User Guide to Thresholds and Classification - Environmental ...

361User Guide for Thresholds and Classificationschemicals with unknown hydrolysis rates and a more detailed test that is performed for chemicals that areknown to be hydrolytically unstable and for chemicals for which the preliminary test shows fast hydrolysis. Inthe preliminary test the concentration of the chemical in buffered solutions at pHs in the range normallyfound in the environment (pHs of 4, 7 and 9) at 50°C is measured after 5 days. If the concentration of thechemical has decreased less than 10% it is considered hydrolytically stable, otherwise the detailed test maybe performed. In the detailed test, the overall hydrolysis rate is determined at three pHs (4, 7 and 9) bymeasuring the concentration of the chemical as a function of time. The hydrolysis rate is determined atdifferent temperatures so that interpolations or extrapolations to environmentally relevant temperatures canbe made. The OPPTS 835.2130 test is almost identical in design to the OECD Test Guideline 111, thedifference mainly being in the treatment of data.It should be noted that apart from hydrolysis the hydrolysis rate constants determined by the tests include allother abiotic transformations that may occur without light under the given test conditions. Good agreementhas been found between hydrolysis rates in natural and in pure waters (OPPTS 835.2110).PhotolysisAt present, there is a draft OECD guideline on aqueous photodegradation, and a guidance document,concerning aquatic direct photolysis, is available (OECD, 1997). The guidance document is supposed toform the basis for the scheduled guideline. According to the definitions set out in this guidance document,phototransformation of compounds in water can be in the form of primary or secondary phototransformation,where the primary phototransformation (photolysis) can be divided further into direct and indirect photolysis.Direct phototransformation (photolysis) is the case where the chemical absorbs light and as a direct resulthereof undergoes transformation. Indirect phototransformation is the case where other excited speciestransfer energy, electrons or H-atoms to the chemical and thereby induces a transformation (sensitisedphotolysis). Secondary phototransformation is the case where chemical reactions occur between thechemical and reactive short lived species like hydroxy radicals, peroxy radicals or singlet oxygen that areformed in the presence of light by reactions of excited species like excited humic or fulvic acids or nitrate.The only currently available guidelines on phototransformation of chemicals in water are therefore OPPTS835.2210 Direct photolysis rate in water by sunlight and OPPTS 835.5270 Indirect photolysis screening test.The OPPTS 835.2210 test uses a tiered approach. In Tier 1 the maximum direct photolysis rate constant(minimum half-life) is calculated from a measured molar absorptivity. In Tier 2 there are two phases. InPhase 1 the chemical is photolysed with sunlight and an approximate rate constant is obtained. In Phase 2,a more accurate rate constant is determined by using an actinometer that quantifies the intensity of the lightthat the chemical has actually been exposed to. From the parameters measured, the actual directphotodegradation rate at different temperatures and for different latitudes can be calculated. Thisdegradation rate will only apply to the uppermost layer of a water body, for example, the first 50 cm or lessand only when the water is pure and air saturated which may clearly not be the case in environment.However, the results can be extended over other environmental conditions by the use of a computerprogram incorporating attenuation in natural waters and other relevant factors.January 2012 EPA0109