cerium dioxide as a new reactive sorbent for fast degradation of parathion methyl and some other...

JOURNAL OF RARE EARTHS, Vol. 32, No. 4, Apr. 2014, P. 360

Foundation item: Project supported by Czech Science Foundation (P106/12/1116)

* Corresponding author: Pavel Kuran (E-mail: [email protected]; Tel.: +420-475 309 256)

DOI: 10.1016/S1002-0721(14)60079-X

Cerium dioxide as a new reactive sorbent for fast degradation of parathion methyl and some other organophosphates

Pavel Janos1, Pavel Kuran1,*, Martin Kormunda2, Vaclav Stengl3, Tomas Matys Grygar3, Marek Dosek1, Martin Stastny1, Jakub Ederer1, Vera Pilarova1, Lubos Vrtoch1

(1. Faculty of the Environment, University of Jan Evangelista Purkyne, Kralova Vysina 7, 400 96 Usti nad Labem, Czech Republic; 2. Faculty of Science, Uni-versity of Jan Evangelista Purkyne, Ceske Mladeze 8, 400 96 Usti nad Labem, Czech Republic; 3. Institute of Inorganic Chemistry AS CR v.v.i., 25068 Rez, Czech Republic)

Received 22 July 2013; revised 3 December 2013

Abstract: Cerium dioxide was used for the first time as reactive sorbent for the degradation of the organophosphate pesticides para-thion methyl, chlorpyrifos, dichlofenthion, fenchlorphos, and prothiofos, as well as of some chemical warfare agents—nerve gases soman and O-ethyl S-[2-(diisopropylamino) ethyl] methylphosphonothioate (VX). CeO2 specimens were prepared by calcination of basic cerous carbonate obtained by precipitation from an aqueous solution. The CeO2 samples containing certain amounts (1 wt.%–5 wt.%) of the neighboring lanthanides (La, Pr, Nd) were prepared in a similar way from pure lanthanide salts. It was shown that ceria accelerated markedly the decomposition of parathion methyl causing the cleavage of the P-O-aryl bond in the pesticide molecule. A similar reaction mechanism was proposed for the degradation of other organophosphate pesticides and nerve agents. The degradation times (reaction half-times) were in an order of minutes in the presence of CeO2, compared to hours or days under common environ-mental conditions. The reaction in suitable organic solvents allowed conversions of about 90% for parathion methyl loading of 20 mg pesticide/g CeO2 within 2 h with a reactant half-life in the order of 0.1 min. The key parameter governing the degradation efficiency of CeO2 was the temperature during calcination. At optimum calcination temperature (about 773.15 K), the produced ceria retained a sufficiently high surface area, and attained an optimum degree of crystallinity (related to a number of crystal defects, and thus poten-tial reactive sites). The presence of other lanthanides somewhat decreased the reaction rate, but this effect was not detrimental and permitted the possible use of chemically impure ceria as a reactive sorbent. A fast organophosphate degradation was demonstrated not only in non-polar solvents (such as heptane), but also in polar aprotic solvents (acetonitrile, acetone) that are miscible with water. This opens new possibilities for designing more versatile decontamination strategies. The cleavage of phosphate ester bonds is of a great importance not only for the degradation of dangerous chemicals (chemical weapons, pesticides), but also for interactions of ceria (es-pecially the nano-sized one) in biologically relevant systems.

Keywords: cerium dioxide; carbonate precursor; lanthanides; organophosphate pesticide; parathion methyl; chemical warfare agents; hydrolysis; non-aqueous solvents; rare earths

In the 1990s, unique surface properties of some (nano) crystalline metal oxides (MgO, CaO, Al2O3, ZnO) were discovered[1] and subsequently exploited for the adsorp-tive removal and chemical destruction of hazardous ma-terials, including acid gases, polar organics, and organo-phosphorus compounds[2,3]. More recently, these materi-als (usually called “destructive/reactive (ad)sorbents” or “stoichiometric reagents”) have been prominently applied to the destruction of chemical warfare agents[4]. Some metal-oxide based reactive sorbents (Fe and Mn oxides, doped titania) are capable to decompose these extremely dangerous chemicals (e.g., sulfur mustard or soman) into nontoxic or less toxic compounds very quickly, often in less than one hour[5–8].

In the present work, we examined the application of reactive sorbents to the degradation of organophosphate

pesticides that exhibit a certain degree of structural simi-larity to some warfare agents. To this end, we developed a new kind of reactive sorbent based on cerium dioxide and tested its degradation efficiency on the organophos-phate pesticide parathion methyl—which is still used in great quantities in some countries[9]—as well as on sev-eral other similar pesticides.

Cerium dioxide belongs to the most important rare earths with numerous applications as a catalyst or co- catalyst for automotive emission control (as a part of the so-called three-way catalysts)[10] or for the catalytic in-cineration and degradation of aromatic hydrocar-bons[11–13]. CeO2 has been used also in biomedical appli-cations[14]. In recent time, cerium dioxide gained a great attention because of its ability to interact with phosphate ester bonds in biologically relevant molecules including

Pavel Janos et al., Cerium dioxide as a new reactive sorbent for fast degradation of parathion methyl and some … 361

ATP or DNA[14]. These unique properties (enhanced catalytic activity, enzyme-mimetic ability) are usually at-tributed to the specifically designed nanosized forms of cerium dioxide—nanoceria[14–16]. We have shown in this work that highly effective cerium dioxide can be pre-pared by an ordinary precipitation/calcination procedure. For the first time (to our knowledge), we demonstrated an applicability of cerium dioxide as a reactive sorbent for the degradation of organophosphates under mild (ambient) conditions.

