Preferential binding properties of carboxyl and hydroxyl groups with aluminium salts for humic acid removal

Chemosphere 234 (2019) 478e487

Contents lists available at ScienceDirect

Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

Preferential binding properties of carboxyl and hydroxyl groups with aluminium salts for humic acid removal Jina Song a, b, Xin Jin b, Xiaochang C. Wang b, Pengkang Jin b, * a b

College of Energy and Environmental Engineering, Hebei University of Engineering, Handan, Hebei Province, 056038, China School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi Province, 710055, China

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


Preferential binding properties of COOH and OH with Al for HA removal were discovered.
Number of COOH in benzene ring and binding environment greatly influenced HA removal.
The coordination of COOH with Al was restricted by the substituted position of OH.
Sweep coagulation occurred on Al(OH)3 surface by COOH complexation and OH H-bonding.
Other structural features improved coagulation due to hydrophobic effects.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 March 2019 Received in revised form 9 June 2019 Accepted 14 June 2019 Available online 15 June 2019

To systematically elucidate the removal characteristics of humic acid (HA), which are highly dependent on the molecular structure of HA, a series of representative HA model compounds containing different numbers and positions of carboxyl and hydroxyl groups were selected, and the chemical reaction behaviour between HA and aluminium coagulants was investigated. The results indicated that the number of carboxyl groups in the benzene ring and binding environment had great effects on the Albinding properties of HA molecules. Under weakly acidic conditions, coordination occurred between carboxyl and Al ions, and the complexing capacity was restricted by the substituted position of hydroxyl groups. Under neutral conditions or at higher coagulant dosages, sweep coagulation occurred by surface complexation of aluminium hydroxide by carboxyl groups and hydrogen bonding between the hydroxyl group and aluminium hydroxide; this process was dependent on the substitutive pattern of the functional groups. Moreover, increased aliphatic chain length and benzene ring size could enhance hydrophobicity, and hence resulted in higher coagulation efficiency. This study provided new insight into the mechanism of the interaction between HA and aluminium coagulants. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Xiangru Zhang Keywords: Humic acids Preferential binding Carboxyl and hydroxyl groups Chemical configuration

1. Introduction

* Corresponding author. E-mail address: [email protected] (P. Jin). https://doi.org/10.1016/j.chemosphere.2019.06.107 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

Numerous studies on the application of coagulation for removing natural organic matter (NOM) have been over 100 years. Historically, coagulation has been employed for the treatment of

J. Song et al. / Chemosphere 234 (2019) 478e487

drinking water to decrease turbidity and colour and to remove €a € et al., 2018). Humic acid (HA) has become the pathogens (Sillanpa focus of attention in water treatment since it was reported to react with free chlorine to produce trihalomethanes (THMs) and other byproducts in 1970s (Bellar et al., 1974; Rook, 1974). People tried to use traditional coagulation mechanisms to explain the removal of HA, and then found that colloid theory was inapplicable to HA. Therefore, many researchers have made great efforts to explain the difference between coagulation of HA and inorganic particles (Hundt and O'Melia, 1988; Huang and Shiu, 1996; Duan et al., 2002; Angelico et al., 2014). It is generally believed that two distinct domains of 80% filtered removal of humic substance (HS) by coagulation were evident (Hundt and O'Melia, 1988). One domain was that HS formed soluble complexes with various hydrolyzed aluminium species with lower Al dosage under weakly acidic conditions, which was significantly different from inorganic particle removal. The other domain was Al(OH)3(s) precipitation for HS removal with higher Al dosage under weakly acidic conditions, as well as that at pH  7 (Hundt and O'Melia, 1988). As analytical techniques and methods progress, many studies have found that complexation reactions are an important chemical interaction in the coagulation process under weakly acidic conditions (Benschoten and Edzwald, 1990; Cheng and Chi, 2002; Lin et al., 2014; Lu et al., 1999). The complexation of aluminium species with HA can be regarded as a complex chemical reaction process that is related to the structure of HA. HA is an extremely complex and heterogeneous mixture of a variety of organic compounds with varying molecular sizes and functional groups. Among the functional groups of HA, carboxyl and hydroxyl are the most important functional groups that dominate the coagulation interaction between HA and aluminium salts (Provenzano et al., 2004). However, many studies have shown that not all carboxyl and hydroxyl groups are unavailable for metal ion binding, due to steric ndez hindrance, proton competition or electrostatic effects (Herna et al., 2006; Iglesias et al., 2003). Our previous studies demonstrated that there was a selective complexation between HA and Al probably due to different affinities and binding capacities of carboxyl and hydroxyl groups with the Al species (Jin et al., 2018a; Song et al., 2019). Coagulation removal efficiency is assumed to be rather distinct due to the differences of chemical binding environment and substitutive pattern of functional groups. When the chemical conditions remained unchanged, the deprotonation characteristics of functional groups (pKa) and complexing capacities were distinct and influenced by the diversity of position and number of carboxyl and hydroxyl substituents on the benzene ring. Therefore, HA with different numbers and positions of carboxyl and hydroxyl groups may show remarkably different coagulation behaviour with Al salts. However, limited attention has been paid to their corresponding behaviours. As is well known, HA can be enmeshed by amorphous hydroxide precipitate and effectively removed under neutral conditions or at higher coagulant dosages. This process has become known as ‘sweep flocculation’, in which particles are ‘swept out’ of water (Duan and Gregory, 2003). In fact, sweep coagulation is a precipitate adsorption that results from chemical interactions such as hydrogen bonding, coordination reactions, covalent bonding, and ion exchange reactions (Stumm and O'Melia, 1968). Shin et al. (2008) reported that adsorption occurs by surface complexation. Our previous studies also found that surface complexation could occur, in which carboxyl and hydroxyl groups adsorbed on Al(OH)3 and formed complexes with Al (Jin et al., 2018b). Considering this point, it can be deduced that the extent of adsorption depends on the number and position of carboxyl and hydroxyl groups on the HA. The role of number, position of carboxyl and hydroxyl groups on surface complexation and structural characteristics involved in

