Zeta Potential of Deposit Components at Elevated Temperatures

Transcription

1 P R E P R I N T ICPWS XV Berlin, September 8 11, 2008 Zeta Potential of Deposit Components at Elevated Temperatures Victor Rodriguez-Santiago a,b, Sonja Vidojkovic b, Mark V. Fedkin b, Serguei N. Lvov a,b,c, and David J. Wesolowski d a Department of Energy and Mineral Engineering, b The EMS Energy Institute, c Department of Materials Science and Engineering The Pennsylvania State University, University Park, PA 16802, U.S.A. d Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA lvov@psu.edu High temperature zeta potential data (up to 150 o C) were obtained for magnetite and silica, which are common components of the deposits in fossil fuel and nuclear power plant systems. A high temperature electrophoresis cell developed in our laboratory was used to measure the electrophoretic mobilities of the nanosize magnetite and silica particles suspended in dilute electrolyte aqueous solutions. The experimentally obtained isoelectric points (IEP) of magnetite at 25 and 100 C are in good agreement with the values found in the literature. The zeta potential of silica was negative throughout the ph and temperature ranges studied (from 25 to 150 C), and no IEP value was observed. The electrokinetic data obtained in this study provide important information for understanding the mechanism of the deposit formation and controlling these processes in power plants. Introduction One of the major concerns in fossil fuel, nuclear, and geothermal power plants is the formation of deposits of corrosion products in various areas of the water steam cycle. This process causes overheating and rapture of boiler tubes, loss of efficiency of turbines, damages other segment of power units and, in the case of nuclear plants, it can cause radioactive contamination in cooling-water systems. Deposits can also enhance the corrosion processes in all listed types of plants. The damage caused by the deposits can significantly influence the performance of power plants. The deposits are mainly composed of various oxide phases, such as silica (SiO 2 ), and particulate corrosion products, such as magnetite (Fe 3 O 4 ). Dissolved or colloidal oxides tend to precipitate on the inner surfaces of the process lines [1-4]. Understanding the electrokinetic behavior of the oxide particles at the parameters of power plant environments is very important for understanding the mechanism of the deposit formations and further finding ways for its control and prevention. Examination of numerous power plants made evident the presence of crud particles that generally behave as colloids [4-7]. The colloidal characteristics of crud particles may have a decisive role in the mechanism of deposit formation. Accordingly, the deposition process can be explained by the electrostatic interaction between the metal surface of the pipe walls and the colloidal particles based on the electrical charge present on their surfaces. Electrostatic and chemical interaction at the solid/solution interface results in the formation of the electrical double layer (EDL). One of the important parameters of the EDL is the zeta potential (ζ, or ZP), which is defined as the surface potential at the plane of shear where the bulk solution slips along the particle surface. Hypothetically, the shear plane divides the hydrodynamically immobile fluid layer adsorbed at the solid surface and the bulk (mobile) fluid. The zeta potential can be experimentally obtained using a variety of electrokinetic techniques that induce relative movement of solution against the solid surface. Ultimately, it is the magnitude of the zeta potential that determines the extent of the interaction between colloids and other surfaces. That is, high zeta potential values provide greater colloidal stability (by way of increased electrostatic repulsion between charged surfaces). The sign of the zeta potential depends on the balance of the adsorbed species. The point at which the zeta potential switches its sign from negative to positive is called the isoelectric point (IEP), and it is in this area that the suspensions possess the lowest stability, and hence the greater chance of aggregation or deposition. By knowing the magnitude and sign of the zeta potential of the deposit components at elevated temperatures, the

