Examining the efficiency of muffle furnace-induced alkaline hydrolysis in determining the titanium content of environmental samples containing engineered titanium dioxide particles

A novel muffle furnace (MF)-based potassium hydroxide (KOH) fusion digestion technique was developed and evaluated for different titanium dioxide materials in various solid matrices. Digestion of different environmental samples containing sediments, clay minerals and humic acid with and without TiO₂ particles was first performed utilizing the MF-based KOH fusion technique and its dissolution efficacy was compared to a Bunsen burner (BB)-based KOH fusion method. The three types of TiO₂ particles (anatase, brookite and rutile) were then digested with the KOH fusion techniques and microwave (MW)-based nitric (HNO₃)-hydrofluoric (HF) mixed acid digestion methods. Statistical analysis of the results revealed that Ti recoveries were comparable for the KOH fusion methods (BB and MF). For pure TiO₂ particles, the measured Ti recoveries compared to calculated values were 96%, 85% and 87% for anatase, brookite and rutile TiO₂ materials, respectively, by the MF-based fusion technique. These recoveries were consistent and less variable than the BB-based fusion technique recoveries of 104%, 97% and 72% and MW-based HNO₃-HF mixed acids digestion recoveries of 80%, 81% and 14%, respectively, for anatase, brookite and rutile. Ti percent recoveries and measurement precision decreased for both the BB and MF methods when TiO₂ was spiked into sediment, clay minerals, and humic acid. This drop in efficacy was counteracted by more thorough homogenization of the spiked mixtures and by increasing the mass of KOH in the MF fusion process from 1.6 g to 10.0 g. The MF-based fusion technique is consistently superior in digestion efficiency for all three TiO₂ polymorphs. The MF-based fusion technique required 20 minutes for digestion of 25 samples (based on in-house Lindberg MF capacity) compared to 8 hours for the same number of samples using the BB-based fusion technique. Thus, the MF-based fusion technique can be used to dissolve a large number of samples in a shorter time (e.g., 500 samples per 8 hours) while conserving energy and eliminating health and safety risks from methods involving HF.