479

surface complexation between HA and Al(OH)3(s) are far from well illustrated, the understanding of which may provide new insight into surface complexation for HA removal. To gain a better understanding of the effect of the number and position of carboxyl and hydroxyl groups of the HA component on the interaction between HA and aluminium salts, a series of representative HA model compounds containing different numbers and positions of carboxyl and hydroxyl groups were chosen to investigate the removal behaviour during coagulation at different pH values. Based on the experimental data, this study may provide additional insight into reaction characteristics between HA and Al salts. 2. Materials and methods 2.1. Materials and reagents HA used in the experiments was a commercialized product bought from Sigma-Aldrich. Stock HA solution was prepared by adding 1 g HA into 1 L 0.1 mol/L NaOH solution. After stirring for 12 h, the samples were filtered through a 0.45 mm membrane to remove suspended materials. All HA model compounds used in this study were obtained from Sigma-Aldrich. The structures of HA model compounds are illustrated in Table S1. The concentration of all stock solutions was 0.005 mol/L. 2.2. Jar test procedure All coagulation studies were performed in a conventional jartest apparatus, equipped with six 1-L beakers. The stock solution was diluted with ionized water to reach the designated concentration (DOC ¼ 10 mg/L). Aluminium chloride (AlCl3$6H2O) was used in the study. The calculated volume of coagulant (to achieve the required dosage) and 0.1 M NaOH solution (to achieve the required pH) was added to the solution. The coagulation procedure involved rapid mixing at 200 r/min for 1 min, followed by slow stirring at 20 r/min for 30 min. A 60-min settling period followed. The HA flocs were separated from the solution by centrifugation and lyophilized for pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) analyses. The supernatant was filtered through 0.45-mm membrane to perform Potentiometric titrations. The flocs of HA model compounds were separated from the solution by centrifugation and lyophilized for fourier transform infrared (FT-IR) analyses. The supernatant was filtered through 0.45-mm membrane to record EEM and measure DOC. For each set of experimental conditions (pH, dosage), tests conducted in triplicate and the mean value of DOC was determined. 2.3. Fluorescence measurements Three-dimensional fluorescence spectra in the form of excitation-emission matrix (EEM) plots were recorded using an FP6500 fluorescence spectrophotometer (Jasco, Japan). The EEM spectra of the samples were scanned over an excitation range of 220e400 nm (increments of 5 nm) and an emission range of 250e550 nm (increments of 2 nm) with a 5 nm slit. The scan rate was set to 12000 nm/min. EEM plots were generated from fluorescence spectral data using Origin software. 2.4. FT-IR analysis A mixture of a 0.5 mg sample and 50 mg of KBr was ground and then compressed. The pellets were analysed using a Fourier transform infrared (FT-IR) spectrometer (Model Nicolet 6700,

480

J. Song et al. / Chemosphere 234 (2019) 478e487

Thermo Fisher Scientific) covering a frequency range of 4000500 cm1. 2.5. Potentiometric titrations Potentiometric titrations were performed on HA solutions using an automatic Metrohm Tiamo titrator. For the titrations, the stock HA solution that diluted with ionized water to reach the concentration of 10 mg/L (DOC) and supernatant after coagulation were acidified with HNO3 (resulting in a pH of 3) under an N2 atmosphere to protonate the acid groups, and filtered through 0.45-mm membrane. Then, the filtrate was adjusted to 0.01 M with 5 M NaNO3. A 50 mL sample was allowed to equilibrate under flowing N2 for 30 min, and then titrated with 0.1 M NaOH from approximately pH 3 to 10. A pH of 8.0 was usually arbitrarily chosen as the carboxyl equivalence point. The alkali consumption from pH 3 to 8 was considered to be the concentration of carboxylic acid groups. The phenol content was estimated as twice the alkali consumption between pH 8.0 and 10.0. 2.6.

1

H NMR analysis

The HA solution was adjusted to different pH values and pretreated by solid phase extraction (SPE) to extract dissolved organic matter (DOM) from the solution. The extracted organic materials were finally dissolved in D2O solution and tested for 1H NMR. The extraction process and experimental parameters were described by Jin et al. (2018a). 2.7. Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) Py-GC-MS was performed on freeze-dried HA flocs using a pyroprobe 5200 filament pyrolyser (CDS, Oxford) connected with a gas chromatograph mass spectrometer (QP2010 plus, Shimadzu). The samples were placed directly in the quartz tube. The quartz tubes underwent flash pyrolysis, and the platinum filament was programmed to a final temperature of 600  C and held at this temperature for 20 s. The pyrolysis products were separated on an Rtx-5MS column (30 m  0.25 mm, film thickness ¼ 0.25 mm) using the following temperature conditions: 60  C (1 min isothermal), raised to 300  C at a rate of 10  C/min, and held at 300  C for 10 min. Helium was the carrier gas (1.0 mL/min), the injector temperature was 300  C, and the split injection mode had an 80:1 split ratio. Pyrolysis products were detected by a mass spectrometer in the EI mode (70 eV) scanning from 30 to 500 amu.

