Experimental investigation of trihalomethanes removal in chlorinated
drinking water sources with carbon nanomaterials
Article in Fresenius Environmental Bulletin · December 2016
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6202
EXPERIMENTAL INVESTIGATION OF
TRIHALOMETHANES REMOVAL IN CHLORINATED
DRINKING WATER SOURCES WITH CARBON
NANOMATERIALS
Kadir Ozdemir*
Department of Environmental Engineering, Bulent Ecevit University, Incivez, 67100 Zonguldak, Turkey.
ABSTRACT
In recent years, carbon nanomaterials have been used widely in water treatment technology. This study investigates to the removal of THMs from chlorinated drinking water sources by combined
coagulation process using single-walled carbon
nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). Terkos Lake water (TLW), Buyukçekmece Lake water (BLW) and Ulutan Lake water (ULW) were used as drinking water source in
this study. Conventional coagulation using
aluminum sulfate (alum) and ferric chloride (FeCl3)
was also conducted using TLW, BLW and ULW samples. Also, the chlorination of raw water samples within a reaction time of 168 hours was conducted in
accordance with Standard Methods 5710 B. The
maximum removal percentages of TTHMs (92%)
was observed with combined SWCNTs and FeCl3 in
chlorinated TLW, followed by BLW (82%) and ULW (78%). In BLW and ULW, TTHMs removal
(86% and 83%) was highest when using FeCl3
+MWCNTs. The TTHMs removal was lowest in ULW (39% for Alum and 45% for FeCl3). On the
other hand, the removal of TTHMs increases in the acidic pH levels whereas decreases alkaline pH levels. In the meantime increases the ionic strength result in decreasing the TTHMs removal for chlorinated three water sources. The results reveal
that combined coagulation using carbon
nanomaterials is effective for the removal of THMs from various types of chlorinated water source waters.
KEYWORDS:
Chlorination; Trihalomethanes; Carbon Nanotubes; Coagulation; Water Treatment.
INTRODUCTION
Chlorination has been widely used for disinfection in many countries to destroy waterborne
pathogenic organisms [1-3]. As the chlorine reacts
with natural organic matter (NOM), disinfection
byproducts (DBPs) form [4-6]. Of the DBPs formed
in chlorinated water, trihalomethanes (THMs) represent a significantly high fraction of these chlorination by-products. Also, these products may have adverse health effects on human beings and are
considered potentially carcinogenic [7-14].
Therefore, many countries have established strict regulations to control these disinfection byproducts
[15-18]. For instance, the United States
Environmental Protection Agency (USEPA) has set a maximum contamination level of 80 µg/L for trihalomethanes and 60 µg/L for five haloacetic acids; and the European Community regulates the
levels of four brominated/ chlorinated
trihalomethanes (chloroform,
dichlorobromomethane, dibromochloromethane and bromoform) often called total trihalomethanes
(TTHMs) at 100 µg/L [19]. Furthermore, the
trihalomethane limit in Turkey is 100 µg/L (as total
trihalomethanes) [20]. Due to the adverse health
effects on humans, several treatment alternatives have been proposed and developed to achieve the removal of DBPs from drinking waters.
Activated carbon adsorption, coagulation, electrocoagulation and biosorption have been used for the removal of DBPs like THMs. Nevertheless,
these methods [21, 22] suffer from high cost and the
removal efficiency was significantly small. For example; due to the low adsorption affinity for low molecular weight polar organic matter, the use of activated carbon has an important drawback. On the other hand, the coagulation process has been used
widely in water treatment applications. Natural organic matter (NOM) and DBPs in water sources have often been removed with aluminum sulfate (Alum) or ferric chloride; enhanced coagulation, whereby a higher dose of coagulant is used, is also a common procedure [23, 24]. Although it was very simple and easy to operate, DBPs and NOM cannot be effectively removed with the coagulation process alone [25, 26]. In recent years, nanotechnology has introduced different types of nanomaterials to the water industry that may have promising outcomes. Nanosorbents such as carbon nano tubes (CNTs),
6203 polymeric materials (e.g. dendrimers) and zeolites have exceptional adsorption properties; and are applied for the removal of heavy metals, organics and biological impurities [27]. In other words, the applications of CNTs for NOM and removal of chlorinated organics have been developed to replace or enhance conventional treatment processes in recently. Further, CNTs strongly adsorb many of these polar organic compounds due to the diverse
contaminant CNT interactions including
hydrophobic effects, pep interactions, hydrogen bonding, covalent bonding, and electrostatic interactions [28]. The Π-Π electron-rich CNT surface allows pep interactions with organic molecules with C-C bonds or benzene rings, such as polycyclic aromatic hydrocarbons (PAHs) and polar aromatic compounds [29, 30]. CNTs are relatively new adsorbents for adsorption of trace pollutant from water.
