Journal of Environmental Sciences 2010, 22(12) 1910–1915
Simultaneous elution of polycyclic aromatic hydrocarbons and heavy metals from contaminated soil by two amino acids derived from β-cyclodextrins Chengjian Yang1,2 , Qingru Zeng2,∗, Yunzhong Wang1 , Bohan Liao2 , Jian Sun3 , Hui Shi1 , Xingdou Chen1 1. School of Environment & Municipal Engineering, Xi’an Architecture Science Technology University, Xi’an 710055 , China. E-mail: [email protected]
2. Department of Environmental Science, Hunan Agricultural University, Changsha 410128, China 3. Ministry of Education Key Laboratory of Pollution Control and Ecological Remediation for Industrial Agglomeration Area, Department of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China Received 23 January 2010; revised 22 April 2010; accepted 29 May 2010
Abstract Two highly water-soluble amino acids, which derived from β-CDs, i.e., glutamic acid-β-cyclodextrin (GluCD) and ethylene-diamineβ-cyclodextrin (EDCD), were synthesized and were examined for their eﬀect on solubilization of anthracene (ANT), complexation of cadmium (Cd2+ ), and elution removal of ANT and Cd2+ in soil. The results showed that GluCD and EDCD were powerful complexant for ANT and Cd2+ . In the presence of 10 g/L GluCD and EDCD, the solubilization of ANT increased by 47.04 and 23.85 times compared to the control, respectively. GluCD resulted in approximately 90% complexation of Cd2+ while 70% complexation was observed for EDCD. Simultaneously, GluCD and EDCD could greatly enhance the elution removal of ANT and Cd2+ from soil. GluCD resulted in the highest elution eﬃciency of ANT and Cd2+ . With the addition of 10 g/L GluCD, 53.5% of ANT and 85.6% of Cd2+ were eluted, respectively. The ANT had a negligible eﬀect on the Cd2+ removal due to diﬀerent complexing sites of ANT and Cd2+ , while Cd2+ enhanced the ANT removal under the addition of GluCD because Cd2+ neutralized the –COOH group of GluCD. Adversely, the removal of ANT was decreased with Cd2+ under the addition of EDCD, this was due to the fact that Cd2+ enhanced the polarity of EDCD molecule and inhibited the complexation between ANT and EDCD. The study suggested that GluCD could be preferred and be successfully applied to remediation of heavy metals or organic compounds in contaminated soil. Key words: anthracene; cadmium; cyclodextrin; solubilization; elution; soil DOI: 10.1016/S1001-0742(09)60338-7
Introduction Organic and inorganic contaminations such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals have occurred as a result of various anthropogenic activities (discharge of industrial waste, wood treatments, metallurgical operations, burning of fossil fuels, etc.). The accumulation of both PAHs and heavy metals in the environment not only results in potential environmental risk but also poses a serious threat to human health due to their toxic, mutagenic, and carcinogenic nature, and their long-lasting sorption by soils (Wang and Brusseau, 1995a; Brusseau et al., 1997). The remediation of PAHs and heavy metals contaminated sites is an important environmental issue. Various physical, chemical, biological, and their combined technologies have been developed to remediate PAHs and heavy metals contaminated soils (Menzie et al., 1992; Wagrowski and Hites, 1997; Fountain et al., 1991). Owing to the high cost and disruption of treatments, traditional remediation of contaminated soils such as exca* Corresponding author. E-mail: [email protected]
vation and ex-situ treatment and disposal has promote the development of in situ alternatives (Fountain et al., 1991; Martin and Ruby, 2003). Several methods relate to in situ remediation have been explored, including vitriﬁcation, soil washing/acid extraction, phytoremediation, soil ﬂushing (Brusseau et al., 1997), and bioremediation (Lovley and Coates, 1997). Co-occurrence of PAHs and heavy metals usually complicate the cleanup of sites (Wang and Brusseau, 1995b). However, little eﬀorts were conducted on simultaneous removal of heavy metals and PAHs from soils. Several publications indicated the potential of surfactants for the removal of PAHs and heavy metals from soil (Mulligan et al., 2001; Park and Bielefeldt, 2003). As a commonly used surfactant, ethylene diaminete-traacetic acid (EDTA) was proved eﬀective in removing heavy metals from soil (Sun et al., 2001). However, most of surfactants are toxic and generally interact strongly with soil (Wolf and Feijtel, 1998; Zhu et al., 2003). The high cost of EDTA also limited its application on a large scale. Given the limitation associated with classical extracting
Simultaneous elution of polycyclic aromatic hydrocarbons and heavy metals from contaminated soil······