increased from 3 to 7. With increasing pH, Al ions was more strongly bound to the binding sites as the deprotonation of carboxyl and hydroxyl groups increased, and that resulted in a greater the amount of bound aluminum. At pH > 7, the binding sites occupied by Al ions decreased due to the negatively charged Al(OH)-4 ions formed. In addition, the number of binding sites associated with carboxyl groups was larger than the values of hydroxyl groups in all cases, indicating that the carboxyl groups considered as the most binding sites on a humic moiety. And the complexation of Al mainly involved carboxyl binding sites under acidic conditions, while carboxyl and hydroxyl binding sites play a major role in complexation at pH  7. Therefore, the degree of proton migration could change due to protonation/deprotonation of functional groups of HA, bringing about the distinct in the carboxyl and hydroxyl binding sties at different pH values. Fig. 1 shows the 1H NMR spectra of HA at different pH values. The 1H NMR spectra of HA are divided into five resonance ranges: aliphatic dH ~0.0e1.9 ppm, HC-C-C-; “acetate analogue” and carboxyl-rich alicyclic material (CRAM) dH ~ 1.9e3.1 ppm, H-C-C-O; “carbohydrate-like” and methoxy dH ~ 3.1e4.9 ppm, HCO; olefins dH ~ 5.3e7.0 ppm, HC¼C, HCO2; and aromatic dH~ 7.0e9.0 ppm, Har NMR resonances (Cortes- Francisco et al., 2014). The resonance at 4.7 ppm corresponds to a chemical shift for D2O. As shown in Fig. 1, the spectra clearly showed different patterns with pH value changes, indicating different compositions of organic matter. The distinct variation in resonance patterns is mainly in the aromatic (7.0e9.0 ppm), “acetate analogue” and CRAM (1.9e3.1 ppm), and carbohydrate-like and methoxy resonance (3.1e4.9 ppm) regions. The fully protonated HA was characterized at pH 3, and the 1H NMR spectrum showed a higher abundance of aromatic resonances than that at higher pH values. The deprotonation of HA increased as pH increased, bringing about the proton migration of functional groups. Therefore, the aromatic proton signal intensities gradually decreased, and both the “acetate analogue” and CRAM and the carbohydrate-like and methoxy resonances increased. The degree of proton migration of these functional groups depends largely on the pH value of the solution. Therefore, the distribution of non-exchangeable protons of HA changed at different pH conditions. Based on the above results, the deprotonation degree and proton migration of HA were distinct at different pH values, bringing about diverse reaction behaviour with Al. Therefore, a series of smaller and well-characterized molecules containing the functional groups of HA were chosen to investigate the effect of the structural features of HA on coagulation.

2.8. Other analytical measurements 3.2. The coagulation characteristics of the protonated groups of HA DOC analysis was described by Song et al. (2019). The complexing capacities and the stability constants of Al complexes with HA model compounds were described elsewhere (Plaza et al., 2006) and shown in supporting information. 3. Results and discussion 3.1. The rules of proton migration of HA Carboxyl and hydroxyl groups are the major oxygen-containing acidic functional groups in HA. At optimal dosage of 6 mg/L Al, the removal of carboxyl and hydroxyl binding sites estimated from potentiometric titration at different pH values are illustrated in Table 1. From the results presented in Table 1, the total carboxyl and hydroxyl binding sites in HA was 9.26 and 2.99 meq/g C, respectively. The binding sites occupied by Al ions increased as the pH

3.2.1. Effects of number and position of carboxyl groups The influence of coagulation on the removal of six HA model compounds with varying numbers and positions of carboxyl groups, namely, benzoic acid, phthalic acid, hemimellitic acid, trimellitic acid, trimesic acid and pyromellitic acid, was evaluated at pH 5 and pH 7 (Fig. 2). As shown in Fig. 2, at pH 5, benzoic acid exhibited no removal over the coagulant dosage range investigated, and removal of phthalic acid ranged between 5.0 and 30.0%. The removal effect of pyromellitic acid was the best, and the maximum coagulation efficiencies of DOC were up to more than 80.0%. Each of the three acids discussed above has more carboxyl groups than the previous one. These results indicated that the removal increased as carboxyl groups were added. The pKa values for these acids are presented in Table S2. At pH 5, the compounds with different pKa values are considered to have varying degrees of dissociation. The greater the

J. Song et al. / Chemosphere 234 (2019) 478e487

481

Table 1 The distribution of binding sites in HA estimated from potentiometric titration at different pH values. Sample

Total binding sites (meq/g C)

Carboxyl binding sites (meq/g C)

Hydroxyl binding sites (meq/g C)

HA pH ¼ 3 pH ¼ 4 pH ¼ 5 pH ¼ 6 pH ¼ 7 pH ¼ 8 pH ¼ 9

11.55 2.17 4.70 5.82 6.01 6.17 3.52 1.71

9.26 1.98 4.41 4.68 4.89 4.29 2.49 1.07

2.29 0.19 0.29 1.14 1.12 1.88 1.03 0.64

Fig. 1. 1H NMR spectra of humic aicds at different pH values.

Fig. 2. The coagulation effects of HA model compounds with carboxyl groups at pH5 (a) and pH7 (b).

