Modified macromolecules in the prevention of silica scale

Reagents and chemicals

Sodium silicate pentahydrate Na₂SiO₃·5H₂O was purchased from Sigma-Aldrich. Ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O) was obtained from Alfa-Aesar and oxalic acid (H₂C₂O₄·2H₂O) from EM Science (Merck). Sodium hydroxide (NaOH) and hydrochloric acid (HCl) 37% were purchased from Sigma-Aldrich. Deionized water from an ion-exchange resin was used for all experiments and stock preparations. This water was tested for molybdate-reactive silica (interference tests) and was found to contain negligible amounts.

Solution preparation

Sodium silicate stock solution: A solution containing silicate (500 ppm as SiO₂) was prepared by dissolving 4.08 g of Na₂SiO₃·5H₂O in 2 L of deionized water (a non-glass container must be used to avoid silicate leaching from glass), followed by overnight rigorous stirring.

Inhibitor stock solutions: Stock solutions of the additives in water (PPEI in various phosphonated forms) were 1% w/v (10 000 ppm).

Reagent solutions for the silicomolybdate method. The following solutions were prepared for the silicate spectrophotometric detection test: (a) An ammonium molybdate solution: 10 g of ammonium molybdate was dissolved in 100 mL of water, and its pH was adjusted between 7 and 8 with NaOH to avoid precipitation of ammonium molybdate. (b) A hydrochloric acid solution: one volume 37% HCl was mixed with equal volume water. (c) An oxalic acid solution: 8.75 g of oxalic acid was dissolved in 100 mL of water. All solutions were kept in polyethylene containers. HCl and ammonium molybdate were kept in the fridge, while silica and oxalic acid solution were kept at room temperature.

Silicic acid polycondensation protocol (“control” experiment)

Hundred milliliter from the 500 ppm sodium silicate stock solution (see above) was placed in a polyethylene beaker and the initial pH was found 11.8. The pH is then adjusted to 7.00±0.1 by addition of HCl and/or NaOH, as needed (the change in the resulting volume was minor and did not affect any of the calculations). Then, the beaker was covered with plastic membrane and set aside without stirring. The solutions were checked for molybdate-reactive silica by the silicomolybdate method every 1 h for the first 8 h or after 24, 48, and 72 h after the pH adjustment (see below).

Silicic acid polycondensation in the presence of PPEI inhibitors

Hundred milliliter portions of the 500 ppm sodium silicate stock solution (see above) were placed in polyethylene containers. In each container, different volumes of PPEI (from the prepared 10 000 ppm stock solutions) were added to achieve desirable concentration. After that, the same procedure as the “control” protocol was followed.

Determination of molybdate-reactive silica (silicomolybdate method)

Molybdate-reactive silica (mainly mono- and some disilicic acid) was quantified using the well-established silicomolybdate spectrophotometric method [43]. As in our previous studies, we used the “yellow molybdate” method (using Spectrophotometer HACH DR/890,) as follows: 2 mL from the working solution is filtered through a 0.45 μm syringe filter and then diluted to 25 mL in a special cylindrical cell of 1 cm path length, made of quartz. Next, 1 mL of ammonium molybdate stock solution and 0.5 mL of HCl (1:1 dilution of the concentrated solution) are added to the sample cell, the solution is shaken well and left standing for 10 min. Afterward 1 mL of oxalic acid solution is added and the cell contents are mixed well. The solution is set aside for 2 min. The photometer is now set to “zero absorbance” using a sample of deionized water (“blank”). Finally, the sample absorbance is measured (at 452 nm) and is expressed as “ppm SiO₂”. The detectable concentration range for this specific protocol is 6–75 ppm. To calculate the concentration in the original solution, an appropriate dilution factor (×27.5/2) is applied. The basic working principal of the silicomolybdate test is that ammonium molybdate reacts only with mono- and disilicic acid and any phosphate present and forms yellow-colored complexes (see Fig. 4).

Fig. 4:

Structure of the yellow silicomolybdic acid cage-like cluster. The silicon atom (gray sphere at the center) is encaged within 12 MoO₆ octahedra (oxygen atoms in white) [44].

This reaction requires acidic environment in order to take place, and this is why the hydrochloric acid is added to the samples. It should be noted that colloidal silica does not participate in the reaction and thus does not affect the intensity of yellow color, which is proportional to the concentration of the reactive silica present in the sample experiment. Oxalic acid is added to destroy any molybdophosphoric acid formed, leaving the silicomolybdate complex intact, and thus eliminating any color interference from phosphates.

Inhibitory activity of PPEI derivatives

PPEI was tested in “long-term” (up to 3 days) and “short term” (8 h) experiments. Long term experiments are an important time frame for scale inhibitors, as it evaluates the ability of a certain inhibitor to prevent scale formation for a prolonged time period. Short term experiments examine the silica polycondensation in its early stages.

At first, PPEI (full, i.e. 100% phosphonomethylation degree, in mole percent) was tested as a silica scale inhibitor in long term (3 days) experiments at various concentrations (20–80 ppm), see Table 1 and Fig. 5. Results obtained with PEI as the inhibitor are also presented for comparison in Table 1 and Fig. 5 (upper). PEI, due to its high positive charge density is not active at concentrations above 40 ppm, whereas, at 20 ppm dosage, it enhances silicic acid solubility by only 53 ppm above the control. By introducing the phosphonate moieties, inhibitory activity is dramatically increased in PPEI, see Fig. 5, lower. This is particularly evident at inhibitor dosages >40 ppm. Specifically, at 60 ppm dosage PPI (full) is able to enhance silicic acid solubility more than 200 ppm above the control after 24 h. This behavior of PPEI may be explained by the “relief” the anionic phosphonate groups exert on the cationic backbone. By balancing the excess cationic charge, they most likely prevent inhibitor entrapment in the forming silica matrix.

