A key marketing point for environmentally friendly lubricants is ready biodegradability. The idea is that, in the event of a spill, these products disappear quickly from the environment, causing little harm. But according to research at Germanys Hermann Bantleon GmbH, in cases of large spills rapid degradation itself places an enormous burden on the environment that could have detrimental effects on the plant and animal life in the area.
In a presentation at the Lubmat Conference in Bilbao, Spain, last June, Laboratory Head and Head of Product Development Ben Mller-Zermini explained that the problem is that biodegradation uses oxygen. While a lot of oxygen is present in sewage treatment plants, said Mller-Zermini, there is often a lack of oxygen in the environment. If a fast biodegradable substance gets into an ecosystem, nature has to cope with severe oxygen deprivation.
Ulm-based Mller-Zermini contended that the race for fast biodegradability has made people forget other environmentally relevant properties. He then went on to describe more realistic criteria for an environmentally acceptable lubricant from a holistic point of view.
Biodegradable in Europe
The approach to biodegradable oils has taken many forms in Europe. Very early on, said Mller-Zermini, there was a trend to use rapeseed oil. However, while these oils were successful in some applications, particularly chain saws, they were not robust enough for more demanding applications.
In hydraulic systems, for example, they often lacked oxidation stability and were prone to hydrolysis. Chemically modified rapeseed oils had better oxidation stability. But they still had a problem with hydrolysis, and users were not satisfied.
The demand for bio-based oils pushed researchers to develop new molecules that had one thing in common with vegetable oils: an ester bond. In most cases, Mller-Zermini said, the ester bond leads to faster biodegradation because it makes the molecule more polar and, therefore, more water soluble. However, this also makes esters sensitive to hydrolysis in applications where water is present. In addition, the high polarity of ester fluids often causes problems with seals and hoses.
An alternative biodegradable fluid is based on low-viscosity polyalphaolefin (PAO). These oils do not have an ester bond and, therefore, are not sensitive to hydrolysis. The polarity of PAOs is similar to that of mineral oil, even a little lower, noted Mller-Zermini. Therefore, most elastomers used in seals and hoses are compatible with PAO-based lubricants.
Although PAOs are usually more expensive than ester or mineral based lubricants, Mller-Zermini noted that their other properties often offset the initial higher cost. For example, they usually have longer lifetimes, good cold-start performance, low pour point, and a good viscosity-temperature relationship.
Mller-Zermini added, The advantages of PAO-based fluids include longer machine life, less fuel consumption and less oil consumption. These properties provide an economical advantage despite PAOs higher purchase price.
The environmental benignity of a product is usually rated in terms of biodegradability, acute fish toxicity, acute toxicity to small crustaceans called daphnia and bacterial inhibition. The toxicity tests for fish, daphnia and bacteria are run according to test method ASTM D 6081.
Mller-Zermini explained that these tests arent really challenging for most lubricants. In addition, the tests do not simulate an oil spill in nature. Another issue, he added, is that the standards measure toxicity to water organisms but ignore terrestrial plants and animals.
The problem with all test methods, Mller-Zermini said, is that measuring biodegradation rates of poorly water soluble products is very difficult. A round-robin test for the [Organization for Economic Cooperation and Developments] OECD 301B test with water soluble chemicals, for example, showed very poor reproducibility.
Today, the CEC-L-33-A-93 test, developed by the Coordinating European Council for the Development of Performance Tests for Fuels, Lubricants and Other Fluids, is the only method developed specifically for measuring biodegradation rates of poorly water soluble lubricants. Mller-Zermini noted that the latest round robin test for the CEC test was carried out using base oils as reference material in 1997. Reproducibility in this round robin was better than that for the OECD test. CEC working group TDG-L-103 recently revised the test method, and a new round robin test is in progress.
The Oxygen Gap
There are many different methods to determine biodegradability, Mller-Zermini said. They all have in common that they consider the degradation of very little oil in a large excess of water and oxygen.
However, the reality in most accidents is different. The amount of oil is usually very high, and the available oxygen is very low. In addition, the environment may be too dry or too cold, so that almost no degradation takes place. Therefore, Mller-Zermini said, even if the spilled oil is rapidly biodegradable, it remains a pollutant to the environment.
Mller-Zermini contended that laboratory tests tell researchers little about biodegradation in the environment. It is not enough to test biodegradability and aqueous toxicity of a few droplets of oil to guarantee that there will be no harm to the environment if hundreds of liters of oil are spilled. These tests may be reliable to predict performance in waste water systems, he continued, but nature is more complex than a sewage treatment plant.
Mller-Zermini explained that the consequences of an oil spill vary significantly in soil, stagnant water, flowing water, sewage treatment plants and mixed environments. Every environmental [biome] reacts differently to the introduction of oil. And some compartments do not contain enough oxygen to ensure complete biodegradation without harming plants and animals.
Another consideration is the nature of the oil spill. Mller-Zermini identified three different scenarios: wastewater treatment, minor lubrication loss and major accidents.
