Effects of metal ions on disinfection byproduct formation during chlorination of natural organic matter and surrogates

Chemosphere 144 (2016) 1074e1082

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Effects of metal ions on disinfection byproduct formation during chlorination of natural organic matter and surrogates Yu Zhao a, Hong-wei Yang a, Shi-ting Liu a, Shun Tang a, Xiao-mao Wang a, *, Yuefeng F. Xie a, b a b

State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China Civil and Environmental Engineering Programs, The Pennsylvania State University, Middletown, PA 17057, USA

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Primary catalytic mechanism of calcium and cupric were complexation and redox reaction, respectively.
Catalytic effect of calcium was less sensitive to molecular structure or weight.
Catalytic effect of cupric was greatly dependent on molecular structure.
Catalytic effect of ferric and ferrous iron critically depended on molecular weight.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 April 2015 Received in revised form 25 August 2015 Accepted 25 September 2015 Available online xxx

The effects of calcium, cupric, ferrous and ferric ions on the formation of trihalomethanes (THMs) and haloacetic acids (HAAs) were investigated using natural organic matter (NOM), small molecular weight NOM surrogates and natural water samples. The results showed that the effects were greatly dependent on the disinfection byproduct (DBP) precursor structure and molecular weight, and metal ions species. While using NOM as precursors, addition of 4.00 mM calcium ions increased the formation of THMs, dihaloacetic acids (DHAAs) and trihaloacetic acids (THAAs) by 24e47%, 51e61% and 15e25%, respectively. Addition of cupric ions at 0.02 mM increased the formation of THMs and DHAAs by 74e83% and 90 e100%, respectively, but decreased the formation of THAAs by 26e27%. Similar effect was not observed when 0.04 mM ferrous or ferric ions were added. The effects of calcium and cupric ions on DBP formation were generally more evident for the NOM surrogates than that for NOM. The primary catalytic effect of calcium ions was due to complexation and less sensitive to molecular structure or weight, while that of cupric ions was attributed to redox reactions and greatly dependent on molecular structure. Both ferric and ferrous iron had substantial effects on the DBP formation of surrogates (citric acid and catechol in particular), which implied that the catalytic effects of ferric and ferrous iron mainly depended on molecular weight. The catalytic effect of cupric ions was also observed on natural water samples, while the effects of calcium, ferrous and ferric ions on natural water samples were not evident. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Catalytic effect Model compounds Trihalomethanes (THMs) Haloacetic acids (HAAs) Complexation

1. Introduction

* Corresponding author. E-mail address: [email protected] (X.-m. Wang). http://dx.doi.org/10.1016/j.chemosphere.2015.09.095 0045-6535/© 2015 Elsevier Ltd. All rights reserved.

Disinfection byproducts (DBPs) in drinking water are unintentionally formed by reactions between chemical disinfectants and the natural organic matter (NOM) that are ubiquitous in water.

Y. Zhao et al. / Chemosphere 144 (2016) 1074e1082

DBPs are a very complex mixture, which include 600 þ identified species and much more species that are yet to be identified (Krasner et al., 2006; Bond et al., 2012). Many DBPs (e.g. halobenzoquinones) have been associated with various human cancers and reproductive complications (Hrudey and Charrois, 2012). Trihalomethanes (THMs) and haloacetic acids (HAAs) are among the few DBPs currently regulated around the world (Wang et al., 2015) partly because they are usually the two predominant DBP groups by mass and molarity and can to some extent represent the total DBP content, and partly because both THMs and HAAs are the primary decomposition end-products of many unstable and potentially more toxic intermediate DBPs (Zhai and Zhang, 2011; Bond et al., 2012). The formation and control of THMs and HAAs have been investigated in many studies (Westerhoff et al., 2004; Reckhow et al., 2004; Hua and Reckhow, 2007; Liu et al., 2008). Because of the complex chemical composition and molecular structure of NOM (Kornegay et al., 2000; Sutton and Sposito, 2005), simple organic compounds are usually used as NOM surrogates to determine the mechanisms of DBP formation (Boyce and Hornig, 1983; Chang et al., 2006; Bond et al., 2009; Zeng and Arnold, 2013; Hua et al., 2014). Previous studies have suggested that activated aromatics, amines and b-dicarbonyl species are among the main functionalities of NOM that are more responsible for the formation of THMs and HAAs (Reckhow and Singer, 1985; Zhai and Zhang, 2011; Bond et al., 2012; Pan and Zhang, 2013). The inorganic components of natural water affect the formation of DBPs during the disinfection process. The most notable inorganic component, which has been extensively studied, is bromide ions (Hua et al., 2006; Hu et al., 2009; Shi et al., 2013). Several other inorganic anions, such as nitrite and chloride, can also alter DBP concentrations and speciation during chlorination (Navalon et al., 2008; Hu et al., 2009). Furthermore, metal ions, which are ubiquitous in natural water and water distribution systems and may be deliberately added for water treatment, react with DBP precursors and disinfectants. NOM can form complexes with hardness ions (e.g. calcium, ferrous and ferric ions) (Stumm and Morgan, 1996; Liu et al., 2007), which may substantially change the reactivity of the NOM to the disinfectants. NOM can also be chemically or physically bound to the surface of ferric or aluminum coagulant flocs. Some transition metal ions (e.g. cupric ions) can catalyze the decomposition of the disinfectants leading to the formation of more powerful oxidants, which react with NOM and change their properties. The concentrations of various metal ions could differ drastically among different water sources (Crittenden et al., 2012). For example, the calcium and ferrous iron concentrations can vary from ~1 to >50 mg L1 and from <0.05 to >0.5 mg L1, respectively. The dissolved ferric iron concentration is usually very low (<1 mg L1), which greatly depends on the NOM concentration and properties. Primarily due to anthropogenic contamination, the cupric ion concentration in surface water can be up to several mg L1 (Liu et al., 2007). The study of the effects of metal ions on DBP formation and speciation is therefore of both theoretical and practical significance. A few studies have been conducted regarding metal ions effects. Among the metals, copper has drawn particular attention due to its significant catalytic effect. Duggirala (1996) first reported that the presence of a small amount of cupric ions remarkably increased the formation of THMs and proposed that the increase may be attributed to a complexation between the cupric ions and DBP precursors. Fu et al. (2009) further investigated the role of cupric ions in chlorination and found that copper fortifies the formation of hydroxyl free radicals, which in turn break down the large NOM molecules. Calcium, magnesium and ferric ions were also found to be capable of influencing the formation of DBPs by complexation with DBP precursors. Navalon et al. (2009) found that the formation

