XPS Characterization of Films Formed on Distribution Systems Using Additives to Control Pb/Cu Levels in Drinking Water

Transcription

1 Bulletin 17 XPS Characterization of Films Formed on Distribution Systems Using Additives to Control Pb/Cu Levels in Drinking Water B.E. Scheetz Materials Research Laboratory, The Pennsylvania State University, University Park, PA PQ Corporation is a privately held global enterprise operating in 20 countries, with annual revenues in excess of $500 million. PQ is a leading producer of silicate, zeolite, and other performance materials serving the detergent, pulp and paper, chemical, petroleum, catalyst, water treatment, construction, and beverage markets. Potters Industries, PQ s wholly owned subsidiary, is a leading producer of engineered glass materials serving the highway safety, polymer additive, fine abrasive, and conductive product markets. J. LaRosa Thompson PQ Corporation, R&D Center, Conshohocken, PA and P.J. Delaney Carus Chemical, Co., Peru IL Presented at the Water Quality Technology Conference, November 9-12, 1997, Denver, CO INTRODUCTION Silicates have been used by the drinking water utilities for decades (Shuldener and Sussman, 1960). Initially, silicates were used for red water complaints in the distribution system. Control of zinc [from galvanized iron] and aluminum corrosion by silicates has also been described by early researchers (Lehrman and Shuldener, 1951, 1952). More recently, silicates have been found effective for reducing red and black water complaints resulting from the oxidation of naturally occurring iron and manganese in groundwaters. The Lead and Copper Rule states that silicate treatment, phosphate treatment, and ph and alkalinity adjustment, are best available methods for controlling lead and copper in drinking water. However, detailed field and experimental data of the use of silicates for lead and copper control are scarce. In cases were silicate treatment has been or is being employed, application appears to be more of an art based on previous iron corrosion control experiences rather than on solid scientific knowledge. In the past, silicate manufacturers have attempted to sell their product on the theory that silicates form a smooth, glassy, protective coating on the surface of distribution system materials. Typically, an initial start-up dosage is recommended, followed by a drop to a maintenance dose after passivation has occurred. These mechanisms and recommendations lack sufficient scientific evidence to support them, however. Questions about the mechanism of corrosion control, water quality conditions favoring silicate use, and dosage requirements along with the lack of well documented experiences and chemical cost have led to a general lack of consideration by water utilities. Probably the largest problem encountered through observations and literature reviews is that the mechanism by which silicates work is unclear. The problem primarily arises from the difficulty in clearly separating ph Registered trademarks of PQ Corporation

2 and silicate effects resulting from sodium silicate addition. N sodium silicate (which is most commonly used in drinking water for corrosion control) is a basic solution (ph > 11). Depending on silicate dosage and a water s alkalinity, it can increase the ph of the source water by more than 1 ph unit. A ph increase of that magnitude would more than likely be beneficial to corrosion control. Also, documented identification of silica-based films or solids on lead, copper, and brass surfaces that have been exposed to silicate inhibitors over significant time periods are very difficult to locate. The most significant question that needs to be answered is whether silicate is incorporated into protective films on copper and lead surfaces. If so, it will be important to identify the composition, form, and nature of the film, the conditions that favor its formation, and the mechanism by which the film forms. A literature review shows that most of the previous detailed examinations of silica-based films on metal surfaces have been done on aluminum, zinc (or galvanized), and iron surfaces rather than lead and copper (Lehrman and Shuldener, 1951, 1952; McCune, 1959; Lane et al., 1977a, 1977b; Shuldener and Lehrman, 1957). In addition, hot water is often used. The film-forming mechanism in most of the cases is described as adsorption of silica to existing -oxide or -hydroxide films on the metal surface as mentioned. The general thought is that the silica acts as a webbing on the surface, filling in the voids on the film and adding a thin layer to the surface. Theoretically, the film reinforcement provided by the silica results in additional protection and corrosion reduction. These films are typically described as being very thin. Because silica apparently adsorbs onto existing films, once all the existing surface sites are occupied, the film does not build on itself, and for this reason manufacturers suggest dropping to a lower maintenance silica dosage after silica start-up dosage has reduced metal levels. If the silica film is not completed quickly, partial coverage may make corrosion worse because uncovered sites would have more concentrated corrosion going on. Laboratory studies are often inadequate in supplying sufficient, useful information primarily because of time constraints. There is also the problem that the silicate deposit may be difficult to identify if it does not form a distinct crystalline solid phase. Further, it may affect the formation of other solid phases on the pipe surface, so general characterization of all corrosion products may be necessary to discover the mechanism of action of the silicate. Examining pipe surfaces that have been extracted from distribution systems employing silicate treatment and have relatively well documented water quality records will be critical in identifying a mechanism. There are a variety of techniques available to identify films on the surface of pipes. Deposition of films within water pipes that result from either natural precipitation or precipitation induced by the addition of agents to control the concentrations of regulated aqueous metal ions have been characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray analysis (EDX) in an attempt to understand the chemistry and structure of the films. However, no one method provides a full understanding of the nature of the films. The objective of the research presented in this paper was two-fold: first, utilize x-ray photoelectron spectroscopy (XPS) to detect and characterize films on the interior of distribution pipes; and secondly, use XPS to

