How good are the predictions of mobility of aged polychlorinated biphenyls (PCBs) in soil? Insights from a soil column experiment
Graphical abstract
Introduction
Polychlorinated biphenyls (PCBs) are globally distributed recalcitrant contaminants extensively used in many industrial applications (e.g. dielectric fluids, plasticizer, lubricants, joint sealant) till their restriction in the ‘80 and regulation with the Stockholm Convention on POPs (Persistent organic pollutants) (IARC, 2016). Even if the primary emissions of POPs have been reduced, environmental reservoirs can act as secondary sources (Nizzetto et al., 2010). Soil is the main reservoir of PCBs in terrestrial ecosystems (Cabrerizo et al., 2011) due to the marked affinity of such hydrophobic chemicals for soil organic matter (Armitage et al., 2006; Cousins et al., 1998). The mobility and bioavailability of PCBs in contaminated soils are generally limited because of their hydrophobicity, the aging processes (e.g. formation of bound residues) and the slow desorption kinetics (Luthy et al., 1997; Pignatello and Xing, 1995; Xing and Pignatello, 1997). However, PCB mobility in soil may be enhanced by the association to dissolved organic carbon (DOC) and mobile organic carbon (OC) coated fine particles/colloid particles (from now on called particles/colloids) (Cousins et al., 1999a; Moeckel et al., 2008) that could play a crucial role in driving the infiltration fluxes of hydrophobic chemicals (Enell et al., 2016; Persson et al., 2008). In the last decade, many environmental fate models have started to implement DOC as mobility enhancer in chemical transport (Ghirardello et al., 2010; Komprda et al., 2009; Lindim et al., 2016; Morselli et al., 2018b; Nizzetto et al., 2016; Terzaghi et al., 2018, Terzaghi et al., 2017), showing its potential in influencing infiltration, runoff, redistribution and fluxes of contaminants towards ground and surface waters. This can be crucial when evaluating the long/term fate of POPs or the bioremediation potential, e.g. in contaminated sites. Recently, Terzaghi et al. (2018) in estimating the time required for PCB natural attenuation using the SoilPlusVeg model (Terzaghi et al., 2017), underlined the need of accurately predict the role of DOC and its dynamics in soil to better estimate the PCB bulk water concentrations and possibly their bioavailability. Several authors (Cousins et al., 1999a, Cousins et al., 1999b; McLachlan et al., 2002) suggested that the PCB vertical transport in solid phase is a key feature to perform a correct evaluation of the PCB fluxes (e.g. towards atmosphere). However, they referred to a very surficial soil (up to ~30 cm) and the process was explained as being caused by bioturbation, even though the addition of the transport associated to DOC, colloids/small particles was recommended for future developments (Cousins et al., 1999b).
Furthermore, in order to retain simplicity, many of these models may underestimate the role of several factors regulating the cycle of the soil organic carbon and, as consequence, the chemical mass balance.
Contact time among phases (soil/water, water/DOC), for example, affects both the chemical desorption from soil towards the water (Pignatello and Xing, 1995) and the sorption on DOC (Krop et al., 2001). The time to reach equilibrium partitioning on DOC for hydrophobic chemicals can be brief (minutes) (Krop et al., 2001; Pörschmann et al., 1997; Pörschmann et al., 1998) or relatively long (days) (Haitzer et al., 1999) (Xie et al., 2009) depending on chemical properties and DOC source (Haitzer et al., 1999). Furthermore, DOC quality varies with time (Kamjunke et al., 2017) with potential effects on binding affinity for chemicals (Chin et al., 1997; Chiou et al., 1986, Chiou et al., 1987).
Additionally, variables that are generally considered negligible in the variability of the soil/water partitioning processes (e.g. moisture conditions, temperature) can deeply influence the DOC fluxes and particle release, affecting chemical concentrations in mobile phases. Natural occurring wetting-drying cycles, for example, can determine an accumulation of dissolved organic carbon (Huang and Lee, 2001) and promote the mechanical disaggregation of soil particles (Jablonowski et al., 2012). Temperature affects the production and release of DOC from soil due to the mineralization of organic matter, with concentrations generally higher in summer than in winter (Kalbitz et al., 2000).
In the present study, a number of column leaching experiments were performed to investigate the influence of different environmental conditions (soil/water contact time, temperature, saturation) on mobility of PCBs in an aged contaminated soil. Measured concentrations were compared to data predicted by a dynamic air-vegetation-litter-soil model (SoilPlusVeg) (Terzaghi et al., 2017). The goal was to measure the chemical fluxes, focusing on PCB transport associated to DOC and small particles, to gain insights to improve PCBs (as well as other POPs) movement modelling.
Section snippets
Reagents
Calibrants (PCB 28/31, 52, 101, 138, 153, 180, 209, purity ≥99%) and internal standards (PCB 30, 155, purity ≥99%) were purchased from Sigma-Aldrich (St. Louis, MO, U.S.A), while recovery standards (tetrabromobenzene (TBB), purity 98%; hexabromobenzene (HBB), purity 99%) were purchased from Sigma-Aldrich (St. Louis, MO, U.S.A) and Riedel-de Haën (Seelze, Germany) respectively. Acetone and cyclohexane (pesticide residue grade) were purchased from Fluka Analytical (Sigma-Aldrich, St. Louis, MO,
Soil and leachate characteristics
The soil was characterized by a sandy loam texture and organic carbon content of 15.00 (±1.96) g/kg (dry weight) (1.5%) (n = 4). DOC concentrations (Table S5.4) in TW leachates at 25 °C were roughly similar (~25–35 mg/L) for brief CT (2, 4, 7 days) but significantly enhanced (ANOVA, p < 0.001) for long CT (48 days) (~50 mg/L). DOC concentrations were smaller in LT samples (~10 mg/L) compared to RT samples (Tables S5.1, S5.4) probably due to the lower microbiological activity and its influence
Conclusions
The present study investigated how several relevant environmental variables (contact time, DOC and particle/colloid transport, soil saturation and temperature) can influence the movement of PCBs in soil and their relevance for modelling purposes by comparing measured data in column leaching experiments with predicted concentrations by using the SoilPlusVeg model.
The results suggest that some key features need to be embedded in models to simulate more realistic scenarios and gain more reliable
Acknowledgements
Simone Anelli of ERSAF (Ente Regionale per i Servizi all'Agricoltura e alle Foreste) is acknowledged for the sampling of the contaminated soil. Angelo Maspero (University of Insubria) is kindly acknowledged for the help in the experiment setup. Luca Bechini (University of Milan) and Melissa Morselli (University of Insubria) are kindly acknowledged for their suggestions on early versions of the manuscript. Gabriele Tartari (ISE-CNR, Verbania Pallanza, Italy) is also acknowledged for TOC
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