482

J. Song et al. / Chemosphere 234 (2019) 478e487

number of carboxyl groups presented, the greater the deprotonation of model compounds, which suggested that more complexes between functional groups and hydrolyzed Al species could form. Moreover, for compounds with multiple functional groups, more than one functional group can be involved in complexation. Three different tricarboxylic acid isomers, namely, hemimellitic acid (three adjacent carboxylic groups), trimellitic acid (two adjacent groups), and trimesic acid (no adjacent groups), were studied to understand the influence of the carboxyl group positions. It was observed that removal decreased in the order trimesic acid > trimellitic acid > hemimellitic acid. Under the same conditions, because the pKas for trimesic acid are all below the experimental pH of 5, the three carboxyl functional groups were capable of complexing with Al species. The removal of trimesic acid and pyromellitic acid was similar; although pyromellitic acid has four carboxylic groups, only three groups could undergo deprotonation at pH 5. At pH 7, the coagulation efficiencies increased with increasing number of carboxyl groups. Pyromellitic acid exhibited the highest percent removal over the dosage range. At the higher pH, the removal efficiencies of the three isomers trimesic acid, trimellitic acid and hemimellitic acid were different from those at pH 5. The removal efficiency of hemimellitic acid was the best, followed by trimellitic acid and trimesic acid. The pKas for these isomers were all below pH 7, and they had significantly different removal. As is known, the dominant coagulation mechanism is adsorption of the organic species on amorphous Al(OH)3 at pH 7. This fact suggested that the adsorption of model compounds on aluminium hydroxide was influenced by the position of carboxyl groups. Compounds with carboxyl groups in ortho positions more easily formed surface complexes than compounds with single or meta or para carboxyl groups. Evanko and Dzombak (1998) suggested that in cases where carboxyl groups were adjacent, additional complexes may be formed that involve more than one carboxyl group. The benzene ring is a rigid planar structure, and the s angle between ortho groups was much smaller than that of meta or para groups; thus, bidentate complexes were more likely to form. The presence of carboxyl groups in the ortho position can increase the electronwithdrawing inductive effect and electron density within the functional group, thereby making the interaction between carboxyl and aluminium hydroxide stronger. 3.2.2. Effects of number and position of hydroxyl groups Fig. 3 shows the results for the removal of the model HA compounds with varying numbers and positions of hydroxyl groups, namely, phenol, catechol, pyrogallic acid, phloroglucinol, salicylic acid, and a series of 3,4-dihydroxybenzoic acid (3,4-DHBA), 2,3dihydroxybenzoic acid (2,3-DHBA), 2,5-dihydroxybenzoic acid

(2,5-DHBA) and 2,6-dihydroxybenzoic acid (2,6-DHBA), at pH 5 and pH 7. As shown in Fig. 3 (a), at pH 5, phenol, catechol, pyrogallic acid, and phloroglucinol (not shown in the diagram), which all lack a carboxyl functional group, exhibited no removal over the coagulant dosage range investigated. Complexation between hydrolyzed aluminium species and functional groups is the main mechanism at pH 5. All groups of these compounds could not undergo deprotonation. Therefore, there was no complexation or removal at pH 5. The addition of a carboxyl group in the ortho position of phenol (salicylic acid) increased the coagulation efficiency, in which the maximum coagulation efficiencies of DOC for salicylic acid were up to 15.0% (Fig. 3 (a)). The addition of a carboxyl group in the ortho position (2,3-DHBA) and para position (3,4-DHBA) of catechol could increase the removal efficiency. The phenolic groups have a strong electron-donating resonance effect, which is obvious at the ortho and para positions and outweighs the weaker electronwithdrawing inductive effect. Therefore, the presence of phenolic groups in the ortho position to the carboxyl can increase the electron density within the carboxyl group, thereby favouring the formation of the complex (Guan et al., 2006). The removal effect of four dihydroxy-benzoic acids decreased in the order 3,4-DHBA > 2,3-DHBA > 2,5-DHBA > 2,6-DHBA. The removal efficiencies of 3,4DHBA and 2,3-DHBA all reached 40.0%, while the removal efficiency of 2,5-DHBA was less than 10.0%. According to the pKa values (Table S2), only carboxyl groups can undergo deprotonation and be involved in complexation for these five organic acids. The different removal efficiencies may be related to the complexing capacity of model HA compounds. At pH 7 (Fig. 3 (b)), phenol was not removed by coagulation (not shown in the diagram). The addition of a hydroxyl group (catechol) or a carboxyl group (salicylic acid) in the ortho position could increase the removal efficiency. However, the effect of functional group addition on coagulation depended on the type of functional group added. The removal of catechol and salicylic acid were higher than 50.0% and 30.0%, respectively. Therefore, the addition of a phenolic group increased coagulation efficiency compared with the addition of a carboxyl group in the ortho position. Mitani et al. (1995) suggested that the adsorption of benzoic acid derivatives on chitosan beads was improved in the presence of additional hydroxyl. The addition of a phenolic group in the ortho position relative to catechol and salicylic acid (as with pyrogallic acid and 2,3-DHBA) increased removal significantly. The removal of pyrogallic acid and 2,3-DHBA reached a maximum of 81.0% and 87.0%, respectively. These results indicated that the removal efficiency increased with increasing number of phenolic groups in the ortho position. In addition, the addition of a phenolic group in the meta (2,6-DHBA) and para (2,5-DHBA) positions relative to salicylic acid

Fig. 3. The coagulation effects of HA model compounds with hydroxyl groups at pH5 (a) and pH7 (b).

