PH: S0273-1223(98)00195-4

8) Pergamon War. Sci. Tech. Vol. 37, No. 6-7, pp. 157-164, 1998.© 1998 IAWQ. Published by Elsevier Science Ltd

Printed in Great Britain.0273-1223198 $19'00 + 0-00

CHARACTERISATION OF PAH•CONTAMINATED SEDIMENTS IN AREMEDIATION PERSPECTIVE

M. P. Cuypers, 1. T. C. Grotenhuis and W. H. Rulkens

Department ofEnvironmental Technology, Wageningen Agricultural University,P.O. Box 8129, 6700 EV, Wageningen. The Netherlands

ABSTRACf

To suppon the choice of a remediation technique for the decontamination of PAH contaminated dredgedsediments a characterisation test is needed, that takes into account the physical state in which PAHs occur.Here, solvent extraction is proposed as such a characterisation test. Solvent extraction experiments wereperformed with a III acetone-water mixture on (particle size fractions of) PAH-contaminated harboursludge. The PAHs were more easily dissolved from fine than from coarse fractions. which indicates that thePAHs in the fine fractions are better (bio)available. The results further suggest that PAHs were present inpanlculate form. absorbed in organic matter and dissolved in oil. The performed extraction experimentsprovide a basis for funher development of existing models. © 1998 IAWQ. Published by Elsevier ScienceLtd

KEYWORDS

Characterisation; PAH; particle size distribution; physical state; remediation; solvent extraction.

INTRODUCTION

In the Netherlands approximately 500 million cubic meters of sediment have been or still have to be dredgedfor maintenance and remediation purposes between the years 1991 and 20 IO. Thirty percent of this sedimentis slightly to heavily contaminated (Hofstra, 1995), in many cases with PAHs.

If PAH concentrations are too high to allow reuse, dredged sludges can either be cleaned or disposed of inControlled storage sites. Permanent disposal is no long-term solution, since large amounts of sediment willhave to be dredged in the near future, Therefore, decontamination is an important sanitation alternative.

At present, a number of techniques are available for the decontamination of PAH-contaminated sediments.These techniques focus either on concentration of PAHs in a limited volume or on chemical ormicrobiological conversion of the contaminant. Of these techniques, classification/polishing andlandfarming are operational (Stokman, 1995). Some other techniques successfully removed PAH insituations varying from lab-scale to full-scale operations. Among these are thermal treatment (KEMA,1996), microbiological slurry processes (Oostenbrink et al., 1995), flotation (Bruning, 1995), wet oxidation(Rienks et al.• 1995), and solvent extraction (Koken et al., 1996).

157

IS8 M. P. CUYPERS tit al.

Unfortunately, characteristics of different contaminated sediments usually vary considerably. Consequently,it is not necessarily so that a remediation technique which is successful for one sediment will be equallysuccessful for other ones. Therefore, a characterisation test is needed which can support both the selection oftreatment technologies and the choice of proper process conditions. Nowadays, the importance of such a testis widely recognised (Pruijn and Groenendijk, 1993; Stokman, 1995; Rulkens and Bruggeman, 1996), whichstimulated research in the field of sediment characterisation (Feenstra et al., 1995; Joziasse et al., 1995).

In general, characterisation of contaminated dredged sludge focuses both on the contaminant and on thesediment itself. Characterisation usually includes measurement of the contaminant concentration, the organicmatter and carbonate content, and the particle size distribution. Sometimes, the distribution of organic matterand contaminants over the particle size fractions is also measured.

A characteristic that, to our knowledge, is rarely taken into account for the characterisation of PAH•contaminated sediments is the physical state of the contaminants. The physical state might be a main factorin determining the clean-up possibilities, especially in biological treatment processes,classification/polishing, flotation processes, and solvent extraction. PAHs can be present in 6 basic forms: asa particulate pollutant, as a liquid film, adsorbed to sediment particles, absorbed in organic matter, dissolvedin pore water, and as a solid or liquid phase in pores (Rulkens and Bruning, 1995; Rulkens et al., 1996).Additionally, PAHs can be present in thin oil layers around sediment particles if the sediment contains largeamounts of oil. Until now, no standard procedure has been available that can reveal the physical state ofPAH contamination. In this report we propose controlled solvent extraction as a proper method.

