Biodegradable surfactants and the OECD 301 series

Ready biodegradability is the strict category. It uses a small inoculum, no adaptation period and a short test window, so passing implies the substance will degrade rapidly in most environments. Inherent biodegradability is a weaker statement: it shows a substance can degrade under favourable conditions, not that it will do so quickly in a real receiving water.

Ultimate biodegradation means full mineralisation to carbon dioxide, water and mineral salts, with no persistent fragment left behind. Primary biodegradation, by contrast, only means the parent molecule lost its surface-active structure, which is a much lower bar.

All six measure ready biodegradability, but they differ in what they track, which threshold applies and which substances they suit. Poorly soluble or volatile materials rule out some methods, which is why the chosen test matters when comparing two data sets.

In 301B the test substance is the only carbon source for a small inoculum, CO2-free air is bubbled through the vessels, and the carbon dioxide the culture releases is trapped and quantified. The result is expressed as a percentage of theoretical carbon dioxide, ThCO2, so it measures mineralisation directly rather than through a proxy.

301B is the method most often requested for surfactants in Europe, and it avoids the oxygen artefact that nitrogen release from amine-containing substances causes in respirometric methods. NEXOVANT F Eco reached 74.7 % ThCO2 in 28 days under 301B in a GLP study.

OECD 301A (DOC Die-Away) and OECD 301E (Modified OECD Screening) follow dissolved organic carbon leaving the solution and require 70 % removal rather than 60 %. They are simple and inexpensive when the substance is fully soluble and does not adsorb.

For surfactants the DOC family is used with care: surface-active molecules adsorb onto biomass and vessel walls, so carbon can disappear from solution without being degraded and the apparent result overstates biodegradation.

301F measures the oxygen a culture consumes in a closed respirometer and reports it as a percentage of theoretical oxygen demand, ThOD. It handles coloured, turbid or poorly soluble samples well and produces a continuous curve, which makes verifying the 10-day window straightforward.

The caveat is nitrogen: if the substance releases ammonium, nitrification consumes oxygen that has nothing to do with degrading the carbon skeleton, so a 301F result on an amine-containing substance should state whether it is calculated on an ammonium basis.

A substance is readily biodegradable when it reaches 60 % of theoretical oxygen demand or carbon dioxide evolution, or 70 % dissolved organic carbon removal, within 28 days. The thresholds sit below 100 % by design, because part of the carbon is assimilated into microbial biomass instead of being respired or removed.

The validity controls matter as much as the number. The reference substance must degrade normally, the inoculum blank must stay low, and the toxicity control must show that the substance did not inhibit the organisms meant to degrade it.

The threshold must be met within a 10-day window that starts once degradation passes 10 %. The window separates substances that degrade rapidly on first contact from substances that need a long acclimation.

That is where many marketing claims quietly fail. A substance reaching 65 % on day 27 after a slow start has not passed the ready criterion, even though the headline number looks convincing. For multi-component substances the window can be waived when components are assessed individually, and a report relying on that allowance should say so.

Work through a report in this order and the headline percentage becomes much easier to trust or discard.

NEXOVANT F Eco meets the ready-biodegradability criterion with 74.7 % ThCO2 in 28 days under OECD 301B (GLP). F Pro is under final assessment: a manometric respirometry test (OECD 301F) gave 78.9 % ThOD on an ammonium basis, but the report is not yet signed, and an earlier OECD 301B study reached 47.4 % ThCO2, below the 60 % threshold. The same GLP programme confirmed the material is non-mutagenic (OECD 471) and characterised aquatic toxicity under OECD 201 and OECD 202.

Biodegradability and toxicity are separate questions and both matter. A molecule can degrade quickly and still be acutely toxic on the way. Aquatic toxicity is reported as EC50 values: algae (OECD 201, 72 h) 11.9 mg/L for F Eco and 1.49 mg/L for F Pro, against 0.47 mg/L for a conventional lauryl amine oxide; Daphnia magna (OECD 202, 48 h) 42.9 mg/L and >5 mg/L against 2.4 mg/L. The reports are in finalisation, so no multiplier is published.