J. Song et al. / Chemosphere 234 (2019) 478e487

did not increase the removal. In addition, two different isomers, pyrogallic acid (three phenolic groups in ortho positions) and phloroglucinol (three phenolic groups in meta positions), had significantly different removal efficiencies. The maximum pyrogallic acid removal efficiency was 80.0%, while the maximum phloroglucinol removal was only 4.5%. These results proved once again that the position of the functional group was more important than the number of groups. Four different dihydroxybenzoic acid (DHBA) isomers were studied for the influence of the position of hydroxyl groups. The coagulation efficiencies of 3,4-DHBA and 2,3-DHBA were similar and were higher than the others; their structural features included two adjacent hydroxyl group substituents on the benzene ring and differences in the position of the carboxyl group. The coagulation efficiencies of 2,6-DHBA and 2,5-DHBA were similar and significantly lower than those of 3,4-DHBA and 2,3-DHBA. One adjacent carboxyl and hydroxyl group was the common structural feature. Moreover, two hydroxyl groups were not adjacent to each other in any DHBA molecule. In addition, salicylic acid, 2,6-DHBA, and 2,5DHBA exhibited similar removal efficiencies. When one carboxyl and hydroxyl group was adjacent, the addition of a hydroxyl group in the meta or para position did not improve the removal efficiency. 3.2.3. Effects of other structural features Fig. S1 shows the results for the removal of simple monocarboxylic acids with various aliphatic chains, namely, phenyl propionic acid, phenyl pentanoic acid and phenyl hexanoic acid, at pH 5 and pH 7. Compounds containing shorter aliphatic chains exhibited no removal at pH 5 and pH 7. Coagulation efficiencies increased with longer aliphatic chains (>(CH2)5). However, the removal efficiency was relatively low, only approximately 5.0%. Coagulation is generally more effective at removing high molecular € et al., 2018). weight organics and hydrophobic fractions (Sillanp€ aa The molecular weight and hydrophobic effect of the molecule increased with aliphatic chain length, resulting in higher coagulation efficiency. Coagulation of 3,4-diaminobenzoic acid, 2-aminosalicylic acid, o-phenylenediamine, 3,4-dihydroxybenzoic acid, salicylic acid, and catechol were investigated to allow comparison between coagulation of compounds with amino and hydroxyl functional groups. The coagulation removal at pH 7 is shown in Fig. S2. Compounds containing amino functional groups exhibited no removal at pH 5. 2aminosalicylic acid is similar to salicylic acid but has an amino group, and at pH 7, the removal efficiency only increased by approximately 5.0%. 3,4-diaminobenzoic acid and 3,4dihydroxybenzoic acid (and o-phenylenediamine and catechol) exhibited similar structures, containing either two adjacent amino or hydroxyl groups, respectively; the compounds containing adjacent hydroxyl groups showed much higher coagulation performance. This result indicated that the effect of the amino group on coagulation was relatively small compared with that for other functional groups (carboxyl, hydroxyl). Coagulation of homophthalic acid, tropic acid, phthalic acid and salicylic acid was investigated to compare the coagulation of compounds with functional groups on the benzene ring and aliphatic chain. The coagulation removal is shown in Fig. S3. As shown in Fig. S5, the removal efficiencies of homophthalic acid at pH 5 and pH 7 were 7.2e21.7% and 3.0e18.0%, respectively. Phthalic acid achieved coagulation removals of 14.4e35.6%, 9.5e32.0% at pH 5 and pH 7, respectively, which were higher than those in homophthalic acid by approximately two times. The removal of tropic acid by coagulation was less than 4% at pH 5 and pH 7, while it reached approximately 7.1e17.0% and 6.0e33.7% for salicylic acid at pH 5 and pH 7, respectively. The results demonstrated that compounds with functional groups on the benzene ring showed

483

superior coagulation performance compared to functional groups on the aliphatic chain. The benzene ring is a large p-bond structure with six centres and six electrons, and carboxyl is also a p-bond structure with three centres. As a result, a delocalized conjugated system can be formed when carboxyl or hydroxyl groups are directly connected to the benzene ring. Meanwhile, the carboxyl or hydroxyl groups exhibited strong electron-withdrawing inductive effects. Thus, the electron density in the oxygen atoms of carboxyl or hydroxyl increased, facilitating the formation of complexes between the functional group and Al. However, the delocalized conjugated system could be disrupted due to the existence of methylene between the functional group and the benzene ring. Thus, the electron-withdrawing effect became weaker, and the electron density remained unchanged. As a consequence, coordination could not easily occur. In addition, although homophthalic acid increased the length of the aliphatic chain, the presence of a shorter aliphatic chain had a negligible effect on coagulation when other important structural features were present. 3.3. Mechanism analysis 3.3.1. Complexing capacities and stability constants of compounds The fluorescence EEM spectra of the HA model in the absence and presence of aluminium at a total concentration of 10 mM are shown in Fig. S4. All EEM plots were dramatically altered when Al3þ was added to the above aqueous solution. When Al3þ was added to the HA model solutions, there was a very significant redshift or blueshift in the wavelength of maximum excitation or emission. These results were indicative of the HA model functional groups that were involved in aluminium complexation and suggested a marked modification of the HA model electronic structures. Furthermore, the addition of Al3þ to the solutions caused varying intensity changes. For example, Al3þ addition resulted in quenching of salicylic acid, 3,4-DHBA and 2,6-DHBA but enhancement of 2,3-DHBA and 2,5-DHBA. Fig. S5 presents the experimentally determined relative fluorescence intensity of the main fluorophore as a function of aluminium concentrations. The highest fluorescence intensity (FI) decrease or increase occurred at low aluminium concentrations, while lower FI changes were observed at higher Al concentrations. When no further changes in FI were measured, the maximum complexing capacity of aluminium by organic acids was attained. The nonlinear regressions generated by fitting these data in Fig. S5 with the model of Ryan-Weber (Ryan and Weber, 1982) and the corresponding parameters obtained for organic acids, i.e., the correlation coefficients of predicted vs. measured fluorescence intensity (r), the fluorescence intensities (IML) of metal-ligand complexes, the stability constants (log K) of metal-organic acid complexes, and the metal complexing capacities (CC), are listed in Table 2. As shown in Table 2, the large values found for the correlation coefficients (r) indicate that the model fitted very well to the experimental data. The stability constants (log K) of aluminium complexes with the HA model appeared relatively high. The large stability constants may be related to the high content of acidic functional groups and other O-containing ligand groups and their