The industrial production and rapidly emerging potential applications of CNTs in many fields, including water treatment and purification, are a welcome discovery. However, this may be offset by the concern that CNTs are toxic and may become a new class of hazardous pollutants that threaten human health. The release of CNTs into the environment could have harmful impacts on our natural ecosystem [31]. Furthermore, there are concerns that CNTs may interfere or damage DNA, and could have harmful effects on organs if introduced into the body [32, 33]. Also, the toxicity of CNTs on mammalian cells is a function of physical dimension of the tubes [34, 35], physical state of the tubes, i.e. whether dispersed or
agglomerated [35], presence of amorphous
impurities in CNTs and type of treatment of as prepared CNTs [34, 36]. Interactions between CNTs and NOM are likely to alter trends in DBP formation. In addition to direct reaction with chlorine-based disinfectants, CNTs are likely to influence DBP production through their ability to concentrate NOM on their surfaces through sorption [37, 38]. Thus, it is reasonable to expect that these same surface functionalities may also react with chemical disinfectants to yield undesirable by-products, with potentially adverse effects on human health. Many of them have been classified as possible human carcinogens and have been regulated by several international regulatory agencies worldwide [39, 40]. In the meantime, environmental CNTs could be defined as those nanotubes which have leaked from a water purification column during operations and subsequently been transported into the surrounding water, soil and air. For example; nanomaterials can react with humic acids resulting in nanoscale coatings [41], which are comparable to protein coronas in mammalian systems [42]. This strongly modifies CNT aggregation, deposition, and toxic properties [43, 44]. Interactions of CNTs with other biomolecules such as humic substances and
polysaccharides modified nanotube's properties and
overall behaviors [45]. This may cause
homoaggregation and heteroaggregation of
nanotubes leading to different toxicity phenomena for living organisms. The toxicity of well-dispersed CNTs is less compared to that of CNT agglomerates [35].
Nevertheless, several methods could be applied to remove environmental CNTs. Firstly; membrane filtration has been used for eliminating CNTs from solution [46]. In filtration, a solution containing CNTs is allowed to pass through a membrane. However, the major challenge that one can consider is membrane pore size, which should be less than the nanotube size. Secondly, ultracentrifugation can be used for precipitating CNTs from the solution. Here a solution of CNTs is rotated at high speed in a vessel and the particles move freely within the solution by an external field of acceleration caused by the ultra-centrifugal field, resulting in by CNT precipitation. Thirdly, the simple coagulation technique has been found to be effective to recollect CNTs after use [47]. However, this has some difficulties. For instance, the smallest size CNTs do not precipitate or settle under normal gravity force. According to the Health and Safety Executive (HSE) [55], waste containing CNTs must be classified and labeled as hazardous waste. Therefore, after purification of CNTs used in experimental studies, the CNT waste must be sealed carefully using double layers of polyethylene bags. Combustion of waste containing CNTs is preferred, as pyrolysis above 500C completely oxidizes the CNTs.
Although many studies have focused on CNTs and their adsorption properties, there are a limited number of articles about the removal of halogenated
compounds like THMs from the aquatic
environment by using nanoparticles as adsorbent materials. For example; Long and Yang [48] reported that significantly higher dioxin removal efficiency was found with CNTs than with activated carbon. Bina et al. [49] determined that the removal efficiency for ethylbenzene using single-walled and multi-walled carbon nanotubes and hybrid carbon nanotubes and to rank their ethylbenzene removal abilities. Hu et al. [51] investigated the adsorption process of roxarsone on multi-walled carbon nanotubes (MWCNTs), such as adsorption kinetics, thermodynamics, and the effects of various experimental parameters providing a potential solution to the roxarsone and other organometallic compounds in contaminated wastewater. Peng et al. [50] indicated that CNTs are good adsorbents to remove 1,2-dichlorobenzene from water and can be used in a wide pH range of 3–10. Chen et al. [52] investigated the adsorption of chlorophenols on pristine and functionalized SWCNTs (hydroxylated
SWCNTs (SWCNT-OH) and carboxylated
SWCNTs (SWCNT-COOH). Lu et al. [53] investigated the effect of temperature change in the
6204 range 5–35 °C (in 10 °C increments) on the adsorption of THMs from chlorinated drinking water by HNO3/H2SO4 purified MWCNTs.
