Do lagoon area sediments act as traps for polycyclic aromatic hydrocarbons?

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Do lagoon area sediments act as traps for polycyclic aromatic hydrocarbons?

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Author names and affiliations

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Dr Mauro Marini (corresponding author) National Research Council (CNR) Institute of Marine Science (ISMAR) Largo Fiera della Pesca, 2 60125 Ancona ITALY [email protected] tel +39 71 2078840 fax +39 71 55313

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Dr Emanuela Frapiccini National Research Council (CNR) Institute of Marine Science (ISMAR) Largo Fiera della Pesca, 2 60125 Ancona ITALY [email protected]

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1. Introduction

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Several natural and anthropogenic processes can lead to the formation of polycyclic

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aromatic hydrocarbon compounds (Wakeham et al., 1980), whose main inputs are

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pyrolytic and petrogenic (Means et al., 1980; Lipiatou and Saliot, 1991). Each source

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generates a characteristic PAH distribution pattern due to the different chemical-

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physical behaviour of these compounds (Mitra et al., 1999). PAH behaviour in a marine

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system is the result of different factors, such as PAH sources and physicochemical

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properties, water and sediment movement, size fraction and environmental conditions

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(Baumard et al., 1999; Wang et al., 2001; King et al., 2004). Through the study of the

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probable source of these compounds, it is possible to identify PAH distribution in a

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certain area (Baumard et al., 1998; Mitra et al., 1999; Franco et al.,2006). Once PAHs

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appear in the marine environment, they are present in the water column then, due to

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their high hydrophobicity and molecular mass (Mackay, 1991), they tend to accumulate

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in sediment and biota. In the marine environment they can be studied mainly in three

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matrices: water column, marine organisms and sediments. Sedimentary hydrocarbons

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have received special attention because these compounds are readily sorbed onto

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particulate matter, in fact bottom sediments are considered as a reservoir of hydrophobic

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contaminants (Medeiros et al., 2005). The level of PAH in sediments varies, depending

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on the proximity of the sites to areas of human activity and on the PAH biodegradation

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(Bihari et al., 2007). The study of these compounds is needed because they have shown

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differences in their stability, transport mechanisms and fate, because of their physical-

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chemical properties, distribution constants, half-life times and origin (Bouloubassi and

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Saliot, 1993). Various studies have been carried out on PAHs in Mediterranean and

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Adriatic marine sediments (Baumard, et al., 1998; Alebic-Juretic, 2011; Bouloubassi, et

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al., 2012), in particular, this work is focused on the Italian Adriatic coast, since it is

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characterised by the presence of several rivers that discharge organic compounds (Tesi

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et al., 2007) in the sea and by transitional areas such as lagoons. Coastal lagoons are

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vulnerable systems, located between the land and the sea, enriched by both marine and

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continental inputs and are among the most productive aquatic ecosystems (Nixon,

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1998). The coastal lagoon that has been examined in this study is the Lesina lagoon

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(Fig.1). This area has been frequently investigated in the last few years (Roselli et al.,

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2009; Specchiulli et al., 2009; Specchiulli et al., 2010; Lugoli et al., 2012; D’errico et

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al., 2013). However, the effects of the coastal lagoon characteristics on PAH sorption in

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sediment haven’t been studied yet. Up to now, several studies on the different sorption

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properties of the sediments, the sorption kinetics and the various influencing factors

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have been performed (Karickhoff et al., 1979; Barret et al., 2010; Yang and Zheng,

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2010). It has been demonstrated that the changes in salinity are significant for increase

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in equilibrium sorption constants (Means, 1995, Tremblay et al., 2005). Xia et al.

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(2006) have been focused on the PAH sorbed which increases with the sediment

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content. The purpose of this work is to understand the PAH behaviour in the lagoon

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areas through the determination of PAH distribution and PAH sorption. Specifically,

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how certain characteristics of the lagoon sediments such as particle-size, organic matter,

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salinity and vegetative sediments, may affect the PAH behaviour in the transitional

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areas compared to their behaviour in the open sea. For this reason two different areas

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have been compared: a closed transitional environment (Lesina lagoon) and a coastal

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marine environment (offshore Ravenna harbour) in order to see how PAHs behave

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before they reach the sea in crossing a transitional lagoon area.

