Polymorph-dependent titanium dioxide nanoparticle dissolution in acidic and alkali digestions

Abstract

Multiple polymorphs (anatase, brookite and rutile) of titanium dioxide nanoparticles (TiO₂-NPs) with variable structures were quantified in environmental matrices via microwave-based hydrofluoric (HF) and nitric (HNO₃) mixed acid digestion and muffle furnace (MF)-based potassium hydroxide (KOH) fusion. The environmental matrices included stream bed sediments, kaolinite and bentonite. The percentage of titanium (Ti) recovered from the mixed acid digestion was not statistically different from KOH fusion when anatase and brookite TiO₂-NPs were blended in all three environmental matrices. However, the percentage of Ti recovery of rutile TiO₂-polymorph from the samples digested using the mixed acid digestion method was significantly lower [23 (±5), 12 (±6), 11 (±0.6)] than those digested using KOH fusion method [74 (±4), 53 (±7), 75 (±2)]. The recovery percent values reported are for Ti in sediment, kaolinite, and bentonite matrices, respectively. Exposing the TiO₂-NP spiked samples to elevated heat and pressure reduced the recovery of Ti from all three polymorphs via mixed acid digestion. In contrast, Ti recoveries from KOH fusion improved after heat and pressure treatment. A narrowing of the X-ray diffraction (XRD) peaks for anatase and brookite after heat and pressure treatment indicated an increase in the aggregation or particle interaction of the TiO₂-NPs. The XRD peaks for rutile TiO₂-NP polymorph was similar before and after heat and pressure treatment. In summary, regardless of the selected environmental matrix type, the mixed acid digestibility of TiO₂-NPs is polymorph-dependent; whereas, the KOH-fusion digestibility is polymorph independent. Therefore, when analyzing environmental samples containing TiO₂-NPs with unknown polymorphs, a KOH-fusion digestion method is recommended for yielding consistent results.