Prior studies have indicated that the subsurface transport of Cryptosporidium parvum oocysts is diminished in sediments containing iron oxides and that inner-sphere complexation of oocyst surficial carboxylate plays a role in the retardation. However, the impacts of natural organic matter (NOM) remain poorly understood. In this study, we used a model anionic surfactant, sodium dodecyl sulfate (SDS), as a surrogate for amphiphilic NOM components to examine the impacts of amphiphilic components on oocyst adhesion mechanisms. We employed in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to determine the effects of SDS on the molecular bonds that mediate interactions between oocyst surficial biomolecules and hematite (alpha-Fe2O3) surface functional groups over a wide range of solution pH. The results show that the presence of SDS significantly diminishes Fe-carboxylate complexation, as indicated by progressive decreases in intensity of asymmetric and symmetric stretching vibrations of carboxylate [nu(as)(COO-) and nu(s)(COO-)] with reaction time. In addition, one of the nu(s)(COO-) bands shifted from 1370 to 1418 cm(-1) upon SDS introduction, suggesting that SDS also changed the complexation mode. The data indicate that competition from the sulfonate groups (OSO3-) of SDS at alpha-Fe2O3 surface sites is a primary mechanism resulting in decreased Fe-carboxylate complexation. Sorptive competition from amphiphllic NOM components may therefore increase the mobility of C. parvum oocysts in the environment through disruption of interfacial pathogen mineral surface bonds.