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Determination of the total iodide content in desalinated seawater permeate
Desalination 261 (2010) 251–254
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Determination of the total iodide content in desalinated seawater permeate S.J. Duranceau University of Central Florida, Department of Civil, Environmental and Construction Engineering, Orlando, FL, 32815-2450, USA
a r t i c l e
i n f o
Article history: Received 29 March 2010 Received in revised form 17 June 2010 Accepted 19 June 2010 Available online 10 July 2010 Keywords: Desalination Synthetic membrane processes Permeate iodide concentration Seawater Catalytic reduction analysis
a b s t r a c t An investigation was conducted to determine the iodide content of permeate collected from several operating facilities reliant upon synthetic membrane processes for seawater desalination. A possible, yet unintentional impact for communities that employ synthetic membrane processes for seawater desalination is the introduction of permeate streams containing iodide into their water supply, that then may result in the formation of iodinated disinfection by-products. To evaluate this potential, the iodide content of desalinated seawater permeate streams were measured using an analytical procedure based on the catalytic reduction of ceric sulfate by arsenious acid in a sulfuric acid solution. It was determined that iodide concentrations in permeate samples collected from seawater desalination facilities were less than the catalytic reduction method detection limit of 4.0 μg/L for membrane feed seawaters that ranged between 51.1 μg/L and 35.8 μg/L of total iodide. Results of this investigation indicated that synthetic membrane processes can remove greater than 89% of the total iodide from the feedwater of seawater based on an iodide detection limit of 4.0 μg/L. © 2010 Elsevier B.V. All rights reserved.
1. Introduction 1.1. Overview and motivation Potable water purveyors are increasingly turning to advanced membrane technologies to augment existing unit operations to improve water quality and allow reliance on poorer source waters. Synthetic reverse osmosis membrane processes, made practical and possible by the pioneering work of Sidney Loeb and Srinivasa Sourirajan during the late 1950s at the University of California, Los Angeles, offers today's water purveyors an economical method for sustainable augmentation using impaired water sources [1]. Many coastal communities are, or are intending to, employ synthetic membrane processes to desalinate seawater because of the need to supplement dwindling fresh water supplies due to expanding population centers, environmental degradation, extended drought and climate change. The work described herein would not have been possible without the contribution of Sidney Loeb, for without his many research contributions (most important of which was the development of an anistropic membrane film), we may not have been able to enjoy the benefits of a mature reverse osmosis industry that so many in the drinking water community rely upon today. Sidney Loeb was a talented researcher that spent the majority of his life seeking answers to the many problems associated with the world's water supply. In an effort to honor Sidney Loeb's lifetime achievements, the work described herein provides new information that aids to further
E-mail address: [email protected]. 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.06.039
understand the application of reverse osmosis for treating impaired water supplies. In more recent times, there is concern that the use of new and/or alternative water supplies may impart secondary impacts to the community's overall finished water quality. A potential, yet unintentional impact for communities that employ seawater desalination to solve their water supply problems is the introduction of permeate streams that contain iodide into their existing water supply, that then may result in the formation of iodinated disinfection by-products (DBPs). There is new concern that iodinated-DBPs may be more toxic than brominated or chlorinated compounds, predicated on evidence that brominated-DBPs are much more toxic (and carcinogenic) than their corresponding chlorinated analogs, and that iodine appears to be more reactive than bromine, particularly in mammalian cell toxicity studies [2]. Consequently, determining possible secondary impacts of placing on-line desalination facilities with regards to the amount of iodide available for the formation of iodinated-DBPs is of importance to the water community. It is known chloride and bromide permeate synthetic membrane processes used for seawater desalination; however, the extent of iodide permeation has not been well documented. Bromide and iodide in water supplies have been identified as inorganic species that impact disinfection by-product (DBP) formation during chemical disinfection processes [3]. When disinfecting with chlorination processes, iodide is rapidly oxidized by free chlorine to form hypoiodous acid (HOI). The hypoiodous acid is either further reduced to nontoxic iodate or reacts with the organic matter to produce iodinated trihalomethanes (I-THMs) [4]. Chloramine, unlike chlorine, does not completely oxidize iodide to iodate, IO− 3 , allowing for a greater reaction time between hypoiodous acid and organic matter [5].