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2. Material and methods

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2.1. Study areas

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The lagoon of Lesina (Fig. 1), situated on the Southern Adriatic coast of Italy (41.88 °N

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and 15.45 °E), is characterised by shallow water (0.7 – 1.5 m) and limited exchanges

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with the sea. Due to its shallow depth, the Lesina lagoon is strongly influenced by

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meteorological and climatic conditions, continental inputs and low tidal exchange. The

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lagoon is connected to the sea by two tidal channels: one to the west (about 2 Km long)

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and the other to the east (about 1 Km long) (Roselli et al., 2009). It receives freshwater

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inputs from urban wastewaters, intensive aquaculture and agricultural activities,

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determining a very important input of organic and inorganic contaminants, which cause

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eutrophication events, characteristic in the coastal lagoon (Specchiulli et al., 2009). To

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find out which characteristics of the lagoon affect the PAH accumulation in the

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sediment, the Lesina lagoon has been divided into two basins: a western and an eastern

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one, showing well known different hydrological and physical-chemical characteristics.

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Indeed, about 80% of the annual freshwater budget is discharged into the eastern part of

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the lagoon, consequently, a trophic and salinity gradient from the western to the eastern

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part of the basin was established (Roselli et al., 2009). For a better understanding of

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PAH behaviour in transitional environment such as the Lesina lagoon, also the

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accumulation, distribution of the PAHs in a coastal sea area sediment, offshore Ravenna

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harbour in the Northern Adriatic Sea, (Fig. 1) have been evaluated. This area has been

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chosen since it is strongly influenced by the contribution of fresh water that flows along

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the western Adriatic coast and by river water from the Po Valley (Marini et al., 2002;

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Campanelli et al., 2011). Some characteristics of the two study areas have been

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described in Table 1.

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2.2. Sample collection and preparation

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The Lesina lagoon marine sediments were collected in autumn 2010. Because of the

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Lesina lagoon shallowness and the high heterogeneity of the area, thirteen sediments

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were sampled: five in the western basin and eight in eastern one (Fig. 1). For a

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comparison purpose we also studied sediment samples taken in the western Adriatic

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Sea. Precisely, offshore Ravenna harbour (5 Km from Ravenna) six marine sediment

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samples were collected, in autumn 2010, by a box corer at a depth inferior to 20 cm.

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The sediment samples collected in both study areas were homogenized and were stored

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at –18 °C prior to process in the laboratory. Fig. 2 shows the sediment sample

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classification according to Shepard (1954). The sediment samples were air-dried and

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then 10 g of dry sediment was weighed with an analytical balance. For each sediment

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sample (Lesina Lagoon and offshore Ravenna harbour) the water content and the PAH

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concentration were defined. The sorption experiments were carried out in one Ravenna

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sediment (R1) and in two Lesina lagoon sediments (Les2 and Les9 respectively for the

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western and eastern basin). These sediment samples were chosen because they were the

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less contaminated ones (Fig. 3).

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2.3. PAH extraction and chemical analysis

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The PAH extraction from sediment samples was carried out with methylene chloride

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solvent (20 mL) by three cycles of 15 min each of ultrasonic baths. The PAH enriched

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solvent was centrifuged (1500 rpm for 15 min) and the suspended part was then

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removed by rotary evaporation (35 °C). The dry residue was recovered with acetonitrile

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(0.5 mL). This process was followed by the chromatographic analysis. The PAHs were

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analysed with a high performance liquid chromatography (HPLC Ultimate3000,

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Dionex). A mixture of PAHs was separated on a 4.6 x 150 mm analytical reverse phase

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column C16 3µm 120 Å. Eluting PAHs were detected with a fluorescence detector

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(RF2000, Dionex) for the quantitative analysis and together with (in line) a PDA-100

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Photodiode Array Detector for the qualitative analysis. Acenaphthylene cannot be

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analysed with fluorescence detection, so it is analysed with a PDA-100 Photodiode

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Array Detector. A mixture of acetonitrile and water (from 40:60 to 90:10), distilled and

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further purified by a Mill-Q system (Millipore, Billerica, MA, USA), was used as the

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mobile phase, delivered with a gradient program at 1.5 mL min-1 (IOC-Unesco, 1982).

