Accepted Manuscript Removal of fluoride and natural organic matter removal from natural tropical brackish waters by nanofiltration/reverse osmosis with varying water chemistry Isaac Owusu-Agyeman, Michael Reinwald, Azam Jeihanipour, Andrea Iris Schäfer PII:
S0045-6535(18)31996-9
DOI:
https://doi.org/10.1016/j.chemosphere.2018.10.135
Reference:
CHEM 22397
To appear in:
ECSN
Received Date: 8 May 2018 Revised Date:
16 October 2018
Accepted Date: 18 October 2018
Please cite this article as: Owusu-Agyeman, I., Reinwald, M., Jeihanipour, A., Schäfer, A.I., Removal of fluoride and natural organic matter removal from natural tropical brackish waters by nanofiltration/ reverse osmosis with varying water chemistry, Chemosphere (2018), doi: https://doi.org/10.1016/ j.chemosphere.2018.10.135. 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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ACCEPTED MANUSCRIPT Removal of fluoride and natural organic matter removal from natural tropical
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brackish waters by nanofiltration/reverse osmosis with varying water chemistry
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Isaac Owusu-Agyeman, Michael Reinwald, Azam Jeihanipour, Andrea Iris Schäfer*
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Membrane Technology Department, Institute of Functional Interfaces (IFG), Karlsruhe Institute of
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Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
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*corresponding author: Andrea Iris Schäfer, +49 (0)721 6082 6906,
[email protected]
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submitted to Chemosphere
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May 2018
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revision October 2018
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Abstract
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In the context of decentralised brackish water treatment in development applications, the
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influence of water quality on membrane separation was investigated with real waters. High ionic
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strength (low net driving pressure) on fluoride (F) retention by nanofiltration (NF) and reverse
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osmosis (RO) was investigated over a wide pH range (2-12). Further, the influence of pH on the
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permeation of natural organic matter (NOM) fractions, in particular low molecular weight (LMW)
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neutrals, was elucidated. Natural and semi-natural waters from Tanzania with similar F
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concentrations of about 50 mg L−1 but varying NOM and inorganic carbon (IC) concentration were
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filtered with an NF and RO, namely NF270 and BW30.
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F retention by NF270 for the feed water with highest ionic strength and IC concentration
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was lower and attributed to charge screening. This parameter further reduced at high pH due to co-
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ions (F− and CO32−) interactions and combined (synergistic) effect of high salt concentration and
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pH on F. High NOM resulted in higher membrane zeta potential in comparison with low NOM
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natural water. However, there was no significant difference in F retention due to the fact that F
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retention enhancement was annulled by deposit formation on the membrane. The fraction of NOM
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found in NF/RO permeates was dominated by LMW neutrals. This was attributed to their size and
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uncharged nature, while their higher concentration at low pH remains unexplained. More humic
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substances (HS) of higher molecularity and aromaticity permeated the NF270 when compared with
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BW30, which can be explained with the different membrane molecular weight cut off (MWCO).
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The study highlights the complexity of treating tropical natural waters with elevated F and
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NOM concentrations. In order to develop appropriate membrane systems that will achieve optimal
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F and NOM removal, the influence of water quality parameters such as pH, NOM content, ionic
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strength and IC concentration requires understanding. Seasonal variation of water quality as well as
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operational fluctuations, which occur in particular when such treatment processes are operated with
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renewable energy, will require such challenges to be addressed. Further, given the high
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permeability of low molecular weight (LMW) organics significant permeate side fouling may be
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expected.
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Keywords: water quality; tropical water; carbonaceous water; humic substances; liquid
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chromatography organic carbon detection (LC-OCD)
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1. Introduction
Excessive fluoride (F) is a major drinking water quality issue in many global regions, and in
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particular in the East African Rift Valley where some of the world's highest F concentrations occur
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(Smedley et al., 2002). The occurrence of naturally high fluoride levels originates from the
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interaction of water with fluoride-rich volcanic rocks in this region (Fawell et al., 2013). Abnormal
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F levels occur through chemical weathering of fluorine bearing minerals and it are associated with
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high CO2 pressure and low Ca content (Frencken, 1990; Gizaw, 1996). Subsequent evaporation in
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waterways further increases the F concentration, alkalinity, and salinity of the water (Gizaw, 1996).
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The high F concentration of surface- and ground waters in Northern Tanzania is associated
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with sodium-bicarbonate (Na-HCO3) type of water and varying pH values (3−12) (unpublished
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data, (Gizaw, 1996). This is evidence in the high Na+ (≈ 6600 mg L−1), HCO3− (≈ 4300 mg L−1) and
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CO32− (≈ 7200 mg L−1) concentrations as well as high ionic strength accompanying fluoride-rich
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soda lakes in this region (Nanyaro et al., 1984). Likewise, Magadi, a salt from soda lakes such as
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Lake Natron in the region, which is used by locals for cooking, is found to have F content of up to
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9 mg g−1 and consists mainly of trona (Na2CO3·NaHCO3·2H2O) (Nielsen, 1999; Kaseva, 2006).
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A moderate concentration of F in drinking water (1 mg L−1) is beneficial to humans by
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preventing tooth decay (Fawell et al., 2006), while excessive F intake results in various forms of
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dental and skeletal fluorosis (Fewtrell et al., 2006). The World Health Organization (WHO)
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recommends guideline value of 1.5 mg L−1 of F in drinking water (Fawell et al., 2006). The high F
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concentration in natural waters coupled with the unavailability of alternative water sources forced
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the Tanzanian authorities to raise the F limit in drinking water to 8.0 mg L−1 in 2007. This was
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reduced to 4.0 mg L−1 in 2010 (Government of Tanzania, 2007; African Development Bank, 2010).
