Abstract

Titanium (Ti) occurs naturally in soils and as highly purified titanium dioxide (TiO2) in many commercial products that have been used for decades. We report for the first time the occurrence, characterization, and removal of nano- and larger-sized Ti at wastewater treatment plants (WWTPs). At one WWTP studied in detail, raw sewage contained 100 to nearly 3000 μg Ti/L. Ti larger than 0.7 μm accounted for the majority of the Ti in raw sewage, and this fraction was well removed by WWTP processes. Ti concentrations in effluents from this and several other WWTPs ranged from <5 to 15 μg/L and were nearly all present in the <0.7 μm size fraction. As Ti was removed, it accumulated in settled solids at concentrations ranging from 1 to 6 μg Ti/mg. Ti-containing solids were imaged in sewage, biosolids, and liquid effluent as well as in commercial products containing engineered TiO2. Single nanoparticles plus spherical aggregates (50 nm to a few hundred nanometer in size) composed of sub-50 nm spheres of Ti and oxygen only (presumably TiO2) were observed in all samples. Significantly larger silicate particles containing a mixture of Ti and other metal atoms were also observed in the samples. To support the field work, laboratory adsorption batch and sequencing batch reactor experiments using TiO2 and activated sludge bacteria verified that adsorption of TiO2 onto activated sludge biomass occurs. Monitoring for TiO2 in the environment where WWTP liquid effluent is discharged (rivers, lakes, oceans) or biomass disposed (landfills, agriculture and soil amendments, incinerator off-gas or residuals) will increase our knowledge on the fate and transport of other nanomaterials in the environment.