Differences in the reported PZC may also come from differences in the method applied to deter-mine the PZC, the order of titration process, and sorption of anions used in the titration.The presence of monovalent or divalent cations controlled the stability of non-coated nanoceria in aquatic system.These authors measured the aggregation kinetic of nanoceria and compared to the theoretical prediction of Derjaguin–Landau–Verwey– Overbeek . At pH 11, the experimental critical coagula-tion concentration was higher for the monovalent , than the divalent salts, 80 mM and 16 mM respectively. They showed that DLVO theory could predict quite well the stability of nanoceria at this pH. However, this model fails to explain aggregation behavior as solution conditions become more environmentally relevant and non- DLVO forces may also play important roles between particles.In a water-saturated column packed with sand, water composition has also been shown to control the transport and deposition of nanoceria.Transport was significantly hindered at acidic conditions and high ionic strengths , and the deposited nanoceria may not have been re-entrained by increasing the pH or lowering the ionic strength of water. At neutral and alkaline conditions , and lower ionic strengths ,planting in pots ideas partial breakthrough of nanoceria was observed and particles could be partially detached and re-entrained from porous media by changing the solution chemistry.In a more complex system, hetero aggregation, i.e. between a nanoparticle and another particle in the environment, is more likely to occur due to the greater concentration of environmental particles.
It has been shown that in various solutions, the agglomeration and sedimentation rate of nanoceria were dependent on NOM content and ionic strength. In freshwater, with a high TOC, and low ionic strength, nanoceria dispersion were stable with a low rate of sedimentation.In algae medium, Quick et al.showed that the sedimentation decreased with increasing NOM content. The fraction of nanoceria that remained suspended in algae medium increased with increasing NOM content. The main mechanism explaining the increased stability is the adsorption of NOM to the particle surface. Recently, Li and Chen mea-sured and modeled the aggregation kinetic of nanoceria in the presence of humic acid , in monovalent and divalent solutions. HA has been shown to stabilize nanoceria in all KCl concentration. However at high CaCl2 concentration HA enhanced the aggregation of nanoceria probably owing to the bridging attraction between nanoceria, which is induced by the HA aggregates formed through intermolecular bridging via Ca2+ complexation. The stability and mobility of nano-ceria in dilute NaCl solution was also greatly enhanced in the presence of humic acid, fulvic acid, citric acid, alginate and CMC due to electrostatic effect.Even in the presence of NOM in the media, homo-aggregation was measured in several studies. Keller et al.measured >500 nm aggregates formed in sea water whereas ~300 nm aggregates were stable in suspension for a high TOC. Van Hoecke et al.measured nanoceria aggregation in algal test media, between 200 and 1000 nm but the extend of the agglomeration was dependent on pH, NOM, IS. Increasing pH and ionic strength enhanced aggregation, while NOM decreased mean aggregate sizes. Organic molecules that can adsorb onto the particle surfaces provide a barrier to aggrega-tion but were not able to overcome the van der Waals forces holding small nanoparticles aggregates together.Available reports on the behavior of nanoceria in complex natural ecosystem are scarce. In a simulated freshwater eco-system in laboratory, sediments were measured as the major sink of nanoceria with a recovery of 75.7% of total nanoceria after 15 days.
