The sorption of nonionic organic solutes to humic acid-mineral complexes

Better prediction of the environmental fate of nonionic organic solutes (NOS) is dependent on the characterization of the mechanisms of solute to solid interactions of single and multiple solute systems. The objective of this research was to determine: the effect of organic C content (f{dollar}sb{lcub}rm oc{rcub}{dollar}) on the C normalized distribution coefficient (K{dollar}sb{lcub}rm oc{rcub}{dollar}), the effect of mineral type on NOS sorption to humic acid-mineral complexes, the sorption characteristics of binary NOS systems, and the extent to which NOS sorption at low f{dollar}sb{lcub}rm oc{rcub}{dollar} is due to adsorption versus partitioning. Three different minerals, a geologic calcite, a reagent CaCO{dollar}sb3{dollar}, and a montmorillonite (SWy-1) were treated with Aldrich{dollar}spcircler{dollar} humic acid (HA) to achieve f{dollar}sb{lcub}rm oc{rcub}{dollar} ranging from 3 {dollar}times{dollar} 10{dollar}sp{lcub}-6{rcub}{dollar} to 0.025.; The sorption of naphthalene, anthracene, and pyrene to the HA-mineral complexes indicated that: K{dollar}sb{lcub}rm oc{rcub}{dollar} derived from octanol-water partition coefficients (K{dollar}sb{lcub}rm ow{rcub}{dollar}) greatly underestimated the sorption of NOS in low {dollar}fsb{lcub}rm oc{rcub}{dollar} systems ({dollar}<{dollar}3 {dollar}times{dollar} 10{dollar}sp{lcub}-5{rcub}{dollar}), adsorption to bare mineral surfaces did not adequately account for the NOS surface excess observed in sorption isotherms, and adsorption of NOS to widely distributed surface bound HA was most likely occurring. Broad distribution of HA across the mineral surface would offer adsorption sites for NOS and would account for both the large surface excess of NOS observed and the competition effects observed in binary NOS systems. Estimations of transport of organic solutes, like NOS, through low organic C sediments and aquifers could be incorrect if it is assumed that only partition type interactions occur. The research presented here indicates the use of K{dollar}sb{lcub}rm ow{rcub}{dollar} based K{dollar}sb{lcub}rm oc{rcub}{dollar} in low {dollar}fsb{lcub}rm oc{rcub}{dollar} systems significantly underestimates the interactions of NOS with low organic C surfaces. The underestimation of solute surface excess could lead to longer solute transport times than predicted and longer remediation time by either "pump and treat" or biodegradation methods than expected due to more solute-surface interactions than would be accounted for by partitioning alone. Adequately accounting for NOS surface excess in these systems will likely involve the use of an adsorption parameter exclusive to, or in conjunction with, a K{dollar}sb{lcub}rm ow{rcub}{dollar} based partition coefficient.