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Pergamon

PII: SO0456535(98)00127-l

Chemosphere, Vol. 37, No. I, pp. 1299-1305, 1998 0 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0045.653519

8 $19.00+0.00

SOLUBILIZATION

OF POLYCYCLIC

AROMATIC

HYDROCARBONS

BY P-CYCLODEXTRIN

AND CARBOXYMETHYL-fl-CYCLODEXTRIN

Gao Shixiang, Wang Liansheng * , Huang Qingguo, Han Sukui National Key Laboratory of Pollution Control and Resource Reuse Department of Environment Science and Engineering

Nanjing University; Nanjing 2 10093 China
(Received in Germany 11 November 1997; accepted 26 March 1998)

Abstract
The solubilization cyclodextrin(CMCD) of six polycyclic aromatic hydrocarbons(PAHs) by B-cyclodextrin and carboxymethyl+

has been investigated.

The presence of P-CD and CMCD greatly enhance the solubility of is caused by formation of inclusion complexes of PAHs

PAHs in water The solubilization effect of cyclodextrins with cyclodextrin.

The degree of solubilization power is related to the “tit” of PAH structure within the cavity of power. B-CD shows higher

D-CD or CMCD. The better the tit of the molecule, the greater the solubilization solubilization power than CMCD at low concentration,
01998 Elsevier

Science Ltd. All rights reserved

Key words. solubilization,

P-cyclodextrin,

carboxymethyl-B-cyclodextrin,

PAHs

Polycyclic aromatic

hydrocarbon(PAHs)

are poorly soluble, hydrophobic

organic compounds

that have been

released to the environment

on a large scale. They are distributed in air, water, and soil. Because many PAHs are is undesirable. Especially PAHs in soil, which can be accumulated

carcinogens, their presence in the environment

and transport into agricultural plants, may cause raised PAH contents in grains.

The cleanup of soil contaminated cosolvents or surfactants,

by low polarity organic compounds

usually use the method of solubilization of from soil into the water is usually not

which can improve the transport has its drawbacks.

speed of contaminants The solubilization

phase.“-*’ However,

each of these methods

of cosolvents

significant until their volume-fraction

concentrations

are above 10%; and surfactants

may form high viscosity as alternative

emulsions that are difficult to remove. It has been reported that cyclodextrins solubilization reagents for soil cleanup. Hydroxypropyl-B-cyclodextrin

may be considered

can enhance the water solubility of several

1299

1300 low-polarity cyclodextrins organic compounds significantly’“‘. Because the extremely low solubility of PAHs in water,

and their derivatives may be attractive reagents for enhancing water solubilities of PAHs.

Cyclodextrins,

first isolated in 1891t4’ are cyclic oligomers of glucose produced by the action of certain bacterial , have a doughnut-shaped structure with a relatively hydrophilic exterior and a to

enzymes on starch”]. Cyclodextrins

relatively hydrophobic interior “hole”‘“’ Low-polarity the hole form inclusion complexes organic compound-cyclodextrin the apparent solubility

organic substances with a size and shape complementary Because of the hydrophilic

with cyclodextrins”‘ .

exterior, the solubility of hence increasing

complexes are usually much higher than the pure compounds, in water Cyciodextrins

of organic compounds
in activated

have found several other applications in the

environmental area. P-Cyclodextrin to suppress toxicity

was used to form inclusion complexes with pesticides and related compounds sludge system, leading to enhanced by contacting biological detoxification of industrial

wastewater’“‘. PAHs could be removed from wastewater insoluble media”’ Hydroxypropyl-fl-cyclodextrin through soil”“‘ .

with cyclodextrins

anchored on water organic compounds

could enhance the transport of low-polarity

The purpose of this study is to investigate the solubilization power of a-cyclodextrin carboxymethyl-fl-cyclodextrin ( CMCD ) to several PAHs

and

one of its derivative

Materials

and Methods

Materials:

PAHS used in this study are phenanthrene, and P-bromonaphthalene, Scientific. P-naphthalene

2_methylphenanthrene,

tluorene, 1-methylfluor-ene.