In several types of soil, Cornelis et al.showed, by investigated the retention of nanoceria, that nanoceria retention in soil is low. The retention of nanoceria in soils was proposed to be associated with naturally occur-ring colloids, such as Al, Si, and Fe oxides.Contrary to some other manufactured nanoparticles , nanoceria have an inherently low solubility. Negligible solubility was reported; e.g. in freshwater sys-tem over 72 h,in moderately hard reconstituted water for48 h2 or in algal medium for 3 days.Similarly, Röhder et al. measured a low dissolved Ce concentration in different algae exposure media ranging from 0.01 to 0.11% total Ce, and 0.47 to 1.13% in the presence of EDTA. However, they show that the dissolved Ce may be responsible for the observed toxicity in Chlamydomonas reinhardtii. The dissolution of nanoceria has been shown to be very low in 16 different types of soil spiked with nanoceria.Dissolution of nanoceria studied in an artificial soil solution was only significant at pH 4 and was less than 3.1% of total Ce. Ce redox state is affected by environmental transformation. A reduction of CeIJIV to Ce in nanoceria has been observed during the contact between nanoceria and E. coli, in C. elegans, 2 in cucumber plants,40 and to a lesser extent in corn66 and soybean.The Ce reduction may explain the tox-icity induced by these nanoparticles by suggesting oxidative damage of macromolecules or generation of ROS.The reduction of Ce was not observed in all studies: Ce was found as Ce in the roots seedlings of cucumber, alfalfa, tomato, corn and soybean seedling exposed to 4000 mg l−1 of nanoceria.However, nanoceria interaction with HA and with biological media induced a decrease of Ce proportion measured by EELS.70 This may indicate that nanoceria had been oxidized in the presence of humic substances and biological media. The presence of phosphate in media can modify nanoceria properties. Zhang et al.identified the formation of cerium phosphate from a nanoceria suspension, KH2PO4 and a reducing substance . Singh et al.suggested that the interaction of nanoceria with phosphate may have caused the formation of cerium phosphate at the particle surface, in which cerium is mainly present as Ce.
They showed that binding of phosphate anions to nanoceria leads to the complete disappearance of superoxide dismutase activity and concomitant increase in catalase mimetic activity.To summarize, the few available studies showed that the properties of environmental media modifies the stability and the chemical state of nanoceria. But we lack sufficient knowledge to understand and predict the extent of transformations in the environment and the risks associated with the release of nanoceria on biological systems.Wastewater treatment plants are an important intermediate pathway for NP to soil and water.NPs may undergo transformations before being discharged with treated effluent or biosolids. Transformations of two varieties of nanoceria, pristine and citrate-functionalized, were followed in an aerobic bioreactor simulating wastewater treat-ment by conventional activated sludge.This study indicates that the majority of nanoceria was associated with the solid phase where a reduction of the CeIJIV NPs to Ce occurred. After 5 weeks in the reactor, 44 ± 4% reduction was observed for the pristine nanoceria and 31 ± 3% for the citrate-functionalized nanoceria, illustrating surface function-ality dependence. The authors suggest that the likely Ce phase generated would be Ce2S3. At maximum,growing blueberries in pots 10% of the CeO2 will remain in the effluent and be discharged as CeO2, a Ce phase.Nanoceria can also be toxic and/or provoke changes in the microbial communities involved in wastewater treatment therefore affecting the performance of the wastewater treat-ment process. Garcia et al.evaluated the effect of nanoceria on the activity of the most important microbial communities of a WWTP: ordinary heterotrophic organisms, ammonia oxidizing bacteria, and thermophilic and mesophilic anaerobic bacteria. A great inhibition in biogas production and a strong inhibitory action of other bio-masses were caused by nanoceria coated with hexamethylene-tetramine . On the contrary, the study of Limbach et al., 2008,showed that an ordinary heterotrophic organisms biomass from a municipal WWTP in Switzerland was not affected by 1000 mg l−1 of non-coated nanoceria. This dis-crepancy could be related to differences in the characteristics of the bacterial community and the nanoparticles properties used in both studies.Ma et al.were among the first to investigate potential nano-ceria phytotoxicity. The authors reported that the seed germi-nation of 7 different species was completely unaffected by 2000 mg l−1 of nanoceria suspension. Similarly, subsequent root elongation tests with these plant species was largely unaf-fected by nanoceria; only lettuce root growth was suppressed by 34% at this concentration. Lopez-Moreno et al.also showed that nanoceria at 2000–4000 mg l−1 had no overt toxic-ity on soybean, although particles were detected within root tissue by synchrotron X-ray absorption spectroscopy . The authors did report genotoxicity as measured by random amplified polymorphic DNA assay; however, the precise nature of the molecular effects is not known. In a follow up study, the same group reported the effects of 0–4000 mg l−1 nanoceria exposure on alfalfa, corn, cucumber and lettuce growth.The germination and root elongation of several of the species were enhanced at lower concentrations but were significantly inhibited at 2000 and 4000 mg l−1 .Interestingly, shoot elongation was enhanced in nearly all cases. ICP-OES was used to confirm ceria presence within the seedlings, although root and shoot tissues appear to not have been separated prior to analysis. After dilute acid rinsing, XAS confirmed that the oxidation state was unaltered in the root tissues of these four plant species. Zhang et al.reported a concentration-dependent sorption of nanoceria to cucumber roots in a 14 day hydroponic exposure.
