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

Chemistry of Organophosphonate Scale Growth lnhibitors: 3. Physicochemical Aspects of 2Phosphonobutane-l,2,4-Tricarboxylate (PBTC) And Its Effect on CaCO Crystal Growth Konstantinos D. Dcmadis* and Panos Lykoudis Department of Chemistry, University of Crete, 300 Leoforos Knossos, Heraklion, Crete, Greece GR-71409 GRAPHICAL ABSTRACT This study reports various physicochemical aspects of the chemistry of PBTC that include inhibition of CaCO3 crystal growth and modification properties under severe conditions of high CaCO.. supersaturation, stability towards oxidizing microbiocides and tolerance towards precipitation with Ca 2/. CaCO3 crystals (Untreated) CaCO3 crystals (treated with PBTC) Bar = 20 ix Bar = 10 ix ABSTRACT 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 Part 2, See preceding paper. Phone" +30 2810 393651, fax: +30 2810 393601, e-mail" 135 Vol. 3, Nos. 3-4, 2005 Chemistry of Organophosphonate Scale Growth Inhibitors: 3 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-l,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. Keywords: phosphonates, carboxylates, PBTC, calcium carbonate, crystal modifiers, inhibition, biocides Glossary HEDP AMP PBTC BCDM Calcium tolerance Cycles of concentration hydroxyethylidenephosphonic acid amino-methylene-tris-phosphonic acid Phosphonobutane-l,2,4-Tricarboxylic acid -bromo-3-chloro-5,5-dimethyl-hyd antoin ability of an inhibitor to remain soluble in the presence of Ca2/ Concentration increase of ions as compared to their initial concentration in the raw water Dispersancy Blowdown Biocides "Bromine" "Chlorine" Stabilized Halogens Prevention of scale deposition on a surface Designed loss of process cooling water and its replacement with "fresh" water in order to maintain a certain pre-specified level of conductivity Water additives that control microbiological growth. They can be either oxidizing or non-oxidizing. Term in the water treatment sector that is associated with BrO, not Br2 Term in the water treatment sector that is associated with C10, not C12 XO (X halogen) with additives that render them less aggressive 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 136 K.D. Demadis and P.Lykoudis Bioinorganic Chemistry and Applications 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 Ca2/ 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. Stock solutions were prepared in deionized water as follows: CaC12"2H_O 10,000 ppm (as CaCO3); CaCIz’2H20 20,000 ppm (as CaCO3); MgSO4-7H20 10,000 ppm (as CaCO3); NaHCO3 20,000 ppm (as CaCO3); PBTC 2,000 ppm (as PBTC). Instrumentation A DR-2000 Spectrophotometer from the Hach Co. (Loveland, Co, U.S.A.) was used for halogen and phosphate analyses. Protocols were followed according to the literature/14/. Ca 2/ was measured by Atomic Absorption Spectroscopy. 137 Vol. 3, Nos. 3-4, 2005 Chemistry of Organophosphonate Scale Growth Inhibitors: 3 OH ,....... H HO /OH HO oPN, :.COOH HOOC N COOH PBTC HO’" / \.,____ I’./_. HO" )H 0 HO HEDP AMP OH HO ...,.I, 0 /r HO ,., .]’/" HO H x’-----Pl/--.O OH (...truncated)