Sorption of Surfactants onto Sediment at Environmentally Relevant Concentrations: Independent-Mode as Unifying Concept
At low surfactant concentrations often non-linear sorption processes are observed when more complex natural adsorbents like sediment or soil are involved. This sorption process is often explained by a Dual-Model (DM) model, which assumes sorption to the organic matter fraction to be based on a combined electrostatic and hydrophobic sorption interaction term. An Independent-Mode (IM) model, however, could treat surfactant sorption as two independent sorption processes to which the hydrophobic and electrostatic features of the surfactant molecule contribute differently. For both models the overall true partition coefficient, Kptotal, 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 Independent-Mode (IM) only was able to explain the reported sorption phenomena. We also show that when one interaction is dominating, e.g. hydrophobic over electrostatic, the ΔsG0total of the IM model can be approximated by the sum of the different ΔsG0 values, the ΔsG0total of the DM model. The true partition coefficient, Kp(Cw) (L/kg) = dCs(mmol/kg)/dCw(mmol/L), is turning each sorption isotherm into a partition isotherm that provides the Kp values required in environmental risk assessment models.