Biodegradability

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Volunteer image author Weinstock

When a compound biodegrades, it returns to compounds found in nature complete its natural cycle. There are ways we can make this happen without waiting for it to happen naturally. We can either find the cause of resistance to biodegradation or enhance it. Understanding how biodegradation happens on the molecular level also helps.

Molecular features generally increasing resistance to aerobic biodegradation are given as follows:

  1. Halogens : Especially Cl and F.
  2. Chain branching, especially Quaternary C and N or extensive branching such as in surfactants derived from tri- or tera-propylene.
  3. Nitro, nitroso, azo and arylamino groups.
  4. Polycyclic residues, especially with more than 3 rings.
  5. Heterocyclic residues, for example, pyridine rings.
  6. Aliphatic ethers.

Biodegradability may be in direct contrast to the functionality/performances of the chemical. Biodegradability enhancing factors:

  1. Presence of potential sites of enzymatic hydrolysis, for example, esters and amides.
  2. Introduction of oxygen in the form of hydroxyl, aldehydic or carboxylic groups. Now, this is the most important one, since during the biodegradation of many compounds, the biodegradation of say, hydrocarbons, starts with the enzymatic insertion of oxygen into the structure.
  3. Presence of unsubstituted linear alkyl chains (>=4 C) and phenyl rings.
  4. Water solubility. Let’s talk about the effects of solubility in biodegradation.

Effects of solubility in biodegradation:

  1. Microbial bioavailability: Insoluble compounds tend to remain partitioned in the activated sludge, sediments and soil.
  2. Rate of solubilization: Many microorganisms secrete biosurfactants that enhance the rate of solublization.
  3. Low aqueous phase concentration is responsible for inefficient performance by cellular enzymes.

Examples of Biodegradable chemicals:

  1. LABS (Sodium dodecylbenzenesulfonate)
  2. Dialkyl Quaternaries (dialkyl dimethyl ammonium salts, imidazolium quaternary)

Biodegradability increases in the following order: Linear molecule, branched ortho, para. Since microbial degradation is the major loss mechanism for most organic chemicals in soil, water and sewage treatment, biodegradability should be induced as a factor in product design along with function and economics.

Biodegradability should lead to non-toxic products. I wonder if biodegradability leads to toxic products in any case? Does it break down into simplest forms like carbon dioxide and water?

In order for claims to be made about a products biodegradability, they must be sent to an outside laboratory where special tests are performed on the products raw materials. Tests include: Toxicity Studies with Daphnia Magna fish and Element Analysis for Oxygen, Carbon, Hydrogen, Nitrogen and Sulfur. It is important to remember that just because a product is biodegradable, does not mean it is non-toxic. The ‘Aquatic Toxicity’ level will tell you if it is toxic, this information can be provided by the manufacturer. – EMC Canada

Last edited: December 19th 2017

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