Accepted Manuscript Accurate and self-consistent procedure for determining pH in seawater desalination brines and its manifestation in reverse osmosis modeling Oded Nir, Esra Marvin, Ori Lahav PII:
S0043-1354(14)00498-9
DOI:
10.1016/j.watres.2014.07.006
Reference:
WR 10765
To appear in:
Water Research
Received Date: 24 March 2014 Revised Date:
24 June 2014
Accepted Date: 2 July 2014
Please cite this article as: Nir, O., Marvin, E., Lahav, O., Accurate and self-consistent procedure for determining pH in seawater desalination brines and its manifestation in reverse osmosis modeling, Water Research (2014), doi: 10.1016/j.watres.2014.07.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Accurate and self-consistent procedure for determining pH in seawater desalination
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brines and its manifestation in reverse osmosis modeling
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Oded Nira*, Esra Marvinb and Ori Lahava
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Faculty of Civil and Environmental Engineering, Technion – ITT, Haifa, 32000, Israel; b
Faculty of Environmental Engineering, Helsinki Metropolia University of Applied Sciences, Finland;
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*corresponding author, E-mail:
[email protected]
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Abstract
Measuring and modeling pH in concentrated aqueous solutions in an accurate and
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consistent manner is of paramount importance to many R&D and industrial applications,
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including RO desalination. Nevertheless, unified definitions and standard procedures
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have yet to be developed for solutions with ionic strength higher than ~0.7 M, and
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implementation of conventional pH determination approaches may lead to significant
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errors. In this work a systematic yet simple methodology for measuring pH in
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concentrated solutions (dominated by Na+/Cl-) was developed and evaluated, with the
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aim of achieving consistency with the Pitzer ion-interaction approach. Results indicate
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that the addition of 0.75 M of NaCl to NIST buffers, followed by assigning a new
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standard pH (calculated based on the Pitzer approach), enabled reducing measured errors
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to below 0.03 pH units in seawater RO brines (ionic strength up to 2 M). To facilitate its
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use, the method was developed to be both conceptually and practically analogous to the
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conventional pH measurement procedure. The method was used to measure the pH of
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seawater RO retentates obtained at varying recovery ratios. The results matched better the
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pH values predicted by an accurate RO transport model. Calibrating the model by the
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measured pH values enabled better boron transport prediction. A Donnan-induced
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phenomenon, affecting pH in both retentate and permeate streams, was identified and
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quantified.
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Key Words: pH, brine, Pitzer, Phreeqc, reverse-osmosis
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1. Introduction The pH value is a parameter of major significance in reverse osmosis (RO) applications.
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Most significantly, it affects the permeation rate of potentially toxic weak-acid elements,
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e.g. boron (Tu et al., 2010), NH3 (Hurtado and Cancino-Madariaga, 2014) and arsenic
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(Teychene et al., 2013) and at the same time controls the development of chemical
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scaling of minerals e.g. calcite and brucite (Nir et al., 2012). Additionally, the pH affects
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the membrane’s lifespan (Donose et al., 2013) and may induce a change in its physical
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properties and thereby in its performance (Wang et al., 2014). Despite this, an accurate
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measurement procedure and a reliable predictive model for pH in desalination brines are
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currently lacking in the literature. Regarding the measurement procedure, the knowledge
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gap is associated with theoretical and practical difficulties arising from standardization of
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pH in high ionic strength (I) solutions (Buck et al., 2002). Regarding modeling, the gap is
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attributed to the high complexity of transport and reaction processes, affecting the
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evolution of pH in seawater-brines in full-scale RO operations (Nir and Lahav, 2013).
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To date, a widely accepted definition and a measurement procedure are available for both
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dilute solutions (I < 0.1 mol kg-1) and seawater, but not for seawater desalination brines
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(approximately twice SW concentration). In practice, pH is regularly measured in
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desalination feeds and brines by a combined glass electrode, calibrated by standard NIST
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buffers (NIST stands for U.S. National Institute of Standards, which develops and
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maintains pH standards; the value measured by this procedure is termed pHNIST).
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However, this concept, which was developed for dilute solutions (see brief discussion in
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the supporting material file), encompasses two assumptions which are theoretically
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invalid for concentrated solutions: (1) In the process of assigning primary pH standards,
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the activity coefficient of chloride ions can be estimated by the Bates-Guggenheim
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convention; (2) The liquid junction potential term, appearing in the process of measuring
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pH using the glass electrode arrangement, is practically cancelled out as a result of the
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calibration procedure (Buck et al. 2002). As a result of relying on these erroneous
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assumptions, measuring pHNIST in concentrated solutions invariably leads to considerable
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errors.
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Extensive work has been dedicated to measurement, interpretation and standardization of
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pH in seawater (see brief discussion in the supporting material file). Although high
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precision pH measurements have been shown feasible by potentiometric and
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spectrometric methods, interpretation and standardization are still under debate (Marion
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et al. 2011). Overall, the seawater pH scale approach is based on the relatively constant
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composition of seawater and therefore cannot be readily extended to desalination brines
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of varying compositions, nor to seawater brines at salinity >45‰ or pH values
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significantly different from 8.1 (Millero et al., 2009).
