Sorption of Ionizable Organic Amines on Soil and

Water Air Soil Pollut (2015) 226:257 DOI 10.1007/s11270-015-2506-3

Sorption of Ionizable Organic Amines on Soil and Their Effects on Phenanthrene Sorption Hongwen Sun & Fei Wang & Biting Feng & Wenling Wu & Lei Wang

Received: 22 March 2015 / Accepted: 22 June 2015 # Springer International Publishing Switzerland 2015

Abstract Sorption of four ionizable organic amines, nhexylamine, trimethylamine, 1-naphthylamine, and phenylamine, on a soil sample were measured, and their effects on the sorption of phenanthrene (PHE) to the same adsorbent were studied. The aim of this study was to better clarify sorption mechanisms of chemicals with different polarity and ionization characteristics in a single-solute system and in a polar/nonpolar binary system. In the single system, cationic organic amines exhibited greater sorption than those in a neutral form, and the sorption increased with hydrophobicity for amines with the same form. In the binary system, the sorption of PHE was promoted in the presence of nhexylamine and the solid-water distribution coefficient (Kd) increased with increasing amine concentrations. This may be explained by the elevated amount of hydrophobic organic sites provided by the head-on adsorption of cationic n-hexylamine to the negatively charged sorbent surface, which are probably more favorable for the sorption of PHE compared with natural organic matters. Contrarily, the neutral amine, 1-naphthylamine, might compete with PHE for the mutually available hydrophobic sites and hence inhibited PHE sorption. H. Sun (*) : F. Wang : B. Feng : W. Wu : L. Wang MOE Key Laboratory of Pollution Processes and Environmental Criteria, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China e-mail: [email protected] W. Wu Technical Center, China State Construction Engineering Corporation, 15 Linhe Street, Shunyi District, Beijing 101300, China

On the other hand, both trimethylamine and phenylamine had little effects on PHE sorption due to their relatively high solubility and weak hydrophobicity. Therefore, either in single or binary system, both the form and the solubility/hydrophobicity of the compound play important roles in the sorption of ionizable organic amines and their effects on the sorption of nonpolar cosolute. Keywords Sorption . Co-sorption . Ionizable organic amines . Phenanthrene . Interaction mechanism

1 Introduction Interface distribution, especially sorption, has a critical effect on the fate, transport, reactivity, and bioavailability of organic pollutants in soil/sediment-water systems. To date, plenty of literature has focused on the sorption behavior of non-ionic hydrophobic organic chemicals (HOCs), such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), and non-ionic polar organic chemicals, like pesticides (Chefetz et al. 2000, 2004; Chen et al. 2004; Yang et al. 2013). It has been found that there are mainly three kinds of mechanisms for the sorption of HOCs in soil/sediments, including (1) solute partitioning into the solid organic phase (Chiou et al. 1979); (2) surface adsorption, which is the sorption of organic pollutants on the fixed sites in the soil or sediment through specific interactions, such as van der Waals force (Pignatello and Xing 1995); and (3) micropore filling (Zhang et al.

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2014). For polar compounds, the surface interaction may be enforced by specific interactions, such as hydrogen bond and electron transfer (Xing et al. 1996; Vasudevan et al. 2013). Recently, different kinds of emerging chemicals, such as pharmaceutics and environmental endocrine disruptors, have become a hot spot in the field of environmental science. Many of these compounds are ionizable. Ionic organic compounds (IOCs) differ from non-ionic chemicals and can exist either as nondissociated or dissociated forms in an aqueous phase, which is dependent on hydrochemical conditions, especially the pH of the solution. The dissociated forms, either as cations or as anions, may have other interactions with the sorbent except those above, and electrostatic repulsion (or attraction) may become the primary process for sorption (Fabrega et al. 1998; Li et al. 2001). Organic amines are an important group of ionizable organic chemicals in the environment and have drawn much environmental concern as odorous pollutants. In this paper, four main organic amines with different molecular structure and property, including trimethylamine, n-hexylamine, 1-naphthylamine, and phenylamine were studied. Trimethylamine is one of the priority pollutants listed under odorous pollution control due to its large quantity of dispose and low olfactory threshold. What is more, it causes human respiratory and eye irritation and corneal lesions (Akesson et al. 1989). The n-hexylamine is commonly applied in organic synthesis and also causes harmful effects to the skin and eye (Tsubaki et al. 2002). The 1-naphthylamine is one of the top-priority contaminants and of the most important substructures of potentially carcinogenicity, which is discharged from pharmaceutical, dyestuff, photographic, agrochemical industries, and cigarette smoke (Valero-Navarro et al. 2011). Phenylamine, as one of the parent compound of an important family of industrial chemicals, is widely used for the manufacture of agrochemicals, pharmaceuticals, rubber, varnishes, and dyestuffs (Cai et al. 2005). It is highly toxic and may cause adverse effects on the spleen and blood with repeat exposure (EPA 1994). Moreover, it has strong chemical and biological stability and can transform into more stable substances, such as azobenzene, nitrobenzene when interacting with humic acids (Thorn et al. 1996; Weber et al. 1996). Hence, the four amines were selected based on their environmental concerns and, more importantly, on their different