The present study is the first attempt in Turkey to investigate to the removal of TTHMs from chlorinated drinking water sources by a combined coagulation process using CNTs. Single - walled carbon nanotubes (SWCNTs) and Multi – walled carbon nanotubes (MWCNTs) will be used as CNTs for investigating to the removal efficiency of TTHMs in the coagulation process. Conventional coagulants (Alum + FeCl3) will also be investigated
for comparison. Three major potential drinking water sources were used in this study. Terkos Lake and Buyukçekemece Lake are the main surface water sources for Istanbul, providing nearly 1 million m3
raw water to the drinking water treatment plants. Ulutan Lake is another drinking water source that provides nearly 35,000 m3 of raw water to the
drinking-water treatment plant of Zonguldak city, Turkey. Each of these raw water types consists of varying water quality characteristics, including conductivity, pH, UV254 (Ultraviolet absorbance at
254 nm) TOC (Total organic carbon) and SUVA (Specific ultraviolet absorbance) which have the potential to influence the removal of THMs. In this study, removal mechanisms and ideal water conditions for maximum THMs removal will be also discussed. The novelty of this study is to remove THM compounds in chlorinated water sources by a combined coagulation processes using SWCNTs and MWCNTs.
MATERIALS AND METHODS
Source water and sampling. During this
study, water samples were taken from Terkos Lake water (TLW) and Buyukçekmece Lake water (BLW) in Istanbul city and also Ulutan Lake water (ULW) in Zonguldak city, Turkey. The sampling was done in all four seasons from 2014 to 2015 (with seasons starting in September 2014, January 2015, May 2015, and August 2015). Approximately 1 million m3 of drinking water per day is provided by the TLW
and BLW reservoirs in Istanbul. In addition, ULW is a reservoir that provides nearly 35,000 m3 of raw
water to the drinking-water treatment plant of Zonguldak. Raw water samples were collected as a grab sample, shipped to the laboratory on the same day and kept in the dark in a refrigerator at 4 °C to retard biological activity prior to use.
Coagulation procedure. Prior to the jar test,
stock solutions containing 5,000 mg/L of the SWCNTs and MWCNTs were prepared by adding 1 g of the CNTs to 200 mL of DI water and stirring with a magnetic stirrer at 600 rpm. The applied coagulant doses ranged from 0 to 100 mg/L. Coagulation of TLW, BLW and ULW was carried
out by using a Phipps and Bird six-paddle jar test apparatus. The jars were round beakers with 1-L capacity. Rapid mixing was at 150 rpm for 2 min; flocculation was carried out at 40 rpm for 30 min. The ferric chloride and alum was consistently used for THMs removal at similar dosages as coagulant. On the other hand, the coagulant dose was varied in accordance with the NOM content of the source water, related to hydrophobicity. Source waters with low organic content, including ULW, and BLW, and those with high organic content, including, TLW were treated with coagulants in the ranges of 0–100 mg/L and 0–50 mg /L, respectively. For low SUVA waters, the optimal coagulant was 80 mg/L, while the optimal dose in high SUVA waters was 50 mg/L. Once the jar tests were completed, the treated water samples were collected and passed through 0.45 μm-membrane filters for DOC and THM analysis.
Purified CNTs. One gram of raw CNTs was
dispersed into a 100-ml flask containing 40 ml of mixed acid solutions (30 ml of HNO3 +10ml of
H2SO4) for 24 h to remove metal catalysts (Ni
nanoparticles). After cleaning, the CNTs were again dispersed in a 100-ml flask containing 40 ml of the mixed acid solutions, which were then shaken in an ultrasonic cleaning bath (Branson 3510 Ultrasonic Cleaner, Connecticut, USA) and heated at 80 °C in a water bath for 2 h to remove amorphous carbon. After cooling to room temperature, the mixture was filtered with a 0.45-µm glass-fiber filter, and the solid was washed with deionized water until the pH of the filtrate was 7. The filtered solid was then dried at 80 °C for 2 h to obtain the purified CNTs. This procedure for purifying CNTs has been used by other researchers in previous CNT studies [54]. After purifying the CNTs, a simple coagulation process with application of Alum was used for precipitating CNTs from the solution and thus, CNTs particles were recollected. Then, the residual CNT waste was sealed carefully using double layers of polyethylene bags and transported to solid waste incineration plants with other hazardous wastes from the laboratory [55].