The application of a solvent extraction process for characterising contaminated sediment is illustrated in Fig.I. The figure shows that the PAH extraction efficiency gives a qualitative indication of the PAH availabilityin different particle size fractions. The availability can be quantified by linking the dissolution-time profileto theoretical dissolution models which are based on the physical state of the contaminant. Ultimately, theintegration of extraction results and the particle size distribution enables prediction of the sedimentremediation perspective.

PAll contaminated sediment

tfractionation

tsolvent extraction

/ ~extraction efficiency dissolution kinetics

+ + ~theoretical dissolution models

quantitative indicationavailability

~

qualitative indicationavailability,,

,,~

Remediation perspectiveFigure I. Application of solvent extraction for the characterisation of PAH-contarninated sedimenL

PAH solubilisation in a solvent extraction process can be mathematically described for particles with a weIl•defined diameter if experiments are carried out under specific hydrodynamic and environmental conditions.The mathematical description strongly depends on the physical state of the PAH contaminants (Rulkens andBruning, 1995; Rulkens et aI., 1996). Table 1 shows the dissolution time for PAHs in 4 different physicalstates. It should be noted that the particle diameter (RO>, the organic matter diffusivity (D m>, and the PAHsolubility (Cs) are important factors which strongly influence solubilisation time. Furtherm~re, the choice ofa certain value for the parameters P$' 0 1, 0om' and Cs is not straightforward because PAHs are mostlypresent as a mixture of components.

Charaterisation of PAH-contaminated sediments 159

In this research, we investigated the applicability of solvent extraction for the characterisation of dredgedsediments. For this purpose experiments were performed on sludge from the Amsterdam Petrol Harbour (PHsludge). Characterisation starts with a brief investigation of the history of the spot, because the processesthat caused contamination are expected to relate to the physical state in which a contaminant occurs.Furthermore, the distribution of mass, organic matter, and PAHs over the particle size classes is determined,after which two size fractions were further investigated by solvent extraction. The solvent extractionexperiments primarily aim at a qualitative assessment of PAH availability in remediation processes, thusgiving a first indication of the remediation perspective of dredged PH sediment.

Table I. Dissolution time of PAHs in different physical states

physical state

adsorbed film

pIR PAH pa1icle

!lOUd in pore

absorbed 10 OM

dissolution time conditions

Re<1

Re<1

k» D1

L

Dk»~

Ro

t.,. =solubilisation time (s)tw.99%= time needed for 99.99"10 solubilisation (s)PI =density solid PAH (kgIm')Ro =particle tadius (m)d= film thickness (m)L =pore length (m)n. = molecular diffusivity in liqUid phase (rrrls)Dom =molecular diffusivity in organic matter (ml/s)

C. - solubility ofPAH (kglm')k - mass transfer coefficient in liquid film around particle (mls)

Assl.ll'lljXions: pollutant particles are spherical; JXlI'OOity ofporous soil particles is equal throughout the particle; the tortuosity ofthepores is equal throughout the particle; PAH bulkcorv:.entration remains:zero; mixture ofPAHs is regarded as one PAH comporentwith constant diffusivity both in particle am in water (Ru1kens & 8rlJ1ing, 1995; Rulkens et aI. 1996).

MATERIALS AND METHODS

Petrol Harbour sJudl:e (Feenstra et af.. 1995: Tiihuis and Godefrooij. 1995)

The investigated sludge was dredged from the Amsterdam Petrol Harbour, which was constructed forstorage and transshipment of petroleum and coal. Around the Petrol Harbour, industrial activities developedand oil tanks were built. At the beginning of World War II, oil storage tanks were destroyed and largequantities of oil leaked into the harbour, causing major oil contamination of the sediment. Diverse othersources, such as industrial discharges, shipping, and tanker cleaning have also contributed to contaminationof the sediment. As a consequence, PH sludge is contaminated with oil, PAHs, and heavy metals. Typicalconcentration ranges are one thousand to ten thousand mg/kg (dry matter) for oil and a few hundred to a fewthousand mg/kg (dry matter) for PAHs.

