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Aging Effects on the Fate and Bioavailability of Cerium Oxide Nanoparticles to Radish (Raphanus sativus L.) in Soil Weilan Zhang, Yongbo Dan, Honglan Shi, and Xingmao Ma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00724 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 14, 2016

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ACS Sustainable Chemistry & Engineering

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Aging Effects on the Fate and Bioavailability of Cerium Oxide Nanoparticles to Radish

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(Raphanus sativus L.) in Soil

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Weilan Zhang1, Yongbo Dan2, Honglan, Shi2,3, Xingmao Ma1,3 *

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Station, TX 77843-3136, USA

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and Technology, Rolla, MO, 65409, USA

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University of Science and Technology, Rolla, MO, 65409, USA

Zachry Department of Civil Engineering, Texas A&M University, TAMU 3136, College

Department of Chemistry and Environmental Research Center, Missouri University of Science

Center for Single Nanoparticle, Single Cell, and Single Molecule Monitoring (CS3M), Missouri

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*For correspondence:

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Xingmao Ma

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Zachry Department of Civil Engineering

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Texas A&M University

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TAMU 3136

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College Station, TX 77843-3136

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USA

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Email: [email protected]

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Abstract The fate and impact of cerium oxide nanoparticles (CeO2NPs) on soil-grown plants have

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been intensively studied. However, all previous studies were performed in freshly prepared soils,

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without considering the temporal changes of the properties of CeO2NPs in the environment.

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Increasing evidences are suggesting that the properties of CeO2NPs will alter with aging and

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therefore it is essential to understand how the aging process affect the fate and bioavailability of

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CeO2NPs in the environment. In this study, the effects of aging on the fractionation of CeO2NPs

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in a silty loam soil and their bioavailability to radish were investigated. The results indicated that

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seven months of aging did not affect the fractionation of CeO2NPs in soil. However, the aging

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process significantly increased the concentration of Ce3+ in soil. The soil with aged CeO2NPs

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contained 40.5% higher Ce3+ than soil with fresh CeO2NPs. The aging process also resulted in

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significantly higher Ce concentration in the radish shoots (87.1% higher) grown in aged soil than

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in freshly contaminated soil, even though the Ce concentration in radish storage root and fine

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roots were comparable between plants grown in these soils. The radish growth and nutritional

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status were unaffected by either the fresh or the aged CeO2NPs. This study provides the first

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comprehensive evaluation on the aging effect of CeO2NPs on its fate and impact on soil-grown

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plants.

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Keywords

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cerium oxide nanoparticles, ionic cerium, radish, aging, fractionation, bioavailability

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Introduction Cerium oxide nanoparticles (CeO2NPs) are widely used as fuel-borne catalyst, UV-

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blocker and polishing agent due to its high oxygen storage capacity, UV absorption ability and

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high hardness and reactivity 1. As a result, it is very likely that CeO2NPs will continue to build

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up in the environment and such prospect of high CeO2NPs concentrations in the environment has

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attracted considerable attention in their potential fate and impact on the ecosystem. Even though

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there have been many studies on the interactions of CeO2NPs with plants in the literature, all of

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the previous studies were conducted with freshly prepared CeO2NPs. However, a more likely

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scenario is that CeO2NPs will persist in the environment for a long period of time and increasing

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evidences are showing that the properties of CeO2NPs may change with time due to various

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physical, chemical and biological processes in the environment 2. Therefore, it is critical to

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understand the aging effect of CeO2NPs on their fate and bioavailability to obtain a realistic

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understanding on the fate and transport of CeO2NPs in the environment.

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Current understanding on the aging process of engineered nanoparticles (ENPs) is still in

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its fledgling stage but is increasing rapidly. Coutris et al. 3 reported that the bioaccessibility of

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bare AgNPs increased due to the continued release of Ag+ from nanoparticles. Diez-Ortiz et al. 4

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also reported that the bioavailability of bare AgNPs to earthworm in soil increased during aging

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(52 weeks in soil) as a result of dissolution. While CeO2NP has low solubility and is not

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expected to dissolute extensively in bulk soil 5, higher dissolution of CeO2NPs has been reported

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in the rhizosphere of plant roots and in environments with high concentrations of fatty acids 6.