In treatment plants, he said, both organic input and oxygen level are very high. Oxygen saturation is sustained artificially by introducing air. And the faster the oil is biodegraded, the faster the water is purified and can be discharged into nature.
In minor lubrication loss, for example, with outboard motors and chain saws, oil droplets are released into the environment slowly and in small quantities. The environment tolerates rapid degradation of this small amount of oil because the oxygen depletion can be compensated quickly.
In a major spill, however, large quantities of oil are suddenly released into the environment. During biodegradation, said Mller-Zermini, oxygen depletion from such a large amount of oil is enormous.
In principle, the oil can be degraded by bacteria, but in a major oil spill, bacteria are not enough. Rapid degradation causes rapid oxygen depletion, said Mller-Zermini, which harms organisms. He noted that oxygen is scarce in water, especially stagnant water. At 10 degrees C, a maximum of about 11 milligrams of oxygen is dissolved in a liter of water. Oxygen solubility drops as temperature rises, and at 23 degrees C, only about 8 mg/L of oxygen is soluble in the water.
The complete degradation of a single droplet of oil by oil-consuming bacteria requires the amount oxygen in 80 liters of water, he said. Completely degrading 1 kg of spilled oil requires the oxygen from 400,000 liters of water.
As a result, oxygen concentration would no longer be sufficient to sustain higher aquatic animals, and they would suffocate. In turn, the reduced oxygen content activates anaerobic bacteria, which produce toxic hydrogen sulfide. The net result is eutrophication — excessive nutrient levels leading to death of aquatic animal life.
Death of a River
To illustrate the effects of a major spill he cited a 2006 accident where 20,000 liters of biodegradable oil were spilled into a small river. The oil, which was biodegradable according to industry-accepted laboratory tests, spread so thoroughly in the water that retrieval was very difficult.
On the day of the accident, the chemical oxygen demand on the river water was measured at 35,000 mg/L (compared to the normal demand of 7 to 25 mg/L). The long-term consequences of the accident were terrible, said Mller-Zermini. The river was contaminated for years. Fish and fresh-water crabs could not breed. Over a distance of 10 kilometers, almost all organisms that live and feed on dead organic matter suffocated. The whole food chain was interrupted.
In Mller-Zerminis view, this example shows that the primary aim in the event of an oil spill must be nature conservation and not necessarily fast biodegradation. Spilled bio oil should not cause the death of flora and fauna. Rather, plant and animal life should continue to exist as unchanged as possible, and nature should recover as quickly as possible.
Mller-Zermini also pointed out that the effect of oil degradation on the growth of terrestrial plants is rarely considered. He noted that a 1995 study by Newcastle Universitys Department of Agriculture and Environmental Science showed that esters of dicarboxylic acids, which are rapidly biodegradable, inhibit the growth of wheat.
In the study, soil was treated with two esters, a mineral oil and a vegetable oil. A year later, wheat was planted in the treated beds, and the number of germinated plants counted. Plant germination in the beds treated with mineral oil, vegetable oil and TMP esters was nearly identical to that of untreated soil. However, not a single plant grew in the soil treated with esters of dicarboxylic acids.
As a result, he continued, the preferred solution is the physical removal of spilled oil as quickly and completely as possible. The more completely the oil is removed from the environment, the less damage is subsequently caused by oxygen depletion during degradation of oil residues.
In the event of a spill, Mller-Zermini contended, a better approach is to retrieve and remove the spilled oil. However, this is possible only if the fluid is water insoluble and if appropriate oil binders are available. When as much of the spill as possible is removed mechanically, remaining oil residues can be degraded by microorganisms. In this scenario, said Mller-Zermini, the rate of degradation is not as important as the complete removal of the oil.
Mller-Zermini noted that some eco labels require renewable content in lubricants. But, he pointed out that bio-based oils can hurt agricultural yields more than crude oil. Also, lubricants are very complex, and extensive chemical transformation is needed to make them from plant oil. Adding transportation costs to these production costs increases the price of making lubricants from renewable resources.
Considering all these factors, Mller-Zermini suggested that it may not make sense to make lubricants from renewable resources. We think that the conventional point of view, where the main criterion for a bio oil is rapid biodegradability, is no longer workable. Biodegradability is only a small part of the picture.
To determine which products are really environmentally acceptable, he advocated a life cycle analysis that includes technical requirements, component compatibility, energy consumption, biodegradability and ecotoxicity not only for water organisms, but also for terrestrial plants and animals.
This type of analysis evaluates every input and output from the beginning to the end of a products life. It includes production, utilization and end-of-life, said Mller-Zermini. And it provides an evaluation of the effects on issues such as energy consumption, raw material consumption, greenhouse effect, acidification, waste, etc.
A true environmentally acceptable lubricant must exhibit both technical performance and environmental compatibility, Mller-Zermini said. We can call a product environmentally friendly only if this combination leads to a positive life-cycle analysis.