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structures and properties (e.g., pKa, the acid dissociation constant). The form of organic amines in a solution phase depends on the pH-pKa relationship (Zhu et al. 2005). Generally, the differences in molecular structure often result in the variance in the solubility and pKa values of chemicals, and thus, the form of organic amines may dramatically change with different hydrochemical conditions and exhibit a distinct sorption behavior. Sheng et al. (2010) have investigated the adsorption of 1-naphthylamine on oxidized MWCNTs and found that the sorption increased by over one order of magnitude when the pH increased in the range of 3–7 and yet slightly decreased when the pH further increased. Li et al. (2001) reported that, for amines, the sorption usually reduced with increasing system pH due to amine speciation that shifted to the non-dissociation form, and the distributed parameter model used in that study showed that cationic exchange plays a significant role in aromatic amine sorption even at pH values two to three units greater than the pKa. In the actual environment, multiple pollutants usually co-exist as mixture, especially in abandoned chemical parks. In China, it is quite common in the recent years that chemical industries move to a new park due to city expansion, leaving heavily polluted site with multiple pollutants. Some of these sites have been ranked as sites that need remediation, and some heavily polluted soil have been disposed in landfill yard. Hence, it is important to grasp knowledge of the environmental behavior in a mixture pollution so as to make an accurate risk assessment or propose a suitable remediation plan. Previous works mainly focused on the sorption of organic amine in a single-solute system; however, the influence of the co-existence of amines on the sorption of other pollutants has seldom been studied (Donaldson and Nyman 2006; Sheng et al. 2010; Hu et al. 2011; Li et al. 2011). Several studies on the adsorption of phenol/ phenylamine mixture in a binary system conducted by Zhang et al. (2007a, b, c) have shown enhanced sorption of the two compounds resulting from the lateral acidbase interaction. In the present study, we examined the sorption of the four organic amines on a loamy clay soil sample in a single-solute system and the effect of these amines on the sorption of phenanthrene (PHE) on the same soil. The aim of the study was to better understand the sorption mechanism of chemicals with different polarity and ionization characteristics


in a single-solute system and in a polar/nonpolar binary system.

2 Materials and Methods 2.1 Materials Trimethylamine, n-hexylamine, 1-naphthylamine, and phenylamine were purchased from Tianzheng Fine Chemical Reagent Company (Tianjin, China), Jinke Fine Chemicals Corporation (Tianjin, China), Guangfu Fine Chemicals Institute (Tianjin, China), and Kemiou Chemical Research Center (Tianjin, China), respectively. The structure and primary properties of the four amines are listed in Table 1. The properties of the four compounds varied a lot, with a logKow of 0.16–2.06, Sw of 1.7–890 g/L, and pKa of 3.92–10.64. Phenanthrene (PHE) was purchased from Acros Corporation (NJ, USA). The primary properties of PHE are also listed in Table 1. All solvents were of analytical grade. A soil sample was collected as the geosorbent from the top 5 cm of an estuary area of Tianjin in China, far from any industrial area. The soil was ground and sieved through a 75-μm sieve after air-dried and then stored in the dark at room temperature. The main characteristics of the soil were measured according to the corresponding standard methods in China, which can be referred to in our previous work (Sun et al. 2009). In summary, the pH of the soil is 8.0 and the texture is loamy clay with an organic carbon content of 2.42 %. The cation-exchange capacity (CEC) of the adsorbent is 10.8 cmol/kg. 2.2 Sorption Experiments Batch experiments were conducted to measure equilibrium sorption isotherms of the four organic amines as well as the sorption isotherms of PHE in the presence and absence of the amines. The sorption experiments were conducted in accordance with the Organisation for Economic Co-operation and Development (OECD) guideline 106 (OECD Guidelines for the Testing of Chemicals, Adsorption–desorption using a batch equilibrium method 2000). Briefly, 0.1 g of the geosorbent was weighed into 22-mL glass vials with Teflon-lined caps (US EPA) followed by a 20-mL background solution. The solution contained 5 mM CaCl2 to maintain the ionic strength and 200 mg/L NaN3 to avoid biodegradation of the target compounds during the experiment.