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Numerous previous research has shown that the bioavailability of dissolved metallic ions in soil

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changes with time because of the various physical and chemical processes occurred between the

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soil and metal contaminants 2. Examples of the interactions between the soil and metal 3 ACS Paragon Plus Environment


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contaminants include adsorption and precipitation. The adsorption of heavy metal ions onto soil

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particle surface occurs through three mechanisms: inner-sphere surface complexation due to

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siloxane cavity, outer-sphere surface complexation, and adsorption due to diffuse-ion swarm (or

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diffuse layer) 7. The first mechanism, which is also described as “specific adsorption”, takes

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place in permanent charge clay minerals. The latter two, which are relatively weaker and are

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generally described as “non-specific adsorption”, occur at pH dependent mineral surfaces,

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including the edges of individual kaolinite platelets, Fe-Mn oxides, aluminum hydroxide, and

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organic matter. Precipitation, as another mechanism affecting the bioavailability of metals in soil,

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can immobilize the metal ions by decreasing the solubility of the metal compounds. Because of

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these metal ion retention mechanisms, the bioavailability of contaminant metal ions usually

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decreases during the aging process 8, 9. In addition to dissolution, CeO2NPs also display unique

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surface redox chemistry between Ce3+ and Ce4+ on the NP surface, depending on the surrounding

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environment. For example, Kuchibhatla et al. 10 found that Ce3+ could be formed from Ce4+ in

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the presence of hydroperoxy species (Ce4+ + HO2 Ce3+ + H+ + O2). Those processes which

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could take place after the release of CeO2NPs suggest that aged CeO2NPs may display highly

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different fate and impact on the environment than fresh CeO2NPs. However, no studies have

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been performed to examine the aging effects of CeO2NPs on the fate and bioavailability of

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CeO2NPs to plants.

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The primary objectives of this study were (1) to investigate the aging effects of CeO2NPs

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on its fractionalization in soil; (2) to determine the aging effect on the bioavailability of

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CeO2NPs to radish, and (3) to examine the effects of fresh and aged CeO2NPs on the nutritional

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status of radish bulb. Radish (Raphanus sativus L.) was used as a model plant because they are

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grown and consumed widely around the world and bear higher risks of CeO2NPs accumulation


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in the edible bulbs (storage roots) than aboveground crops and vegetables due to their direct

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contact with CeO2NPs in soil. The fractionation of CeO2NPs in soil was determined with the

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modified BCR (Bureau Commune de Reference of the European Commission) sequential

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extraction method 11. This method separates metals in soil into four fractions: the exchangeable,

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water/weak acid soluble fraction (F1); the reducible fraction bound to Fe-Mn oxides (F2); the

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oxidizable fraction associated with soil organic matter (F3); and the residue fraction bound to

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silicate minerals (F4). Previous studies demonstrated that metal contaminants retained by soil

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through non-specific adsorption (first three fractions) are all bioavailable, while F1 represents the

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most active, mobile and bioavailable phase of the metal 12, 13.

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Materials and Methods

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Nanoparticles Uncoated CeO2NPs powder (10-30 nm) was purchased from US Research Nanomaterials

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(Houston, TX). The shape and size of NPs were confirmed by a Tecnai G2 F20 transmission

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electron microscope (TEM) (FEI, Hillsboro, OR) and are shown in Figure 1a. Most

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nanoparticles have irregular polygonal shape. A few nanorods were also found in the powder.

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The size distribution and average size were obtained by measuring 185 individual NPs with an

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image processing software ImageJ 1.49. Most CeO2NPs fell in the size range of 6-24 nm (Figure

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1b), with an average size of 10 nm. An X-ray photoelectron spectroscope (XPS) (Omicron

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multiprobe MXPS system, Scienta Omicron, Germany) was used to investigate the surface

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speciation of CeO2NPs. The results (Figure 1c) indicated that about 8% of the Ce on the

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nanoparticle surface was in the form of Ce3+, as determined by the XPS peak fitting software

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XPSPEAK 4.1.



