Article pubs.acs.org/est
Capping Efficiency of Various Carbonaceous and Mineral Materials for In Situ Remediation of Polychlorinated Dibenzo-p-dioxin and Dibenzofuran Contaminated Marine Sediments: Sediment-to-Water Fluxes and Bioaccumulation in Boxcosm Tests Sarah Josefsson,†,# Morten Schaanning,*,‡ Göran S. Samuelsson,§ Jonas S. Gunnarsson,§ Ida Olofsson,§ Espen Eek,∥ and Karin Wiberg†,⊥ †
Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden Norwegian Institute for Water Research (NIVA), Gaustadalléen 21, NO-0349 Oslo, Norway § Department of Systems Ecology, Stockholm University, SE-106 91 Stockholm, Sweden ∥ Norwegian Geotechnical Institute (NGI), P.O. Box 3930 Ullevål Stadion, NO-0806 Oslo, Norway ⊥ Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences (SLU), Box 7050, SE-750 07 Uppsala, Sweden ‡
S Supporting Information *
ABSTRACT: The efficiency of thin-layer capping in reducing sediment-to-water fluxes and bioaccumulation of polychlorinated dibenzo-p-dioxins and dibenzofurans, hexachlorobenzene, and octachlorostyrene was investigated in a boxcosm experiment. The influence of cap thickness (0.5−5 cm) and different cap materials was tested using a three-factor experimental design. The cap materials consisted of a passive material (coarse or fine limestone or a marine clay) and an active material (activated carbon (AC) or kraft lignin) to sequester the contaminants. The cap thickness and the type of active material were significant factors, whereas no statistically significant effects of the type of passive material were observed. Sediment-to-water fluxes and bioaccumulation by the two test species, the surface-dwelling Nassarius nitidus and the deepburrowing Nereis spp., decreased with increased cap thickness and with addition of active material. Activated carbon was more efficient than lignin, and a ∼90% reduction of fluxes and bioaccumulation was achieved with 3 cm caps with 3.3% AC. Small increases in fluxes with increased survival of Nereis spp. indicated that bioturbation by Nereis spp. affected the fluxes.
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INTRODUCTION Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/ Fs) are ubiquitous contaminants in the global environment. They are formed as unintentional byproducts in various combustion processes and during the production of chlorinated chemicals and ferrous and nonferrous metals.1,2 One area with high levels of PCDD/Fs is the Grenland fjord in southern Norway, a branched fjord system that covers a surface area of ∼40 km2 and where depths frequently exceed 100 m. A magnesium production plant operating from 1951 to 2002 emitted PCDD/Fs directly to the fjord, with discharges peaking at 10 kg of toxic equivalents (TEQ) per year in the mid1970s.3,4 Even though the direct emission from the magnesium plant has ceased, concentrations of PCDD/Fs remain high in the sediment, and a strong concentration gradient of dissolved © 2012 American Chemical Society
PCDD/F from the sediment pore water to the overlying water column has been observed.5 The sediment thus functions as a source of contaminants to the water and the aquatic organisms in the area. To decrease levels in biota, remedial actions are considered. For large contaminated seabed areas such as the Grenland fjord (tens of km2), thin-layer capping has been proposed as the only feasible remediation method. Capping is an in situ remediation technique, where the sediment is covered with a clean material (cap) to prevent the contaminants from being released to the water column and Received: Revised: Accepted: Published: 3343
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to three large cooling tanks with flow-through seawater pumped from 60 m depth, maintaining a temperature of 8−10 °C and a salinity of ∼34 throughout the experimental period. The water level in the tanks was about 1 cm below the rim of the boxes. The 60 m seawater was also pumped into a header tank from which it was distributed to all boxes (n = 26) at an average (±1 standard deviation) flow of 0.94 ± 0.05 mL min−1, corresponding to a turnover time of 8.6 days for the overlying water in the boxcosms. This low flow rate was maintained during the water sampling to not dilute the POP levels in the water. During an initial phase (3 months), a 10 times higher flow was used to provide good conditions for added macrofauna. An air-diffusing system, consisting of an airstone diffuser placed in a perforated Plexiglas tube in the center of each box, was used for stirring and aerating