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Little is known about the mobility of engineered nanoparticles (ENPs) in granular media at environmentally relevant concentrations of surfactants, which represents a critical knowledge gap in employing ENPs for in situ remediation of contaminated groundwater. In this study, transport and retention of alizarin red S (ARS)-labeled hydroxyapatite nanoparticle (nHAP) were investigated in water-saturated sand at environmentally relevant concentrations of surfactants: anionic sodium dodecyl benzene sulfonate (SDBS, 0-50 mg L-1) and cationic cetyltrimethylammonium bromide (CTAB, 0-5 mg L-1). Transport of ARS-nHAP increased with increasing SDBS concentration because of enhanced colloidal stability and reduced aggregate size arising from enhanced electrostatic, osmotic, and elastic-steric repulsions between ARS-nHAP and sand grains. In contrast, transport decreased significantly with increasing CTAB concentration due to reduced surface charge and enhanced aggregation of ARS-nHAP. Osmotic and elastic-steric repulsions played a minor role in enhancing ARS-nHAP colloidal stability in CTAB tests. Retention profiles of ARS-nHAP exhibited hyperexponential-shapes for all conditions tested and became more pronounced as CTAB concentration increased. The phenomenon was attributed to the aggregation and ripening of ARS-nHAP in the presence of surfactants, particularly CTAB. Overall, the present study suggests that surfactant type and concentration may be important considerations in employing nHAP for engineered in situ remediation of metal- and radionuclide-contaminated groundwater. (C) 2014 Elsevier B.V. All rights reserved.