2 conditions that enable deposit formation can be modified (namely, ph control). Furthermore, by measuring the zeta potential of collected samples their components can be identified. Electrophoresis is the method that allows determining the zeta potential of colloidal particles by measuring their electrophoretic mobility, μ E, which is defined as the velocity of a charged particle normalized to the applied electric field strength. mobility measurements are quite common at 25 C, and a number of commercial instruments are available for this analysis. However, only a few attempts to measure the zeta potential above room temperature have been reported in the literature. For example, Jayaweera and Hettiarachchi [8] used the high temperature streaming potential technique to measure the zeta potential of oxide materials up to 235 C. Zhou and co-authors [9] developed a system to measure the zeta potential of particulate materials up to 220 C using the microelectrophoresis technique. This system was used to study the zeta potential of TiO 2 and ZrO 2 phases in aqueous solutions up to 200 C [9, 10]. Nevertheless, consistent zeta potential data for oxides at elevated temperatures is scarce. A number of studies provide the surface charge data and point of zero charge of magnetite that have been measured as a function of temperatures, although not necessarily close to power plant operation conditions. Tewari and co-authors [11] reported the temperature dependence of the point of zero charge of magnetite between 25 and 90 C applying and addition method. Blesa and co-authors [12] determined point of zero charge up to 80 o C also using the method of potentiometric. Jayaweera [8] provided data of zeta potential and zero charge of magnetite at 235 C employing streaming potential technique. Wesolowski and coauthors [13] investigated the surface charge of magnetite from 25 up to 290 C by potentiometric. Many experimental studies of surface properties of magnetite had been performed at ambient temperature [14-22], with alkali metal nitrates, chlorates and halides used as background electrolytes [23]. For silica, surface charge and IEP measurements are limited to temperatures below 100 C [24-28]. In this paper we present new data for the electrophoretic mobility and zeta potential for nanoparticulate magnetite and silica over the temperature range up to 150 C. Experimental approach Amorphous silica (SiO 2 ) particle size standards (Polysciences, Inc.) ± 30 nm - and magnetite powder prepared by hydrothermal synthesis [29] in Oak Ridge National Laboratory (ORNL) were used in this study for the electrophoretic mobility measurements. The SiO 2 suspensions were prepared with mol/kg NaCl(aq) as background solution, with solid loading of ~1.7 g/l. The ph of the silica suspensions was adjusted with HCl(aq) and NaOH(aq), respectively. The magnetite suspensions were prepared in mol/kg (aq) as background solution except at 25 C were the suspensions were prepared in mol/kg NaCl(aq) with solid loading of ~1.7 g/l. The ph of the magnetite suspensions was adjusted with HNO 3 (aq) and KOH(aq), respectively. The ph measurements were performed using a high precision glass ph electrode (±0.005 ph units, Cole Palmer) at 25 C, and the ph at high temperatures was calculated from the room temperature value. The suspensions were ultrasonicated for one hour before each measurement, and the size of the particles was measured at 25 C using a dynamic instrument (Nanosizer, Malvern Instruments). The average particle size for magnetite was determined at 402 nm. X-ray diffraction (XRD) analyses showed that the magnetite material was pure and free of hematite. Figure 1: Schematic of the high temperature electrophoresis cell designed for electrophoretic mobility measurements up to 260 o C. (A) window assembly; (B) stud heater; (C) quartz capillary; (D) Pd electrodes; (E) coil pre-heater. The electrophoretic mobility measurements of Fe 3 O 4 and SiO 2 were conducted using a hightemperature capillary electrophoresis cell designed and constructed in our laboratory (Figure 1), and the details have been described elsewhere [9]. Experiments were performed at three different temperature/pressure conditions: 25 C and 1 bar, 100 C and 20 bar, and 150 C and 20 bar. At each temperature, the pressure was adjusted above the liquid-gas phase boundary to prevent the solution from boiling. A constant voltage between 3 9 V was applied on the Pd-electrodes. The electrophoretic mobility, μ E, was obtained from the slope of the dependence velocity vs. electric field for each temperature and ph according to the following expression: v e = μ E E + v 0 (1) where v e is the electrophoretic velocity of the particles in m s -1, E is the applied electric field 2

3 strength in V m -1, and v 0 is the background velocity (noise), which may be observed prior to the application of the external electric field. In the ideal case, the background velocity should be equal to zero, when the suspension is perfectly stationary. However at elevated temperatures, parasitic forces, such as thermal convection or Browninan motion of particles, may interfere with the velocity measurements. In the present study, we found that, although minimal, some non-electrophoretic particle motion may become noticeable at temperatures above 100 C. This influence was taken into account by measuring the particle velocity at several different electric field strengths with subsequent linear fitting to determine the μ E value as the slope of this dependence. Also switching polarity on the electrodes and measuring the particle velocity in both directions also helped in most cases to eliminate the noise component. The zeta potential of SiO 2 and Fe 3 O 4 was calculated from the obtained electrophoretic mobilities using the numerical treatment developed by O Brien and White (OW), which has been described in detail elsewhere [30]. Briefly, the OW treatment takes into account the effect of the relaxation of the electrical double layer (EDL) around the moving particle. This relaxation effect arises from the drag the ions forming the EDL experience as the particle moves under the influence of the applied electric field, and which ultimately affects the electrophoretic mobility of the particles. with respect to magnetite even at high concentrations and temperatures. [31] On the other hand, it has been shown that magnetite forms strong complexes with chlorides especially at high temperatures [32, 33]. Figure 2: Zeta potential of magnetite vs. ph at 25 C (closed squares) and 100 C (open squares). Results and discussion Zeta potential vs. ph data for magnetite at 25 and 100 C, and for silica at 25, 100, and 150 C are presented in Figures 2 and 3. The zeta potential curve for magnetite at 25 C shows the typical inverted s-shaped behavior, which is typical of many other oxides. Fitting of the experimental points over the ph range yields an isoelectric point at the ph of ~5.7; however, this result is preliminary as the number of points in the neutral area is insufficient. Nevertheless, this value is consistent with the IEP and points of zero charge (PZC) for magnetite reported in the literature (Table 1). The zeta potential curve for magnetite at 100 C exhibits a very steep drop at ph near the IEP (ph ~6). This value (which can be taken as IEP at 100 o C) also compares reasonably well with some values of PZC reported in the literature (Table 2). The measurements are continued to obtain better resolved zeta potential curves at these temperatures and to estimate the IEP values with high accuracy. Some variations in the values of IEP and PZC can be attributed to differences in sample preparation and pretreatment, sample composition (synthetic or natural), and/or impurities. Various electrolytes have been used for magnetite studies, but nitrates ( ) is often preferred as an inert electrolyte Figure 3: Zeta potential of silica vs. ph of silica at 25 C (square), 100 C (triangles), and 150 C (circles). Table 1: IEP and PZC data for Fe 3 O 4 at 25 C IEP or PZC Method Electrolyte Ref mm [22] 0.1 M NaNO 3 [16] NaClO 4 [21] 0.1, 0.01, M [20] NaCl and [19] DI water, NaCl, and [14] 5.7 Electrophoresis 0.1 mm NaCl This study The zeta potential curves for silica show an irregularly shaped behavior with an sharper 3

