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At low surfactant concentrations often non-linear sorption processes are observed when natural adsorbents like sediment or soil are involved. This sorption process is often explained by a Dual-Model (DM) model, which assumes sorption to an adsorbent to be based on a combined ionic-polar and non-polar sorption interaction term. An Independent-Mode (IM) model, however, could treat surfactant sorption as two independent sorption processes to which the non-polar and ionic-polar features of the surfactant molecule contribute differently. For both models the overall exact partition coefficient, Ktotalp, and its corresponding total standard free enthalpy of adsorption, ΔsG0total, are derived. We tested the outcome of both models against multiple published experimental sorption data sets by, (i) varying the organic carbon fraction, (ii) constructing sorption and partition isotherms over different concentration ranges, (iii) removing the organic carbon fraction, (iv) applying different types of mixtures of surfactants, and (v) explaining sorption hysteresis in desorption studies based on either continuous and successive washing steps. It turned out that only the IM model was able to explain the reported sorption phenomena. We also show that when one interaction is dominating, e.g. non-polar over ionic-polar, the ΔsG0total of the IM model can be approximated by the sum of the different ΔsG0 values, the ΔsG0total of the DM model. The exact partition coefficient, Kp(Cw) (L kg-1) = dCs (mmol kg-1)/dCw (mmol L-1), is turning each sorption isotherm into a partition isotherm that provides the Kp values required in environmental risk assessment models.

Sorption of surfactants onto sediment at environmentally relevant concentrations:independent-mode as unifying concept