Pollutant and organic matter content in sediment particle size fractions

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1 Freshwater Contamination (Proceedings of Rabat Symposium S4, April-May 1997). IAHSPubl.no. 243, 1997 Pollutant and organic matter content in sediment particle size fractions MARCELL SCHORER Department of Hydrology, University of Trier, D Trier, Germany Abstract In a partly urbanized catchment to the south of Trier, Germany, selected sediment samples were separated into different size fractions ranging from <2 urn to 200 urn. Each sample fraction was analysed for heavy metals, poly chlorinated biphenyls (PCB), poly cyclic aromatic hydrocarbons (PAH) and organic carbon. The results show a linear correlation between heavy metal content and particle size. The organic content of the samples does not play an important role in heavy metal adsorption. In contrast, the organic micropollutants (PAHs and PCBs) do not correlate with the particle size distributions but with the content of the organic material. The maximum concentrations of organic micropollutants and organic carbon are not associated with the clay fraction but are bimodally distributed. The bimodal distribution can be explained by the presence of two types of organic material in the sediment. INTRODUCTION River sediments are a major potential sink for hydrophobic pollutants in the aquatic environment (Karickhoff et al, 1979; Means et al, 1980; Voice & Weber, 1983). The organic matter content of river sediment has been shown to be an important factor determining the extent of sorption (Means et al, 1980; Baughman & Paris, 1981; Karickhoff, 1981; Calmano & Fôrstner, 1996). In the literature, the occurrence of organic pollutants in river sediments has been correlated with the abundance of clay. It has been assumed that the efficacy of inorganic exchange sites of the clay and its associated organic matter are responsible for the amount and the behaviour of the sorbed substances (Karickhoff & Brown, 1978). Only a few studies discuss the behaviour of organic micropollutants and the influence of different grain size fractions. Even within these studies, the lack of standardization in particle fractionation methods aggravates the comparison and generalization of acquired data (Karickhoff et al, 1979; Readman et al, 1984; Umlauf & Bierl, 1987; Evans et al, 1990). Since sediments are potential sinks and sources of contaminants in the aquatic environment it is of overriding importance to determine the partition of pollutants to different particle size fractions, because different particle sizes exhibit different remobilization and transport processes. In the present investigation, sediment size classes ranging from <2 urn to 200 um were examined with the aim of determining the relationships between the organic material, clay content, and the individual and total PAH, PCB and heavy metal content in each particle size fraction.

60 Marcell Schorer MATERIAL AND METHODS Study area The study area, which comprises the 39 km 2 drainage basin of the Olewiger Bach, is located to the south of Trier in the western part of Germany.

2 60 Marcell Schorer MATERIAL AND METHODS Study area The study area, which comprises the 39 km 2 drainage basin of the Olewiger Bach, is located to the south of Trier in the western part of Germany. Devonian shales of the Hunsriick Mountains with quartz and diabase veins dominate the geology. Pleistocene terraces of the River Mosel overlie the solid geology in the northern part of the drainage basin. Land use is predominantly agriculture with grassland and arable farming. Settlement covers about 10% of the area. Runoff from several roads, effluents from small industries and untreated waste water from solitary farms influence river water quality. Sediment sampling Freshwater surface sediments from two sites were sampled weekly from August 1993 to December For sediment sampling, several concrete slabs (each 0.1 m 2 in area) were placed into the river at the sampling sites. Care was taken to keep the slab surface even with the river sediment. For a better understanding of the highly dynamic process of pollutant enrichment in sediments, it is necessary to sample frequently (Schorer et al, 1994; Schorer et al, 1995; Symader et al., 1994), and therefore material was collected on a weekly basis. Interactions with the underlying bed were prevented by the complete removal of the sediments from the slabs. Biogeochemical changes following deposition were minimized. The sediments were placed directly into solvent-washed, 2 1 glass bottles with glass cutting lids. Fresh samples were wet sieved (200 jam and 63 um) within 2 h. The fraction smaller 63 um was centrifuged to separate the sediment particles from the aqueous phase, and the and < 63 urn fractions were freeze dried. Particle size fractionation Selected sediment samples were separated into the following size fractions: <2 urn (clay), um (fine silt), urn (finer middle silt), um (coarser middle silt), urn (coarse silt) and um (fine sand). The um fraction was wet sieved and the different particle sizes < 63 urn were obtained using an elutriation system (Muller & Tisue, 1977; Umlauf & Bierl, 1987). The sedimentological and soil science literature abounds with particle size separation techniques, including wet and dry sieving, normal and centrifugally accelerated sedimentation in several media, filtration, ultrafiltration and elutriation. The choice of method depends on the demands of the particular case. Sieving cannot separate particles of <20 um, and centrifugation requires special equipment. Elutriation procedures seem to avoid these difficulties. Elutriation involves suspending the sample in an upward water flow. Particles that settle at velocities greater than the upward velocity remain in the separation

