Environ. Sci. Technol. 2009, 43, 736–742
Attenuation of Polychlorinated Biphenyl Sorption to Charcoal by Humic Acids A L B E R T A . K O E L M A N S , * ,†,‡ BRENDO MEULMAN,† THIJS MEIJER,† AND MICHIEL T. O. JONKER§ Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands, Wageningen-IMARES, Haringkade 1, 1976 CP Ijmuiden, The Netherlands, and Institute for Risk Assessment Sciences, Utrecht University, P.O. Box 80177, 3508 TD Utrecht, The Netherlands
Received October 9, 2008. Revised manuscript received December 9, 2008. Accepted December 11, 2008.
Strong sorption to black carbon may limit the environmental risks of organic pollutants, but interactions with cosorbing humic acid (HA) may interfere. We studied the attenuative effect of HA additions on the sorption of polychlorinated biphenyls (PCBs) to a charcoal. “Intrinsic” sorption to HA-amended charcoal was calculated by subtracting the sorption contribution of HA from the total sorption to charcoal and HA. Association of PCBs with HA was proportional to hydrophobicity. However, the planar PCBs 77 and 126 had an additional 2-4 times stronger association than expected from hydrophobicity alone. Sorption isotherms for the raw charcoal fitted slightly better to a threeparameter Polanyi-Dubinin-Manes model than to a twoparameter Langmuir model. Preloading the charcoal with 1-75 mg of HA/g of charcoal increasingly attenuated sorption to charcoal with up to a factor of 10. The resultant isotherms could be described adequately with the Freundlich model. Isotherm nonlinearity increased with HA loading, suggesting increased sorption competition between HA and PCBs. Attenuation was negligible in the PCB picogram per liter to nanogram per liter range and increased at higher PCB concentrations, which points to saturation of binding sites on the charcoal. Attenuation was highest for planar congeners, which suggests an additional site blockage mechanism. These variations due to HA loading and PCB concentration can explain the variability in attenuation reported in earlier work and imply that the use of constant “attenuation factors” to adjust sorption coefficients determined for pure carbonaceous materials in order to apply them to field situations may not be warranted.
Introduction Recently, it has been shown that carbonaceous materials (CMs), such as black carbon (BC; charcoal, soot) may play a dominant role in the immobilization of hydrophobic organic chemicals (HOCs) in aquatic sediments (1-4). However, in sediments, association with either particulate organic carbon * Corresponding author phone: +31 317 483201; fax: +31 317 484411; e-mail:
[email protected]. † Wageningen University. ‡ Wageningen-IMARES. § Utrecht University. 736
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(POC) or dissolved organic carbon (DOC) phases may reduce the number of accessible sorption sites for HOCs on the CM surface, thereby attenuating HOC-CM sorption. For a manmade CM, i.e., activated carbon (AC), it has been reported that especially so-called “reactive” humic acid (HA) fractions are responsible for sorption attenuation because of sorption competition and/or blockage of the AC’s pores (5-7). Knowledge on this subject is crucial because AC amendments to sediments are currently being tested with respect to their possible role in contaminated sediment remediation and risk reduction (8, 9). For the natural CMs charcoal and coal, it has been shown that sorption of polychlorinated biphenyls (PCBs) could be attenuated by up to a factor of 50 when mixed with sediment (10). Most probably, this attenuation was due to interactions with DOC originating from the sediment. In this study (10), however, the DOC concentration was constant and POC was present as well, so that the attenuation effect could not be attributed unambiguously to one of the phases. Later studies reported (i) a factor of 9 attenuation of phenanthrene sorption to environmental BC by comparing isotherms for combusted and noncombusted BC-containing sediment (11), (ii) the complete absence of attenuation due to precipitated HA (mimicking sedimentary POC) on sorption of phenanthrene-d10 (12), (iii) a factor of 2-53 attenuation of phenanthrene sorption to AC, when added to various harbor sediments (9, 13), and (iv) a decline in benzene sorption and the surface area of a wood char due to pore blockage by HA lipid fractions (14). Another study appearing during peer review of our paper showed a similar decline in sorption and the surface area of a char in HA solutions and char-HA coflocculates (15). On the basis of the above, it can be expected that sorption attenuation is highly variable and dependent on the amount and type of sorbent (organic matter and CM), the type and concentration range of the adsorbate, and the presence of possible competitors (DOC, ions, metals, other HOCs). It is this variability that currently limits a proper assessment of the expected impact of CM on HOC immobilization, and thus (i) its role in HOC fate or food-chain accumulation modeling (3, 4) and (ii) the potential of risk reduction and remediation of contaminated sediments with amendments of CMs (3, 4, 8, 9, 13). Most aforementioned sorption attenuation studies addressed only a few compounds and used phenanthrene. PCBs might behave differently because of their different properties and environmental concentration ranges. The primary aim of the present study was to assess the effect of preloading charcoal with HA on subsequent sorption of PCBs to this charcoal. The most important merits of this work are (i) that PCB-HA competition isotherms are reported, including an assessment of sorbate planarity effects, (ii) that a wide and realistic range of HA concentrations is applied, and (iii) that a mixture of 11 PCBs spanning a log Kow range of 5.24-7.42 is used, at realistically low concentrations of 10-5-10-1 µg/L.
Materials and Methods Chemicals. PCBs used (IUPAC no. 18, 28, 52, 72, 77, 101, 118, 126, 138, 156, and 169) all had a declared purity of g98% and were obtained from Promochem (Wesel, Germany), except for PCB 72, which was purchased from ULTRA Scientific (North Kinsttown, RI). Aldrich humic acid (HA; molecular formula HOC6H3(OCH3)CH2CO2H; molar weight 182 g/mol) was obtained from Sigma Aldrich, Germany. Polyoxymethylene (POM) was obtained from Vink kunstoffen BV, Didam, The Netherlands. The plate with a thickness of 0.5 mm was cut into strips of desired dimensions. Before use, these strips 10.1021/es802862b CCC: $40.75
2009 American Chemical Society
Published on Web 01/06/2009
were cold-extracted with hexane (30 min) and three times with methanol (30 min), after which they were air-dried. Pulverized charcoal (from combustion of bark, particles