THMFP procedure. Samples of filtered raw and coagulated-settled, filtered water obtained from the jar tests were used in the THMFP tests. THMFP was measured according to the Standard Methods for the Examination of Drinking Water [56]. The pH of all water samples was adjusted to 7.0 (±0.2) with phosphate buffer. Reactions were performed in headspace free vials and in the absence of light. Chlorine solution was added at a dose that produced a free chlorine concentration of 1–2 mg/L as Cl2 after
a reaction time of 7 day at 25 °C. After seven days, the residual chlorine and THM concentrations were determined.
6205 water samples was conducted in accordance with Standard Methods 5710 B [57]. Before chlorination, sample pH values were adjusted to 7 by addition of HCl or NaOH solution as appropriate. The chlorinated samples were placed into 125 mL amber glass bottles with polypropylene screw caps and TFE faced septa. The chlorination process was conducted for a given chlorine dosage (10 mg/L), fixed pH (pH 7), and room temperature (20 °C). After chlorination, the water samples were incubated at 20 °C for the desired contact time (168 h). At the end of the reaction period, a quenching agent (sodium sulfite solution) was added to each of the chlorinated water samples for the analysis of THM formation.
Analytical procedure. All water samples were analyzed based on procedures described in the Standard Methods [57]. All standard solutions were prepared in ultra pure water (Sartorius Co., Germany). Further, raw water samples were filtered using 0.45 μm cellulose acetate filters before analyses and chlorination. DOC analysis was conducted by the high temperature combustion method according to 3510B using a Shimadzu-5000A TOC analyzer equipped with an auto-sampler [57]. The minimum quantification limit of the analyzer was 0.1 mg/l. UV254 absorbance readings were carried out by a Shimadzu 1601 UV Visible spectrophotometer at a wavelength of 254 nm [57]. THM measurement was conducted using EPA Method 551.1 of liquid–liquid extraction (LLE) with pentane [18]. For Haloacetic acids (HAAs) analysis, EPA Method 552.3 acidic methanol esterification was performed [18]. THM calibration standards were prepared using certified commercial mix solutions (AccuStandard, Inc., purity N99%). The
four THM species were chloroform,
bromodichloromethane, dibromochloromethane,
and bromoform. Both THM and HAA analyses were performed with the HP 6890 Series II Gas Chromatograph equipped with a micro Electron Capture Detector (GC-μECD). A capillary column of DB-1 (30 m×0.32 mm I.D.×1.0 μm, J&W Science) was used. Injections of samples were made in split/ splitless mode, with helium as carrier gas and nitrogen as makeup gas.
RESULTS AND DISCUSSION
Characterization of the raw water quality. Table 1 presents the average measured raw water
quality parameters in surface water supplies of Istanbul and Zonguldak city in Turkey The ranges throughout the year were as follows: pH: 7.43–8.43; Turbidity: 1.6–13.5 NTU; Conductivity: 250–685
µS/cm; Total hardness: 101–179 mgCaCO3/L;
Bromide: 60–250 µg/L, and temperature: 5.4–25.3 °C. The NOM surrogate parameters TOC, UV254,
SUVA, and trihalomethane formation potential (THMFP) had ranges of 3.87–6.42 mg/L, 0.07–0.17 cm-1, 1.85–3.14 L/mg.m, and 169.5–389.2 µg/L,
respectively. As can been seen in Table 1, the highest UV254 absorbance was observed in Terkos Lake
Water (TLW) (0.17 cm−1), followed by
Buyukcekmece Lake Water (BLW) (0.16 cm−1),
while Ulutan Lake Water (ULW) showed the lowest level of UV254 absorbance value of 0.07 cm−1. On the
other hand, the highest SUVA value (the ratio UV254/DOC of water) was observed in TLW (3.14
L/mg.m), BLW (2.54 L/mg.m), and ULW (2.48 L/mg.m), respectively. SUVA has been found to be a good predictor of the carbon aromaticity content of the NOM and DBP formation in water [58]. SUVA values of about 4–5 L/mg m represent highly aromatic NOM [59]. On the other hand, Ozdemir et al. [60] have found that the THMs yields increased during chlorination as SUVA values increased. Similarly, the maximum THMFP concentration was measured to be 389.2 µg/L in chlorinated TLW, due to the highest SUVA value (3.14 L/mg.m). Besides, the THMFP values of BLW, and ULW were detected in 302.6 and 277.8 μg/L, respectively. As shown in Table 1, the highest bromide concentration (0.25 mg/l) was observed in BLW. Previous studies in this region indicated that the BLW source is located near the Marmara Sea coastlines, and therefore, usually there is some sea water intrusion to this water source [56]. Bromide concentration in BLW was followed by TLW (0.12 mg/L), and ULW (0.09 mg/L), respectively.