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Especially in tropical regions, natural waters with high F can have simultaneously elevated
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concentration of natural organic matter (NOM). A typical example is a water from the Ngare
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Nanyuki River, the main and most important river crossing the densely populated Ngarenanyuki
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Ward in the Arumeru District of northern Tanzania (Istituto Oikos, 2011). The river water has been
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reported to have F and NOM concentrations as high as 60 mg L−1 and 114 mg C L−1, respectively
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(Owusu-Agyeman et al., 2018). NOM in natural waters can be aquagenic (originating from water)
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or pedogenic (originating from soil). The origin of NOM determines its properties and/or
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composition which inevitably affects treatability. It has been shown that NOM from pedogenic
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origin has higher molecularity and aromaticity than aquagenic NOM (Filella, 2009). NOM can be
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fractionated using liquid chromatograph organic carbon detection (LC-OCD), where fractions
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include humic substances (HS), biopolymers, building blocks, low molecular weight (LMW)
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organic acids and neutrals. LMW neutrals typically have a complex composition, and consist
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mainly of alcohols, aldehydes, ketones, sugars, and amino acids (Huber et al., 2011). Pesticides,
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pharmceutical and agro-chemicals with no charge can further contribute to LMW neutrals. The
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catchment area of Ngare Nanyuki is known of using pesticides such as in the farmlands which can
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potentially be tranportant to the river through runoff. Photobleaching can decrease the size of high-
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molecular weight fractions of NOM to LMW constituents and change their ultravoilet absorbance
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capacity (Brinkmann et al., 2003; Lou and Xie, 2006). Such organic compounds cannot always be
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detected by UV-persulphate organic carbon detectors. Photobleaching is influenced by pH,
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temperature and salinity (Song et al., 2017). Due to the complex nature of LMW neutrals, as well
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as their regrowth potential, there is a critical need to investigate the removal of such materials in
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water treatment.
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NOM quality and quantity of water in tropical regions are different from non-tropical
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waters (Findlay and Sinsabaugh, 2003; Johnson et al., 2011). This is evidenced by a spectroscopic
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investigation that has indicated that NOM in tropical waters is mostly of pedogenic origin with the
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biggest fraction being humic substances. This results in higher molecular weight and aromatic ring
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content than NOM in waters from temperate regions (Oliveira et al., 2006). Tropical waters,
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therefore, have higher specific ultraviolet absorbance (SUVA), where a value above 2 L mg−1 cm−1
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is an indication of high disinfection by-product formation potential (Environmental Protection
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Agency Ireland, 2012). For waters from northern Tanzania, high aromatic contents with SUVA
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values in the range of 2−6 L mg−1 m−1 have been reported (Shen and Schäfer, 2015; Aschermann et
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al., 2016). Nanofiltration/reverse osmosis (NF/RO) are suitable for defluoridation and NOM removal
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from water with high removal efficiency in comparison with other methods (Schäfer et al., 2004;
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Ayoob et al., 2008; Song et al., 2011). High molecular weight (HMW) fractions of NOM are to a
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large extent rejected by NF/RO and have been reported to be responsible for flux decline (Schäfer
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et al., 2004; Fan et al., 2008). LMW organics remain a challenge even though the overall organics
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rejection is high. While charged LMW molecules are retained to some extent by electrostatic
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repulsion, LMW neutrals permeate more easily and may cause regrowth (Meylan et al., 2007).
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Fluoride removal by NF/RO is influenced by a number of factors including pH, initial F
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concentration, solute-solute interaction, and ionic strength (Hu and Dickson, 2006; Shen and
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Schäfer, 2015). A positive influence of NOM on F removal has been observed (Shen and Schäfer,
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2015). In view of this beneficial observation and the simultaneous occurrence of F and IC; the
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mechanisms of humic acid (HA) and IC impact on F retention by NF/RO have been explored using
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synthetic waters (Owusu-Agyeman et al., 2017). However, little is known about the influence of
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pH, IC speciation and ionic strength on NOM and F removal from real natural surface waters by
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NF/RO membranes. This work contributes to the understanding of the complex and interlinked
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processes of natural waters containing different quantities of NOM and IC at high F over a broad
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pH range.
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The main research questions will be (i) what is the impact of extreme ionic strength (low
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net driving pressure) on F retention at variable pH (and hence speciation) and (ii) how does pH
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affect the permeation of NOM fractions, specifically LMW neutrals?
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2. Materials and Methods
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2.1. Chemicals and Milli-Q water The pH of natural waters was adjusted with 1 M HCl (VWR Chemical, Germany) and 1 M
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NaOH (Merck, EMD Millipore Corporation, Germany). Total ionic strength adjustment buffer
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(TISAB) for F analysis was prepared using trans-1,2-Diaminocyclohexane tetraacetic acid (CDTA)
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(Acros Organics, USA), NaCl (99.9%, VWR Chemicals, Germany) and glacial acetic acid (100%,
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Merck, EMD Millipore Corporation, Germany). Milli-Q water having a conductivity of 0.05 µS
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cm−1 (Milli-Q® Type 1 water, Merck Millipore, Germany) was used for the preparation of all
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solutions.
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2.2. Water sampling
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A semi-natural water containing Magadi (a salt containing trona and high F from soda
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lakes) and two natural waters were studied. The sampling region of the waters was the Northern
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region of Tanzania (see Figure 1). For the semi-natural water, two types of Magadi were purchased
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from road side sellers at the Southwest of the Lake Natron (north of Engare Sero) during a field trip
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in 2012. The two samples, different in composition, were mixed 1:1 (w/w) and dissolved in Milli-Q
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water to prepare a solution with F concentration of around 50 mg L−1, which is similar to the two
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natural waters.
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Figure 1. Pictures of A) Mdori water source, B) Magadi sellers at Lake Natron (north of
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Engare Sero) and C) Ngare Nanyuki water source (© Schäfer).
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The two natural waters studied were from Mdori and Ngare Nanyuki (Olkungwado). The
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water from Mdori was taken from a borehole in the location S03°47.273' E035°51.138' (Figure 2)
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in December 2013. The Ngare Nanyuki water samples were collected from two locations
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S03°10.929' E036°51.676' and S03°11.141' E036°51.738' in October 2013 and mixed 1:1 (v/v).
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All natural water samples were collected in 5 L plastic containers. The containers were washed
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thoroughly with the natural water before filling. Samples were sealed and stored at room
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temperature before airlifting to Karlsruhe Institute of Technology (KIT), Germany. Upon arrival in
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Germany, samples were stored at a temperature of 4 °C until used.