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of THMs was enhanced by 32% when 30e100 mg L1 magnesium or calcium ions were added to natural water or model compound solution before chlorination. They suggested that the complexation between the dissolved metal ions and DBP precursors was the crucial step in the metal catalysis, although they did not study the effect of calcium on HAA formation. Liu et al. (2011) investigated the presence of ferric ions during NOM chlorination and found that 0.5 mg L1 ferric ion increased THM levels by 10% in springtime Yangtze River water. Liu et al. (2012) revealed that several metal ions (magnesium, ferrous, manganese, cupric and aluminum) enhanced HAA formation but inhibited THM formation when using tannic acid as the DBP precursor. The authors proposed that two different mechanisms (metal complexation and enhancement of hydroxyl radical generation) were involved in the chlorination process. The effects of metal ions on the chlorination of algae has also been investigated, and iron and manganese were found to reduce HAA yields during the chlorination of Chlorella vulgaris due to a metal hydroxide/oxide coating on the algae (Ge et al., 2011). Previous studies suggested that the calcium, cupric and ferric ions had catalytic effect on DBP formation, the degree of which however differed among the water sources (i.e. contained NOM) and the metal ions. In addition, two general mechanisms, complexation and free radical generation, were proposed for the interpretation of the catalytic effects exerted by all the metal ions. NOM are inherently heterogeneous in terms of the molecular weight and molecular structure. In this study, the catalytic effects of four selected metal ions (calcium, cupric, ferrous and ferric ions) on both macromolecular NOM (i.e. fulvic acid and humic acid) and small molecular weight NOM surrogates that represent different NOM fragments were investigated. The objective of this study was to investigate the role of NOM structure and size in the metal ioninduced catalytic effects on the formation and speciation of THMs and HAAs, and to explore the primary catalytic mechanisms for each selected metal ion. 2. Materials and methods 2.1. Original water samples NOM (humic acid and fulvic acid) solutions, NOM surrogate solutions and natural water samples were used in this study. All the NOM and NOM surrogate working solutions were freshly prepared by diluting their respective stock solutions. A humic acid (HA) stock solution of 1.0 g L1 was prepared by dissolving commercial HA (SigmaeAldrich, Switzerland) into an alkaline solution (NaOH, pH ¼ 12), then the pH of the solution was gradually adjusted to neutral using 1.0 M hydrochloric acid (A.R., Beijing Chemical Works, China). The solution was then filtered through a 0.45 mm membrane filter (HAWP04700, Millipore, USA) and was stored in a refrigerator (4  C) until use for a maximum of 30 d. The HA working solution (obtained by diluting the stock solution by 100-fold) had a total organic carbon (TOC) concentration of 1.6 mg L1. A fulvic acid (FA) stock solution was prepared by directly dissolving commercial FA (JONLN, China) into ultrapure water. The FA stock solution was diluted 100-fold to obtain the working solution, which had a TOC concentration of 1.1 mg L1. The NOM surrogates were selected primarily based on the conceptual molecular structure for FA/HA (Fig. S1 in Supplementary Information). They include two aliphatic carboxylic acids (pyruvic acid and citric acid) and three dihydroxybenzenes (catechol, resorcinol and hydroquinone). The chemical structures for the five selected surrogate compounds are illustrated in Fig. 1. All surrogate compounds were obtained in analytical grade (Sinopharm Chemical Reagent Co. Ltd, China). All stock solutions of surrogate compounds were prepared at concentration of 1.0 g L1. The working

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Y. Zhao et al. / Chemosphere 144 (2016) 1074e1082

Fig. 1. Molecular composition and structure of selected model compounds.

