- Email: [email protected]
Effects of organic matters coming from Chinese tea on soluble copper release from copper teapot
SC IE N CE OF T H E TOT AL E N V I RO N ME N T 3 8 9 ( 2 00 8 ) 2 0 2–2 07
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / s c i t o t e n v
Effects of organic matters coming from Chinese tea on soluble copper release from copper teapot Lixiao Nia , Shiyin Lib,⁎ a State Key Laboratory of Hydrology, Water Resources and Hydraulic Engineering, College of Environmental Science and Engineering, Hohai University, Nanjing 210098, China b School of Chemistry & Environmental Science, Nanjing Normal University, Nanjing, 210097, China
AR TIC LE I N FO
ABS TR ACT
Article history:
The morphology and elemental composition of the corrosion products of copper teapot's
Received 2 April 2007
inner-surface were characterized by the scanning electron microscopy and energy
Received in revised form
dispersive X-ray surface analysis (SEM/EDS), X-ray powder diffraction (XRD) and X-ray
17 August 2007
photon spectroscopy (XPS) analysis. It was revealed that Cu, Fe, Ca, P, Si and Al were the
Accepted 24 August 2007
main elements of corrosion by-products, and the α-SiO2, Cu2O and CaCO3 as the main
Available online 27 September 2007
mineral components on the inner-surface of copper teapot. The effects of organic matters coming from Chinese tea on soluble copper release from copper teapots in tap water were
Keywords:
also investigated. The results showed that the doses of organic matter (as TOC), temperate
Copper
and stagnation time have significant effects on the concentration of soluble copper released
Chinese tea
from copper teapots in tap water.
Release
© 2007 Elsevier B.V. All rights reserved.
Copper teapot Tap water
1.
Introduction
Application of copper and its alloys in dietetic utensils in China could be dated back to the Bronze Age, about 3000 years ago. Even today, copper teapots, hollowwares, bowls and cups are used in some rural areas, and also urban families in some cases. For example, at some rural areas in the Anhui Province in China, copper bowls, teapots, cups are used for cooking, drinking and so on. Undoubtedly, the by-products of copper corrosion might be released from dietetic utensils and go into food and drinking water when copper dietetic utensils are used. The local and systemic toxicity might be considered if copper release from tableware enters diets for long-term. The use of copper dietetic utensils could cause health problem due to the release of copper corrosion by-products when copper tableware contacts with chloride, sulphate and other natural organic materials (Atlas et al., 1982; Stone et al., 1987; Singh and Mavinic, 1991; Edwards and Ferguson, 1993;
Edwards et al., 1999; Hong and Cauley, 1998). Some recent studies have highlighted the importance of temperature, chlorine, natural organic matter in corrosion by-products release from copper pipe in drinking water (Rehring and Edwards, 1996; Korshin et al., 1996; Broo et al., 1998; Boulay and Edwards, 2001). It is necessary to understand the behavior of copper release from copper appliances in our daily life, and investigate and evaluate the systemic toxicity of using copper appliances since copper is essential, nutritional element and the potential for adverse effects from too little (deficiency) as well as excess ingestion. However, few studies have focused on the release of copper from dietetic utensils. It is known that tea and wine cultures are the components of dietetic culture in China. Tea contains abundant organic matters, for example polyphenols and diverse organic acids (Suresh and Subramanyam, 1998), which may affect copper corrosion by-products release when copper teapots are used. This study has concentrated on the behaviors of copper release
⁎ Corresponding author. Tel.: +86 25 85891397; fax: +86 25 85891364. E-mail address: [email protected] (S. Li). 0048-9697/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2007.08.039
S CIE N CE OF T H E TOT AL E N V I RO N ME N T 3 8 9 ( 2 00 8 ) 2 0 2–2 07
203
Table 1 – Main parameters of tap water of Nanjing City in China Parameter PH Alkalinity (as CaCO3) (mg/L) Temperature (°C) DOC range (mg/L) DIC (mg/L) TOC (mg/L) Sulfate (mg/L) Nitrate (mg/L) Chloride (mg/L)
Value 7.84 ± 0.02 186 10–15 0–7 44.7 4.6 27.73 9.48 5.19
from copper teapots under certain simulated dietetic circumstances.
2.
Experiment
Copper release experiments were carried out using the tap water of Nanjing City in China. Table 1 shows the main parameters of tap water used in the experiment. Tap water used in the experiment was obtained by flushing tap water at flow rate of ∼ 15 L/min for 10 min. Copper teapots were purchased from local junk shops. According to the results of XPS analysis of copper teapots' inner material, copper teapots were made with pure copper. The Chinese tea used was a local product (Lulang Tea Company in Nanjing, China). Prior to exposures, all copper teapots were orderly rinsed with detergent, tap water, distilled water, ethanol and acetone to eliminate all possible residual contamination and greases. XRD and XPS were primary techniques used for surface characterization of copper inter-surface in this investigation. Information was collected using the SEM/EDS analysis. The copper corrosion by-products of copper teapot's inter-surface were harvested, sawed into 0.5 × 0.5 cm section and stored in the desiccators to remove any adsorbed moisture. SEM/EDS was primarily used as a complementary technique to obtain quantitative information on morphology and on elemental composition of the corrosion products of copper
Fig. 2 – EDS micro-analysis of patina samples from the innersurface of copper teapot.
