Chemistry of Organophosphonate Scale Growth lnhibitors: 3. Physicochemical Aspects of 2-Phosphonobutane-1,2,4-Tricarboxylate (PBTC) And Its Effect on CaCO3 Crystal Growth

Industrial water systems often suffer from undesirable inorganic deposits, such as calcium carbonate, calcium phosphates, calcium sulfate, magnesium silicate, and others. Synthetic water additives, such as phosphonates and phosphonocarboxylates, are the most important and widely utilized scale inhibitors in a plethora of industrial applications including cooling water, geothermal drilling, desalination, etc. The design of efficient and cost-effective inhibitors, as well as the study of their structure and function at the molecular level are important areas of research. This study reports various physicochemical aspects of the chemistry of PBTC (PBTC = 2-phosphonobutane-1,2,4-tricarboxylic acid), one of the most widely used scale inhibitors in the cooling water treatment industry. These aspects include its CaCO3 crystal growth inhibition and modification properties under severe conditions of high CaCO3 supersaturation, stability towards oxidizing microbiocides and tolerance towards precipitation with Ca2+. Results show that 15 ppm of PBTC can inhibit the formation of by ∼35 %, 30 ppm by ∼40 %, and 60 ppm by ∼44 %. PBTC is virtually stable to the effects of a variety of oxidizing microbiocides, including chlorine, bromine and others. PBTC shows excellent tolerance towards precipitation as its Ca salt. Precipitation in a 1000 ppm Ca2+ (as CaCO3) occurs after 185 ppm PBTC are present.


INTRODUCTION
Calcium carbonate /1/ and calcium phosphate(s) /2/ are the most frequently encountered deposits in industrial water systems. Their accumulation greatly diminishes effective heat transfer, interferes with fluid flow, facilitates corrosion processes, and can worsen microbiological fouling/3/. These phenomena are most critical in cooling water applications, where incoming water passes through a heat exchanger, cools a "hot" process and is sent back to repeat the same cooling process after it is cooled by forced evaporation/4/. This water loss by evaporative cooling results in high supersaturation levels of dissolved ions. Eventually, massive precipitation of sparingly soluble mineral salts can occur, either in bulk or on a surface that, in some cases, causes catastrophic operational failures. These usually require chemical and/or mechanical cleaning of the adhered scale, in the aftermath of a scaling event. Silica and silicate salts are such examples/5/.
Scale growth can be mitigated by use of scale inhibitors. They are key components of any chemical water treatment added to process waters in "ppm" quantities and usually work synergistically with dispersant polymers/6,7/. Phosphonates belong to a fundamental class of such compounds/8/used extensively in cooling water treatment programs /9/, oilfield applications /10/ and corrosion control /11/. PBTC, HEDP (hydroxyethylidenediphosphonate) and AMP (amino-tris-methylenephosphonate) are "popular" and effective commercial scale inhibitors (Figure 1) /12/. Phosphonates are thought to achieve scale inhibition by adsorbing onto specific crystallographic planes of a growing crystal nucleus after a nucleation event. This adsorption prevents further crystal growth and agglomeration into larger aggregates/13/.
Study of phosphonates is attracting additional interest due to their potential uses in sequestering toxic metal ions in industrial effluents. Moreover, their established use as bone resorption agents and in treatments for osteoporosis makes them desirable from a biological/pharmaceutical perspective.
Understanding the function of scale inhibitors requires a closer look at the molecular level of their possible function. The present study aims toward this direction and reports the inhibition properties of 2phosphonobutane-l,2,4-tricarboxylic towards CaCO3 crystal growth inhibition under high supersaturation conditions, as well as its stability towards oxidizing biocides and Ca 2/ precipitation.

EXPERIMENTAL SECTION Preparations
All phosphonates were obtained from Solutia UK (Newport, United Kingdom). PBTC is available in acid form under the commercial name Dequest 7000 as 50 % w/w solution in water and was used as received. Aqueous solutions of PBTC are infinitely stable if common preservation practices are applied.