$Pollutant and organic matter content in sediment particle size fractions 61 chamber; those that settle at lower velocities are carried out upward to the next separation chamber.$

3 Pollutant and organic matter content in sediment particle size fractions 61 chamber; those that settle at lower velocities are carried out upward to the next separation chamber. The separation apparatus consists of a magnetic stirrer to keep the initial sample in suspension. A peristaltic pump, which can be set at different speeds and is linked to separation cylinders of varying sizes, provides the required upward velocities. The settling velocities for each particle size fraction can be calculated using Stoke's law. Since the settling behaviour of a particle is a function of its density, shape and other hydrodynamic parameters, an examination of the fractionation efficiency can be determined. For referencing, the grain size distribution of a small part of each fraction was determined using a laser particle analyser (CIS, Fa. GALAIS/LOT). Samples were dispersed in M Na 4 P and stirred for 1 h. After flushing the separation tubes with water, the peristaltic pump was activated. The separation is finished, when the cylinders become clear (after about 30 h). The isolated fractions in the cylinders are flushed through a valve at the bottom of the separation tubes and the finest fraction is centrifuged using a continuous flow centrifuge. All fractions were freeze dried and weighed. Analyses Each fraction of the sediment samples was investigated for selected heavy metals (Pb, Cu, Zn, Fe, Mn), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and organic carbon content. Heavy metals were analysed with an atomic absorption spectrometer (AAS), after decomposition under pressure with concentrated nitric acid. Nitrogen and organic carbon were determined with an element analyser. For the analysis of PAHs and PCBs, the samples were spiked with internal standards and were solvent extracted using Aceton/Hexan 1:1 in a Soxhlet system for 8 h. After rotary evaporation almost to dryness, the solvent extracts were purified by column chromatography. Identification and quantification were achieved by gas chromatography/mass spectrometry operating in the selected-ion-monitoring mode. RESULTS AND DISCUSSION Selected river sediments that have been taken weekly from August 1993 to December 1995 from two sampling sites were fractionated. Table 1 presents the percentage organic carbon content in each size fraction. Many previous field studies suggest that the organic matter content of river sediment increases with decreasing particle size (Evans et al, 1990). In the present study, highest organic matter contents were not found in the clay fraction, but in a bimodal distribution showing peaks in the fine silt and fine sand fractions. This bimodal distribution of organic carbon can be explained by the presence of different types of organic material in the sediment, which is reflected in its ratio. The organic material associated with different size fractions of the sediment in the Olewiger Bach has different origins. Condensed humic substances form coatings and complexes with particles in smaller fractions, while organic material in the larger fractions comprises floes of algae and small

4 62 Marcell Schorer Table 1 Organic carbon (%) and PAH and PCB concentrations Og kg' 1 ) in size fractions of selected sediment samples. Sample IR06 IR26 IR27 IR28 IR29 IR30 IR39 KG 06 KG 12 KG 39 KG 41 Parameter <2 ]\.m im ]xm (im im \xm

5 Pollutant and organic matter content in sediment particle size fractions 63 pieces of degraded plant material. Sorption behaviour is known to differ according to the origin and composition of the organic material (Chen et al., 1995; Flemming et al, 1996). Figure 1 plots the heavy metal content in the different particle sizes for selected samples. Heavy metals are more adsorbed by the smaller particles, especially clay, than by the coarser fractions. The organic content of the samples plays a minor role in heavy metal adsorption. In contrast to the organic content, the heavy metal content decreases continuously from the clay to the coarse silt fraction. In the fine sand fraction, however, there is a weak, but unexpected, increase in heavy metals which is an artefact of the fractionation techniques used in this study. Wet sieving without dispersion for the urn fraction does not remove all <63 um inorganic particles from the fine sand fraction, and these aggregate to floes of >63 um with Pb Cu KG 41 O O Pb O O Zn ^ Zn Pb Cu <s IR27 O O Pb b à Cu o ozn *A Zn Grain sizes (//m) Grain sizes (//ml Pb Cu ' O O Pb 4 A Cu o OZn Zn N / 20 IR Grain sizes dim) Grain sizes (//m) Fig. 1 Heavy metal content (mg kg" 1 ) of sediment size fractions for selected samples.

