The claim that a base oil is biodegradable is frequently promoted as a desirable attribute of a hydrocarbon base stock. This is especially true of some of the newer so-called green base stocks that are often made from nonpetroleum derived hydrocarbon feedstocks.

However, biodegradable has little meaning unless it is properly qualified. Almost all hydrocarbon base stocks biodegrade eventually. Its just a matter of timescale. The term must be properly qualified with a prefix such as inherently or readily biodegradable.

The mass of a base stock that is officially classified as inherently biodegradable – which many hydrocarbon grades are – will biodegrade by 20 to 60 percent, measured as evolved carbon dioxide, when subjected to sunlight, microbial activity and water in 28 days or less. This is not a massive hurdle and can be met, for example, by some API Group I distillate base stocks.

Base stocks are considered readily biodegradable when they can convert 60 percent or more of their mass to CO₂ when subjected to sunlight, water and microbial activity in 28 days or less. These levels of biodegradation must preferably be achieved within a 10 day window in the 28 days. Only a limited number of hydrocarbon base stocks can meet this requirement.

These criteria are supported by well-defined test methods, using prescribed environments, solid supports and bacterial sources. Of course, these aerobic tests cannot represent all environment and should be taken only as guides for real-life performance.

Factors such as chain length and degree of branching have a significant influence for hydrocarbon base stocks. As a rule of thumb, the longer the average chain length – that is the heavier the grade – the longer the base stock will take to biodegrade and the less likely it is to be biodegradable by any definition.

Also, for some reason, the bacteria that generally degrade base stocks aerobically seem to prefer a lower degree of hydrocarbon chain branching to work on. This is probably one reason why polyalphaolefins and waxier, high pour point Group IIIs – with their lower levels of branching – tend to perform directionally better in biodegradation tests. However, this is just a rule of thumb, and many other factors can influence biodegradation in the environment compared to a laboratory test.

Should we expect biolubes to perform inherently better in biodegradation tests? If the finished base stocks are saturated hydrocarbons, they should follow the same basic trends as petroleum-derived stocks. Certainly, some renewable hydrocarbon feedstocks such as farnesene and botryococcus oils are quite highly branched with methyl groups.

While this branching provides good fluidity and pour point, these stocks will likely exhibit biodegradation performance similar to that of highly isomerized Group III+ stocks of comparable grade. This is because once they are processed and saturated to finished base stocks, the basic branching architecture will not have changed. In fact, incorporating linear alpha olefins into biobased feedstocks could assist with biodegradability because this will increase the carbon number without notably increasing branching.

Ester base stocks have a head start in biodegradation because the oxygenate ester group is rather easily cleaved in a number of biochemical pathways. This is reflected in their outstanding performance as, usually, readily biodegradable base stocks. Hence, esters are the base stocks of choice for lubricants in environmentally sensitive areas such as hydraulic fluids in agricultural and forestry machinery or as chain saw lubricants.

A common assumption until the late 1980s was that most hydrocarbons, including heavier oils like base stocks, could not biodegrade in the absence of oxygen. But then it was shown that biodegradation can occur even in the absence of oxygen by so-called anaerobic biodegradation of hydrocarbons, brought about by specific bacteria that operate in the absence of oxygen. In fact, there are probably more anaerobic hydrocarbon biodegradation pathways than oxygen-dependent pathways.

This discovery assisted the soil remediation industry, which uses specific bacterial types injected directly into the ground where spills have occurred in refineries. Such an approach can help clean up really awkward spills of heavy, nonvolatile hydrocarbons (at ambient temperatures) such as base oil distillates and the heavier base oil residual fractions without recourse to expensive and sometimes impractical soil removal procedures.

Soils, with or without oxygen, can function as catalysts to promote the biodegrading environment. And this effect can be boosted with appropriate bacteria and promoters – either naturally occurring or specifically added.

All of this tends to suggest that relatively recent technical developments support my original premise that most base stocks are biodegradable given time and the right environment.