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For the single-solute experiment, designated amounts of the stock solutions of trimethylamine, n-hexylamine, 1naphthylamine, and phenylamine in HPLC-grade methanol were spiked into the water-soil suspension, respectively. The initial concentrations of these amines were over two to three orders of magnitude, being 100– 1500 mg/L for trimethylamine and n-hexylamine, 5– 50 mg/L for 1-naphthylamine, and 100–1000 mg/L for phenylamine. In the binary systems, PHE with initial concentrations of 0.5, 1, 2, 3, 4, and 5 mg/L were added following 500 or 1000 mg/L of the amines, except for 1naphthylamine, which concentrations were 5 and 50 mg/L. The vials were then shaken at 20±0.5 °C and 300 rpm in the dark for 48 h to reach apparent equilibrium. The solution and soil were then separated by centrifugation at 3500 rpm for 15 min. An appropriate aliquot of the supernatant was removed, and the concentrations of the sorbates were analyzed. Control tubes were prepared to assess the losses due to evaporation, photochemical decomposition, and sorption to the vials. The results showed that the losses were totally less than 3 % and can be neglected. All the tests were conducted in triplicate, and the data in the figures are the average of the three replicates. More details can be referred to our previous studies (Sun et al. 2009; Wu and Sun 2010). 2.3 Analytical Method Aqueous trimethylamine and n-hexylamine were quantified by the dissolved organic nitrogen, which was measured by an UV-spectrophotometer (Hitachi U-3210, Japan) with a standard method (Water qualityDetermination of total nitrogen-Alkaline potassium persulfate digestion-UV spectrophotometric method) (GB11894-89 1989). The chemicals were measured by transforming the nitrogen in nitrogen compounds into nitrate. The absorbance (A) of the chemical was calculated by A ¼ A220 −KA275, where A220 and A275 represent absorbance at 220 nm and 275 nm, respectively. The nitrate has the maximum absorption at 220 nm, and generally, dissolved organic matter (DOM) has absorption in the whole UV segment. Hence, the difference between the absorption at 220 nm (nitro+DOM) and the absorption at 275 nm (DOM) could eliminate the effect of organics on the measurement. K is a coefficient ratio, and the measured K is 2 in this method. High-performance liquid chromatography (HPLC) with a reverse-phase column (VP-ODS Kromasil C18,

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Table 1 Selected physicochemical properties of four organic amines and PHE

MW

Chemicals

CAS

n-Hexylamine

111-26-2

101

Trimethylamine

75-50-3

59.1

(g/mol)

Structure

LogKow

NH+ 2.06a

N

Sw, g/L

pKa

12a

10.6a

0.16b

890b

9.81b

2.25c

1.7c

3.92c

4.63c

NH2

1-Naphthylamine

134-32-7

143

NH2

Phenylamine

62-53-3

93.1

0.9c

34c

Phenanthrene

85-01-8

178

4.57d

0.0011d

-e

MW: molecular weight, log Kow : octanol-water partition coefficient, Sw: water solubility, pKa: acidity Coefficient a