agents, there is an interest in examining the ability of other materials to facilitate the elution of PAHs and heavy metals contaminants for applications in subsurface remediation. Cyclodextrins (CDs) are cyclicoli-gosaccharides, containing six, seven, or eight D-(+)-glucopyranose units (α-, β-, and γ-cyclodextrin) attached by α-1,4-linkage. CDs can trap the lipophilic organic compounds as guest in a cage-like meshwork, thereby enhancing its solubility and dissolution rate (Blanco et al., 1991). β-Cyclodextrin (βCD) appears to be the best natural cyclodextrin due to its eﬃcient organic compound complexation and availability in pure form (Cheng and Li, 2001). However, natural β-CD can be modiﬁed to improve the low aqueous solubility. Previous studies demonstrated the enhanced performance of β-CD derivatives, such as 2-hydroxypropyl-βcyclodextrin (HPCD), methyl-β-cyclodextrin (MCD), and carboxylmethyl-β-cyclodextrin (CMCD) on removal of PAHs and heavy metals from soils (Cheng and Li, 2001; Chatain et al., 2004). β-CD derivatives present several advantages over β-CD ﬂushing agents such as high water solubility stability and complexing capacity (Chatain et al., 2004; Albers and Muller, 1992). In the present study, two novel amino acid derived β-CDs were synthesized, and were examined for their capability for elution and complexation of PAHs and heavy metals in contaminated soil.
1 Materials and methods 1.1 Materials β-CD was obtained from Chemical Reagent Factory of Shanghai (China) with a purity of 99%. The solubilities of natural CDs in water are diﬀerent with the number of glucose units (6, 7, or 8, respectively for α-, β-, and γ-CD). β-CD is the cheapest in cost, but weak in water solubility (15.8 mmol/L). β-CD derivatives, such as glutamic acid-β-cyclodextrin (GluCD) and ethylenediamine-β-cyclodextrin (EDCD), containing amino groups and carboxyl groups are excellent in water-solubility. Anthracene (ANT) was obtained from Yuanhang Chemical Reagent Factory of Shanghai, China (purity 99%). ANT was chosen in this study due to its various physicochemical properties and its wide spread presence in contaminated sites (Wang and Brusseau, 1995a; Wagrowski and Hites, 1997). The properties of ANT and β-CD are shown in Table 1. Cadmium (cadmium nitrate, purity 99%) was selected as representative of heavy metal. Tested soil was aseptically sampled from Jinyun Mt. (29◦ 49 N, 106◦ 22 E) located about 32 km northwest of Chongqing, China. The soil was classiﬁed by the Chinese soil taxono-
my as yellow soil, which was dominant in southern China. The sampling site was forested and practically undisturbed by land-use activities. After collection, the soil sample was transported to the laboratory in cooler, and air-dried. Before use, the soil was crushed and sieved through a 2-mm sieve. The soil was characterized with respect to an organic carbon content of 1.66%, a pH of 6.25 (1 g soil to 1 mL solution), a cation exchange capacity (CEC) of 13 mmol/100 g, a Cd content of 0.26 mg/kg, and a ANT content of 2.23 μg/kg. 1.2 Methods 1.2.1 Synthesis of amino acid derived β-CD Epichlorohydrin (10.2 g) was added dropwise to the solution of β-CD (8.1 g), potassium hydroxide (6.7 g) and sodium glutamine (16.9 g) or ethylene diamine (6.0 g) in 70 mL distilled water at 50°C. The mixture was then reacted at 60°C for 1 hr. The pH was adjusted to 5–6 using 10% (V/V) vitriol liquor after the solution was cooled to room temperature and then left to stand overnight. The mixture was dehydrated by using anhydrous ethanol (150 mL) and was pumped through a column packed with neutral alumina. Ethanol (60%, V/V) was used as ﬂushing agent and was repeatedly passed through the column. The eﬄuent was left to stand for 4–5 hr and supernatant was moved away. With the addition of enough anhydrous methanol to the residue, white GluCD or EDCD precipitate was obtained and the precipitate was then ﬁltrated, freezedried and stored for further use. 1.2.2 Solubilization of ANT by CDs The generator column approach was used to determine the solubilization of ANT by CDs (Wang et al., 1993). The generator column used was a 40-cm long glass chromatography column packed with pre-washed quartz sands coated with excess ANT (0.05–0.1 g). The column was plugged with glass wool at both ends and contained a large pore-diameter fritted disc sealed at the outlet. Twentyﬁve milliliter of distilled water, or solutions containing diﬀerent concentrations of CDs were passed through the column, and a fraction of the eﬄuent was immediately analyzed for solute concentration. The remaining eﬄuent was then repeatedly passed through the column so as to obtain a relatively constant eﬄuent concentration. All experiments were carried out at room temperature (25°C). The concentration of ANT was measured by UV-Vis spectrophotometry (Shanghai Chemical Instrument Inc., China). The wavelengths used for UV detection were 250 nm for ANT. During the solubility measurement, 1 mL eﬄuent was withdrawn and diluted with a methanol/water (1/1, V/V) solution in 10 mL volumetric ﬂasks. The role
Table 1 Physico-chemical properties of ANT and β-CD*
Molecular weight (g/mol)
Solubility in distilled water (mg/L)
Molecule volume (nm3 )
0.075 1.85 × 104
ANT: anthracene; β-CD: β-cyclodextrin. * Data were cited from Wang and Brusseau, 1995a; Brusseau et al., 1997.