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Figure 2. Geographical location of sampling sites (map adapted from Tracks4Africa)
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2.3. NF/RO filtration system and protocol
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A stainless steel stirred cell set up was used for the filtration process (Figure 3). The cell
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had internal membrane diameter of 7 cm and effective membrane surface area of 38.5 cm2. The
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internal volume of the cell is 990 mL and was equipped with a magnetic stirrer assembly
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(Millipore, UK), set at a stirring speed of 400 rpm. Pressure and feed temperature were measured by
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a pressure transducer (PX209-300GV5) and a thermocouple (TJ2-CPSS-M60U-250-SB) purchased
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from Omega Engineering, UK. Pressure regulator Pressure sensor Stirred cell setup 3
Stirred cell setup 2
P T
Data acquisition
Synthetic air Stirred cell
Magnetic stirrer
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Membrane Rpm
Stirred cell setup 1
Temperature sensor
Magnetic stirrer assembly
Support
CPU/ LabView
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Pressure regulator
Sample vials
25.0 g
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Balance
Figure 3. Schematic of stirred cell experimental apparatus with data acquisition
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For each experiment, a new membrane coupon from the same membrane sheet was used.
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Membranes were soaked in 10 mM NaCl for one hour prior to use. Pressure was set at 9.8 bar for
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all experiments: compaction using Milli-Q water (1 hour), pure water flux determination (30
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minutes), experiment with 400 mL of feed solutions, and post-filtration pure water flux
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measurement (30 minutes). Normalized flux was determined as the ratio of permeate flux (J)
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during filtration to initial pure water flux (JW0). Recovery was calculated as per equation (1) and
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eight permeate samples of 25 mL each (total permeate volume of 200 mL) were collected, resulting
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in a recovery of 50% for Mdori and Ngare Nanyuki waters. Filtration of Magadi waters was
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performed at a recovery of 30% due to the high ionic strength resulting in high osmotic pressure.
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Recovery =
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∑ VP VF
(1)
where: VP:
volume of permeate (mL) 8
VF:
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volume of feed (mL)
Osmotic pressure (П) of the feed water is calculated from the equation (2) (2)
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Where: R:
universal gas constant (L bar mol−1 K−1),
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T:
absolute temperature (K),
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∑Ci:
the sum of concentrations of solutes (mol L−1).
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In the experiments pH was varied from pH 2 to 12, if possible. This is in such real waters
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limited by the degassing at low pH, scaling at high pH and accompanied by a significant increase
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of conductivity at pH values <4 and >10. The decision to vary pH of such natural water was taken
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to investigate the impact of solute speciation on nanofiltration performance and to determine the
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limitation to treat waters of similar nature but different pH. Significant variation of natural water
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pH has been observed in prior work (Rossiter et al., 2010).
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2.4. Membrane types and characteristics
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Two commercial membrane types, BW30 and NF270, were selected. Both membranes are
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polyamide thin film composite (TFC) membrane types supplied as flat sheet samples by Dow
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FilmTechTM (Minneapolis, MN, USA). BW30 is a fully aromatic polyamide based RO membrane
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used in brackish water applications, whereas NF270 is a piperazine based polyamide NF membrane
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with semi-aromatic, weakly acidic COO¯ groups (Khan et al., 2011). The main selection criteria
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was to compare a tight (BW30) and a loose (NF270) membrane with very different salt retention
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characteristics to understand mechanisms of inorganic and organic retention and to determine
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limiting factors such as concentration polarization or fouling and scaling in treatment. The
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incomplete fluoride retention displayed by the NF270 membrane allows to study the influence of
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organic matter on fluoride retention better than if retention is high.
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2.5. Streaming potential measurement
Streaming potential measurements were performed with an electrokinetic analyser
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(SurPASS, Anton Paar GmbH, Austria) to determine the zeta potential of the membrane in Mdori
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and Nagre Nanyuki waters. Streaming potential measurements were done by Anton Paar, Austria.
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Membrane samples were mounted on two rectangular sample holders (2 cm by 1 cm). Sample
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holders were placed in the adjustable gap cell at a gap height of approximately 100 µm. The system
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was filled and rinsed with the natural water to be used as electrolyte before measurement.
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2.6. Analytical methods
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Electrical conductivity (EC) was measured using an InoLab Cond Level 2 meter with a
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TetraCon® 325 electrode (WTW, Germany). The pH-measurement was conducted using a WTW
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InoLab pH720 meter with a SenTix81 electrode (WTW, Germany). Fluoride concentration was
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measured using an ion F800 selective electrode (ISE) and pH/ION 3310 meter (WTW, Germany).
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TISAB buffer was used (sample to TISAB ratio of 1:1 (v/v)) for F measurements to mask chemical
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interferences resulting from pH and EC and hence increase accuracy.
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Inorganic carbon (IC) and total organic carbon (TOC) were measured with a TOC analyser
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(Sievers M9 portable TOC analyser with autosampler, GE Analytical Instruments, UK).
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Absorbance was determined with a UV-Vis spectrometer (UV/VIS Spectrometer Lambda 25,
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PerkinElmer, USA). All samples were measured against a Milli-Q water reference using a 1 cm
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quartz cuvette. The UV absorbance at 254 nm (UV254) was used to calculate the specific ultraviolet
217
absorbance (SUVA) as in equation (3):
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SUVA =
UV254nm (L mg-1 m-1 ) DOC
(3)
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Organic carbon in the Ngare Nanyuki water samples was characterised using liquid
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chromatography – organic carbon detection (LC-OCD) (DOC Labor, Germany) equipped with UV
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(UVD), organic carbon (OCD) and organic nitrogen (OND) detectors. This method characterizes
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the organic carbon fractions, separated by a size exclusion chromatography column (Huber et al.,
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2011). Discrepancies between organic carbon concentration measured with the LC-OCD and TOC
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analyser (Sievers M9) are discussed in the supporting information (SI) (Fig. SI 6). The aromaticity of NOM in the water samples and the molecularity of HS were determined
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using LC-OCD. Aromaticity is defined as the ratio of spectral absorption coefficient (SAC) to the
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concentration of HS in the dissolved organic carbon (DOC) (see equation (4)). Aromaticity is,
227
therefore, the specific UV absorption of the HS peak.
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Aromaticity (L mg m ) =
SAC (HS)(m-1 ) DOC (HS) (mg L-1 )
(4)
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Molecularity of HS was calculated on the basis of calibration of the column with reference
229
HS of the International Humic Substances Society (IHSS) (Huber et al., 2011). The molar mass
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calibration of the column is derived from equation (5).
