TiO₂ nanoparticle sorption to sand in the presence of natural organic matter

The sorption properties of TiO₂ nanoparticles (NPs) on sand were investigated in the presence of Suwannee River humic acid (SRHA) as a surrogate for natural organic matter. SRHA drastically reduced the sorption preference regardless of the initial pH and ionic strength of the solution. Despite the higher SRHA loading onto sand versus the TiO₂ NPs, the consequent zeta (ζ)-potential drop was more significant in the TiO₂ NPs, indicating that the decreased sorption should be primarily ascribed to the fraction of SRHA sorbed onto the TiO₂ NP surfaces. The hindering effect of SRHA on the sorption increased with SRHA concentration, but stabilized at the concentration equivalent to full surface coverage of the TiO₂ NPs and sand in the reaction system. Interaction force measurements indicated that SRHA inhibits the approach of the TiO₂ NPs to the sand surface, as expected by the Derjaguin–Landau–Verwey–Overbeek calculation. However, the adhesion force of TiO₂ NPs to sand was not critically affected by the presence of SRHA. TiO₂ NP sorption to natural sand was limited as to the acid-treated sand in the presence of SRHA, implying that the organics dissolved from natural sand might facilitate the transport of TiO₂ NPs in aquifers by inhibiting TiO₂ NP sorption to aquifer materials.