; ((b) (b')): = 8.

(T = 140˚C - 150˚C) (1)

Then:

(T > 170˚C) (2)

This result is an agreement with the previous studies [19] [20] .

By increasing 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 ammonia when molar ratio reached 8. These observations showed that the decomposition of struvite, in one step, is facilitated by increasing its amount for molar ratio of 8.

4. Conclusions

After this study, we can conclude as follow:

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

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

Figure 13. Heat flux in function of temperature for different molar ratios.

- 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 (Al3+) 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 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.

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NOTES

*Corresponding author.