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Soil characterization

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An agricultural soil was collected from a farmland in Carbondale, IL. The percentages of

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sand, silt, and clay in the soil were determined through wet sieve analysis and hydrometer test 14,

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and is shown in Table S1. Based on the weight percentages of sand, silt, and clay, the soil was

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classified as silty loam according to the USDA soil texture classification. The soil routine

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analyses, including the measurements of pH, conductivity, Nitrate-N, and some macro- and

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micro-nutrients, were conducted in the Soil, Water and Forage Testing Laboratory at Texas

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A&M University following established protocols. Briefly, soil pH and conductivity were

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determined with a slurry of 1:2 soil-deionized water ratio. The slurry was vigorously stirred and

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then settled for a minimum of 30 minutes at room temperature before the measurements. Both

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the pH and conductivity was determined with an Orion Star A325 pH/conductivity portable

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multiparameter meter (Thermo Scientific, Beverly, MA). The Nitrate-N was extracted from soil

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with 1 N KCl solution and reduced to nitrite through a cadmium column, and then quantified by

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a UV-Vis spectrophotometer in a FIAlab-2500 analyzer system (FIAlab Instruments, Inc.

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Bellevue, WA). The micronutrients were extracted using a solution containing 5 mM diethylene

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triamine pentaacetic acid (DTPA), 10 mM CaCl2 and 100 mM triethanolamine. The extracted

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micronutrients were then determined by an inductively coupled plasma optical emission

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spectroscopy (ICP-OES) (SPECTRO Analytical Instruments, Kleve, Germany). The organic

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matter content was measured by measuring the loss in mass during the combustion at 440 °C for

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24 hours, following the ASTM D 2974 method (Standard Test Methods for Moisture, Ash and

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Organic Matter of Peat and Organic Soils). The average organic matter content was 2.21% ±

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0.04% (average ± standard error, n=3).

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Soil preparations 6 ACS Paragon Plus Environment

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Three treatment scenarios using the above mentioned silty loam soil were prepared:

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control soil (no CeO2NPs addition), soil with 1000 mg freshly spiked CeO2NPs as Ce element

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per kg dry soil (Fresh soil), and soil with the same concentration of CeO2NPs but was aged for

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seven months (Aged soil). The CeO2NPs aging was performed as follows: the soil was first

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mixed with known amount of CeO2NPs manually to achieve the targeted concentration (1000

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mg/kg) and shaken on a shaker table at 250 rpm for two months. The mixture was then stored in

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a covered container for five more months. The Fresh soil was prepared by mixing the soil with

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CeO2NPs similarly and shaking on a shaker table at 250 rpm for 24 hours before the experiment.

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Cerium fractionation in soil

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Before seed sowing, the fractionation of CeO2NPs in the above-mentioned soils was

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analyzed following an established BCR extraction protocol with slight modifications 11. Half

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gram of dry soil was randomly collected from each prepared soil and transferred to a 50 mL

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centrifuge tube. The soil was mixed with 20 mL of 0.11 M acetic acid solution by shaking at 250

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rpm for 16 hours at room temperature. The mixture was then centrifuged at 3,000 g for 20

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minutes. The Ce in the supernatant was considered as the exchangeable fraction (F1). The

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residue was then resuspended and extracted with 20 mL of 0.5 M hydroxylamine hydrochloride

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solution at pH 1.5 by shaking and centrifuging in similar conditions as described above to obtain

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the reducible fraction (F2). The residue from F2 was again resuspended, and mixed with 30%

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(w/v) hydrogen peroxide (H2O2). The mixture was kept at room temperature with loosely closed

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cap for an hour. The mixture was then closed and heated at 85±2 °C in a water bath for another

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hour. Afterwards, the volume of the mixture was reduced to less than 1.5 mL by further heating

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in a water bath without cap. Another aliquot of 5 mL of 30% (w/v) H2O2 was added and the