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The detection limit (estimated as two time background noise) of the method was 0.04 –

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0.4 ng g-1 for PAH.

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2.4. Sorption to sediment

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The sediment samples of both study areas were employed for sorption experiments to

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evaluate the water-sediment distribution coefficient (Kd). For each sediment sample

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(R1, Les2 and Les9) a batch test was prepared using five different initial solute

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concentrations (559 mg L-1, 224 mg L-1, 112 mg L-1, 56 mg L-1 and 28 mg L-1). They

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were performed in the following ratio 1:10, 1:25, 1:50, 1:100 and 1:200 from a standard

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PAH solution (EPA 610 PAH Mix), using methyl chloride as solvent. The mass of dry

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sediment which was used in each batch test was of 1.0 ± 0.1 g with a final solution

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volume of 10 mL (Means, 1995), comprising 0.4% formalin to inhibit microbial

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activity. The samples, inserted into glass tubes without caps and sealed with parafilm,

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were equilibrated in a shaking table in the dark and at a temperature of 25.0 ± 0.5 °C.

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The equilibrium time wasn’t calculated by sequential sampling but according to the

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equilibrium achievement of the low-water-solubility compounds, generally achieved

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within 24 h (Karickhoff et al., 1979; Barret et al., 2011). After reaching the equilibrium,

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the glass tubes were centrifuged for 30 min at 3000 rpm. The PAH extraction from the

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aqueous phase was performed by methylene chloride, then a liquid-liquid separation

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was made. The solution was concentrated on a rotary evaporator and the dry residue

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was recovered with acetonitrile. Analysis of extracts were performed using HPLC as

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explained above (2.3). Blank samples containing sorbate solution but no sediment were

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also prepared in triplicate for each concentration. The quantity of PAH sorbed to the

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sediment phase in the samples was calculated by the difference between the PAH

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concentrations in the water phase in the blank samples and those from the sorption

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samples containing sediment (Kohl and Rice, 1999). Sorption isotherms were

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established for all the 16 PAHs. The curve was fitted by the three isotherm equations:

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the linear model, Freundlich model and Langmuir model (Trevisan et al., 1995;

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Businelli et al., 2000; Yang and Zheng, 2010). Kd represents the sorption capacity of the

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whole sediment. Instead, if only one characteristic of the sediment is considered, for

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example the organic carbon, Kd is substituted by Koc. Koc is the partition coefficient

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corrected for organic carbon content of the sediment (Means, 1995).

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2.5. PAH quantitation

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PAH quantitation was performed using the external standard calibration procedure.

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Calibration curves were established using a serial dilution (1:50, 1:100 and 1:200 with

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methylene chloride) from a standard PAH solution (EPA 610 PAH Mix), purchased

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from Supelco, Bellefonte, PA, USA. Standard PAH solution (1:1) contains a mixture of

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sixteen priority pollutant PAHs, with a known concentration. Methods employed were

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validated by intercalibration. Recovery rates were obtained for each individual PAH on

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two sediment samples certified for PAH: IAEA code 383 (IAEA/MEL/65, 1998) and

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IAEA code 408 (IAEA/MEL/67, 1999). These certified samples were extracted and

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analysed following the same procedure as for the sediment samples. PAH recoveries on

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sediment samples certified IAEA code 383 varied between 42% (for acenaphtene) and

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93% (for indeno[1,2. PAH,3-c, d]pyrene)recoveries on sediment samples certified

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IAEA code 408 varied between 51% (for benz[a]anthracene) and 88% (for anthracene),.

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The percentage standard deviations varied between 2% (for benzo[b]fluoranthene) and

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24% (for anthracene). All concentrations were expressed on a dry weight basis and no

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corrected with the recovery data.

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3. Results and discussion

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3.1. PAH molecular distribution

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An average PAH molecular distribution in the two study areas has been carried out to

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explain the contribution of each compound on total PAH load (Fig. 4a). The standard

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deviation of the relative percentage values has been calculated for the thirteen sediment

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samples of the Lesina lagoon and for six sediment samples of 5 Km offshore Ravenna

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harbour. Sediment samples of the Lesina lagoon have recorded that the PAH load is

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dominated by 4 ring compounds (Fig. 4b): fluoranthene and pyrene, which together

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have reached about 44% of the total. Three sites (Les4, Les7 and Les8) have revealed a

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PAH molecular distribution dominated by 5/6 ring PAHs, although lower total PAH

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concentration has appeared (≤ 100 ng g-1 d. w., Fig. 3). While, naphthalene,

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acenaphthylene, acenaphthene and fluorene have been recorded below detection limits.