solutions were prepared by diluting the respective stock solutions by 1000-fold. Two natural water samples, SW1 and SW2, were prepared for this study. SW1 was a reservoir water sample collected in Beijing, China, and SW2 was a river water sample collected in Shandong Province, China. Their water quality parameters are listed in Table 1. Both the original working solutions and natural water described above acted as controls versus the fortified water samples which would be described in section 2.2. 2.2. Metal ion-fortified water samples To investigate the effects of the four different metal ions separately, an aliquot of the respective concentrated metal ion solutions (2.5 M for calcium and 0.2 M for the remaining three metals) was added to each of the water samples described in Section 2.1. All the concentrated solutions were prepared from analytical grade chloride salts (SCRC, China). The dosages of calcium, cupric, ferrous and ferric ions to the water samples were 4.00, 0.02, 0.04 and 0.04 mM, respectively. These dosages are within typical ranges for natural surface water (Liu et al., 2007; Crittenden et al., 2012). Metal ions were added at least 12 h before chlorination to ensure the reach of their complexation equilibria. Both calcium and cupric ions were added directly to the water samples. Before ferrous iron was added, the samples were degassed with nitrogen gas for 1 h; through this process, the oxidation of ferrous to ferric iron was largely prohibited. Before ferric iron was added, the water samples were adjusted to pH ¼ 2 using hydrochloric acid to prevent hydrolysis. The pH of the water samples was then gradually increased to pH ¼ 8 using 10.0 M sodium hydroxide. The pH-adjusting process lasted 2 h to ensure the reach of complexation equilibrium of the ferric ions with organic compounds. 2.3. DBP formation procedure The Uniform Formation Conditions (UFC) test was adopted with slight modification for the evaluation of DBP formation. Details of

the UFC test are found in the literature (Summers et al., 1996). In brief, each of the water samples was added with 2 mL L1 borate buffer (1.0 M boric acid and 0.26 M sodium hydroxide, pH ¼ 8.0) and, if required, adjusted to pH ¼ 8.0 ± 0.2 by using a 1.0 M sulfuric acid solution or a 1.0 M sodium hydroxide solution. A relatively high dose of sodium hypochlorite (12 ± 0.5 mg L1 as free chlorine) was used in this study to ensure a sufficient excess of free chlorine over the organic compounds. After an incubation time of 24 h, the pH value and residual free chlorine were measured. A maximum pH value variation of 0.1 was allowed and a minimum free chlorine residual of 4.0 mg L1 must be kept after incubation (Table 2). The incubated water samples were then quenched by an excess of sodium thiosulfate (over free chlorine residual by 0.5 mg L1) before DBP concentration measurement. All tests were performed in triplicate for each water sample. Blanks with added metal ions but no DBP precursors were processed following the same protocol to investigate the effects of metal ions on free chlorine degradation. 2.4. Analytical methods THMs and HAAs were analyzed using gas chromatographs with electron capture detectors (7890A, Agilent, USA) according to USEPA methods 551.1 and 552.3, respectively (USEPA, 1996; USEPA, 2003). Mixed THM standard and mixed HAA standard were both purchased from Supelco (USA). The limit of detection (LOD) was 0.1 mg L1 and 0.5 mg L1 for the four THMs and the nine HAAs, respectively. Free chlorine was detected using a Hach chlorine test kit according to the DPD method (Method 10070, Hach, USA), and the LOD was 0.02 mg L1. The pH value was measured using a pH meter (Orinon Star A211, Thermo, USA). The TOC was measured according to Standard Method 5310 B by a TOC analyzer (Shimadzu, TOC-V cph, Japan), and the LOD was 0.1 mg L1. The ultraviolet absorbance at 254 nm (UV254) was measured by a spectrophotometer (Napada, UV-1800, China) based on Standard Method 5910B. The concentrations of iron, copper and bromine in the water samples were measured according to Standard Method 3120 by a Thermo Fisher analyzer (iCAP6300, Thermo Fisher, USA), and the concentrations of calcium and magnesium were measured by ion chromatography (ICS-3000, DIONEX, USA). The LOD values for calcium, magnesium, iron, copper and bromine were 1, 1, 0.1, 5 and 5 mg L1, respectively. 3. Results and discussion 3.1. Effects on free chlorine consumption Free chlorine, including hypochlorous acid and hypochlorite ion in water (Morris, 1978), can self-decompose under illumination or heat. Cupric ions also catalyze the decomposition of free chlorine in water (Gray et al., 1977; Zhang and Andrews, 2012). In the presence of 0.02 mM cupric ions, the concentration of free chlorine decreased by 49% and 66% after incubation times of 24 and 48 h, respectively (Fig. 2). During the reaction, free chlorine decomposed into oxygen and chloride ions, and the cupric ions underwent redox

Table 1 Water quality parameters of the water samples.

HA FA SW1 SW2

TOC (mg L1)

UV254 (cm1)

Ca (mg L1)

Mg (mg L1)

Fe (mg L1)

Cu (mg L1)

Br (mg L1)

1.64 1.12 3.74 3.89

0.15 0.033 0.036 0.043

0.093 0.25 35.16 37.63

0.051 0.18 14.71 30.72

0.036 ND 0.19 ND

0.043 0.027 ND ND

ND ND 0.11 0.073

Notes: HA: humic acid solution; FA: fulvic acid solution; SW1: reservoir water sampled in Beijing; SW2: river water sampled in Shandong Province; ND: not detected.