M = exp ( −
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tr − A ) B
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where:
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retention time (min)
A, B: matching coefficients calculated from calibration of the column with IHSS humic acid (HA) and fulvic acid (FA)
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tr:
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(5)
The average molecular mass (Mn) of the HS fraction is then determined by equation (6).
Mn =
∑i(ni Mi ) ∑ i ni
(6)
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where: n:
number of molecules
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M:
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molar mass (g mol−1)
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The radius (r) of HS fraction of NOM was calculated from equation (7) with the assumption
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that the molecules are spherical. The equation was developed from the Stokes Einstein equation
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(Worch, 1993). r = 2.037⋅ 10-9 ⋅ Mn0.53 (nm) 3. Results and discussion
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3.1. Characteristics of water samples
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(7)
The characteristics of the three waters studied are presented in Table 1. Magadi is a salt
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formed by natural evaporative processes from natural water streams flowing into the inland soda
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lakes such as Lake Natron (see Figure 2). The inflowing water gradually evaporates and forms salt
249
crusts at its shore (Nielsen, 1999). Magadi consists of the carbonate mineral trona
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(Na2CO3·NaHCO3·2H2O) mixed with minor contents of halite (NaCl) and high F concentration up
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to 9 mg g−1 (Nielsen, 1999). Magadi from Lake Natron has been shown to have one of the highest F
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concentrations in comparison with other East African Magadis (Nielsen, 1999). A semi-natural
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water sample was prepared from the Magadi to have a final F concentration similar to the other
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waters. This resulted in a pH 10, a high IC (1120 mg L−1), salt content (EC = 12430 µS cm−1) and F
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(49 mg L−1), similar to typical water samples found in the Northern regions of Tanzania (Shen and
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Schäfer, 2015).
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Mdori water is a hot borehole water and characterized by high F content (56.2 mg L−1) with
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an IC concentration of 524 mg L−1. The high F concentration in the Mdori waters can be attributed
259
to dissolution of fluoride containing rocks. The conditions favourable for the high fluoride
260
concentration were high IC and Na+ and low Ca2+ concentrations (Rafique et al., 2015). The
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detailed compositions of the water samples are provided in the supporting information (Table SI 1).
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The salinity was low in comparison with the waters prepared from Magadi. The Mdori natural
263
water had NOM concentration of 11.4 mg L−1. The samples had a sulphuric smell indicating 12
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hydrogen sulphide content. This smell vanished with longer exposure to air. The Ngare Nanyuki
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water shows similar characteristics in terms of F and IC as the Mdori water. In addition to the high
266
F levels, the Ngare Nanyuki water also had high NOM content (160 mg L−1) (Table 1). The
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presence of high NOM results in a reddish brown colour, given the river its name Ngare Nanyuki
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(meaning “red water” in the Maasai language). In a typical tropical region, the high NOM
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concentration evolves mainly from the decomposition of plants. The alkaline nature (pH 8.6) of the
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Ngare Nanyuki water can be a contributing factor for the high NOM due to increase in the
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dissolution of NOM from peat soils (Shen et al., 2016).
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Table 1. Characteristics of feed water samples Fluoride
EC
IC
TOC
Osmotic pressure
(mg F L− 1)
(µS cm−1)
(mg C L−1)
(mg C L−1)
(bar)
Magadi
10.0
49.2
12430
1120
1.5
6.7
Mdori
9.4
56.2
5110
524
11.4
3.0
Ngare Nanyuki
8.6
53.7
3990
437
160.0
2.3
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3.2. Zeta potential measurement of membranes in Mdori and Ngare Nanyuki waters
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Streaming potential results with Ngare Nanyuki and Mdori water as electrolyte solutions
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were compared with 10 mM NaCl to determine the influence of the natural waters on the
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membrane surface charge (Figure 4). BW30 membrane gave the highest negative zeta potential in
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Ngare Nanyuki waters when compared with 10 mM NaCl and Mdori water and was attributed to
279
adsorption of NOM on the membrane surface (Childress and Deshmukh, 1998). However, for
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NF270 which originally had a high zeta potential at high pH, the zeta potential was higher in Ngare
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Nanyuki water than 10 mM NaCl only at pH <5. Unlike the 10 mM NaCl, the natural waters
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contained some concentration of multivalent ions which may result in charge screening (Childress
283
and Elimelech, 1996). Mdori water showed somewhat abnormal behaviour for BW30. The zeta
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potential remains slightly positive without showing an isoelectric point (IEP) in the pH range below
285
the intrinsic pH of this water source (Figure 4A). Since the result was different to all other
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observations and especially different to the effect of Mdori on membrane NF270 (see Figure 4B),
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the zeta potential analysis was repeated using a new sample of BW30. The repetition shows a
288
slightly negative zeta potential. This discrepancy (± 8 mV) between first and second measurement
289
was attributed to measurement error in such complex real samples. Further analysis of the membranes was required in order to exclude the influence of water
291
ionic strength on membrane zeta potential and to verify membrane surface modification by NOM
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deposition. Zeta potential analysis was performed in the presence of a standard electrolyte NaCl at
293
a low but fixed ionic strength of 1 mM. For this purpose, the membrane was kept mounted in the
294
adjustable gap cell after each measurement with the Tanzanian natural waters and rinsed together
295
with the electrolyte circuit with 500 mL pure water. After exchanging the electrolyte solution to 1
296
mM NaCl, the zeta potential of the membrane was measured only in the acidic pH range 2−5.5
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(Figure 4C and D). For both membranes the surface charge was higher after exposure to the high
298
NOM Ngare Nanyuki water than the Mdori water.
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20
BW30
A
0 -20 10m NaCl Ngare Nanyuki Mdori Mdori rep
-40 -60 20 C
D
Mdori Ngare Nanyuki Clean
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NF270
B
0 -20 -40 -60 2
6 pH
8
10 2
4
6 pH
8
10
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Zeta potential (mV)
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Figure 4: Influence of Tanzania natural waters on membrane surface charge. Zeta potential
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of (A) BW30 and (B) NF270 measured in the natural waters, and zeta potential of (C)
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BW30 and (D) NF270 measured 1 in mM NaCl after exposure to natural Tanzania waters
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3.3. NF/RO of fluoride-rich semi-natural water with high IC content and high ionic strength (Magadi)
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F removal of the fluoride-rich semi natural water with high ionic strength was performed at
307
reduced recovery (30%) due to a high osmotic pressure built-up during the experiments (Figure 5).
