TiO<inf>2</inf> nanoparticle sorption to sand in the presence of natural organic matter

The sorption properties of TiO<inf>2</inf> 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 TiO<inf>2</inf> NPs, the consequent zeta (ζ)-potential drop was more significant in the TiO<inf>2</inf> NPs, indicating that the decreased sorption should be primarily ascribed to the fraction of SRHA sorbed onto the TiO<inf>2</inf> 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 TiO<inf>2</inf> NPs and sand in the reaction system. Interaction force measurements indicated that SRHA inhibits the approach of the TiO<inf>2</inf> NPs to the sand surface, as expected by the Derjaguin–Landau–Verwey–Overbeek calculation. However, the adhesion force of TiO<inf>2</inf> NPs to sand was not critically affected by the presence of SRHA. TiO<inf>2</inf> 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 TiO<inf>2</inf> NPs in aquifers by inhibiting TiO<inf>2</inf> NP sorption to aquifer materials.