Most of the adsorbed nanoceria were only loosely bound to the root surface and more than 85% of the nanoparticles could be washed off with deionized water. Translocation of the particles to shoot tissue was minimal but measurable, and interestingly, 7 nm size particles were found at significantly higher amounts than 25 nm nanoceria. In a follow up 21 day hydroponic study, exposure to 2000 mg l−1 bulk CeO2 and nanoceria resulted in no toxicity to cucumber.Although minimal root to shoot translocation was noted, soft X-ray scanning transmission microscopy and XANES were used to show measur-able biotransformation to CePO4 in roots and cerium carbox-ylates in shoot tissue. Notably, the authors hypothesize that root exudate mediated dissolution of nanoparticles precedes ion uptake, subsequently followed by in planta reduction to nanoceria and/or bio-transformed products. Similarly, Schwabe et al.observed plant exudate induced changes in solution pH, nanoceria agglomeration and particle size. How-ever, they reported no phytotoxicity to pumpkin and wheat after 8 day exposure at 100 mg l−1 nanoceria; no cerium was detected in wheat shoots but minimal translocation in pump-kin yielded tissue levels of 15 mg kg−1 . Interestingly, the association of cerium with the roots of both plant species was reduced in the presence of NOM. Rice exposed to nanoceria at 63–500 mg l−1 experienced no visible signs of phytotoxicity, although altered lipid peroxi-dation, electrolyte leakage, and other enzyme activity suggested possible oxidative stress.Wang et al.noted that tomato seeds harvested from plants previously exposed to nanoceria yielded a “second generation” of individuals that produced less biomass, transpired less water, possessed differential root morphology, and exhibited overall higher levels of reactive oxygen species that did seeds from unexposed plants.Birbaum et al.were the first to report on nanoceria expo-sure to terrestrial plants under soil conditions. The authors reported that after 14 day exposure with the nanoceria in the irrigation water , no ceria was found in the leaves or sap of corn plants. However, no mention was made of toxicity or of root ceria content. Interestingly, the authors included an aerial exposure on leaves and although nanoceria could not be washed from the tissue, the particles were not internalized or transferred to new growth. Similarly, Wang et al.grew tomato in the presence of nanoceria-amended irrigation water and reported either no impact or slight enhancements in plant growth and yield. The authors did observe ceria in the shoots, including edible tissues, which suggests transloca-tion, but the mechanism and form of element transfer is unknown. Zhao et al.observed that after one month of growth in soil, corn roots accumulated significantly greater quantities of alginate coated nanoceria than uncoated particles but no mention was made of toxicity. These authors also noticed that soils with high organic matter generally enhanced the association of nanoceria with roots but reduced the translocation to shoots, regardless of the surface proper-ties of nanoceria. However, the effect of soil organic matter was more significant on uncoated nanoceria than alginate coated nanoceria. Although translocation in general was low, μXRF did confirm the presence of nanoparticles within vascu-lar tissues, as well as in epidermal and cortex cell walls, suggesting an apoplastic uptake pathway. A separate study with cucumber showed that up to 800 mg nanoceria/kg soil did not demonstrate any adverse effect on a suite of plant physiological indictors such as the net photosynthesis rate, leaf stomatal conductance, but nanoceria at this concentration did lower the yield of cucumber by 31.6%.