Hollow-Fiber Microporous Membrane Liquid-Liquid Extraction for Determination of Polybrominated Diphenyl Ethers at Trace Levels in Sewage Sludge with Gas Chromatography-Electron Capture Detection

doi:10.4236/jep.2011.22023

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Journal of Environmental Protection, 2011, 2, 198-203

doi:10.4236/jep.2011.22023 Published Online April 2011 (http://www.SciRP.org/journal/jep)

Hollow-Fiber Microporous Membrane

Liquid-Liquid Extraction for Determination

of Polybrominated Diphenyl Ethers at

Trace Levels in Sewage Sludge with Gas

Chromatography-Electron Capture Detection

Azadeh Sharifi Aghili, Jan Åke Jönsson

Center for Analysis and Synthesis, Department of Chemistry, Lund University, Lund, Sweden.

Email: Jan_Ake.Jon[email protected]

Received January 20th, 2010; revised February 3rd, 2011; accepted March 16th, 2011.

ABSTRACT

A two-phase hollow-fiber (HF) liquid-phase microextraction (LPME) method followed by gas chromatography was

developed for quantification of 8 major polybrominated diphenyl ethers at trace level in sewage sludge. In this method

the porous polypro pylene hollow fib ers filled with a few microliters of organic so lvent, were immersed in aqueous sam-

ples of wet sludge which were spiked with the PBDEs at ng/l level. Parameters such as salt concentration, stirring

speed, extraction time and pH were optimized and the optimum extraction conditio ns were then applied to the determi-

nation of PBDEs in sewage sludge from Källby sewage treatment plant in Lund. The optimized method allowed detec-

tion of 5.1 µg kg–1 and 0.43 µg kg–1 of BDE-47 and 183, respectively, in dried sludge. The findings were compatible

with the results from recent research on PBDEs level in sewage sludge samples from Sweden. Although BDE-209 was

expected to have the highest level, it was not detected. Limit of detection, photodegradation or/and biodegradation of

BDE-209 during treatment or experiment are major reasons. Low organic solvent consumption, low sample volume

requirement, high preconcentration factor, simplicity without using expensive instrument for extraction and excellent

sample clean-up are some important factors that make this sample preparation technique very useful for determination

of PBDEs in sludge.

Keywords: PBDE, Gas Chromatography, Sewage Sludge, Membrane Extraction, LPME

1. Introduction

Of all the pollutants released into the environment every

year, persistent organic pollutants (POPs) are among the

most dangerous chemical pollutants. These are used as

pesticides, used by industry, or generated unintentionally

as by-products of industrial processes. They accumulate

through the food chain in environmental and biota sam-

ples [1-9] and pose a risk of causing adverse effect to

human health and environment. Polybrominated diphenyl

ethers (PBDEs) are a group of persistent organic pollut-

ants, which are used as flame retardants in plastics in

consumer products such as computers, television sets and

polyurethane foam. Animal studies have shown that

PBDEs cause very serious health effects such as estro-

genic [10] and, neurobehavioral [8,11] effects, thyroid

dysfunctions [12] and cancer [13,14]. Exponential in-

crease of PBDEs in human [4] and biota samples as well

as their adverse health effects on animals have raised

concerns over potential health effects of PBDEs in hu-

mans and many studies have been conducted to deter-

mine these persistent pollutants in environmental sam-

ples such as sewage sludge and treatment plant effluents,

which are major sources of contamination by PBDEs.

Conventional methods for extraction of PBDEs from

sewage sludge such as Soxhlet extraction or accelerated

solvent extraction (ASE) need a clean-up step prior to

analysis by chromatographic methods and consume a lot

of time and extraction solvent, which leads to a signifi-

cant expense. Hollow fiber microporous membrane liq-

uid-liquid extraction (HF-MMLLE) [15] also known as

Hollow-Fiber Microporous Membrane Liquid-Liquid Extraction for Determination of

199

Polybrominated Diphenyl Ethers at Trace Levels in Sewage Sludge with Gas Chromatography-Electron Capture Detection

liquid phase micro-extraction (LPME) is an alternative

technique that can be used for extraction of contamina-

tion from different types of matrices.

