ReviewHexachlorobenzene in the global environment: Emissions, levels, distribution, trends and processes
Introduction
This paper presents a review of hexachlorobenzene (HCB) in the global environment and uses it as a ‘model chemical’ to try to understand environmental fate and cycling of persistent organic pollutants (POPs) in general. POPs continue to attract considerable scientific and regulatory interest. Questions remain, for example, over the extent and proportion of a given POP which undergoes long-range atmospheric transport (LRAT), the extent to which they undergo repeated air-surface exchange or ‘hopping’ to become globally dispersed, the balance between primary and secondary sources in maintaining ambient levels, and their ultimate sinks in the environment (e.g., Dachs et al., 2002, Meijer et al., 2003a, Meijer et al., 2003b, Jaward et al., 2004a, Dalla Valle et al., 2004).
HCB is an interesting chemical to study because of its prior use history, persistence and physicochemical properties. HCB has long half-lives in air, water and sediment (Mackay et al., 1992) and is therefore extremely persistent in the environment. However, its octanol/air and octanol/water partition coefficients are lower than for many other POPs, which indicate it is more likely to undergo environmental re-cycling than–for example–PCBs. Atmospheric degradation of HCB is extremely slow and is not an efficient removal process. In air, HCB is found almost exclusively in the gas phase, with less than 5% associated with particles in all seasons except winter, where levels are still less than 10% particle bound (Cortes et al., 1998). These results suggest that HCB can be transported great distances in the atmosphere before removal by deposition or degradation. Van Pul et al. (1998) modelled the atmospheric residence time of HCB. The transport distance (the distance over which 50% of the chemical is removed) for HCB was calculated to be 105 km. Due to this long atmospheric residence time, it is distributed widely on national, regional or global scales.
HCB in the troposphere can be removed from the air phase via atmospheric deposition to water and soil (Bidleman et al., 1986, Ballschmitter and Wittlinger, 1991, Lane et al., 1992a, Lane et al., 1992b). The hydrophobic nature of HCB results in its preferential partitioning into sediment, soil and plant surfaces from water or air. Reported log KSA (soil/air partition coefficient) values for HCB range from 5.0 to 7.3 (Hippelein and McLachlan, 1998, Meijer et al., 2003c). Thus, although the global volume of soil is considerably less than the volume of air, at equilibrium, soil will contain a much greater mass of HCB than air. Significant amounts of HCB can also be found in water in rivers, lakes and, in particular, seas since, although the concentrations of HCB in seawater are low, the volume of the oceans is very large, and therefore, oceans may be important sinks for HCB. In the aquatic environment, HCB may remain in the solution, may undergo photolysis, may be adsorbed on solid surfaces (sediments and biota), or may get reversibly transferred into the atmosphere by volatilisation. The adsorption of HCB onto particulate matter and sediment is an important mechanism for its removal from the water column. Consequently, the sediment component of aquatic ecosystems can be a significant sink of HCB. Suspended particles entering slow-moving waters, such as large water bodies and estuaries, settle out, and their associated burden is added to the existing sediment load. Chemical or biological degradation is not considered to be important for the removal of HCB from water or sediments (Mansour et al., 1986, Mill and Haag, 1986, Oliver and Carey, 1986). Although HCB is not readily leached from sediments, slow desorption does occur and may be an important source of HCB back into other environmental compartments (Oliver et al., 1989).
HCB has a high volatility and only moderate partition coefficients compared with other POPs, which means it can move around the environment in multiple hops in a manner that has been described as the ‘grasshopper effect’ (Wania and Mackay, 1996). HCB undergoes the iterative process of deposition, remobilisation into the atmosphere and re-deposition. This remobilisation may occur as a result of several processes, including volatilisation from the Earth's surface under temperatures warmer than during initial deposition; volatilisation from water surfaces into a formerly ‘dirty’ but now cleaner atmosphere; and re-suspension from sediments and subsequent volatilisation from water surfaces. Eventually, HCB is likely to become deposited in seafloor sediments, but perhaps only after cycling through the water, biota, sediments and ice, perhaps many times. Due to its properties, it is hypothesised that HCB will reach a global equilibrium comparatively quickly compared with other POPs, and thus, it may be possible to use it to predict the ultimate fate of other chemicals.
Section snippets
Production and emission of HCB
Historically, HCB had several uses in industry and agriculture. HCB was first introduced in 1933 as a fungicide on the seeds of onions, sorghum and crops such as wheat, barley, oats and rye. It is believed that agricultural use of HCB dominated its emissions during the 1950s and 1960s. At its peak, thousands of tonnes of HCB were used each year. Estimates of global HCB production in the mid-1970s range from 1000 to 2000 tonnes (Courtney, 1979). The peak of HCB production was the late 1970s and
Geographical trends
The large number of articles reporting HCB concentrations makes it possible to develop a picture of the spatial distribution of HCB in the environment. These geographical trends may provide an insight into the transport processes that redistribute POPs like HCB.
Temporal trends
Levels of HCB in the environment have declined significantly since emissions of HCB were reduced. The following section reviews the different rates of decline that have been found in different media from many locations over the past 30 years.
Global mass balance calculation
Significant quantities of HCB are present in the atmosphere, the surface oceans and the terrestrial environment. The volume of the accessible terrestrial environment of soils and vegetation is relatively small (effective depth 5–30 cm) compared to that of the atmosphere (approximately 8000 m deep) and the oceans (effective depth of mixed layer 100 m). However, the hydrophobic nature of HCB results in its preferential partitioning into soil and plant surfaces from water or air. Using the
HCB degradation rates
Although HCB is very persistent, it does degrade at a slow rate in all environmental compartments. For modelling purposes, HCB was assigned approximate half-lives of 17,000 h (1.9 years) in air and 55,000 h (6.3 years) in water and sediment by Mackay et al. (1992). However, these were ‘default’ values, assigned to HCB based on its categorisation as a ‘non-degrading’ chemical. Reported degradation rates in different media are included in the following section.
Conclusions
It has been suggested that because of the general HCB decrease, and similar HCB concentrations in North Sea and Antarctic fish (Weber and Goerke, 1996), then the global distribution of HCB could be close to equilibrium (Aono et al., 1997). However, the increasing levels of HCB in some Arctic media, and the wide variation of HCB concentrations of background soils and sediments, suggest that this is not the case. The combination of a mass balance calculation, existing multimedia fate models and
Acknowledgements
We thank Euro Chlor and the UK Department of the Environment, Food and Rural Affairs (DEFRA) Air Quality Division for financial support. We also thank Kees Booij from the Netherlands Institute of Sea Research (NIOZ) for access to unpublished seawater HCB concentration data. We are also grateful for the comments of two anonymous referees.
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