Sources and Persistence of POPs
Because of the long-range transport and harmful effects on human, flora and fauna, several global efforts have been made to reduce future environmental impacts of POPs. These include the 1998 Aarhus Protocol on POPs and the 2001 Stockholm Convention on POPs to eliminate or control their releases. Prior knowledge about sources and emission rates of POPs into the environment is essential if environmental burdens are to be reduced and quantitative source-receptor relationships at regional and global scales are to be developed. Most POPs can be broadly classified according to the source of generation as intentionally produced or accidentally produced . Intentionally produced POPs may be subdivided into several subgroups such as pesticides and industrial chemicals such as chlordane, DDT, PCBs, PBDEs and others. Accidentally produced POPs are usually separated between combustion and chemical-industrial processes. Some examples are polychlorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDFs) and polycyclic aromatic hydrocarbons (PAHs) .
Hexachlorobenzene (HCB) and PCBs are examples of POPs that are used as industrial chemicals but are also inadvertently formed as by-products in combustion and other processes [6-8]. The Stockholm Convention on POPs lists HCB and PCB not only as intentionally produced POPs but also as unwanted by-products and therefore requires identification and quantification of their sources and establishment of release inventories from non-intentional production .
POPs may also be classified into the following categories according to the source of its generation :
- • Chemicals that are subject to elimination of production and use (e.g., Aldrin, hexachlorobenzene, chlordane, HCB, PCB)
- • Chemicals that are restricted in production and use (e.g., DDT, perfluorooctanesulfonic acid)
- • Chemicals that are unintentionally produced (e.g., pentachlorobenzene, HCB, PCB)
- • Chemicals under investigation for persistence (e.g., PAHs, chlorinated naphthalenes, hexachlorobutadiene)
In addition to forestry, agriculture, horticulture, municipal, industrial and medical activities, natural sources such as volcanic activities and vegetation fires also contribute these chemicals to the ecosystem. Further, water and direct contact sources are also responsible for POPs contamination [4,9].
The lipophilic property of POPs is responsible for long-time persistence, transport from one organism to another and bioaccumulation at higher concentrations. Some POPs, such as perfluorooctanesulfonic acid (PFOS), are water soluble, which has caused their detection in municipal wastewater and drinking water samples . Flalogenated POPs are more resistant to degradation reactions, have the ability to associate with aerosols and, hence, transport across long distances. The fate of transportation depends on meteorological conditions, physicochemical properties, and the removal process by photochemically driven reactions . The transfer of POPs from Earth’s surface to the atmosphere takes place in two steps, that is, transformation from liquid or solid state to vapors and dispersion by mixing [11,12]. Thus, POPs can reach at far distances from their sources to even remote locations such as Antarctica and the Arctic Circle.