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The Pitzer approach, has been long recognized as a potential framework within which the
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definition and traceability of pH values could be soundly extended to higher ionic
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strengths (Covington, 1997; Ferra, 2009; Millero 2009). Based on statistical mechanics
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approach, the Pitzer equations for ion activity coefficients take into account both long-
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range interactions, represented by the Debye-Huckel term, and short-range specific
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interactions between dissolved species (Pitzer 1973). Since the establishment of a new
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pH reference system requires extensive and precise analytical work (which, ultimately,
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will have to be conducted in institutions responsible for standards) and extensive
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uncertainty analysis (Spitzer et al., 2011), the progress in this direction is slow.
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Meanwhile, industries such as desalination or oil&gas are in urgent need for a practical
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solution, which will allow for improved modeling and better process monitoring and
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design. Different approaches introduced for pH measurement in NaCl brines include the
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use of a liquid-junction free cell coupled with ion selective electrode (Knauss et al.,
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1990), calibration techniques involving acid/base titrations (Mesmer, 1991) and
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spectroscopic methods (Millero et al. 2009). These procedures were, by and large, not
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adopted by desalination professionals, probably due to the relatively large disparity
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between them and the conventional NIST concept in terms of both measurement
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complexity and the pH scale employed.
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The problem of consistency between the measured pH and the applied thermodynamic
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model was already recognized by Harvie et al. (1984) whose Pitzer-based model formed
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the basis for many studies on thermodynamic properties of desalination brines. In their
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work, this issue was addressed by adopting the extended Macinnes convention (i.e. the
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chloride ion activity coefficient is equal to the mean activity coefficient of a KCl solution
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of the same ionic strength) for the representation of activity coefficients. The Macinnes
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scale was set as a default in the implementation of the Pitzer model in the geochemical
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program PHREEQC (Plummer et al., 1988). Although not considered superior from the
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thermodynamic standpoint, the extended Macinnes convention is acknowledged the most
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appropriate due to its higher compatibility with the NIST scale at high ionic strengths, as
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compared with other conventions (e.g. Bates-Guggenheim) (Harvie et al., 1984).
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However model-measurement discrepancies resulting from liquid junction potential are
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still apparent in determining pH of concentrated solutions (Plummer et al., 1988).
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The Pitzer model has been increasingly applied to desalination brines mainly for the
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assessment of scaling tendency (Azaroual et al., 2004; Huff, 2004; Schausberger et al.,
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2009; Radu et al. 2014; Sousa et al. 2014) and energy requirements (Mistry et al., 2013).
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In a previous work the writers developed a computer simulation code for predicting the
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transport and equilibrium state of weak acid-base species within SWRO streams (Nir and
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Lahav, 2013; Nir and Lahav, 2014). The simulation was based on a reactive-transport
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approach, i.e. membrane transport equations (solution-diffusion-film model) coupled
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with elaborated thermodynamic and chemical-equilibrium calculations, facilitated by the
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use of the Pitzer concept, as implemented in PHREEQC. By performing a full species
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distribution analysis at each numerical brine recovery step the simulation code enabled
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the prediction of the pH evolution in the rejected solution as it flowed through a full-scale
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membrane train. This approach was shown to improve the modeling predictions of boron
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permeation (Nir and Lahav, 2013), compared to the approach applied in most of the
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works published thus far on boron membrane transport, in which the hypothesis is that
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the pH of the reject remains constant from the raw seawater to the brine produced at the
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outlet of the SWRO step (e.g. Taniguchi et al., 2001; Sagiv and Semiat, 2004; Mane et
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al., 2009). However, considerable differences in pH values were measured at the feed and
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brine in many SWRO applications (Waly et al., 2011; Andrews et al., 2008).
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Being a key parameter in acid-base equilibria, the exact knowledge of the pH value
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throughout the retentate path within the membrane train is imperative in any predictive
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model, both as an input parameter and throughout the membrane train path, as means of
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calibrating and assessing model predictions. However, little attention has been thus far
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given to errors associated with the measured pH in the context of desalination brines. In
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the current work a heuristic measurement procedure for desalination brines dominated by
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Na+/Cl- was developed and evaluated. Similarly to Nordstrom et al. (1999), a computer
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program implementing the Pitzer approach was used to assign pH values to non-standard
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buffer solutions according to the Macinnes convention. Subsequently, the significance of
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the new measurement procedure to SWRO modeling was demonstrated by comparing
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simulated and experimental pH values.
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2. Material and Methods
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2.1 Experimental
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pH measurements were made using the Metrohm Aquatrode Plus (6.0257.600) combined
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glass electrode with integrated Pt 1000 temperature sensor and a Metrohm780 pH meter.
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Temperature was kept constant at 25±0.6⁰C with a MRC BL-30 circulating bath. Sample
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measurements and calibrations were carried out in mixed 25 ml beakers. Certified
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secondary standard buffers phthalate (pH=4.01), equimolal phosphate (pH=6.86), and
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carbonate (pH=10.01) from Merck were used for calibration when the pH was measured
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in the NIST scale (see results in Fig.1 and part of the results in Fig.3). For the NaCl-
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buffers: the abovementioned NIST buffers were prepared in the lab using analytical grade
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chemicals. Prior to NaCl addition, the mV of the prepared buffers were compared with
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the certified value (margin of error ∆mV