Table 2 Fitting parameters of the Ryan-Weber. Organic acids

r

IML

logK

CC/(mmol/g)

salicylic acid 2,3-DHBA 3,4-DHBA 2,5-DHBA 2,6-DHBA

0.981 0.996 0.993 0.997 0.990

58.23 586.75 158.65 254.28 31.78

5.51 5.90 5.54 5.59 5.15

2.1 3.1 3.85 1.23 1.13

484

J. Song et al. / Chemosphere 234 (2019) 478e487

aromatic character (Plaza et al., 2006). In addition, an aromatic carboxyl group and adjacent phenolic hydroxyl group or two adjacent aromatic carboxyl groups are known to form highly stable salicylate-like and phthalate-like bidentate complexes with metal ions (Elkins and Nelson, 2002). The aluminium ion complexing capacity of any organic acid decreased in the order 3,4-DHBA >2,3DHBA > salicylic acid >2,5-DHBA >2,6-DHBA, which was in agreement with the trends of coagulation efficiency. 3.3.2. Effect of the position of the functional group on coagulation The second derivative infrared spectra of salicylic acid, 3,4DHBA, 2,3-DHBA, 2,5-DHBA and 2,6-DHBA before and after coagulation at pH 7 are shown in Fig. 4. The second derivative infrared spectra can find the exact location of absorption peaks and shoulder peaks. If the first derivative is zero and the second derivative is not zero for a point in the curve, this point is the extreme point in the curve. When the second derivative is more than zero, the point is a minimum, and vice versa. Therefore, the peak-valley of the second derivative corresponds to the peak and shoulder position of the original spectrum. In the infrared spectra, the appearance of new bands and band shifts are expected for inner-sphere surface complexes (Nordin et al., 1997). As shown in Fig. 4, the antisymmetric COO band

shifted from 1590 cm1 to 1605 cm1, and the symmetric COO band centred at 1385 cm1 shifted to 1397 cm1 upon adsorption of salicylic acid on Al(OH)3(s), indicating the formation of inner-sphere surface complexes to carboxyl at the aluminium hydroxide surface. In addition, the phenolic hydroxyl (Ph-OH) stretching band at 1250 cm1 shifted downward to 1214 cm1. According to Yost et al. (1990), the Ph-OH band shifts to lower frequencies are indicative of coordination to the surface. These changes demonstrated that salicylate formed a chelate structure with aluminium involving a carboxylic oxygen and phenolic group. Upon the adsorption of 2,3DHBA, 3,4-DHBA, 2,5-DHBA and 2,6-DHBA on Al(OH)3(s), the antisymmetric COO peak shifted from 1569 cm1, 1539 cm1, 1532 cm1, and 1523 cm1 to 1554 cm1, 1501 cm1, 1505 cm1, and 1552 cm1, respectively, and the symmetric COO peak shifted from 1382 cm1, 1400 cm1, 1421 cm1, and 1400 cm1 to 1390 cm1, 1369 cm1, 1403 cm1, and 1395 cm1; moreover, the Ph-OH stretching bands at 1290 cm1, 1275 cm1, 1257 cm1, and 1289 cm1 shifted to 1279 cm1, 1268 cm1, 1268 cm1, and 1272 cm1, respectively. This evidence revealed the involvement of carboxyl and Ph-OH groups in the complexation reaction and the formation of inner-sphere complexes at the aluminium hydroxide surface. Among all compounds, the coagulation efficiency of 3,4-DHBA

Fig. 4. Second derivative FT-IR spectra of organic acids before and after coagulation at pH 7.

J. Song et al. / Chemosphere 234 (2019) 478e487

and 2,3-DHBA was the best. As the chelating complexes possessed greater stability than the open-chain analogues, two deprotonated phenolic groups in ortho positions represent very effective binding sites (Guan et al., 2006). Therefore, 3,4-DHBA and 2,3-DHBA could form inner-sphere complexes on the aluminium hydroxide surface via the two adjacent phenolic groups coordinated with one aluminium atom. Moreover, the removal efficiency of catechol was better than that of salicylic acid, 2,5-DHBA and 2,6-DHBA, which proved this point. The substance 3,4-DHBA serves to illustrate the surface complexes formed on the aluminium hydroxide. In addition, salicylic acid, 2,6-DHBA and 2,5-DHBA exhibited similar coagulation efficiencies. One carboxylic oxygen and the ortho phenolic oxygen could coordinate one Al atom of the aluminium hydroxide surface, forming chelating structures. 3.3.3. Comparison with HA To advance our understanding of the mechanisms of HA removal, Aldrich HA flocs were collected after coagulation at pH 5 and pH 7 and freeze-dried to obtain Py-GC-MS (Fig. 5). The individual compounds identified by Py-GC-MS in coagulation at pH 5 and pH 7 were different. The GC-MS data indicated that coagulation selectively removed polycyclic aromatic hydrocarbons (methylnaphthalene, pyrene, fluorene, phenanthrene, etc.), heterocyclic aromatics (benzofuran, dibenzofuran, 9-octadecenamide, etc.) and chain hydrocarbon (butylnonane, methylheptadecane, eicosane, etc.) at pH 5. Alphatic hydrocarbons (methoxypentene, hexatriene, methylheptadecane, tetrapentacontane, etc.), fatty acids (octadecanoic acid, tridecanoic acid, tetradecanoic acid, etc.) and amides (9-octadecenamide, octadecanamide, etc.) appeared to be more readily removed in coagulation at pH 7. Although similar removal could be achieved (Wang et al., 2002), the structure of