Fig. 5:

Silica inhibition by PEI (upper) and PPEI (full, lower) in long-term (3-day) experiments.

The results presented in Table 1 and Fig. 5 establish that by grafting anionic phosphonate moieties onto the PEI backbone, thus creating the zwitterionic PPEI polymer, profoundly increases four-fold the inhibitory activity. However, this is true for the fully grafted PPEI (100% phosphonomethylation). Hence, in order to evaluate the effect of the extent of phosphonometylation on inhibitory activity, a series of partially grafted PPEI’s were studied. For direct comparison reasons, the 60 ppm dosage was studied for all inhibitors. These results are shown in Table 2 and Fig. 6.

Fig. 6:

Effect of PPEI% phosphonomethylation degree on silicic acid inhibitory activity of all PPEI forms synthesized, at 60 ppm PPEI concentration, in long-term (3 days) experiments. Upper: 3-day results. Lower: linear dependence of the 24-h measurements.

There appears to be an almost linear dependence of inhibitory activity on % grafting degree. The maximum inhibition performance is achieved when PPEI is fully (100%) substituted. Specifically, it stabilizes 202 ppm of silicic acid higher than the control after 24 h. Its inhibitory activity drops after 48 and 72 h, as is expected from our past experience of silica inhibitors, but still PPEI 100% is able to stabilize 171 ppm and 127 ppm, respectively above the control (Fig. 6 upper).

The concentration dependence of the PPEI (40, 60, 80, 100%) inhibitors was also studied, and the results are shown in Fig. 7. In all grafting degrees inhibitory activity increases as the PPEI concentration increases. For the grafting degrees 40, 60, and 80%, the enhancement is very mild as concentration increases from 0 to 20, to 40 ppm. In contrast, the same concentration increase for PPEI 100% causes a drastic enhancement in silicic acid stabilization. All PPEI inhibitors reach a plateau after the 60 ppm concentration.

Fig. 7:

Dependence of inhibitory activity on concentration. Measurements include those after the first 24 h.

Based on the above-mentioned “long-term” (24 h) results, we decided to further study the silica inhibition event at its earlier stages, i.e. during the first 8 h. For this purpose, we selected all phosphonated forms of PPEI (40, 60, 80 and 100%), which were tested in short – term measurements at 60 ppm concentration. These results are presented in Table 3 and plotted in Fig. 8. The same dependence of inhibitory performance on % grafting was noted, as with the long-term experiments. Specifically, as the grafting degree increases, silicic acid stabilization ability increases. Again the fully substituted PPEI is the most efficient inhibitor. It can stabilize 400 ppm (246 ppm above the control) silicic acid after 8 h.

Fig. 8:

Silica inhibition measurements with PPEI 8%, 40%, 60%, 80% and 100% (full) at a constant concentration 60 ppm in short-term experiments (8 h).

Characterization of precipitated silica

Although PPEI derivatives are efficient inhibitors of silica polycondensation, there is always some colloidal silica formed. This appears as a white flocculant precipitate in the working solutions. Its appearance and severity is variable and depends on the actual PPEI form and is a result of the competition of two concomitant processes: the silica condensation and the inhibition by PPEI. As shown in Fig. 9, as the phosphonomethylation% degree increases, the solution turbidity decreases. This is reasonably rationalized by the excessive cationic charge of the PPEI at lower substitution degrees, that causes flocculation of the formed silica particles at pH=7. It is well established that the pKa of surface silanols decreases as the silica polyconcensation degree increases [45]. In other words, the silanol groups on a silica particle are much more acidic (pKa~4) than silicic acid (pKa~9.3) [46]. Hence, cationically charged macromolecules at appropriate concentrations can act as flocculants in silica containing solutions, and, thus, create silica-inhibitor precipitates through electrostatic interactions and hydrogen bonding. This has been observed before [19], [21], [29]. However, when the negatively-charged phosphonate groups are introduced, the cationic charge density is lowered, hence, the tendency to generate silica-inhibitor precipitates is eased.

Fig. 9:

Visual demonstration of the turbidity results at 60 ppm inhibitor concentration at pH=7.0, after 8 h, at 25°C. 1: PPEI 8%, 2: PPEI 40%, 3: PPEI 80%, 4: 100%.

The visual observations presented in Fig. 9 were quantified by turbidity measurements taken over the course of 8 h at pH 7, shown on Fig. 10. Increase of the phosphonomethylation degree on the PPEI backbone has two significant effects: (a) the PPEI inhibitor becomes more efficient (see Figs. 5 and 6), and (b) the tendency of formation of silica-PPEI precipitates is substantially reduced.

Fig. 10:

Evolution of turbidity over time in solutions containing PPEI inhibitors with various phosphonomethylation degrees. All PPEI inhibitors are at 60 ppm concentration.

Selected silica precipitates were studied by scanning electron micrpscopy (SEM), in order to evaluate morphological effects of the PPEI inhibitors on the silica particle morphology and texture. The SEM images are shown in Fig. 11. SEM images were recorded on precipitates formed at pH 7.0, after 8 h and after 3 days of polycondensation time, in the presence of various concentrations of fully phosphonomethylated (100%) PPEI, as indicated. The solids appear as aggregates of smaller, nearly spherical primary particles of submicron size.

Fig. 11:

SEM images of silica precipitates in the absence (control) and presence of PPEI inhibitors.