Depending on the analyte hydrophobicity, a two or

three phase extraction system is applied. In two-phase

LPME that is also called microporous membrane liq-

uid-liquid extraction (MMLLE), the organic solvent

filled in the lumen and the pores of the hollow fiber acts

as acceptor phase and is used to extract hydrophobic

analytes from an aqueous sample which is donor phase.

The acceptor phase can then be analyzed with gas chro-

matography or HPLC in the normal phase mode. The

three phase system (aq/org/aq), or supported liquid

membrane (SLM) extraction, involves extraction of polar

compounds from an aqueous sample matrix, through an

organic phase in the pores of the hollow fiber into a new

aqueous phase inside the lumen of the hollow fiber.

Analytical techniques such as reversed phase HPLC and

capillary electrophoresis can be used for analysis of ex-

tracts from three-phase LPME [16,17]. In the present

study a two phase hollow fiber microextraction method is

applied to extract PBDEs from sewage sludge samples

and gas chromatography (GC) with electron capture de-

tector was used as final analysis. The aim was to propose

an inexpensive and simple method for environmental

laboratories, where thousands of samples are analyzed

annually.

2. Material and Methods

2.1. Reagents and Standards

The PBDE congeners 28, 47, 99, 100, 153, 154, 183 and

209 were obtained from Accustandard (BDE-EPA-SET;

New Haven, CT, USA) as stock solutions (50 µg mL-1 )

in isooctane. They were stored in refrigerator and pro-

tected against light. Acetone and n-undecane (analytical

grade) were from Sigma-Aldrich (Steinheim, Germany).

Sulfuric acid (> 95%) and sodium hydroxide were pur-

chased from Acros (New Jersey, USA) and Scharlau

(Barcelona, Spain) respectively. Sodium chloride was

from Merck (Darmstadt, Germany). Working solutions

were prepared by appropriate dilution of stock solutions

in acetone. All aqueous solutions for optimization pro-

cedure were prepared by using reagent water purified by

a Milli-Q system (Millipore, Bedford, MA, USA). A

standard solution containing 4000 µg·L−1 BDE-209 and

2000 µg·L−1 of the remaining seven BDEs was prepared

in acetone and was used to spike into aqueous sample

solutions containing sludge. Solutions were stored in

dark at 4˚C.

2.2. Instruments

Sample analyses were carried out using a Hewlett-Pack-

ard 6890 series (Agilent, Palo Alto, CA, USA) gas

chromatograph system equipped with a Hewlett-Packard

6890 autosampler and a DB-5MS fused silica capillary

column 15 m × 0.25 mm × 0.1 µm (J&S Scientific, Fol-

som, CA, USA) connected to a siltek deactivated reten-

tion gap from Restek Corp (Bellefonte, PA, USA) (5 m ×

0.32 mm) by a siltek deactivated universal glass press-fit

connector (Restek Corp, Bellefonte, PA, USA). The

temperature program was: 50˚C hold 2 min; rate 20 to

final temperature of 325˚C, hold 15 min. Injection was

made on-column. The injection volume was set at 3 µL.

Helium was used as a carrier gas at a flow rate of 30 cm/s.

The µECD (Hewlett Packard) temperature was 350˚C

and nitrogen was used as make-up gas at a flow of 60

ml/min.