Cited from Brust (2001)

b

Cited from http://wwwp.ymparisto.fi/scripts/Kemrek/Kemrek_uk.asp?Method=MAKECHEMdetailsform&txtChemId= 2161

c

Cited from Fabrega et al. (1998)

d

Cited from Weber et al. (2002)

e

Not available

150 mm×4.6 mm×5 μm) and an ultraviolet (UV) detector (Hitachi U-3210, Japan) were used to analyze the concentration of 1-naphthylamine (at 254 nm) and phenylamine (at 285 nm), respectively. For 1-naphthylamine, the mobile phase was 0.02 M NaH2PO4 aqueous solution (A) and methanol (B) (1:3, v:v) with a flow rate of 0.4 mL/min and the injection volume was 20 μL. For phenylamine, the phase A of the mobile phase was milli-

Q water and phase B was methanol (35:65, v:v) with the same flow rate and injection volume as above. Aqueous PHE was determined by the same RP-HPLC system but equipped with a fluorescence detector with excitation and emission wavelengths of 250 nm and 364 nm, respectively. The mobile phase was a mixture of milliQ water (A) and acetonitrile (B) with a volume ratio of 20:80, at a flow rate of 1 mL/min.


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2.4 Data Analysis The solid-phase adsorbate concentrations (Cs, mg/kg) under the equilibrium condition were calculated by mass balance C s ¼ ðC 0 −C e Þ V =m

ð1Þ

where C0 and Ce are the initial and equilibrium concentrations of the adsorbate in the aqueous phase (mg/L), respectively, V is the volume of solution (L), and m is the mass of the sorbent (kg). All the sorption data in this study were fitted by the Freundlich isotherm: C s ¼ K f C ne

ð2Þ

where Kf is the Freundlich affinity coefficient ((mg/kg)/ (mg/L)n), and n is the Freundlich linearity index (unitless). The solid-water distribution coefficient (Kd, L/kg) is defined as the adsorbed-to-solution concentration ratio: Kd ¼

Cs Ce

ð3Þ

2.5 Statistical Analysis The significance of the differences between the adsorption amounts of PHE in the presence and absence of the organic amines were identified by Mann-Whitney nonparametric tests using the SPSS statistical software package version 21.0 (SPSS, 2012).

3 Results and Discussion 3.1 Isotherms of Organic Amines in a Single-Solute System The adsorption isotherm can provide the most important information, which shows how adsorbate molecules are distributed between the liquid phase and solid phase when the adsorption process reaches equilibrium. Figure 1 presents the adsorption isotherms of the four amines, trimethylamine, n-hexylamine, 1-naphthylamine, and phenylamine, respectively. The sorption data of the four amines followed Freundlich isotherm well with correlation coefficients (R2) being 0.964–0.998, with the aliphatic trimethylamine being the most linear

and the aromatic 1-naphthylamine being the most nonlinear. Because of the isotherm nonlinearity, the solidwater distribution coefficient (Kd) derived at equilibrium solute concentrations of 0.2 Sw and 0.8 Sw were used for comparison. The Kd varied a lot, ranging from 0.164 to 23.9 (Table 2). The Kd values of 1-naphthylamine and phenylamine were lower than those of previous studies, where the pH of geosorbents was lower (Li et al. 2001; Zhu et al. 2005). The existence form of IOCs in an aqueous phase is dependent on the pH-pKa relationship. The percentage of a cationic form for organic amines (α) in the solution at a certain pH can be calculated by: α¼

1 1 þ 10ðpH−pKaÞ

ð4Þ

Under the experimental conditions (pH=8.0) in the present study, the α of trimethylamine, n-hexylamine, 1naphthylamine, and phenylamine were 98.5, 99.8, 0.0083, and 0.043 %, respectively. Hence, in the present paper, trimethylamine and n-hexylamine existed mainly as a cationic form, whereas for 1-naphthylamine and phenylamine, neutral molecule was their dominant form. The different forms resulted in the great discrepancy in the sorption capacity of the four organic amines (Table 2). When organic amines are cationic, the sorption is a combination of electrostatic attraction, polar interaction, and hydrophobic partition since the soil particles were negatively charged (ζ-potential = −21.24 at pH of 8). When they are in a neutral form, only hydrophobic partition together with polar interaction is the main mechanism, which is in agreement with previous observations (Fabrega et al. 1998; Zhu et al. 2003). Studies on other organic ionizable compounds also confirmed our results. For example, studies on adsorption of tributyltin (TBT) on sediments or black carbon all demonstrated that the sorption is pHdependent and TBTOH was adsorbed to hydrophobic sites, whereas TBT+ was adsorbed by the anionic sites on the particle surfaces (Burton et al. 2004; Fang et al. 2010). Bi et al. (2006) reported that specific interactions, such as complexation of surface-bound cation rather than partition, were the dominant mechanism in the sorption of indole, 2-hydroxyquinoline, and benzotriazole. Additionally, though electrostatic repulsion existed due to its negative charge, 2-naphthol could also enter sediment with the same charge by the partition due to its high hydrophobicity with a great hydrophobic

