Effects of sodium dodecyl sulfate and solution chemistry on retention and transport of biogenic nano-hydroxyapatite in saturated porous media

Biogenic nano-hydroxyapatite (bHAP) has shown great potential in the remediation of heavy metal contaminated soil. However, there is very little understanding about the impact of anionic surfactants on transport of bHAP in saturated porous media, challenging the in-situ remediation using bHAP. To fill this knowledge gap, we explored mobility of bHAP in saturated sand columns with sodium dodecyl sulfate (SDS) under various environmental conditions. Results showed that the bare bHAP particles were basically immobile in sandy porous media, while in the presence of SDS (0.05%-0.5%), significant amounts (C/C0 = 0-0.71) of bHAP were transported at pH 7.0 when Na+ was the main background electrolyte. Mobility of bHAP increased with the increasing SDS concentration and pH, due to the increased stability of bHAP and repulsive interaction between bHAP and quartz sands. However, transport of bHAP was hindered in the presence of higher ionic strengths. Particularly, the retention of bHAP was more sensitive to the increase in concentration of divalent Ca2+ than monovalent Na+. Under different environmental conditions (SDS concentrations, pH, flow velocities, ionic strength, and ionic composition), the peak value of the bHAP breakthrough curves (BTCs) varied greatly, and the bHAP BTCs exhibited tailing characteristics. A time fractional convection-dispersion-deposition equation (tFCDDE) was proposed to describe the transport of bHAP in the sand column. Furthermore, the DerjaguinLandau-Verwey-Overbeek (DLVO) theoretical calculation accurately reflected the interaction of bHAP and quartz sand. This study provided a comprehensive understanding of the fate and transport behaviors of bHAP affected by SDS, which was necessary to evaluate the efficacy of bHAP in the remediation of contaminated soil and groundwater.