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heating process was repeated. The oxidizable fraction (F3) was then extracted by 25 mL of 1 M 7 ACS Paragon Plus Environment


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ammonium acetate solution at pH 2 by shaking at 250 rpm for 16 hours and centrifuged at 3,000

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g for 20 minutes at room temperature. The ISO 11466 protocol was applied to extract the

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residual fraction (F4) by further digesting the residue from F3 using aqua regia. A mixture of 4.5

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mL HCl (12.0 M) and 1.5 mL HNO3 (15.8 M) was added to the centrifuge tube that contained

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the residue from F3. The residue was resuspended in the aqua regia and transferred to a 50 mL

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reaction vessel that connected to a reflux condenser. The reaction vessel was heated on a hot

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plate for two hours under reflux. Afterwards, the condenser was rinsed with HNO3 (0.5 M). The

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rinsing solution and additional HNO3 (0.5 M) were collected and added to the reaction vessel

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until the solution level reached 50 mL. The Ce in the supernatant of each fraction was quantified

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by an inductively coupled plasma-mass spectrometry (ICP-MS) (DRCII ICP-MS, Perkin Elmer,

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Waltham, MA). Three replicates were prepared and analyzed for each treatment.

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In addition, the concentration of Ce3+ in each soil was determined through a separate

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ethylenediamine tetraacetic acid (EDTA) partial extraction. Previous research has shown that

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both the dissolved Ce3+ ions and the Ce3+ adsorbed or precipitated with PO43- on the surfaces of

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CeO2NPs and soil particles can be successfully extracted by EDTA 6, 15. Briefly, another 0.5 g of

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dry soil was randomly collected from each prepared soil and transferred to a 50 mL centrifuge

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tube. Twenty mL of 100 µmol/L EDTA acid disodium salt dehydrate was added to the soil. The

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mixture was shaken at 250 rpm for seven days at room temperature. Then the mixture was

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centrifuged at 3,000 g for 20 minutes. The supernatant was collected and filtered through a 10

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kDa Amicon Ultra-4 Centrifugal Filter unit (EMD Millipore, Darmstadt, Germany) for 45 min at

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4,000 rpm. The soluble Ce in filtrate was quantified by ICP-MS. For each soil, three replicates

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were analyzed.

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Seed germination and growth conditions 8 ACS Paragon Plus Environment

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The growing pots were established by adding 150 g of dry soil to a plastic container

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(~266 mL total volume). Deionized water was added to the growing pots to saturate the soil to

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100% of field capacity. Radish seeds [Cherriette (F1)], purchased from Johnny’s Selected Seeds

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(Winslow, ME), were geminated in Petri dishes for five days. Healthy seedlings were transferred

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to the growing pots incubated on a growth cart with a 16-hour photoperiod at 28 °C. The light in

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the growth cart was provided by four T5 florescent bulbs. The light intensity was approximately

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104 µmol m-2 s-1 at the height of growing pots. Radishes in the growing pots were irrigated daily

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with a quarter strength Hoagland’s solution to a constant mass (230 g after irrigation for the

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whole pot) from Day 6 to Day 30 after sowing. Daily Hoagland’s solution usage was recorded to

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estimate the accumulative transpiration of plants. Nine radish seedlings were grown for each

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treatment.

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Cerium uptake and accumulation in plants

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At Day 30 after sowing, plants were carefully removed from soil and rinsed with

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deionized water and then 5 mM CaCl2 solution for three times to remove the adhering soil

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particles and Ce element on the root surface. A separate study in our lab suggested that the

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adopted washing procedure can remove approximately 93.9 and 67.0 % of Ce from the surface

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of storage root and fine roots respectively (Data not published). After washing, the radishes were

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divided into shoots, storage root, and fine roots.