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Fig. 1 Sorption of the four organic amines (filled square): n-hexylamine (a), trimethylamine (b), 1-naphthylamine (c), and phenylamine (d) fitted by Freundlich model

functional group (Zhao and Yang 2002). Although compared to 1-naphthylamine, n-hexylamine has higher solubility and lower logKow (Table 1), its sorption was greater because of the complexation and ionic exchange at the anionic sites of the soil. Hence, the electrostatic force and hydrophobic force act together to control the sorption of an ionizable compound. Cationic organic amines showed greater affinity for the soil than neutral forms. For the amines with the same form, the sorption was relatively stronger for the amine with greater hydrophobicity (Table 2). Both of the n-hexylamine and

trimethylamine are cations at pH 8, but the adsorption of n-hexylamine was significantly greater than that of trimethylamine, as indicated by the larger Kd values of the former, which were 14.1 times (at 0.2 Sw) and 12.3 times (at 0.8 Sw) of the later, respectively. This is because the high solubility and weak hydrophobicity of trimethylamine did not benefit for its sorption; while for n-hexylamine, it can interact with the adsorbent not only via ionic exchange (CEC of the adsorbent is 10.8 cmol/kg) but also by partitioning into the organic matter of the soil. Similarly, the Kd value of 1-naphthylamine was much higher than phenylamine due to its greater

Table 2 Sorption isotherm parameters of the four organic amines fitted by Freundlich model Parameter

n-Hexylamine

Trimethylamine

Kf ((mg/kg)/(mg/L)n)

77.3±17.1

3.18±1.86

5.89±0.420

0.670±0.597

n

0.849±0.033

0.948±0.091

0.756±0.021

0.863±0.136

R2

0.996

0.980

0.998

0.964

0.2 Sw

23.9

1.70

1.42

0.198

0.8 Sw

19.4

1.58

1.01

0.164

Kd (L/kg)

1-Naphthylamine

Phenylamine


hydrophobicity (Table 1). This is also similar to the previous observation showing the stronger affinity of cation exchange sites for 1-naphthylamine than for phenylamine when pH ≤pKa and the prior selectivity for the compound with the increasing number of fused rings (Li et al. 2001). 3.2 Effect of Cationic Organic Amines on the Sorption of PHE Sorption isotherms of PHE in the presence of 0, 500, 1000 mg/L of n-hexylamine were low nonlinear (Fig. 2a), and the Kd values are shown in Table 3. Clearly, the sorption of PHE was promoted by nhexylamine and increased with increasing amine concentrations. The enhancement of Kd were 5.30 % (500 mg/L) and 56.8 % (1000 mg/L) at 0.2 Sw, 24.8 % (500 mg/L) and 56.9 % (1000 mg/L) at 0.8 Sw as compared to that in the absence of n-hexylamine (Table 3). The n value of PHE was low nonlinear in the presence of n-hexylamine, which indicated that partitioning into organic matter was still the primary Fig. 2 Effect of n-hexylamine(a), trimethylamine (b), 1-naphthylamine (c), phenylamine (d) on the sorption of PHE. Organic amines 0 mg/L (filled circle), 500 mg/L (empty triangle), and 1000 mg/L (empty diamond)

