Phthalates and Nonylphenols in Roskilde Fjord

6.1.2 Sorption and solvationIn the model set-up it is assumed that the adsorption-desorption processesare instantaneous, reversible and linear in concentrations. Comparedto other processes in the system the sorption reactions reachchemical equilibrium quickly and the kinetic relationships related to adsorptioncan therefore be assumed to be steady-state. The reversibility istrue in some cases, but for the phthalates the adsorption is very strongand it is questionable whether the desorption is of any importance. 1 st orderirreversible adsorption has been shown to describe soil adsorptionmore satisfactorily (Sørensen, 1999).Different regimes for simulating adsorption capacities can be employed.The Langmuir adsorption model is defined by a maximum adsorption capacitythat is related to a monolayer coverage of surface sites, which isrepresentative of a wide range of equilibrium sorption isotherms for organicadsorbates in natural waters (Schnoor, 1996).When the number of available sites is large compared to the number ofoccupied sites there is usually a linear relationship between the concentrationof dissolved substance, C (mg ⋅ litre -1 ), and the concentration ofadsorbed substance, C X (mg ⋅ litre -1 ). For a specific adsorbate the concentrationof available sites can be expressed through the particulate drymatter concentration, C XB (kg DM litre -1 ). The adsorption constant specificfor this adsorbate, is expressed through the constant K d .⎡ mg ⎤CX= Kd ⋅ C ⋅ CX B ⎢ ⎥(2)⎣litre⎦K d [litre ⋅ (kg DM) -1 ] is a measure of the actual partition in natural watersthat can be empirically derived from the fraction of organic carbon presentin the particulate matter, f oc , and the organic carbon/water partitioncoefficient for the compound, K oc . This is valid when the organic carboncontent is larger than 0.05 - 0.1 % in environmental matrices. When theorganic content is lower there is an increasing tendency for adsorption tothe inorganic parts of the matrix. This phenomenon is more pronouncedfor polar organic substances.The K d value is characteristic for specific adsorbates. It is assumed thatthere exist different “species” of particulate matter in the system, i.e. atthe WWTP outlet and in the water phase of the Fjord the organic fractionof the particulate matter is relatively high and in the sediment it is lowerdue to degradation. This implies a lower K d value in the sediment. However,K d is defined under steady-state conditions and since it is questionablewhether the retention time in the water is long enough for steadystateto occur, a reduction of K d is appropriate here. On the other hand,the substance entering the Fjord through discharge and deposition will alreadybe adsorbed to DOM and therefore it is more a problem of desorptionin the diluted Fjord water.At the sediment surface the organic fraction will be higher than in thedeeper layers. The sediment density is, however, assumed to be constantthroughout the depth although there will be an increase caused by pressurebuild-up. The former will result in a decreasing retention factor andthe latter in an increasing retention factor for increasing depths.57