I ,2-benzofluorene,

in which phenanthrene was produced by

was purchased from Aldrich; fluorene was provided by Ultra Shanghai First Reagent Plant; 2methylphenanthrene,

l-

methylfluorene,

and 1,2-benzofluorene

were prepared in this laboratory. P-Cyclodextrin

was purchased from the

Food and Fermentation

Institute of Jiangsu Province. CMCD was synthesized by reacting P-CD with chloroacetic Its average degree of carboxymethyl substitution is 1 3, and its average molecular

acid followed by purification weight is 1210.

Method.

The solubilization experiments

were conducted by use of the generator column method. The column used was a

40cm long, 2.5cm diameter glass column with a sintered glass bed at the outlet end. The PAH samples (0.05-o. Ig) were dissolved in 20ml ethanol, and the solutions were mixed with analytical pure silica sand with a size of 0.050 I mm. followed by evaporating of the ethanol. The sand coated with samples were packed into the column with glass wool placed above the glass bed to prevent the loss of silica sand. The column was flushed with 2OOML distilled water before use.

1301 1he solutions with P-CD or CMCD were passed through the column and a fraction of the effluent was taken in each 0.5 hours, and analyzed immediately to determine the concentration of the solute. The remaining effluent was obtained. The samples for

was repeatedly passed through the column until a constant solute concentration concentration measurement

were about 0.5-5.0ml, prediluted with same volume methanol and diluted to lOm1 solution. The role of methanol is to decompose UV spectrum of the compounds the complexes unchanged. of cyclodextin-PAH, were

with I : 1 methanol-water

thereby keeping the aqueous-based measured by 751G UV-Vis maximum absorption

The concentrations

Spectrophotometer

(manufactured

by Shanghai Analytical Instrument

Plant). The and fluorene,

wavelength

used for detection

of each PAH were 260nm for phenanthrene 263nm for 1,2_benzofluorene,

254nm for 2methyphenanthrene, P-bromonaphthalene.

265nm for I-methylfluorene,

and 230nm for

CMCD solution has a weak response

in the UV range. This effect was eliminated by

subtracting the UV response of blank CMCD solution from the total UV response of the sample solution.

Results And Discussion

Solubilizatiou

effect of P-CD and CMCD to PAHs

The solubilization etTect of P-CD and CMCD on selected

(I

i

(0

g/L

III

15

Fig. 1.

Solubilization

of

P-CD

to

Fig.2. Solubilization of P-CD to fluorene,

phenanthrene and 2-methylphenanthrene St is the equilibrium aqueous

1-methylfluorene. and 1.2benzofluorene
St is the equilibrium aqueous

concentration presented;

of PAH with cyclodextrins of

concentration presented:

of PAH with cyclodextrins

So is the water solubility cyclodextrins;

So is the water solubility of cyclodextrins: Co is the

PAH without

Co is the

PAH without

concentration of P-CD

concentration of P-CD

PAHs was shown cyclodextrin

in Figure

l-5. Linear regression gives very good

of the relative aqueous factors for five

concentration PAHs studied

of PAH versus except for o-

concentration

correlation reagent.

bromonaphthalene following equation:

with P-CD as solubilization

These linear relationship

could be expressed

by the

1302 St/So=KcwCo+l in which St is the equilibrium aqueous concentration of PAH

with cyclodextrins presented; So is the water solubility of PAH without cyclodextrins; PAH partitioning concentration Kcw is the equilibrium constant of the cavity; Co is the

in water and cyclodextrin

of P-CD or CMCD. In figure I-S Kcw is the line which reflects the solubilization
II 2 co 4 K/I6 8

slope of the regression

power of cyclodextrin to PAH

Fig.3

Solubilization of P-CD to paqueous wth

The linear relationship concentrations(St/So) P-CD and CMCD

between

the relative aqueous-phase concentrations of of

bromo-naphthalene St is the equilibrium concentration cyclodextrins water of presented; of PAH