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PAH molecular distribution obtained in this work may be to compared to other

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Mediterranean coastal lagoon, where the PAH group profile substantiates a

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predominance of high molecular weight over low molecular weight PAHs (Frignani et

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al., 2003; Culotta et al., 2006; Perra et al., 2009). A different PAH molecular

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distribution has been shown in offshore Ravenna harbour marine sediments, in fact, 2/3

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ring PAHs have been found in these sites, in particular, the phenanthrene compound has

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contributed with 21% to total PAH load; fluoranthene and pyrene compounds follow.

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3.2. Sediment PAH concentration

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The total PAH concentration was determined by the sum of each organic compound

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concentration (∑PAH) in the surface sediment layers of the Lesina lagoon and of 5 Km

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offshore Ravenna harbour. ∑PAH in Lesina lagoon sediments was found to vary

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between 4 – 4486 ng g-1 dry weight (mean 866 ± 1236 ng g-1 d. w.), besides, it varies

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quite widely between stations (Fig. 3). In the western basin of the Lesina lagoon the

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PAH concentration has appeared higher along the southern shore (Les1 e Les3). . These

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discharge areas have represented the dominant vector of PAH inputs in the lagoon. The

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site Les1 near livestock farms and fish farms follows with 1081 ng g-1. On the contrary,

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in the eastern basin of the Lesina lagoon the southern shore sediments have shown a

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lower PAH concentration (≤ 100 ng g-1 d. w.), except for Les5 (929 ng g-1 d. w.) and

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Les10 (1390 ng g-1 d. w.) that are located near drainage pumping stations, which may

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have increased the PAH level. Therefore, eastern basin central area stations have

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resulted as the ones with the highest PAH concentration, showing a PAH level of 1559

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and 1189 ng g-1 dry weight, respectively for Les11 and Les12. The total PAH

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concentrations obtained in this work are medium low compared with the other

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Mediterranean coastal lagoons (Specchiulli et al., 2009)

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∑PAH in offshore Ravenna harbour sediments has appeared lower than the lagoon one,

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showing a less variable range: 130 – 550 ng g-1 dry weight (mean 321 ± 187 ng g-1 d.

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w., Fig. 3). The main difference between the two study areas has been the 2/3 ring PAH

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concentration, dominant in all Ravenna stations suggesting probable oil inputs, but

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absent in the Lesina sediments (Fig. 4b). This may be explained by the nearness of the

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Ravenna harbour and by oil spill from boats, which could be responsible for the release

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of petroleum in the surrounding environment (De Luca et al., 2004; King et al., 2004).

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The Les 2, Les 9 and R1sediments were chosen because they were the less

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contaminated ones (Les 2 and Les 9 < 100 ng g-1 d.w. while R1 was 130 ng g-1 d.w.,

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Fig.3).

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3.3. Sorption studies

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From Table 2 it can be seen that the sorption isotherms of 16 PAHs are all well fitted

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with the three equation models. Since the fitting results of the linear isotherm model are

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the best, only the Kd values calculated with the linear model have been considered. The

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carried out sorption tests suggested that the Kd values changed depending on the

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different PAH compounds. For this reason, a distinction between lower molecular

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weight PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene and

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anthracene) and higher molecular weight PAHs (fluoranthene, pyrene, crysene,

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benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benz[a]pyrene,

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dibenz[a,h]anthracene, indeno[1,2,3-cd]pyrene and benzo[ghi]perylene) has been made.

