grown plants might depend significantly on the standard of water-soluble fraction
(Zhang et al. 2015). There have also been some progresses in assessing the
bioaccumulation and trophic transfer of CNPs within terrestrial and specifically
agricultural food chains. Findings showed that CNPs may be taken up by plants
and transferred to consumers along food chains, which may affect food safety
(Hawthorne et al. 2014; Ma et al. 2018; Majumdar et al. 2016a, b). Majumdar
et al. further suggested that the bioaccumulation and biomagnifications of Ce in
the mature stages of primary and secondary consumers (Mexican bean beetles and
spined soldier bugs) feeding on kidney bean plants exposed to CNPs were observed,
due to the higher content of ingestion than excretion (Majumdar et al. 2016a, b).
To reveal the favorable reaction path to make CNPs more bioavailable, the
chemical fate of CNPs in both soil and plants was evaluated. Arai and Dahle showed
that CNPs of 30 nm and 78 nm were strongly adsorbed (>98%) by soils (Arai and
Dahle 2017). Under the oxic condition, >90% of CNPs remained as Ce(IV)O 2, and a
small portion of CePO 4 were identified in XANES analysis. Interestingly, under
anoxic conditions, the reduction was more pronounced in small CNPs, which is
considered to be highly related to bioavailability. However, the greater concentration
of exchangeable Ce(III) in large CNPs facilitated the formation of Ce(III)P/oxalate
surface precipitates, suppressing the bioavailable and thus reducing cytotoxicity
(Arai and Dahle 2017). Upon uptake, both the geochemical modeling and XANES
analysis of the roots failed to correlate the CNPs phytoavailability with different Ce
speciation in the soil solution, since Ce is mainly present as Ce(IV)O 2 inside the root
tissues (Layet et al. 2017; Ma et al. 2018; Zhao et al. 2012b). Nevertheless,
biotransformation of CNPs to Ce(III)P/carboxylate complexes has been observed
in root in hydroponic cucumber plants (Rui et al. 2015; Zhang et al. 2012). These
results further reveal the lower levels of bioavailability, transformation, and phytotoxicity of CNPs in soil than in aqueous media (Layet et al. 2017; Majumdar et al.
2016a, b; Zhang et al. 2015; Zhao et al. 2012a, b). Notably, in the trophic transfer
experiment, the reduction of Ce(IV) to Ce(III) did not occur in plants and snail faces,
and did only in the digestive gland of snail (Ma et al. 2018), which raises the
question whether this reduction creates potential risk to humans through the food
supply.
In short, though hydroponics studies provided valuable information about the
chemical fate and plant uptake of CNPs, the soil properties (porosity, pH, IS, NOM,
and mineral composition) played decisive roles in the bioavailability, migration,
chemical transformation, and their interactions at the nano-bio interface in soils.
Therefore, it is imperative to study the behavior of CNPs in the terrestrial system in
realistic conditions to determine if CNPs can be put in the food chain threatening the
ecosystems and human health.
3.3 Uptake and Toxicity of CNPs Through Air Exposure
As the increasing release of CNPs into the atmosphere through vehicles emissions
(Hong et al. 2014), plants could be exposed to unusual high concentrations of CNPs
180
G. You et al.
(Zhang et al. 2015). There have also been some progresses in assessing the
bioaccumulation and trophic transfer of CNPs within terrestrial and specifically
agricultural food chains. Findings showed that CNPs may be taken up by plants
and transferred to consumers along food chains, which may affect food safety
(Hawthorne et al. 2014; Ma et al. 2018; Majumdar et al. 2016a, b). Majumdar
et al. further suggested that the bioaccumulation and biomagnifications of Ce in
the mature stages of primary and secondary consumers (Mexican bean beetles and
spined soldier bugs) feeding on kidney bean plants exposed to CNPs were observed,
due to the higher content of ingestion than excretion (Majumdar et al. 2016a, b).
To reveal the favorable reaction path to make CNPs more bioavailable, the
chemical fate of CNPs in both soil and plants was evaluated. Arai and Dahle showed
that CNPs of 30 nm and 78 nm were strongly adsorbed (>98%) by soils (Arai and
Dahle 2017). Under the oxic condition, >90% of CNPs remained as Ce(IV)O 2, and a
small portion of CePO 4 were identified in XANES analysis. Interestingly, under
anoxic conditions, the reduction was more pronounced in small CNPs, which is
considered to be highly related to bioavailability. However, the greater concentration
of exchangeable Ce(III) in large CNPs facilitated the formation of Ce(III)P/oxalate
surface precipitates, suppressing the bioavailable and thus reducing cytotoxicity
(Arai and Dahle 2017). Upon uptake, both the geochemical modeling and XANES
analysis of the roots failed to correlate the CNPs phytoavailability with different Ce
speciation in the soil solution, since Ce is mainly present as Ce(IV)O 2 inside the root
tissues (Layet et al. 2017; Ma et al. 2018; Zhao et al. 2012b). Nevertheless,
biotransformation of CNPs to Ce(III)P/carboxylate complexes has been observed
in root in hydroponic cucumber plants (Rui et al. 2015; Zhang et al. 2012). These
results further reveal the lower levels of bioavailability, transformation, and phytotoxicity of CNPs in soil than in aqueous media (Layet et al. 2017; Majumdar et al.
2016a, b; Zhang et al. 2015; Zhao et al. 2012a, b). Notably, in the trophic transfer
experiment, the reduction of Ce(IV) to Ce(III) did not occur in plants and snail faces,
and did only in the digestive gland of snail (Ma et al. 2018), which raises the
question whether this reduction creates potential risk to humans through the food
supply.
In short, though hydroponics studies provided valuable information about the
chemical fate and plant uptake of CNPs, the soil properties (porosity, pH, IS, NOM,
and mineral composition) played decisive roles in the bioavailability, migration,
chemical transformation, and their interactions at the nano-bio interface in soils.
Therefore, it is imperative to study the behavior of CNPs in the terrestrial system in
realistic conditions to determine if CNPs can be put in the food chain threatening the
ecosystems and human health.
3.3 Uptake and Toxicity of CNPs Through Air Exposure
As the increasing release of CNPs into the atmosphere through vehicles emissions
(Hong et al. 2014), plants could be exposed to unusual high concentrations of CNPs
180
G. You et al.