Controlled Natural Degradation Carbon Dioxide Determination: Principles, Standards, and Testing Meth

Controlled natural degradation carbon dioxide (CO2) determination is a respirometric test method used to evaluate the ultimate aerobic biodegradability of plastic and other organic materials in soil under controlled laboratory conditions. Based on recognized standards such as ASTM D5988, ISO 17556, and GB/T 22047, the method quantifies the CO2 evolved by microbial metabolism as a direct and scientific indicator of mineralization. This article explains the technical principles, sample requirements, test items, procedures, validity criteria, and typical applications of this widely used biodegradability assessment.

Technical Overview

Controlled natural degradation CO2 determination measures the amount of carbon dioxide released when a test material is degraded by microorganisms in a natural soil medium maintained under controlled laboratory conditions. The test simulates the natural soil environment—regulating temperature, humidity, aeration, and microbial activity—so that the biodegradation process is reproducible, measurable, and comparable between laboratories. Because the evolved CO2 is the end product of microbial mineralization of organic carbon, its cumulative quantity provides a scientifically robust basis for calculating the biodegradation rate of the material.

During the test, the test material is mixed with a well-aerated, biologically active soil and incubated in sealed bioreactors. The CO2 generated by microbial respiration is captured and measured at regular intervals over an extended period, typically several months. The measured CO2 is corrected against a blank control (soil without test material) and compared with the theoretical CO2 (ThCO2) that would be produced if all organic carbon in the sample were completely converted to CO2. The percentage of biodegradation is then calculated and plotted as a biodegradation curve over time.

The method is primarily governed by the following standards:

• ASTM D5988 – Standard Test Method for Determining Aerobic Biodegradation of Plastic Materials in Soil, by CO2 evolution measurement.
• ISO 17556 – Determination of the ultimate aerobic biodegradability of plastic materials in soil by measuring the oxygen demand in a respirometer or the amount of carbon dioxide evolved.
• GB/T 22047 – Chinese national standard for determining the ultimate aerobic biodegradability of plastic materials in soil by measuring evolved CO2, technically aligned with ISO 17556.
• EN 17033 – Specification for biodegradable mulch films, which references soil biodegradation testing as part of its conformity requirements.

The test is distinguished from controlled composting tests such as ISO 14855 and ASTM D5338 by its use of natural soil at ambient temperatures rather than mature compost at elevated thermophilic temperatures, making it particularly relevant for products intended to biodegrade in soil, agricultural, and general terrestrial environments.

Testing Samples

Biodegradable Plastic Resins and Compounds

Polymer resins, masterbatches, and compounded formulations—such as PLA, PBAT, PBS, PHA, PPC, PGA, and starch-based or cellulose-based blends—can be tested to characterize their intrinsic biodegradation behavior before commercial formulation. Samples are typically supplied as granules or ground powder to increase surface area and ensure uniform microbial access.

Finished Products and Articles

End products such as mulch films, agricultural films, planting pots, seedling trays, bags, food service items, and disposable hygiene products can be evaluated either as-is or after size reduction. Testing finished products verifies that processing aids, inks, adhesives, and structural modifications do not prevent biodegradation.

Films, Sheets, Coatings, and Foams

Thin films, laminates, coatings on paper or fiber substrates, and foamed structures are common sample types because they are widely used in soil-contact and litter-prone applications. Specimens are cut into defined dimensions or reduced to fragments to standardize the exposed surface area-to-mass ratio.

Additives, Fillers, and Bio-Based Materials

Plasticizers, pro-oxidants, chain extenders, mineral fillers, natural fibers, and other additives may be tested individually or within a formulation to determine their influence on the overall biodegradation profile of the host polymer.

Testing Items

The core deliverables of a controlled natural degradation CO2 determination program include:

• Cumulative CO2 evolution of the test material, blank control, and positive reference control over the full incubation period.
• Theoretical CO2 (ThCO2) calculation based on the total organic carbon or elemental carbon content of the sample.
• Percentage of biodegradation at each measurement interval and at the end of the test, calculated as (net sample CO2 / ThCO2) × 100.
• Complete biodegradation curve, including lag phase, active biodegradation phase, and plateau phase.
• Ultimate aerobic biodegradability conclusion, reported against the pass threshold defined by the applicable standard or specification.
• Validity parameters, including reference material performance, blank CO2 background, test system pH, moisture content, and temperature stability.