4 increase of the zeta potential at higher ph values (above ph ~5.5). This might indicate a change in the surface charging mechanism, surface relaxation, or progressing hydrothermal dissolution of silica. This behavior is especially pronounced at high temperatures. More measurements are necessary to confirm and to explain these trends. One important observation related to the silica data is the absence of an isoelectric point the surface of the particles is charged negatively for all temperatures and ph values used in this study. Based on the available literature, IEP values for SiO 2 are typically acidic and may vary a few ph units with the average value falling around ph 2 at 25 C [24-27]. There are virtually no literature data available on the electrophoretic mobility, zeta potential, or IEP of SiO 2 at temperatures above 100 C to compare with the data obtained in this study. Table 2: IEP and PZC data for Fe 3 O 4 at elevated temperatures IEP T or Method Electrolyte Ref. ( C) PZC , Electrophoresis Streaming potential *sodium trifluiromethanesulfonate Conclusion 5 mm to 0.5 M [11] 0.03, 0.3 M NaTr* 0.1 mm 1 mm [13] This study [8] This study represents the first experimental data on the zeta potential of magnetite and silica at elevated temperatures (above 100 o C). The zeta potential of magnetite exhibits an abrupt drop at 100 o C, which is different from its behavior at ambient conditions. Silica, on the other hand, possessed negatively charged surface over the whole ph range studied at all temperatures up to 150 o C. This material did not exhibit IEP in this range. These electrokinetic data for magnetite and silica may suggest that slightly acidic environment in a high temperature aqueous system can promote deposition of silica colloids on oxidized steels (e.g. those with magnetite surface layer) due to sharp differentiation of zeta potentials (range of +60 mv for magnetite and range of -60 mv for silica). On the contrary basic conditions may not only promote faster dissolution of silica nanoparticles, but also trigger re-dispersion of the deposit phases due to anomalously high zeta potential on silica particles at temperatures o C and higher. Future experiments will focus on extending the temperature and ph ranges for the zeta potential measurements towards the representative conditions found in power plants. Acknowledgements The authors gratefully acknowledge the financial support of this work by the U.S. Department of Energy, Office of Basic Energy Sciences, through a grant to Oak Ridge National Laboratory (DE-AC05-00OR22725) and the National Science Foundation (EAR ). The authors also acknowledge use of facilities at the PSU site of NSF NNIN. References [1] R. B. Dooley and W. P. McNaughton, Boiler Tube Failure, Theory and Practice, EPRI. Palo Alto, CA, TR , (1996). [2] W. Leye and E. Maughan, Removal of Magnetite to Protect On-Line Analysis Equipment, Power Plant Chemistry, 8, 349 (2006). [3] H. M. Shalaby, On the mechanism of formation of hot spots in boiler tubes, Corrosion, 62, 930 (2006). [4] R. L. Tapping, C. W. Turner, R. H. Thompson and R. H. Thompson, Steam-Generator Deposits - a Detailed Analysis and Some Inferences, Corrosion, 47, 489 (1991). [5] P. Benezeth, D. J. Wesolowski, D. A. Palmer, M. K. Ridley and C. Ciao, Effect of Amines on the Surface Charge Properties of Iron Oxides, Power Plant Chemistry, 8, 132 (2006). [6] C. W. Turner, D. A. Guzonas and S. J. Klimas, Surface Chemistry Intervention to Control Boiler Tube Fouling, EPRI. Palo Alto, CA, TR , (1999). [7] C. W. Turner and S. J. Klimas, The Effect of Alternative Amines on the Rate of Boiler Tube Fouling, EPRI. Palo Alto, CA, TR , (1997). [8] P. Jayaweera and S. Hettiarachchi, Determination of Zeta-Potential and Ph of Zero Charge of Oxides at High-Temperatures, Review of Scientific Instruments, 64, 524 (1993). [9] X. Y. Zhou, X. J. Wei, M. V. Fedkin, K. H. Strass and S. N. Lvov, Zetameter for microelectrophoresis studies of the oxide/water interface at temperatures up to 200 degrees C, Review of Scientific Instruments, 74, 2501 (2003). [10] M. V. Fedkin, X. Y. Y. Zhou, J. D. Kubicki, A. V. Bandura, S. N. Lvov, M. L. Machesky and D. J. Wesolowski, High temperature microelectrophoresis studies of the rutile/aqueous solution interface, Langmuir, 19, 3797 (2003). [11] P. H. Tewari and A. W. McLean, Temperature Dependence of Point of Zero 4

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