Here is a breakdown of how surfactants are degraded:
1. Primary vs. Ultimate Biodegradation
When discussing degradation, it is important to distinguish between these two stages:
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Primary Biodegradation: The surfactant loses its “surface-active” properties. The molecule is altered just enough that it can no longer emulsify or foam.
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Ultimate Biodegradation (Mineralization): The surfactant is completely broken down by microorganisms into basic inorganic compounds like water (), carbon dioxide (), and mineral salts.
2. Mechanisms of Degradation
The most common way surfactants break down is through microbial action in soil or water treatment plants.
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Aerobic Degradation: In the presence of oxygen, bacteria use enzymes to “clip” the long hydrophobic carbon chains. Straight-chain surfactants (like Linear Alkylbenzene Sulfonate, or LAS) are much easier for bacteria to digest than branched-chain surfactants.
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Anaerobic Degradation: In environments without oxygen, such as deep river sediments or septic tanks, some surfactants are much harder to break down and can persist for long periods.
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Chemical Degradation: Some surfactants can be broken down by hydrolysis (reaction with water) or photolysis (exposure to UV light), though this is usually less efficient than biological means.
3. Structural Influence on Degradability
The “shape” of the molecule determines how fast it disappears:
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Linear Chains: Generally highly biodegradable. Most modern laundry detergents use linear chains for this reason.
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Branched Chains: Harder for enzymes to “grip” and break down. Many older surfactants were phased out in the 1960s because their branched structures caused foam to build up permanently in rivers and sewage plants.
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Aromatic Rings: Surfactants containing benzene rings or stable ring structures often take longer to degrade than simple fatty acid-based surfactants.
4. Environmental Impact of Non-Degradation
If a surfactant does not degrade quickly, it can cause several issues:
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Aquatic Toxicity: Persistent surfactants can coat the gills of fish, interfering with oxygen exchange.
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Eutrophication: If a surfactant contains phosphorus (common in some industrial chelating agents or older builders), its degradation can release nutrients that lead to harmful algal blooms.
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Endocrine Disruption: Some degradation byproducts (like nonylphenol ethoxylates) are actually more toxic than the original surfactant and can interfere with the hormones of aquatic life.