Chengjian Yang et al.
of methanol is to inhibit the formation of CD-solute complexes (Wang and Brusseau, 1995a). The eﬀects of CDs on the UV spectra of the ANT were negligible within the range of experiments. 1.2.3 Complexation of Cd2+ by GluCD or EDCD The complexation of Cd2+ by GluCD or EDCD was performed by standard batch method. Aliquots (100 mL) of 10 mg/L Cd2+ solutions, containing control and varying concentrations of GluCD or EDCD, were added to 200mL ﬂasks and equilibrated on a reciprocating shaker for at least 2 hr at 25°C. After equilibration, the samples were analyzed by a cadmium selective electrode (Therm Orion 9448BN, USA), with the assumption that the electrode measures the activity of the free (un-complexes) Cd2+ . 1.2.4 Simultaneous elution experiments of ANT and Cd2+ A 40-cm long glass chromatography column was packed with 60 g soil. The column was plugged with glass wool at both ends and contained a large pore-diameter fritted disc sealed at the outlet. After packing, the columns were slowly wetted from the bottom with electrolyte solution (0.01 mol/L CaCl2 ). To prepare the contaminant-elution experiments, solutions containing ANT (0.05–0.07 mg/L), cadmium (100 mg/L), or a combination of both were pumped into the soil-packed column. To ensure uniform and complete contamination of the soil columns, the solution was continuously pumped into the columns until the eﬄuent concentrations of both cadmium and ANT were equal to the inﬂuent concentrations. An atomic absorption spectrophoto-meter (WFX-810, Beijing Rayleigh Analytical Instrument Co., Ltd., China) was used to monitor the concentration of cadmium in the eﬄuent fractions. The initial concentrations of ANT and cadmium sorbed by the soils for each set of experiments were 4 and 101.2 mg/kg soil, respectively. Immediately after contamination, distilled water or solution containing either 10 g/L GluCD or 10 g/L EDCD was pumped through the columns. The ANT and Cd2+ removed by distilled water was a blank control group.
Firstly, amino group is reacted with epichlorohydrin under basic condition to form 1-amino-2-hydroxyl-3chloropropane. Secondly, 3-amino-1,2-epoxypropane is formed after the cyclization of 1-amino-2-hydroxyl-3chloropropane. Thirdly, β-CDs is reacted with 3-amino1,2-epoxypropane to form amino acids derived β-CDs. These reactions for each set of synthesis experiment were performed in aqueous solution, and the structures of the ﬁnal products were determined through Fourier transform infrared (FT-IR-8400, Shimadzu, Japan) (FT-IR). Figure 1 shows that some diﬀerent bands arose in the spectra of GluCD and EDCD. At 1719 cm−1 , a new peak appeared in the spectra of GluCD attributed to C=O stretching. In addition, an adsorption bound boosted up in the range of 1400–1500 cm−1 for GluCD could be ascribed to N–H deformation and C=N stretching. These results supported the conclusion that the synthetic compound was GluCD. A distinct diﬀerence was observed between spectra of β-CD and EDCD. At about 3382 cm−1 , the EDCD presented a new strong adsorption which mainly ascribe to NH2 stretching indicated the synthesis of EDCD. 2.2 Solubilization and complexation studies for ANT and Cd2+ The solubilization eﬀects of β-CD, GluCD and EDCD on ANT are plotted in Fig. 2. The results showed that the apparent aqueous solubility of ANT was linearly increased with the concentrations of β-CD, GluCD and EDCD which might be ascribed to the formation of 1:1 inclusion complexes in solution (Wang and Brusseau, 1995b). The linearity can be expressed by the following Eq. (2): S t = S 0 (1 + KSC0 )
where, St is the aqueous-phase concentration of ANT with β-CD, GluCD or EDCD, S0 is the concentration of ANT without β-CD, GluCD or EDCD, C0 is the initial concentration of β-CD, GluCD or EDCD, and KS is the stability constant of inclusion complexes for ANT with β-CD, GluCD or EDCD. In Eq. (2), it was assumed that the concentration of β-CD, GluCD or EDCD is not depleted to an appreciable extent by complexing with
2 Results and discussion 2.1 Synthesis and characterization of amino acid derived β-CD The synthesis of amino acid derived β-CD is shown as the following (Dai et al., 1998):
Fig. 1 FT-IR spectra of amino acid derived β-CD. Line a: β-CD; line b: GluCD; line c: EDCD.