3 quantify elemental compositions of the films in order to supplement the data from the more conventional characterization methods. XPS was chosen as an analytical tool in order to attempt to retrieve more detailed compositional information from films. With this technique, all elements, with the exception of H and He, can be detected and can provide semi- and quantitative results on the elemental composition of the interrogated sample. It is a non-destructive technique so the samples are available for additional analysis. XPS characterizes films to a depth of 0.5 to 5nm and can be used for depth profiling to about 5nm. It has a depth resolution of a few nm and a lateral resolution of 75mm to 5mm. XPS has an advantage over conventional XRD and SEM/EDX because it examines a much smaller volume of material and is, therefore, especially suited for films. The disadvantage of XPS, however, is that it operates under a very hard vacuum. BASIS FOR THE XPS ANALYSIS Analysis of the experimental results of the XPS data is based on the Einstein Photoelectric Law: KE = hu -BE where: KE = kinetic energy of the photoelectron hu = energy of the photon BE = binding energy of the electron. By controlling the energy of excitation photon and measuring the kinetic energy of the expelled photoelectron, the energy associated with the bonding electrons from the elements in the surface of the test specimen can be determined. Because the technique examines the electrons directly, these methods can be used to identify the valence state of elements, the immediate ligand environment around an atom, i.e. CH 2, CH 3. CF 3 and CO appear as distinct carbons in the XPS analysis of CF 3 COCH 2 CH 3 and can be used to imply structure to the bonding pattern of the element, i.e. bridging vs. non-bridging. SAMPLE SELECTION AND EXPERIMENTAL CONDITIONS Samples: Two sets of samples were examined; one from a controlled experiment conducted at the University of South Carolina s experimental copper pipe loop and the other from a copper service line in Rochester, NY. The Rochester samples also represented a copper pipe loop that functioned without the addition of any additives, a control, then followed by the use of a phosphate admixture and finally by the addition of silicate. The samples used in this study are summarized in Table 1. TABLE 1. SUMMARY OF TEST SPECIMENS Location Sample ID Source Pipe Type Treatment Type Univ SC PQ3; PQ7 pipe loop copper silicate: high & low Rochester, NY PQR1 pipe loop copper none PQR2 pipe loop copper phosphate PQR3 pipe loop copper silicate