308
pH did not have an observable effect on the normalized flux of the BW30 membrane. However, for
309
NF270, a flux decline was observed at higher pH (see Figure 5A), which was attributed to the
310
decrease in the net driving force at high pH due to the increase in ionic strength due to pH
311
adjustment (see Figure SI 4B). For the BW30 membrane no such flux decline was observed,
312
presumably due to the fact that there was no significant change in the net driving force with pH
313
(see Figure SI 4A). At pH >7, the concentration of IC in the feed was low due to degassing
314
(H2CO3/CO2). IC retention was low at pH 7−9 and increased at pH ≥10 (see Figure 5B), a trend
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315
that has been explained with a shift in IC speciation from HCO3− to CO32− (Simpson et al., 1987;
316
Owusu-Agyeman et al., 2017). As expected F retention by the BW30 was higher than NF270 membrane. In comparison
318
with a similar study with less complex and low ionic strength (EC = 1500 µS cm−1) synthetic
319
waters, the F retention was lower (30−70 % at pH >6) for the more open NF270 membrane. F
320
retention of 80−90 % was achieved by NF270 at pH >6 in the previous study (Owusu-Agyeman et
321
al., 2017). The lower F retention in the more complex Magadi water can be attributed to the high
322
ionic strength which resulted in high osmotic pressure as well as membrane charge screening
323
(Bejaoui et al., 2014; Zaidi et al., 2015). Fluoride retention by NF270 increased initially with pH
324
but decreased towards more basic pH (Figure 5C). The initial increase in retention was as a result
325
of a shift in F speciation from HF to F− (Richards et al., 2009) and increase the membrane surface
326
charge (Teixeira et al., 2005; Mänttäri et al., 2006). The further decrease in F retention by NF270 at
327
higher pH can be explained by (i) co-ions (F− and CO32−) interaction (Owusu-Agyeman et al.,
328
2017) (ii) charge concentration polarisation (Verliefde et al., 2008) and a synergistic effect of
329
higher salt concentration and pH (Freger, 2004; Nilsson et al., 2008; Luo and Wan, 2013). The co-
330
ions interaction occurred between F− and CO32−. At high pH, IC existed as CO32− which is better
331
rejected than HCO3−. This created a charge deficit at the permeate side of the membrane, hence
332
forcing F− ions through the membranes to compensate for the shortage (Yaroshchuk et al., 2011).
333
Secondly, membrane negative charge increases with pH which in turns leads to an increase in the
334
attraction of positively charged ions, resulting in an increase in the membrane wall salt
335
concentration at high pH (Verliefde et al., 2008). The difference between bulk feed concentration
336
and wall concentration was higher for the NF270 than the BW30 membrane, especially at pH >5
337
(see Table SI 2). In consequence, the combined (synergistic) effect of salt and pH arose because of
338
the electrostatic screening by salt ions due to the stronger attraction between the negatively charged
339
membrane and the counter ions. This scenario resulted in the decrease in electrostatic repulsion
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340
between F− ions and membrane. However, the decrease in F retention at high pH was not observed
341
for the BW30 membrane, probably due to the fact that size exclusion is the main F retention
342
mechanism for this membrane (Richards et al., 2013; Shen and Schäfer, 2015). The overall EC retention by both BW30 and NF270 increased with pH (Figure 5D). The
344
increase in EC retention for both membranes was attributed to the shift in IC speciation from
345
HCO3− to CO32−. CO32− has a higher charge and hydrated radius (see Table SI 1) and consequently
346
it is more easily retained.
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J/JW0 (%)
80
40
80 60 40 20 0 0
0 100
4
6
8
10
pH
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2
80 60 40 20 0
4
6
8 pH
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60
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D
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0 100 C
80
40
Degassing
20
100 IC retention (%)
B
EC retention (%)
BW30 NF270
SC
100 A
RI PT
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Figure 5. Influence of pH on (A) normalized flux (J/JW0), (B) IC retention, (C) F retention,
349
and (D) EC retention at 30% recovery of the semi-natural water (Magadi) with high IC
350
and high ionic strength (pressure 9.8 ± 0.1 bar and temperature 20 ± 2 oC)
351 352
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3.4. NF/RO of natural surface water with low NOM and high fluoride content (Mdori)
353
The Mdori water was chosen due to the fact that it contained only a small amount of NOM
354
but high F concentration of around 50 mg L−1. Effects of NOM on the removal of F at different pH
17
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355
were therefore expected to be insignificant compared to the natural water with high NOM content
356
(Ngare Nanyuki). Normalized flux was relatively high compared with the Magadi waters especially for the
358
BW30 membrane (Figure 6A), and no significant change was observed as a function of pH. The
359
relatively high normalized flux (≈ 60%) was attributed to the lower osmotic pressure difference
360
between the bulk and permeate side of the membrane (Table 1). As a result of the relatively low
361
osmotic pressure, the net driving force of the Mdori water was higher than that of the Magadi water
362
(see Fig. SI 4). This was less pronounced for the NF270 due to the lower EC retention of this
363
membrane.
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D
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60
40
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0 2
4
6
8
10
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2
4
6
8
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12
pH
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F
TOC retention (%)
0 E 6
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SUVA (L mg m )
EC retention (%)
80
20
364
IC retention (%)
BW30 NF270
80
0 100 F retention (%)
A
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J/JW0 (%)
100
Figure 6. (A) Normalized flux (J/JW0), (B) IC retention, (C) F retention, (D) EC retention,
366
(E) SUVA of permeates, (F) TOC retention as a function of pH at 50% recovery of the
367
Mdori water (pressure 9.8 ± 0.1 bar and temperature 20 ± 2 oC)
368
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IC and F retention (Figure 6B and C) of the Mdori were higher than that of the semi-natural
370
Magadi waters (Figure 5). At pH >4, F retention of ≈80% and >95% was achieved for NF270 and
371
BW30, respectively. Higher IC and F retention in comparison with that of the Magadi water can be
372
attributed to the relatively lower ionic strength and hence both a higher driving force and reduced
373
charge screening. EC retention increased with pH. The increase in EC with pH was explained with
374
an increase in membrane surface charge. Surface charge affects solute partitioning into the active
375
layer and in consequence an increase in rejection of ions through Donnan electrostatic exclusion
376
(Coronell et al., 2013; Wang et al., 2017). UVA values of both BW30 and NF270 permeates were <
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2 L mg C−1 m−1, which is an indication of low aromatic NOM content (Environmental Protection
378
Agency Ireland, 2012). Retention of NOM was greater than 95% over the entire pH range, which
379
confirms previous results (Shen and Schäfer, 2015), even though it must be noted that the NOM of
380
this water was very low.