Calcium Tolerance Procedure
An amount of the Ca 2/ stock solution (400 mL) was placed in a glass container, which in turn was placed in a water bath and allowed to achieve the desired temperature of 54 C (monitored by a thermosensor, built into the pH probe) under continuous stirring (300 rpm). The solution pH was kept at 9.00 + 0.05 units using a pH controller coupled with a syringe pump supplying 0.1 N NaOH (set at 0.50 mL/min). NaOH addition was used for pH adjustment purposes only. Upon temperature equilibration, the PBTC solution was continuously fed into the test solution at 0.50 mL/min rate, using a different syringe pump. During PBTC addition solution pH was kept at 9.00 + 0.05 units. Solution turbidity (at 420 nm), conductivity and pH were monitored.
Signals were stored in a computer at 1 data point/min. At a certain critical PBTC concentration the slope of transmittance started, to decrease, indicating turbidity increase due precipitate formation. Calcium tolerance was calculated based on the amount of PBTC (in ppm) at the onset of precipitation. Calculations also took into account dilution effects from external addition of solutions. The same procedure was followed for HEDP and AMP.

CaCO3 Scale Inhibition Test
Ca2/, Mg2+, and HCO3 are expressed as ppm CaCO3, whereas PBTC as ppm actives. Appropriate amounts of stock solutions were used to achieve final concentrations of Ca 2+ (800 ppm), Mg 2/ (200 ppm), and HCO3 (800 ppm). In a volumetric flask Ca2/, Mg 2/ were mixed and then the appropriate amount of PBTC was added. Finally, the desired amount of NaHCO3 was added and the remaining volume was made up with de-ionized water. The final volume of the test solution was 100 mL. The solution was then transferred to an Erlenmeyer flask. The flask was covered and placed in a water bath maintained at 43 C. The solution inside the flask was under constant stirring with a magnetic stirring bar. pH 9.0 was maintained by addition 0.1 N NaOH via an auto-titrator. After a time period of 2 h the flask was removed from the water bath and a sample was filtered through a 0.45 la filter. Analysis by atomic absorption spectroscopy gave the concentration of soluble Ca2+. The remaining solution was covered and stored unstirred at room temperature. A second set of samples was withdrawn from just below the surface 24 h after the pH was first raised to 9.0. The analytical results of these unfiltered samples yielded the dispersed Ca 2+ concentration. No further pH adjustment was made before the 24 h measurement.
It should be noted that the experimental water used in the present study cannot cover the whole range and variability of natural waters used for cooling purposes. It represents, however, a good "model" for the precipitation chemistry taking place in most process cooling waters. Although other metal ions can form foulant salts in supersaturated cooling waters they were not included in the study because they represent a minority of problems. Such sparingly soluble electrolytes will be subjects of future reports.

Biocide Resistance Procedure
Appropriate amounts of stock solutions were used to achieve final concentrations of Ca -/ (200 ppm), Mg 2+ (100 ppm), and HCO3" (200 ppm). In five different volumetric flasks Ca2+, Mg 2+ were. mixed and then the appropriate amount of PBTC was added to give 5 ppm actives. Each biocide was added to the appropriate flask at a 5 ppm dosage (as total C12). Solution temperature was 25 C. pH was adjusted and maintained at 8.3.
An aliquot was withdrawn after 1 h and was analyzed for phosphate (o-PO43) by the molybdophosphoric method/14/.

RESULTS AND DISCUSSION
Inhibitory Effect of PBTC on CaCO3 Crystal Growth and Crystal Modification Inhibitors are often required to control scale formation under very stressful conditions of metal ion and carbonate concentration and pH. Instrument malfunction occasionally results into increase of cycles of concentration well beyond the specification of the scale inhibition program used. pH sensors left uncalibrated for long periods often give erroneous measurements. As a result, operating conditions of high supersaturation levels and high pH are not unusual. Scale inhibitors can protect the system from such operational upsets and, consequently, unwanted deposits.
In the present study high hardness conditions were used to model a situation where there is uncontrollable increase of the cycles of concentration. This could very well occur when a malfunctioning conductivity meter does not properly allow supersaturated water to escape from the system (blowdown) and be replaced with make-up (fresh) water.
CaCO3 is the only insoluble salt precipitating under the conditions studied. Although Mg 2/ ions are added in the system (to "model" realistic process cooling waters) they do not cause precipitation of Mg(OH)2, which forms at pH regions above 10. Precipitation of MgCO3 is not a possibility, since its solubility is much higher than that of CaCO3. Under the experimental conditions studied, PBTC can inhibit the formation of CaCO3 up to 350 ppm at a 60 ppm dosage level (Table 1, Figure 2). Its performance is comparable to that of HEDP and AMP under similar conditions/12a/. Its effectiveness shows an upward trend as the actives level increases, Figure 2.
Influence of PBTC on crystal morphology of CaCO3. Crystallization of without additives (left, bar 20 [a), and with 15 ppm PBTC (right, bar 10 t).