4 TABLE 2. CONDITIONS OF THE UNIVERSITY OF SOUTH CAROLINA PIPE LOOP EXPERIMENT Avg. ph Alkalinity Hardness TDS Silicate mg/l mg/l mg/l mg/l CaCO 3 CaCO 3 SiO 2 Loop 3 - PQ Loop 7 - PQ Table 2 summarizes the controlled conditions of the USC pipe loop experiment. In order to eliminate the potential ph effects of the addition of aqueous silicate to the pipe loop, the ph was independently controlled at approximately 7.7 while almost tripling the silicate concentrations between test #7 and test #3. Table 3 presents the composition of the waters and dosages of phosphate and silicate from the Rochester, NY specimens. TABLE 3. CONDITIONS OF THE ROCHESTER, NY SERVICE LINE SAMPLE Sample ID ph Alkalinity Hardness Treatment mg/l mg/l CaCO 3 CaCO 3 PQR wks no additions PQR ZnPO 4 1mg/L mg/L PQR silicate 2wks@20mg/L 22wks@12mg/L 11wks@8mg/L Instrumentation: The six samples used in this study were characterized with the aid of an environmental scanning electron microscope (ESEM), a conventional SEM equipped with energy dispersive x-ray analyses, x-ray diffraction and with x-ray photoelectron spectroscopy (XPS). Both SEM s provided photographs of the surface structures of the specimens. The environmental SEM was chosen for the initial characterization of the interior of the pipes and was operated at 10 torr of water vapor pressure as a precaution to prevent modification to the surfaces resulting from exposure to the high vacuum environment of the conventional SEM and the XPS. The XPS utilized soft x-rays generated from a magnesium Kα target as the excitation source. The Kratos instrument was operated at an acceleration potential of 14 KeV with an anode current of 20mA. The sample area surveyed for the XPS analysis measured 3mm x 7mm. Occasionally specimens were examined with a 700mm magnification which corresponded to a circular iris opening of 1 mm. Data was collected in 0.3eV increments with a dwell time at each step of 500 milliseconds. Quantification was conducted on the observed elements as detailed in Table 4. The technique has the capability of differentiating between Si and Si bonded to adjacent oxygens. Although in the above table the analysis was reported as Si, and is used in this report in that manner, it is clearly bonded to an oxygen network.

5 TABLE 4. ELECTRONIC STRUCTURE AND BINDING ENERGIES FOR SELECTED ELEMENTS Element electron structure binding energy (ev) O 1s C 1s Si 2p 105 Cu 2p 938 Pb 4f 141 Cl 2p 201 RESULTS University of South Carolina Samples: Sections approximately 1cm square were cut from the copper pipes which were removed from the pipe loop and forwarded for analysis. Each section was carefully flattened for characterization. Both samples exhibited a noticeable film on the interior surface of the entire pipe including the sampled section. The amount of the film varied between the two specimens; PQ 3 exhibiting a more heavily developed film that graded from light to heavy and sample PQ 7 possessed a film that was just noticeable with the naked eye and exhibited distinct color zonations varying of white to yellow. Figure 1 is a micrograph taken from PQ 7 which reveals the lightly developed film and damage to the film that was caused by flattening the copper pipe section. A more detailed close up of this surface in Figure 2 reveals the open porous nature of the film. Detailed examination of the film developed on PQ3, Figure 3 designated light zone, shows the development of a base film [cracking similar to Figure 1 is readily visible] with secondary 1000 to 2000mm foil-like growths. As the film develops into a heavy encrustation, the secondary foil-like growths thicken and colas as shown in Figure 4. A more detailed examination of the foil-like growths begin to reveal micron-size objects that appear to be subeuhedral crystal growth, Figure 5. X-ray mapping and EDX analysis of the films supports a composition that is substantially Si (see Figure 6). X-ray diffraction of these samples exhibited the scattering halo typical of amorphous materials. FIGURE 1: Silica-rich coating on interior of University of South Carolina s experimental pipe loop, sample ID-PQ7. The cracking was introduced when the curved copper pipe specimen was flattened. FIGURE 2: An enlargement of the surface of the film for sample PQ7 revealing the porous nature of the surface. Scale bar is equal to 1 micron.