381
3.5. NF/RO of natural surface water with high NOM and fluoride content (Ngare Nanyuki)
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The Ngare Nanyuki water was chosen to investigate the influence of high NOM content on
383
F removal. Given the fact that the ionic strength and IC content are comparable to those of the
384
Mdori water, this makes for a very interesting comparison.
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Figure 7A indicates a decrease in flux towards lower pH values. The decline in flux at low
386
pH (2−4) can be attributed to the increase in deposition of NOM molecules due to the reduction of
387
electrostatic repulsion between the membrane surface and NOM (Hong and Elimelech, 1997). Thus
388
at low pH, the membrane surface charge is no longer positively charged (see Fig. SI 5) and NOM
389
molecules are protonated. Further, NOM molecules have a more compact configuration at acidic
390
pH due to reduced charge density (Ghosh and Schnitzer, 1980), which results in increased
391
deposition.
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IC retention (Figure 7B) was similar to that of the Mdori waters (Figure 6), following the
393
trend of IC speciation as discussed in section 3.4. Fluoride retention increased with pH (Figure 7C).
394
This follows the increase in membrane charge and change in F speciation with pH. F retention by
395
BW30 increased sharply from 42% at pH 2 to 90% at pH 4. Thus F retention increased sharply
396
after a change in F speciation from HF to F− (pKa of HF is 3.2). However, for NF270, F retention
397
was about 20% at pH 2−4 and increased to 80% at pH 6. F retention for the Ngare Nanyuki waters
398
was significantly higher than that of the high ionic strength semi-natural Magadi waters, while only
399
a slight increase at higher pH when compared with Mdori (see Table 2) could be observed. EC
400
retention increased with pH due to the increase in the surface charge of the membrane (Figure 7D).
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100
60
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Degassing
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40
20
20 C
0 100
D
80
80
60
60
40
40
E
F
6
4
2
0 2
4
6
401
pH
8
10
M AN U
−1
−1
SUVA (L mg m )
0
20
SC
20
EC retention (%)
0 100 F retention (%)
80
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J/JW0 (%)
80
IC retention (%)
A
12
2
4
6
pH
8
0 100 80 60 40 20
TOC retention (%)
100
0 10
12
Figure 7. (A) Normalized flux (J/JW0), (B) IC retention, (C) fluoride retention, (D) EC
403
retention, (E) SUVA, (F) TOC retention as function of pH at a recovery of 50% of the
404
Ngare Nanyuki water (pressure 9.8 ± 0.1 bar and temperature 20 ± 2 oC)
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Based on previous observations it was expected that the high NOM Ngare Nanyuki water
407
has a higher F retention than the low NOM Mdori water. This is due to an increase in charge of
408
membrane surface by NOM (Shen and Schäfer, 2015). However, a previous study using synthetic
409
water has shown that F retention enhancement by NOM can be annulled by a high concentration of
410
NOM due to membrane deposit formation (Owusu-Agyeman et al., 2017). In view of this, there
411
was indeed no significant difference between F retention for Mdori and Ngare Nanyuki waters,
412
with the exception of NF270 where an increase of 4-5% was consistent (Table 2).
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Table 2. Fluoride retention as a function of pH for all waters and membranes used pH
Fluoride retention (%) Magadi
Mdori
BW30
10
95.1± 4
8.5 (Original)
BW30
NF270
98.9 ± 4
80.6 ± 3
98.8± 4
85.4 ± 4
64.6 ± 3
98.7 ± 4
87.2± 4
98.5± 4
91.0 ± 4
88.8 ± 4
71.2 ± 3
98.8 ± 4
85.8± 4
98.3± 4
89.6 ± 4
8
90.0 ± 4
73.3 ± 3
98. ± 4
86.9± 4
98.8± 4
91.0 ± 4
7
88.3 ± 4
78.3 ± 3
98.2 ± 4
89.1± 4
98.7± 4
87.7 ± 4
6
63.8 ± 3
59.7 ± 3
96.9 ± 4
81.0 ± 3
96.4± 4
85.8 ± 4
4
72.3 ± 5
8.7 ± 1
92.3 ± 6
20.0 ± 1
90.8± 6
29.6 ± 2
2
13.2± 1
12.9 ± 1
60.0± 6
47.5± 5
42.3± 4
29.6 ± 3
a
32.7 ± 1
NF270
pH 11
415
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93.2 ± 4
BW30 a
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NF270 a
Ngare Nanyuki
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Permeate SUVA values by both NF270 and BW30 were below 2 L mg−1 m−1 except for low
417
pH 2 and 4. This shows that both membranes retained the aromatic content of NOM better at pH >
418
4, while the somewhat higher SUVA values at pH 2 and 4 cannot be explained. TOC retention was
419
above 99 % over the entire pH range. The higher TOC retention observed for Ngare Nanyuki (high
420
NOM) waters is attributed to the fact that size exclusion is the main NOM retention mechanism
421
(Shen and Schäfer, 2015). Thus increase in feed NOM concentration did not significantly change
422
NOM concentration in the permeate (≈ 1 mg C L−1 as TOC) and hence retention remained high.
423
3.6. Characteristics of NOM in NF/RO permeate as a function of pH
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424
The NOM in the membrane permeates and original Ngare Nanyuki feed water were further
425
analysed using LC-OCD. This was done in order to understand the characteristics of the NOM
426
permeating the NF/RO membranes. Figure 8 and Figure 9 show the results of the LC-OCD analysis
427
of the original feed water as well as BW30 and NF270 permeates at different pH values.