The Q3/2 Accurel PP Q3/2 polypropylene hollow-fiber

membranes (HF) (200 μm wall-thickness, 600 μm inner

diameter, 0.2 μm pore size) obtained from Membrana

GmbH (Wuppertal, Germany) were used to extract

PBDEs from sludge. The effective length of the mem-

brane was 5.5 cm. LC Microsyringes with 50 μl volume

from Agilent Technology were applied to inject the or-

ganic solvent (n-undecane) in the lumen of hollow fibers

which were sealed from one side and to keep the mem-

brane in extraction solutions. The membrane pores were

impregnated un-decane by dipping the hollow fiber

pieces in it for 15 min. The solution pH was measured

with a 211 microprocessor pH-meter (Hanna instruments,

Kungsbacka, Sweden). A five-position magnetic stirrer

(RO 5 power, IKA-WERKE GmbH & Co. KG, Staufen,

Germany) was used to stir the samples during extrac-

tions.

2.3. Sample Collection

The sludge sample was collected during May 2009 from

Källby sewage treatment plant which treats sewage from

the city of Lund in the south of Sweden and kept in dark

condition at 4˚C until analysis. The waste water treat-

ment in STP consists of four steps: a primary mechanical

pre-treatment, a biological treatment step, a chemical

treatment step and a sludge treatment step. The aim of

mechanical treatment is to remove different types of

suspended solids. In the biological process activated

sludge which contains large amount of microorganisms is

added to waste water to degrade organic matter in water

and in chemical treatment process phosphate is removed

by addition of iron or aluminum salt. Finally in the

sludge treatment process, the sludge collected from the

first step (sedimentation) undergoes dewatering; an an-

aerobic digestion process and various dewatering proc-

esses for reducing the amount of sludge.

Hollow-Fiber Microporous Membrane Liquid-Liquid Extraction for Determination of

200

Polybrominated Diphenyl Ethers at Trace Levels in Sewage Sludge with Gas Chromatography-Electron Capture Detection

3. Results and Discussion

3.1. Analytical Challenges

Analysis of high molecular weight PBDE (nona- and

decaBDE) congeners poses some problems. These com-

pounds are sensitive to light and adsorb to glass surfaces

and small dust particles. In general, the photochemical

reaction rate increases with increasing number of bro-

mine substituent, so PBDE-209 is the most sensitive. To

avoid photochemical degradation, samples should be

kept in dark and UV filters may be placed at windows

and below fluorescent lights. In addition, all glassware

should be covered with aluminum foil to prevent adsorp-

tion of dust. They are also sensitive to temperature and

degrade at high temperature required for GC analysis.

Therefore relatively short columns should be used for the

GC analysis of nona- and decaBDEs [18].

The injection technique, type of retention gap, press-fit

connector and stationary phase also significantly affect

the yield of PBDEs from chromatographic systems.

Concerning these aspects the recommendations given in

the literature were followed. [19].

3.2. Optimization of Extraction Procedure

The optimization step was performed using four variable

factors, involving pH of donor phase, salt (NaCl) addi-

tion, stirring speed and extraction time. The choice of

organic solvent (n-undecane) as the acceptor phase was

based on selection of the best solvent with high stability

and selectivity in extraction of PBDEs (high hydropho-

bicity) in a previous study on environmental water [20].

To study the extraction process the variable taken as re-

sponse was extraction efficiency E, that is the ratio of the

number of moles of analyte in the acceptor phase (nA)

after extraction to that initially present in the donor phase

(nS).

Enn (1)

To evaluate the effect of parameters on extraction effi-

ciency, a series of 100 ml aqueous slurry samples con-

taining 1 g of wet sludge were spiked to final concentra-

tions of 0.100 µg·L−1 of BDE-209 and 0.050 µg·L−1 of

each the remaining seven BDEs and then pH was ad-

justed at 5.1 according to a preliminary experiment.

Spiked slurry samples were stirred for 5 min before

starting the extraction procedure. All determinations and

experiments were performed in triplicate and the pre-

sented results are the average values of three determina-

tions.