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S.J. Duranceau / Desalination 261 (2010) 251–254
Therefore, chloramination leads to higher concentrations of I-THM's and iodinated haloacids than chlorination or ozonation. As part of a Water Research Foundation (Denver, CO) funded research project entitled “Post-Treatment Stabilization of Desalted Permeate,” an accompanying investigation regarding the amount of iodide in desalted seawater permeate was conducted. Three of the participant utilities that were operating seawater desalination facilities desired to obtain additional information regarding seawater permeate iodide content since existing information was not readily available. Typically, membrane manufacturers do not collect iodide rejection data yet can provide bromide rejection values for specific membrane elements [6]. As a result, three of the participant utilities submitted water samples to the University of Central Florida's environmental engineering laboratories for iodide determination. The three water purveyors included Tampa Bay Water (Tampa, FL), City of Long Beach Water Department (Long Beach, CA), and Consolidated Water Company, Ltd. (Caymen Islands). 1.2. Iodide and iodate in sea and fresh water supplies Dissolved iodine in seawater exists primarily as iodide, iodate, and organic iodine. Concentrations of iodate have been shown to vary between b0.1 and 0.45 μM while iodide concentrations range from b0.01 to 0.3 μM [7]. A negative correlation between depth and iodide was found in the Gulf of Mexico and other oceans, suggesting that seawater intakes may be impacted in quality [8]. Wong and colleagues have studied the speciation of total iodine in an estuary near where the Chesapeake Bay mixes with the Atlantic Ocean and found that iodide was the predominant species over that of iodine [9]. Independent research suggested that seawater from the North Sea and the English Channel underwent a slow conversion rate between iodide and iodate in the open sea [10]. A survey of North American and European rivers indicated that the total iodide concentrations varied from 5 μg/L to 212 μg/L; however, the work did not evaluate process streams [11]. With respect to freshwater, it had been shown that iodide and iodate are two main stable iodine species with iodate being the more thermodynamically favorable form in oxygenated waters; hence, the formation of iodinated-DBPs may result in blended water supplies [7]. However, a comprehensive literature review of the subject matter did not provide an indication of the iodide content of desalinated permeate streams used for potable water production [9]. Hence there was a need to investigate the total iodide content of desalinated process permeate streams to obtain more definitive data since existing information in the literature is lacking. 2. Experimental 2.1. Approach to the analytical determination of iodide Iodide content of coastal seawater has been found to be as high as 60 μg/L; however, permeate iodide values are not readily available [10,11]. The information regarding seawater iodide content was used to determine an appropriate method for total iodide concentrations expected to be found in permeate. As iodide concentrations are attributed to natural untreated water it was hypothesized that permeate iodide concentrations would be expected to be at least 10 μg/L, as total iodide. This expectation was based on an existing understanding of the rejection of bromide and chloride by reverse osmosis membranes. In order to analyze permeate iodide content, two analytical methods were deemed appropriate, the “catalytic reduction method” and the “voltammetric method.” The catalytic reduction method would be applicable when evaluating samples with iodide concentrations of 80 μg/L or less, and the voltammetric method was useful for iodide in the range of 5 μg/L to 10.2 μg/L, respectively [12,13]. Although the voltammetric method was the more sensitive of these two options, it was noted that sulfide can interfere with voltammetric analyses. This was important because Consolidated Water had indicated that their deep Caribbean seawater
supplies contained appreciable levels of sulfide, which meant that it would be necessary to treat the submitted samples for sulfide in order to utilize the more sensitive analytical method. Consequently, the catalytic reduction method was selected for the reasons stated above and because of its relative simplicity, availability and repeatability in addition to the fact that the voltammetric method required more sophisticated analytical equipment that was not accessible at the time of the study. Iodide can be determined by using iodide's ability to catalyze the reduction of ceric ions by arsenious acid in a sulfuric acid solution. The reaction is nonlinearly proportional to the amount of iodide present, where the reaction is terminated after a specific time interval by the addition of ferrous ammonium sulfate. Eq. (1) illustrates the ceric reaction. 4+
2Ce
3+
+ As
3+
→2Ce
5+
+ As
ð1Þ
When iodides are present in solution they act as a catalyst for this reduction reaction. As iodide concentrations increase, ceric sulfate reduction also increases. The ceric sulfate solution produced in this method has a distinct yellow color, and as the ceric ions are reduced the yellow colors steadily dissipates. Under conditions of constant temperature and reagent concentration, the time until disappearance of this yellow color could be used to determine total iodide concentration. However, a more convenient method has been developed in which the reduction reaction time is held constant and the non-reduced ceric ion concentration are measured. It would be impractical, using this approach, to measure this ion concentration while the reaction is still taking place. The addition of ferrous ions arrests the reaction and allows for more consistent and accurate readings by immediately reducing the remaining ceric ions. The remaining ferric ion concentration is proportional to that of the remaining ceric ion concentration present before the reduction reaction is stopped. This is shown in Eq. (2). 4+
Ce
2+
+ Fe
3+
→Ce
3+
+ Fe
ð2Þ
Addition of a thiocyanide solution then produces a stable, dark red color that is inversely proportional to the iodide concentration [12,13]. This color intensity can then be measured with respect to a set of standards by means of a color photometer or spectrometer. Results 2.2. Design of analytical testing for total iodide Iodide concentrations were measured per the procedure first established by Rogina and Dubravcic [12] and as further defined by Method 4500-I− C. Catalytic Reduction Method [13]. In order for this method to be accurate and repeatable, several precautions were taken that included: (1) control of the temperature by use of a water bath set at 30 °C±0.5 °C; (2) stringent control of the time variable from the point of reduction reaction initiation until the addition arresting agent; and (3) uniformity and accuracy of reagent concentration/addition to laboratory samples. These variables, along with the accuracy of the synthesized iodide standards, were found to have the most profound effect on overall accuracy and repeatability. Reagents were prepared on a monthly basis (except ferrous ammonium sulfate, which was prepared daily) per the catalytic method. High-performance liquid chromatography (HPLC) reagent-grade water was used for sample and standard dilutions after problems encountered with distilled water during initial experimentation was identified. A sodium chloride (NaCl) reagent solution was prepared by dissolving 200 g of NaCl in 1 L of HPLC reagent-grade water. Formation of non-catalytic forms of iodide such as silver and mercury can have inhibitory effects on iodide readings, so NaCl is used to reduce such effects. Arsenious acid, which is used for the reduction of ceric sulfate in sulfuric acid, was prepared by heating to dissolve 4.946 g of As2O3 and 0.20 mL H2SO4 in 1 L of HPLC water. Standard solution 36 N sulfuric acid was used for acid additions. Ceric ammonium was made by dissolving 13.38 g of Ce
