Temporal variability in TiO2 engineered particle concentrations in rural Edisto River.

Titanium dioxide (TiO2) is widely used in engineered particles including engineered nanomaterial (ENM) and pigments, yet its occurrence, concentrations, temporal variability, and fate in natural environmental systems are poorly understood. For three years, we monitored TiO2 concentrations in a rural river basin (Edisto River, < 1% urban land cover) in South Carolina, United States. The total concentrations of Ti, Nb, Al, Fe, Ce, and La in the Edisto River trended higher during spring/summer compared to autumn/winter. Upward trending Ti/Nb ratio in the spring/summer compared to near-background autumn/winter ratios of 255.7 ± 8.9 indicated agricultural preparation and growing-season-related increases in TiO2 engineered particles. In contrast, downward trending of the Ti/Al and Ti/Fe ratios in the spring and summer compared to the near-background autumn/winter ratios of 0.05 indicated greater mobilization of Fe and Al, relative to Ti during spring/summer. Surface-water concentrations of TiO2 engineered particles varied between 0 and 128.7 ± 3.9 µg TiO2 L-1. Increases in TiO2 concentrations over the spring/summer were associated with increases in phosphorus, orthophosphate, nitrate, ammonia, anthropogenic gadolinium, water temperature, suspended sediments, organic carbon, and alkalinity, and with decreases in dissolved oxygen. The association between these contaminants together with the timing of the increases in their concentrations is consistent with diffuse wastewater sources, such as reuse application overspray, biosolids fertilization, leaking sewers, or septic tanks, as the driver of instream concentrations; however, other diffuse sources cannot be ruled out. The findings of this study indicate spatially-distributed (non-point source) releases can result in high concentrations of TiO2 engineered particles, which may pose higher risks to rural stream aquatic ecosystems during the agricultural season. The results illustrate the importance of monitoring seasonal variations in engineered particles concentrations in surface waters for a more representative assessment of ecosystem risk.

Temporal variation in TiO2 engineered particle concentrations in the Broad River during dry and wet weathers.

Titanium dioxide (TiO2) engineered particles are widely used in the urban environment as pigments in paints, and as active ingredients in photocatalytic coatings. Consequently, studies are necessary to quantify TiO2 engineered particle concentrations and their temporal variability in surface waters to gain better understanding about their abundance and environmental fate in order to minimize their potential environmental impacts. The objective of this study was to determine the temporal variability in the concentration of TiO2 engineered particles in the Broad River, Columbia, South Carolina, United States during dry and wet weather conditions and to examine the relationship between flow discharge, water quality indicators, and the concentration of TiO2 engineered particles. TiO2 engineered particle concentration in the Broad River water was determined by mass balance calculation using bulk titanium concentration and the increase in Ti/Nb ratio above the natural background ratio. The relative abundance of single metal and multi-metal Ti-bearing particles was determined by single particle-inductively coupled plasma-time of flight-mass spectrometer (SP-ICP-TOF-MS). Additionally, the elemental ratios of Ti/Nb, Ti/Al, and Ti/Fe within multi-metal Ti-bearing particles were determined at the single particle level. Discharge, bulk elemental concentrations (e.g., Ti, Al, Fe, and Nb), bulk elemental ratios (e.g., Ti/Al, Ti/Fe, and Ti/Nb), TiO2 engineered particle concentration, and turbidity displayed the same trend of rise and fall following storm events. Linear relationships were established between turbidity and TiO2 engineered particle concentrations in the Broad River for different flow regimes. However, no correlation was observed between TiO2 engineered particle concentrations and flow discharge, dissolved oxygen, pH, or ionic strength. The established correlations between turbidity and TiO2 engineered particle concentrations are important as they can be used to translate the continuously monitored turbidity to TiO2 concentrations.

Episodic surges in titanium dioxide engineered particle concentrations in surface waters following rainfall events.

Quantifying and characterizing engineered particles in environmental systems is key for assessing their risk but remains challenging and requires the distinction between natural and engineered particles. The objective of this study was to characterize and quantify the concentrations of titanium dioxide engineered particles in the Broad River, Columbia, South Carolina, United States during and following rainfall events. The elemental ratio distributions of Ti/Nb, Ti/Fe, and Ti/Al, determined on a single particle basis using inductively coupled plasma-time of flight-mass spectrometry (SP-ICP-TOF-MS), were similar between samples during the different rainfall events, indicating that naturally occurring particles had the same elemental ratios and origin. Therefore, the changes in the Ti/Nb ratios in the bulk water samples were attributed to the introduction of titanium dioxide engineered particles into the Broad River with urban runoff during and following rainfall events. The total concentrations of Ti, Fe, Al, Nb, Ce, and La in the Broad River followed the same trend of rise and fall as the discharge/runoff. The elemental ratios of Ti/Nb were higher (e.g., 330 to 565) than the average crustal values (e.g., 320) and the natural background elemental ratios in surface waters in Columbia, SC (e.g., 266.4 ± 8.9), suggesting contamination with titanium dioxide engineered particles. The concentration of titanium dioxide engineered particles were estimated by mass balance calculations using total titanium concentrations and increases in Ti/Nb ratios above the natural background ratios. The concentrations of titanium dioxide engineered particles in the Broad River varied between 20 and 140 µg TiO2 L-1 following rainfall events. The source of titanium dioxide was attributed to urban runoff due to the absence of sewage contamination as indicated by the low size of the gadolinium anomaly. The findings of this study demonstrate that urban runoff is a major source of titanium dioxide engineered particles to urban rivers, which results in episodic high concentrations of titanium dioxide engineered particles, which may pose environmental risks during and following rainfall events. This study also highlights the importance of determining the temporal variations in engineered particle concentrations in surface waters for a more comprehensive risk assessment of engineered particles.