Seasonal variations of THM formation potentials in water reservoirs. Figure 1 shows the seasonal variation of THM formation potential in chlorinated TLW, BLW and ULW, for a reaction time of 168 hours. For all water studied, the average highest THM concentrations were observed in winter season, while the lowest THM concentrations were obtained in summer season. It was concluded that seasonal variations of THMs concentration were dependent on the changes in NOM content and characteristics of water sources. A higher DOC level is associated with more THMs.
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TABLE 1
Physicochemical characteristics of Ulutan raw water samples.
FIGURE 1
Seasonal variations of THM formation for three water sources.
As shown in Fig. 2, an increase of TOC concentrations was observed during the fall and winter season in general. These observations indicate that the type of NOM present in any particular water source can vary seasonally owing to stormwater
runoff after rainfall events, snowmelts in
mountainous regions, or extreme weather events such as flooding and droughts, thus complicating its removal [61-63]. On the other hand, a high SUVA value indicates that the organic matter is largely composed of hydrophobic, high MM organic material. 999
A low SUVA value indicates that the water contains mainly organic compounds which are hydrophilic, of low MM and have low charge density [64, 65]. For the current study, TLW, BLW and ULW samples contain mainly organic compounds which are hydrophilic in the summer season as 2.37
L/mg.m. 2.12 L/mg.m and 1.88 L/mg.m,
respectively. Nevertheless, in winter, NOM in TLW (3.14 L/mg.m), BLW (2.64 L/mg.m) and ULW (2.49 L/mg.m) was composed of high molecular weight material and were more hydrophobic than the other seasons. Also, The NOM found in the natural water samples represents medium-molecular-weight NOM and is somewhat hydrophobic, based on its low SUVA254 value in fall and spring seasons.
FIGURE 2
Seasonal variations of DOC levels for water supplies.
On the other hand, the formation of THM species in chlorinated TLW, BLW and ULW samples is demonstrated in figure 3. As shown Fig. 3, chloroform (CHCl3) was the dominant species
among the four THM compounds for chlorinated TLW, BLW and ULW samples. The maximum concentration of CHCl3 in chlorinated raw water
samples was observed in TLW samples to be 255.6 µg/L, followed by ULW and BLW samples as 168.69 µg/L and 120.82 µg/L, respectively. On the other hand, due to the moderate concentration of bromide ions (250 µg in BLW), the concentrations
of dichlorobromoform (CHClBr2) and
dibromochloroform (CHCl2Br) were higher than that
of CHCl3 for BLW within the reaction time of 168.
These results have also shown that the chlorination of high levels of bromide-containing water modifies the chlorination process, i.e., bromide is rapidly oxidized to bromine and directly affects the formation and distribution of trihalomethane species. As shown in Fig. 4, Bromoform (CHBr3) has the
lowest concentration within the other trihalomethane species in all water samples. Although the maximum CHBr3 concentration was measured to be 5.99 µg/L
in chlorinated BLW within the reaction time of 168h, TLW and ULW showed the concentrations ranging from 1.36 to 0.45 µg/L. Also, these results are confirmed by several studies [66-68].