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mechanism of PHE sorption (Ling et al. 2009). Thus, we could speculate that the sorbed nhexylamine molecules, especially those adsorbed by head-on sorption on negatively charged surface, may provide more hydrophobic organic sites, which are more homogeneous and more effective for the sorption of PHE compared with natural organic matter (Fig. 3). In our previous study about the effects of a cationic surfactant, dodecyltrimethylammonium (DDTMA) on the sorption of PHE, which is also composed of a hydrophilic head and a hydrophobic tail, we found that DDTMA could remarkably enhance PHE sorption (Sun et al. 2009). Surprisingly, the Kd of PHE was increased up to 5.43 times of that without DDTMA in that study; while it was just increased by 5.3 % (0.2 Sw) and 24.8 % (0.8 Sw) in the presence of n-hexylamine at the same concentration in this study. This seems can be explained by the more organic carbon provided by DDTMA. Chefetz and Xing (2009) have proposed that hydrophobic organic compounds, like PHE, have a strong affinity for aliphatic soil organic matter domains. The

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Table 3 Effect of organic amines on PHE sorption fitted by Freundlich model Parameter

Phenanthrene

n-Hexylamine

Cspiked (mg/L)

Without co-solute

500

1000

500

1000


107±2

136±15

168±7

100±6

98.1±0.7

n

0.860±0.033

0.985±0.143

0.862±0.042

0.849±0.094

0.865±0.011

R2

0.997

0.969

0.996

0.980

0.999

0.2 Sw

132

139

207

126

120

0.8 Sw

109

136

171

102

100

Kd (L/kg)

Trimethylamine

1-Naphthylamine

Phenylamine

Cspiked (mg/L)

5

50

500

1000


85.8±1.1

82.3±0.6

102±1

101±1

n

0.838±0.027

0.870±0.018

0.842±0.029

0.895±0.014

R2

0.998

0.999

0.997

0.999

0.2 Sw

110

100

129

118

0.8 Sw

87.6

83.7

104

102

fractional organic carbon content (ƒoc, mol C/kg) of the sorbed n-hexylamine when ω combined is with the sorption amounts by the following equation:

percentage of carbon (ω) in amine molecules was calculated according to the following equation: ω¼

12 nc 100% M

ð5Þ f oc ¼

where nc is the number of carbon for the chemical of interest and M is the molecular weight of the chemical. For n-hexylamine, which contains six carbon atoms and the molecular weight is 101, the ω is thereby 71.3 % according to the equation above. Likewise, we can work out that the ω of DDTMA is 68.2 %. Furthermore, we can also calculate the

ωQ 100% 106

ð6Þ

where Q is the sorption capacity (mol/kg) of nhexylamine on the soil. With the addition of 500 and 1000 mg/L of n-hexylamine, the Q was 9.3×10−2 and 1.85×10−2 mol/kg, respectively; thus, the foc was calculated to be 6.62 × 10−5 and 1.31 × 10−4 mol C/kg,

Hydrophobic domain

Partition

Fig. 3 Schematic illustration for the proposed interactions between the co-existing sorbates as well as with the geosorbent

NH +

—

NH +

—

NH +

—

+ + NH NH + NH + NH + NH + NH NH +

—

—

—

—

—

—

—

NH + NH +

—

—

NH +

—

Charge neutralization

Kd (L/kg)


respectively. While for DDTMA, with the same additional concentrations, the Q was 3.20×10−2 and 6.40× 10−2 mol/kg, respectively, and the foc value was 2.20× 10 − 5 and 4.36 × 10 − 5 mol C/kg, respectively. Consequently, n-hexylamine could provide more organic carbon than DDTMA, so the reason for weaker enhancement of PHE’s sorption was not the less amount of organic carbon provided by n-hexylamine and there must be other reasons for the difference. As DDTMA contains a long hydrophobic tail of 12 carbon atoms, when the surfactant is sorbed to the soil, the hydrophilic head groups are neutralized and a stable zone is formed by the tail far away from the surface of the sorbent, which is thought more favor for the hydrophobic interactions of surfactant bilayers with PHE molecules (Sun et al. 2009). Another cationic organic amine, trimethylamine, showed distinctly different effect on PHE adsorption (Fig. 2b). As it can be seen from Table 3, Kd values of PHE decreased only slightly compared to that without trimethylamine (P>0.05). The Kd value of PHE in the presence of trimethylamine was significantly lower than those in the presence of n-hexylamine (Table 3). Yet, it is understandable for trimethylamine to have a little effect on the enhancement of PHE sorption, and this is due to its weak sorption ability (Table 2). In addition, in the view of molecular structure, unlike n-hexylamine with a six-carbon linear alkyl chain, trimethylamine is only composed of three methyl groups and has no long hydrocarbon chain. Therefore, it cannot generate a new phase of hydrophobic organic environment for PHE sorption (Fabrega et al. 1998). 3.3 Effect of Neutral Organic Amines on the Sorption of PHE Figure 2c depicts the effect of 1-naphthylamine on the adsorption of PHE. In the presence of 1-naphthylamine, the Kd values of PHE on the soil were lower than those of the single PHE. At a concentration of 5 mg/L or higher, 1-naphthylamine showed a significant inhibitory effect on PHE adsorption (P < 0.05). As discussed above, under the experiment pH (pH=8), the neutral form was the dominant speciation for 1-naphthylamine and only 0.0083 % of 1-naphthylamine existed as cation. Thus, 1-naphthylamine might compete with PHE for the mutually available hydrophobic sites and reduced its sorption. Short-chain organic acids, such as citric acid, oxalic acid, and so on, also had the similar

