Toxicological and Environmental Chemistry, Vol. 76, pp. 171-178 © 2000 OPA (Overseas Publishers Association) N.V.Reprints available directly from the publisher Published by license underPhotocopying permitted by license only the Gordon and Breach Science

Publishers imprint.Printed in Malaysia.

THE ADSORPTION OF PCBs TOYANGTSE RIVER SEDIMENT

HUALI YUANa, SHIFEN XUa,*, YONGRUI TANa, XIN JIANGb,JIANFANG FENGa, SHUOKUI HANa and LIANSHENG WANGa

a State Key Laboratory of Pollution Control and Resources Reuse,Department of Environmental Science and Engineering, Nanjing University,

Nanjing 210093, P.R. China; bInstitute of Soil Science,Chinese Academy of Sciences, Nanjing 210008, P.R. China

(Received 10 November 1999; Revised 3 January 2000)

The adsorption of six PCB congeners to natural sediments from the Yangtse River was inves-tigated. Experiments were conducted at different PCB concentrations to get the adsorptionisotherm for each compound. Analytical determinations were carried out using gas chromato-graphy with ECD detector. Results showed that the adsorption mechanism of PCBs on YangtseRiver sediment fitted the best to the Langmuir adsorption model. It was also found that theadsorption rate decreased as the solubility increased or the octanol-water partition coefficientdecreased for each compound.

Keywords: Adsorption; the Yangtse River; sediment; PCBs; GC-ECD

INTRODUCTION

Polychlorinated biphenyls (PCBs) are a group of ubiquitous contaminantsof environmental and toxicological importance, generally characterizedby high octanol-water partition coefficient. Because PCBs contaminantsintroduced into aquatic systems are hydrophobic, they can be transportedwith the suspended solids and eventually incorporated into the bottomsediments. Therefore studying the adsorption of PCBs to sediments is ofparamount importance for predicting the transport and fate of PCBs inaquatic environment.

* Corresponding author.

171

172 HUALI YUAN et al.

Concern has increased over the past decade with regard to ecotoxico-logical implications of PCBs because of their environmental persistence[1,2]. Meanwhile, a major concern has emphasized the adsorption of PCBsto sediment which is the secondary contamination of linked aquatic envi-ronment because the PCBs compounds absorbed by sediment might bereleased again into the environment or accumulate in organism, and finally inhumans [3,4].

The Yangtse River, the longest river in China, has a large run-off and itcan incorporate pollutants in large quantities. Thus using adsorption behav-ior as an example, we can explore the removal process of PCBs in water body.This work just mainly focused on the adsorption of PCBs to Yangtse Riversediment. Six studied PCB congeners, representing a wide range of chlori-nated degrees (from 3 to 7 chlorine atoms) that display differences in theirsubstitution patterns and physical properties, were selected for their potentialtoxicity and their occurrence in the environmental samples.

MATERIALS AND METHODS

All organic solvents (including hexane; nonane; acetone and dichloro-methane) used were of HPLC grade, and were obtained from Aldrich com-pany. The stock solution of each PCBs (including PCB28, PCB52, PCB101,PCB138, PCB153 and PCB180) was prepared in iso-octane at a concentrationof 10.0 mg/L. All PCB congeners were purchased from Dr. Ehrenstorfer,Germany.

The surface sediment (0-20 cm) was collected from the Yangtse Riverin eastern China. It was homogenized, air-dried and milled using zirconia-milling vessels. The resulting samples were stored in brown glass bottlebefore use. The organic carbon content of the prepared soil was 1.52%while the sediment pH was 6.57.

Batch tests were used to determine the isotherm adsorption betweenPCBs and sediment. Weighted sediment samples (1 g) were placed intolOmL vessels, together with lOmL of an aqueous solution of PCBs, thenwere capped. The samples were shaked for 24 h at 25°C to reach adsorp-tion equilibrium, then centrifuged at 3000 rpm for lOmin. Known amountsof supernatants were removed, and extracted with the 10:8 of n-hexane:dichloromethane solvent for three times. After dehydration with anhydrousNa2SO4 (2g), the extracts were concentrated to 2mL by rotary evapora-tion. Then the extracts were evaporated to 0.2 mL after addition of 150uLnonane, and were further concentrated to lOO^L using gentle nitrogenstream before GC-ECD analysis.

ADSORPTION OF PCBs TO YANGTSE RIVER SEDIMENT 173

The experiments were all made in triplicates. The seven concentrations ofPCBs solution ranging from 60% to 90% solubility were tested for eachadsorption isotherm. Moreover, adsorption blanks were made with sedimentand CaCl2 solution in the vessels, but no PCBs inside, and were used as abackground adsorption correction. Control experiments containing testchemicals but no sediment were conducted according to the above steps.