0 100 200 300 400 TH MF P (μ g/L ) Season TLW BLW ULW 0 2 4 6 8
Winter Spring Fall Summer
DO C ( mg /L ) Season TLW BLW ULW
Parameters Units Terkos Lake
Water (TLW)
Buyukçekmece Lake Water (BLW)
Ulutan Lake
Water (ULW
Range Range Range
pH 7.52-8.33 7.44-8.43 7.43-8.12
Turbidity NTU 2.1- 13.5 1.6-7.3 3,1- 6.4
Conductivity µS/cm 390-685 375-575 312-530
Total Hardness mgCaCO3/L 101-162 110-179 105-165
Alkalinity mgCaCO3/L 83-132 97-163 82-145 Temperature °C 6.3-25.3 5.8-24.6 5.4-23.8 Br- mg/L 0.08-0.12 0.14-0.25 0.06-0.09 TOC mg/L 4.12-6.42 3.87-5.92 4.01-5.87 UV254 cm-1 0.08-0.17 0.075-0.16 0.07-0.15 SUVA L/mg.m 2.88-3.14 1.85-2.54 1.88-2.48 THMFP µg/L 200.1-389.2 181.1-302.6 169.5-277.8
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FIGURE 3
Formation of THM species in three chlorinated water sources.
TTHMs removal by coagulation using SWCNTs. Figure 4 compares the removal of TTHMs by increasing doses of SWCNTs with the addition of conventional coagulants for three chlorinated water sources in the jar-testing procedure. As can be seen in Figure 4, among the chlorinated water samples, the largest percentage of
maximum TTHMs removal using only SWCNTs
was observed in TLW (72%) followed by BLW(66%), and ULW (63%) within the reaction time of 168 hours. As mentioned, the highest TTHMs removal was accounted for by the optimum SWCNTs dose of 50 mg/L in chlorinated TLW water, while the BLW and ULW samles had the lower removal efficiency. This outcome is explained that since the hydrophobic content of NOM contained more activated carbon structures than the hydrophilic content, the highest TTHMs removal
was determined in TLW samples with the SUVA level of 3.14L/mg.m. On the other hand, for three chlorinated water sources, the addition of Alum yielded a significiant removal (>15%) of TTHMs. The removal percentage of TTHMs was recorded in 80% or higher in TLW. Similar trends were observed in ULW and BLW. Several previous studies have shown that as comparing the effectiveness of the Alum and FeCl3, FeCl3 is more effective than alum because of the higher charge density of ferric coagulants [75]. After the conventional coagulants have been applied with SWCNTs, the removal ratio of TTHMs increased for three water sources.
With the addition of alum, the removal percentages of TTHMs remained constant at SWCNT doses of 50 mg/L or greater, with 85% for TLW, 72% for BLW, 76% and 73% for ULW (Figure 4). With the application of FeCl3, the
removal percentages of TTHMs became consistent at SWCNT doses greater than 50 mg/L were 92% in TLW, 82% in BLW and %78 in ULW, respectively. The greatest percentage of TTHMs removal was determined in TLW (>90%) with combined coagulation by FeCl3. On the other hand, the lowest percentage of TTHMs removal using combined coagulation with SWCNTs was observed in ULW (73%), followed by BLW (76%). These results have demonstrated that since the aromatic portion of NOM was compared to that of the hydrophilic portion during the coagulation process, TLW had the higher ratio of TTHMs removal. This finding is also confirmed by other published research [69-70].
FIGURE 4
Removal of TTHMs by SWCNTs using jar test for (a) TLW, (b) BLW, (c) ULW. Optimum coagulant dose = 50 mg /L.
0 100 200 300 400 TLW BLW ULW TH M sp ecie s (μ g/L) Source water CHBr3 CHBr2Cl CHCl2Br CHCl3
6208
FIGURE 5
Removal of TTHMs by MWCNTs using jar test for (a) TLW, (b) BLW, (c) ULW. Optimum coagulant dose = 50 mg /L.
Removal of TTHMs by MWCNTs. Figure 5
shows the removal percentage of TTHMs by increasing doses of MWCNTs, with conventional coagulants during jar-testing.
As shown in Fig. 5, the removal ratio of TTHMs was lower in TLW using only MWCNTs (68%) than those using SWCNTs (72%). Also, maximum TTHMs removal was slightly higher in BLW (73%) and in ULW (70%) using only MWCNTs than when using SWCNTs being nearly 66% and 63%, respectively. This observation revealed that the hydrophobic NOM detected in the majority of the source waters used in this study was more easily removed by SWCNTs than by MWCNTs, while the hydrophilic NOM detected in BLW and ULW was more easily removed by MWCNTs than by SWCNTs. Previous studies have shown that NOM with low molecular weights are difficult to remove using adsorption [71]. On the other hand, three source waters experienced increases in the percentage of TTHMs removal during coagulation when FeCl3 and Alum was used.