S.J. Duranceau / Desalination 261 (2010) 251–254
(NH4)(SO4)4•4H2O with 44 mL of H2SO4 in 1 L of HPLC water. Ferrous ammonium sulfate reagent was prepared by dissolving 1.50 g of Fe(NH4)2 (SO4)2•6H2O in 100 mL of HPLC water with an addition of 0.6 mL of H2SO4 and potassium thiocyanate solution was prepared by dissolved 4.0 g of KSCN in 100 mL of HPLC water. For standard iodide solution initial preparations, using the standard method outline did not produce feasible results. In turn, standard iodide solutions were produced using an anion standard iodide with a concentration of 1000 mg/L NaI. Initially a 5000 μg/L standard stock solution was prepared by diluting 0.5 mL of the 1000 mg/L NaI anion iodide standard to 100 mL using HPLC water. Standards were prepared using a 5000 μg/L iodide stock solution. A 100 μg/L iodide standard solution was used for preparation of the 0.5, 1.0, 2.5, and 8.0 μg/L standard solutions. Water samples with unknown iodide concentrations were collected from several seawater facilities throughout the Caribbean and facilities in Florida and California. For the experiment, approximately 10 mL of sample volume was required for each test tube. Raw and concentrated water were diluted by using 20 mL of the sample and diluting to 100 mL with HPLC reagent-grade water corresponding to a ratio of 1:4. One duplicate and a 10 μg/L spike were analyzed for quality control and assurance. For the standard solutions 10 mL of each were used to for analysis. 2.3. Analytical methodology Initially 1.00 mL of NaCl, 0.50 mL of arsenious acid, and 0.50 mL of concentrated sulfuric acid were added (in this order) to each of the 10 mL test tube sample. Test tubes with samples were capped and placed in a water bath at 30 ± 0.5 °C, along with a test tube containing ceric ammonium sulfate. The samples and reagent were given 20 to 30 min to acclimate to a constant temperature, after which the caps were removed from the samples and 1 mL of ceric ammonium sulfate solution was added to each sample in 1-minute intervals and mixed by inversion. A yellow color was observed upon addition of ceric ammonium sulfate. After 20± 0.1 min, the first sample was removed from the water bath, 1.00 mL ferrous ammonium sulfate was immediately added, and the yellow color disappeared. Once the color disappeared, 1.00 mL of potassium thiocyanate (KSCN) was added with mixing, and a red color was produced because ferrous ammonium sulfate addition arrests the reduction reaction of ceric ammonium sulfate by arsenious acid. The KSCN produces a red color that has intensity inversely proportional to iodide concentration. Samples were allowed to reach ambient room temperate, which took approximately 45 min. A spectrophotometer set at 525 nm was used to measure absorbance of the samples using 10 mm cuvette. The spectrometer was calibrated with the blank standard and a standard curve was developed from the standard samples evaluated during experimentation. 3. Results and discussion 3.1. Characterization of seawater raw, feed and combined permeate water quality Table 1 presents the general characteristics of water quality of the seawater raw, feed and combined permeate sample locations, and are representative of the conditions experienced at the facility. The water qualities show slight variations in trends of raw water pH, conductivity and temperature because of the differing facility locations and represent typical characteristics of the western Caribbean (Belize), Tampa Bay (Florida) and Pacific Coast (Long Beach) seawater supplies. The data presented in Table 1 are for illustrative purposes, and may not represent all conditions experience at each of the facilities and as such may vary further or differ depending on season. Corresponding data for the Consolidated Water Company, Ltd. facilities located in the Bahamas were not available for reporting purposes.
253
Table 1 Typical seawater raw, feed and combined permeate water quality characteristics. Facility name and sample location Tampa Bay Water, Tampa, FL Raw Feed Combined Permeate City of Long Beach, Long Beach, CA Raw Feed Combined Permeate Consolidated Belize Raw Feed Combined Permeate
Temperature, °C
pH
Conductivity
27.1–38.5 28.7–40.8 30.2–39.1
7.3–7.6 7.1–7.5 6.4–6.7
33.7–41.9 mS 31.9–42.3 mS 680–770 μS
16.7–19.5 15.7–21.2 18.2–22.3
7.4–7.6 7.0–7.5 8.1–8.5
39.3–47.5 mS 40.5–51.9 mS 210–345 μS
26.1–32.3 27.9–34.3 29.4–30.3
7.1–7.4 7.0–7.6 6.2–6.5
49.1–57.2 mS 48.2–56.3 mS 699–762 μS
3.2. Seawater feed and permeate iodide concentrations Data collected from the experiments were use to develop standard calibration curve using statistical regression following a three-parameter exponential decay equation. The decay equation was used to determine concentration for each sample containing unknown amount of iodide. The standard curve data fit equations are presented with the respective R2 values shown in Table 2. The standard calibration curve is presented in Fig. 1 for the experiments conducted in this study. Statistical regression was used to evaluate standard error of the three-parameter exponential decay model, with a goal of maintaining a standard error of less than 0.015 and a corresponding standard deviation below 0.1; values determined to fall above the established standard error and standard deviation goals were considered to not be within acceptable limits established for the experimental plan. These values were used to determine that the method detection limit for these experiments was 4.0 μg/L. The results of the analytical testing and experimental program used to determine total iodide in seawater feed and desalted permeate are provided in Table 3. Feed water ranged from 35.8 μg/L for Tampa Bay (which is slightly lower than seawater) and 51.1 μg/L for Gulf of Mexico water that serves as the source for the Belize facility. Long Beach source water is taken off a power plant canal, and as such, would be expected to be less than Pacific Ocean water. These results indicate that the synthetic membranes used in seawater treatment, based on the conditions experienced in this study, remove greater than 89% of the total iodide in the feed water based on a method detection limit of 4.0 μg/L. Additional experimental work is required to ascertain iodide levels below 4.0 μg/L in the permeate of seawater desalination facilities employing synthetic membrane processes for salinity control. 3.3. Quality assurance and quality control A minimum of one duplicate and one spike sample were performed for every ten samples that were analyzed. Duplicate samples were Table 2 Total iodide standard curve regression model equations. Experimental test run
Regression model (I− ≥ 5.0)
1 2 3 4 5 6 7 8 9 10 11 12
y = − 1.2695 + 1.2589 y = − 1.3371 + 1.3364 y = − 1.4645 + 1.4645 y = − 1.4945 + 1.4884 y = − 1.3171 + 1.3247 y = − 1.3429 + 1.3396 y = − 1.3725 + 1.3713 y = − 1.4246 + 1.4256 y = − 1.3559 + 1.3538 y = − 1.4302 + 1.4321 y = − 1.3132 + 1.3116 y = − 1.5841 + 1.5747
e(− 0.1121∙x) e(− 0.1165∙x) e(− 0.1376∙x) e(− 0.1213∙x) e(− 0.1010∙x) e(− 0.1177∙x) e(− 0.1339∙x) e(− 0.1139∙x) e(− 0.1130∙x) e(− 0.1192∙x) e(− 0.1088∙x) e(− 0.1275∙x)
R2 0.9984 0.9995 0.9981 0.9994 0.9994 0.9997 0.9989 0.9998 0.9997 0.9996 0.9997 0.9988
Note: In these equations, the independent variable (x) is indicative of the iodide concentration (μg/L), while the dependent variable (y) is representative of the corresponding absorbance at 525 nm.
