The Role of Black Carbon in Environmental Fate of Persistent Organic Pollutants (Pops) in Soils and Their Effect on Food Safety
Total Page：16
File Type：pdf, Size：1020Kb
THE ROLE OF BLACK CARBON IN ENVIRONMENTAL FATE OF PERSISTENT ORGANIC POLLUTANTS (POPS) IN SOILS AND THEIR EFFECT ON FOOD SAFETY Rai S Kookana CSIRO Land and Water, PMB 2, Glen Osmond, 5064, Australia E-mail: [email protected] ABSTRACT Partitioning of POPs into different compartments of environment (soil, air, water, sediment) determine their environmental fate, transport, accumulation and impact on biota and food quality. Physico-chemical properties of soil and sediments, especially the content and the chemistry of organic carbon, have a major bearing on not only the sorption and accumulation of POPs but also their degradation and plant uptake. It is now well known that “black carbon (BC)” (partially combusted material such as soot and charcoal) which has a strong affinity for POPs is ubiquitous in soils and sediments. However, the role of soil and sediment organic carbon, especially of BC, has not been fully appreciated or accounted for in assessment of distribution and impact of POPs on food safety. This paper demonstrates the importance of the chemistry of organic carbon as a strong moderating factors for several processes governing the accumulation, distribution and plant uptake of POPs, namely, sorption-desorption, degradation and bioavailability to plants/microorganisms. Recognizing that, as a response to climate change, biochar amendments of soils is attracting increasing interest globally and “black is seen as a new green”, the paper highlights the need for better understanding of the long term implications of BC in determining the fate and behaviour of POPs in soils and their potential impact on food safety. Key words: Soil organic matter, black carbon, partitioning, plant uptake, food safety, bioavailability, POPs. Abbreviations: BC, black carbon; HCB, hexachlorobenzene; HCH, hexachlorocyclo- hexane; Koc, organic carbon normalised sorption coefficient; MIR, midinfrared spectroscopy; NMR, nuclear magnetic resonance spectro- scopy; OCDD, octachlorodibenzodioxin; PAH, polynuclear aromatic hydrocarbon, PCDD/Fs, dibenzo-p-dioxins and dibenzofurans; POPs, persistent organic pollutants; PCBs, polychlorinated biphenyls; SOC, soil/sediment organic carbon INTRODUCTION Lohmann et al. 2007). Therefore, their partitioning properties play a crucial role in determining their Persistent organic pollutants (POPs) produced environmental fate, transport, accumulation, and used in industrialized nations are a cause of impact on biota and food safety. great concern globally due to their persistent, Soil has been identified as a one of the bioaccumulative and toxic nature and their most important repositories for fluxes of POPs propensity to travel long distances to affect even between soil and air and soil processes can remote, uninhabited parts of the globe. POPs have a large bearing on their future distribution have diverse physical-chemical properties and are patterns (Meijer et al. 2003, Sweetman et al. 2005, released in the environment from varied sources. Lohmann et al. 2007). While the importance of soil Based on their partitioning properties in different biogeochemistry and the role of soil or sediment environmental compartments (e.g. air, water, organic carbon (SOC) was recognized some thirty soil) POPs have been classified by Wania (2006) years ago (Kirckhoff et al. 1979), it is only in the as “swimmers” (e.g. HCHs), “single to multiple last decade, SOC has been recognized as the key hoppers” (e.g. PCBs, organochlorine pesticides, parameter governing spatial variability of POPs chlorobenzenes) and “flyers” (emerging polar in soils (Cousins et al. 1999) and partitioning in POPs such as perfluorinated octyl sulfonamides, the marine environment (Lohmann et al. 2005). 1 However, most POP studies so far have not to be expected depending on the partitioning and incorporated the complex biogeochemistry of SOC degradation processes and the inherent properties in terrestrial and aquatic ecosystems (Lohmann et of POPs. Meijer et al. (2002) using soils from al. 2007). a transect from the UK and Norway showed Lohmann et al. (2007) compared the that PCB concentrations varied up to 4 orders of predicted potential reservoirs of POPs with magnitude and SOC played an important role in the measured data on soils. The predicted soil their spatial distribution. Sweetman et al. (2005) maximum reservoir capacities for POPs ranged further examined the data and found the strongest from 60 ◦N to 80◦N, and were mainly a function of correlation between HCB concentrations in soil and SOC content and temperature (Fig. 1). However, the SOC, followed by PCB-52 (Fig. 2). However, the actual measurements by Meijer et al. (2003) the correlation between OCDD