teapot. Patina morphology was determined through SEM, with a Hitachi-X650 microscope, and energy dispersive X-ray surface analysis with PV9100-EDS coupled (EDAX Comp. America), for in situ determination. The samples were observed in plan and in cross section, polished up to 0.25 μm diamond paste. The range of elemental concentration is 1–99.9%. The energy of beam is 20.0 kV. The distinguishing ratio is 10 nm. The XPS scan was carried out using a ESCALB MK-II spectrometer. The base pressure during analysis was 10− 7 Torr and the power for Mg Kα X-radiation (1253.6 eV) was 300 W. The energy of beam is 15.0 kV. The distinguishing ratio is 1.1 eV/ 2 × 105 CPS. Any charging shift produced by samples was carefully removed by using a B.E. scale referred to C(1s) B.E. of the hydrocarbon part of the adventitious carbon line at 285.0 eV. Non-linear least square curve fitting was performed using a Gaussian/Lorentzian peak shape after background removal. XRD technique was used to identify crystalline phases in the corrosion by-products by means of D/Max-RA (Rigaku Company, Japan) instrument with Rotating Anode Generators (50 kV, 40 mA) and strictly monochromatized Cu Kα radiation. X-ray diffraction patterns were recorded for the scan angles from 15° to 75° to identify crystalline phases in 0.02° scanning steps at a scanning step frequency 10.00 deg/min.
Fig. 1 – SEM image of the inner-surface of copper teapot: (A) low magnification, and (B) high magnification.
204
SC IE N CE OF T H E TOT AL E N V I RO N ME N T 3 8 9 ( 2 00 8 ) 2 0 2–2 07
Table 2 – The results of XPS analyses of copper corrosion by-products on the inner-surface of copper teapot Peak ID
Intensity N(E)
FWHM (eV)
Area
Area (%)
Sensitivity factor
C1s O1s Si2p Zn2p Cu2p
293.40 539.70 110.35 1030.30 941.85
4.15 3.00 2.75 25.15 22.80
8883 29241 854 4588 52276
38.52 48.02 3.43 1.04 8.99
0.25 0.66 0.27 4.80 6.30
In order to investigate the copper release from a copper teapot when the Chinese tea was served in the copper teapot, simulative experiments were processed in the lab. In the experiments, 500 ml of boiled tap water and the Chinese tea (1 or 2 g) were added into the glass appliances and infused for 10 min. Then, the tea water was cooled and filtered. The filtered water was transferred into another glass appliance to store. The concentration of organic matters in tea water was measured as the amount of total organic carbon (TOC). The control tests were also designed to eliminate trace copper release from the tea. In order to obtain the quantitative TOC of tea water, the original tea water was diluted with the boiled tap water. The quantitative tea water was transferred into the copper teapot. The contact area between the quantitative tea water and copper teapot was controlled ∼ 300 cm2. The reactive system was stirred continuously with a glass electric stirrer at 120 rpm and controlled at certain temperature with an auto-heater made of quartz glass tube and resistance coil. Repetitive experiments were considered. Soluble copper was operationally defined using filtration through a 0.45 μm pore size syringe filter according to standard methods (APHA, 1998). In preliminary experiments, copper was analyzed by an inductively coupled plasma emission spectrophotometer (ICP-ES) according to standard method 3120 or an AAS 3111B (APHA, 1998). The results from the ICP-ES and AAS were comparable and there were no significant difference. Soluble copper was measured by an atomic absorption spectrometry spectrophotometer (AAS) with graphite furnace sampling system (Hitachi Z-8100). Commercial stan-
dard stock Cu solution was used. The experimental conditions of AAS are as follows: Dry temperature (80–140 °C within 30 s); Ash (600 °C for 30 s); Atomizing (2700 °C for 7 s); Absorb wavelength at 324.7 nm. The results for each sample were the average of the three readings, considered with the respective coefficients of variation or percentages of standard deviations. Inorganic carbon (IC) and total carbon (TC) were quantified using a SHIMADZU TOC-5000. Hydrochloric acid (made in SHIMADZU Corporation) was used to take out carbon dioxide dissolved in the samples. Total organic carbon (TOC) equals to the difference between TC and IC. Measurement of pH was carried out with a pH meter at temperature 25 °C (Microcomputer pH/mv/TEMP Meter 6171, made in China for JENCO U.S.A.).
3.
