Impact of montmorillonite clay on the homo- and heteroaggregation of titanium dioxide nanoparticles (nTiO2) in synthetic and natural waters

Junyu Wang; Xiaoli Zhao; Fengchang Wu; Zhi Tang; Tianhui Zhao; Lin Niu; Mengyuan Fang; Hongzhan Wang; Fanfan Wang

doi:10.1016/j.scitotenv.2021.147019

Impact of montmorillonite clay on the homo- and heteroaggregation of titanium dioxide nanoparticles (nTiO₂) in synthetic and natural waters

Sci Total Environ. 2021 Aug 25:784:147019. doi: 10.1016/j.scitotenv.2021.147019. Epub 2021 Apr 10.

Authors

Junyu Wang¹, Xiaoli Zhao², Fengchang Wu³, Zhi Tang⁴, Tianhui Zhao⁴, Lin Niu⁴, Mengyuan Fang⁴, Hongzhan Wang⁴, Fanfan Wang⁴

Affiliations

¹ College of Water Sciences, Beijing Normal University, Beijing 100875, PR China; State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China.
² State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China. Electronic address: zhaoxiaoli_zxl@126.com.
³ State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China. Electronic address: wufengchang@vip.skleg.cn.
⁴ State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China.

PMID: 34088034
DOI: 10.1016/j.scitotenv.2021.147019

Abstract

The homoaggregation of titanium dioxide nanoparticles (nTiO₂) and their heteroaggregation with ubiquitous natural clay colloids are crucial processes affecting the environmental transport and fate of nTiO₂, whereas the latter has received less attention. In this study, the effects of pH, electrolytes, natural organic matter (NOM), and montmorillonite on the homo- and heteroaggregation of nTiO₂ were systematically investigated. The isoelectric point of bare nTiO₂ was 6.98, whereas TiO₂-montmorillonite mixtures remained negative charged due to the reduced particle surface potential by heteroaggregation. Homoaggregation of nTiO₂ was mainly affected by anions, whereas heteroaggregation in TiO₂-montmorillonite mixtures was mainly affected by cations. Heteroaggregation between nTiO₂ and montmorillonite involved the adsorption of CC/CH. Intensive aggregation of nTiO₂ was observed with 4 mg/L montmorillonite, whereas with 20 mg/L montmorillonite, the aggregation was significantly inhibited by the over-coverage of montmorillonite. NOM was attached to the surface of nTiO₂ with the adsorption of functional groups involving CC/CH and OCO. The addition of NOM effectively reduced the homo- and heteroaggregation of nTiO₂, and the stabilizing effect was enhanced with the increased molecular weight and aromatic/aliphatic fraction in NOM. Besides electrostatic repulsion, steric repulsion could also be one of the main stabilization mechanisms of NOM. With Ca²⁺ in the solutions, the stabilizing effect of NOM was significantly weakened through cation bridging. The addition of montmorillonite could facilitate the aggregation of nTiO₂ in the presence of NOM. The homo- and heteroaggregation of nTiO₂ were also observed in 7 different types of natural waters. Homoaggregation predominated in waters with high ionic strength and low NOM contents (seawater and groundwater), whereas heteroaggregation predominated in surface freshwater and wastewater systems. The results reflect the instability of nTiO₂ in natural aquatic environments and the potential risk they pose to benthic organisms.

Keywords: Heteroaggregation; Homoaggregation; Montmorillonite; Natural waters; nTiO(2).