of the PAHs and indicates

that the solubilization

So IS the without

solubility

PAH

PAHs by P-CD and CMCD are partition model solubilization of proposed NOCs

in accordance with the linear cyclodextrins; Co is the concentration colleagues for the of P-CD organic mater and

by Chiuo and by dissolved

Fig -I

Solubilizatlon

of

CMCD

to and

Fig 5 Solubili&on of CMCD to fluorene .
I-methylfluorene. St
IS

Phenanthrene. 2-methylphennnthrene. p-bromonaphthalene St is the equilibrium

and 1,2-benzofluorcne

the

equilibrium

aqueou

aqueous

concentration of PAH with cyclodextrm presented; So is the water solubility o PAH without cyclodextrins; concentration of CMCD Co is th

concentration of PAH with cyclodestnns presented: So is the \\ater solubllity o PAH \\lthout cyclodestrins: Co is the

Concentration orCMCD surfactants”” complexation Because the solubilization of of organic compounds cyclodextrins to low polarity organic compounds are caused by

with cyclodextrin cavities. the complexation

process can also be considered

as partitioning of PAH from water into cyclodextrin complexes can be considered as partition coefficient

cavity, and the stability constants KS of cyclodextrin-P&4 Kcw when the concentration of cyclodextrins is much

greater than the water solubility of PAH’“‘. The partition coefficients of water-cyclodextrins

(Kcw) for six PAHs,

1303 obtained from linear regression of St/So versus cyclodextrin concentrations Table I. Solubilization PAH Kcw (P-CD) Co, are listed in Table 1,

Data of P-CD And CMCD To PAHs Kcw (CMCD) SW m&L width length 0.89 0.74 0.78 0.89 0.98 0.94 IogKow Molecule Size(nm)

fluorene P-bromonaphthalene phenanthrene

0.865

0.544 1.480

1.68 5.34

4.18 4.3 4.53 4.97 5.10 5.68

0.48 0.49 0.58 0.58 0.58 0.73

1.005 0.872 1.698 0.324

0.698 0.425 1.312 0.152

I .60
1.09 0.27 0.045

1-methylfluorene
2-methylphenanthrene l,2-benzofluorene

Influence of PAH structure

on the solubilization

Inspection

of the Kcw for six PAHs reveals that the

solubilization of PAHs by P-CD and CMCD are I strongly / solubilization :‘ I’ I’ effect is obtained from pbromonaphthalene-CMCD Although experiments @@ a b Complexes the data from system (Kcw=l 48). the solubilization structure dependent. Greatest

of P-CD to P-bromonaphthalene the cause of

did not give a linear relationship, this phenomenon of insoluble

Ftg.4 Structure of Cyclodextrin-Phenanthrene

was attributed to the formation aggregates complexes From the at of /3-CD-Phigh low D-CD P-CD effect of and a

bromonaphthalene I\ concentration. concentration

data strong

solubilization

could still be seen. The least effectiveness solubilization CMCD @ Ftg 7 Structure of Cyclodextrin-Fluorene Complex greatest to were achieved from P-CD has

l,2-benzofluorene

which

molecular

size. These results indicate

that the mechanism for solubilization of PAH by cyclodextrins are different from solubilization of and surfactants Because

NOCs by cosolvents the cyclodextrin structure cavity has a fixed size , only the compounds

with a similar molecular

size and appropriate

could be included into the cavity. Other compounds

with a bigger or smaller size or inappropriate solubilization effect. Possible structures of p-

structure could only achieve partial inclusion, causing decreased cyclodextrin-phenanthrene inclusion complexes

and P-cyclodextrin-tluorene

inclusion complexes

are shown in

1304 Figure 6 and Figure 7. From Figure 6. we can see phenanthrene can be included in two ways In structure (a)

phenanthrene can tit into the P-CD cavity completely, which will give a equilibrium constant of the complexation. In structure (b) phenanthrene phenanthrene. can only achieve a partial inclusion. The structure of fluorene is different from