6 FIGURE 3: Silica-rich coating on encrusted interior of University of South Carolina s experimental pipe loop, sample ID-PQ um sized foil-like deposits have formed on the film surface. FIGURE 4: Heavy encrustation of PQ3 exhibiting thickening and colasing of the foil-like growths. FIGURE 5: Detailed image of the foil-like growths on PQ3 revealing micron-sized sub-euhedral grain growth. FIGURE 6: Si X-ray map of the image presented in Figure 5. The results of XPS analyses for the surfaces of both of these samples and for each of the observed zonations are reported in Table 5. TABLE 5. XPS RESULTS FOR UNIVERSITY OF SOUTH CAROLINA PIPE LOOP SAMPLES [MOLE %] Sample ID O C Si Cu Pb Cl PQ 3 heavy zone light zone PQ 7 yellow zone white zone

7 Qualitatively, the sample with the heaviest encrustation corresponded to the specimen with the highest silica concentration in the circulating water. Furthermore, there appears to be an inverse correlation to the alkalinity of the circulating water and the carbon [presumably as CO 3 2- ] concentration of the films. The copper and lead concentrations of both films appear to be similar. Rochester, NY Samples: This set of samples represented a long-term experiment from the service lines of the City of Rochester, NY. The 80-week experiment was conducted in three phases. ph, alkalinity and hardness were monitored during the entire test period and are reported only in summary term that are presented in table 3; ph , alkalinity of 60 mg/l CaCO 3 equivalent and hardness of 80 mg/l CaCO 3 equivalent. Initially the system was run for five weeks without the addition of any Cu/Pb suppressing ingredients. The second phase utilized zinc orthophosphate for two weeks at a dosage rate of 1 mg/l followed by 33 weeks at a rate of 0.4 mg/l. The final stage of the experiment utilized the soluble silicate at a dosage rate of 20 mg/l for two weeks followed by 12 mg/l for 22 weeks and finally at 8 mg/l for 11 weeks. At the end of each of the three intervals, a fresh section of copper pipe was removed from the distribution system for analysis. SEM characterization of the control and the zinc orthophosphate samples revealed very little while the soluble silicate sample appeared similar to that reported in Figure 1. The XPS data are summarized in Table 6. As would be anticipated, the control sample and the zinc orthophosphate samples contained no detectable silica but both possessed comparable levels of copper and lead. From these analyses the corresponding oxygen for both samples were also comparable. In contrast, the soluble silicate sample contained correspondingly less oxygen and the silicone concentrations appeared comparable to levels observed in the laboratory pipe loop samples at comparable dosages. It is interesting to note that no phosphorous was identified in the zinc orthophosphate treated specimens. TABLE 6. XPS ANALYTICAL RESULTS FOR THE CITY OF ROCHESTER, NY SERVICE LINE SAMPLES Sample ID O C Si Cu Pb control-pqr phosphate-pqr silicate-pqr A feature of XPS that can be exploited is the interpretation of the next-nearest-neighbor environment of surface atoms in these samples. Figure 7a, b and c presents the actual spectral data for the bonding oxygen atoms associated with samples in which soluble silicate was used as a treatment agent. In panels A and B, it is clear that the O 1s binding energy is perturbed as represented by shoulders on the emission peaks. That is, there appears to be two unique types of oxygen. Panel C, exhibits an O 1s spectra that is more typical of a single type of oxygen. Hochella and Brown (1988) have interpreted the presence of shoulders on the O 1s emission peaks as being attributed to non-bridging and bridging