22
BW30
ACCEPTED MANUSCRIPT LMWA LMWN BB HS BP TOC
−1
DOC (mg C L )
6 A 4
NF270
B
2 0 2
4
6
pH
8
10
12
2
4
6
pH
8
10
12
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428
Figure 8. Characteristics of (A) BW30 and (B) NF270 permeate NOM of Ngare Nanyuki
430
water as a function of pH (LMWA: LMW acids, LMWN: LMW neutrals, BB: building
431
blocks, HS: humic substances, BP: biopolymers measured with the LC-OCD and TOC:
432
TOC measured with Sievers M9 analyser)
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The main fraction of NOM in permeates of both membranes was LMW neutrals (Figure 8).
434
The LMW neutrals concentration of >4 mg C L−1 was recorded for the BW30 and NF270
435
permeates at pH 2−4 and pH 2, respectively. However, these values were not confirmed by TOC
436
analysis with Sievers M9 TOC analyser that measured TOC concentrations of ≈ 1 mg C L−1 over
437
the entire pH range (see Figure 8). This is a very curious discrepancy that triggered significant
438
further investigations and ongoing discussions.
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There is a clear indication from Figure 8 and Figure 9 that both BW30 and NF270
440
membranes show selectivity towards certain fractions of NOM. HS, building blocks, LMW acids,
441
and biopolymers are almost completely rejected by both NF270 and BW30 membranes over the
442
entire pH range. This is due to the fact these fractions consist of large and/or charged molecules
443
that can be rejected through either size exclusion or charge repulsion (Meylan et al., 2007). On the
444
other hand, LMW neutrals are smaller in size and uncharged, and hence could not be rejected by
445
charge interaction and steric hindrance. LMW neutrals are rarely found in natural waters and were
446
not expected in the feed and permeate. It was suspected (after thorough exclusion of the possibility
447
of sample contamination) that the LMW neutrals in the feed could be as a result of pesticides and
448
other agrochemicals used by farmers in the catchment of the Ngare Nanyuki River (Kihampa et al.,
449
2010). Pesticide analysis of the feed and one permeate (NF270, pH 2) showed that there are indeed
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small concentrations of pesticides in µg/L range in the feed water which also permeated the
451
membrane, notable among them were Carbendazim, DEET, Diuron, Fenuron, and Monuron (Table
452
SI 3). However, the concentration of the pesticides analysed was orders of magnitude lower than
453
the LMW fraction concentrations. The possibility that the LMW neutrals in the feed could be as
454
results of contamination from agricultural activities in the catchment of the river remains. LMW
455
neutrals have been found in NF permeates by other studies and has been attributed to leaching from
456
the membrane and natural components of the NOM (Schäfer et al., 2004). Likewise, LMW neutral
457
peaks in natural waters were reported to permeate NF/RO membranes which was explained by the
458
uncharged nature and smaller size of LMW neutrals (Meylan et al., 2007). Furthermore, Figure 8
459
shows that the amount of LMW neutrals in permeate of BW30 was lower than NF270 permeates at
460
pH ≥ 6 due to fact that the molecular weight cut off of BW30 (98−100 Da) (Boleda et al., 2010;
461
Richards, 2012) is smaller than that of the NF270 membrane (155−180) (Boussu et al., 2006;
462
Richards, 2012). The LMW neutrals signals were higher at low pH 2−4 for both membranes and
463
the higher permeability of LMW neutrals at the acidic pH range could not be explained. Other
464
studies have recorded low retention of LMW organics by NF/RO membranes at low pH (below
465
pka) and attributed the low retention to change of structure of such solutes (Ozaki and Li, 2002;
466
Bellona and Drewes, 2005). As the compounds could not be identified, such claims remain
467
speculative and further investigation is required to identify processes such as photobleaching and
468
microbial degradation that possibly produce such LMW compounds in such tropical water. A
469
further field requiring more in depth study is the consequences of permeation of such organics that
470
are very biodegradable. In a membrane application this would no doubt contribute to permeate side
471
biofouling which is observed on occasion, where the ‘nutrients’ are delivered through the
472
membrane to the permeate channel.
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473
Both the OCD and OND showed signals for the LMW neutrals (see Figure 9). This suggests
474
that LMW neutrals that permeated the NF/RO membranes included nitrogen bound organic
24
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compounds. However, the UVD of the LC-OCD did not show any signal for the LMW neutrals.
476
LMW neutrals are hydrophilic to amphiphilic and show a low or little response in UVD due to their
477
low UV absorptivity (Leenheer and Croué, 2003; Huber et al., 2011).
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25
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Relative signal
pH 12 pH 10
M AN U
pH 8.5 pH 8 pH 7 pH 6
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OCD OND
Permeates - BW30
pH 4
Permeates- NF270
pH 12
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Biopolymer
B
Building blocks
HS
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Relative signal
A
pH 6 pH 4 pH 2
0 478
20 40 60 80 Retention time (min)
100
479
Figure 9. LC-OCD chromatograms of Ngare Nanyuki water (A) Feed and (B) BW30 and
480
(C) NF270 permeates as a function of pH
481 26
482
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3.7. Characteristics of HS in NF/RO permeates
The aromaticity and molecular weight of HS in the NF/RO permeate was plotted on the
484
humification diagram in order to compare the permeate HS with the feed and IHSS standards (see
485
Figure 10). It was observed that at low pH 2 and 4, the aromaticity of HS in the permeate was high
486
(13−20 L mg−1 m−1) for both membranes. Both the aromaticity and Mn of permeate HS for BW30
487
were lower than that of the NF270 at pH >6 (Figure 10A). At pH >6, the molecular weights of HS
488
that permeated the BW30 were 60−85% less than that of the feed, while those of the NF270 were
489
only 30−50% less. The lower Mn and aromaticity of BW30 than NF270 was attributed to the fact
490
that BW30 have smaller MWCO than NF270 (Cho et al., 2000). In consequence, the equivalent
491
radius of HS that permeated the BW30 membrane was lower than that of the NF270 (Figure 10B).
492
The estimated HS radius (>0.44 nm) shows that the permeating HS molecules were larger than the
493
reported pore radius of both membranes (0.30−32 nm for BW30 and 0.38−0.42 nm for NF270)
494
(Hilal et al., 2005; Richards, 2012; Simon et al., 2013; Dražević et al., 2014). This suggests that the
495
mode of transport of HS molecules through the membrane was through diffusion and charge
496
interaction. However, this suggestion is made with caution as the pore sizes are average effective
497
values and retention/transport is likely to be dominated by a small number of larger pores.
