This is the first of two articles in which we will be discussing the chemistry of bitumen from a health, safety and environmental perspective. Bitumen is one of, if not the most widely used compounds in construction having also been used in various forms and applications for many years. Some of the most common uses are as a sprayed seal (hot or ambient) for road surfaces or as a binder in asphaltic concrete. It is also growing in used a component in foam bitumen stabilisation and even as a dust or erosion control agent. In this first article we will examine the chemistry of bitumen and draw out why, when not managed or understood, it can be hazardous (acutely or chronically) to those applying the product, the users of infrastructure or the receiving/surrounding environment.
Bitumen consists of saturates, aromatics, resins all together known as maltenes and asphaltenes as its main chemical constituents. Saturates are mainly branched and long with aliphatic chains and traces of heteroatoms and few crystalline n-alkanes. Aromatics have dominant chemical structures slightly aliphatic in nature and a carbon skeleton with lightly condensed aromatic rings. Resins are polar peptizers that act as dispersing agents for asphaltenes and are made up of 2-4 fused rings. Asphaltenes possess condensed polycyclic aromatic rings and traces of transition metals nickel, vanadium and iron in the form of metallo-porphyrins. Bitumen’s rich chemistry from its varying chemical compounds definitely creates short-term and long-term hazards depending on the mode of contact. Potential modes of contact in humans involve aerosol and fumes of bitumen on its spraying in spray seals and mixing in with aggregates in asphalt and base course in foam bitumen stabilization. The solubility of bitumen chemical constituents in water is another mode of exposure to aquatic flora and fauna.
The unfavourable effects of bitumen on living organisms form the basis of its toxicology. Bitumen has been considered as a possible carcinogen owing to its polycyclic aromatic hydrocarbons. Occupational exposure to bitumen vapours and aerosols occurs during handling, manufacture and laying of bitumen as no measurable emissions occur below 80°C. Bitumen fumes have been demonstrated to be mutagenic in mammals and carcinogenic in humans. Skin contamination with bitumen fume condensate can result in uptake of condensate and metabolites into lymphocytes in the lungs. Urinary concentration of polynuclear aromatic hydrocarbon metabolites has been found to be lower than typical high exposure workplaces. Inflammation resulting from irritative effects on the upper and lower airways is also prevalent in groups exposed to bitumen fumes.
Blood samples have revealed a spike in oxidative DNA damage in bitumen exposed workers. Metabolites of polyaromatic hydrocarbons mono-oxygenation are often very highly reactive and more toxic than the parent compound and are related to chemically induced cancers and oxidative stress in vertebrates. Metabolism can be definitely associated with harm. Predominantly enzymes producing unstable and reactive metabolites include oxygenases, epoxide hydrolases, peroxidases and Aldo-keto reductases. Most of these metabolites include reactive oxygen species, diol-epoxides, o-quinone derivatives, phenolic derivatives and benzylic alcohols.
Bitumen contains a wide range of hydrocarbons with different aqueous solubilities. Mobility of these compounds may lead to surface water and groundwater systems. Toxicity in early-stage development of fish has been linked to the concentration of 3-5 ringed alkyl polycyclic aromatic hydrocarbons, metals and naphthenic acids in bitumen. Degradation kinetics of naphthenic acids have shown that the presence of unsaturation and cyclization can lead to rapid degradation compared to linear and single ring naphthenic acids. Therefore, it can be confirmed that the chemical structure and size of different chemical constituents in bitumen determines their half-life in water. Acute toxicity in diluted bitumen is associated with lower molecular weight hydrocarbons, naphthalene, alkyl naphthalene and short-chain alkanes. Chronic toxicity on the hand is related to polycyclic aromatic hydrocarbons and alkyl-polyaromatic hydrocarbons.
The hydrophilicity of alkyl-polyaromatic hydrocarbons relative to their unsubstituted counterparts has led to more accumulation in aquatic and terrestrial flora. Although the solubility of compounds leading to chronic toxicity is low compared to those related to acute toxicity over time and with continued residence in water their effects can affect fish adversely. Accumulation of polyaromatic cyclic hydrocarbons in fish and aquatic species lipid membranes leads to distortion of membrane structure and function. 3 to 5 ringed alkyl polycyclic aromatic hydrocarbons seem to be the most toxic in fish. Their sources are bitumen especially in diluted form and over a prolonged exposure can lead to embryotoxicity. In amphibians such as frogs, lower mitochondrial oxygen consumption is indicative of toxic polyaromatic hydrocarbons whereas in birds impaired lipid homeostasis is observed.
In soil and sediments, di- and tri-aromatic compounds of lower molecular weight tend to be volatile even when bound enabling their easy re-release into air or water. On the contrary, higher molecular weight polyaromatic compounds are more tightly bound to soil and sediment for much longer periods. Aquatic macrophytes can be coated by bitumen and in the process impairing photosynthesis by blocking light. Coating of shoots can block gas exchange and coated leaves may lead to the occlusion of stomata thus prevent carbon dioxide uptake as well as the release of transpiration water vapour. The complexity stemming from the longevity of their residence in soil contributes to different exposure patterns and duration amongst animal species in the environment. Exposure to UV light leads to photodegradation of polyaromatic compounds and the resulting heterocyclic compounds and their reactivity with other chemicals producing even more toxic products. In general, half-lives of polyaromatic cyclic hydrocarbons in tissues vary among taxa, ranging between a day to a month in marine invertebrates.
The acute exposure and injury (we could call think of this as safety risks) due to hot bitumen and asphaltic concrete in the form of burns is well understood and on the whole, considering the extreme hazards involved, well managed. However, the acute and chronic health and environmental impacts are much harder to manage and quantify as the degrees of bioaccumulation vary depending on the volume of bitumen exposed and the metabolic capacity for biotransformation varies from one species to the other. Polyaromatic cyclic hydrocarbons are complicated because they are inevitably found as complex mixtures with other contaminants and photo-transformed products. Debunking the misconceptions of bitumen on the health of workers, the community, animals, vegetation exposed to its hazards will require further investment and a systematic approach and in an attempt to evaluate all the possible risks and in minimising considerable effects on human health and the environment.
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