8 Intensity / Counts A 0 1s Binding Energy / cv Intensity / Counts Intensity / Counts B 0 1s Binding Energy / cv C 0 1s Binding Energy / cv FIGURE 7: XPS spectra for 01s atoms collected on silicate films showing the presence of non-bridging oxygen atoms in panels A and B [top to bottom] and just non-bridging oxygen atoms in panel C [bottom]. Panel D schematically displays the bridging and non-bridging components of the 01s oxygen atoms. oxygen atoms. They attribute the bridging oxygen to the larger binding energy and the non-bridging oxygen to the smaller binding energy. The data in panel C would suggest that the silica tetrahedra in that particular film is essentially fully coordinated, one silica linked to another via an oxygen ion. When other cations, such as copper and lead in these samples, are incorporated into the films, the bonding pattern is disrupted which results in some silica being coordinated to an oxygen which is not in turn connected to another silica. In principal, the relative proportion of the two types of oxygen in the films could then be determined which would assist in the interpretation of the structural nature of the film. D bo nbo Binding energy (ev) CONCLUSIONS The objective of this study was to begin to supply the necessary information that will serve to develop a theoretical understanding of how soluble silicate functions to suppress corrosion and Cu/Pb in drinking water. The use of x-ray photoelectron spectroscopy has provided initial

9 insight into this process. This study has shown that on all pipe loop and service line specimens examined that have been treated with soluble silicate, detectable silica can be found on the interior surfaces of the pipes at the scale of several nanometers, suggesting a film development. In those specimens, ESEM and SEM/EDX examination, typically to a depth of several microns, also confirmed the presence of films on the specimens surfaces. Data from the XPS has presented a compositional picture of the films possessing a silicone concentration varying between 10 and 20 mole percent and oxygen concentration between 65 and 75 mole percent. Additional support for the presence of a film can be derived from the nextnearest-neighbor behavior of the oxygen atoms that were characterized in the surfaces as containing both non-bridging and bridging oxygens. Clearly more needs to be done in order to establish the exact mechanism for how soluble silicate works to minimize corrosion and Cu/Pb in municipal water systems. REFERENCES Lehrman, L. and H.L. Shuldener, The role of soluble silicate in inhibiting corrosion by film formation on water piping,, J. AWWA, Vol. 43, (1951). Lehrman, L. and H.L. Shuldener, Action of soluble silicate as a corrosion inhibitor in water piping, Industrial & Engineering Chemistry, Vol. 44, (1952). McCune, H.W., Corrosion product and inhibitor films on aluminum, J. Electrochem. Soc., Vol 106, #1, (1959). Lane, R.W. T.E. Larson and S.W. Schilskly, Silicate treatment inhibits corrosion of galvanized steel and copper alloys, Materials Performance, Vol 12, #4, (1977). Lane, R.W. T.E. Larson and S.W. Schilskly, The effect of ph on the silicate treatment of hot water in galvanized piping, J.AWWA. Vol. 69, (1977). Shuldener, H.L., and L. Lehrman, Influence of bicarbonate ion on inhibition of corrosion by soluble silicate in a Zn-Fe system, J. AWWA, Vol. 49, (1958). Shuldener, H.L. and S. Sussman, Thirty years experience with silicate as a corrosion inhibitor in water system, Corrosion, Vol. 16,#7, (1960). Although the information and suggestions in this brochure ("information") are believed to be correct, PQ Corporation makes no representations or warranties as to the completeness or accuracy of the information. The information is supplied upon the following conditions: The persons receiving the information will determine its suitability for their purposes; PQ Corporation will not be responsible for damages of any nature whatsoever resulting from the use of, or reliance upon, the information or the materials, devices or products to which the information refers; No information is to be construed as a recommendation to use any product, process, equipment or formulation in conflict with any patent; PQ Corporation makes no representation or warranty, express or implied, that the use thereof will not infringe any patent; and NO REPRESENTATIONS OR WARRANTIES, EITHER EXPRESS OR IMPLIED, OF MERCHANTABILITY, FIT- NESS FOR A PARTICULAR PURPOSE OR OF ANY OTHER NATURE ARE MADE HEREUNDER WITH RESPECT TO INFOR- MATION OR THE MATERIALS, DEVICES OR PRODUCTS TO WHICH THE INFORMATION REFERS. Reprinted from Proceedings of 1997 AWWA Water Quality Technology Conference, by permission. Copyright 1997, American Water Works Association Copyright 2003 by PQ Corporation. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher and copyright holder.