Because the length of fluorene molecule is greater than the diameter of the B-CD cavity, it could

only be included in one way as in Figure 7 Hence, it could only achieve a partial inclusion. That is why the solubilization power of D-CD and CMCD for phenanthrene for fluorene and 1methylfluorene. and 2methylphenanthrene are much greater than that

Wang’“’ has reported organic compounds experiments

that the solubilization

power of hydroxypropyl-P-cyclodextrin(HPCD) partition coefficients deference

to low polarity

was parallel to their octanol-water Structure

IogKow. The present result of our factor to the

does not verify this tendency

seems to be a more important

solubilization of PAHs by @CD and CMCD.

Difference

in solubilization

power

of P-CD and CMCD

on PAHs

Comparing

the Kcw of PAHs

solubilized by O-CD and that of CMCD reveals that the Kcw with B-CD is much greater than that with CMCD for the same PAH, which indicates that the solubilization power of P-CD is greater than its derivatives CMCD at the same concentration carboxymethyl This phenomenon may be explained by the special structure of CMCD. Substitution of

group at the outside of the cavity may impede the diffusion of PAH molecule into the cavity,

making the formation of inclusion complex difficult, decreasing the solubilization power of CMCD. Although the solubilization power of P-CD is greater than that of CMCD, unfortunately P-CD has only a limited water In this study the was IO g/L,

solubility of about I8 7 g/L at 23°C. and its complexes are thereby only slightly water soluble greatest solubilization of P-CD was achieved with 2-methylphenanthrene the water solubility of 2-methylphenanthrene has unlimited water solubility, its concentration When CMCD concentration

When D-CD concentration

was only increased about I8 fold. Comparing

with P-CD, CMCD

used for- solubilization of PAHs could be very high For example, water solubility of fl-bromonaphthalene could be

reaches SO g/L. the apparent

increased more than seventy times. Because CMCD could be easily synthesized from P-CD, it might be a better choice for the cleanup of PAHs in the environment

Conclusion
The results from this study indicate that P-CD and CMCD could significantly increase the apparent concentration water of PAH

solubilities of PAHs. Linear relationship was observed between the relative aqueous-phase and the concentration

of P-CD and CMCD for all PAHs studied. The solubilization power of P-CD and CMCD

to PAHs are dependent on the structure of PAH. The solubilization power of P-CD is greater than CMCD to the same PAH. Because of the solubility deference between D-CD and CMCD, CMCD is an attractive solubilization reagent for PAHs Further studies should be focused on removal of PAHs from soil by CMCD and the influence

1305 of CMCD on the biodegradation and photodegradation of PAHs

Acknowledgments
This research was financially supported by the National Science Foundation of China.

References
I. J. F.McCarthy. J.M.Zachara
2. D.E. Kile, C.T.Chiou Ettvirow. Ettvimtt. Sci. Twhtd 23,832( Techtrd. 23,496( 1989). 27.282 l-2825( 1989).

Sci. Techttol. ~tivirotr. ki.

3 Xiaojiang Wang, M.L.Brusseau 4. A. Villiers

1993)

C.R.Accad.

Sci. 112,536-538(1891) 17,207-209( 1904) 1988 D.D.MacNicol, Eds.; Academic

5 F Schardinger 6 J Szejtli

Wien. Klin. Wochenschr.

Cyclodextrin Technology;

Kluwer: Dordrecht

7 R.J.Bergeron.

In Inclusion Compounds,

Vol 3; J.L. Atwood, J.E.D.Davies,

Press. London, 391-443( 1984) 8 J Olah, T Cserhati. J.Szejtli 9 Rolf Kleine. P. Hamera Water Res. 22, 1345-1351(1988)

LaborPraxis

18(6), 28,30,33-35(1994)
Ettvirott. Sci. Techtd. Ski. Techrro~. 28, 952-956( 20,502(1986) 1994)

IO. M.L. Brusseau, Xiaojiang Wang, Qinhong Hu

11 C.T. Chiou. R.L. Malcolm, T.I. Brinton, DE Kile

Ettvirott.


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