PH sludge has an organic matter content of about 10% and contains a large amount of small particles (>50%smaller than 20 ~m). Tar particles were present in all size fractions.

fractionation and analysis of fractions

PH sludge was separated into JO particle classes (<32 ~m, 32-45 ~m, 45-63 ~m, 63-90 ~m, 90-125 ~,125-200 ~m, 200-5OO~, 500-1000 ~m, 1000-2000 ~m and >2000 ~m) by wet sieving. The dry matter andorganic matter content of the fractions were determined by weight loss during heating. First, samples weredried for 24 h at 104·C. Second, organic matter was oxidised for 5 h at 6OO·C. Distribution of dry matter(DM) and organic matter (OM) over the sieve fractions was calculated. PAHs were extracted from thesediment fractions with N-methylpirrolidinone (NMP), following a method described by Noordkamp et al.

\60 M. P. CUYPERS et al.

(1997). Approximately 4 g of wet sludge was extracted with 22 ml of NMP for one hour at 130·C. Theextractions were performed in a microwave oven (CEM, MDS-21(0). Extracts were centrifuged (5 min,10000 r.p.m.) and analysed for 16 EPA PARs by photo diode array detection, after separation over an HPLCcolumn (Vydac 5, CI8 Rev. phase, 250x4.6 mm).

Solyent extraction

Raw PH sludge and material from the particle classes 45-90 J.lm and 125-200 J.lm were extracted with a 1/1(volJvol.) acetone-water mixture. Extractions were performed in 500 ml vessels which were filled with theacetone-water mixture and wet sludge (fractions) in such a way that the vessel contained 400 ml of solvent.The solid/solution ratio was In.5 glml in each experiment. The suspension was stirred intensively (900r.p.m.; Euro ST-P-DV, IKA Labortechnik), and the increase of the PAR concentration in the solvent wasmonitored by frequent sampling (1.5 ml) and subsequent HPLC analysis. During the 12Q-minuteexperiments the temperature of the slurry was kept at 20·C. Extractions were performed in duplicate.

RESULTS AND DISCUSSION

Fractjonation

Figure 2 shows the dry matter distribution (2a), organic matter distribution (2b), and PAH distribution(2c+d) over the particle size fractions of PH sludge.

Figure 2a shows that the (dry) mass distribution over the particle size classes is clearly inhomogeneous. Thefractions <32 J.lm and 125-200 J.lm contain a major part of the total mass, which is in agreement with themass distribution in PH sludge as found by Feenstra et al. (1995). The particles >500 J.lm contribute onlymarginally to the total mass.

Figure 2b shows that the organic matter content of the dry material differs for the different particle sizefractions. OM is lower than 10% in the size range 63-500 J.lm, it varies between 10% and 17.5% for particlessmaller than 63 J.lm, and it is 20% to 35% for particles larger than 500 J.lm. The organic matter content intotal PH sludge is 10.8%. Here it should be noted that the use of a loss-on-ignition method for thedetermination of OM will generally lead to an overestimation of OM because this method also causes thedisintegration of carbonate and the evaporation of sulphides and crystal water. Carbonate, which contributesapproximately 5.8% to the mass of PH sludge (unpublished data), may have disturbed the OMmeasurements. Feenstra et al. (1995) corrected their loss-on-ignition data on PH sludge by multiplying OMwith a factor 0.9. Their uncorrected results correspond well to the data in Fig. 2b.

Figure 2c shows the PAH concentrations in the ten particle size fractions. In the fractions with particlessmaller than 200 J.lm the concentrations are 2Q00-4ooo mglkg. In the fractions with larger particles (>200J.lm) the concentrations are 7000-16000 mglkg. The average PAH concentration in PH sludge is 2603 mglkg.It is remarkable that the coarse fractions have high PAH concentrations, since it is generally assumed thatthe sand fraction is relatively clean. Most likely, coarse tar particles are responsible for the high PARconcentrations. Such particles were actually observed by stereo microscope (40x magnification), and theirpresence in PH sludge has also been reported by Feenstra et al. (1995).