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S.J. Duranceau / Desalination 261 (2010) 251–254 Table 3 Iodide concentrations in seawater feed and desalted permeate. Seawater desalination facility
Feed water iodide Permeate iodide (μg/L) (μg/L)
Tampa Bay Water, Tampa, FL
35.8
City of Long Beach, Long Beach, CA Demonstration Plant Pilot
40.3
Consolidated Belize RO Plant, Belize 51.1 Consolidated Blue Hill Plant, – Bahamas Consolidated Windsor Plant – Bahamas
1st pass = b 4.0 2nd pass = b4.0 Combined permeate = b 4.0 Concentrate = 72.6 1st pass = b 4.0 2nd pass = b4.0 Combined permeate = b 4.0 1st pass = b 4.0 1st pass = b 4.0 1st pass = b 4.0 2nd pass = b4.0
Fig. 1. Iodide calibration curve showing one standard deviation interval.
analyzed to determine variance within samples and to check the reproducibility of the method. Spikes of 10 μg/L of iodide using a 100 μg/L iodide standard were performed to determine the percent recovery (ratio of product feed to water produce) of the method. Percent recovery for the samples ranged from 103% to 75.4%. The reproducibility (RPD) of samples at a 95% confidence interval ranged between 3.0% and 5.8%. Typically it is desired to have the lowest RPD possible within a range of 1.0% to 10% as an indication of acceptable quality control. The RPD values for the samples indicate the method used for determining iodide concentrations was acceptable for reporting purposes. A three-parameter exponential decay equation was used to fit the data and develop a calibration curve, in accordance to the technique established by Rogina and Dubravcic [12]. From the results of the analysis it is shown that by using the catalytic reduction method concentrations of iodide were detected in feed and concentrated water; however, the method detection limit could not be established less than 4.0 μg/L of total iodide for the work reported upon herein, as shown in Fig. 1. 4. Conclusions An investigation was conducted to determine the iodide content of permeate collected from several operating facilities reliant upon synthetic membrane processes for seawater desalination. The total iodide content of desalinated seawater permeate streams was measured using an analytical procedure based on the catalytic reduction of ceric sulfate by arsenious acid in a sulfuric acid solution. It was determined that the total iodide content of permeate samples collected from seawater desalination facilities were less than the catalytic reduction method detection limit of 4.0 μg/L for membrane feed seawaters that ranged between 51.1 μg/L and 35.8 μg/L of total iodide. Results of this investigation indicate that synthetic membrane processes can remove greater than 89% of the total iodide from the feedwater of seawater facilities based on an iodide detection limit of 4.0 μg/L. Additional work is required to ascertain total iodide concentrations in desalinated seawater membrane permeate below 4.0 μg/L for refinement of detail; however, it has been shown that seawater being treated by a synthetic membrane desalination process results in a significant reduction in total iodide concentrations to relatively low levels. The information developed in this investigation would indicate that there would be possible impacts on iodinated-DBP formation; however, the extent of this impact on iodinated-DBP formation potential of blended native supplies remains unknown and should be explored further explored. Acknowledgements This research was funded by the Water Research Foundation (6666 West Quincy Avenue, Denver CO 80235) under project agreement
4079, Post-Treatment Stabilization of Desalted Permeate. The contents do not necessarily reflect the views and policies of the sponsors nor does the mention of trade names or commercial products constitute endorsement or recommendation. The comments and opinions expressed herein may not necessarily reflect the views of the officers, directors, affiliates or agents of the Foundation, the University of Central Florida (UCF), or participant utilities. Without the assistance of C. Owen, Tampa Bay Water, R. Cheng and J. Allen, City of Long Beach Water Department, and J. Countz and W.B. Van Doren, Consolidated Water Company, LTD, Inc. this work would not have been possible; their efforts are greatly appreciated. Special thanks are provided to D. Khiari who served as the Foundation's Project Manager and the Project Advisory Committee who provided valuable technical reviews of this work. The author is grateful to the contributions and assistance of UCF students R. Pfeiffer, V. Trupiano, C. Boyd, and S. Douglas, who contributed in varying capacities to the work. In addition, the author wishes to thank I.C. Watson of RosTek Associates, Inc., Tampa, FL, for his advice during the conduct of this study.