Y. Zhao et al. / Chemosphere 144 (2016) 1074e1082

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Table 2 Concentrations of free chlorine after UFC tests (24 h) in the different water samples in the absence and presence of metal ions. Free chlorine dose (mg L1)

DBP precursors

HA FA SW1 SW2 Pyruvic acid Citric acid Catechol Resorcinol Hydroquinone

12.0 12.0 11.9 12.5 11.7 12.5 11.5 11.5 11.5

Residual free chlorine (mg L1)

(0.0) (0.0) (0.1) (0.6) (0.1) (0.0) (0.1) (0.1) (0.1)

Without metal ions

With Ca2þ

With Cu2þ

With Fe2þ

With Fe3þ

9.3 9.0 9.9 7.9 10.1 10.9 9.6 7.4 9.3

9.1 9.2 9.9 7.9 10.2 11.3 9.3 7.6 9.4

8.1 7.2 9.1 7.5 6.0 6.1 7.2 4.2 7.2

9.5 9.1 9.7 8.0 9.9 11.0 9.6 7.6 9.3

9.5 8.7 9.7 7.8 9.9 11.1 9.4 7.4 9.3

(0.3) (0.0) (0.1) (0.1) (0.1) (0.2) (0.0) (0.0) (0.1)

(0.1) (0.3) (0.1) (0.1) (0.3) (0.1) (0.1) (0.3) (0.1)

(0.1) (0.0) (0.1) (0.3) (0.1) (0.0) (0.0) (0.0) (0.0)

(0.1) (0.1) (0.1) (0.0) (0.0) (0.2) (0.1) (0.1) (0.0)

(0.1) (0.3) (0.1) (0.4) (0.1) (0.3) (0.3) (0.3) (0.1)

Note: The numbers in parentheses represent the standard deviations.

metal ion complexation had an insignificant influence on the free chlorine consumption by the organic substances.

-1

Concentration of free chlorine (mg L )

12

3.2. Effects on DBP formation of humic and fulvic acids

10

SW1 + Cu

8

SW1 + Cu

2+

2+

HA + Cu

2+

2+

FA + Cu

2+

Catechol + Cu

6

blank 2+ Fe 3+ Fe 2+ Ca 2+ Cu

4

2+

Citric acid + Cu

2+

Pyruvic acid + Cu

2+

Resorcinol + Cu

0

8

16

24

32

40

48

Time (h) Fig. 2. Degradation of free chlorine in the absence (as blank) and presence of calcium, cupric, ferrous and ferric ions. The free chlorine residuals after 24 h in the presence of both cupric ions and DBP precursors were also included for comparison.

cycling between the divalent and trivalent states (Gray et al., 1977). When organic substances coexist with cupric ions, the free chlorine residuals may be higher, lower, or similar to those with cupric ions only (Table 2 and Fig. 2). Organic substances acting as DBP precursors also consume free chlorine during the incubation process. Among the two kinds of NOM and five NOM surrogates, resorcinol consumed free chlorine the fastest, whereas citric acid consumed it the slowest. If there were no interactions (e.g., complexation) between the organic substances and cupric ions, the free chlorine consumption rate would be higher than with no organic substances. The reduced free chlorine consumption rate in the presence of cupric ions coexisted with HA, FA, NOM and surrogates may indicate that cupric ions interacted with these organic substances. Therefore, cupric ions, when coexist with organic substances, could have a reduced catalytic effect on free chlorine decomposition due to the decreased concentration. In contrast, free chlorine was fairly stable in the presence of 4.00 mM calcium, 0.04 mM ferrous iron and 0.04 mM ferric iron (Fig. 2). Although free chlorine should be consumed through oxidation of ferrous iron, this was not observed probably because the stoichiometric consumption was too small. When these metal ions coexist with organic substances, the free chlorine residuals differ only marginally from those with organic substances without metal ions (Table 2). We anticipated that the added metal ions had been at least partially complexed with the organic substances. The small differences in free chlorine residuals might indicate that the

No brominated DBPs were detected in the chlorinated HA or FA solution. It was not out of expectation because no bromide ions were added to either solution and no bromide was detected (Table 1), and the high free chlorine dose favored the formation of the more chlorinated DBPs. Additionally, very low metal ion concentrations were detected in the original HA and FA solutions (Table 1), both of which were used as the control in the investigation of metal ions effects. When the HA solution was chlorinated with excess free chlorine for 24 h, the yields of trichloromethane (TCM), dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) were 47.0, 16.0 and 33.9 mg mg1 (all in terms of TOC hereafter), respectively (Table 3). Other DBP concentrations (e.g., monochloroacetic acid) were much lower. The DBP yields were generally in agreement with those rerodes et al., 2003; ported in previous studies (Chang et al., 2001; Se Zhang et al., 2013). When 4 mM calcium ions was added to the HA solution, the yields of TCM, DCAA and TCAA increased by 47%, 61% and 25%, respectively (Table 4, Fig. S2). In comparison, when 0.02 mM cupric ions was added, the TCM and DCAA yields increased by 74% and 90%, respectively, and the TCAA yield decreased by 27%. The overall HAA yield (the sum of DCAA and TCAA) increased by 11%. The DBP yield was not significantly changed by the addition of 0.04 mM ferrous or ferric iron. The DBP yields in the pristine FA were generally similar to those in the pristine HA. When the pristine FA solution was chlorinated, the TCM, DCAA and TCAA yields were 40.1, 25.2 and 21.8 mg mg1, respectively (Table 3), making the DCAA yield higher and the TCAA yield lower in comparison to HA. Similarly, when 4 mM calcium was present in the FA solution, the TCM, DCAA and TCAA yields all increased but only by 24%, 51% and 15%, respectively (Table 4, Fig. S2). Regarding the cupric ion-fortified FA solution, the TCM and DCAA yields increased by 83% and 100%, respectively, whereas the TCAA yield decreased by 26%. As in the HA solution, both ferrous and ferric iron had negligible effects on the DBP yields in the FA solution. NOM has a strong binding capacity to calcium ions (Kinniburgh et al., 1996, 1999), and the catalytic effect of calcium on DBP formation is generally attributed to its complexation with NOM molecules (Fu et al., 2009; Navalon et al., 2009; Liu et al., 2011). Calcium ions bind to the carboxylate and hydroxyl groups, changing the electron densities of these active sites and increasing the reactivity of the molecule with regard to chlorination. Calcium ions exerted the greatest effect on DCAA formation, followed by TCM and TCAA, in both NOM and NOM surrogates, which are described later.