485

organic matter removed by coagulation was different at pH 5 and pH 7. 3.3.4. Mechanisms of HA removal As is well known, the removal of HA was due to complexation between aluminium species and HA under acidic conditions. At pH 5, aluminium chloride (AlCl3) can be hydrolyzed to form 2þ 4þ 5þ Al13O4(OH)7þ 24 , Al2(OH)4 , Al4(OH)8 , and Al5(OH)10 species, which have high positive charges. The higher the valence of an Al ion is, the more empty the 3p orbital of the metal ion is. Therefore, Al ions can accept more electron lone pairs to form a stable complex. According to the pKa value, carboxyl groups can be protonated, thus allowing carboxyl to coordinate onto the Al ions. However, not all carboxyl groups are available for metal ion binding. It was observed that the number of sites occupied by the copper ion represented only 20% of the total concentration of acid groups ionized on fulvic acid (FA) (Iglesias et al., 2003). Differences in the number and positions of functional group substituents on the benzene ring resulted in diverse complexing capacities between HA and hydrolyzed aluminium species. The complexing capacity was strongly related to the acidity and electron density of the functional group. The coagulation efficiency increased with increasing number of carboxyl groups in the benzene ring due to changes in the acidity of the compounds. On the other hand, the complexing capacity was restricted by the substituted position of hydroxyl groups. The hydroxyl groups have a strong electron-donating conjugation effect, which is pronounced at the ortho and para positions and outweighs the weaker electron-withdrawing inductive effect. Thus, the presence of hydroxyl groups in the ortho or para position to the carboxyl group can increase the electron density within the carboxyl group, thereby favouring metal-carboxylate

Fig. 5. Pyrochromatogram of freeze-dried flocs after coagulation at pH 5 and pH 7.

486

J. Song et al. / Chemosphere 234 (2019) 478e487

complexation. At pH 7, the dominant mechanism is adsorption of HA on amorphous Al(OH)3(s). The extent of adsorption depends on the type and number of functional groups on the HA and the positions of the functional groups. According to the pKa value, carboxyl groups were deprotonated, thus allowing HA to adsorb to the Al(OH)3(s) surface and form inner-sphere surface complexes. However, the proton was not easier to dissociate from hydroxyl groups. As a result, the lone pair electrons of oxygen atoms from Al(OH)3 could form hydrogen bonds with hydrogen atoms of hydroxyl groups. Therefore, hydrogen bonding between the hydroxyl group and aluminium hydroxide played an important role. Accordingly, sweep coagulation occurred mainly by surface complexation of the carboxyl group with aluminium hydroxide and hydrogen bonding between the hydroxyl group and aluminium hydroxide. According to the above analyses, the coagulation process exhibited distinct DOC removal efficiency for HA with different numbers and positions of carboxyl and hydroxyl groups on the benzene ring. The aromatic halogenated DBP removal could significantly reduce the DBP levels in finished drinking waters and limited the formation of regulated THMs and haloacetic acids (HAAs) (Jiang et al., 2017, 2018). As we all known, HA is an important precursor of disinfection byproducts. Therefore, successfully removing the aromatic content of HA could significantly reduce the levels of halogenated aromatic DBPs formed in the chlorine disinfected water.

4. Conclusions In this study, a series of representative HA model compounds were selected to investigate the preferential binding properties of carboxyl and hydroxyl groups with aluminium salts for the removal of HA. The results indicated that the number of functional groups in the benzene ring and binding environment had a great effect on the preferential binding properties of the HA molecule with Al. Under weakly acidic conditions, it was revealed that the coordination reaction between carboxyl and hydrolyzed aluminium species dominated coagulation. The removal efficiency may be related to the dissociation constant pKa and the complexing capacity of HA model compounds as observed by fluorescence spectroscopy. Under neutral conditions, adsorption occurred by surface complexation of carboxyl with Al(OH)3 and hydrogen bonding between OH and Al(OH)3. Moreover, the position of the functional groups influenced coagulation more than the number of groups. Compounds containing two adjacent carboxyl or hydroxyl groups on the benzene ring have better removal efficiency than both groups in the meta or para position. In addition, FTIR analysis indicated that molecules with similar structural characteristics exhibited the same coagulation behaviour. The important structural characteristics included adjacent phenolic groups and adjacent carboxylic and phenolic groups.