$64 Marcell Schorer organic material. This aggregated fine inorganic material is responsible for the increase in heavy metal content associated with the fine sand fraction.$

6 64 Marcell Schorer organic material. This aggregated fine inorganic material is responsible for the increase in heavy metal content associated with the fine sand fraction. Dispersion and particle size analysis of the latter revealed a significant portion of <63 um particles. The behaviour of total PAH and PCB contents, however, is different to that of the heavy metals. They show no correlation with the particle size distributions but are correlated with the abundance of organic material. Maximum contents of the organic pollutants and organic carbon are not found in the clay fraction but are bimodally distributed (Table 1). The PAH and PCB loadings do not vary in a simple way with particle size. For example, the PCB content of sample IR06 increases from the clay to fine silt fraction, decreases as the fractions coarsen to coarse silt, but increases again in the fine sand fraction (Table 1). The PAH content of sample IR06, however, increases from the clay to the coarse middle silt fraction, decreases to the coarse silt fraction but increases again in the fine sand fraction. In general, the PCB content (sum of the congeners 28, 52, 101, 138, 153 and 180) follows the organic carbon content so that PCBs are bimodally distributed with maxima in the fine silt and in the fine sand fraction. The number of possible sorption sites, in contrast to qualitative aspects of the organic material, seems to be decisive for the sorption of PCB. In comparison, the total PAH content (sum of the 16 PAHs of the US-EPA priority pollutants list without Naphthalene and with Benzo(e)pyren) demonstrate a different sorption behaviour. They are also bimodally distributed, but the first maximum occurs in the coarser middle silt fraction, in contrast to the fine silt fraction for PCBs. It is clear from Table 1, that the total PAH content and the ratio is similarly distributed between the different size fractions. Like the PAHs, the ratio increases from clay to the coarser middle silt fraction, decreases to the coarse silt fraction, and increases again in the fine sand fraction. It is not the highly condensed organic substances with an enriched aromatic fraction forming coatings and films on clay (Schmitt et al., 1996) and characterized by low values that adsorb the highest PAH content, but rather the less decomposed organic matter with high values, such as fragmentary plant material. Other studies of PAHs and sediments have found two general types of organic material with different affinities to PAHs in different particle size fractions (Evans et al., 1990). In the present investigation, the chemical composition and structure, and not the quantity of the organic material in sediments, are the decisive factors affecting sorption of PAHs. The distribution of PAHs and PCBs in the different grain sizes suggests that the time of adsorption of the pollutants onto particles is a leading factor. The PAHs represent a later period of adsorption, since they are adsorbed on growing organic material such as biofilms and floes with incorporated plant material. The PCB congeners, however, are adsorbed a long time before the PAHs are adsorbed onto the sediment particles. They are associated with highly condensed organic material, which is distributed regularly in all fractions. Figure 2 demonstrates the relationship between selected individual PAH and PCB compounds and organic carbon. Results from all samples collected from one sampling site (Irsch) were used to construct the graphs. Regression analyses (Table 2) show a highly significant linear relationship between individual PAHs and PCBs and organic carbon. The individual PCB congeners, such as PCB 138, show a stronger correlation with organic carbon than the individual PAHs (Phen, Flua, Chry, I(cd)P, B(ghi)P). For individual PAHs, it is noticeable that the scatter in the

7 Pollutant and organic matter content in sediment particle size fractions 65 Wkgl ' x x, * / x /x - x m xx 50' Phenanthren n org. C (%) org. C {%) (//g/kgl / x 50- % ^x x * / # V / * x *,/xx xj^x x/ lndeno(cd)pyren n org. C (%) org. C (%) org. C (%) Fig. 2 Relationship between selected individual PAHs and PCBs and organic carbon from the sampling site of Irsch. relationship with organic carbon decreases with increasing molecular weight (from Phenanthren to Benzo(ghi)perylen). The hydrophobicity, which increases with molecular weight, seems to be responsible for the stronger sorption on the organic matter of the sediment size fractions. The slope of each regression line however is

8 66 Marcell Schorer Table 2 Regression relationships between selected PAHs and PCBs and organic carbon at the sampling site of Irsch. Pollutant Regression equation Correlation coefficient Significance (%) Phenanthren Fluoranthen Chrysen Indeno(cd)pyren Benzo(ghi)perylen PCB 138 y = 44.86* y = 126.9x y = 58.47* y = x y = 29.12x y = 4.43x not dependent on the binding capacity of the organic matter. The supply or the absolute amount of each pollutant in the environment is responsible for the slope of the regression lines. Thus, the organic pollutant compounds with low regression slopes (I(cd)P, B(ghi)P) are present generally in low concentrations, while other pollutants with steeper regression slopes (Flua) occur in higher concentrations. CONCLUSIONS - The organic material content in the different particle size fractions is bimodally distributed with maxima in the fine silt and fine sand fractions reflecting two types of organic material. - Heavy metals increase with decreasing particle size, due to the higher number of inorganic exchange sites in the clay fraction. - The PCB content of the particle fractions is bimodally distributed and correlated with the organic carbon distribution. - PAHs show a comparable affinity to the organic material, but the chemical composition and structure of the organic material plays a leading role in the sorption process. The chemical and biological alteration of organic material and humic substances can change the adsorption properties significantly (Abbt-Braun & Frimmel, 1996). - There is a positive linear relationship between individual PCBs and PAHs and organic carbon. The particle size distribution plays a minor role for the adsorption of organic micropollutants. - The partition of pollutants to different size fractions is of overriding importance, because sediments could be a source for contaminants. On the one hand changing hydrodynamic conditions (e.g. flood waves) can mobilize and transport variable particle sizes from the sediment. Therefore, an enrichment of individual contaminants must be expected through this sorting of the particles. On the other hand, a resuspension of particle sizes with high pollutant contents into the water column can lead to an increased bioavailability. REFERENCES Abbt-Braun, G. & Frimmel, F. H. (1996) Interaction of pesticides with river sediments and characterization of organic matter of the sediments. In: Sediments and Toxic Substances (ed. by W. Calmano), Springer, Berlin.