Gas chromatography was performed through the experiment with aHewlett-Packard 6890 GC system in the splitless mode. The 30 m x 0.32 mmi.d. HP-5 capillary column with a film thickness of 0.25 um at a pressure of50 kPa was employed to analyze PCBs. Helium was used as carrier gas at2.0 mL/min. The oven temperature was programmed as follows: set point at60°C and was immediate ramped to 140°C at a rate of 12°C/min, followed byramping to 280°C at 8°C/min, held for 5min. The injector and the detectorwere set at 220°C and 280°C respectively. Identification of the PCBs wasbased on comparison of their retention times with those of standard referencecompounds. An internal standard method was used and data analysis wasconducted on Chemstation (Window NT).

RESULTS AND DISCUSSION

Twenty-four hours were chosen as equilibrium time for the adsorption ofPCBs to Yangtse River sediment. No other influence factors in batch test wereprobed since change in factors such as pH should not affect the adsorption ofnonionic organic compounds on natural sediment.

The adsorption isotherms were constructed from adsorption data and theconformity to a linear model, nonlinear Freundlich equation and Langmuirequation, was checked. Table I lists molecular formulas of these investigatedcompounds. The adsorption results are shown in Table II.

It can therefore be concluded that the equilibration adsorption data werereasonably described by the above three conventional adsorption models, butit still turns out that the Langmuir equation fitted the best with its correlationcoefficients being the greatest of the three types. The results are consistentwith those described by Yuanhui Zhao and Peizhen Lang [5] that L-type wasfound to be the most suitable to study the adsorption of nonionic compoundsto natural sediments.

Table III lists the logarithm of adsorption coefficient (̂ TOc) of six PCBsfrom the Freundlich equations:

(1)

(2)

174 HUALI YUAN et al.

TABLE I Molecular formulas for six polychlonnated biphenyls

No. Compound No. Compound

1.PCB28 4. PCB138

2. PCB52 5. PCB153

3. PCB101 6. PCB180

TABLE II The adsorption coefficient and correlation coefficients of the adsorption of PCBsto Yangtse River sediment describing by three conventional sorption equations

No. Adsorption equations

Langmuir equation Freundlich equation Henry equation

123456

G°

88.5029.0771.9414.2759.5222.82

A

1.77 x2.91 x5.76 x5.71 x2.38 x6.84 x

10"'io-5

io-»10"6

io-4

io-'

r

0.9260.9730.9220.9900.9590.994

K

4.64 x1.98 x2.65 x6.46 x5.09 x7.45 x

IO2

IO3

104

IO4

IO4

10lu

n

4.9982.1071.4591.3460.9470.952

r

0.9630.9350.8970.9840.9460.976

K

1.45 x9.57 x5.17 x2.00 x2.72 x9.72 x

10'104

10'10'10'10'

r

0.8960.9080.8860.9640.9440.863

TABLE III Some descriptors of the six PCBs

No.

123456

l0g^ o w

5.626.096.807.627.62

12.27

log 5

-0.47-1.07-1.87-2.78-2.89-3.55

logtfoe

4.495.116.256.637.53

12.69

ADSORPTION OF PCBs TO YANGTSE RIVER SEDIMENT 175

The experimental water solubility and octanol-water partition coeffi-cients of PCBs cited from literature [6] were also given in Table III. Theinfluence of the chemical structures of PCBs on the adsorption processwas investigated. From the data shown in Table III for the investigatedsix PCBs, which differ in the chloro-substitution pattern and number, we canfind that as the molecular size increases with an increasing number of chlo-rine atoms, the adsorption coefficient Koc also increases. Actually the morethe chlorine substitution atoms of PCBs are, the easier the adsorption ofPCBs to the sediment is.

The relationship between \ogKov/, log S and logAToc were investigated.Regression analysis of these data yields the following equations:

log AToc = 2.704-2.095 logS(3)

n = 6 r = 0.842 SE = 1.771 F = 9.767 P = 0.035

log Koc = -2.254 + 1.221 \ogKoy/

(4)n = 6 r = 0.995 SE = 0.340 F= 368.672 P = 0.00043

where n represents the number of compounds, r is the regression coeffi-cient, F denotes the F-test value, SE is the standard deviation and P is thesignificance level. The Fand P values show that the correlation is significant.Due to the great r and small SE, the equations can be used to predict theadsorption coefficient of PCBs to the Yangtse River sediment under theexperimental condition.

Regression plots of log Koc vs. \ogKow and log Ace vs. log 5 for all testedcompounds are shown in Figure 1. It shows that the log^Toc is negativelyrelated to log 5 of PCBs whereas the log^oc is positively related to logATow.It also demonstrates that the adsorption capacities of PCBs are negativelyrelated to their solubility when they are adsorbed by sediments from theYangtse River and the adsorption capacities of PCBs increases as theoctanol-water partition coefficient increased for every compound.