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The separated plant tissues of three replicates were randomly chosen from each treatment

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and were dried in an oven at 95 °C for 7 days before the dry weight measurement. The dry

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tissues were digested using a DigiPREP MS hot block digester (SCP science, Clark Graham,

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Canada). Four mL of 70% (v/v) nitric acid (Certified ACS Plus) was mixed with the ground

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plant tissues and stayed at room temperature for overnight to predigest the tissues. Then the 9 ACS Paragon Plus Environment


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mixture was digested in the hot block at 95 °C for 4 h. After cooling down to room temperature, 2

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mL of 30% (w/v) H2O2 was added and the mixture was continued to be digested at 95 °C for 2

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more hours. Finally, the Ce in mixture was quantified by ICP-MS.

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Plant nutritional status

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The storage roots of another three replicates were randomly chosen from each treatment

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and dried similarly as described above. The measurement of the total protein (calculated from N

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percentage) and some common minerals in the dry tissues was conducted at the Soil, Water and

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Forage Testing Laboratory at Texas A&M University. The plant nitrogen is determined by high

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temperature combustion process. Nitrate in plant tissues was first extracted with 1 M of KCl

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solution and then reduced to nitrite through a cadmium column before it was quantified by a UV-

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Vis spectrometer in the same FIAlab-2500 analyzer system as mentioned above. Plant minerals

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were determined by a SPECTROBLUE FMX26 ICP-OES after digestion with 70% (v/v) nitric

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acid.

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Data analysis

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The experimental data are presented as the mean ± standard error of three or more

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replicates. One-way ANOVA and Duncan’s test for post hoc comparisons were performed by

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IBM SPSS Statistics 22. Statistical significance was attained when p-value is less than 0.05.

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Results and Discussion

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Cerium fractionation in soil

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Cerium is the most abundant rare earth element in the earth’s crust 16. As expected, the control soil contained high background concentration of Ce. The summation of four fractions of


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CeO2NPs extracted by BCR method was 39.2 ± 1.22 mg/kg dry soil. The percentages of different

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Ce fractions in soils are illustrated in stacked columns in Figure S1. F1 was considered to

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include both the suspended CeO2NPs and Ce3+. It was the smallest fraction and only accounted

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for less than 0.73% in all treatments due to the low solubility of CeO2NPs and its low dissolution

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rate. The measured concentrations of individual fractions for each treatment are presented in

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Figure 2. Both the Fresh soil and Aged soil have significantly higher Ce concentrations than

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control soil in F1, yet no significant difference was observed between these two treated soils.

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Rare earth phosphates, including cerium phosphate, have been reported as insoluble

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materials in soil because of their very low solubility products (logKsp0 = -26.2±0.15 for CePO4) 17.

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The phosphorus concentration in the soil used in this study was 16.33 mg/kg (Table 1). Inorganic

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phosphorus in soil was mainly in phosphate form 18. So the Ce3+ dissolved in the liquid phase or

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on the CeO2NPs surface (Figure 1c) could potentially form particulate CePO4 and precipitate out

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of the solution. This part of Ce3+ in the CePO4 may not be accounted for in the determination of

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F1 due to the low solubility of CePO4 in water or acetic acid 15. However, this fraction of Ce3+

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could still be available to plants and our previous study demonstrated that while the sum of the

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first three fractions of Ce based on BCR extraction can be bioavailable to plant roots, it is the F1

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that are most likely to be transported to plant shoots and the total concentration of Ce3+ reflects

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the transportable Ce from roots to shoots 19. Hence, F1 could not be a sufficiently reliable

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indicator of Ce bioavailability to plant shoots when studying aging effects. Because of this

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deficiency of BCR method, an independent EDTA partial extraction was conducted to determine

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the total concentration of Ce3+ in different soils. Golden and Wang 20 reported that the cerium-

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acetic acid complex formation constant at 25 °C (log Kf) is 1.68, while the log Kf of Ce-EDTA

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complex is 16.80. So EDTA can strongly bond to all Ce3+, including the Ce3+ in CePO4 (if



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present) and those attached to CeO2NPs surfaces, and form soluble Ce-EDTA complex 15. The

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results from the EDTA partial extraction showed that the aged soil had the highest Ce3+

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concentration, followed by the Fresh soil. Control soil had the lowest Ce3+ concentration (Figure

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3). The differences of Ce3+ in these three soils were significant (p

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