Results and discussion
SEM images showed that the highly uniform corrosion occurred on the inner-surface of copper teapot, and the dimension of granule was un-symmetrical at the precipitates zone (Fig. 1). According to the results of EDS micro-analysis (Fig. 2), it illustrates that aluminosolicate solids precipitate slowly, and soluble Al and silica can pass through a treatment plant and “post-precipitate” in the distribution and drinking water system. Aluminosilicates and other Al based solids can coat plumbing and vessel materials and accumulate irregularly (Rehring and Edwards, 1996). In view of Chinese tea containing some aluminum, and therefore, that could be another source for the aluminum. The results of EDS micro-analysis of copper teapot's innersurface revealed that Cu was the main element of corrosion by-products on the inner-surface of copper teapot. Fe, Ca, P, Si and Al were found on the coupon surface through other EDS spectra as shown in Fig. 6. Calcium, silica and iron could have come from formed deposits on the pilot-scale water distribution systems. It indicates that aluminosilicates deposit onto copper teapot materials in drinking water system. Precipitation of silica with aluminum hydroxide [Al(OH)3] is a common phenomenon in water treatment. Coagulants are typically the
Fig. 3 – XRD pattern of corrosion by-products of surface layer of copper teapot.
S CIE N CE OF T H E TOT AL E N V I RO N ME N T 3 8 9 ( 2 00 8 ) 2 0 2–2 07
source for aluminum (Al) in these solids. Silica is naturally present in waters at 5–50 mg/L as silicon dioxide (SiO2) (Rehring and Edwards, 1996). Phosphorus is the component of new copper materials or formed scales (Xiao et al., 2007). Zinc and iron corrosion by-products attached on the innersurface of copper teapot may be due to the corrosion of iron pipe or zinc pipe used at the nearby water distribution system, and transported with the flow of tap water. Copper corrosion by-products on the inner-surface of teapot were analyzed by XPS and the results are presented in Table 2. There was a large surface carbon levels as is common with XPS measurements. These high surface carbon levels are attributed to hydrocarbon adsorption on the surface from the organic matter coming from Chinese tea as controlled XPS experiments by Cousens et al. (2000) have shown that atomically clean surfaces pick up substantial hydrocarbon layers within 1 min of exposure to normal laboratory air. To try to overcome this inevitable carbon contamination, each sample was analyzed after a 5 min argon ion etch at an ion current of 3 μA. This is attributed to the rough patina surface and the porous nature of the patina. The presence of carbon and silicon is consistent with the EDS detection. Si is assumed to originate from suspending solids in drinking water, whereas it is attributed to dirt or sand on the patina surface or as inclusions within the patina. Zn is sure to come from drinking water distribution system. The zinc pipe is still used in old urban zone, whereas the zinc corrosion by-products are transferred to the other regions. XRD pattern for corrosion by-product of copper teapot (Fig. 3) gives proof for the presence of α-SiO2, Cu2O and CaCO3 as the main mineral components. Some peaks are difficult to be identified on the pattern due to the complexity of copper corrosion by-products. These results are consistent with the EDS and XPS detection. Fig. 4 shows the effect of different levels of organic matters on soluble copper release from copper teapots in tap water at a temperature of 50 °C after 30 min stagnation. The results indicate that soluble copper increases markedly at low level of organic matter. This idea has precedence in the “trace organic” effect noted by Campbell (1971), in which very low levels of organic matter completely change the type of calcite solids precipitated on the copper surface. The maximum concentration of soluble copper is 0.64 mg/L between TOC 75.3 and
Fig. 4 – Effect of organic matters on soluble copper release from copper teapots in tap water at a temperature of 50 °C after 30 min stagnation.
205
Fig. 5 – Exposed time versus soluble copper release from copper teapots in tap water with (as TOC 602.5 mg/L) or without Chinese tea at a temperature of 50 °C.
602.5 mg/L. In the experiment, visible insoluble precipitate was produced, which could be removed by filtration mostly. Under the condition of temperature 50 °C, exposed times of 60 min and TOC 602.5 mg/L, the total copper in water reached 0.91 mg/L. About 30% of coppers were in the formation of particulate copper organic solids. Precipitation involves many processes such as super saturation, nucleation, particle growth, Ostwald ripening, recrystallization, and agglomeration. These processes are significantly affected by water temperature, aging time, pH, alkalinity, anion species and physical and chemical factors. These interactions contribute to illdefined solid phased and inconsistent solubility constants (Feitknecht and Schindler, 1963). It might be due to the low solubility of the organic–copper complex with relatively low formation constants (Campbell, 1971). The sorption of soluble organic matter onto copper teapots surfaces during stagnation, which decreases the solution's complexation capacity for copper (Edwards and Sprague, 2001). The effects of exposed times on soluble copper release from copper teapots in tap water with (as TOC 602.5 mg/L) or without Chinese tea at temperature 50 °C are shown in Fig. 5. The results show that soluble copper in tap water with Chinese tea obviously increase with the prolongation of exposed times, and slightly
Fig. 6 – Soluble copper release from copper teapots in tap water with (as TOC 602.5 mg/L) or without Chinese tea at different temperatures.
