Carbon black retention in saturated natural soils: Effects of flow conditions, soil surface roughness and soil organic matter

- We studied the retention of carbon black (nCB) in two saturated natural soils.
- The nCB deposition increased as soil organic matter was removed by preheating.
- Lowering ionic strength and raising pH released only small portion of deposited nCB.
- The maximum retention capacity of nCB in soils was not controlled by flow velocity.
- Kelvin probe force microscopy indicated that nCB was retained on surface roughness.

We evaluated factors affecting the transport, retention, and re-entrainment of carbon black nanoparticles (nCBs) in two saturated natural soils under different flow conditions and input concentrations using the two-site transport model and Kelvin probe force microscopy (KPFM). Soil organic matter (SOM) was found to create unfavorable conditions for the retention. Despite an increased flow velocity, the relative stability of the estimated maximum retention capacity in soils may suggest that flow-induced shear stress forces were insufficient to detach nCB. The KPFM observation revealed that nCBs were retained at the grain boundary and on surface roughness, which brought about substantial discrepancy between theoretically-derived attachment efficiency factors and the ones obtained by the experiments using the two-site transport model. Thus, decreasing ionic strength and increasing solution pH caused re-entrainment of only a small fraction of retained nCB in the soil columns.