the water. No water was pumped through the sediment to simulate the abiotic movement of pore water (e.g., groundwater discharge). This is typically a process of less importance for sediment-towater POP fluxes than bioturbation,18 particularly in areas with thick deposits of low-permeable clay sediment, such as on the deep fjord bottoms. Experimental Organisms. Six benthic species were collected in the field and added to the boxes as bioturbators. The clam Abra nitida and the brittle stars Amphiura filiformis and Amphiura chiajei (collectively termed Amphiura spp.) were collected in the Gullmar fjord (Fiskebäckskil, Sweden). Two polychaetes, the ragworm Hediste (Nereis) diversicolor and the sandworm Nereis (Alitta) virens (collectively termed Nereis spp.), and the netted dog whelk Nassarius nitidus (previously Hinia reticulata) were collected from tidal flats in Fiskebäckskil and the outer Oslo fjord, Norway, respectively. The species were kept in cold, flow-through marine water until addition to the boxes (20 cm depth into the sediment.24 Capping Treatments. Three days after the sediment had been placed in the boxes, the caps were prepared by mixing active and passive cap materials, the overlying water was temporarily siphoned off, and the caps were added on top of the sediment. In total, the experiment consisted of 26 boxes, where the thickness of the cap and the active and passive capping materials were varied in accordance with a fractional factorial experimental design, with triplicates for three treatments (Table 1).
accumulated by biota. The cap increases the path length that contaminants have to traverse to diffuse out to the water, decreases the resuspension of contaminated particles, and provides a cleaner habitat for the benthic fauna, thereby decreasing the food web transfer. In active capping, a strong sorbent (active material) is added to the cap to increase the sorption of the contaminants and reduce their pore-water concentrations. By adding active materials, thinner caps can be used. Activated carbon (AC) has recently been suggested as an efficient active material for sediment remediation of persistent organic pollutants (POPs) on the basis of research showing strong sorption of POPs to soot and carbon particles.6,7 Several laboratory studies have demonstrated reduced aqueous concentrations in sediment and reduced bioaccumulation of POPs by various benthic species after mixing of sediment with 2−4% of AC.8−14 Reductions in soil pore-water concentrations and earthworm bioaccumulation of PCDD/F have also been observed.15 The effect on sediment-to-water fluxes has been less studied, but AC amendment to contaminated marine sediment decreased the flux of dissolved polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs) in quiescent systems up to 90%.9 Recently, AC was used in a field project for remediation of PCB-contaminated sediment in the San Francisco Bay by mixing AC into the sediment on a tidal flat.16,17 In the deep bottoms of the Grenland fjord, it is more convenient to place a thin cap on top of the sediment, and the present study was initiated to provide more information on the efficiency of the thin-layer cap approach. In this study, we measured the capping efficiency of thin caps with or without active material added. The experiment was conducted in boxcosms with bioturbating macrofauna to resemble real-world conditions. The objective was to investigate the efficiency of different capping materials and layer thicknesses (0.5−5 cm) in reducing the sediment-to-water flux of dissolved POPs and in reducing the bioaccumulation by benthic fauna. Bioaccumulation was measured in the surfacedwelling Nassarius nitidus and the deep-burrowing Hediste (Nereis) diversicolor and Nereis (Allita) virens. The capping materials consisted of three types of passive mineral materials (coarse or fine limestone material or a marine clay) and two active sorbents (AC or kraft lignin). The analyzed POPs were the seventeen 2,3,7,8-substituted PCDD/F congeners, hexachlorobenzene (HCB), and octachlorostyrene (OCS). In addition, Hg was determined in sediment samples to function as a tracer of POP contamination. Studies on capping to decrease the flux of PCDD/Fs are lacking, and the analyzed POPs are present at high concentrations in the Grenland fjord. Together they span over a wide range of physicochemical properties, thus enabling a thorough investigation of the capping efficiency of the different materials and thicknesses for a range of hydrophobic chemicals.