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Y. Zhao et al. / Chemosphere 144 (2016) 1074e1082

Table 3 DBP formation yield during the chlorination of different precursors. Precursors

THMs (mg g1 TOC)

DHAA (mg g1 TOC)

THAA (mg g1 TOC)

HA FA SW1 SW2 Pyruvic acid Citric acid Catechol Resorcinol Hydroquinone

47.0 40.1 28.4 52.3 30.1 36.9 14.8 1151.2 44.7

16.1 25.2 10.2 10.7 4.0 19.7 1.9 6.3 4.6

33.9 21.8 9.4 7.7 33.4 2.3 11.0 51.4 17.5

(2.5) (0.3) (0.9) (10.5) (0.8) (1.4) (0.3) (37.1) (1.7)

(0.0) (0.5) (0.1) (0.2) (0.1) (0.5) (0.1) (0.2) (0.1)

(0.4) (0.4) (0.9) (1.1) (2.1) (0.0) (0.1) (2.7) (0.0)

Note: The numbers in parentheses represent the standard deviations.

The catalytic effects of cupric ions were generally consistent with previous studies (Zhang and Andrews, 2012; Yuan et al., 2013). Cupric ions also complex with NOM molecules (Voelker and Sulzberger, 1996; Kinniburgh et al., 1999). The complexation increases the reactivity of the involved functional groups. However, this mechanism alone cannot explain the substantial impact of cupric ions on DBP formation, given that the catalytic effects were similar to those of calcium but the cupric ion concentration (0.02 mM) was much lower than that of the calcium ions (4 mM). As illustrated by the rapid free chlorine concentration decrease in presence of the ions (Fig. 2), cupric ions catalyze the oxidation of specific compounds by undergoing redox cycling between the divalent and trivalent states (Gray et al., 1977; Demmin et al., 1981). Although the free chlorine concentration decreased somewhat slower when NOM and cupric ions coexisted in the solution, oxidation was also very likely to occur due to the higher free chlorine consumption rate than when cupric ions were not present. Oxidation of NOM can substantially modify the structures and/or functional groups of the molecules and therefore change the DBP yield and speciation. The effect of cupric ions on DBP formation is, however, greatly dependent on the properties of the DBP precursors. In the HA and FA cases, the TCAA yield was reduced, the TCM yield was increased but, as a sum, the TCAA and TCM yield was increased. Previous studies revealed that THMs and trihaloacetic acids (THAAs) share the same group of intimate precursors (Reckhow and Singer, 1985). The presence of cupric ions might vary the percentages of THMs and THAAs formation from the intimate precursors by changing the density of the oxidizable functional groups. The percentage of THMs was increased for HA and FA. However, other possible reasons for the reduced yield of TCAA, e.g. the change of free chlorine concentration (Hua and Reckhow, 2008), could not be excluded. The effect of calcium ion on HA was stronger than that on FA, which was in contrast with the effect of cupric ion. Considering the different structural characteristics of the two organic acids, it was assumed that calcium ions had stronger capacity to affect aromatic DBP precursors than aliphatic precursors, while the effect of cupric ions on aliphatic DBP precursors was more significant than that on aromatic precursors. Catalytic effect of ferrous or ferric iron was not observed in either HA or FA solutions, which is inconsistent with previous reports (Liu et al., 2011, 2012). The discrepancy was likely due to differences in iron concentrations and NOM properties (Fujii et al., 2008). In our case, an environmentally similar concentration (0.04 mM) was adopted. Both ferric and ferrous ions are not stable in natural water because they undergo hydrolysis and oxidation reactions, respectively. Even if substantial amounts of ferric and ferrous ions were complexed with NOM molecules, their effects on DBP formation would not be significant unless other catalytic mechanisms exist because the iron concentration was much lower than the calcium concentration. The effect of ferric/ferrous iron on

