Lumping Analysis for Sorption of Neutral Organic Compounds in Mixtures to Simulated Aquifer Sorbents

The concept of using lumping analysis to reduce the complexity associated with the sorption of 12 neutral organic compounds (NOCs) comprising complex mixtures to simulated aquifer sorbents was evaluated. The sorbates covered a wide range of octanol–water partitioning coefficients, $K_{ow}$ (i.e., $-0.24\le \log K_{ow}\le 4.23$), and the sorbents included different types of mineral surfaces and humic acid–mineral complexes with different fractions of organic carbon, $f_{oc}$ (i.e., $0.006\%\le f_{oc}\le 0.221\%$). Both a priori lumping criteria (i.e., the aqueous activity coefficient at saturation ${\gamma }_w^{\mathrm{sat}}$, and the organic carbon partitioning coefficient, $K_{oc}$) and experimentally derived lumping criteria (i.e., Freundlich sorption parameters) were evaluated as potential lumping criteria. The results indicated that the sorption behavior of the 12 NOCs contained in mixtures could be approximated reasonably well by lumping them into a fewer number of pseudocompounds (four to six) with similar sorption behaviors based on Freundlich sorption parameters, and that both ${\gamma }_w^{\mathrm{sat}}$ and $K_{oc}$ could be used as viable a priori lumping criteria. However, the number and compositions of the pseudocompounds were a complex function of the hydrophobicity (i.e., ${\gamma }_w^{\mathrm{sat}}$ and $K_{oc}$) of the NOCs and varied as a function of the type of mineral surface (e.g., uncoated, $\alpha$-FeOOH-coated, and ${\mathrm{Al}}_2{\mathrm{O}}_3$-coated sands) and the $f_{oc}$ of the sorbent.