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The higher molecular weight PAHs were sorbed more in sediments in comparison with

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the lighter ones (Witt, 1995). The sorption capacity of the lower molecular weight

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PAHs in various sediments follows this sequence: Kd eastern > Kd western > Kd

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offshore Ravenna harbour. While, the sorption capacity of the higher molecular weight

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PAHs follows this other sequence: Kd western > Kd eastern > Kd offshore Ravenna

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harbour (Table 3). This last sequence can be compared with TOC sequence: TOC

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western > TOC eastern > TOC offshore Ravenna harbour (Table 1). Indeed, only the

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higher molecular weight PAHs have shown a significant correlation between Kd and

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TOC (r = 0.998; n= 3; p < 0.05). Since the sorption capacity was correlated with the

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TOC, the Kd values of the higher molecular weight PAHs could be normalized to TOC

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(Means, 1995). By the Koc calculation it has been observed that the sorption capacity

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was affected by the TOC of the sedimentary matrix. Therefore, the TOC was the most

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significant factor which controlled the PAH sorption in sediment. The sediment Koc

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values of the eastern basin resulted higher compared with the western basin ones, for all

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the analysed PAHs (Fig. 5). While, the offshore Ravenna harbour sediments showed

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higher Koc values in lower molecular weight PAHs in comparison with the western

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basin ones, suggesting a greater molecular persistence in the coastal sediments.

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3.4. Sediment particle size and organic carbon content

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The partitioning of PAH in sediments is linked to several more or less strong

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correlations with different sediment textural features. It has been demonstrated that the

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concentration of PAHs in sediments was affected by the chemical composition of the

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samples such as the organic matter and water content (Kim et al., 1999). A more muddy

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sediment is characterized by high values of PAHs (Belahcen et al., 1997). Other studies

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underlined the role of grain-size fractions (Readman et al., 1982). The sorption results

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have generally accepted that organic carbon content (TOC) is important to control the

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accumulation of organic pollutants in sediments. Moreover, a positive correlation (r =

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0.839, n = 6, p < 0.05) between the concentration of total PAHs and TOC has been

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observed in the Lesina lagoon sediments (Les 3; Les 5; Les 7; Les 11; Les 12; Les 13).

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While, with regard to the correlations between the concentration of PAHs and the

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particle size (sand, silt or clay), the obtained values showed a correlation no statistically

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significant (r = 0.282, n = 13). This may be explained by a lack of correlation between

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pelite and organic matter and by the TOC distribution and by the presence of the

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different PAH sources in the Lesina lagoon.

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In the eastern basin central areas and in the nearest urban center site (Les3) the TOC

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increased. In fact, in the Les3 site high TOC equivalent to 5.28% was recorded, while,

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in the eastern basin central area a range between 3.80% – 4.67% TOC was observed

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(Specchiulli et al., 2010). Furthermore, in the highest TOC areas, a major PAH

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concentration was present (Fig. 4a). A different situation can be noticed in the sampled

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coastal marine environment, where offshore Ravenna harbour sediments have revealed

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a strong correlation between PAH and fine grain-size (r = 0.972, n = 6, p < 0.01). The

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TOC in offshore Ravenna harbour sediments ranges 0.83% ± 0.15 and increases

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offshore (Tesi et al., 2007). The highest PAH concentration was found in the R2 site

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and in the more offshore sediments (R4 and R5), where the result of the clay and silt

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percentage was above 90% (Fig. 4b). A positive correlation (r = 0.967, n = 6, p < 0.01)

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of the organic carbon with the finest sediment fraction was confirmed in this area. Thus,

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PAH accumulation has proved to be strongly associated with the finest sediment

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fraction. This strong correlation showed that clay or clay and silt have had a great

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influence on PAH distribution, confirming their preferential sorption to organic material

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and sediments with high clay percentage, as demonstrated by Zang et al. (2004).

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3.5. Salinity

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Salinity is one of the most fluctuating environmental factors that might affect PAH

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degradation and PAH accumulation in sediments (Tam et al., 2002). It is acknowledged

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that when the sea water salt concentration increases the PAH solubility decreases;

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causing a PAH transfer from the aqueous phase to the solid one (Xia and Wang, 2008),

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consequently the microorganism degrading skill decreases (Nedwell, 1999). The salinity

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variability is significant in a shallow lagoon environment. The Lesina lagoon salinity

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has been determined by freshwater input, precipitation, evaporation, morphology

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(Marolla et al., 1995) and the exchange efficiency of the tidal channels (Fabbrocini et

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al., 2005). In this work, according to Specchiulli et al. (2010) and Roselli et al. (2009),

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a lower salinity has been observed in the eastern basin of the lagoon (7 – 16, in winter)

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in comparison with the western basin (> 19, in winter). This probably happens because

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the eastern area of the lagoon receives the freshwater inputs, mainly along the southern