with SOC was the showed the maximum soil concentrations at 60 ◦N, weakest. HCB being the most volatile of the three, but ranging from 50 ◦N to 70 ◦N, suggesting that a near steady-state condition between air and SOC other processes such as differences in sources and via recurring soil-air exchange was suggested as the other biogeochemical and global dynamics were mechanism. However, for octachlorodibenzodioxin playing a role. Meijer et al., (2003) also highlighted (OCDD) the opposite was the likely scenario. the likelihood of a much retarded transport (grass- Heywood et al 2006 also reported no correlation hopping of POPs) due to strong sorption to SOC between SOC content and the sum of total or heavy than what has been taken into account in such PAH concentrations. A weak, but nonetheless predictions. Studies have shown that temperature significant (P < 0.05), positive correlation was and SOC are the drivers of the observed variability found with the total light PAHs congeners. Wilcke in the distribution of POPs. and Amelung (2000) found a strong positive correlation between total PAH concentration and RELATIONSHIP OF FATE AND DISTRIBUTION OF SOC content in soil from remote sites in North POPS WITH SOIL ORGANIC CARBON America. Notably the PAH profiles at these sites were dominated by lighter compounds. In all of the Due to their hydrophobic nature, most POPs above studies, the partition coefficient determined have a strong affinity for SOC. Therefore, global the strength of the relationship. surface soils have been recognized to serve as an Ren et al. (2007) assessed the distribution important reservoir for POPs, representing an of PCBs load in 52 Chinese surface soils (0- integration of several processes over a time scale of 20cm) collected from different regions and found years to decades (Sweetman et al. 2005). A strong a statistically significant but weaker (p <0.005) relationship between soil concentration and SOC is correlation with SOC (Fig. 3). However, the Fig. 1. Predicted maximum soil reservoir capacity of POPs in top soils and surface mixed layer of ocean in comparison to that in the atmosphere. Source Lohmann et al. 2007 based on Dalla Valle et al. 2005. 2 correlation improved significantly (p <0.001) when THE ROLE OF SOIL ORGANIC CARBON the longitude was considered. They also noted CHEMISTRY IN DETERMINING THE a longitudinal fractionation of PCBs in Chinese FATE AND EFFECTS OF POPS background/ rural soil from east to west (Fig. 3). Clearly, both the proximity of the source region and Not only the content of SOC but its chemistry SOC determined the distribution of PCBs in this has a strong influence on the environmental fate study. Recently Koblickova et al. 2008 reported and behaviour of POPs in soils and sediments. a similar relation between spatial distribution of Soil organic matter consists of heterogeneous PCBs and SOC in soils from the Czech Republic. mix of organic materials often classified as Considering the importance of SOC in an amorphous, gel-like “soft carbon” matrix dynamics, distributions and long range transport of or domain and a condensed, glasslike “hard POPs, in recent years several workers have called carbon” matrix or domain (Weber et al. 1992), for a greater understanding and accounting of role including altered and relatively unaltered aliphatic of SOC in fluxes, distributions and long range polymers, polysaccharides (e.g., cellulose), lignin transport of organic contaminants (Lohmann et al. and lignin degradation products, fats, proteins, 2007, Scheringer, 2009, Koblickova et al. 2008). pectines (Schumann 2006). Highly carbonaceous Fig. 2. Correlation between soil concentrations of POPs (HCB and PCB-52) and soil organic carbon contents. Source: Sweetman et al. 2005. Fig. 3. Correlation of total PCB load in 52 Chinese soils with soil organic carbon (upper) and with organic carbon and location (lower panel). Source: Ren et al. 2007 3 organic matter from partial combustion processes assessed the aquatic fate of dibenzo-p-dioxins and represents another important constituent of organic dibenzofurans (PCDD/Fs) and made a comparison matter in soils and sediments. This includes chars, of model predictions with observed organic carbon soots, and other highly carbonaceous material normalized sediment-water partition coefficients commonly termed as “black carbon” (Accardi-Dey (Koc) and dissolved water concentrations. They and Gschwend, 2003). Black carbon (BC) fraction found that for estimation of Koc or dissolved water in soils and sediments can be as high as 45-50% concentrations, the sorption models inclusive of of total organic carbon (Allen-King et al. 2002; black carbon provided greatly improved estimates Schmidt et al. 2002). compared to the amorphous carbon only model. Highly carbonaceous BC acts like activated In Fig. 5, the typical log Koc values observed