9 Pollutant and organic matter content in sediment particle size fractions 67 Baughman, G. L. & Paris, D. F. (1981) Microbial bioconcentration of organic pollutants from aquatic systems a critical review. Critical Reviews in Microbiology 8, Calmano, W. & Fôrstner, U. (1996) Sediments and Toxic Substances. Springer, Berlin. Chen, J. H., Lion, L. W., Ghiorse, W. & Shuler, M. (1995) Mobilization of adsorbed cadmium and lead in aquifer material by bacterial extracellular polymers. Wat. Res. 29, Evans, K. M., Gill, R. A. & Robotham, P. W. J. (1990) The PAH and organic content of sediment particle size fractions. Water, Air, and Soil Pollution SI, Flemming, H.-C., Schmitt, J. & Marshall, K. C. (1996) Sorption properties of biofilms. In: Sediments and Toxic Substances (ed. by W. Calmano), Springer, Berlin. Karickhoff, S. W. & Brown, D. S. (1978) Paraquat sorption as a function of particle size in natural sediments. J. Environ. Qual. 7, Karickhoff, S. W., Brown, D. S. & Scott, T. A. (1979) Sorption of hydrophobic pollutants on natural sediments. Wat. Res. 13, Karickhoff, S. W. (1981) Semi-empirical estimation of sorption of hydrophobic pollutants on natural sediments and soils. Chemosphere 10, Means, J. C, Wood, S. G., Hassett, J. J. & Banwart, W. L. (1980) Sorption of polynuclear aromatic hydrocarbons by sediments and soils. Environ. Sci. & Technol. 14, Muller, R. N. & Tisue, G. T. (1977) Preparative-scale size fractionation of soils and sediments and an application to studies of Plutonium geochemistry. /. Soil Sci. 124, Readman, J. W., Mantoura, R. F. C. & Rhead, M. M (1984) The physico-chemical speciation of PAH in aquatic systems. Fresenius Z. Anal. Chem. 319, Schmitt, J., Gu, B., Schorer, M., Flemming, H.-C. & McCarthy, J. J. (1996) The role of natural organic matter as a coating on iron oxide and quartz. Arch. Hydrobiol. Spec. Issues Advanc. Limnol. 47, Schorer, M., Bierl, R. & Symader, W. (1994) Zeitliche Verânderung von Schadstoffgehalten in FluBsedimenten (Temporal variations of pollutant contents in river sediments) (in German with an English summary). Vom Wasser 83, Schorer, M., Symader, W., Nagel, N. & Udelhoven, T. (1995) Zeitliche Dynamik von Schadstoffen in mobilen FluBsedimenten (Temporal dynamic of pollutants in mobile river sediments). Short Communications of the German Water Chemistry Conference 1995, 15. Symader, W., Schorer, M. & Bierl, R. (1994) Temporal variations of pollutants in channel sediments. In: Variability of Stream Erosion and Sediment Transport (ed. by L. J. Olive, R. J. Loughran & J. A. Kesby) (Proc. Canberra Symp., December 1994), IAHS Publ. no Umlauf, G. & Bierl, R. (1987) Distribution of organic micropollutants in different size fractions of sediment and suspended solid particles of the river Rotmain. Z. Wasser-Abwasser-Forsch. 20, Voice, T. C. & Weber, W. J. (1983) Sorption of hydrophobic compounds by sediments, soils and suspended solids. Wat. Res. 17,

Diffuse Pollution Conference, Dublin 2003 EFFECT OF SALINITY ON HEAVY METALS MIGRATION AMONG DISSOLVED AND SOLID PHASES IN ESTUARY SEDIMENT

Diffuse Pollution Conference, Dublin 23 EFFECT OF SALINITY ON HEAVY METALS MIGRATION AMONG DISSOLVED AND SOLID PHASES IN ESTUARY SEDIMENT *TsaiL. J. 1, Yu.C. 1, Ho S.T. 2, Chang J. S. 1, Wu T. S. 1 1 Department