Comparing the PAH concentration distribution in Fig. 2c and the OM distribution in Fig. 2b we concludethat the PAH concentration did not correlate well to the organic matter content. Linear regression showed alow r2 of 0.22. which indicates that absorption of PARs in OM does not dominate speciation, as is mostlythe case in soil and sediment material (Karickhoff et al., 1979; Chiou, 1990). Therefore, a considerableamount of the PAHs must be present in other physical states. Obviously, some of the PARs are present inthe particulate form. Besides that, it may be expected that some of the PARs are dissolved in oil, which is amain co-contaminant of the sediment.

Charaterisation of PAH-contaminated sediments )6)

(A) OM (% of total OM) (B) OM (% of OM)

45 3540 303530 25

25 20 -

20 15 -15

10105 5-

0 0~ on C') S on 8 ~ § § § N on C')

~on 8 8 § § §~ :g N C')

~ :g NV M~ :J: v ... :J: on

C') • ID

~ ~N N • 8 8 ~ ~

N

~ ~ " ~ "N ...particle cIa.. (micrometer.) particle cl... (micrometer.)

(C) PAH concentration (mglkg OM) (0) PAH distribu~ion (% of total)

16000 3014000

2512000

10000 20

8000 156000 -

104000

2000 5-

0 0~

It)

~ Sl It) 8 ~ § § § N on C') Sl on 8 § § § §"'t N C') "'t :g NV~ 13 ~

N V 13 ~N• oil 8 ~

N N N oil 8 ...~

N

~ " C') •~ "~ N ~ N

particle cl... (micrometer.) particle cl... (micrometer.)

Figure 2. Distribution of dry matter, organic matter and PAHs over the particle classes: (A) dry matter; (8) organicmatter; (C) PAUs, concentration (16 EPA PAHs); (D) PAHs, distribution of tota) amounL

Figure 2d shows the distribution of the total amount of the PAHs over the particle size fractions.Approximately 75% of the PARs is concentrated in the size fractions <32 J.1m and 125-500 J.1m, whichcontain 65% of the dry matter.

Combining the data in Figs 2a-d, we conclude that it is not possible to produce a relatively clean sedimentfraction containing a significant part of the dry matter by separating certain size fractions from the bulk ofPH sludge. However, separation of coarse and fine fractions can still be beneficial, because coarse fractionsmay be relatively simply polished in upflow or flotation processes if PARs are mainly present in particulateform, dissolved in oil, or absorbed in organic matter.

SOlyent extraction

For controlled solvent extraction the fractions 45-90 J.1m and 125-200 J.1m were selected. Criteria forselection were: a significant contribution to the total mass (>10% of DM), a difference in organic mattercontent between the two fractions, and an adequately high PAH concentration.

162 M. P. CUYPERS et al.

A

;.2&3 rings

104 rings65&6 rings

x total

120100

x

o6

•

80

••lEI

IJ,.

60time (min)

x

o•

40

x

o6

•x

•o6

20

x

Cii 400 ,...----------------------;::r300 r~200 x.o ,.

~ 100 ~ 0

~ 0 1&6 6o

B

.2&3 rings

:04 rings

65&6 rings

x total

12010080

o

•

60time (min)

•

40

••

20

Ci 1400 ,...-----~--"V""*:---:x:------------,~ 1200 x x x5 1000 >!'~ 800 x••5 600 ~~400~ 000 ID:r 200 Z 6 & 6~ 0 -WIID.IilI!!:-.:~-=6:....-=6---=---.;;...------_----'I

o

c

.2&3 rings"

04 rings

65&6 rings

x total

120100

x

80

xx

40

x

20 60time (min)

Figure 3. Amount of PAHs (EPA 16) extracted from PH sludge (fractions), during extraction with III acetone•water mixture: (A) 125-200 11m; (B) 45-90 11m; (C) tota! PH sludge. Initial PAH concentrations in the samples were

5720 mglkg, 3870 mglkg and 3990 mglkg, respectively.

