The biodegradation pathway of various surfactants in the natural environment is a complex process affected by many factors.
1. Primary biodegradation
Oxidation reaction
For most various surfactants, oxidation is an important starting step of biodegradation. Under aerobic conditions, enzymes in microorganisms (such as monooxygenases and dioxygenases) will act on various surfactant molecules. Taking linear alkylbenzene sulfonate (LAS) as an example, its alkyl chain may be oxidized first. Under the catalysis of enzymes, oxygen combines with the alkyl chain to form intermediates such as alcohols and aldehydes. This process is like opening a "gap" in the molecule, allowing subsequent degradation reactions to proceed.
For various surfactants containing unsaturated bonds, such as the unsaturated bonds in the polyethylene oxide chain in some nonionic various surfactants, they will also be oxidized, causing changes in the molecular structure.
Hydrolysis reaction
Ester bonds and amide bonds are functional groups that are easily hydrolyzed. For example, some ester-type Various surfactants containing ester bonds break their ester bonds under the action of moisture in the natural environment and esterases secreted by microorganisms, breaking the molecules into smaller fragments. The hydrolysis reaction reduces the complexity of the molecules of Various surfactants and makes them easier to be further metabolized by microorganisms.
2. Intermediate biodegradation
β-oxidation
For the products after primary degradation, if they contain fatty acid chains, β-oxidation is a common metabolic pathway. In this process, the fatty acid chain, under the action of enzymes, starts from the carboxyl end and removes two carbon atom units each time to form acetyl CoA. Acetyl CoA is an important intermediate in cell metabolism and can enter the tricarboxylic acid cycle and be completely oxidized and decomposed into carbon dioxide and water.
Ether bond cleavage
For non-ionic Various surfactants containing polyoxyethylene ether structures, the cleavage of ether bonds is a key step. Special enzymes in microorganisms can recognize and cut ether bonds, gradually decompose polyoxyethylene chains, and generate short-chain intermediates such as alcohols and aldehydes, which are further degraded through other metabolic pathways.
3. Final biodegradation
Mineralization
After a series of biodegradation reactions, the carbon, hydrogen, oxygen and other elements of Various surfactants are finally converted into inorganic substances such as carbon dioxide, water and inorganic salts. This process is called mineralization. For example, under ideal degradation conditions, LAS will eventually be completely mineralized, realizing the transformation from organic pollutants to harmless inorganic substances, and thus completely removed from the natural environment.
However, the biodegradation pathways of Various surfactants are affected by many factors such as molecular structure, temperature, pH value, microbial community, etc., and the degradation process and speed of different types of Various surfactants in the natural environment are also quite different.