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EXPERIMENTAL SECTION Boxcosm Setup. Sediment was collected with a van Veen grab from 48 m depth at a heavily polluted site in the Frierfjord, in the Grenland fjord area (Southeast Norway, 59°6′768″ N, 9°36′963″ E). The sediment was transported to the Solbergstrand Marine Research Station (Drøbak, Norway) and stored at 5−10 °C in the dark until use. After homogenization with a concrete mixer, the sediment was placed in polycarbonate boxes (0.32 × 0.28 × 0.40 m) with lids, leaving 13 cm of the box filled with seawater (Figure S1, Supporting Information). The boxes were randomly allocated 3344
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freely dissolved fraction, which is considered to be the bioavailable fraction, and the use of SPMDs was regarded as a more robust option than pumping water from each box to filters and sorbents, as this has associated risks in the form of leaks, breakthrough, and clogged filters. Sediment and Biota Sampling. The experiment was terminated shortly after the removal of the SPMDs (on day 189). O2 profiles were measured in sediment cores (i.d. ≈ 15 mm) from each box using a Unisense Ox-100 Clark-type microelectrode with an internal reference and a guard cathode.26 Another core (i.d. ≈ 45 mm) from each box was sliced to measure the vertical contamination profile. In each slice, redox potentials were measured using a platinum electrode (Radiometer P101) with a silver/silver chloride reference electrode (Radiometer ref 201). The POP content was determined for 10 slices selected from different boxes and depths, while the Hg content was determined in a high number of samples (26 boxes × 6 slices) to function as a tracer for the POP contamination. The remaining sediment was sieved (>1 mm) to recover the macrofauna. The animals were left in clean seawater for 24 h to purge their gut and then blotted dry before their wet weight was determined (individuals of each species were pooled from each box). After killing by freezing, the shells of N. nitidus were removed, and soft tissues of N. nitidus and Nereis spp. were homogenized using a PRO250 homogenizer (PRO Scientific Inc., Oxford, CT). N. nitidus from all treatments were analyzed for POPs (n = 26), whereas Nereis spp. were analyzed from all treatments except 0.5 cm caps with FI and CO as passive materials (n = 22). Chemical Analyses. POPs were analyzed in sediment, cap material, biota, and SPMD samples after Soxhlet−Dean−Stark extraction (sediment and cap materials), cold-column extraction (biota), or liquid extraction under agitation (SPMDs). The cleanup and instrumental analysis followed the procedure of Josefsson et al.27 The instrumental analysis was performed on an Agilent 6890N GC instrument coupled to a Waters Micromass Autospec Ultima HRMS instrument using the isotope dilution method for quantification. Total Hg in sediment and cap materials was analyzed by atomic absorption spectroscopy using an RA-915+ mercury analyzer coupled to a PYRO-915+ pyrolyzer (Lumex Ltd., St. Petersburg, Russia). Further details on the POP and Hg analyses, and on the quality assurance/quality control (QA/QC) procedure, can be found in the Supporting Information. Calculations and Statistical Analyses. The flux (N, pg m−2 day−1) of dissolved POPs from the sediment to the water column was defined as
Table 1. Experimental Design, with the Numbers of Boxes for Each Combination of the Three Factors Passive Material,a Active Material,b and Cap Thickness (0−5 cm) passive material
active material
0 cm
No CL CL CL FI FI FI CO CO CO
No No AC LG No AC LG No AC LG
3
0.5 cm
1.0 cm
2.0 cm
3.0 cm
5.0 cm
1 3 3 1
1
1
1 1 1 1 1 1 1 1 1
1
1 1 1
a
CL = clay, FI = fine limestone material, and CO = coarse limestone material. bNo = no active material, AC = activated carbon, and LG = kraft lignin.