References [1] Y. Cohen, J. Glater, A tribute to Sidney Loeb — the pioneer of reverse osmosis desalination research, Desalination and Water Treatment 15 (2010) 222–227. [2] S.D. Richardson, Disinfection by-products and other emerging contaminants in drinking water, TrAC Trends in Analytical Chemistry 22 (10) (2003) 666–684. [3] G. Hua, D.A. Reckhow, J. Kim, Effect of bromide and iodide ions on the formation and speciation of disinfection byproducts during chlorination, Environmental Science & Technology 40 (9) (2006) 3050–3056. [4] Y. Bichsel, U. Von Gunten, Oxidation of iodide and hypoiodous acid in the disinfection of natural waters, Environmental Science & Technology 33 (22) (1999) 4040–4045. [5] Y. Bichsel, U. Von Gunten, Formation of iodo-trihalomethanes during disinfection and oxidation of iodide-containing waters, Environmental Science & Technology 34 (13) (2000) 2784–2791. [6] V.G. Molina, M. Busch, P. Selin, Cost savings by novel seawater reverse osmosis elements and design concepts, Desalination and Water Treatment 7 (1–3) (2009) 160–177. [7] G.T.F. Wong, The marine geochemistry of iodine, Reviews in Aquatic Sciences 4 (1) (1991) 45–73. [8] K.A. Schwehr, P.H. Santschi, Sensitive determination of iodine species, including organo-iodine, for freshwater and seawater samples using high performance liquid chromatography and spectrophotometric detection, Analytica Chimica Acta 482 (1) (2003) 59–71. [9] G.T.F. Wong, X. Cheng, Dissolved inorganic and organic iodine in the Chesapeake Bay and adjacent Atlantic waters: speciation changes through an estuarine system, Marine Chemistry 111 (3–4) (2008) 221–232. [10] X. Hou, A. Aldahan, S.P. Nielsen, G. Possnert, H. Nies, J. Hedfors, Speciation of super (129)I and super(127)I in seawater and implications for sources and transport pathways in the north sea, Environmental Science & Technology 41 (17) (2007) 5993–5999. [11] J.E. Moran, S.D. Oktay, P.H. Santsch, Sources of iodine and iodine-129 in rivers, Water Resources Research 38 (8) (2002) Art. No. 1149. [12] B. Rogina, M. Dubravcic, Micro-determination of iodides by arresting the catalytic reduction of ceric ions, The Analyst 78 (1953) 594–599. [13] American Public Health Association, American Water Works Association, Water Environment Federation. Standard Methods for the Examination of Water and Wastewater, 21st edAmerican Public Health Association, Washington, D.C, 2005 Section 4500.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Determination of the total iodide content in desalinated seawater permeate S.J. Duranceau University of Central Florida, Department of Civil, Environmental and Construction Engineering, Orlando, FL, 32815-2450, USA
a r t i c l e
i n f o
Article history: Received 29 March 2010 Received in revised form 17 June 2010 Accepted 19 June 2010 Available online 10 July 2010 Keywords: Desalination Synthetic membrane processes Permeate iodide concentration Seawater Catalytic reduction analysis
a b s t r a c t An investigation was conducted to determine the iodide content of permeate collected from several operating facilities reliant upon synthetic membrane processes for seawater desalination. A possible, yet unintentional impact for communities that employ synthetic membrane processes for seawater desalination is the introduction of permeate streams containing iodide into their water supply, that then may result in the formation of iodinated disinfection by-products. To evaluate this potential, the iodide content of desalinated seawater permeate streams were measured using an analytical procedure based on the catalytic reduction of ceric sulfate by arsenious acid in a sulfuric acid solution. It was determined that iodide concentrations in permeate samples collected from seawater desalination facilities were less than the catalytic reduction method detection limit of 4.0 μg/L for membrane feed seawaters that ranged between 51.1 μg/L and 35.8 μg/L of total iodide. Results of this investigation indicated that synthetic membrane processes can remove greater than 89% of the total iodide from the feedwater of seawater based on an iodide detection limit of 4.0 μg/L. © 2010 Elsevier B.V. All rights reserved.
1. Introduction 1.1. Overview and motivation Potable water purveyors are increasingly turning to advanced membrane technologies to augment existing unit operations to improve water quality and allow reliance on poorer source waters. Synthetic reverse osmosis membrane processes, made practical and possible by the pioneering work of Sidney Loeb and Srinivasa Sourirajan during the late 1950s at the University of California, Los Angeles, offers today's water purveyors an economical method for sustainable augmentation using impaired water sources [1]. Many coastal communities are, or are intending to, employ synthetic membrane processes to desalinate seawater because of the need to supplement dwindling fresh water supplies due to expanding population centers, environmental degradation, extended drought and climate change. The work described herein would not have been possible without the contribution of Sidney Loeb, for without his many research contributions (most important of which was the development of an anistropic membrane film), we may not have been able to enjoy the benefits of a mature reverse osmosis industry that so many in the drinking water community rely upon today. Sidney Loeb was a talented researcher that spent the majority of his life seeking answers to the many problems associated with the world's water supply. In an effort to honor Sidney Loeb's lifetime achievements, the work described herein provides new information that aids to further
E-mail address: [email protected]. 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.06.039
understand the application of reverse osmosis for treating impaired water supplies. In more recent times, there is concern that the use of new and/or alternative water supplies may impart secondary impacts to the community's overall finished water quality. A potential, yet unintentional impact for communities that employ seawater desalination to solve their water supply problems is the introduction of permeate streams that contain iodide into their existing water supply, that then may result in the formation of iodinated disinfection by-products (DBPs). There is new concern that iodinated-DBPs may be more toxic than brominated or chlorinated compounds, predicated on evidence that brominated-DBPs are much more toxic (and carcinogenic) than their corresponding chlorinated analogs, and that iodine appears to be more reactive than bromine, particularly in mammalian cell toxicity studies [2]. Consequently, determining possible secondary impacts of placing on-line desalination facilities with regards to the amount of iodide available for the formation of iodinated-DBPs is of importance to the water community. It is known chloride and bromide permeate synthetic membrane processes used for seawater desalination; however, the extent of iodide permeation has not been well documented. Bromide and iodide in water supplies have been identified as inorganic species that impact disinfection by-product (DBP) formation during chemical disinfection processes [3]. When disinfecting with chlorination processes, iodide is rapidly oxidized by free chlorine to form hypoiodous acid (HOI). The hypoiodous acid is either further reduced to nontoxic iodate or reacts with the organic matter to produce iodinated trihalomethanes (I-THMs) [4]. Chloramine, unlike chlorine, does not completely oxidize iodide to iodate, IO− 3 , allowing for a greater reaction time between hypoiodous acid and organic matter [5].