Simultaneous elution of polycyclic aromatic hydrocarbons and heavy metals from contaminated soil······
Fig. 3 Schematic illustration of hypothesized conﬁguration of Cd2+ ANT-GluCD (a) and Cd2+ -ANT- EDCD complex (b). Fig. 2 Solubilization curves of ANT by β-CD, EDCD and GluCD. βCD: S t /S 0 = 1.733C0 + 1.0504, r2 = 0.994; EDCD: S t /S 0 = 2.735C0 + 1.0022, r2 = 0.998; GluCD: S t /S 0 = 4.602C0 + 1.0177, r2 = 0.997.
ANT. The relative aqueous-phase concentrations (S t /S 0 ) of the ANT are plotted against the concentration of β-CD, GluCD or EDCD. The regression equations and correlation coeﬃcients of solubilization cures of ANT with β-CD, GluCD and EDCD are listed in Fig. 2. Figure 2 reveals that GluCD resulted in the most notable solubilization of ANT followed by EDCD and β-CD. The prerequisite for solutes ﬁtting completely in the cavity of cyclodextrin and forming an inclusion complex is that the molecule must be of appropriate in shape and size (Wang et al., 1993). Table 1 shows that the molecule volume of the ANT was smaller than this cavity volume for β-CD. Therefore, they can enter into their respective cavities for β-CD, GluCD and EDCD and inclusion complexes. In addition, the watersoluble of the compounds and CDs pose a strong eﬀect on the stability. As a result, the solubilization capability of β-CD for ANT was greater than that of naphthalene due to the weaker water-soluble of naphthalene than ANT (Wang and Brusseau, 1995a). In addition, the ANT was oriented coincident with the cavity axis when it extended through the β-CD cavity. The reason might be that the length of ANT was larger than the diameter and the depth of the βCD cavity (Table 1), However, the length of naphthalene is only slightly smaller than the diameter of the β-CD cavity, maximum contact with the β-CD cavity was obtained when the short axis of naphthalene was parallel to the axis of the cavity (Wang and Brusseau, 1995b). In this study, inclusion complexes of ANT with GluCD or EDCD might be formed when the ANT extended through the GluCD or EDCD cavity (Fig. 3). Higher water-solubility of GluCD and EDCD compared to the β-CD was based on: (1) the strong hydrophilic groups –COOH or –NH2 in GluCD and EDCD; (2) the break of reticulated CD polymer formed by hydrogen bonding interactions. Other than –NH2 group, hydrophilicity of GluCD was enhanced by the –COOH group which contributed to much stronger water-solubility of GluCD as compared to EDCD (Dai et al., 1998). In this study, the same results were obtained. Addition of 10 g/L GluCD resulted in 47.04 times increase in solubilization of ANT but only 23.85 times increase for EDCD at equal
concentration. The complexation results of Cd2+ by GluCD or EDCD at various concentrations are shown in Table 2. GluCD and EDCD complexed approximately 90% and 70% of the Cd2+ , respectively and the complexation capacity increased with cyclodextrin concentrations. Previous study suggested that the functional groups of –COOH and –NH2 in GluCD and EDCD played important roles in complexation (Wang and Brusseau, 1995b). For example, hexamethylene-diamine and EDTA, –NH2 of EDCD combines with Cd2+ to from four coordinate bonds, and GluCD-Cd2+ complexation contains Cd2+ attached by six coordinate bonds to –COOH and –NH2 (Fig. 3). In this case, GluCD is comparable to fulvic acids and anionic surfactants in complexation capacity for heavy metals (Xue and Sigg, 1999; Doong et al., 1998). 