Phthalates and Nonylphenols in Roskilde Fjord

The diffusivity of a molecule depends on the random movements(Brownian motion) which again is dependant on the kinetic (or internal)energy. The kinetic energy is defined as ½ ⋅ m ⋅ v 2 stating that largermolecules have lower mean velocities than smaller molecules, givenequal energy levels. Furthermore the increased cross sectional areas reducethe mean free path in the carrier medium resulting in a lower mobilityof larger molecules compared to smaller molecules.This relationship between molecular size and diffusivity can be used topredict diffusivities of compounds from known diffusivities of othercompounds based alone on the molecular masses, cf. Equation 7(Schwarzenbach et al., 1993)DDunknownknownmassknown≈ (7)massunknownThis requires that the carrier media are the same and that the chemicalstructures are related. Benzene can be used as a model compound forcalculating the diffusivity of DEHP that also contain a benzene ring inaddition to the alkyl chains and ester groups. Schwarzenbach et al.(1993) have found a diffusion coefficient in water for benzene (M w = 78g ⋅ mol -1 ) of approximately 10 -9 m 2 ⋅ sec -1 .DDEHP= Dbenzene⋅mmbenzeneDEHP=1 ⋅ 10-92msec.⋅78391= 5 ⋅ 10-102msec.Fick’s 1st law can describe the vertical substance flux in the sedimentdCS⎡ mg ⎤sz= θ ⋅ Ddisp,sz ⋅dz ⎢⎣m⋅ sec. ⎥(8)⎦N2where θ is the porosity of the sediment and D disp,sz is the dispersion coefficient,defined as2⎡ m ⎤Ddisp, sz= α ⋅ vp+ Dsz⎢ ⎥(9)⎣sec.⎦where α is the longitudinal dispersivity, v p is the advective pore watervelocity (≈ 0) and D sz is the molecular diffusion coefficient.Schwarzenbach (1993) suggests the following empirical relationshipD=D=5100.55=2102msec.1.5-101.5-10(10)disp, szszAdditionally, under favourable conditions, the redistribution of sedimentparticles by benthic macro fauna can occur by a variety of processes.This bioturbation can influence the transport and fate of hydrophobic xenobioticsin the surface layer. The particle reworking and the pumping ofpore water can create a mixed layer where the adsorbed and dissolved62