Calcium Pyrophosphate Crystal Salt forms and the Influence of Phosphocitrate (original) (raw)
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The inhibition of calcium carbonate precipitation in aqueous media by organophosphorus compounds
Journal of Colloid and Interface Science, 1992
The formation of insoluble calcium carbonate scale is often undesirable and the use of water additives which prevent or retard the deposition of this salt is widespread as the alternative solution of water acidification would cause severe corrosion problems. In the present work, the effect of four synthetically prepared compounds, 1,2-dihydroxy-l,2-bis(dihydroxyphosphonyl)ethane (DDPE), 2-dihydroxyphosphonyl-2-hydroxypropionic acid (DHHPA), 1,3-bis[( 1-phenyl-l-dihydroxyphosphonyl)methyl]-2-imidazolidinone (BPDMI), and 2,3-bis(dihydroxyphosphonyl)-l,4-butanedioic acid (BDBA), on the spontaneous precipitation of calcium carbonate in aqueous media was investigated. The precipitation process was studied in batch reactors at 25°C and pH 8.50, under conditions of sustained supersaturation. Two of the organophosphorous compounds investigated, DDPE and BDBA, contained P--C--C--P bonds in their molecules. In a third, BPDMI, the two P--C bonds were separated by a bulky fivemembered ring, and the other, DHHPA, contained only one P--C bond. All organophosphorus compounds examined in the present work were synthesized in the laboratory and were found to be resistant to hydrolysis even at elevated temperatures, typical of those existing in geothermal wells. In all cases, the induction periods preceding the onset of the spontaneous precipitation of calcium carbonate were significantly increased depending on the concentration of the additives examined, while the concomitant precipitation rates were markedly slower at concentration levels as low as 2 × 10 -8 mol dm -3. It is possible that the inhibiting effect of the organophosphorus compounds tested is due to the blocking of the active growth sites of the crystallites formed after the induction period. The existence of the P--C--C--P bonds in the molecules of the organophosphorus compounds resulted in the most effective inhibitors, while the separation of the two phosphonyl groups by the bulky five-membered ring resulted in the same inhibitory activity as with the compound possessing only one phosphonyl group.
Addition of carboxylate-containing polymeric materials to a metastable supersaturated calcium carbonate solution greatly reduced calcite crystal growth rates at constant supersaturation and pH = 8.5. Calcite crystallization rates were decreased to half their value in pure solutions by a tannic acid concentration of about 0.3 ppm (parts per million); a fulvic acid concentration of about 0.2 ppm; and a poly(acrylic acid) concentration of about 0.0175 ppm. An equation relating the calcite crystallization rate and the additive concentration follows an expression based on a Langmuir adsorption model. However, the Langmuir isotherm plot has two linear segments suggesting that these polyelectrolyte inhibitors may selectively adsorb initially at the fastest growing crystal faces. This relation between polyelectrolyte concentration and calcite growth rates implies inhibition by carboxylate-containing polymeric materials involves blockage of crystal growth sites on the calcite surface.
The use of natural hard waters in industrial water systems (e.g., cooling, boiler, desalination, oil production, etc.) can cause severe scaling and corrosion of equipment surfaces and pose serious technical and economic challenges. The scales commonly encountered are sulfates, carbonates, and phosphates of calcium, magnesium, and barium. The precipitation and deposition of scale on equipment surfaces are influenced by various factors including feed and re-circulating water chemistry, pH, temperature, flow velocity, heat exchanger metallurgy, and types of additive used in the treatment program. Such scale deposits significantly reduce heat transfer efficiency, constricts flow (e.g., reduces internal pipe diameters), increase the operating pressure of pumps, and enhance the probability of corrosion damage. In many cases, the removal of deposits leads to discontinuous operation of the system, resulting in higher operating costs. In addition, treatment chemicals (i.e., flocculants and coagulants) used upstream to treat feed water also influence the performance of additives used to prevent the precipitation of scale forming salts.
Inhibitory Effects of “Green” Additives on the Crystal Growth of Sparingly Soluble Salts.
In this chapter the effects of environmentally friendly additives on formation and growth of sparingly soluble salts are presented in a concise way. Specifically, the influence of a biodegradable, environmentally friendly polysaccharide-based polycarboxylate, carboxymethyl inulin (CMI), on the crystal growth kinetics of calcium oxalate has been studied. The spontaneous crystallization method was used to delineate the crystallization kinetics of calcium oxalate (CaC 2 O 4 , CaOx). The results demonstrate that the retardation in crystal growth is controlled by the carboxylation degree of the CMI and its concentration. These studies also show that CMI additives direct calcium oxalate crystallization from calcium oxalate monohydrate (COM) to calcium oxalate dihydrate (COD). Colloidal silica is known to be a substantial problem in silica-laden process waters. Although silica "technically" is not a salt, it is often categorized with them, as it brings about the same problems as mineral scale deposits (reduced heat transfer, etc.). We also report a strategy to retard silicic acid condensation in supersaturated aqueous solutions by using non-toxic, "green", zwitter-ionic phosphonomethylayed chitosan (PCH). An overview of the use of green additives in water treatment is also presented.
