The precipitation and dissolution of calcium sulfate dihydrate (CaSO4·2H2O, gypsum) is of particular interest because of its importance in desalination, in water treatment processes, in geothermal and oilfield drilling, and phosphoric acid production. The calcium salts lead to the scale formation on metallic surfaces. The scale formation reduces heat transfer effectiveness over time, leading to unacceptable heat transfer rates , , , .
Several methods of preventing of deposits formation have been developed. One of the most effective methods for controlling the unwanted deposition is addition of polymeric compounds at very low concentration. Studies have shown that inhibitory efficiency of the polymers strongly depends on polymer structure such as nature of their functional groups, molecular weight, and chemical composition , , , .
Despite all these investigations, the effect of inhibitors on deposition formation is still not yet fully disclosed. The presence of organic molecules , inorganic cations , macromolecules , polyelectrolytes ,  and cations  in a crystallizing solution are known to play an important role in the crystallization process. They may affect nucleation or crystal growth stages of mineral formation.
A large number of studies have appeared in the literature on the effects of functional groups like carboxylic acids, sulfonic acids, phosphoric acids, copolymers of acrylic acids, and high molecular weight polymers with acidic groups on crystallization of sparingly solubleinorganic salts process , , . Although there are many reports about water soluble polymers, there is only one example of the mineralization control by a rigid carboxylated polymer in literatüre. Neira-Carrillo et al.  investigated the influence of conductive polymers based on carboxylated polyaniline on CaCO3 crystallization. Their results show that the structural placement of the carboxy group in the polymer is determinant of its influence on the crystallization.
In this work, the effects of conductive copolymers of vinylphosphonic acid (VPA) and 4-vinylimidazole [poly(4-VIm-co-VPA)] (Fig. 1) on CaSO4·2H2O crystallization, was investigated by using spontaneous crystallization method. The degree of inhibition of cystallization was measured as an increase in induction time and the reduction in crystallization rate. The inhibition increases with VPA content of the copolymer. The fit of the Langmuir adsorption model to the experimental data support a mechanism of inhibition through molecular adsorption of polymers on the surface of growing crystals.
Calcium chloride dihydrate (CaCl2·2H2O) and sodium sulfate (Na2SO4) Merck) (reagent grade) were from Merck. Copolymers of VPA and 4-vinylimidazole-(4-VIm), poly(4-VIm-co-VPA) synthesis and polymer characterization and properties can be found in the literature , .
Crystal growth experiments
Crystal growth experiments were performed in mechanically stirred water-jacketed cells of 1 L capacity at 30±0.1°C. The supersaturated solutions for growth experiments were prepared by mixing equal volumes of CaCl2 and Na2SO4 solutions. Polymers were added to medium with Na2SO4 solution. The pH, temperature and conductivity of the reaction solutions were monitored by computer with appropriate software during the crystallization. Samples were collected at definite time intervals and quickly filtrated through millipore filters of 0.22 μm pore size. Ca2+ concentration in the aqueous phase was determined by atomic absorption spectroscopy (Perkin-Elmer Analyst 200). Induction period (ti ) and R0/Ri values used to determine the inhibition effect of copolymers on CaSO4·2H2O crystallization. The time between the generation of a supersaturated state and the first observed change in Ca2+ concentration was defined as the induction period. R0/Ri is the ratio of the rate of crystallization of the pure solution (R0, mol/L min) to the rate of crystallization in the presence of additive (Ri , mol/L min) at the same concentration and temperature. The rates reported were the initial rapid growth rate calculated from the slope of the ion concentration versus time plots for each experiment. The rates were determined from at least three separate experiments, and only the average values were reported. The induction period was found by monitoring the variations in the Ca2+ concentrations. The crystals which were filtered were examined by scanning electron microscopy (SEM) (JEOL JSM-SEM) and FT-IR (Bruker α-P).
Results and discussion
Table 1 summarize the conductive copolymers used in this study and the effect of added copolymer on the crystallization rate. The results show that the presence of a small amount of copolymers resulted in an initial inhibition, followed by precipitation at a rate comparable to the rate of crystallization from pure solutions. The effect of copolymer on the rate of crystallization was determined over a concentration range of 0.5 mg/L–2 mg/L.
Effect of conductive copolymer concentrations on growth rate of CaSO4·2H2O.
|Copolymer||VPA (mole %)||Polymer concentration, Ci (mg/L)||R0/Ri||Induction time, ti (min)|
The prolongation of induction period and increasing of R0/Ri ratio depends on the nature of the inhibitor and its effective concentration at a given condition. For a given inhibitor, generally the higher the inhibitor concentration, the longer induction period and bigger R0/Ri ratio. According to Table 1, is clear that the effectiveness of poly(4-VIm-co-VPA) copolymers increases with increasing VPA content of copolymers. Induction time increases from 85 min to 136 min for copolymers of poly(4-VIm-co-VPA) at 1 mg/L as the VPA content increases from 48 to 58. The experimental results in the present study show that not only induction time but also R0/Ri ratio increases with increasing VPA content. But increasing is not significiant as induction time for R0/Ri ratio. R0/Ri ratio increases from 1.023 to 1.260 as the VPA content increases from 48 to 58.