Also, increases in TTHMs removal were observed for TLW, BLW and ULW with the application of chemical coagulants. For example; the sole use of Alum provided the highest removal percentage of TTHMs from TLW (51%), followed by the combined use of alum and MWCNTs (68%). A similar trend was observed for TTHMs removal using FeCl3, in which coagulation only and its
combined use with MWCNTs produced the highest TTHMs removal (59%), followed by the use of MWCNTs (77%) (Fig.5).
Comparing of removal efficiencies between
coagulation processes. Figure 6 compares the
removal percentages of TTHMs using only conventional and combined coagulation processes. For three chlorinated water sources, high TTHMs removal percentages (>90%) were observed when using the combined coagulation. Higher TTHMs removal was observed when using only FeCl3 than with alum. Altough the TTHMs removal percentages were low for both alum (42%) and FeCl3 (49%) in BLW, high TTHMs removal (86%) was observed using MWCNTs and FeCl3. Similar trend was determined in ULW (Fig.6). The highest TTHMs (92%) removal was obtained by combining coagulation with FeCl3 +SWCNTs, in TLW. This outcome has shown that THMs generating with chlorination of hydrophobic NOM in TLW was more easily removed by SWCNTs than by MWCNTs, whereas THMs including hydrophilic NOM in BLW and ULW than in TLW was more easily removed by MWCNTs than by SWCNTs. On the other hand, TTHMs removal in ULW was lower than in other sources with removal percentage of 39% when alum was used, while it was 46% when FeCl3 was used. The use of FeCl3 with CNTs provided the highest removal percentage of TTHMs in TLW (92% and 77%), followed by in BLW (82% and 86%) and in ULW (78% and 82%). The ionic strength of the source waters plays an important role in THMs removal by CNTs. Several studies have found that increases in the ionic strength result in increased NOM adsorption, which could be caused by changes in the physical and chemical properties of NOM with CNTs [72].
6209
FIGURE 6
Comparison of TTHMs removal using conventional coagulation (only alum and FeCl3) and combined coagulation processes. Optimum alum and FeCl3 dose = 80 mg/L and optimum
combined coagulant dose = 50 mg/L.
FIGURE 7
Effect of pH on the removal of TTHMs by a.) SWCNTs, b.) MWCNTs.
Compared to the water sources, the significant increase in the removal capacity of the MWCNTs detected in ULW could be the result of the ionic strength. The ionic strength in ULW (conductivity = 684 mS/cm) is higher than that of TLW (conductivity =611 mS/cm) and BLW (conductivity= 511 mS/cm). Therefore, the increasing ionic strength generally resulted in increased DOC removal with MWCNTs. Moreover, the higher ionic strength resulted in reduced electrostatic interactions with the CNTs. Thus, MWCNTs are more effective in the removal of the hydrophilic portion of NOM. These observations are consistent with other studies on removal of NOM [76, 77].
The effects of pH and ionic strength on coagulation by CNTs. Figure 7 shows the effects of
pH on removal of TTHMs by coagulation for chlorinated water sources within a reaction period of 168 hours. It is obvious that the removal of TTHMs on SWCNTs and MWCNTs increases slightly in the pH range of 3-6. As can be seen Fig. 7, the changes of TTHMs removal as a function of pH shows a
similar trend for SWCNTs and MWCNTs. This can be explained by the SWCNTs and MWCNTs employed have been purified by acid solution to develop their properties which can enhance the resistance of SWCNTs and MWCNTs to acid environment. On the other hand, the removal of TTHMs decreases for alkaline pH levels. This result is due to the fact that more oxygen containing groups on the CNTs surface are ionized at higher pH values. This finding is also confirmed by the results of Peng et al. [50]. For example; the removal percentage of TTHMs for all chlorinated water sources increased to the range of 15 to 25 in the acids pH values (pH 3 to pH 6) whereas the removal of TTHMs decreased gradually the range of 20 to 30 at higher pH values. The ionic strength of the source waters plays an important role in THMs removal by CNTs. Several studies have found that increases in the ionic strength result in increased NOM adsorption, which could be caused by changes in the physical and chemical properties of NOM with CNTs [72]. On the other hand, there have been few studies to investigate the effect of ionic strength on the
6210
FIGURE 8
Effect of Ionic strength on the removal of TTHMs by a.) SWCNTs, b.) MWCNTs.
