OECD 106 – Adsorption-desorption using batch equilibrium (OECD, 2000b)
This batch equilibrium method is appropriate for the determination of the adsorption-desorption distribution coefficient of ionisable and neutral substances that have a variety of functional groups, pKa values and log KOW values. The OECD 106 was originally intended to evaluate the sorption characteristics in 5 standardised soil types, but can be easily modified for the analysis of sediment and wastewater treatment sludge. The method may be limited in part by compounds that have low solubility, or that significantly bind to the test apparatus. The method is amenable to 14C-labeled test materials thereby facilitating analyses at low concentrations and a complete mass balance/ internal validation of the experiment. The test protocol is conducted in phases that allow for: 1) the optimisation of solids levels to ensure appropriate analytical sensitivity for measurement of test substance in both the solid and aqueous phases; 2) confirmation of the equilibrium period required for the test substance to be in equilibrium between the solid and aqueous phases; and 3) confirmation that the test substance is stable during the equilibrium period. Once fully optimised, the sorption- desorption properties of the test substance is then evaluated over a concentration range to construct the sorption- desorption isotherm. The isotherm relates the amount of substance adsorbed to the concentration of the substance in solution at equilibrium.
To determine a sorption distribution coefficient, replicate samples of 0.01 M CaCl2, sorbent and test compound are equilibrated with each other via mechanical shaking. The two phases are then separated by centrifugation and the concentrations of the test compound in the two phases, aqueous and solid phase, are determined using an appropriate analytical technique.
While the standardisation of measuring soil pH in 0.01 M CaCl2 is well known and established (ISO, 1995; Nilsson et al, 2005), and that it approximately represents the ionic strength characteristic of soils (Nilsson et al, 2005) and is used in the method to improve centrifugation and minimise cation exchange (OECD 106), it should be recognised that the approach has the potential to underestimate sorption for those compounds where cation exchange is the main mechanism of sorption (Nicholls and Evans,1991) by displacing cationic compounds at surface of the sorbent. For such cases it may be helpful to conduct sorption determinations in both distilled water and 0.01 M CaCl2 to assess whether a potential cation exchange mechanism is relevant.
OPPTS 835.1110 – Activated sludge sorption isotherm (US EPA, 1998)
This batch equilibrium method follows that of the OECD 106 in respect to the underlying principles, how the test is generally conducted and any potential restrictions. The method provides pragmatic guidance on how to collect and prepare activated sludge for the sorption test that is not found in the OECD 106 guidance. Activated sludge is settled, washed and then lyophilised prior to its use to remove unwanted matrix related materials and to limit the microbial activity of the matrix.
OECD 121 – Estimation of adsorption coefficient (KOC) on soil and sewage sludge using high pressure liquid chromatography (HPLC) (OECD, 2001)
This HPLC method ‘estimates’ the sorption coefficient of a test material based on its retention (capacity factor) in a standard HPLC column. The test material interacts with the polar and non-polar sites on the column in a similar fashion as with the organic matter in soil and sludge matrices. This relationship allows for the correlation of the capacity factor to KOC as established by the use of reference substances in a calibration curve. While this method does not require the use of 14C-labeled test material, it does require the material to have some chromophore or other moiety detectable by standard HPLC detection systems; and the material must be stable in solvent/ buffer systems used in the HPLC. For ionisable substances, the retention should be evaluated under at least two conditions where the substance is non-ionised and where it’s at least 10% ionised to fully assess the impact of pH on sorption. The method does not adequately estimate sorption coefficients when the mechanism of sorption is predominately associated with clay and/ or other soil constituents (OECD 121). It assumes that Kd normalised by the fraction of organic content is appropriate for reading across from soil and sludge matrices when applied to ionisable substances. This method may be of particular value for estimating KOC when the test substance is volatile, highly sorptive to the test chamber, or very insoluble.