206
SC IE N CE OF T H E TOT AL E N V I RO N ME N T 3 8 9 ( 2 00 8 ) 2 0 2–2 07
Fig. 7 – The change of pH (25 °C) for tap water with (as TOC 602.5 mg/L) or without Chinese tea in glass vessel or copper teapots at different exposed temperatures after 15 min stagnation.
increase in the system of no Chinese tea. The maximum concentration of total copper release (0.91 mg/L) from copper teapots in tap water at temperature 50 °C after 60 min stagnation in the presence of Chinese tea (as) was still lower than WHO's Drinking Water Action Level for copper (2 mg/L). In order to investigate the influence of temperature on soluble copper release from copper teapots in the presence of Chinese tea, temperatures 25, 50 and 100 °C are selected (Fig. 6). According to the main parameters of tap water of Nanjing City in China in Table 1, the tap water contains much more inorganic carbon that might play an important role in the composition of tap water. Average concentration of TC in tap water of Nanjing City was 49.3 mg/L, but IC (44.7 mg/L) was much more than TOC (4.6 mg/L). The effects of carbonates and bicarbonates had been extensively studied in relation to the corrosion of copper (Edwards et al., 1994; Cruse and Pomeroy, 1974; Cohen and Lyman, 1972; Campbell, 1971). The apparent formation of surface film, for instance, calcium carbonate and malachite, is one of the possible reasons for the corrosion inhibition. Therefore, the soluble copper increase indistinctively with the exposed temperature elevation in tap water without added organic matters. Compared with the experimental system of no Chinese tea, temperature has markedly affected on soluble copper release under the condition of TOC 602.5 mg/L. The concentration of total copper released from copper teapot reaches to 5.04 mg/L at temperature 100 °C, which is greatly beyond WHO's Drinking Water Action Level (2 mg/L). The results illustrate that organic substances released from tea obviously elevate copper corrosion by-products release. It may be due to the increase of solubility of organic–copper complexes at high temperate. This effect of organic matter is thought to be primarily related to the formation of soluble Cu2+-organgic complexes with relatively high formation constants (Korshin et al., 2000). Fig. 7 shows pH variation for tap water with or without Chinese tea in glass vessel or copper teapots at different temperatures after 15 min stagnation. The pH of tap water without Chinese tea in glass vessel increases with the system's temperature elevation. According to the main parameters of tap water of Nanjing City in China in Table 1, the tap water contains much more inorganic carbon that might play an important role in the change of pH values. The apparent formation
of surface film, for instance, calcium carbonate and malachite, is one of the possible reasons for the pH increase. Compared with the tap water in glass vessels, it is found that the pH values of tea water in copper teapots at temperature 25 °C decrease when the reactive system's temperature is increased. It shows that temperature plays an important role in the interaction of copper corrosion by-products and organic matters. Li (2001) had studied the chelating effect of various Chinese tea extracts. Similar phenomena were found that the initial pH values of reactive system of Cu2+ and Chinese tea extracts decrease with the prolongation of reactive time. Phenolic compounds and amine compounds form very strong complexes with 10%–30% of the Cu2+ in teas. Boulay and Edwards (2001) found that both gum xanthan and alginate reduced the pH of the copper pipe solution during the threeday stagnation period. Some detailed investigation is still being processed by electrochemistry and other analytical methods.
4.
Conclusion
The results of SEM/EDS and XRD analysis showed that α-SiO2, Cu2O and CaCO3 were the main mineral components of copper teapot's corrosion by-products. Cu, Fe, Ca, P, Si and Al were the main elements of corrosion by-products on the inner-surface of copper teapot. Chinese tea tended to cause relatively high copper release from copper teapot in tap water. Organic matters released from Chinese tea dramatically increase copper corrosion by-product release from copper teapot. Levels of organic matters (as TOC) and high temperature have significant effects on the concentration of soluble copper. The solubility of the complex of organic–copper may be related to the level of soluble copper in tap water.
Acknowledgment This work was supported by the International Copper Association (ICA) under grant H-AS-01-01. The authors would like to thank Mr. Qiu Hong, chief representation of ICA in China for his contribution to provide relative data and assistance in the procedure of project.
REFERENCES APHA. Standard methods for the examination of water and wastewater. 20th ed. Baltimore, Maryland: United Book Press; 1998. Atlas D, Coombs J, Zajicek OT. The corrosion of copper by chlorinated drinking waters. Water Res 1982;16:693–8. Boulay N, Edwards M. Role of temperature, chlorine, and organic matter in copper corrosion by-product release in soft water. Water Res 2001;35:683–90. Broo AE, Berghult B, Hedberg T. Copper corrosion in water distribution systems—the influence of natural organic matter (nom) on the solubility of copper corrosion products. Corros Sci 1998;40:1479–89. Campbell HS. Corrosion, water composition and water treatment. Water Treat Exam 1971;20:11–34.
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Cohen A, Lyman WS. Service experience with copper plumbing tube. Mater Prot Perform 1972;11:48–52. Cousens DR, Wood BJ, Wang JQ, Atrens A. Implications of specimen and of surface contamination for the measurement of the grain boundary carbon concentration of steels using X-ray microanalysis in an UHVFESTEM. Surf Interface Anal 2000;29:23–32. Cruse H, Pomeroy RD. Corrosion of copper pipes. J Am Water Works Assoc 1974;67:479–83. Edwards M, Ferguson JF. Accelerated testing of copper corrosion. J Am Water Works Assoc 1993;85:105–13. Edwards M, Sprague N. Organic matter and copper corrosion by-product release: a mechanistic study. Corros Sci 2001;43:1–18. Edwards M, Meyer T, Rehring J. Effect of selected anions on copper corrosion rates. J Am Water Works Assoc 1994;86:73–81. Edwards M, Jacobs S, Dodrill D. Desktop guidance for mitigating Pb and Cu corrosion by-products. J Am Water Works Assoc 1999;91:66–77. Feitknecht W, Schindler P. Solubility constants of metal oxides, metal hydroxides, and metal hydroxide salts in aqueous solution. Pure Appl Chem 1963;6:130–99. Hong PK, Cauley MY. Corrosion and leaching of copper tubing exposure to chlorinated drinking. Water Air Soil Pollut 1998;108:457–71.