Testing Methods

Test Setup and Incubation

The test material is weighed, characterized for total organic carbon, and mixed with a standardized quantity of sieved, biologically active natural soil of documented microbial activity. The soil-material mixture is placed in sealed or semi-sealed bioreactors (glass jars or respirometer flasks) and incubated under controlled conditions, typically at 20–28 °C, with moisture maintained near field capacity and continuous or periodic aeration free of CO2 contamination. Each test is run in replicate, together with a blank control (soil only) and a positive reference control (such as microcrystalline cellulose) to confirm the viability of the inoculum.

CO2 Measurement Techniques

Evolved CO2 is quantified using one of several recognized analytical approaches, selected according to laboratory capability and standard requirements:

• Alkaline absorption followed by titration, in which CO2 is trapped in a sodium hydroxide or barium hydroxide solution and determined by acid titration.
• Direct infrared (IR) gas analysis of the headspace or exhaust air of each bioreactor.
• Gas chromatography (GC) with thermal conductivity or flame ionization detection for precise headspace CO2 concentration profiling.
• Continuous automated respirometry, in which oxygen uptake and CO2 evolution are monitored in real time by a closed-circuit respirometer.

Data Evaluation

At each interval, the net CO2 of the sample is obtained by subtracting the blank control value from the sample reactor value. The cumulative net CO2 is divided by the theoretical CO2 content of the sample to obtain the biodegradation percentage. Results are reported as a time-dependent biodegradation curve, and the test is typically continued until the curve reaches a plateau or the maximum duration specified by the standard, whichever occurs first.

Validity Criteria

The test is considered valid when the positive reference material (cellulose) reaches a biodegradation plateau exceeding approximately 60–70%, the blank CO2 background remains within acceptable limits, and the replicates show acceptable deviation. Accredited third-party laboratories apply these criteria strictly so that the final report is suitable for certification, regulatory submission, and product claims.

Applications

Controlled natural degradation CO2 determination supports a wide range of scientific, regulatory, and commercial objectives:

• Research and development of biodegradable polymers, enabling formulation screening and comparative material selection based on real soil biodegradation data.
• Verification of biodegradability claims for agricultural inputs such as biodegradable mulch films and crop protection products, supporting conformity with specifications such as EN 17033.
• Supporting evidence for compostable and soil-biodegradable product certifications submitted to certification bodies and accredited third-party laboratories.
• Regulatory compliance and market access documentation for plastic products, packaging, and disposable articles subject to environmental labeling requirements.
• Academic publication, life cycle assessment (LCA) input data, and environmental impact studies requiring quantitative mineralization results.
• Quality control for manufacturers confirming batch-to-batch consistency of biodegradable product lines.

Because the method reflects realistic terrestrial degradation conditions, its results are particularly valuable for products designed to enter soil environments at end of life, including mulch films, landscaping products, geotextiles, and horticultural containers.

FAQ

What is the difference between controlled natural degradation and controlled composting CO2 tests?

Controlled natural degradation tests (ASTM D5988, ISO 17556, GB/T 22047) use natural soil as the inoculum and medium at ambient temperatures of roughly 20–28 °C, simulating biodegradation in terrestrial environments. Controlled composting tests (ISO 14855, ASTM D5338) use mature compost at elevated thermophilic temperatures of approximately 58 °C and simulate industrial composting conditions. The two methods serve different end-of-life scenarios and are not interchangeable.

How long does the test take?

The test typically runs for several months, commonly up to six months, and continues until the biodegradation curve reaches a plateau or the maximum duration allowed by the standard is reached. Materials with slow degradation kinetics may require longer incubation to establish a definitive plateau.

What is the pass criterion for biodegradation?

Acceptance thresholds depend on the applicable standard or certification scheme. Many specifications require at least 90% absolute or relative biodegradation versus the cellulose reference within a defined period, while others accept the plateau value reported against the specific product specification. Laboratories report the measured percentage and leave threshold judgment to the relevant specification.

How should samples be prepared and how much is required?

Samples are generally reduced to small particles or fragments of uniform size to ensure reproducible surface area exposure, and their total organic carbon content must be determined for the ThCO2 calculation. Typical requirements range from tens to hundreds of grams of material depending on carbon content and replicate design; accredited third-party laboratories provide specific sampling instructions before testing begins.

Which controls are included in the test?

Each study includes a blank control containing only soil to quantify background respiration, and a positive reference control such as cellulose to demonstrate that the soil inoculum is microbiologically active. Additional controls, such as an inert reference material, may be added to further validate the test system.