The active materials were activated carbon (AC) and kraft lignin (LG). The AC powder (AquaSorb CP1; derived from coconuts, specific surface area 1158 m2 g−1, total pore volume 0.539 cm3 g−1, 95% C on dry weight (dw) basis) was purchased from Jacobi Carbons Ltd., Leigh, U.K. The LG powder (softwood kraft lignin from coniferous trees obtained through the LignoBoost process;25 63% C on dw basis; lignin, carbohydrate, and ash contents were 93%, 2.1%, and 1.1%, respectively) was provided by Innventia AB, Stockholm, Sweden. Kraft lignin is a byproduct from pulp and paper production and was investigated as an alternative to AC due to its lower production costs. Both active materials were soaked in seawater before being mixed with the passive material. Three passive materials (marine silty clay (CL, 0.05) showed that the increased Nereis spp. survival in the treatments without active material did not lead to increased particle mixing into the cap layer. However, increased bioirrigation could explain the increased fluxes with higher Nereis spp. survival (Figure 3). The construction of burrows by Nereis spp. increases the sediment−water interface area36 and decreases the path length that buried contaminants have to traverse to diffuse from the sediment. Also, continuous ventilation of the burrows with cleaner overlying water will maintain a concentration gradient between pore water and burrow water, which facilitates the diffusive flux across the sediment−water interface. The interaction between contaminant remobilization and the benthic species present is important for thin-layer capping. Improved conditions for benthic fauna after capping, which should be a general objective in capping operations, could lead to increased fluxes of contaminants through the cap. On the other hand, bioturbation could also improve the capping with active materials over time by increasing the contact between the contaminants in sediment and the active materials, thereby decreasing contaminant bioavailability. Hydrophobicity Trends in the Flux Reduction. The reduction in the flux due to capping was inversely related to the KOW of the substances, with larger reductions for substances with lower hydrophobicity (HCB, OCS, less chlorinated PCDD/Fs compared to the more chlorinated PCDD/Fs; 3349
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University, Sweden), Ivar Dahl, Nasir Hamdan El-Shaikh, and Joachim Törum (NIVA, Norway), Berne Petersson and KarlErling Alexandersson (Sven Lovén Centre for Marine Sciences, Sweden), Elisabeth Sjöholm (Innventia AB, Sweden), Gerard Cornelissen (NGI, Norway), Caroline Raymond (Stockholm University, Sweden), and Kjell Leonardsson (SLU, Sweden).
Figure 1; Figure S9, Supporting Information). This is in accordance to what has been observed previously after AC amendment in laboratory studies for the bioaccumulation and aqueous concentrations of PCDD/Fs in earthworm and soil15 and of PCBs in various benthic species and sediment.8,9,11,13,14 Similar results were also found in field tests, where the reduction in PCB uptake in SPMDs in an AC-amended sediment plot compared to a control plot was largest for tetraCBs (76%) and decreased to 42% for hepta-CBs.16 The less efficient reduction of higher chlorinated congeners can be explained with their slower mass transfer rates between sediment and AC.9,11 More hydrophobic congeners are retained more strongly in the original sediment and are sequestered more slowly by the added sorbent. Thus, their bioavailability is less affected by the added sorbent, and they require longer time to achieve bioavailability reductions similar to those of less chlorinated congeners. However, in a study where the AC was placed as a cap layer on top of the sediment, the hydrophobicity had less influence on the reduction of bioavailability, as the reduction in bioaccumulation was >60% for all PCB homologue groups.14 In the present experiment, with thin-layer capping, the trend of less efficient reduction for more hydrophobic compounds was seen for all treatments (LG, AC, or no active material; Figure S9). It is somewhat surprising that the same trend was present for caps without active material, but the passive mineral materials also have the capacity to sorb contaminants, albeit not as efficiently as carbonaceous materials. From the present 6 month experiment it is clear that, over the same time frame, an increased cap thickness is required to achieve the same cap efficiency for the hydrophobic PCDD/Fs compared to less hydrophobic compounds such as HCB and OCS.
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ASSOCIATED CONTENT
* Supporting Information S
Additional tables, figures, and information on chemical analyses and calculations. This material is available free of charge via the Internet at http://pubs.acs.org/.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Phone: +47 99 230782; fax: +47 22 185200; e-mail: morten.
[email protected]. Present Address #
Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences (SLU), Box 7050, SE-750 07, Uppsala, Sweden. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This experiment was a cooperation between the Scandinavian projects CARBOCAP (funded by Formas-Vinnova, Grant 2102007-282), THINC (funded by Norsk Hydro), and OPTICAP (funded by the Norwegian Research Council, Climate and Pollution Agency, and industrial project partners Hustadmarmor, NOAH, Agder marine, and SECORA). The Ph.D. position of S.J. was financed by the Centre for Environmental Research in Umeå and the Ph.D. position of G.S. by Formas. We thank Karin Lindgren and Peter Axegård (Innventia AB, Sweden) for providing the kraft lignin. We also thank Maria Hjelt, Rolf Andersson, and Sara Sjöstedt de Luna (Umeå 3350
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