Just how safe is bitumen? : Part Two

An evaluation of bitumen chemistry and its use as a dust and erosion control product 

Following on from last week’s article, this second article focuses on use of bitumen from a health, safety and environmental perspective in non-traditional applications such as dust and erosion control. 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. In particular as an emulsion it has grown as a dust or erosion control agent. In this field, bitumen (because it is in emulsion form) is often portrayed as environmentally friendly and safe – but is it really? In this article we will critically examine the chemistry of bitumen and bitumen emulsions in order to lay out clearly the risks associated with using these technologies. 

Bitumen Emulsion

Bitumen emulsions are ambient derivatives consisting of bitumen dispersed in water or water dispersed in bitumen in the presence of a surface-active agent. Phase chemistry of bitumen emulsions is based on the premise of either oil in water or water in oil emulsions with a dispersed and continuous phase. Bitumen emulsions were designed as low temperature applications of bitumen allowing for easier use in cold in place mix-in technologies. Repetitive use of bitumen emulsion in the industry without thought of implications associated with its residence time and toxicity as it accumulates in layers and potential health and environmental effects to its repeated use are a cause for concern. 

How do bitumen emulsions work to control dust?

Due to their ready solubility, bitumen is utilised/promoted by some companies as an easy to use dust or erosion control product. Typically it is mixed in a water truck and either very frequently sprayed at low dilutions, or used in higher concentrations and mixed into the soil profile, and less frequently sprayed as a surface maintenance coat. Bitumen emulsion in dust and erosion control works through the successive steps that lead to film formation and coating begins with heteroflocculation and densification of bitumen droplets to create distribution and continuity of the bitumen film as bitumen droplets begin to be in close proximity. Gradually, coalescence of bitumen droplets develops from water draining between bitumen droplets then surfactant film breaks leading to droplet fusion in the process of demulsification.

Why emulsion chemistry poses a risk?

Fundamentally the very existence of bitumen emulsion in liquid form poses a potential solubility catastrophe. Low molecular weight aliphatic and aromatics of less than ten carbon atoms are mostly water soluble and volatile. The decrease in concentration because of up 24 hours half-life leads to more acute related conditions. Bioaccumulation is more prevalent with increase in molecular size in as much as hydrophobicity is a challenge for their partitioning in water. Mixture with rainwater for instance accelerates mobility to areas on the roadbed and including potential penetration which spreads the potential toxicity of the polyaromatic hydrocarbons to depths in the ground with possible contamination of ground water tables as well. 

Surfactants concentrate and are active at the surface showing strong tendencies to migrate to  the bitumen-water interface and orientate to harness polar-apolar duality. Anionic, cationic, non-ionic and zwitterionic surfactants are the four different types of surfactants of interest. Anionic surfactants dissociate in water to an amphiphilic anion and an alkaline metal cation usually sodium and potassium or quaternary ammonium. Cationic surfactants dissociate in water into an amphiphilic cation and an anion mostly halogen in nature. Biodegradability of cationic surfactants varies from one cationic surfactant to the next. They are found to be strongly and irreversibly absorbed into soil with potential runoff into waterbodies. For example, tallow polyamine emulsifier produced runoff ten times more toxic than other emulsifiers containing mixtures of imidazoline and amidoamine fat derivatives. 

Non-ionic surfactants consist of non-dissociable hydrophilic group such as an alcohol, phenol, ether, ester or amide and it does not ionize in aqueous solution. The tendency to stay resident in the soil for a very long time is inevitable when it comes to non-dissociable groups in fact they stay longer with potential detrimental effect to the environment particular growth hormones in plants and can even cause imbalance in the water ecosystems if their proportions are not managed. Zwitterionic or amphoteric surfactants consist of a single surfactant molecule that exhibits both anionic and cationic dissociations. Having the potential to even have more metallic ions disposed of in the environment presents another potential escalation of soil pH which can be detrimental to microbes that participate in soil fertility through breakdown of organic matter. The most commonly used surfactants are anionic surfactants although cationic surfactants have recently been more popular in bitumen emulsions. 

Bioaccumulation and poor biodegradability

Bitumen emulsion exhibits persistence and low degradability of when used for dust suppression and binding purposes. Repeated use of bitumen emulsion in mix in applications can lead to its accumulation over time. In some cases when over-used, 4 to 5 liters per m2 of liquid emulsion is utilized and if that is repeated every two months, up to 30 liters per m2 are dumped into the ground annually. Many benefits have been credited to use of bitumen emulsions with omission of the mention of the effects of toxic polyaromatic hydrocarbon precursors over a duration of time. Dealing with the presence of metallic ions from surfactants and deducing the extent to which mobility of bitumen emulsions is even enhanced on contact with water adds to potential risks associated with ecotoxicity. The high volatility of benzene, toluene, ethylbenzene and xylene found in bitumen emulsion pose health risk to workers working with it over a prolonged period. The callousness of justifying how bitumen emulsions build up in the road will result in less frequency of application is the very reason why its use should be questioned. The connotations imply its use as a form of dumping under the pretext of achieving dust and erosion control to say the very least. 

Understanding the risk

It cannot be emphasized enough that chemical toxicity of bitumen emulsions is often understated in the casual mention of the binding and dust suppression properties it can offer. Repeated and consistent use of bitumen emulsions without due diligence on the health hazards and environmental implications is rampant given common acceptability of bitumen emulsion use. We highlight reasons why a blind eye should not be turned to bitumen emulsions and their use (particularly in dust and erosion control) without stringent thought into short term and long-term threats it poses to workers, neighboring communities and the environment at large. 

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REFERENCES 

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