Theoretical and practical experience of calcium phosphate inhibition in RO waters
The increasing trend over the last ten years to use waste water as a feed source for reverse osmosis membrane plant has resulted in an increase in calcium phosphate scaling on membrane surfaces. This is due to much higher levels of phosphate in waste water typically 10-30 mg/l as compared with a groundwater source which may have only 1-5 mg/l of phosphate. These higher phosphate levels resulting from effluent breakdown in primary sewage treatment causes calcium phosphate solubility to be exceeded in reverse osmosis membrane systems resulting in scale formation. Conventional antiscalant chemistry based on threshold inhibition and crystal distortion has had poor results in high phosphate high pH waters. Dosing large quantities of acid to lower the scaling tendency is no longer an acceptable approach. This paper investigates the complexities of multiple species of calcium phosphate with varying solubility's all contributing to an amorphous deposit. Research conducted by the medica...
Myo-inositol hexakisphosphate(phytate) inhibits calcium carbonate crystallisation in hard water
A batch system and a flow system with synthetic water were used to study calcium carbonate precipitation and phytate crystallisation inhibitory effects. Afterwards, phytate inhibitory effects on calcium carbonate crystallisation were tested in a real system, working with a cistern filled with hard water. Finally, the effects of phytate on calcium carbonate crystallisation were compared with another phosphate derivative and with a chelating agent.
Systematic Structural Determinants of the Effects of Tetraphosphonates on Gypsum Crystallization.
In this study, the effect of phosphonate additives on the crystallization of calcium sulfate dihydrate (CaSO 4 3 2H 2 O, gypsum) has been investigated in aqueous solutions. Ethylenediamine-tetrakis(methylenephosphonic acid) (EDTMP), hexamethylenediamine-tetrakis(methylenephosphonic acid) (HDTMP), octamethylenediamine-tetrakis-(methylenephosphonic acid) (ODTMP), and dodecamethylenediamine-tetrakis(methylenephosphonic acid) (DDTMP) have been used as additives. It was found that they are very effective retardants for the crystallization of calcium sulfate dihydrate. The inhibition efficiency is directly proportional to the number of methylene groups in the organic chain that connects the aminobis(methylenephosphonate) moieties. The degree of inhibition of crystallization was measured as an increase in induction time and reduction in crystallization rate. Particle size and crystal morphology were determined with a particle-sizer and scanning electron microscopy. According to experimental results, phosphonate additives tested in this study are very effective retardants for the formation of calcium sulfate dihydrate scale. The crystal structure of [Ca(EDTMP)(H 2 O) 2 ] 3 H 2 O is also reported. This is a one-dimensional coordination polymer in which EDTMP acts as both a bidentate chelate and a bridge for Ca 2þ centers.
Journal of Crystal Growth, 2000
The rate of growth of a stable form of calcium pyrophosphate tetrahydrate, found to be monoclinic and not orthorhombic as previously proposed, has been studied at pH 5 and 7. At low supersaturation (S(2), the rate of growth at pH 5 is about 20 times higher than at pH 7 for the same supersaturation. At pH 5 the growth rate increases with S as expected, with simultaneous spontaneous precipitation occurring at S '2. At pH 7 the growth rate increases with S up to S +3, but thereafter growth is increasingly inhibited as S increases. Spontaneous precipitation occurs after longer induction times at pH 7 than at pH 5. Contrary to other ionic species, concentrations of CaP P\ and P O\ ions in solution are considerably higher in experiments at pH 7 than at pH 5, and the concentration of CaP P\ increases strongly with S for S '3 at pH 7. This indicates that chelation of Ca> and P O\ on the crystal surface inhibits growth and precipitation at pH 7. To our knowledge such an auto-inhibition of crystal growth has not been observed previously.
The influence of the presence of humic, fulvic, and polyacrylic acid on the nucleation and crystal growth of calcium carbonate in aqueous supersaturated solutions was investigated in batch reactors at 25°C and pH = 8.50. The nucleation of calcium carbonate was investigated by free drift methods, and the crystal growth was investigated with seeded crystal growth experiments at constant supersaturation. In all cases calcite was found to form exclusively, and the presence of all tested compounds at concentrations between 0.1 – 1.0 ppm prolonged the induction time preceding the spontaneous formation of calcite. Humic acid at concentration up to 0.5 ppm inhibited the growth of calcite seeds up to 95% and polyacrylic acid at concentrations up to 0.1 ppm gave the same degree of inhibition. Polyacrylic acid was found to be stronger inhibitor. A concentration of 0.25 ppm of polyacrylic acid completely stopped crystal growth of calcite. Humic acid at concentration 1.0 ppm completely stopped crystal growth of calcite seed crystals. The retardation was explained by the adsorption of the polyelectrolytes onto the active growth sites of the crystals. Application of a Langmuir-type adsorption model on the kinetics data obtained in the presence of the inhibitors tested yielded a higher affinity constant of polyacrylic acid for the calcite seed crystals.