DBP formation is discussed further below. 3.3. Effects on DBP formation with surrogate compounds NOM is an inherently complex polydisperse mixture of polyfunctional organic molecules containing hydroxyl groups, carboxyl groups, carbonyl groups, aromatic rings, etc. The complex nature of NOM is not only the primary reason for the difference in the degree of the catalytic effect of any given metal ion but also makes it difficult to distinguish detailed catalytic mechanisms of different metal ions in DBP formation. Five small molecular weight organic compounds (Fig. 1) are therefore used as NOM surrogates to explore the role of NOM structure (and molecular weight) in presence of catalytic metal ions and respective DBP formation during chlorination. Among these NOM surrogates, pyruvic acid and citric acid were selected to study aliphatic fragments and catechol, resorcinol and hydroquinone were used as substitutes for aromatic fragments. HA and FA molecules were assumed to be composed of these molecular fragments, in addition to others, in different fractions. Chlorination of the NOM surrogates showed that resorcinol yielded substantially more TCM (1151.2 mg mg1) than DCAA and TCAA (at 6.3 and 51.4 mg mg1, respectively; Table 3). For the remaining four surrogates, chlorination of pyruvic acid, citric acid, catechol and hydroquinone yielded less DCAA, TCAA, DCAA and DCAA, respectively, than TCM by approximately one order of magnitude. The DBP yields were generally in agreement with those reported in previous studies (Gallard and von Gunten, 2002; Dickenson et al., 2008). The addition of 4 mM calcium ions into the NOM surrogate solutions increased the DBP yields in various degrees, except for the TCAA yields of pyruvic acid, catechol and hydroquinone, which decreased after the addition of the calcium ions (Table 4, Fig. S2). The most notable increases were found in the DCAA yields of pyruvic acid, catechol and hydroquinone, although the DCAA yields in original solutions were relatively low (Table 3). For NOM surrogates and the NOM (HA and FA), the addition of calcium ions generally increased the DCAA yield the most, followed by the TCM and TCAA yields. Navalon et al. (2009) attributed the enhanced DBP formation effect of calcium ions to complexation between the calcium ions and DBP precursors. Based on the results (Table 4, Fig. S2) we can conclude that complexation enhances DBP formation with little selectivity to molecular structure and molecular weight of precursors. The addition of 0.02 mM cupric ions enhanced DBP yields during the chlorination of the five NOM surrogates, except the TCAA yield of pyruvic acid, which decreased by 64% (Table 4). The most notable increases were found in the TCAA yield of citric acid (which increased by 9336%), the TCM yield of citric acid (increased by 3043%), the DCAA yields of catechol and hydroquinone (increased by 1418% and 971%, respectively), the TCAA yield of hydroquinone and the DCAA yield of citric acid (increased by 638% and 535%, respectively). Unlike the calcium ions, there is no trend in the yields

0.96 1.08 0.81 0.97 0.23 1.39 18.27 1.13 11.65 1.08 1.05 0.96 1.06 4.31 2.81 29.59 1.45 9.53 (0.02) (0.03) (0.03) (0.07) (0.00) (0.13) (2.00) (0.02) (1.26)

THM

1.00 1.00 0.91 0.98 1.32 2.01 8.60 1.08 4.35 (0.00) (0.01) (0.04) (0.07) (0.00) (0.05) (0.02) (0.08) (0.00)

THAA

0.99 0.95 0.99 1.14 0.89 2.23 3.13 1.10 1.27 (0.00) (0.00) (0.01) (0.04) (0.03) (0.05) (0.11) (0.03) (0.02) 0.99 0.98 1.11 1.10 2.15 6.48 5.38 1.35 1.54

DHAA THM

(0.01) (0.01) (0.01) (0.14) (0.02) (0.07) (0.04) (0.05) (0.01) 0.73 0.74 0.48 0.60 0.36 94.36 5.75 2.38 7.38 1.90 2.00 1.41 1.23 1.37 6.35 15.18 3.22 10.71

DHAA

(0.00) (0.06) (0.04) (0.09) (0.09) (0.70) (0.24) (0.00) (0.20) Note: The numbers in parentheses are the standard deviations.

1.74 1.83 1.26 0.99 2.43 31.43 2.91 1.07 2.29

THM

(0.01) (0.02) (0.07) (0.05) (0.04) (0.07) (0.01) (0.00) (0.02)

THAA

1.25 1.15 1.00 1.38 0.69 1.25 0.75 1.00 0.81 HA FA SW1 SW2 Pyruvic acid Citric acid Catechol Resorcinol Hydroquinone

1.47 1.24 1.05 0.99 1.97 2.82 1.34 1.07 1.21

(0.02) (0.03) (0.02) (0.07) (0.07) (0.25) (0.05) (0.03) (0.05)

DHAA

(0.01) (0.02) (0.15) (0.00) (0.27) (0.06) (0.11) (0.04) (0.05)

THM

1.61 1.51 0.91 1.08 8.10 2.52 9.04 3.46 4.39

Cupric Calcium Precursors

Table 4 Ratios of DBPs formed in the presence and absence of calcium, cupric, ferrous and ferric ions.

(0.03) (0.02) (0.02) (0.03) (0.19) (0.27) (0.29) (0.14) (0.02)

THAA

(0.01) (0.08) (0.01) (0.01) (0.07) (0.36) (1.13) (0.01) (0.04)

Ferrous

1.00 0.95 0.79 1.03 1.14 5.35 2.28 1.07 1.16

Ferric

DHAA

(0.01) (0.01) (0.04) (0.20) (0.01) (0.14) (0.02) (0.11) (0.13)

THAA

(0.12) (0.03) (0.04) (0.19) (0.00) (0.07) (0.27) (0.19) (0.00)

