contribute to the biological properties of CNPs, but its effects on Ce(III) /Ce
(IV) redox reactions need to be further clarified.
Collectively, the various physicochemical properties tailored by synthesis
methods and tested organism types reflect the different environmental effects of
CNPs. Nevertheless, there is no consensus on which CNPs characteristic is primarily
responsible for the observed effects. Perhaps the dynamic and stochastic transformation of CNPs under environmental scenarios is more realistic in controlling
CNPs’ ecological processes and effects, as discussed next.
3 Phase Transformation of CNPs in Different
Environmental Scenarios: Impact on Bioaccumulation
and Toxicity
In the case of CNPs to readily chemically transform between Ce(IV) and Ce(III), the
physicochemical transformations that accompany engineered and/or incidental colloids/coatings, as well as pursuant reactions in different environmental scenarios,
strongly complicate the understanding and evaluating on risks in relation to the
release of CNPs in the environment (Barton et al. 2014; Dahle et al. 2015; Merrifield
et al. 2013, 2017). To correctly forecast the environmental and ecological risks, it is
necessary to increase the knowledge of the transformations of CNPs in various
environmental scenarios.
The Hard/Soft Acid/Base theory predicts the tendency of Ce to strongly bond
with hard base ligands like hydroxyl ions and P over hard acidic species such as
sulfates and hydrids. Generally, Ce(III) is more soluble than Ce(IV), for Ce(IV) is
known to undergo hydrolysis reaction to generate insoluble Ce(OH) 4 (solubility
product of K sp ¼ 2 Â 10
À48 ), whereas K sp of Ce(OH) 3 is 1.6 Â 10
À20 (Channei et al.
2017; Dahle et al. 2015). Nevertheless, Ce(III) could form the insoluble P compound
of CePO 4 with K sp of 1.0 Â 10
À23 (Xu et al. 2018). Some other common O-donor
organic (oxalate and tartrate) complex with Ce(III) to form products of
Ce 2 (C 2 O 4 ) 3 Á9H 2 O (K sp : 3.2 Â 10
À26 ) and Ce 2 (C 4 H 4 O 6 ) 3 (K sp : 1.0 Â 10
À19 )
(Dahle et al. 2015). These resulting transformations of CNPs will impact their fate,
transport, and toxic properties. The physicochemical and biological conditions
favoring the formation of complex are reviewed in the following sections considering various environmental scenarios.
3.1 Speciation and Ecological Effect of CNPs in Aquatic
Environments
As Ce exhibit various possible redox states, the chemical stability of CNPs can vary
in aquatically environmental conditions, as schematically illustrated in Fig. 2.
Surface Properties and Environmental Transformations Controlling the. . .
175
(IV) redox reactions need to be further clarified.
Collectively, the various physicochemical properties tailored by synthesis
methods and tested organism types reflect the different environmental effects of
CNPs. Nevertheless, there is no consensus on which CNPs characteristic is primarily
responsible for the observed effects. Perhaps the dynamic and stochastic transformation of CNPs under environmental scenarios is more realistic in controlling
CNPs’ ecological processes and effects, as discussed next.
3 Phase Transformation of CNPs in Different
Environmental Scenarios: Impact on Bioaccumulation
and Toxicity
In the case of CNPs to readily chemically transform between Ce(IV) and Ce(III), the
physicochemical transformations that accompany engineered and/or incidental colloids/coatings, as well as pursuant reactions in different environmental scenarios,
strongly complicate the understanding and evaluating on risks in relation to the
release of CNPs in the environment (Barton et al. 2014; Dahle et al. 2015; Merrifield
et al. 2013, 2017). To correctly forecast the environmental and ecological risks, it is
necessary to increase the knowledge of the transformations of CNPs in various
environmental scenarios.
The Hard/Soft Acid/Base theory predicts the tendency of Ce to strongly bond
with hard base ligands like hydroxyl ions and P over hard acidic species such as
sulfates and hydrids. Generally, Ce(III) is more soluble than Ce(IV), for Ce(IV) is
known to undergo hydrolysis reaction to generate insoluble Ce(OH) 4 (solubility
product of K sp ¼ 2 Â 10
À48 ), whereas K sp of Ce(OH) 3 is 1.6 Â 10
À20 (Channei et al.
2017; Dahle et al. 2015). Nevertheless, Ce(III) could form the insoluble P compound
of CePO 4 with K sp of 1.0 Â 10
À23 (Xu et al. 2018). Some other common O-donor
organic (oxalate and tartrate) complex with Ce(III) to form products of
Ce 2 (C 2 O 4 ) 3 Á9H 2 O (K sp : 3.2 Â 10
À26 ) and Ce 2 (C 4 H 4 O 6 ) 3 (K sp : 1.0 Â 10
À19 )
(Dahle et al. 2015). These resulting transformations of CNPs will impact their fate,
transport, and toxic properties. The physicochemical and biological conditions
favoring the formation of complex are reviewed in the following sections considering various environmental scenarios.
3.1 Speciation and Ecological Effect of CNPs in Aquatic
Environments
As Ce exhibit various possible redox states, the chemical stability of CNPs can vary
in aquatically environmental conditions, as schematically illustrated in Fig. 2.
Surface Properties and Environmental Transformations Controlling the. . .
175