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S.J. Duranceau / Desalination 261 (2010) 251–254
Therefore, chloramination leads to higher concentrations of I-THM's and iodinated haloacids than chlorination or ozonation. As part of a Water Research Foundation (Denver, CO) funded research project entitled “Post-Treatment Stabilization of Desalted Permeate,” an accompanying investigation regarding the amount of iodide in desalted seawater permeate was conducted. Three of the participant utilities that were operating seawater desalination facilities desired to obtain additional information regarding seawater permeate iodide content since existing information was not readily available. Typically, membrane manufacturers do not collect iodide rejection data yet can provide bromide rejection values for specific membrane elements [6]. As a result, three of the participant utilities submitted water samples to the University of Central Florida's environmental engineering laboratories for iodide determination. The three water purveyors included Tampa Bay Water (Tampa, FL), City of Long Beach Water Department (Long Beach, CA), and Consolidated Water Company, Ltd. (Caymen Islands). 1.2. Iodide and iodate in sea and fresh water supplies Dissolved iodine in seawater exists primarily as iodide, iodate, and organic iodine. Concentrations of iodate have been shown to vary between b0.1 and 0.45 μM while iodide concentrations range from b0.01 to 0.3 μM [7]. A negative correlation between depth and iodide was found in the Gulf of Mexico and other oceans, suggesting that seawater intakes may be impacted in quality [8]. Wong and colleagues have studied the speciation of total iodine in an estuary near where the Chesapeake Bay mixes with the Atlantic Ocean and found that iodide was the predominant species over that of iodine [9]. Independent research suggested that seawater from the North Sea and the English Channel underwent a slow conversion rate between iodide and iodate in the open sea [10]. A survey of North American and European rivers indicated that the total iodide concentrations varied from 5 μg/L to 212 μg/L; however, the work did not evaluate process streams [11]. With respect to freshwater, it had been shown that iodide and iodate are two main stable iodine species with iodate being the more thermodynamically favorable form in oxygenated waters; hence, the formation of iodinated-DBPs may result in blended water supplies [7]. However, a comprehensive literature review of the subject matter did not provide an indication of the iodide content of desalinated permeate streams used for potable water production [9]. Hence there was a need to investigate the total iodide content of desalinated process permeate streams to obtain more definitive data since existing information in the literature is lacking. 2. Experimental 2.1. Approach to the analytical determination of iodide Iodide content of coastal seawater has been found to be as high as 60 μg/L; however, permeate iodide values are not readily available [10,11]. The information regarding seawater iodide content was used to determine an appropriate method for total iodide concentrations expected to be found in permeate. As iodide concentrations are attributed to natural untreated water it was hypothesized that permeate iodide concentrations would be expected to be at least 10 μg/L, as total iodide. This expectation was based on an existing understanding of the rejection of bromide and chloride by reverse osmosis membranes. In order to analyze permeate iodide content, two analytical methods were deemed appropriate, the “catalytic reduction method” and the “voltammetric method.” The catalytic reduction method would be applicable when evaluating samples with iodide concentrations of 80 μg/L or less, and the voltammetric method was useful for iodide in the range of 5 μg/L to 10.2 μg/L, respectively [12,13]. Although the voltammetric method was the more sensitive of these two options, it was noted that sulfide can interfere with voltammetric analyses. This was important because Consolidated Water had indicated that their deep Caribbean seawater
supplies contained appreciable levels of sulfide, which meant that it would be necessary to treat the submitted samples for sulfide in order to utilize the more sensitive analytical method. Consequently, the catalytic reduction method was selected for the reasons stated above and because of its relative simplicity, availability and repeatability in addition to the fact that the voltammetric method required more sophisticated analytical equipment that was not accessible at the time of the study. Iodide can be determined by using iodide's ability to catalyze the reduction of ceric ions by arsenious acid in a sulfuric acid solution. The reaction is nonlinearly proportional to the amount of iodide present, where the reaction is terminated after a specific time interval by the addition of ferrous ammonium sulfate. Eq. (1) illustrates the ceric reaction. 4+
2Ce
3+
+ As
3+
→2Ce
5+
+ As
ð1Þ
When iodides are present in solution they act as a catalyst for this reduction reaction. As iodide concentrations increase, ceric sulfate reduction also increases. The ceric sulfate solution produced in this method has a distinct yellow color, and as the ceric ions are reduced the yellow colors steadily dissipates. Under conditions of constant temperature and reagent concentration, the time until disappearance of this yellow color could be used to determine total iodide concentration. However, a more convenient method has been developed in which the reduction reaction time is held constant and the non-reduced ceric ion concentration are measured. It would be impractical, using this approach, to measure this ion concentration while the reaction is still taking place. The addition of ferrous ions arrests the reaction and allows for more consistent and accurate readings by immediately reducing the remaining ceric ions. The remaining ferric ion concentration is proportional to that of the remaining ceric ion concentration present before the reduction reaction is stopped. This is shown in Eq. (2). 4+
Ce
2+
+ Fe
3+
→Ce
3+
+ Fe
ð2Þ
Addition of a thiocyanide solution then produces a stable, dark red color that is inversely proportional to the iodide concentration [12,13]. This color intensity can then be measured with respect to a set of standards by means of a color photometer or spectrometer. Results 2.2. Design of analytical testing for total iodide Iodide concentrations were measured per the procedure first established by Rogina and Dubravcic [12] and as further defined by Method 4500-I− C. Catalytic Reduction Method [13]. In order for this method to be accurate and repeatable, several precautions were taken that included: (1) control of the temperature by use of a water bath set at 30 °C±0.5 °C; (2) stringent control of the time variable from the point of reduction reaction initiation until the addition arresting agent; and (3) uniformity and accuracy of reagent concentration/addition to laboratory samples. These variables, along with the accuracy of the synthesized iodide standards, were found to have the most profound effect on overall accuracy and repeatability. Reagents were prepared on a monthly basis (except ferrous ammonium sulfate, which was prepared daily) per the catalytic method. High-performance liquid chromatography (HPLC) reagent-grade water was used for sample and standard dilutions after problems encountered with distilled water during initial experimentation was identified. A sodium chloride (NaCl) reagent solution was prepared by dissolving 200 g of NaCl in 1 L of HPLC reagent-grade water. Formation of non-catalytic forms of iodide such as silver and mercury can have inhibitory effects on iodide readings, so NaCl is used to reduce such effects. Arsenious acid, which is used for the reduction of ceric sulfate in sulfuric acid, was prepared by heating to dissolve 4.946 g of As2O3 and 0.20 mL H2SO4 in 1 L of HPLC water. Standard solution 36 N sulfuric acid was used for acid additions. Ceric ammonium was made by dissolving 13.38 g of Ce