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16 12
498
BW30 NF270 Feed
1.0 0.8 0.6
Feed
NF270
8
B
0.4
IHSS-HA
BW30
IHSS-FA
4
0.2
Radius of HS (nm)
NF270 BW30 pH 2 pH 2 pH 4 pH 4 pH 6 pH 6 pH 7 pH 7 pH 8 pH 8 pH 8.5 pH 8.5 pH 10 pH 10 pH 12 pH 12
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−1
−1
SAC/DOC (HS) (L mg m )
M AN U
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483
Marine-FA
0.0
0
400 600 800 1000 1200 1400 -1 Mn (gmol )
2
4
6
8
10
12
pH
499
Figure 10. Characteristics of permeate NOM of Ngare Nanyuki water (A) humification
500
diagram (B) molecular radius of HS of the permeate NOM as a function of pH
501
27
502
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4. Conclusions
Examining real and semi-natural tropical waters as a function of pH with an open (NF270)
504
and a dense (BW30) membrane allowed to elucidate the contribution of numerous factors to F and
505
NOM removal. Permeation of NOM fractions and specifically LMW neutrals was examined in
506
detail and a large contribution of difficult to detect LMW neutrals was identified.
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F retention by NF270 particularly for the semi natural waters with high EC and high IC
508
concentration was generally low (60−70%) and further decreased at high pH (down to 20%). The
509
decrease was attributed to co-ions (F− and CO32−) interaction and synergetic effect of high salt
510
concentration (low driving force) and pH. Relatively higher F retention was achieved for the waters
511
with lower EC and IC (Mdori and Ngare Nanyuki). Although high NOM concentration natural
512
water (Ngare Nanyuki) resulted in higher zeta potential in comparison with low NOM water
513
(Mdori), there was no significant difference between F retention with the exception of the loose
514
membrane were a small increase was observed. This was probably due to the fact that F retention
515
enhancement was annulled by deposit formation on the membrane.
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About 60−90% of NOM fractions that permeated the NF/RO membranes were LWM
517
neutrals. There was higher permeability of LMW neutrals at low pH range 2−4 and was
518
unexplained as the specific compounds could not be identified. The HS of NOM that permeated
519
both NF/RO membranes at acidic pH (2−4) were more aromatic in nature. The fractions of HS that
520
permeated the BW30 membrane at pH ≥6, are of lower molecularity and aromaticity than those that
521
permeated the NF270 due to larger MWCO of NF270 than BW30. The permeation of
522
biodegradable organics poses challenges of controlling permeate side biofouling in full scale
523
application.
524
5. Acknowledgements
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The Deutscher Akademischer Austauschdienst (DAAD) is thanked for a Ph.D. stipend for
526
I.O.-A. Funding for IFG-MT was provided by the Helmholtz Association, Germany through the 28
ACCEPTED MANUSCRIPT
Rekrutierungsinitiative. The DOW Chemical Company (USA) kindly supplied the membranes
528
samples. Dr. Thomas Luxbacher and Sandra Zierler, Anton Paar GmbH, Austria performed the
529
streaming potential measurements. Elijah Paul and Lwitiko Pholds are thanked for sampling and
530
transporting Mdori water samples to KIT respectively. Marita Heinle (IFG-KIT) is thanked for
531
technical support with the TOC Analyzer and ICP-OES analysis. Dr. Stefan Huber (DOC-Labor,
532
Germany) performed the LC-OCD analysis and contributed extensive discussions. Dr. Frank
533
Schramm (IFG-KIT) is thanked for technical support with UV-Vis spectroscopy. Authors are
534
grateful to Prof. Dr. Heinz-Jürgen Brauch of the Technologiezentrum Wasser (TZW, Karlsruhe) for
535
pesticide analysis and discussion.
536
6. References
537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565
African Development Bank, 2010. Environmental and social management plan (ESMP) for Tanzania- rural water supply and sanitation programme (RWSSP) II [online]. Final Report, Tanzania (viewed on 17/01/2017) https://www.afdb.org/fileadmin/uploads/afdb/Documents/Project-andOperations/Tanzania%20%20Rural%20Water%20Supply%20and%20Sanitation%20Program%20II%20_AR_%20doc %20%2BMemo%5B1%5D.pdf. Aschermann, G., Jeihanipour, A., Shen, J., Mkongo, G., Dramas, L., Croue, J.-P., Schäfer, A., 2016. Seasonal variation of organic matter concentration and characteristics in the Maji ya Chai River (Tanzania): Impact on treatability by ultrafiltration. Water Research 101, 370381. Ayoob, S., Gupta, A.K., Bhat, V.T., 2008. A conceptual overview on sustainable technologies for the defluoridation of drinking water. Critical Reviews in Environmental Science and Technology 38, 401-470. Bejaoui, I., Mnif, A., Hamrouni, B., 2014. Performance of reverse osmosis and nanofiltration in the removal of fluoride from model water and metal packaging industrial effluent. Separation Science and Technology 49, 1135-1145. Bellona, C., Drewes, J.E., 2005. The role of membrane surface charge and solute physico-chemical properties in the rejection of organic acids by NF membranes. Journal of Membrane Science 249, 227-234. Boleda, M.R., Majamaa, K., Aerts, P., Gómez, V., Galceran, M.T., Ventura, F., 2010. Removal of drugs of abuse from municipal wastewater using reverse osmosis membranes. Desalination and Water Treatment 21, 122-130. Boussu, K., Zhang, Y., Cocquyt, J., Van der Meeren, P., Volodin, A., Van Haesendonck, C., Martens, J.A., Van der Bruggen, B., 2006. Characterization of polymeric nanofiltration membranes for systematic analysis of membrane performance. Journal of Membrane Science 278, 418-427. Brinkmann, T., Sartorius, D., Frimmel, F.H., 2003. Photobleaching of humic rich dissolved organic matter. Aquatic Sciences 65, 415-424.
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ACCEPTED MANUSCRIPT Highlights • Real tropical waters with different IS, IC, NOM but similar F content treated by NF/RO • At high IS & pH, F retention decreases due to IC speciation, pH and salt effect • No enhancement effect of high NOM on F retention was observed • A discrepancy between permeate NOM measured by TOC analyser and LC−OCD was
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• According to LC−OCD results, LMW neutrals accounted for 60−90% of permeate NOM