Resistance to oxidizing biocides
Certain scale inhibitors, such as HEDP, AMP and other aminomethylene phosphonates, have well-known susceptibility to oxidation by chlorine or bromine-based biocides (necessary to control microbiological growth)/16/. Orthophosphate (PO43"), one of the degradation products, can cause calcium phosphate scale deposition.

Calcium tolerance
Certain applications of organophosphonate scale inhibitors are based on their precipitation as insoluble species with ions such as Ca2/, Sr2/, Ba2/, etc. In geothermal wells, for example, precipitation of scale inhibitors as alkaline earth salts is desirable. Large amounts of inhibitor are "squeezed" in the oilfield well and remain there for a specified amount of time, during which the inhibitor precipitates with alkaline earth metals found in the high-salinity brine and eventually deposits onto the rock formation. Once the well is opened again for operation the metal-inhibitor salts slowly dissolve to provide adequate levels of scale inhibitor in solution /17/. Controlled dissolution of these salts is essential, as fast dissolution will lead to chemical wastage and slower dissolution will result in inefficient scale control.
In cooling water applications (particularly in open recirculating sustems) adequate levels of inhibitor at all times is essential. In these systems, occasionally high supersaturation levels of Ca2/, coupled with inhibitor overfeeding may lead to precipitation of insoluble Ca-inhibitor precipitates. Such precipitates can be detrimental to the entire cooling water treatment program for several reasons: (a) They cause depletion of soluble inhibitor, and, subsequently, poor scale control because there is little or no inhibitor available in solution to inhibit scale formation.
(b) They can act as potential nucleation sites for other scales. (c) They can deposit onto heat transfer surfaces (they usually have inverse solubility properties) and cause poor heat flux, much like other known scales, such as calcium carbonate, calcium phosphate, etc.).
(d) If the phosphonate inhibitor in the treatment program has the purpose of corrosion inhibition, its precipitation as a Ca salt will eventually lead to poor corrosion control.
Based on the above, resistance to precipitation is a useful property of a scale inhibitor. Calcium tolerance is defined as the ability of a certain chemical compound to remain soluble in the presence of calcium ions. It usually decreases as pH increases. This is because at higher pH's the degree of deprotonation of inhibitors (usually phosphonates or carboxylates) is higher.
Calcium tolerance becomes very critical as cycles of concentration increase. An efficient inhibitor must interact strongly with Ca2/, but must be sufficiently soluble to remain in the system. Ca-inhibitor salt precipitation is well known for phosphonate as well as carboxylate-based inhibitors/18/. Three of the most commonly used scale inhibitors, AMP, PBTC, and HEDP, were studied comparatively with respect to their Ca tolerance. The results are found in Table 4 and in Figure 6.

CONCLUSIONS
The definition of the ideal scale inhibitor varies according to the particular application. The scale inhibitor operating in open recirculating cooling water applications must possess desirable properties such as: (a) excellent scale inhibition performance, (b) high metal ion tolerance (resistance of metal-inhibitor salt to precipitate out of solution), (c) stability towards oxidizing biocides, (d) thermal stability (for high temperature applications), and (d) low production cost.
The results of the present study are summarized as follows: (a) PBTC is an effective CaCO3 inhibitor under conditions of high hardness and alkalinity.
(b) It is essentially immune to decomposition by oxidizing biocides at normal biocide levels. This makes PBTC the ideal choice in water systems treated with oxidizers for microbiological control.
(c) PBTC possesses high calcium tolerance and its Ca complex salt does not precipitate under conditions of extreme hardness.
The systematic study of a plethora of phosphonates and mixed phosphonates/carboxylates and their inhibitory activity against metal salt scalants under conditions representing various industrial water treatment applications are currently underway in our research efforts/19/.

ACKNOWLEDGMENTS
The help of Dr. Dimitri Kusnetsof with the calcium tolerance experiments is gratefully acknowledged.