S.J. Duranceau / Desalination 261 (2010) 251–254
(NH4)(SO4)4•4H2O with 44 mL of H2SO4 in 1 L of HPLC water. Ferrous ammonium sulfate reagent was prepared by dissolving 1.50 g of Fe(NH4)2 (SO4)2•6H2O in 100 mL of HPLC water with an addition of 0.6 mL of H2SO4 and potassium thiocyanate solution was prepared by dissolved 4.0 g of KSCN in 100 mL of HPLC water. For standard iodide solution initial preparations, using the standard method outline did not produce feasible results. In turn, standard iodide solutions were produced using an anion standard iodide with a concentration of 1000 mg/L NaI. Initially a 5000 μg/L standard stock solution was prepared by diluting 0.5 mL of the 1000 mg/L NaI anion iodide standard to 100 mL using HPLC water. Standards were prepared using a 5000 μg/L iodide stock solution. A 100 μg/L iodide standard solution was used for preparation of the 0.5, 1.0, 2.5, and 8.0 μg/L standard solutions. Water samples with unknown iodide concentrations were collected from several seawater facilities throughout the Caribbean and facilities in Florida and California. For the experiment, approximately 10 mL of sample volume was required for each test tube. Raw and concentrated water were diluted by using 20 mL of the sample and diluting to 100 mL with HPLC reagent-grade water corresponding to a ratio of 1:4. One duplicate and a 10 μg/L spike were analyzed for quality control and assurance. For the standard solutions 10 mL of each were used to for analysis. 2.3. Analytical methodology Initially 1.00 mL of NaCl, 0.50 mL of arsenious acid, and 0.50 mL of concentrated sulfuric acid were added (in this order) to each of the 10 mL test tube sample. Test tubes with samples were capped and placed in a water bath at 30 ± 0.5 °C, along with a test tube containing ceric ammonium sulfate. The samples and reagent were given 20 to 30 min to acclimate to a constant temperature, after which the caps were removed from the samples and 1 mL of ceric ammonium sulfate solution was added to each sample in 1-minute intervals and mixed by inversion. A yellow color was observed upon addition of ceric ammonium sulfate. After 20± 0.1 min, the first sample was removed from the water bath, 1.00 mL ferrous ammonium sulfate was immediately added, and the yellow color disappeared. Once the color disappeared, 1.00 mL of potassium thiocyanate (KSCN) was added with mixing, and a red color was produced because ferrous ammonium sulfate addition arrests the reduction reaction of ceric ammonium sulfate by arsenious acid. The KSCN produces a red color that has intensity inversely proportional to iodide concentration. Samples were allowed to reach ambient room temperate, which took approximately 45 min. A spectrophotometer set at 525 nm was used to measure absorbance of the samples using 10 mm cuvette. The spectrometer was calibrated with the blank standard and a standard curve was developed from the standard samples evaluated during experimentation. 3. Results and discussion 3.1. Characterization of seawater raw, feed and combined permeate water quality Table 1 presents the general characteristics of water quality of the seawater raw, feed and combined permeate sample locations, and are representative of the conditions experienced at the facility. The water qualities show slight variations in trends of raw water pH, conductivity and temperature because of the differing facility locations and represent typical characteristics of the western Caribbean (Belize), Tampa Bay (Florida) and Pacific Coast (Long Beach) seawater supplies. The data presented in Table 1 are for illustrative purposes, and may not represent all conditions experience at each of the facilities and as such may vary further or differ depending on season. Corresponding data for the Consolidated Water Company, Ltd. facilities located in the Bahamas were not available for reporting purposes.
253
Table 1 Typical seawater raw, feed and combined permeate water quality characteristics. Facility name and sample location Tampa Bay Water, Tampa, FL Raw Feed Combined Permeate City of Long Beach, Long Beach, CA Raw Feed Combined Permeate Consolidated Belize Raw Feed Combined Permeate
Temperature, °C
pH
Conductivity
27.1–38.5 28.7–40.8 30.2–39.1
7.3–7.6 7.1–7.5 6.4–6.7
33.7–41.9 mS 31.9–42.3 mS 680–770 μS
16.7–19.5 15.7–21.2 18.2–22.3
7.4–7.6 7.0–7.5 8.1–8.5
39.3–47.5 mS 40.5–51.9 mS 210–345 μS
26.1–32.3 27.9–34.3 29.4–30.3
7.1–7.4 7.0–7.6 6.2–6.5
49.1–57.2 mS 48.2–56.3 mS 699–762 μS
3.2. Seawater feed and permeate iodide concentrations Data collected from the experiments were use to develop standard calibration curve using statistical regression following a three-parameter exponential decay equation. The decay equation was used to determine concentration for each sample containing unknown amount of iodide. The standard curve data fit equations are presented with the respective R2 values shown in Table 2. The standard calibration curve is presented in Fig. 1 for the experiments conducted in this study. Statistical regression was used to evaluate standard error of the three-parameter exponential decay model, with a goal of maintaining a standard error of less than 0.015 and a corresponding standard deviation below 0.1; values determined to fall above the established standard error and standard deviation goals were considered to not be within acceptable limits established for the experimental plan. These values were used to determine that the method detection limit for these experiments was 4.0 μg/L. The results of the analytical testing and experimental program used to determine total iodide in seawater feed and desalted permeate are provided in Table 3. Feed water ranged from 35.8 μg/L for Tampa Bay (which is slightly lower than seawater) and 51.1 μg/L for Gulf of Mexico water that serves as the source for the Belize facility. Long Beach source water is taken off a power plant canal, and as such, would be expected to be less than Pacific Ocean water. These results indicate that the synthetic membranes used in seawater treatment, based on the conditions experienced in this study, remove greater than 89% of the total iodide in the feed water based on a method detection limit of 4.0 μg/L. Additional experimental work is required to ascertain iodide levels below 4.0 μg/L in the permeate of seawater desalination facilities employing synthetic membrane processes for salinity control. 3.3. Quality assurance and quality control A minimum of one duplicate and one spike sample were performed for every ten samples that were analyzed. Duplicate samples were Table 2 Total iodide standard curve regression model equations. Experimental test run
Regression model (I− ≥ 5.0)
1 2 3 4 5 6 7 8 9 10 11 12
y = − 1.2695 + 1.2589 y = − 1.3371 + 1.3364 y = − 1.4645 + 1.4645 y = − 1.4945 + 1.4884 y = − 1.3171 + 1.3247 y = − 1.3429 + 1.3396 y = − 1.3725 + 1.3713 y = − 1.4246 + 1.4256 y = − 1.3559 + 1.3538 y = − 1.4302 + 1.4321 y = − 1.3132 + 1.3116 y = − 1.5841 + 1.5747
e(− 0.1121∙x) e(− 0.1165∙x) e(− 0.1376∙x) e(− 0.1213∙x) e(− 0.1010∙x) e(− 0.1177∙x) e(− 0.1339∙x) e(− 0.1139∙x) e(− 0.1130∙x) e(− 0.1192∙x) e(− 0.1088∙x) e(− 0.1275∙x)
R2 0.9984 0.9995 0.9981 0.9994 0.9994 0.9997 0.9989 0.9998 0.9997 0.9996 0.9997 0.9988
Note: In these equations, the independent variable (x) is indicative of the iodide concentration (μg/L), while the dependent variable (y) is representative of the corresponding absorbance at 525 nm.