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of DBP with cupric ions. This pattern is also in sharp contrast to NOM, for which the TCM and DCAA yields increased and the TCAA yield decreased. The effect of cupric ions on the formation of DBPs during chlorination has been reported in several studies, and the most common explanation involves the complexation of the cupric ions with DBP precursors (Blatchley et al., 2003; Fu et al., 2009; Zhang and Andrews, 2012). However, the tremendous discrepancy between DBPs produced with cupric ions and with calcium ions implies different catalytic mechanisms for the two metal ions, which is supported by the fact that the concentration of cupric ions (0.02 mM) was much lower than that of calcium (4 mM). Copper is a transition metal that is able to adopt multiple oxidation states. Previous studies indicated that copper is capable of catalyzing the conversion of dihydroxybenzene to corresponding benzoquinones in aquatic systems (Demmin et al., 1981; Yuan et al., 2013). Copper may similarly catalyze the conversion of a hydroxyl group to a carbonyl group in aliphatic organic compounds. Since the electronwithdrawing effect of carbonyl group is much stronger than hydroxyl group (Wade, 2011), after conversion the hydrogen atoms of the adjacent methyl group will be more acidic, thus increasing chlorine substitution (Deborde and von Gunten, 2008). In citric acid, for example, the conversion of a hydroxyl group leads to decarboxylation due to the electron-withdrawing effect of the oxygen atom. This process forms 3-carbonyl glutaric acid, which contributes significantly to the formation of THMs and HAAs during chlorination (Streicher et al., 1986). In this process, the trivalent copper ion is reduced to a cupric ion, which is then oxidized back to trivalent copper by free chlorine (Fig. 3). Copper ions have a similar catalytic effect on the oxidation of the other four NOM surrogates (i.e., accelerating the oxidation). However, trivalent copper ions are selective oxidants to special molecular structures, and the DBP precursors with oxidizable hydroxyl groups are more susceptible to their oxidation. In addition, the oxidized products have very different DBP yields during chlorination. The above two facts could explain the irregular catalytic effects of cupric ions on the different NOM surrogates and NOM. In contrast to the negligible effects on HA and FA in DBP formation, ferric ion at a concentration of 0.04 mM substantially affected the DBP yields of the five NOM surrogates (Table 4, Fig. S2). The most notable increase was found for catechol, its TCM, DCAA and TCAA yields increased by 760%, 2859% and 1727%, respectively. The DCAA and TCAA yields of hydroquinone also increased by more than 853% and 1065%, respectively. The TCAA yield of pyruvic acid, however, decreased by 77%. Similar to the case of cupric ions, the DBP yields were irregularly affected by the addition of ferric ions. Great care was taken to enhance the complexation of ferric ion with the different DBP precursors in this study. If the complexed ferric iron behaved similarly to calcium, the contribution of the complex formation to the DBP yield variation would be negligible, considering the much lower ferric iron concentration (0.04 mM) than the calcium ion concentration (4.00 mM). Previous studies have attributed the catalytic mechanism of ferric iron to the formation of carbocations (Xie, 1989). Moreover, ferric iron tends to hydrolyze and form colloidal ferric (hydr)oxide particles. As such, surface catalysis effects also have a significant impact. Dearomatization is another proposed catalytic mechanism, given the tremendous increase in DBP yield due to ferric ions in association with catechol and hydroquinone. The big difference between the NOM (macromolecules) and NOM surrogates (small molecules) suggests that the catalytic effect of ferric ion is greatly dependent on the NOM molecular weight. The addition of ferrous iron also increased the DBP yields of the chlorinated NOM surrogates (except for the TCAA yield of pyruvic acid). However, in comparison to the effects of ferric ions, the

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Fig. 3. Speculated mechanism for the influence of cupric ions on the chlorination of citric acid.

effects of ferrous iron on DBP formation were much smaller for all NOM surrogates except citric acid (Table 4, Fig. S2). An oxygen-free environment was created during the preparation of the ferrous iron-fortified water samples. After free chlorine was added, the ferrous iron, whether complexed or not, was quickly oxidized to ferric iron, which was in turn hydrolyzed to form colloidal ferric (hydr)oxides. Consequently, if no other catalytic mechanism existed, ferrous iron would behave similarly to ferric iron but with a smaller effect. The abnormal result observed for citric acid might be due to its strong binding to ferrous iron. As such, in the citric acid case, ferrous iron behaved somewhat similarly to cupric ions, and the citric acid was oxidized to form 3-carbonyl glutaric acid. In the redox process, ferrous iron was oxidized to ferric or other highvalency iron species. In order to identify whether there were significant correlations among the catalytic effects of the selected metal ions, Pearson correlation analysis was conducted on the ratios of DBPs formed in the presence and absence of metal ions during chlorination of NOM surrogates (Table S1). The result gave two values, r and p, to evaluate the correlation. The r value indicates the linearity of the correlation between two variables, and p represents the confidence level (Sun et al., 2014). In this study all the p values were greater than 0.05, which implied no statistical relevance among the effects of the selected metal ions, thus the catalytic mechanisms of the four metal ions were probably different. Generally, the catalytic effects and underlying mechanisms of calcium and cupric ions on DBP formation in association with NOM were verified using low molecular weight NOM surrogates. HA and FA are complex macromolecules or macromolecule clusters that are composed of structural fractions and functional groups that are similar to the five surrogates used in this study (Fig. S1), although the existence of other fractions or groups cannot be excluded. The concentrations of the various fractions and groups naturally differ in one NOM molecule or among NOM molecules. For example, the concentrations of resorcinol-like structures in an NOM molecule should normally be very low. Moreover, different functional groups might have very different binding strengths and capacities with metal ions. As a result, the effects of metal ions on DBP formation in association with NOM is a joint consequence that is largely dependent on the NOM structural properties. The dependence on molecular structure was more evident for cupric ions than for calcium ions because cupric ions are more specific to certain reaction sites. Structures similar to citric acid are more substantially affected by cupric ions. Compared with calcium and cupric ions, fewer studies have been conducted on the effects of ferrous/ferric iron on DBP formation or the underlying mechanisms. Ferrous/ferric iron takes part in a number of complex reactions, including complexation, redox, hydrolysis, aggregation, nucleation, and adsorption, which greatly complicates its role in DBP formation during chlorination. Because ferric and ferrous iron occur naturally in surface and ground source waters, are widely used as coagulants in drinking water treatment processes and are corrosion products in pipelines