3.3. Influence of Ionic Strength

The use of salting-out effects may greatly enhance the

extraction efficiency of various organic compounds from

aqueous solutions and thereby sensitivity and precision

of their determination. This effect can be especially sig-

nificant for hydrophobic analytes, which bind well to

particles with high organic carbon content in sewage

sludge. In this study different amounts of NaCl (0 - 20%

W/V) were added to aqueous solutions containing sludge

and spiked with 0.100 µg·L−1 of BDE-209 and 0.050

µg·L−1 of remaining BDEs with pH adjusted at 5.1. Fig-

ure 1 depicts that optimal extraction condition was ob-

tained at 10% NaCl for all analytes.

Addition of NaCl to sample solution will alter the par-

tition coefficient of the analytes between dissolved or-

ganic matter content of sludge and water and also be-

tween water and acceptor phase based on the change of

ionic strength and viscosity [21]. Changes in ionic strength

can alter macromolecular structure of humic substances

(HSs) which exist in sewage sludge. At lower ionic

strength HSs have long and flexible structure with avail-

able region for binding to hydrophobic organic molecules

(HOMs) whereas at higher ionic concentrations HSs

forms rigid spherical shape molecules with the hydro-

philic surface and hydrophobic interior where is less ac-

cessible for binding of hydrophobic organic molecules

(HOCs). Therefore an increase in ionic strength results in

a decrease in sorption of HOMs [22,23].

Moreover the high content of salt in water affects hy-

dration and causes decrease in water solubility. It seems

that in 5% NaCl, the solution does not have sufficient

ionic strength required for efficient partitioning to the

organic solvent in the fiber and increased viscosity of the

aqueous phase negatively influences the kinetics of the

process. Lower extraction efficiency at higher ionic

strength (15% and 20% of NaCl) can also be related to

high viscosity and adsorption of analytes to glassware or

polymer [24]. Therefore, the maximum extraction effi-

ciency in selected concentration for experiments at 10%

of NaCl is due to the combined effects of ionic strength

and viscosity.

Figure 1. Effect of donor salt addition on E (%). Stirring

speed 1200, extraction time 60 min, pH 5.1.

Hollow-Fiber Microporous Membrane Liquid-Liquid Extraction for Determination of

201

Polybrominated Diphenyl Ethers at Trace Levels in Sewage Sludge with Gas Chromatography-Electron Capture Detection

3.4. Stirring Speed

The stirring speed has an important effect on transport of

analytes in LPME system thereby reducing extraction

time to reach thermodynamic equilibrium. The stirring of

donor phase was carried out from dial settings 4 to 10

(480 to 1200 rpm). Figure 2 shows the E value for all

PBDE congeners studied and indicates that extraction

efficiency decreases at lower speeds. Therefore, the

highest speed was selected as optimum for subsequent

experiments.

3.5. Extraction Time

The effect of extraction time was evaluated for slurry

solutions stirred with stirring at 1200 rpm and adjusted to

pH at 5.1 with 10% NaCl during 20, 60, 120, 180 and

300 min. As seen in Figure 3, the concentration in the

fiber increased with time for all PBDEs up to 60 min and

then remained rather constant during an hour, indicating

that equilibrium was attained. A decrease in extraction

efficiency observed during prolonged extraction time

most probably was due to solvent losses at the high stir-

ring speed (1200 rpm). On the basis of these finding, we

selected 60 min as optimum extraction time.

Figure 2. Effect of stirring speed on E (%). Extraction time

60 min, 10% NaCl, pH 5.1.

Figure 3. Effect of extraction time on E (%). Stirring speed

1200 rpm, 10% NaCl, pH 5.1.

3.6. Effect of Sample pH

The pH can be an important factor in membrane extrac-

tion especially when analytes of interest are ionisable. It

was found that sludge pH increases with stirring, espe-

cially at lower pH. So to determine the influence of pH

on extraction efficiency, the initial pH of samples was

adjusted to pH 4, 5, 6.6, 7.5 and 9 using H2SO4 and

NaOH. Figure 4 shows the effect of initial pH on the

extraction efficiency of all analytes. Samples with acidic

and basic pH showed lower recoveries but higher and

similar results were achieved for samples with pH values

of 5, 6 and 7.5. Lower recoveries at acidic and basic con-

ditions can be related to changes in sludge properties.

