COMPARISON OF THE AGGREGATION BEHAVIOR OF TiO2 NANOPARTICLES EXPOSED TO FULVIC ACID AND BACILLUS SUBTILIS EXUDATES

The aggregation behavior of engineered TiO₂ nanoparticles depends on solution pH, ionic strength (IS) and dissolved organic matter (DOM) concentration. Most studies of the effects of DOM on the aggregation of nano-TiO₂ have used pedogenic humic and fulvic acids in relatively high DOM-to-particle ratios, and in general, suggest a tendency for less aggregation in the presence of DOM. However, the aggregation behavior of nano-TiO₂ suspensions is likely to differ significantly when exposed to lower DOM-to-particle ratios and/or DOM of aquagenic origin, such as microbial exudates. Hence, we compared the effects of varying concentrations (0.1 mg DOC L^-1 to 10 mg DOC L^-1) of Suwannee River fulvic acid (SRFA) and Bacillus subtilis exudate on the aggregation behavior of 50 mg L^-1 nano-TiO₂ suspensions in high (0.1M) and low (0.01M) ionic strengths at pH 3 to 7.5. We measured sedimentation rates using sequential UV-Vis spectrophotometry over 240 minutes, and compared the measured rates for DOM-bearing samples to those for DOM-free controls at the same pH and IS. While nano-TiO₂ aggregation behaviors varied widely in response to pH, IS, and DOM concentration, suspensions exposed to either SRFA or bacterial exudate at DOC-normalized concentrations exhibited remarkably similar sedimentation rates in approximately half of all solution conditions. In the remaining treatments, nano-TiO₂ aggregates exhibited slightly higher sedimentation rates when exposed to bacterial exudates, relative to SRFA. In high IS treatments, nano-TiO₂ exposed to either SRFA or bacterial exudate generally sedimented rapidly, except in the presence of relatively high DOC concentrations at pH 6 and 7.5. Low IS treatments exhibited a much larger range of effects. In fact, relative to DOM-free controls, sedimentation rates for nano-TiO₂ aggregates in SRFA and bacterial exudate exposures were up to 14 times faster at pH 3, and up to 14 times slower at pH 7.5. This pH-dependent aggregation behavior at low IS in the presence of DOM represents a complete reversal of the aggregation behavior for DOM-free control samples, where nano-TiO₂ suspensions sedimented slowly at pH 3 and quickly at pH 7.5. Collectively, these data suggest that DOM origin and concentration, as well as solution pH and IS, may control the fate and mobility of nano-TiO₂ in geologic systems.