crystalline structure with an oxygen vacancy being created upon the loss of lattice
oxygen atoms (Esch et al. 2005; Fronzi et al. 2009; Tsai et al. 2008). The unique
excellent ability of cerium oxides to shuttle between Ce(III) (Ce 2 O 3 ) and Ce
(IV) (CeO 2 ) makes CNPs of particular interest to microelectronics or semiconductor
industries (Conesa 1995), mechanical polishers (Hoshino et al. 2001; Stanek et al.
2008), pharmacological agents (Pelletier et al. 2010), and fuel additives in diesel
(Johnson and Park 2012; Sajith et al. 2010). The wide production and application of
CNPs will inevitably lead to increasing concentrations in many natural and
engineered compartments such as surface waters (Conway et al. 2014; Zhao et al.
2017), soils (Layet et al. 2017; Liu and Cohen 2015), wastewater (Lazareva and
Keller 2014; Wang et al. 2018b), sewage sludge (Lazareva and Keller 2014; You
et al. 2017), and air (Hong et al. 2014; Johnson and Park 2012). However, there is
concern that CNPs may present hazards to ecological receptor species, due to their
small particle size and intensified reactivity by design (Antoine Thill et al. 2006a;
Nel et al. 2006). As early as 2001, a report about human health risks of cerium oxides
from diesel fuels warrants immediate attention to fully assess the ecological and
environmental effects of CNPs (Antoine Thill et al. 2006a; Liu et al. 2015).
Thereafter, extensive investigations into the toxicity of CNPs have been conducted
(Heckert et al. 2008; Pešić et al. 2015), which is a serious issue requiring high
precaution.
Thill et al. (2006) found that CNPs injured the outer membrane of E. coli cells and
posed a lethal effect. Moreover, CNPs could be one of the prooxidants changing the
intracellular redox status of cells (Miao et al. 2017; Pešić et al. 2015; Xu et al. 2018;
You et al. 2015), which provokes the loss of survival ability. Garcia et al. reported
the strong inhibitory action of CNPs on the anaerobic activated sludge from wastewater treatment plants (WWTPs) and consequently a substantial inhibition in biogas
production (García et al. 2012). However, the study of Limbach et al. showed that
CNPs posed no effects on the heterotrophic microbial agglomerations from a
municipal WWTP (Limbach et al. 2008). Lethal toxicities of CNPs in Daphnia
magna and Cophixalus riparius have been demonstrated at 1 mg/L after exposure
for 96 h (Lee et al. 2009), while in other researches no acute toxicity was observed in
Daphnia magna at 10 mg/L after the same duration exposure (Gaiser et al. 2011) or
even up to 1,000 mg/L after a 48 h exposure (van Hoecke et al. 2009). The list of
studies revealing the effects of CNPs on the environment and potentially on humans
is long and has been extensively reviewed over the past years (Batley et al. 2013;
Collin et al. 2017; Ganguly et al. 2018; Milani et al. 2017; Petosa et al. 2010).
However, few papers have well identified and systematically summarized the critical
factors in relation to the physicochemical properties of CNPs leading to the existed
contradictory results.
Because of the susceptibility of CNPs to environmental transformation, factors
such as pH, redox potential (E H ), as well as the quantity and composition of natural
organic matter (NOM) are likely to influence the transport and simultaneously
transformations of CNPs (Auffan et al. 2009; Collin et al. 2014; Liu et al. 2011).
Interactions and transformations of CNPs under different environmental scenarios
and toxicological media are well known to occur (Louie et al. 2014; Merrifield et al.
158
G. You et al.
oxygen atoms (Esch et al. 2005; Fronzi et al. 2009; Tsai et al. 2008). The unique
excellent ability of cerium oxides to shuttle between Ce(III) (Ce 2 O 3 ) and Ce
(IV) (CeO 2 ) makes CNPs of particular interest to microelectronics or semiconductor
industries (Conesa 1995), mechanical polishers (Hoshino et al. 2001; Stanek et al.
2008), pharmacological agents (Pelletier et al. 2010), and fuel additives in diesel
(Johnson and Park 2012; Sajith et al. 2010). The wide production and application of
CNPs will inevitably lead to increasing concentrations in many natural and
engineered compartments such as surface waters (Conway et al. 2014; Zhao et al.
2017), soils (Layet et al. 2017; Liu and Cohen 2015), wastewater (Lazareva and
Keller 2014; Wang et al. 2018b), sewage sludge (Lazareva and Keller 2014; You
et al. 2017), and air (Hong et al. 2014; Johnson and Park 2012). However, there is
concern that CNPs may present hazards to ecological receptor species, due to their
small particle size and intensified reactivity by design (Antoine Thill et al. 2006a;
Nel et al. 2006). As early as 2001, a report about human health risks of cerium oxides
from diesel fuels warrants immediate attention to fully assess the ecological and
environmental effects of CNPs (Antoine Thill et al. 2006a; Liu et al. 2015).
Thereafter, extensive investigations into the toxicity of CNPs have been conducted
(Heckert et al. 2008; Pešić et al. 2015), which is a serious issue requiring high
precaution.
Thill et al. (2006) found that CNPs injured the outer membrane of E. coli cells and
posed a lethal effect. Moreover, CNPs could be one of the prooxidants changing the
intracellular redox status of cells (Miao et al. 2017; Pešić et al. 2015; Xu et al. 2018;
You et al. 2015), which provokes the loss of survival ability. Garcia et al. reported
the strong inhibitory action of CNPs on the anaerobic activated sludge from wastewater treatment plants (WWTPs) and consequently a substantial inhibition in biogas
production (García et al. 2012). However, the study of Limbach et al. showed that
CNPs posed no effects on the heterotrophic microbial agglomerations from a
municipal WWTP (Limbach et al. 2008). Lethal toxicities of CNPs in Daphnia
magna and Cophixalus riparius have been demonstrated at 1 mg/L after exposure
for 96 h (Lee et al. 2009), while in other researches no acute toxicity was observed in
Daphnia magna at 10 mg/L after the same duration exposure (Gaiser et al. 2011) or
even up to 1,000 mg/L after a 48 h exposure (van Hoecke et al. 2009). The list of
studies revealing the effects of CNPs on the environment and potentially on humans
is long and has been extensively reviewed over the past years (Batley et al. 2013;
Collin et al. 2017; Ganguly et al. 2018; Milani et al. 2017; Petosa et al. 2010).
However, few papers have well identified and systematically summarized the critical
factors in relation to the physicochemical properties of CNPs leading to the existed
contradictory results.
Because of the susceptibility of CNPs to environmental transformation, factors
such as pH, redox potential (E H ), as well as the quantity and composition of natural
organic matter (NOM) are likely to influence the transport and simultaneously
transformations of CNPs (Auffan et al. 2009; Collin et al. 2014; Liu et al. 2011).
Interactions and transformations of CNPs under different environmental scenarios
and toxicological media are well known to occur (Louie et al. 2014; Merrifield et al.
158
G. You et al.