The chromatogram is shown in Figure 5. Among the

analytes, the more hydrophobic compounds (BDE-183,

BDE-209) showed a significant increase in extraction

efficiency when the pH was changed from 5 to 6.6.

Therefore pH 6.6 was selected as optimum pH for sam-

ple preparation of real samples.

3.7. Application to Real Sample

The extraction method developed in this study was applied

to sewage sludge from Källby sewage treatment plant in

Lund (Sweden). 100 ml of spiked and non spiked aqueous

samples containing 1 g of wet sludge (considering of 70%

water content) and 10% NaCl (w/v) were prepared and pH

was adjusted at 6.6 for each sample. Extractions were per-

formed with 5.5 cm polypropylene hollow-fiber at 1200

rpm stirring speed for 1 hour and the amounts of PBDEs

per dry weight were calculated on the basis of 70% water

in wet sludge using a standard addition calibration proce-

dure. It was found that the sludge contained 5.1 µg·kg−1 of

BDE-47 and 0.43 µg·kg−1 of BDE-183. No other conge-

ners were found. The relative standard deviation of the

results was about 15%.

Also, the extraction efficiency of the native PBDE in

the sludge was calculated from the standard addition data.

It was found to be 23% - 32%, which is compensated for

by the standard addition procedure.

Figure 4. The effect of pH on E (%). Stirring speed 1200

rpm, extraction time 60 min, 10% NaCl.

Hollow-Fiber Microporous Membrane Liquid-Liquid Extraction for Determination of

Polybrominated Diphenyl Ethers at Trace Levels in Sewage Sludge with Gas Chromatography-Electron Capture Detection

202

Figure 5. Chromatogram obtained by the developed method for sludge sample spiked with 0.100 µg L-1 of BDE-209 and 0.050

µg L-1of remaining BDEs. Stirring speed 1200 rpm, extraction time 60 min, 10% NaCl, pH 6.6.

An overview of distribution and levels of brominated

flame retardants in sewage sludge samples from 22

wastewater treatment plants in Sweden shows that the

concentration of PBDEs were in the range of n.d. –450

µg·kg−1 wet weight [25]. Previous studies on 9 sewage

sludge samples in Germany also showed total amounts of

tri- to hepta BDEs between 0.49 and 16.25 µg·kg−1 dry

weight sludge [26]. Extraction techniques applied to

these analyses were centrifugation and Soxhlet extraction

respectively, and the concentrations of PBDEs are com-

patible with the results obtained for BDE-47 and 183 by

membrane extraction in this study although BDE-209

which was expected to have the highest level was not

detected. The major reason may be the limit of detection

but photodegradation or/and biodegradation during treat-

ment or experiment are other explanations. On the other

hand there is a correlation between PBDE levels and vi-

cinity of industrial areas, especially textile and electrical

industries (which is not the case in Lund) as significant

local sources of PBDEs. This causes important variation

among different municipal wastewater treatment plants.

4. Conclusions

A simple and novel method has been developed for de-

termination of 8 major polybrominated diphenyl ethers at

trace level in sewage sludge. The method was based on

two phase HF-MMLLE for extraction and gas chroma-

tography analysis. The investigation was focused on the

optimization of the extraction parameters such as salt

concentration, stirring speed, extraction time and initial

pH of donor phase. The employed extraction method

minimizes the consumption of organic solvents (microli-

ters) which are expensive and toxic for environment. In

addition, cheap piece of hollow fiber, no additional

preconcentration prior to final analysis and no require-

ment for sample clean-up step by expensive instruments

make the method simple and more economic for deter-

mination of PBDEs in sewage sludge.

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