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Korshin GV, Perry SA, Ferguson JF. Influence of NOM on copper corrosion. J Am Water Works Assoc 1996;88:36–47. Korshin GV, Ferguson JF, Lancaster AN. Influence of natural organic matter on the corrosion of leaded brass in potable water. Corros Sci 2000;42:53–66. Li B. Chelating effect of various tea extracts. J Tea Bus (China) 2001;23:19–20 [In English]. Rehring J, Edwards M. Copper corrosion in potable water systems: impacts of natural organic matter and water treatment processes. Corrosion 1996;52:307–17. Singh I, Mavinic DS. Significance of building and plumbing specifics on trace metal concentration in drinking water. Can J Civ Eng 1991;18:893–903. Stone A, Spyridakis D, Benjamin M, Ferguson J, Reiber S, Osterhus S. The effects of short-term changes in water quality on copper and zinc corrosion rates. J Am Water Works Assoc 1987;79:75–82. Suresh K, Subramanyam C. Polyphenols are involved in copper binding to cell walls of Neurospora crassa. J Inorg Biochem 1998;69:209–15. Xiao W, Hong S, Tang Z, Seal S, Taylor JS. Effects of blending on surface characteristics of copper corrosion products in drinking water distribution systems. Corros Sci 2007;49:449–68.
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / s c i t o t e n v
Effects of organic matters coming from Chinese tea on soluble copper release from copper teapot Lixiao Nia , Shiyin Lib,⁎ a State Key Laboratory of Hydrology, Water Resources and Hydraulic Engineering, College of Environmental Science and Engineering, Hohai University, Nanjing 210098, China b School of Chemistry & Environmental Science, Nanjing Normal University, Nanjing, 210097, China
AR TIC LE I N FO
ABS TR ACT
Article history:
The morphology and elemental composition of the corrosion products of copper teapot's
Received 2 April 2007
inner-surface were characterized by the scanning electron microscopy and energy
Received in revised form
dispersive X-ray surface analysis (SEM/EDS), X-ray powder diffraction (XRD) and X-ray
17 August 2007
photon spectroscopy (XPS) analysis. It was revealed that Cu, Fe, Ca, P, Si and Al were the
Accepted 24 August 2007
main elements of corrosion by-products, and the α-SiO2, Cu2O and CaCO3 as the main
Available online 27 September 2007
mineral components on the inner-surface of copper teapot. The effects of organic matters coming from Chinese tea on soluble copper release from copper teapots in tap water were
Keywords:
also investigated. The results showed that the doses of organic matter (as TOC), temperate
Copper
and stagnation time have significant effects on the concentration of soluble copper released
Chinese tea
from copper teapots in tap water.
Release
© 2007 Elsevier B.V. All rights reserved.
Copper teapot Tap water
1.
Introduction
Application of copper and its alloys in dietetic utensils in China could be dated back to the Bronze Age, about 3000 years ago. Even today, copper teapots, hollowwares, bowls and cups are used in some rural areas, and also urban families in some cases. For example, at some rural areas in the Anhui Province in China, copper bowls, teapots, cups are used for cooking, drinking and so on. Undoubtedly, the by-products of copper corrosion might be released from dietetic utensils and go into food and drinking water when copper dietetic utensils are used. The local and systemic toxicity might be considered if copper release from tableware enters diets for long-term. The use of copper dietetic utensils could cause health problem due to the release of copper corrosion by-products when copper tableware contacts with chloride, sulphate and other natural organic materials (Atlas et al., 1982; Stone et al., 1987; Singh and Mavinic, 1991; Edwards and Ferguson, 1993;
Edwards et al., 1999; Hong and Cauley, 1998). Some recent studies have highlighted the importance of temperature, chlorine, natural organic matter in corrosion by-products release from copper pipe in drinking water (Rehring and Edwards, 1996; Korshin et al., 1996; Broo et al., 1998; Boulay and Edwards, 2001). It is necessary to understand the behavior of copper release from copper appliances in our daily life, and investigate and evaluate the systemic toxicity of using copper appliances since copper is essential, nutritional element and the potential for adverse effects from too little (deficiency) as well as excess ingestion. However, few studies have focused on the release of copper from dietetic utensils. It is known that tea and wine cultures are the components of dietetic culture in China. Tea contains abundant organic matters, for example polyphenols and diverse organic acids (Suresh and Subramanyam, 1998), which may affect copper corrosion by-products release when copper teapots are used. This study has concentrated on the behaviors of copper release
⁎ Corresponding author. Tel.: +86 25 85891397; fax: +86 25 85891364. E-mail address: [email protected] (S. Li). 0048-9697/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2007.08.039
S CIE N CE OF T H E TOT AL E N V I RO N ME N T 3 8 9 ( 2 00 8 ) 2 0 2–2 07
203
Table 1 – Main parameters of tap water of Nanjing City in China Parameter PH Alkalinity (as CaCO3) (mg/L) Temperature (°C) DOC range (mg/L) DIC (mg/L) TOC (mg/L) Sulfate (mg/L) Nitrate (mg/L) Chloride (mg/L)
Value 7.84 ± 0.02 186 10–15 0–7 44.7 4.6 27.73 9.48 5.19
from copper teapots under certain simulated dietetic circumstances.
