Transport of ARS-labeled hydroxyapatite nanoparticles in saturated granular media is influenced by surface charge variability even in the presence of humic acid

► The transport and retention kinetics of ARS-labeled hydroxyapatite nanoparticles (ARS-nHAP) were investigated over a range of ionic strengths in the presence of humic acid. ► A two-site kinetic attachment model predicted both the breakthrough curves and retention profiles of ARS-nHAP quite well. ► The retention profiles of ARS-nHAP exhibited hyperexponential shapes for all the test conditions. ► Surface charge heterogeneities on the collector surfaces and especially within the ARS-nHAP population contributed to hyperexponential retention profiles.