12.2. Persistence and Degradability

This section provides information on the persistence and degradability of substances in the environment. It is crucial for assessing long-term environmental impacts and determining whether a substance meets the persistence criteria for PBT (Persistent, Bioaccumulative, Toxic) or vPvB (very Persistent, very Bioaccumulative) assessment.

Biodegradation Overview

Biodegradation is the breakdown of organic substances by microorganisms (bacteria and fungi) into simpler compounds, ultimately resulting in carbon dioxide, water, and mineral salts (mineralization). It is a key process for removing chemicals from the environment.

  • Primary biodegradation: The structural change of a substance resulting in the loss of a specific property
  • Ultimate biodegradation: Complete breakdown of a substance to carbon dioxide, water, mineral salts, and new microbial biomass
  • Ready biodegradability: Rapid and complete biodegradation under aerobic conditions
  • Inherent biodegradability: Potential for biodegradation under favorable conditions, but at a slower rate

Biodegradability Classification

Under REACH and CLP regulations, biodegradability is classified as follows:

Classification Criteria Regulatory Significance
Readily biodegradable ≥ 60% degradation within 28 days (CO2 evolution)
≥ 70% degradation within 28 days (O2 depletion or DOC removal)		 Not considered persistent; favorable for environmental classification
Inherently biodegradable > 20% and < 60% degradation within 28 days May be considered persistent depending on other data
Not biodegradable < 20% degradation within 28 days Considered persistent; unfavorable for environmental classification

10-Day Window Criterion:

For ready biodegradability tests, the 10-day window criterion requires that the pass level (60% or 70%) must be achieved within 10 days of reaching 10% degradation. This criterion does not apply to MITI test (OECD 301C) or to complex, multi-component substances.

Standard Test Methods

OECD Test Guidelines for Biodegradability:

  • Ready Biodegradability:
    • OECD 301A: DOC Die-Away Test
    • OECD 301B: CO2 Evolution Test (Modified Sturm Test)
    • OECD 301C: MITI Test (I)
    • OECD 301D: Closed Bottle Test
    • OECD 301E: Modified OECD Screening Test
    • OECD 301F: Manometric Respirometry Test
    • OECD 310: Ready Biodegradability - CO2 in Sealed Vessels (Headspace Test)
  • Inherent Biodegradability:
    • OECD 302A: Modified SCAS Test
    • OECD 302B: Zahn-Wellens Test
    • OECD 302C: MITI Test (II)
    • OECD 302D: Concawe Test
  • Simulation Tests:
    • OECD 303A: Aerobic Sewage Treatment - Activated Sludge Units
    • OECD 307: Aerobic and Anaerobic Transformation in Soil
    • OECD 308: Aerobic and Anaerobic Transformation in Aquatic Sediment Systems
    • OECD 309: Aerobic Mineralization in Surface Water

Abiotic Degradation

Abiotic degradation processes occur without the involvement of living organisms:

  • Hydrolysis: Breakdown of a substance by reaction with water
    • OECD 111: Hydrolysis as a Function of pH
    • Half-lives typically reported at pH 4, 7, and 9
  • Photolysis: Degradation caused by light (usually sunlight)
    • OECD 316: Phototransformation of Chemicals in Water
    • Direct and indirect photolysis mechanisms
  • Oxidation: Reaction with oxidizing agents in the environment
    • Atmospheric oxidation (e.g., reaction with OH radicals)
    • Oxidation in water by reactive oxygen species

Persistence Criteria for PBT/vPvB Assessment

Under REACH Annex XIII, the persistence criteria are:

Environmental Compartment Persistent (P) Very Persistent (vP)
Marine water Half-life > 60 days Half-life > 60 days
Fresh water Half-life > 40 days Half-life > 60 days
Marine sediment Half-life > 180 days Half-life > 180 days
Freshwater sediment Half-life > 120 days Half-life > 180 days
Soil Half-life > 120 days Half-life > 180 days

A substance that meets any of the persistence criteria for a given environmental compartment is considered persistent (P). If it meets any of the very persistent (vP) criteria, it is considered very persistent.

Factors Affecting Biodegradation

  • Substance Properties:
    • Chemical structure (e.g., branching, halogenation)
    • Water solubility
    • Molecular weight
    • Stereochemistry
  • Environmental Conditions:
    • Temperature (optimal range: 20-30°C)
    • pH (optimal range: 6-8)
    • Oxygen availability
    • Nutrient availability
    • Microbial population (diversity and abundance)
  • Bioavailability:
    • Adsorption to soil/sediment
    • Formation of non-extractable residues
    • Aging effects

Example: Biodegradation Data

Substance X Biodegradation Profile:

  • Ready biodegradability (OECD 301B): 65% degradation after 28 days
  • 10-day window criterion: Met (60% degradation achieved within 10 days after reaching 10%)
  • Conclusion: Readily biodegradable

Half-lives in Environmental Compartments:

  • Water: 15-25 days
  • Soil: 30-45 days
  • Sediment: 45-60 days

Abiotic Degradation:

  • Hydrolysis (OECD 111): t1/2 = 120 days (pH 7, 25°C)
  • Photolysis: t1/2 = 5 days (summer sunlight, 50°N latitude)
  • Atmospheric oxidation: t1/2 = 2 days (reaction with OH radicals)

Degradation Pathways

  • Hydrolysis: Significant at pH 7, resulting in cleavage of ester bonds
  • Photolysis: Contributes to degradation in surface waters and air
  • Oxidation: Primary degradation mechanism in the atmosphere
  • Microbial degradation: Active process in water, soil, and sediment

Degradation Products:

  • Primary metabolites: Non-toxic alcohol and acid derivatives
  • Secondary breakdown products: Readily degradable smaller molecules
  • Final products: CO2, H2O, and mineral salts
  • Persistent metabolites: None identified

Reporting Requirements

SDS Section 12.2 should include:

  • Results from biodegradation tests (ready, inherent, simulation)
  • Half-lives in relevant environmental compartments
  • Information on abiotic degradation processes
  • Identification of degradation products (if relevant for classification)
  • Assessment of persistence in relation to PBT/vPvB criteria
  • Test methods used and reference to guidelines

Critical Considerations

Regulatory Implications

Quality Assurance Checklist

  • Verify that biodegradation tests follow recognized guidelines (e.g., OECD)
  • Ensure that test conditions are reported (temperature, pH, inoculum source)
  • Check that the biodegradation conclusion is consistent with the data
  • Confirm that half-lives are provided for relevant environmental compartments
  • Verify that persistence assessment is consistent with PBT/vPvB criteria
  • Ensure that any degradation products of concern are identified