2.
Experiment
Copper release experiments were carried out using the tap water of Nanjing City in China. Table 1 shows the main parameters of tap water used in the experiment. Tap water used in the experiment was obtained by flushing tap water at flow rate of ∼ 15 L/min for 10 min. Copper teapots were purchased from local junk shops. According to the results of XPS analysis of copper teapots' inner material, copper teapots were made with pure copper. The Chinese tea used was a local product (Lulang Tea Company in Nanjing, China). Prior to exposures, all copper teapots were orderly rinsed with detergent, tap water, distilled water, ethanol and acetone to eliminate all possible residual contamination and greases. XRD and XPS were primary techniques used for surface characterization of copper inter-surface in this investigation. Information was collected using the SEM/EDS analysis. The copper corrosion by-products of copper teapot's inter-surface were harvested, sawed into 0.5 × 0.5 cm section and stored in the desiccators to remove any adsorbed moisture. SEM/EDS was primarily used as a complementary technique to obtain quantitative information on morphology and on elemental composition of the corrosion products of copper
Fig. 2 – EDS micro-analysis of patina samples from the innersurface of copper teapot.
teapot. Patina morphology was determined through SEM, with a Hitachi-X650 microscope, and energy dispersive X-ray surface analysis with PV9100-EDS coupled (EDAX Comp. America), for in situ determination. The samples were observed in plan and in cross section, polished up to 0.25 μm diamond paste. The range of elemental concentration is 1–99.9%. The energy of beam is 20.0 kV. The distinguishing ratio is 10 nm. The XPS scan was carried out using a ESCALB MK-II spectrometer. The base pressure during analysis was 10− 7 Torr and the power for Mg Kα X-radiation (1253.6 eV) was 300 W. The energy of beam is 15.0 kV. The distinguishing ratio is 1.1 eV/ 2 × 105 CPS. Any charging shift produced by samples was carefully removed by using a B.E. scale referred to C(1s) B.E. of the hydrocarbon part of the adventitious carbon line at 285.0 eV. Non-linear least square curve fitting was performed using a Gaussian/Lorentzian peak shape after background removal. XRD technique was used to identify crystalline phases in the corrosion by-products by means of D/Max-RA (Rigaku Company, Japan) instrument with Rotating Anode Generators (50 kV, 40 mA) and strictly monochromatized Cu Kα radiation. X-ray diffraction patterns were recorded for the scan angles from 15° to 75° to identify crystalline phases in 0.02° scanning steps at a scanning step frequency 10.00 deg/min.
Fig. 1 – SEM image of the inner-surface of copper teapot: (A) low magnification, and (B) high magnification.
204
SC IE N CE OF T H E TOT AL E N V I RO N ME N T 3 8 9 ( 2 00 8 ) 2 0 2–2 07
Table 2 – The results of XPS analyses of copper corrosion by-products on the inner-surface of copper teapot Peak ID
Intensity N(E)
FWHM (eV)
Area
Area (%)
Sensitivity factor
C1s O1s Si2p Zn2p Cu2p
293.40 539.70 110.35 1030.30 941.85
4.15 3.00 2.75 25.15 22.80
8883 29241 854 4588 52276
38.52 48.02 3.43 1.04 8.99
0.25 0.66 0.27 4.80 6.30
In order to investigate the copper release from a copper teapot when the Chinese tea was served in the copper teapot, simulative experiments were processed in the lab. In the experiments, 500 ml of boiled tap water and the Chinese tea (1 or 2 g) were added into the glass appliances and infused for 10 min. Then, the tea water was cooled and filtered. The filtered water was transferred into another glass appliance to store. The concentration of organic matters in tea water was measured as the amount of total organic carbon (TOC). The control tests were also designed to eliminate trace copper release from the tea. In order to obtain the quantitative TOC of tea water, the original tea water was diluted with the boiled tap water. The quantitative tea water was transferred into the copper teapot. The contact area between the quantitative tea water and copper teapot was controlled ∼ 300 cm2. The reactive system was stirred continuously with a glass electric stirrer at 120 rpm and controlled at certain temperature with an auto-heater made of quartz glass tube and resistance coil. Repetitive experiments were considered. Soluble copper was operationally defined using filtration through a 0.45 μm pore size syringe filter according to standard methods (APHA, 1998). In preliminary experiments, copper was analyzed by an inductively coupled plasma emission spectrophotometer (ICP-ES) according to standard method 3120 or an AAS 3111B (APHA, 1998). The results from the ICP-ES and AAS were comparable and there were no significant difference. Soluble copper was measured by an atomic absorption spectrometry spectrophotometer (AAS) with graphite furnace sampling system (Hitachi Z-8100). Commercial stan-
dard stock Cu solution was used. The experimental conditions of AAS are as follows: Dry temperature (80–140 °C within 30 s); Ash (600 °C for 30 s); Atomizing (2700 °C for 7 s); Absorb wavelength at 324.7 nm. The results for each sample were the average of the three readings, considered with the respective coefficients of variation or percentages of standard deviations. Inorganic carbon (IC) and total carbon (TC) were quantified using a SHIMADZU TOC-5000. Hydrochloric acid (made in SHIMADZU Corporation) was used to take out carbon dioxide dissolved in the samples. Total organic carbon (TOC) equals to the difference between TC and IC. Measurement of pH was carried out with a pH meter at temperature 25 °C (Microcomputer pH/mv/TEMP Meter 6171, made in China for JENCO U.S.A.).
