Article pubs.acs.org/est
Accumulation Kinetics and Equilibrium Partitioning Coefficients for Semivolatile Organic Pollutants in Forest Litter Luca Nizzetto,†,‡,* Xiang Liu,§,∥ Gan Zhang,§ Klara Komprdova,‡ and Jiri Komprda‡ †
Norwegian Institute for Water Research, Oslo, 0349, Norway RECETOX - Research Centre for Toxic Compounds in the Environment, Masaryk University, Brno, 62500, Czech Republic § State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academic of Sciences, Guangzhou 510640, China ∥ Pearl River Delta Research Center of Environmental Pollution and Control, Chinese Academy of Sciences, Guangzhou 510640, China ‡
S Supporting Information *
ABSTRACT: Soils are important stores of environmentally cycling semivolatile organic contaminants (SVOCs) and represent relevant atmospheric secondary sources whenever environmental conditions favor re-emission. The exchange between air and soil is controlled by resistances posed by interfacial matrices such as the ubiquitously distributed vegetation litter. For the first time, this study focused on the experimental characterization of accumulation parameters for SVOCs in litter under real field conditions. The logarithm of the litter-air equilibrium partitioning coefficient ranged 6.8−8.9 and had a similar dependence on logKOA as that of plant foliage and soil data. Uptake and release rates were also KOA dependent with values (relevant for real environmental conditions) ranging 30,000−150,000 d−1 and 0.0004−0.0134 d−1, respectively. The overall mass transfer coefficient v controlling litter-air exchange (0.03−1.4 cm s−1) was consistent with previously reported data of v for foliage in forest canopies after normalization on leaf area index. Obtained data suggest that litter holds the potential for influencing atmospheric fugacity in proximity to soil, likely affecting overall exchange of SVOCs between the soil reservoir and the atmosphere.
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INTRODUCTION Semivolatile organic compounds (SVOC) encompass many substances of human and environmental health concern including most of the known bioaccumulative persistent organic pollutants (POP) and polycyclic aromatic hydrocarbons (PAHs). In terrestrial environments airborne SVOCs engage diffusive exchange with available media including in particular soil and vegetation. To this respect forest soils serve as effective accumulators and long-term reservoirs for many POPs,1,2 possibly behaving as a secondary atmospheric source when environmental conditions favor contaminant revolatilization.3,4 The most important mechanism behind air-soil exchange of gaseous SVOCs (including for example PCBs, organochlorine pesticides (OCPs), and some PAHs) is partitioning, a process governed by the thermodynamics of diffusion and hydrophobic interactions between the nonionic chemicals and environmental media. Such a diffusive exchange is a function of the gradient of fugacities between the compartments and the resistances (1/v, d m−1) present at the interface and controlling the physical transfer. In turn, both fugacity gradients and resistances (or their reciprocal: the mass transfer coefficient v (m d−1)), depend on a number of factors including temperature, media characteristics, compound specific physical chemical properties and the characteristics and properties of the interface. Although © 2013 American Chemical Society
progress has been made in soil fugacity and air−soil equilibrium partitioning coefficient determination,5 the complex interplay of factors acting at the interface of soil and air and controlling v, does not allow resolving exchange fluxes in a simple manner, even when fugacity gradients can be determined with sufficient accuracy. There is a clear demand for investigating characteristics and properties of the soil−air interface to increase confidence on fate model outputs. This interface is in fact crucial since processes that ultimately control fate and distribution of the globally “re-cycling” burden of SVOCs take place at this level. Vegetation litter is a highly dynamic pool of organic matter (OM) nearly ubiquitously distributed (with exception of nonvegetated areas) at the interface between soil and air, especially in forest understories. Nevertheless it is an extremely complex and heterogeneous matrix which can play a crucial role in controlling air−soil exchange6 by (i) providing a physical resistance to the diffusion of the contaminants degassing from soils or depositing from the atmosphere (therefore reducing the value of v) and ii) directly interacting with hydrophobic Received: Revised: Accepted: Published: 420
October 23, 2013 December 4, 2013 December 9, 2013 December 9, 2013 dx.doi.org/10.1021/es4047318 | Environ. Sci. Technol. 2014, 48, 420−428
Environmental Science & Technology
Article
the range of the concentration of native PCBs in the litter,7 therefore spiking did not considerably increase fugacity levels. After a two days predeployment period in which the spiked litter was kept in a close jar for redistribution and equilibration of the RCs,8 the equivalent of 20 g (dw) was added above the upper metal mesh into the PEM in order to mimic characteristics and abundance of the litter naturally present at site (e.g., the same mass of litter present in a forest floor surface equivalent to A is added into the PEM). A subaliquot of this sample was then collected several hours after deployment and analyzed to determine the amount of labeled and native compounds at the beginning of the experiment (Ut0and Ut*0 (ng), respectively). The starred symbols identify parameters for the RCs. The remaining aliquot in the PEM, was left in the field in close contact to the ground for two months. During this period RCs and native PCBs are subject to environmentally mediated multimedia exchange processes including in particular volatilization to air, leaching, downward diffusion from the litter to PUFs, deposition of airborne PCBs and, in theory, degradation. The PEM were retreived at the end of the exposure period (t1) and the litter analyzed to determine Ut1and U*t1 (namely: the amount (ng) of native and labeled compounds in the organic material). Also the PUF disks were retrieved at t1 and analyzed to determine the mass U*PUF of RC that underwent downward export from the litter. Based on the set of parameters derived using the PEM, relevant intercompartment fluxes (SI Figure S1) of the native PCBs can be estimated following the quantitative framework described in detail in ref.7 Briefly, assuming as done in previous studies9,10 negligible degradation for PCBs during the two months exposure time and negligible contribution of wet depositions (as supported by the low value of leaching fluxes of PCBs from the litter observed even for less hydrophobic congeners),7 the change in PCB mass in the PEM litter over the deployment time (ng m−2 d−1) can be expressed as
substances due to its elevated OM content (therefore potentially affecting the value of air concentrations Ca (or air fugacity) in proximity to the soil). The scope of this study was to experimentally derive accumulation parameters for litter (including the litter−air equilibrium partitioning coefficients, uptake/release rate constant and v of the litter-air exchange) for a set of SVOCs (namely: a range of PCBs). This was done by exploiting recent developments in soil-air exchange measurements and two months integrated flux data available from a recent study7 where a new device (the passive exchange meter (PEM)) was used under the real field conditions of a warm rainforest.