~2000

51500 ~ x x1:1

~1000 xo r+ • •• •• •Ul

-8500 DO 0 0 0

~ 0 ~~-'iL-.......--'lo.-_--=6::...._ __:;:6:....--.......-.....;:;;6-......---~o

The choice for a 1/1 acetone-water mixture was based on the expected PAH extraction kinetics in thissolvent. We expected that extraction in a 1/1 acetone-water mixture is neither very fast nor very slow, whichallows accurate observation of the PAH solubilisation.

The PAH desorption during extraction with a 1/1 acetone-water mixture is shown in Fig. 3. Thesolubilisation of low (2 and 3 ring), middle (4 ring), and high (5 and 6 ring) molecular weight PAHs isdistinguished.

Figure 3 shows that PAHs are more easily dissolved from the total PH sludge and the size fraction 45-90 Ilmthan from the size fraction 125-200 Ilm. The extraction efficiencies were 55.7%, 33.7%, and 6.7%,respectively. The amount of 2 and 3-ring PAHs that were dissolved was always higher than the amount of 4•ring PAHs, which in turn was higher than the amount of 5 and 6-ring PAHs. This follows the initialdistribution of the PAHs over these groups in the sludge (fractions), which was approximately 70%-25%•5%.

Charaterisation of PAH-contaminated sediments 163

Comparing the extraction efficiencies for the fractions 45-90 ~m and 125-200 ~m, we concluded that thePAHs were clearly more available in the fine fraction. The fact that PAHs were even more available in thetotal PH sludge can be explained by the presence of a relatively large amount of particles smaller than 32~m.

Considering the differences in extraction efficiencies between the particle size fractions, we expect that inmicrobial remediation processes a higher PAH removal efficiency can be attained for fine fractions. Theremoval of coarse fractions from PH sludge may thus result in higher treatment efficiencies in biologicalprocesses.

The results further indicate that the low molecular weight PAHs will be more easily available in remediationprocesses than high molecular weight PAHs. It has in fact been shown in a slurry decontamination processthat low molecular weight PAHs are preferentially removed and that the degradation rate of PAHs decreaseswith the number of aromatic rings (Steffess et al., 1995).

In order to reveal the physical state of the contaminant, we studied the extraction kinetics more closely. InFig. 3 all curves show a rather similar pattern of PAH solubilisation: a fast initial increase in the PAHconcentration, followed by a slower increase in, and in some cases a stabilisation of the concentration.Because we expected that PAHs were absorbed in organic matter, or present in tar particles, or dissolved inoil, the fast initial desorption is likely to be caused by the preferential dissolution of PAH present in an oilfilm around the sediment particles. The difference between the quantities of easily available PAHs in theparticle classes is then caused by the difference in the amount of oil present, which is possibly related to thespecific surface area. Furthermore, wet sieving has lowered the amount of oil in the two fractions, comparedto the amount of oil in raw PH sludge.

The dissolution of particulate PAHs and PAHs absorbed in OM is slower than dissolution from an oil filmand could only have been completed for particles considerably smaller than 45 ~m.

CONCLUSIONS

The preliminary experiments show that solvent extraction of the sediment size fractions can be used to givea qualitative indication of the availability of PAH contaminants and, based on this, of the physical state inwhich PAHs occur. Solvent extraction can thus be applied for the characterisation of dredged sediments andmay enable a better founded choice for a specific remediation technique and the conditions under which itshould be applied. Integration of the results from solvent extraction and the particle size distribution gives amore profound insight into availability of PAHs than a single extraction of the total sediment can do.

The extraction experiments provide an interesting basis for further model development, allowing a morequantitative determination of the distribution of PAHs over the physical states. Future experiments willfocus on longer dissolution times and lower solid solution ratios. Hydrodynamic conditions will be includedin the model in more detail.

ACKNOWLEDGMENTS

This work was financially supported by the Dutch Foundation for Applied Research on Water Management(STOWA) and the Dutch Organisation for Applied Scientific Research (TNO). The authors thank FrankBoelsma and Mark Koops for their technical assistance.

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