3.
Results and discussion
SEM images showed that the highly uniform corrosion occurred on the inner-surface of copper teapot, and the dimension of granule was un-symmetrical at the precipitates zone (Fig. 1). According to the results of EDS micro-analysis (Fig. 2), it illustrates that aluminosolicate solids precipitate slowly, and soluble Al and silica can pass through a treatment plant and “post-precipitate” in the distribution and drinking water system. Aluminosilicates and other Al based solids can coat plumbing and vessel materials and accumulate irregularly (Rehring and Edwards, 1996). In view of Chinese tea containing some aluminum, and therefore, that could be another source for the aluminum. The results of EDS micro-analysis of copper teapot's innersurface revealed that Cu was the main element of corrosion by-products on the inner-surface of copper teapot. Fe, Ca, P, Si and Al were found on the coupon surface through other EDS spectra as shown in Fig. 6. Calcium, silica and iron could have come from formed deposits on the pilot-scale water distribution systems. It indicates that aluminosilicates deposit onto copper teapot materials in drinking water system. Precipitation of silica with aluminum hydroxide [Al(OH)3] is a common phenomenon in water treatment. Coagulants are typically the
Fig. 3 – XRD pattern of corrosion by-products of surface layer of copper teapot.
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source for aluminum (Al) in these solids. Silica is naturally present in waters at 5–50 mg/L as silicon dioxide (SiO2) (Rehring and Edwards, 1996). Phosphorus is the component of new copper materials or formed scales (Xiao et al., 2007). Zinc and iron corrosion by-products attached on the innersurface of copper teapot may be due to the corrosion of iron pipe or zinc pipe used at the nearby water distribution system, and transported with the flow of tap water. Copper corrosion by-products on the inner-surface of teapot were analyzed by XPS and the results are presented in Table 2. There was a large surface carbon levels as is common with XPS measurements. These high surface carbon levels are attributed to hydrocarbon adsorption on the surface from the organic matter coming from Chinese tea as controlled XPS experiments by Cousens et al. (2000) have shown that atomically clean surfaces pick up substantial hydrocarbon layers within 1 min of exposure to normal laboratory air. To try to overcome this inevitable carbon contamination, each sample was analyzed after a 5 min argon ion etch at an ion current of 3 μA. This is attributed to the rough patina surface and the porous nature of the patina. The presence of carbon and silicon is consistent with the EDS detection. Si is assumed to originate from suspending solids in drinking water, whereas it is attributed to dirt or sand on the patina surface or as inclusions within the patina. Zn is sure to come from drinking water distribution system. The zinc pipe is still used in old urban zone, whereas the zinc corrosion by-products are transferred to the other regions. XRD pattern for corrosion by-product of copper teapot (Fig. 3) gives proof for the presence of α-SiO2, Cu2O and CaCO3 as the main mineral components. Some peaks are difficult to be identified on the pattern due to the complexity of copper corrosion by-products. These results are consistent with the EDS and XPS detection. Fig. 4 shows the effect of different levels of organic matters on soluble copper release from copper teapots in tap water at a temperature of 50 °C after 30 min stagnation. The results indicate that soluble copper increases markedly at low level of organic matter. This idea has precedence in the “trace organic” effect noted by Campbell (1971), in which very low levels of organic matter completely change the type of calcite solids precipitated on the copper surface. The maximum concentration of soluble copper is 0.64 mg/L between TOC 75.3 and
Fig. 4 – Effect of organic matters on soluble copper release from copper teapots in tap water at a temperature of 50 °C after 30 min stagnation.
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Fig. 5 – Exposed time versus soluble copper release from copper teapots in tap water with (as TOC 602.5 mg/L) or without Chinese tea at a temperature of 50 °C.