Concentrations and size distribution of TiO2 and Ag engineered particles in five wastewater treatment plants in the United States.

The growing use of engineered particles (e.g., nanosized and pigment sized particles, 1 to 100 nm and 100 to 300 nm, respectively) in a variety of consumer products increases the likelihood of their release into the environment. Wastewater treatment plants (WWTPs) are important pathways of introduction of engineered particles to the aquatic systems. This study reports the concentrations, removal efficiencies, and particle size distributions of Ag and TiO2 engineered particles in five WWTPs in three states in the United States. The concentration of Ag engineered particles was quantified as the total Ag concentration, whereas the concentration of TiO2 engineered particles was quantified using mass-balance calculations and shifts in the elemental ratio of Ti/Nb above their natural background elemental ratio. Ratios of Ti/Nb in all WWTP influents, activated sludges, and effluents were 2-12 times higher (e.g., 519 to 3243) than the natural background Ti/Nb ratio (e.g., 267 ± 9), indicating that 49-92% of Ti originates from anthropogenic sources. The concentration of TiO2 engineered particles (in µg TiO2 L-1) in the influent, activated sludge, and effluent varied within the ranges of 70-670, 3570-6700, and 7-30, respectively. The concentration of Ag engineered particles (in µg Ag L-1) in the influent, activated sludge, and effluent varied within the ranges of 0.11-0.33, 1.45-1.65, and 0.01-0.04, respectively. The overall removal efficiency (e.g., effluent/influent concentrations) of TiO2 engineered particles (e.g., 90 to 96%) was higher than that for Ag engineered particles (e.g., 82 to 95%). Particles entering WWTPs are in the nanosized range for Ag (e.g., >99%) and a mixture of nanosized (e.g., 15 to 90%) and pigment sized particles (e.g., 10 to 85%) for TiO2. Nearly all Ag (>99%) and 55 to 100% of TiO2 particles discharged to surface water with WWTP effluent are within the nanosize range. This study provides evidence that TiO2 and Ag engineered nanomaterials enter aquatic systems with WWTP effluents, and that their concentrations are expected to increase with the increased applications of TiO2 and Ag engineered nanomaterials in consumer products.

Stormwater green infrastructures retain high concentrations of TiO2 engineered (nano)-particles.

Stormwater conveys natural and engineered (nano)-particles, like any other pollutants, from urban areas to water resources. Thus, the use of stormwater green infrastructures (SGI), which infiltrate and treat stormwater, can potentially limit the spread of engineered (nano)-particles in the environment. However, the concentration of engineered (nano)-particles in soil or biofilter media used in SGI has not been measured due to difficulties in distinguishing natural vs. engineered (nano)-particles. This study reports, for the first time, the concentration and size distribution of TiO2 engineered (nano)-particles in soils collected from SGI. The concentrations of TiO2 engineered (nano)-particles were determined by mass balance calculations based on shifts in elemental concentration ratios, i.e., Ti to Nb, Ti to Ta, and Ti to Al in SGI soils relative to natural background elemental ratios. The concentrations of TiO2 engineered (nano)-particles in SGI soils varied between 550 ± 13 and 1800 ± 200 mg kg-1. A small fraction of TiO2 engineered (nano)-particles could be extracted by ultrapure water (UPW) and Na4P2O7; however, the concentration of TiO2 engineered (nano)-particles was higher in the Na4P2O7-extracted suspensions than in UPW-extracted suspensions. The concentration of TiO2 in the nanosize range increased with the increase in extractant (Na4P2O7) volume to soil mass ratio due to the increased disaggregation of soil heteroaggregates. The size distribution of TiO2 engineered (nano)-particles in the < 450 nm Na4P2O7-extracted suspension from one of the SGI soils was determined by asymmetrical flow-field flow fractionation coupled to inductively coupled plasma-mass spectrometer, and was found to vary in the range of 25-200 nm with a modal size of 50 nm. These results demonstrated that the increase in the Ti to natural tracers (e.g., Nb, Ta, and Al) elemental ratios in the SGI soil relative to bulk soil can be used to estimate the concentration of TiO2 engineered (nano)-particles in SGI.

Detection and quantification of engineered particles in urban runoff.

Urban runoff conveys contaminants including titanium dioxide (TiO2), widely used as engineered nanoparticles (e.g., 1-100 nm) and pigments (e.g., 100-300 nm) in the urban environment, to receiving surface waters. Yet, the concentrations of TiO2 engineered particles (e.g., engineered nanoparticles and pigments) in urban runoff has not been determined due to difficulties in distinguishing natural from engineered TiO2 particles in environmental matrices. The present study examines the occurrence and estimates the concentrations of TiO2 engineered particles in urban runoff under wet- and dry-weather conditions. Urban runoff was collected from two bridges in Columbia, South Carolina, USA under wet-weather conditions and from the Ballona Creek and Los Angeles (LA) River in Los Angeles, California, USA under dry-weather conditions. The concentrations of TiO2 engineered particles were determined by mass balance calculations based on shifts in elemental concentration ratios in urban runoff relative to natural background elemental ratios. Elemental ratios of Ti to Nb in urban runoff were higher than the natural background ratios, indicating Ti contamination. The occurrence of TiO2 engineered particles was further confirmed by transmission electron microscopy coupled with energy dispersive spectroscopy. The concentration of TiO2 engineered particles in urban runoff was estimated to be in the range of 5-150 µg L-1. Therefore, this study identifies urban runoff as a previously unaccounted source of TiO2 engineered particle release to the environment, which should be included in engineered nanoparticle fate modeling studies and in estimating environmental release of engineered nanoparticles.