Figure 2 shows the plots of ti values as a function of polymer concentration for used copolymers. The crystallization in the presence of 0.5 mg/L of poly(4-VIm-co-VPA) copolymer which has higher VPA content is preceded by an induction period of 120 min and R0/Ri ratio of 1.171. ti value increased to 163 min and R0/Ri value changed as 1.503 at 2 mg/L. This means that duration of induction period and R0/Ri ratios are increased by increasing copolymer concentration.
The influence of concentration and VPA/4-Vlm ratio of copolymers on CaSO4·2H2O crystallization is shown in Fig. 3. Inhibition effect increases with concentration and VPA/4-Vlm ratio of copolymers.
The Langmuir model, which was developed for the adsorption of ideal gasses onto solid surfaces, can be used successfully to describe the adsorption of polymers which inhibit crystal growth on crystal surface. It can be explained by preferential adsortion of inhibitor at the active sites of the crystal surfaces.
Since the amounts of polymer in solution are small, it is thought that the inhibition is most likely occured by blocking of the active growth sites on growing crystal surfaces rather than binding to calcium ions in the solution . This assumption is interpreted by applying the kinetic results in a langmuir isotherm .
where KL is the adsorption or affinity constant which is the ratio of the rate constants for adsorption and desorption, kads/kdes, and Ci is the concentration of polymer. Depending on the surface coverage, θ, the crystallization rate in the presence of polymer, Ri , is defined by Eq. (2)
where R0 is the crystallization rate in the absence of polymer. Combination of Eq. (1) and Eq. (2) are shown in Eq. (3).
The linearity of the plots of Eq. (3) for gypsum crystal growth in the presence of copolymer (Fig. 4) suggests that the reason of inhibitory effect is adsorption of copolymers at active growth sites. kads/kdes can be obtained from the slope of the resulting straight line. The values of the affinity constant calculated for C1, C2, and C3 copolymers are 0.02 (L/mg), 0.22 (L/mg), 0.42 (L/mg), respectively. The results showed that the higher affinity was obtained when the VPA content of the copolymers increased. It may be explained that anionic copolymers containing high VPA can inhibit gypsum crystal growth and that such inhibition is directly linked to fractional coverage of adsorption sites because of their higher anionic charge density .
Photographs were also taken by scanning electron microscope (SEM) for the subsequent visual analysis in order to assess the effects of copolymers on crystal shape and size (Fig. 5). The presence of copolymers with different VPA contents in supersaturated solutions affects not only the kinetics of crystal growth but crystal size as well, as shown in Table 2. The needle-like crystals precipitated with average length of around 76.75 μm and width of 2.96 μm in the absence of copolymers. The average length and width of the crystals grown in the presence of the C3 copolymer was less than that of the control sample at 0.1 mg/L copolymer concentration. The average length of the needle-like crystals was reduced to 35.34 μm and width of 1.99 μm. When the copolymer concentration was increased to 2 mg/L, the average length of the needle-like crystals decreased to length of 32.58 μm.
properties of CaSO4·2H2O crystals grown from solution at the end of 5 h processing time.
|Copolymer||Polymer concentration (mg/L)||Needle||Plate|
|L (μm)||W (μm)||L (μm)||W (μm)|
FT-IR spectra was used to analyse the molecular structure of crystals (Fig. 6). The bands near 3600 and 3400 cm−1 are characteristic of H2O vibrations. A weak absorption is observed at 1000 cm−1 that ascribed to coupling of molecular vibrations. A strong doublet is seen near 600 and 669 cm−1 due to the (SO4)2− bending vibrations. The strongest bands are observed as a doublet near 1115 cm−1 due to (SO4)2− stretching vibrations. The (SO4)2− bending vibration is attributed to the band near 420–460 cm−1 .
According to experimental results, poly(4-VIm-co-VPA) copolymers which have different VPA content tested in this study are effective inhibitors for the formation of CaSO4·2H2O. The presence of copolymers inhibited the crystal growth of CaSO4·2H2O possibly through adsorption onto the active growth sites for crystal growth due to their charge and hydrophilic effects. The copolymer concentration and VPA content are found to be important parameters for the controlling of crystallization of CaSO4·2H2O.
The fit of the Langmuir adsorption model to the experimental data supports a mechanism of inhibition through molecular adsorption of polymers on the surface of growing crystals. The crystal size of CaSO4·2H2O was affected by presence of coopolymers. It was observed that copolymers reduce the particle size of crystals.
The results indicate that an increase in copolymer concentration results in an increase of induction period. The effectiveness of copolymer increases with increasing VPA/4-Vlm ratio of copolymers.
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