2.3 Simultaneous elution of ANT and Cd2+ by CDs The elution curves for Cd2+ and ANT removal from single-contaminant or mixed-contaminant soil are shown in Fig. 4. The elution percentage of ANT and Cd2+ using three types of CD elution solutions are reported in Table 3. Obviously, GluCD resulted in the highest removal eﬃciency of ANT and Cd2+ , followed by EDCD and β-CD. The modiﬁed CDs were more eﬀective than the β-CD for ANT and Cd2+ removal. The recovery percentages of ANT and Cd2+ from soil were 41.6% and 39.7%, respectively. It was noticeable that the addition of 10 mg/L GluCD greatly enhanced the removal of ANT and Cd2+ in comparison to EDCD. Approximately 53.5% of ANT and 85.6% of Cd2+ were removed by the GluCD as compared to 5.2% and 6.4% for β-CD, respectively. These results suggested that glutamic-acid-β-cyclodextrin and ethylene-diamine-βcyclodextrin were more suitable for remediation of soils contaminated by cadmium and anthracene. Table 2
Complexation of Cd2+ by GluCD or EDCD at various concentrations (unit: mg/L)
Amino acids derived β-CDs
Rate of free Cd2+ to total Cd2+ 3 6 9 0.15 0.32
12 0.09 0.21
Chengjian Yang et al. Table 3
Elution rate of anthracene and cadmium from single-contaminated soil or mixed-contaminated soil by β-CD, EDCD and GluCD.
Anthracene elution rate (%) β-CD EDCD GluCD + Cd + Cd 53.5
GluCD + Cd
Cadmium elution rate (%) β-CD GluCD + ANT 85.6
EDCD + ANT
GluCD + ANT
EDCD: ethylene-diamine-β-cyclodextrin; GluCD: glutamic-acid-β-cyclodextrin; β-CD + ANT, EDCD + ANT, GluCD + ANT and β-CD + Cd, EDCD + Cd, GluCD + Cd denote mixed-contaminant experiments (anthracene and cadmium).
Elution of soil contaminated by cadmium and anthracene. (a) anthracene elution curve; (b) Cd elution curve.
As shown in Fig. 4 and Table 3, the eﬀect of ANT on the removal of Cd2+ by EDCD and GluCD was unnoticeable, because ANT and Cd2+ were complexed at diﬀerent locations. The most possible reason might be that the group of –NH2 and –COOH in EDCD and GluCD were responsible for the complexation of Cd2+ outside the cavity while ANT was complexed by inclusion in the cavity (Fig. 3). The result was well consistent with previous report (Brusseau et al., 1997). However, the presence of Cd2+ posed a signiﬁcant impact on the removal of ANT by EDCD and GluCD. Much more ANT was removed by GluCD in the presence of Cd2+ compared to control sample. It is possible that the presence of Cd2+ neutralizes the charge of –COOH and reduces the polarity of GluCD molecule, and thus enhance the partition of ANT to the interior of the GluCD. However, noticeable reduction in ANT removal by EDCD in the presence of Cd2+ also indicated the higher polarity of EDCD molecule when Cd2+ was complexed by EDCD (Fig. 3).
and GluCD due to diﬀerent complex locations of EDCD and GluCD for ANT and Cd2+ . Amino acid derived β-CDs could increase or decrease the elution eﬃciency of Cd2+ from soil which mainly contributed to the change of Cd2+ CD complexation polarity. For GluCD, the cyclodextrin simultaneously remove anthracene and Cd2+ from contaminated soil. The mechanism of how the formation of complex of hydrophobe organic compound and heavy metal with cyclodextrin compounds aﬀect the toxicity of hydrophobe organic compound and heavy metals need to be further investigated. Acknowledgments This work was supported by the Public Foundation of National Environmental Protection Department (No. 201009047) and Xi’an Science and Technology Plan Projects (No. CXY09025(3)).
References 3 Conclusions Two β-CDs derived amino acids were synthesized and were proved have high water solubility due to a large hydrophile group. The results of the solubilization and elution experiments showed that EDCD and GluCD could signiﬁcantly enhance the complexation and elution of anthracene and cadmium, which commonly considered as low-polarity organic and heavy metal contaminants. GluCD was found to be more eﬀective than EDCD in eluting anthracene and cadmium from soil under the same concentration. The presence of anthracene in the system had a negligible eﬀect on the removal of Cd2+ by EDCD
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