Acknowledgements We thank Yang Lei from the University of Melbourne, Australia for his help on chemical analysis. This study was financially supported by the National Key Technology Support Program (Grant No. 2014BAC13B06), the National Natural Science Foundation of China (Grant No. 51378414, 51178376), the Program for Innovative Research Team in Shanxi (Grant No. 2013KCT-13) and the Program for New Century Excellent Talents in the University of Ministry of Education of China (NCET-12-1043).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.06.107. References Angelico, R., Cegliea, A., He, J.Z., Liu, Y.R., Palumboa, G., Colombo, C., 2014. Particle size, charge and colloidal stability of humic acids coprecipitated with ferrihydrite. Chemosphere 99, 239e247. Bellar, T.A., Lichtenberg, J.J., Kroner, R.C., 1974. The occurrence of organohalides in chlorinated drinking water. J. Am. Water Work. Assoc. 66, 703e706. Benschoten, J.E.V., Edzwald, J.K., 1990. Chemical aspects of coagulation using aluminum salts-II. Coagulation of fulvic-acid using alum and polyaluminum chloride. Water Res. 24, 1527e1535. Cheng, W.P., Chi, F.H., 2002. A study of coagulation mechanisms of polyferric sulfate reacting with humic acid using a fluorescence-quenching method. Water Res. 36, 4583e4591. s- Francisco, N., Harir, M., Lucio, M., Ribera, G., Martínez-Llado  , X., Rovira, M., Corte Schmitt, K.P., Hertkorn, N., Caixach, J., 2014. High-field FT-ICR mass spectrometry and NMR spectroscopy to characterize DOM removal through a nanofiltration pilot plant. Water Res. 67, 154e165. Duan, J.M., Wang, J.H., Grahan, N., Wilson, F., 2002. Coagulation of humic acid by aluminium sulphate in saline water conditions. Desalination 150, 1e14. Duan, J.M., Gregory, J., 2003. Coagulation by hydrolysing metal salts. Adv. Colloid Interface Sci. 100e102, 475e502. Elkins, K.M., Nelson, D.J., 2002. Spectroscopic approaches to the study of the interaction of aluminum with humic substances. Coord. Chem. Rev. 228, 205e225. Evanko, C.R., Dzombak, D.A., 1998. Influence of structural features on sorption of NOM-analogue organic acids to goethite. Environ. Sci. Technol. 32, 2846e2855. Guan, X.H., Shang, C., Chen, G.H., 2006. ATR-FTIR investigation of the role of phenolic groups in the interaction of some NOM model compounds with aluminum hydroxide. Chemosphere 65, 2074e2081. ndez, D., Plaza, C., Senesi, N., Polo, A., 2006. Detection of Copper(II) and Herna zinc(II) binding to humic acids from pig slurry and amended soils by fluorescence spectroscopy. Environ. Pollut. 143, 212e220. Huang, C., Shiu, H., 1996. Interactions between alum and organics in coagulation. Colloid. Surf. Physicochem. Eng. Asp. 113, 155e163. Hundt, T.R., O'Melia, C.R., 1988. Aluminum-fulvic acid interactions: Mechanisms and applications. J. Am. Water Work. Assoc. 80, 176e188.  pez, R., Fiol, S., Antelo, J.M., Arce, F., 2003. Analysis of copper and Iglesias, A., Lo calcium-fulvic acid complexation and competition effects. Water Res. 37, 3749e3755. Jiang, J.Y., Zhang, X.R., Zhu, X.H., Li, Y., 2017. Removal of intermediate aromatic halogenated DBPs by activated carbon adsorption: A new approach to controlling halogenated DBPs in chlorinated drinking water. Environ. Sci. Technol. 51, 3435e3444. Jiang, J.Y., Li, W.X., Zhang, X.R., Liu, J.Q., Zhu, X.H., 2018. A new approach to controlling halogenated DBPs by GAC adsorption of aromatic intermediates from chlorine disinfection: Effects of bromide and contact time. Separ. Purif. Technol. 203, 260e267. Jin, P.K., Song, J.N., Yang, L., Jin, X., Wang, X.C., 2018a. Selective binding behavior of humic acid removal by aluminum coagulation. Environ. Pollut. 233, 290e298. Jin, P.K., Song, J.N., Wang, X.C., Jin, X., 2018b. Two-dimensional correlation spectroscopic analysis on the interaction between humic acids and aluminum coagulant. J. Environ. Sci. 64, 181e189. Lin, J.-L., Huang, C., Dempsey, B.A., Hu, J.-Y., 2014. Fate of hydrolyzed Al species in humic acid coagulation. Water Res. 56, 314e324. Lu, X.Q., Chen, Z.L., Yang, X.H., 1999. Spectroscopic study of aluminium speciation in removing humic substances by Al coagulation. Water Res. 33 (5), 3271e3280. Mitani, T., Yamashita, T., Okumura, C., Ishii, H., 1995. Adsorption of benzoic acid and its derivatives to swollen chitosan beads. Biosci. Biotechnol. Biochem. 59, 927e928. Nordin, J., Persson, P., Laiti, E., Sjoberg, S., 1997. Adsorption of o-phthalate at the water-boehmite (g-AlOOH) interface: Evidence for two coordination modes. Langmuir 13, 4085e4093. Plaza, C., Brunetti, G., Senesi, N., Polo, A., 2006. Molecular and quantitative analysis of metal ion binding to humic acids from sewage sludge and sludge-amended soils by fluorescence spectroscopy. Environ. Sci. Technol. 40, 917e923. Provenzano, M.R., D'Orazio, V., Jerzykiewicz, M., Senesi, N., 2004. Fluorescence behavior of Zn and Ni complexes of humic acid from different sources. Chemosphere 55, 885e892. Ryan, D.K., Weber, J.H., 1982. Fluorescence quenching titration for determination of complexing capacities and stability constants of fulvic acid. Anal. Chem. 54, 986e990. Rook, J.J., 1974. Formation of haloforms during chlorination of natural waters. Water Treat. Exam. 23, 234e243. €€ €la €inen, M., 2018. Removal of natural Sillanpa a, M., Ncibi, M.C., Matilainen, A., Vepsa organic matter in drinking water treatment by coagulation: A comprehensive review. Chemosphere 190, 54e71. Shin, J.Y., Spinette, R.F., OMelia, C.R., 2008. Stoichiometry of coagulation revisited. Environ. Sci. Technol. 42, 2582e2589.

J. Song et al. / Chemosphere 234 (2019) 478e487 Song, J.N., Jin, P.K., Jin, X., Wang, X.C., 2019. Synergistic effects of various in situ hydrolyzed aluminum species for the removal of humic acid. Water Res. 148 (1), 106e114. Stumm, W., O'Melia, C.R., 1968. Stoichiometry of coagulation. J. Am. Water Work. Assoc. 60, 514e519. Wang, X.C., Jin, P.K., Gregory, J., 2002. Structure of Al-humic flocs and their removal

487

at slightly acidic and neutral pH. Water Sci. Technol. Water Supply 2 (2), 99e106. Yost, E.C., Tejedortejedor, M.I., Anderson, M.A., 1990. In situ CIR-FTIR characterization of salicylate complexes at the goethite/aqueous solution interface. Environ. Sci. Technol. 24, 822e828.