Biodegradability Testing

Why is Biodegradability Testing?

A large amount of chemicals are produced and released into the environment every year, such as pesticides, herbicides, dyes, and antibiotics, a large portion of which could potentially pose threat to the environment. Chemicals such as chlordane and dieldrin have been removed from the market due to their persistence, bioaccumulation, and toxicity (PBT) properties; in the meantime, regulations have been made to restrict the introduction of PBT substances from entering the market in the first place.

Among the PBT properties, persistence is usually the leading reason resulting in slow degradation in the environment. The accumulation of certain chemicals could potentially be harmful due to continuous exposure and increasing concentrations in the surroundings.

Despite many other transportation processes for the removal of these chemicals such as adsorption, photodegradation, and redox reaction, the biodegradation has been considered as the most fundamental process for the ultimate elimination of PBT chemicals from the environment.

Therefore, biodegradation tests before the introduction of these chemicals to the market has been required by a number of organizations, including Organization for Economic Co-operation and Development (OECD), International Organization for Standardization (ISO), Japanese Ministry of International Trade and Industry (MITI), National Institute of Technology and Evaluation (NITE), European Union (EU), and United States Environmental Protection Agency (US-EPA).

Mechanisms of Biodegradability Tests

Aerobic biodegradation is a natural process where complex organic substances are broken down into smaller and simpler compounds by the enzymes produced by the microorganisms when oxygen is present. It usually involves the metabolic and enzymatic processes with the final products of carbon dioxide and water. Although complicated, the biodegradation tests usually measure relatively simple and non-specific parameters such as oxygen uptake, CO₂ production, or dissolved organic carbon (DOC).

The organic compounds can be either electron donors or acceptors depending on their oxidation states, while oxygen usually acts as an electron acceptor or being involved in the activation of the substrate via oxygenation reactions.

The fundamental units of biodegradation are functional groups, as these are the ‘elements’ that undergo the transformation during the course of microbial catabolism. Each functional group may undergo several types of enzymatic transformations. Therefore, different compounds may show substantially different biodegradation behaviors. However, with about 40 most relevant functional groups found in the environmentally important compounds, and perhaps three to four transformation reactions for each, the University of Minnesota Biocatalysis/Biodegradation Database has been successfully collecting such information for two decades.

On the other hand, the functional groups/substructures that are beneficial for biodegradation generally follow the order: ester, amide, anhydride > hydroxyl > carboxyl, epoxide, site of unsaturation > benzene ring, methyl, methylene, while the negative influences were molecular mass, branching, halogenation, and nitrogen heterocycles.

For example, the initial attack on the aromatic ring is mostly assumed to be electrophilic in nature. Nitrogen ([-CN, -NO₂, -NH₃⁺, -CONH₂]) and halogenic substituents strongly deplete the electron density of the aromatic ring and decrease the rate of biodegradation, while in the presence of hydroxyl and carboxyl groups was found to increase the biodegradation.

In addition, compounds that are already partially oxidized are generally considered to be more easily attacked than those which are not, and the initial degradation step is considered to be rate limiting step.

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