in water distribution systems, the effects of ferric and ferrous iron on DBP formation with different precursors is worthy of further investigation. 3.4. Effects on DBP formation of natural waters Two samples (SW1 and SW2) of water from natural sources were used to further investigate the catalytic effects of metal ions on DBP formation. The two samples had similar TOC contents (~4 mg L1) and calcium concentrations (~1 mM, Table 1). SW2 had a higher magnesium concentration than SW1, while a small amount of iron was detected in SW1 (~0.003 mM). Relatively high concentrations of bromide ions (~0.1 mg L1) were detected in both samples. Accordingly, brominated DBPs were formed during chlorination, which were included in the calculation of the DBP yields in the following discussion. The THM, DHAA and THAA yields in SW1 were 28.4, 10.2 and 9.4 mg mg1, respectively, and those in SW2 were 52.3, 10.7 and 7.7 mg mg1, respectively (Table 3). Compared with the NOM (HA and FA), the THM yields were similar, but the HAA yields were much lower. The addition of 4 mM calcium ions to both natural water samples had little effect on DBP formation, though the TCAA yield of SW2 increased by 38% (Table 4), likely because both natural water samples contained fairly high calcium concentrations. Thus, the further addition of calcium had only marginal effect. The effects of calcium and other metal ion concentrations on DBP formation will be further investigated in future study. Neither water sample contained detectable concentration of copper. The addition of 0.02 mM cupric ions into the samples led to substantial variations in DBP formation (Table 4, Fig. S2). The DHAA yield substantially increased, whereas the THAA yield markedly decreased. The THM yield in SW1 also increased. The effects were very similar to those observed with the NOM. The effect of adding 0.04 mM ferric iron into natural water samples was also insignificant, although the THAA yield in SW1 decreased by ~20%. The results were generally consistent with those observed with the NOM. The organic substances in the natural water samples were assumed to be mostly macromolecules and clusters of macromolecules. In comparison, the effects of adding ferrous iron differed between the natural water samples and the NOM solutions. The addition of 0.04 mM ferrous iron increased the DHAA yields in both samples and the THAA yield in SW2 but decreased the THM yield in SW1. This result might arise from the complicated composition and properties of organic and inorganic constituents in natural water. More work is required to better understand this complexity, and the use of NOM and surrogate compounds may be one effective solution to this problem. 4. Conclusions The catalytic effect of calcium, cupric, ferrous and ferric ions on THM and HAA formation was greatly dependent on the DBP precursor structure and molecular weight and the metal ion. When HA

Y. Zhao et al. / Chemosphere 144 (2016) 1074e1082

was used as precursor, addition of 4 mM calcium ion led to an increase of TCM, DCAA and TCAA formation by 47%, 61% and 25%, respectively, and addition of 0.02 mM cupric ion led to an increase of TCM and DCAA formation by 74% and 90%, respectively, but a decrease of TCAA formation by 27%. Both ferrous and ferric ions at 0.04 mM had negligible effects on DBP formation. It was also the case when FA was used, although the influence differed slightly. Complexation and reactive metal ion (Cu(III)) formation were the primary mechanism of the catalytic effects of calcium and cupric ions, respectively. When low molecular weight NOM surrogates were used as precursors, calcium ions at 4 mM increased DCAA yield the most (from 152% to 804%), TCAA yield the least (from 31% to 25%), and TCM yield in between (from 7% to 182%). The catalytic effect of calcium ions had no much selectivity to either molecular structure or molecular weight. Cupric ions at 0.02 mM had the biggest impact on TCAA and TCM formation from citric acid (increased by 9336% and 3043%, respectively), DCAA formation from catechol (increased by 1418%), and DCAA formation from hydroquinone (increased by more than 971%). The precursors which were highly influenced by cupric ions all have oxidizable hydroxyl groups. It suggested that the catalytic effect of cupric ions was greatly dependent on the molecular structure. Both ferric and ferrous ions at 0.04 mM had significant effects on THM and HAA formation from the five NOM surrogates, catechol and hydroquinone in particular, which was in a sharp contrast to HA and FA. It indicated that the catalytic effect of ferric and ferrous ions was critically influenced by molecular weight. The mechanism of the catalytic effects by ferric and ferrous ions need to be further investigated. When natural water samples were chlorinated, addition of calcium, ferric or ferrous ions had only marginal effects on DBP formation. Addition of cupric ions at 0.02 mM demonstrated a similar effect to that when HA and FA were used as precursors. This study has a number of practical implications. First, calcium ions are the major components of hardness. Chlorination of a water of a higher hardness can lead to the formation of more DBPs under otherwise identical conditions. Second, a low concentration of cupric ions below the drinking water standard (about 1 mg L1) can still greatly affect the formation of DBPs. Nevertheless, cupric ions tend to be complexed by NOM and are susceptible to be removed by coagulation and sedimentation. Its effect may be of no much practical significance. Thirdly, ferric/ferrous iron had very different effects on large molecular weight and low molecular weight organic substances. Therefore, pre-chlorination of the raw water can have a considerable influence on DBP formation in the flocculation tank where ferric coagulants are added. In the water distribution systems, the iron-containing corrosion products provide a lot of surfaces where NOM and their oxidation products are bound, which may substantially affect the DBP formation. Acknowledgments Financial support was provided by the Major Program of the National Natural Science Foundation of China (Grant No. 51290284). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2015.09.095. References Blatchley, E.R., Margetas, D., Duggirala, R., 2003. Copper catalysis in chloroform formation during water chlorination. Water Res. 37, 4385e4394.

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