602.5 mg/L. In the experiment, visible insoluble precipitate was produced, which could be removed by filtration mostly. Under the condition of temperature 50 °C, exposed times of 60 min and TOC 602.5 mg/L, the total copper in water reached 0.91 mg/L. About 30% of coppers were in the formation of particulate copper organic solids. Precipitation involves many processes such as super saturation, nucleation, particle growth, Ostwald ripening, recrystallization, and agglomeration. These processes are significantly affected by water temperature, aging time, pH, alkalinity, anion species and physical and chemical factors. These interactions contribute to illdefined solid phased and inconsistent solubility constants (Feitknecht and Schindler, 1963). It might be due to the low solubility of the organic–copper complex with relatively low formation constants (Campbell, 1971). The sorption of soluble organic matter onto copper teapots surfaces during stagnation, which decreases the solution's complexation capacity for copper (Edwards and Sprague, 2001). The effects of exposed times on soluble copper release from copper teapots in tap water with (as TOC 602.5 mg/L) or without Chinese tea at temperature 50 °C are shown in Fig. 5. The results show that soluble copper in tap water with Chinese tea obviously increase with the prolongation of exposed times, and slightly
Fig. 6 – Soluble copper release from copper teapots in tap water with (as TOC 602.5 mg/L) or without Chinese tea at different temperatures.
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Fig. 7 – The change of pH (25 °C) for tap water with (as TOC 602.5 mg/L) or without Chinese tea in glass vessel or copper teapots at different exposed temperatures after 15 min stagnation.
increase in the system of no Chinese tea. The maximum concentration of total copper release (0.91 mg/L) from copper teapots in tap water at temperature 50 °C after 60 min stagnation in the presence of Chinese tea (as) was still lower than WHO's Drinking Water Action Level for copper (2 mg/L). In order to investigate the influence of temperature on soluble copper release from copper teapots in the presence of Chinese tea, temperatures 25, 50 and 100 °C are selected (Fig. 6). According to the main parameters of tap water of Nanjing City in China in Table 1, the tap water contains much more inorganic carbon that might play an important role in the composition of tap water. Average concentration of TC in tap water of Nanjing City was 49.3 mg/L, but IC (44.7 mg/L) was much more than TOC (4.6 mg/L). The effects of carbonates and bicarbonates had been extensively studied in relation to the corrosion of copper (Edwards et al., 1994; Cruse and Pomeroy, 1974; Cohen and Lyman, 1972; Campbell, 1971). The apparent formation of surface film, for instance, calcium carbonate and malachite, is one of the possible reasons for the corrosion inhibition. Therefore, the soluble copper increase indistinctively with the exposed temperature elevation in tap water without added organic matters. Compared with the experimental system of no Chinese tea, temperature has markedly affected on soluble copper release under the condition of TOC 602.5 mg/L. The concentration of total copper released from copper teapot reaches to 5.04 mg/L at temperature 100 °C, which is greatly beyond WHO's Drinking Water Action Level (2 mg/L). The results illustrate that organic substances released from tea obviously elevate copper corrosion by-products release. It may be due to the increase of solubility of organic–copper complexes at high temperate. This effect of organic matter is thought to be primarily related to the formation of soluble Cu2+-organgic complexes with relatively high formation constants (Korshin et al., 2000). Fig. 7 shows pH variation for tap water with or without Chinese tea in glass vessel or copper teapots at different temperatures after 15 min stagnation. The pH of tap water without Chinese tea in glass vessel increases with the system's temperature elevation. According to the main parameters of tap water of Nanjing City in China in Table 1, the tap water contains much more inorganic carbon that might play an important role in the change of pH values. The apparent formation
of surface film, for instance, calcium carbonate and malachite, is one of the possible reasons for the pH increase. Compared with the tap water in glass vessels, it is found that the pH values of tea water in copper teapots at temperature 25 °C decrease when the reactive system's temperature is increased. It shows that temperature plays an important role in the interaction of copper corrosion by-products and organic matters. Li (2001) had studied the chelating effect of various Chinese tea extracts. Similar phenomena were found that the initial pH values of reactive system of Cu2+ and Chinese tea extracts decrease with the prolongation of reactive time. Phenolic compounds and amine compounds form very strong complexes with 10%–30% of the Cu2+ in teas. Boulay and Edwards (2001) found that both gum xanthan and alginate reduced the pH of the copper pipe solution during the threeday stagnation period. Some detailed investigation is still being processed by electrochemistry and other analytical methods.
4.
Conclusion
The results of SEM/EDS and XRD analysis showed that α-SiO2, Cu2O and CaCO3 were the main mineral components of copper teapot's corrosion by-products. Cu, Fe, Ca, P, Si and Al were the main elements of corrosion by-products on the inner-surface of copper teapot. Chinese tea tended to cause relatively high copper release from copper teapot in tap water. Organic matters released from Chinese tea dramatically increase copper corrosion by-product release from copper teapot. Levels of organic matters (as TOC) and high temperature have significant effects on the concentration of soluble copper. The solubility of the complex of organic–copper may be related to the level of soluble copper in tap water.
Acknowledgment This work was supported by the International Copper Association (ICA) under grant H-AS-01-01. The authors would like to thank Mr. Qiu Hong, chief representation of ICA in China for his contribution to provide relative data and assistance in the procedure of project.
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