2-vinylpyridine

Biodegradation in water: screening tests

Description of key information

Biodegradation testing is complicated by 2VP's reactivity and polymerization activity.  Dilution of the stabilizer by the test solution likely causes the substance to react with organic molecules or autopolymerize

Key value for chemical safety assessment

Biodegradation in water:

under test conditions no biodegradation observed

Additional information

2 -Vinylpyridine (2VP) has undergone biodegradation testing in several different protocols (301E Ready Biodegradation screening test, 301C Ready Biodegradation by the MITI protocol, and an inherent biodegradation test protocol where evaporation is prevented (CONCAWE inherent biodegradation OPPTS 835.3215). All test results indicate poor biodegradation. These results are in contrast with a model prediction (BIOWIN, U. S. EPA) based on chemical structure. 

The results of the CONCAWE biodegradation test for 2VP are informative; "zero" biodegradation was reported. One explanation for this extremely low value is that no 2VP was available to be degraded because it reacted with other substances or with itself to autopolymerise. The manufacturer reports that when water is absorbed by 2VP, or when the concentration of the polymerisation inhibitor (e.g., p-tert-butylcatechol, or p-TBC) decreases, autopolymerisation is initiated and rapidly procedes. Reactivity is the hallmark of 2VP’s functionality as a commercial chemical; the substance requires transport and distribution in refrigerated stainless steel containers.   In biological systems, the substance is corrosive to skin and is a strong irritant to the eye. In repeated dose oral-gavage studies, 2VP consistently caused stomach irritation, inflammation, granulation and acanthosis (thickening, overgrowth of surfaces) of epithelial tissue. 2VP caused sensitisation in guinea pigs, suggesting that it binds to proteins. The physical and biological behaviour of 2VP underscores its reactivity.

These physical, chemical and biological properties of 2VP have implications for the fate of 2VP in the environment. In aqueous environments, autopolymerisation of the 2VP will be promoted by water absorption, dilution of the polymerisation inhibitor and the potential interaction with other chemicals that may act as initiators of polymerisation. Any 2VP that does not polymerise is soluble in water based on the water solubility (27,500 mg/L at 20 °C).  From aqueous solution, 2VP will partition to the atmosphere (based on its Henry’s Law Constant of 0.000014 atm m³/mol at 25 °C and 1 hPa), or partition to sediments (based on its Koc of 219.5 L/kg). In the atmosphere, 2VP will rapidly react with hydroxyl radicals, nitrate radicals and ozone, to form primarily 2-pyridinecarboxyaldehyde (i.e., half-life in air is 2 hours). Residual 2VP in soil or sediments is expected to undergo primary biodegradation in a timeframe of days to weeks (based on the timeframe predicted by BIOWIN 4).

In their Guidance on Aquatic Toxicity Testing of Difficult Substances and Mixtures, published in 2000, an expert panel of the OECD recognised complexation as a process which may significantly affect bioavailability and toxicity of a test substance. The panel suggested that speciation models may be used to calculate the concentrations of dissolved and complexed substances from the total nominal concentrations. The panel acknowledged that “analysis methods for quantifying exposure concentrations…may not always be available or economic.” The expert panel further recommended that uncorrected “data from tests…. are likely to be of questionable value for classifying substances” (Section 3.7). Similary, a task force of ecotoxicologists within ECETOC noted that the current methods of environmental risk assessment are inadequate to address chemical substances "tightly associated or bound" to solid matrices within the natural aquatic environment. New methods must be developed not only to characterise this bound fraction, but also to integrate this dispositional information into approaches of risk assessment (ECETOC Workshop Report No. 17, 2010) so that PBT assessment can be more accurate.

All biodegradation test protocols will provide chemical substances which will react with 2VP; therefore, further testing is not indicated.    The pyridine ring is the principal structural component of 2VP, and is the largest structural fragment of 2VP evaluated by the BIOWIN model. To assess biodegradation potential for a difficult substance such as 2VP, computer model predictions may be more helpful than actual test data. The computer models used to assess biodegradation (BIOWIN, as discussed below) used two independent sets of training data to determine mathematical models; the structural features of the compound of interest are evaluated based on 36 pre-selected susbstructures and molecular weight, without the impact of interfering substances during the course of the laboratory test (which may confound the test results).

The BIOWIN model contains seven predictive modules; two of these seven modules (BIOWIN 3 and BIOWIN 5) have been proposed by ECHA (Guidance on information requirements and chemical safety assessment, Chapter R.7B, Endpoint specific guidance, Section R.7.9.4.1; May 2008) to make an overall prediction regarding ready biodegradability. The BIOWIN criteria for the overall prediction of ready biodegradability include a BIOWIN 3 value ≥ 2.75 and a BIOWIN 5 value ≥ 0.5. Version 4.10 of BIOWIN showed that the predicted values for 2VP met the criterion for BIOWIN 3 (i.e., a predicted value of 2.7527), but failed the criterion for BIOWIN 5 (i.e., a predicted value of 0.4173). Although the overall prediction for the ready biodegradability of 2VP was negative, the results from three of the modules were indicative of rapid biodegradability (BIOWIN 1, BIOWIN 3 and BIOWIN 7); the results of a seventh module (BIOWIN 4) indicated that the primary degradation timeframe is days to weeks. The primary structural feature shaping the prediction is the pyridine ring, a substance which has been demonstrated to be biodegradable.

Based on the laboratory tests and BIOWIN model predictions discussed above, it has been determined that 2VP is not readily biodegradable, and should be classified as “Persistent” in the environment. However, there is a preponderance of evidence to show that the material is not likely to meet the criteria as “very Persistent”. The principal structural component of 2VP (i.e., the pyridine ring) has been shown to be rapidly biodegradable in laboratory tests and as predicted by BIOWIN (although marginally failing the BIOWIN 5 criterion for ready biodegradability).  Although the laboratory tests for ready biodegradability and inherent biodegradability of 2VP do not indicate biodegradation, the lack of biodegradation of 2VP in laboratory tests is likely to be the result of complexation and/or polymerisation of 2VP (processes which preclude mineralisation of the 2VP to carbon dioxide). Therefore, based on the BIOWIN modeling and the demonstrated biodegradability of pyridine, it is expected that primary biodegradation of 2VP occurs in the environment. The timeframe of primary biodegradation of 2VP (as predicted by BIOWIN 4) is “days to weeks”, an indicator that 2VP may not meet any of the three criteria for classification as “very Persistent” (i.e.,a half-life greater than 60 days in marine, fresh or estuarine water; a half-life greater than 180 days in marine, fresh, or estuarine sediment; or a half-life greater than 180 days in soil). Therefore, classification of the material as “very Persistent” is premature until the degree of complexation, polymerisation and/or reactivity which may be occurring is elucidated.