Through comparison of the results shown in Figure 1, we find that thepartition effect induced by lipophility (or water solubility) is the main factorcontrolling the adsorption of organic compounds in water-sediment system,and that octanol-water system and water-sediment system are found tohave some qualities in common such as reaction mechanism.

By normalizing the Freundlich K constants to a 1 g, organic matterbasis (Table IV), adsorption characteristics of PCBs were illustrated in

176 HUALI YUAN et al.

> * . Regression

95% confid.

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

15

" » , Regression95% confid.

6 7 8 9 10 11 12 13

LogKow

FIGURE 1 Plot of log AToc vs. log 5 and log K^ vs. log Kow of six PCBs.

TABLE IV The change in free energy for adsorp-tion of PCBs to Yangtse River sediment

No. In Ac AG (J/mol)

123456

2.96 x 10"1.26 xlO 5

1.69 xlO6

4.11 xlO6

3.24 x 107

4.74 x 1012

10.2911.7414.3415.2317.2929.19

-2.55 x 10"-2.91 x 10"-3.55 x 10"-3.77 x 10"-4.28 x 10"-7.23 x 10"

ADSORPTION OF PCBs TO YANGTSE RIVER SEDIMENT 177

a different manner. Thus the new constant, Kom (Kom = Koc* 100/percentorganic matter), was averaged. With the assumption that the organicmatter was the main factor that influence PCBs adsorption to sediment, thechange in free energy for adsorption was calculated using the equationdescribed by Osgerby [7]:

AG = -RT\nKom (5)

where AG is the change of Gibbs free energy (kJ/mol), Kom is adsorp-tion constant per organic matter, R is the gas constant with the valueof 8.134kJ/mol and Tis the absolute temperature.

The changes in Gibbs free energy of adsorption of PCBs to sedimentsare shown in Table IV. From the table we can find the AG values of theadsorption are all negative, which indicates that adsorption of PCBs tosediments is exothermic reaction process and lower temperature should bebetter for the adsorption. Furthermore, all the AG values (except PCB180,this could be due to its low solubility that lead to the error of AG value)are smaller than 4.187 x 104 J/mol, indicating that the adsorption of PCBs tosediments is more or less physical nature of adsorption on the soil organicmatter surface.

CONCLUSION

The adsorption isotherms of 6 PCBs on Yangtse River sediment wereexplored. The results indicated that the adsorption behavior could be rea-sonably described by three conventional adsorption models such as linearmodel, nonlinear Freundlich model and Langmuir model, but it turned outthat Langmuir equation fitted the best with its correlation coefficients forall the experiments being the greatest.

Since log Koc is negatively related to log S of PCBs whereas the log ATOC

is positively related to logATow, the equations containing log S or logATow

can be used to predict the adsorption coefficient of PCBs to the YangtseRiver sediment under the experimental condition. It is reasonable thatwith the increasing chlorination of PCBs, the lower water solubility andhigher octanol-water partition coefficient could be found, thus the com-pound containing more chlorine atoms can be easily adsorbed to theSediment.

The changes of Gibbs free energy of adsorption of PCBs to sediments werealso studied in this paper. The results show that the adsorption of PCBs toYangtse River sediment is a spontaneous process.

178 HUALI YUAN et al.

References

[1] R.E. Alcock, P.A. Behnisch, K.C. Jones and H. Hagenmaier, Dioxin-like PCBs in theenvironment-human exposure and the significance of sources, Chemosphere, 37, 1457-1472(1998).

[2] S. Safe, Toxicology, structure-function relationship, and human and environmental healthsof polychlorinated biphenyls: Progress and problems. Environ. Health Prospect., 100, 259(1992).

[3] C. Pierard, H. Budzinski and P. Garigues, Grain-size distribution of polychlorobiphenyls incoastal sediments, Environ. Sci. Technol., 30, 2776-2783 (1996).

[4] E. Fattore, E. Benfenati, G. Mariani, E. Cools, G. Vezzoli and R. Fanelli, Analysis of organicmicropollutants in sediment samples of the Venice Lagoon, Italy, Wat. Air Soil Pollut., 99,237-244 (1997).

[5] Y. Zhao and P. Lang, Exploring the adsorption pattern of organic pollutants to naturalsediment using Langmuir model, Environ. Chem., 9, 21-24 (1990).

[6] X. Jin, Z. Cheng, N. Xu and H. Li, Organic Pollutants Chemistry, Tsinghua University Press,Beijing, pp. 36-42, 101-117 (1990).

[7] J.M. Osgerby, Sorption and Transport Processes in Soils. Society of Chemical IndustryMonogr., London, Vol. 37, pp. 63-78 (1970).