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shore, collecting agricultural drainage water from a pumping station located south of

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Lesina (Specchiulli et al., 2010). In both Lesina lagoon basins a lack of correlation

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between PAH concentration and salinity values has been recorded. Several physical

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conditions, such as salinity, may have caused the different microorganism adaptation

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patterns (Spain et al., 1980; Xia and Wang, 2008). Tam et al. (2002) have shown that

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the percentage of degraded phenanthrene has varied in relation to the different values of

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salinity, therefore, in the presence of high or low salinity degradation bacteria have been

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inhibited so, it can be presumed that this could happen also in the Lesina lagoon. In

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offshore Ravenna harbour sediments a strong relation between PAH concentration and

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salinity has been recorded (r = 0.816, n = 6, p < 0.05). So, it can be observed that, in

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this area, the salt gradient increases gradually from the coast to the open sea affecting

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the PAH concentration in offshore marine sediments (Marini and Frapiccini, 2013).

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3.6. Vegetative sediments

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The Lesina lagoon is characterized by a community of macrophytobenthos (Ruppia

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cirrhosa and Nanozostera noltii), mainly distributed in the eastern and central parts of

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the basin (Roselli et al., 2009). Here, the two sampled sites (Les11 and Les12) have

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shown a high total PAH concentration: 1559 and 1189 ng g-1, respectively. As

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demonstrated by Zhang et al. (2004) the highest PAH concentration has been recorded

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in vegetative sediment samples. PAH sorption in these central areas may be due to a

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higher presence of TOC values and clay contents in sediments with mangrove

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vegetation than in those without. However, the sorption results have shown how the

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organic carbon contained in the sediment was the most significant factor that controls

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the sorption, to the disadvantage of other less significant factors (particle-size) (Yang

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and Zheng, 2010; Hassett et al., 1980). Therefore, the PAHs discharged in the Lesina

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lagoon are probably sorbed more in vegetative sediments than in the ones without

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vegetation. A minor PAH accumulation has been observed in the Ravenna area since no

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eelgrass prairies are present there (Barletta et al., 2003).

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4. Conclusion

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The present study has compared two separate areas: a coastal lagoon (Lesina lagoon)

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and a coastal marine area (offshore Ravenna harbour) in order to evaluate the PAH

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behaviour in the marine sediments of both areas. It has been demonstrated that in a

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transitional environment such as the Lesina lagoon, where several factors depending on

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area heterogeneity, come into relation, the PAH distribution and sorption have been

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mainly affected by TOC in comparison with the particle size of the sediment. Through

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the Koc calculation, it can be observed that the eastern basin sediment has had a greater

353

sorption ability than the western one. This is due to a higher TOC in the eastern basin of

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the Lesina lagoon, which is also increased by the presence of vegetative sediments.

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These have enabled the PAH sorption in the eastern sediments. Salinity may be

356

considered a factor which affects the PAH behaviour also in the lagoon environment.

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However, comparing both study areas, the salinity gradient effect on PAH accumulation

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has appeared weaker in the lagoon sediments than in the coastal area ones.

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The relations observed between PAH distribution and sorption and the examined

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parameters (grain size, TOC, salinity and vegetative sediments) have resulted stronger

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in the coastal Ravenna marine area compared with the Lesina lagoon one. This is so

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because the transitional environments, as the Lesina lagoon, are greatly heterogeneous

363

areas with a high variability of the abiotic factors. The above-mentioned heterogeneity

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may become a confusing factor and contribute to the influencing of PAH behaviour. In

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fact, in these areas such behaviour is different compared with the well-known marine

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area PAH distribution patterns, confirmed in the offshore Ravenna harbour sediments.

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Therefore, the results have shown that transitional areas contribute to the increasing of

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the PAH accumulation in the sediment turning it into a trap for organic contaminants

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such as PAHs.

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Acknowledgments

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We would like to thank Raffaele D’Adamo for the sediment sampling in Lesina Lagoon

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and Antonietta Specchiulli to compare the total organic carbon values in the Lesina

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Lagoon. This research is supported by the Bandiera RITMARE Project - La Ricerca

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Italiana per il Mare – coordinated by National Research Council and financed by Italian

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University and Research Ministry, National Research Program: 2011-2013.

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