3.2.3. Influence on the Solid Precipitate Microstructure

The effect of foreign salts addition on the solid precipitate was studied. At the end of each FCP experiment, the precipitate obtained was characterized using the X-ray diffraction analyses. Figures 7–9 present, respectively, the XRD patterns of the precipitate obtained in the absence and presence of MgCl2, Na2SO4 and MgSO4 for IS1 and IS2. CaCO3 precipitated in the PCCW is mainly vaterite, with a little fraction of calcite. The formation of these calcite crystals is inhibited when Mg2+ ions (from IS1) and SO4 <sup>2</sup><sup>−</sup> ions are separately added (Figures 7 and 8). Nevertheless, the presence of Mg2+ promotes the aragonite (Figure 7), while SO4 <sup>2</sup><sup>−</sup> has no effect and vaterite remains the main CaCO3 polymorphous (Figure 8). The action mode of magnesium ions can be by (i) the substitution of calcium ions, or (ii) the insertion in or (iii) adsorption on the CaCO3 lattice [45]. If substitution or insertion leads to magnesian calcite, the adsorption at the earlier stage of nucleation and growth affects the CaCO3 crystalline structure, which is the most probable action mode for our case. Indeed, the interactions between Mg and CaCO3 nuclei are strong because they are improved by electrostatic interactions [46]. Moreover, Mg2+ competes with Ca2+ due to having the same charge, which favors the magnesium adsorption on the surface of calcite and vaterite [47]. The amount of Mg2+ being adsorbed onto CaCO3 crystal active growth sites can be influenced by the precipitation rate and the local solution chemistry [22]. In the presence of MgSO4 at IS1, the vaterite is completely transformed into aragonite (Figure 9) as for the precipitate obtained in MgCl2-solution (Figure 7). This confirms the role of magnesium ions in favoring the aragonite instead of vaterite. However, at such IS and in the presence of SO4 <sup>2</sup>−, Mg2+ ions could not inhibit the crystallization of the calcite form (Figure 9). By increasing the ionic strength (IS2) in the case of MgSO4, calcite is gradually inhibited.

**Figure 7.** XRD patterns of the precipitate formed in the presence of MgCl2 by varying IS.

#### *3.3. Effect of Antiscalant*

#### 3.3.1. Influence on the Nucleation Threshold

Calcium carbonate scale deposits in natural water installations are a stimulating issue, mainly by obstructing the water flow. The formation of such undesirable deposits can be inhibited by the addition of antiscalants such as organic sodium salt of polyacrylate (RPI) and inorganic sodium tripolyphosphate (STPP). The results of FCP tests obtained in PCCW-RPI and PCCW-STPP solutions were compared to the results of the PCCW solution.

**Figure 8.** XRD patterns of the precipitate formed in the presence of Na2SO4 by varying IS.

**Figure 9.** XRD patterns of the precipitate formed in the presence of MgSO4 by varying IS.

Figures 10 and 11 report the temporal evolution of pH and ΔResistivity as a function of RPI and STPP antiscalants amount. They show that the presence of both antiscalants RPI and STPP significantly influences the kinetics of the nucleation process. Indeed, tprenuc increased from 12 min for PCCW to 20 min in the presence of 4 ppm RPI and 0.8 ppm STPP, as seen in Table 3. During this time, the resistivity remained invariable corresponding to the prenucleation threshold limit. In addition, the increase of the antiscalant amount added to PCCW led to a higher precipitation time. In addition, STPP greatly delayed the precipitation of CaCO3 compared to RPI. In fact, tprec increased from 46 to 210 min and to 365 min for 4 ppm RPI and 0.8 ppm STPP, respectively. Furthermore, the duration of the stable nuclei formation (Δt=tprec − tprenuc) was longer as the antiscalant amount was larger. Therefore, the distinction between the prenucleation and precipitation thresholds became easier. Moreover, the prenucleation threshold was always attained at Ωprenuc values less than 7. Conversely, the precipitation threshold was reached at very high Ωprec, especially after adding STPP to PCCW solution, which went up to 353 at 0.8 ppm. Consequently, both antiscalants affected the kinetics and thermodynamics of calcium carbonate prenucleation and precipitation with different manners.

**Figure 10.** pH and ΔResistivity vs. time curves as a function of RPI amount.

**Figure 11.** pH and ΔResistivity vs. time curves as a function of STPP amount.

3.3.2. Influence on the Surface Scaling

The influence of the two antiscalants on scale surface deposition was studied by calculating the total precipitation rate and the heterogeneous precipitation percentage using the weight method. As seen in Table 3, after the addition of each antiscalant, the total precipitation rate τprec decreased remarkably from 45% for PCCW to 29% and 33% in the presence of RPI (4 ppm) and STPP (0.8 ppm), respectively. Moreover, for the same antiscalant, τprec decreased by increasing its amount added to PCCW. Furthermore, both antiscalants greatly affected the scale adherence to the cell wall and probes. Indeed, %hete increased from 45%

for PCCW to 79% and to 70% after adding 4 ppm of RPI and 0.8 ppm of STPP, respectively. Despite RPI being organic and STPP being inorganic antiscalants, they both favorably promote precipitation on the cell wall detrimentally to the bulk solution scaling.


**Table 3.** FCP test results by varying the antiscalant amount.

3.3.3. Influence on the Solid Precipitate Microstructure

The X-ray diffraction patterns of the precipitate formed in the presence of RPI and STPP are presented in Figures 12 and 13. The patterns of deposit scale obtained in PCCW reveal the formation of calcite with majority vaterite. After the addition of 4 ppm RPI, the crystallization was orientated towards the formation of calcite and to the complete inhibition of vaterite. In the presence of 0.8 ppm STPP, the phases formed remained vaterite and calcite, with the appearance of a new aragonite phase. Consequently, the two antiscalants acted on the CaCO3 precipitation in different ways. XRD characterizations proved that the presence of antiscalant can affect the morphology and the crystal shape of calcium carbonate [48].

**Figure 12.** XRD patterns of the precipitate formed in the presence of RPI.

**Figure 13.** XRD patterns of the precipitate formed in the presence of STPP.