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MATERIALS AND METHODS Determination of the Litter-Air Equilibrium Partitioning Coefficient (KLA). The dimensionless litter-air equilibrium partition coefficient KLA is defined as the ratio of the litter and air concentration at the equilibrium, as follows: KLA =
C L·ρL CA
(1)
where CL (pg g−1, dw) is the concentration in the litter and ρL (assumed here to be in the order of 500 × 103 g m−3) is the density of dry litter. KLA can therefore be determined from experimental litter and air concentration data only if equilibrium among them can be demonstrated. The PEM method recently introduced to measure air-litter exchange of POPs7 allow assessing the attainment of equilibrium for individual compounds by comparing the magnitude of the deposition (Fdep, ng m−2 d−1) and volatilization (Fvol) fluxes. The following section will focus on elucidating the significance of these PEM derived fluxes. Based on the result of this comparison and considering measurement uncertainties, targeted SVOCs (in this case a set of 32 PCBs) could be divided into three groups: Group 1: Req = ((Fdep − Fvol)/(Fdep)) ≥ 0.5, compounds for which deposition was dominant; Group 2 −0.5 < Req < 0.5, congeners at the equilibrium; and Group 3: Req≤ 0.5, congeners for which volatilization was dominant. Using eq 1, KLA could therefore be calculated for Group 2 (equilibrium) compounds. Flux Measurements through the Passive Exchange Meter (PEM). In order to better understand the experimental frame behind the present determination of litter accumulation parameters and clarify the significance of the flux data used here, the PEM approach to flux measurement (described in detail by Liu et al.7) is summarized here. The PEM consists of an open metal cylinder (i.d. 147 mm, interception area A = 0.0154 m2) with a coarse metal net (1 mm mesh) in proximity of the base (Supporting Information (SI) Figure S1). Below the net, three pre-extracted polyurethane foam disks (PUFA, PUFB and PUFC, from top to down), are piled supporting another metal net. Forest litter collected in situ (28% OC content) was spiked with a range of isotopically labeled reference compounds (RCs), using a validated procedure8 that was shown not to significantly affect mobility of the native compounds present in the matrix and does not significantly alter biophysical properties of the litter OM.8 This assessment focuses on PCBs and a mixture containing 10 13C labeled PCB congeners (namely: PCB 3, PCB 15, PCB 28, PCB 52, PCB 118, PCB 153, PCB 180, PCB 194, PCB 208, PCB 209) (EC-4189-A, Cambridge Isotope Laboratories) was used as RCs. Spiking level for most congeners were lower or in
Fnet =
Ut1 − Ut 0 = Fdep − Floss A·t
(2)
where t (d) is the deployment time, Fdep represents the entering flux of native atmospheric pollutants deposited from the atmosphere and Floss represents the net loss of either native or labeled compounds which, by assuming no significant degradation of the PCBs during 2 months deployment time, can be expressed as the sum of two processes: Floss = Fdown + Fvol
(3)
where, Fdown is the downward export flux (e.g., leaching (including runoff) + downward diffusion from the litter) and Fvol is the volatilization flux from the litter. The downward transport (F*down) of RCs from the litter to the PUF can be estimated from the amount of the labeled congeners sequestered by the upper PUFs during the deployment time, as follows:
* = Fdown
* UPUF A·t
(4)
Assuming negligible atmospheric depositions of the labeled compounds (since 13C PCBs are rare in the environment) eq 2 for the labeled chemicals can be further simplified as * = −Floss * = −(Fdown * + Fvol *) Fnet 421
(5)
dx.doi.org/10.1021/es4047318 | Environ. Sci. Technol. 2014, 48, 420−428
Environmental Science & Technology
Article
Figure 1. A: LogKLA vs log Koa and comparison with literature data for vegetation and soils. Symbols in the brown range refer to equilibrium partitioning coefficient measured for soil samples by previous studies, while those in the green range refer to living plant foliage samples. Values for ref 29 were calculated by dividing soil or litter concentrations by reported air concentrations. B. The same plot after normalization on OC content.
therefore the volatilization flux of the RCs Fvol * can be calculated as follows: * = −(Fnet * + Fdown * ) Fvol
(6)
* Fdown = θ·Fdown
(7)
* Fvol = θ·Fvol
(8)
with:
By introducing the parameter r(t) = (U(t)/U*(t)) defined as the ratio between the mass of the native and labeled PCBs, it is possible to demonstrate7 that the loss fluxes for the native compounds can be derived with an acceptable level of approximation (relative error