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tendency to occupy a large proportion of adsorption sites and decreased the adsorption capacity of pyrene and PHE in soils (Ling et al. 2009; An et al. 2010). Phenylamine has the similar structure to 1naphthylamine and existed as neutral molecules at pH = 8; however, due to its greater solubility, phenylamine showed much lower adsorption as compared to 1-naphthylamine. There was no obvious reduction in PHE adsorption in the presence of phenylamine (Fig. 2d). Partitioning should be related to the solubility of the chemical, and its affinity in the sorption process increases with decreasing chemical solubility. Hence, the high solubility (34 g/L) and low logKow (0.9) of phenylamine might contribute to weaken the partition affinity of phenylamine. The adsorption of both of phenylamine and 1-naphthylamine were hydrophobic partition, but the Kd values of phenylamine were 0.198 (0.2 Sw) and 0.164 (0.8 Sw), much lower than those of 1naphthylamine 1.42 (0.2 Sw) and 1.01 (0.8 Sw). Sorption studies on a binary-solute system of 1-naphthylamine and phenylamine showed that the sorption of 1naphthylamine was not impacted quantitatively by phenylamine under pH=2.7 or 6.7, while phenylamine’s sorption was notably reduced by 1-naphthylamine and the largest reduction occurred in the soil with the lowest pH (Li et al. 2001; Sheng et al. 2010). Donaldson and Nyman (2005) have studied the adsorption of benzidine and 3,3′-dichlorobenzidine to sediments and found that the contribution of partition for 3,3′-dichlorobenzidine adsorption were often greater than that of benzidine, which has relatively higher solubility and lower logKow value than 3,3′-dichlorobenzidine. Likewise, the solubility of phenylamine is 34 g/L, while that of p-chloroaniline is only 3.1 g/L. The competition capacity of the latter on p-nitroaniline is always stronger than that of the former no matter in a neutral form or in a protonated form. Moreover, it is important to notice that competitive capacity of the two organic amines is stronger in the ionic form than in the neutral molecular form (Zhu et al. 2005). Hence, it can be inferred that the sorption capacity of phenylamine to soil was low due to its weak hydrophobicity, thus uncompetitive with PHE in this process.

4 Conclusion The sorption of ionizable organic amines in a singlesolute system and their effects on the sorption of a

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hydrophobic organic compound, PHE, in a binarysolute system were studied in this paper. The following main results were achieved. (1) Generally, cationic organic amines would exhibit stronger sorption capacities than those in the neutral form. Additionally, the sorption increased with hydrophobicity either for the dissociated forms or for the nondissociated forms. (2) Amines existing as cationic forms, like n-hexylamine, can enhance the sorption of PHE, while for trimethylamine with high solubility and without long hydrophobic alkyl chain, there was no significant enhancement on PHE sorption. (3) Those amines existing in neutral forms with strong hydrophobicity could compete with PHE for the sorption sites and inhibited its sorption while those with weaker hydrophobicity could not effectively compete with PHE and had little influence on its sorption. Acknowledgments The authors would like to thank the financial supports provided by the Ministry of Science and Technology (2014CB441104), the Natural Science Foundation of China (No. 41225014), and the Ministry of Education of China as an innovative research team project (No. IRT13024).

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