TTHMs removal by coagulation process with CNTs. In this study, the influence of ionic strength on the TTHMs removal by CNTs was investigated. In the investigation into ionic strength, KCl was used to adjust the ionic strength and four different concentrations of 0.1, 0.2, 0.5, and 1.0 mol/L were applied to adjust the ionic strength. As shown Figure 8, with the increase in ionic strength from 0 to 1.0 mol/L KCl, the TTHMs removal decreased in all water sources. This can be interpreted that the formation of surface complexes of metal ions with functional groups on CNTs increases the repulsive forces between THM compounds and CNTs. This finding is confirmed by previous studies [78-80].
CONCLUSION
In this study, the removal of THMs using CNTs using a coagulation process and the effects of pH and ionic strength on THMs removal with SWCNTs and MWCNTs were examined. Coagulation experiments demonstrated that SWCNTs were more effective than MWCNTs in removing THMs from TLW containing hydrophobic organic matters. This is probably because of the smaller diameter and the larger surface area of the SWCNTs as compared to MWCNTs. Among the chlorinated water sources within a reaction time of 168 hours, the highest THMFP value (255.6 µg/L) was observed in TLW samples, followed by BLW (168.7 µg/L) and ULW (120.82 µg/L).This finding suggests that the hydrophobic structures into the NOM played a greater role in THM formation and also were more susceptible to coagulation process than the hydrophilic portion of NOM with low SUVA values (< 3 L/mg.m) such as the BLW and ULW (Table 1). These experimental outcomes are also consistent with previous studies [58, 64, 65]. In the meantime, the TTHMs formation occurring in chlorinated BLW and ULW was more easily removed by MWCNTs than by SWCNTs, while SWCNTs were more effective in chlorinated TLW samples. For instance; maximum TTHMs removal
was slightly higher in BLW using MWCNTs + FeCl3 (86%) than that of using SWCNTs (82%). A similar trend was determined in chlorinated ULW using MWCNTs +FeCl3 (83%), whereas using SWCNTs (78%). In the meantime, the highest TTHMs removal (92%) using application of FeCl3
and SWCNTs in TLW was determined. Besides, TTHMs removals were found as 76% and 73% in chlorinated BLW and ULW, respectively when Alum combined with SWCNTs.
On the other hand, the pH values and ionic strength of water sources play a greater role in THMs removal by CNTs. The removal of THMs onto SWCNTs and MWCNTs increase with the range of acidic pH values (pH 4 to pH 6) but decrease with pH value as the pH exceeds 7.
This observation is consistent with other studies [73, 74]. Also, as the ionic strength increases, TTHMs removal ratio decreases, as shown this study. Due to the harmful effects on human health and the environment, the CNTs particles were recollected together with Alum coagulation and the CNT waste was transported to solid waste incinerators with other hazardous wastes from the laboratory after a purifying, where they are completely oxidized above 500oC through pyrolysis
[55].
Results from this investigation show that
coagulation using carbon nanomaterials can be effective in the removal of THMs from various types of chlorinated source waters. Therefore, water treatment plant operators may use the CNTs as coagulants or aid-coagulant matter instead of conventional coagulants, such as those described in this paper, to effectively remove THMs and the other disinfection by-products that can have adverse health effects on human health.
ACKNOWLEDGEMENTS
Thanks is given to the Scientific and Technological Research Council of Turkey for supporting this study as a scientific and technological research project under project no. 114Y030.
6211
CONFLICTS OF INTEREST
The author declares no conflict of interest.
ABBREVIATIONS
CNTs: Carbon nanotubes, SWCNT: Single-walled carbon nanotube, MWCNT: Multi-Single-walled
carbon nanotube, THMFP: Trihalomethane
formation potential, TTHMs: Total
Trihalomethanes, THM: Trihalomethane, DOC: Dissolved organic carbon, UV254: Ultraviolet
absorbance at 254 nm, TLW: Terkos Lake water, BLW: Buyukçekmece Lake water, ULW: Ulutan Lake water, SUVA: Specific ultraviolet absorbance, NOM: Natural organic matter, TOC: Total organic carbon, HAAs: Haloacetic acids
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Received: 25.09.2016
Accepted: 15.11.2016
CORRESPONDING AUTHOR Kadir Ozdemir
Department of Environmental Engineering, Bulent Ecevit University, Incivez, 67100 Zonguldak, Turkey.
e-mail: kadirozdemir73@yahoo.com
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