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S.J. Duranceau / Desalination 261 (2010) 251–254 Table 3 Iodide concentrations in seawater feed and desalted permeate. Seawater desalination facility
Feed water iodide Permeate iodide (μg/L) (μg/L)
Tampa Bay Water, Tampa, FL
35.8
City of Long Beach, Long Beach, CA Demonstration Plant Pilot
40.3
Consolidated Belize RO Plant, Belize 51.1 Consolidated Blue Hill Plant, – Bahamas Consolidated Windsor Plant – Bahamas
1st pass = b 4.0 2nd pass = b4.0 Combined permeate = b 4.0 Concentrate = 72.6 1st pass = b 4.0 2nd pass = b4.0 Combined permeate = b 4.0 1st pass = b 4.0 1st pass = b 4.0 1st pass = b 4.0 2nd pass = b4.0
Fig. 1. Iodide calibration curve showing one standard deviation interval.
analyzed to determine variance within samples and to check the reproducibility of the method. Spikes of 10 μg/L of iodide using a 100 μg/L iodide standard were performed to determine the percent recovery (ratio of product feed to water produce) of the method. Percent recovery for the samples ranged from 103% to 75.4%. The reproducibility (RPD) of samples at a 95% confidence interval ranged between 3.0% and 5.8%. Typically it is desired to have the lowest RPD possible within a range of 1.0% to 10% as an indication of acceptable quality control. The RPD values for the samples indicate the method used for determining iodide concentrations was acceptable for reporting purposes. A three-parameter exponential decay equation was used to fit the data and develop a calibration curve, in accordance to the technique established by Rogina and Dubravcic [12]. From the results of the analysis it is shown that by using the catalytic reduction method concentrations of iodide were detected in feed and concentrated water; however, the method detection limit could not be established less than 4.0 μg/L of total iodide for the work reported upon herein, as shown in Fig. 1. 4. Conclusions An investigation was conducted to determine the iodide content of permeate collected from several operating facilities reliant upon synthetic membrane processes for seawater desalination. The total iodide content of desalinated seawater permeate streams was measured using an analytical procedure based on the catalytic reduction of ceric sulfate by arsenious acid in a sulfuric acid solution. It was determined that the total iodide content of permeate samples collected from seawater desalination facilities were less than the catalytic reduction method detection limit of 4.0 μg/L for membrane feed seawaters that ranged between 51.1 μg/L and 35.8 μg/L of total iodide. Results of this investigation indicate that synthetic membrane processes can remove greater than 89% of the total iodide from the feedwater of seawater facilities based on an iodide detection limit of 4.0 μg/L. Additional work is required to ascertain total iodide concentrations in desalinated seawater membrane permeate below 4.0 μg/L for refinement of detail; however, it has been shown that seawater being treated by a synthetic membrane desalination process results in a significant reduction in total iodide concentrations to relatively low levels. The information developed in this investigation would indicate that there would be possible impacts on iodinated-DBP formation; however, the extent of this impact on iodinated-DBP formation potential of blended native supplies remains unknown and should be explored further explored. Acknowledgements This research was funded by the Water Research Foundation (6666 West Quincy Avenue, Denver CO 80235) under project agreement
4079, Post-Treatment Stabilization of Desalted Permeate. The contents do not necessarily reflect the views and policies of the sponsors nor does the mention of trade names or commercial products constitute endorsement or recommendation. The comments and opinions expressed herein may not necessarily reflect the views of the officers, directors, affiliates or agents of the Foundation, the University of Central Florida (UCF), or participant utilities. Without the assistance of C. Owen, Tampa Bay Water, R. Cheng and J. Allen, City of Long Beach Water Department, and J. Countz and W.B. Van Doren, Consolidated Water Company, LTD, Inc. this work would not have been possible; their efforts are greatly appreciated. Special thanks are provided to D. Khiari who served as the Foundation's Project Manager and the Project Advisory Committee who provided valuable technical reviews of this work. The author is grateful to the contributions and assistance of UCF students R. Pfeiffer, V. Trupiano, C. Boyd, and S. Douglas, who contributed in varying capacities to the work. In addition, the author wishes to thank I.C. Watson of RosTek Associates, Inc., Tampa, FL, for his advice during the conduct of this study.
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