4.1 Reductive Dissolution of CNPs and Related Toxicity
Effects
It is generally accepted that CNPs have extremely low solubility, and the dissolved
Ce ions have little or even no impacts on its cytotoxicity. However, numbers of
researches have shown that CNPs undergo partial dissolution under specific conditions (Dahle et al. 2015; Plakhova et al. 2016; Zhang et al. 2012). In addition,
toxicity of CNPs is largely associated with high redox potential (E H ) of tetravalent
and trivalent Ce ions and their ability to oxidize biomolecules. Thus, the dissolution
of CNPs to form Ce ions is one of the main environmental risk issues.
Extensive data indicated that CNPs solubility is strongly dependent on pH
(Auffan et al. 2017b), as shown in Fig. 4a. As an example, CNPs dissolution was
lower than or close to detection limits (0.1 nM) at pH 7 and 9, but 3.1% (i.e., 29 μM)
CNPs dissolved at pH 4 in artificial soil solution (AS) (Cornelis et al. 2011).
Similarly, CNPs dissolution was reported to be significant at pH < 5 and
pH < 4.5 by Dahle et al. (Dahle et al. 2015) and Plakhova et al. (2016), respectively,
indicating the promoted dissolution by proton. According to the measurements of E H
values, the solubility-pH dependence could be associated with differing redox
conditions of pe-pH dependence (Fig. 4b) (Plakhova et al. 2016). Hence, the
different solubility behavior of CNPs at pH 1.5–4.5, pH 4.5–7, and pH > 7 can be
explained by the different redox conditions. It has also been concluded that CNPs
appeared to be more soluble in AS relative to 0.1 mM NaCl, possibly because of both
ionic Ce(III) and Ce(IV) forming pairs with inorganic oxyanions, thus enhancing
solubility (Cornelis et al. 2011). Through complexation, the presence of NOM like
arabic gum and organic ligands like ethylenediaminetetraacetic acid (EDTA) and
CA were found to enhance CNPs dissolution (Schwabe et al. 2014; Zhang et al.
2012) as demonstrated in Fig. 4c.
Cornelis et al. proposed that through geochemical modeling in PHREEQC, Ce
(IV) ion concentration was much higher than that in equilibrium with CNPs and Ce
(OH) 4 . Therefore, it is speculated that part of Ce which dissolved from CNPs may
occur as Ce(III) (Cornelis et al. 2011). The available Pourbaix diagrams shown in
Channei et al. verified that at pH < 7, the predominant cerium species in solution is
Ce(III) (Channei et al. 2017). Namely, the more thermodynamically probable solubility mechanisms of CNPs are reductive dissolution, where Ce(IV) would be
reduced to a more stable soluble species of Ce(III) (Fig. 4a). This phenomenon
was highly consistent with the considerations of Zhang et al. that with the assistance
of reducing substances secreted by roots, CNPs might be first reduced and then
released as Ce(III) (Zhang et al. 2012). Based on the environmental data with pH
ranging from 2 to 7 in Fig. 4a, a reductive dissolution model was proposed by
Plakhova et al. (2016). The solubility process of CNPs can be depicted by the
following equation:
Surface Properties and Environmental Transformations Controlling the. . .
183
Effects
It is generally accepted that CNPs have extremely low solubility, and the dissolved
Ce ions have little or even no impacts on its cytotoxicity. However, numbers of
researches have shown that CNPs undergo partial dissolution under specific conditions (Dahle et al. 2015; Plakhova et al. 2016; Zhang et al. 2012). In addition,
toxicity of CNPs is largely associated with high redox potential (E H ) of tetravalent
and trivalent Ce ions and their ability to oxidize biomolecules. Thus, the dissolution
of CNPs to form Ce ions is one of the main environmental risk issues.
Extensive data indicated that CNPs solubility is strongly dependent on pH
(Auffan et al. 2017b), as shown in Fig. 4a. As an example, CNPs dissolution was
lower than or close to detection limits (0.1 nM) at pH 7 and 9, but 3.1% (i.e., 29 μM)
CNPs dissolved at pH 4 in artificial soil solution (AS) (Cornelis et al. 2011).
Similarly, CNPs dissolution was reported to be significant at pH < 5 and
pH < 4.5 by Dahle et al. (Dahle et al. 2015) and Plakhova et al. (2016), respectively,
indicating the promoted dissolution by proton. According to the measurements of E H
values, the solubility-pH dependence could be associated with differing redox
conditions of pe-pH dependence (Fig. 4b) (Plakhova et al. 2016). Hence, the
different solubility behavior of CNPs at pH 1.5–4.5, pH 4.5–7, and pH > 7 can be
explained by the different redox conditions. It has also been concluded that CNPs
appeared to be more soluble in AS relative to 0.1 mM NaCl, possibly because of both
ionic Ce(III) and Ce(IV) forming pairs with inorganic oxyanions, thus enhancing
solubility (Cornelis et al. 2011). Through complexation, the presence of NOM like
arabic gum and organic ligands like ethylenediaminetetraacetic acid (EDTA) and
CA were found to enhance CNPs dissolution (Schwabe et al. 2014; Zhang et al.
2012) as demonstrated in Fig. 4c.
Cornelis et al. proposed that through geochemical modeling in PHREEQC, Ce
(IV) ion concentration was much higher than that in equilibrium with CNPs and Ce
(OH) 4 . Therefore, it is speculated that part of Ce which dissolved from CNPs may
occur as Ce(III) (Cornelis et al. 2011). The available Pourbaix diagrams shown in
Channei et al. verified that at pH < 7, the predominant cerium species in solution is
Ce(III) (Channei et al. 2017). Namely, the more thermodynamically probable solubility mechanisms of CNPs are reductive dissolution, where Ce(IV) would be
reduced to a more stable soluble species of Ce(III) (Fig. 4a). This phenomenon
was highly consistent with the considerations of Zhang et al. that with the assistance
of reducing substances secreted by roots, CNPs might be first reduced and then
released as Ce(III) (Zhang et al. 2012). Based on the environmental data with pH
ranging from 2 to 7 in Fig. 4a, a reductive dissolution model was proposed by
Plakhova et al. (2016). The solubility process of CNPs can be depicted by the
following equation:
Surface Properties and Environmental Transformations Controlling the. . .
183