Influence of Humic Acid on Stability and Attachment of nTiO2 Particles to Sand at Different pH

Abstract:

Stability of nano-scale or micro-scale titanium dioxide particles (nTiO₂) and their attachment to sediment grains have important implications to the fate and transport of nTiO₂ in subsurface environments. nTiO₂ may carry either positive or negative charges in natural water, therefore, environmental factors such as pH, humic substances, and Fe oxyhydroxide coatings on sediment grains, which are known to control the stability and transport of negatively charged colloids, may influence nTiO₂ in different manners. The objective of this study is to investigate the effects of pH and humic acid (HA) on the stability and attachment of nTiO₂ to sand, with special attention to low HA concentration ranges that are relevant to groundwater conditions. Stability and attachment of nTiO₂ to quartz sand and Fe oxyhydroxide coated quartz sand were experimentally measured under a range of low HA concentrations at pH 5 and 9. Results showed that HA can either promote or hinder nTiO₂ stability, depending on pH and HA concentration. We also found that HA can either enhance or reduce nTiO₂ attachment to Fe oxyhydroxide coating at pH 5, depending on HA concentration. Results further showed that at pH 5, Fe oxyhydroxide coating reduced nTiO₂ attachment to sand in the absence of HA but increased nTiO₂ attachment in the presence of low concentration of HA. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was invoked to analyze particle-to-particle and particle-to-sand interactions in order to elucidate the roles of pH, HA, quartz, and Fe coating in nTiO₂ stability and attachment. Overall, this study showed that changes in zeta potential of nTiO₂ and Fe coating due to pH changes and/or HA adsorption are the key factors that influence stability and attachment of nTiO₂.