Opposite influences of mineral-associated and dissolved organic matter on the transport of hydroxyapatite nanoparticles through soil and aggregates.

The mechanism by which soil organic matter (SOM) controls nanoparticle transport through natural soils is unclear. In this study, we distinguished the specific effects of two primary SOM fractions, mineral-associated organic matter (MOM) and dissolved organic matter (DOM), on the transport of hydroxyapatite nanoparticles (nHAP) through a loamy soil under the conditions of saturated steady flow and environmentally relevant solution chemistry (1 mM NaCl at pH 7). The results showed that MOM could inhibit the transport of nHAP by decreasing electrostatic repulsion and increasing mechanical straining and hydrophobic interactions. Specifically, the presence of MOM reduced the mobility of nHAP in the bulk soil and its macroaggregates by ~4 fold and ~6 fold, respectively, and this hindered effect became further conspicuous in microaggregates (~36 fold decrease). An analysis of extended Derjaguin-Landau-Vervey-Overbeek (abbreviated as XDLVO) interactions indicated that MOM could decrease the primary energy barrier (Φmax1), primary minimum (Φmin1), and secondary minimum (Φmin2) to promote nHAP attachment. Conversely, DOM (10-50 mg L-1) favored nHAP mobility due to an increase in electrostatic repulsion among nHAP particles and between nHAP and soil surfaces. Pre-flushing soil with DOM (causing DOM sorption on soil) increased nHAP mobility by ~2 fold in the bulk soil and its macroaggregates, and this facilitated effect was furthered in microaggregates (~11 fold increase). The results of XDLVO interactions showed that DOM increased Φmax1, Φmin1, and Φmin2, producing an unfavorable effect on nHAP attachment. Mass recovery data revealed that the MOM-hindered effect was stronger than the DOM-facilitated effect on nHAP transport. This study suggested that changing SOM fractions could control the mobility of nanoparticles in the subsurface considerably.