Depending on the intended use, many different methods have been developed for preparing cerium dioxide; Li et al.[17] compares some of these methods, namely the sol-gel method, precipitation method, and homogeneous hydrolysis. However, cerium dioxide is most often pre-pared by the precipitation of sparingly soluble cerous ox-alates or carbonates, with a subsequent calcination in an oxidizing atmosphere. Cerium carbonates with variable composition and morphology may be precipitated from an aqueous solution by alkaline or ammonium carbon-ate/bicarbonate[18–21] or by a gaseous mixture of carbon dioxide and ammonia[22]. It is generally accepted that the temperature during calcination plays a decisive role in governing the usability of the product, affecting directly its surface properties (specific area, presence of active sites), crystallinity, and other physicochemical character-istics[17,18,22–25].

In the present work, we prepared a series of cerium oxides from the carbonate precursor by calcination at various temperatures ranging from 473.15 to 1273.15 K, and tested their degradation efficiency for parathion methyl and some other organophosphate pesticides using a testing procedure similar to the degradation testing of chemical warfare agents[6]. The prepared oxides were characterized by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and Raman spectros-copy. We also examined the effect of the dopants La, Pr, and Nd (neighboring rare earth elements) on the degra-dation efficiency and other properties. The degradation products of pesticides were identified and quantified with the aid of chromatographic and hyphenated (GC-MS) techniques. The as-yet unclear role of the solvent in the reaction rate was also addressed. The mechanism of the pesticide degradation was proposed; knowledge of this reaction mechanism provides a useful guide for the de-velopment of an appropriate decontamination strategy.

1 Experimental

1.1 Chemicals and preparation of reactive sorbents

Cerous nitrate, Ce(NO3)3·6H2O, lanthanum nitrate, La(NO3)3·6H2O, praseodymium nitrate, Pr(NO3)3·6H2O,

and neodymium nitrate, Nd(NO3)3·6H2O were obtained from Sigma-Aldrich (Steinheim, Germany) as reagent grade chemicals with purity 99.9% (trace metal basis); ammonium bicarbonate, NH4HCO3, 99.5%, was ob-tained from the same supplier. Some reactive sorbents were prepared from the ceria-based glass-polishing powder Cerox 1670, produced by Rhodia, La Rochelle, France. This material was dissolved in a hot mixture of concentrated nitric acid and hydrogen peroxide. After proper dilution, it was treated similarly to the other lan-thanide compounds. Parathion methyl, dichlofenthion, fenchlorphos, chlorpyrifos, and prothiofos, as well as their degradation products, 4-nitrophenyl, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and 3,5,6-trichloropyridinol, were obtained from Sigma-Aldrich as chromatographic standards. Unless stated otherwise, HPLC- grade organic solvents and deionized water were used to prepare the solutions including mobile phases for liquid chromatography.

The carbonate precursor was prepared by precipitation of an aqueous solution of cerous nitrate (0.2 mol/L) with an excess of ammonium bicarbonate (0.5 mol/L) under stirring; the completeness of the precipitation was checked by reaction with oxalic acid. After adding the last portion of ammonium bicarbonate, the agitation continued for one more hour and the precipitate was left until the fol-lowing day. Then, the precipitate was separated by filtra-tion, washed with water, and dried overnight at 383.15 K. Cerium carbonates doped with La, Pr, and Nd were pre-pared in a similar fashion from mixed solutions contain-ing appropriate amounts of the respective lanthanide salts. The same procedure was also applied to the solution ob-tained by dissolving the polishing powder Cerox 1670. The prepared carbonates did not have an exact formula; they were a mixture of normal and basic carbonates. Us-ing the X-ray diffraction analysis, a crystalline phase of the Ce2O(CO3)2·nH2O type was identified. In addition, further unidentified crystalline and amorphous compo-nents were present. Reactive sorbents were prepared from these carbonates by calcination at pre-determined temperatures in the range 473.15–1273.15 K for 2 h.

1.2 Methods of characterization

A scanning electron microscope (SEM; Tescan Vega LSU) was used to examine the sorbents in the modes of secondary electrons (SE) and back scattered electrons (BSE).

X-ray diffraction (XRD) measurements were per-formed using an MPD 1880 diffractometer (Philips). The crystallite sizes were calculated from diffraction line broadening using the Scherrer formula[26]:

cos

Kα

β θ

λ= (1)

where K is the shape factor, λ is the wavelength of the applied radiation, β is the broadening of the diffraction

362 JOURNAL OF RARE EARTHS, Vol. 32, No. 4, Apr. 2014

line, and θ is the diffraction angle. The Scherrer Calcula-tor from the X-Pert HighScore Plus SW package was used for these calculations.

The specific surface area of the sorbents was measured by the BET method (N2 adsorption) with a Sorptomatic 1900 Carlo Erba instrument. X-ray photoelectron spec-troscopy (XPS) was used to investigate the surface com-position of the sorbents. The XPS instrument was based on the SPECS Phoibos 100 and XR50 components.

Diffuse reflectance Fourier transform spectroscopy (DRIFTS) was employed to measure the IR spectra using a Nicolet Impact 400 D spectrometer. Raman spectra were acquired with a DXR Raman microscope (Thermo Fischer Scientific, Inc., Waltham, MA) using a 780 nm laser.

1.3 Pesticide degradation on reactive sorbents

The testing procedure was derived from those used to examine chemical warfare degradation[6,7]. In this proce-dure, constant amounts (50 mg) of the sorbent were weighed into a series of glass vials (Supelco, 4 mL), and an exact volume (150 μL) of the pesticide solution (6660 mg/L) in heptane was added to each vial (corresponding to a dosage of 1 mg of pesticide per 50 mg of sorbent). The vials were sealed with caps and covered with alu-minum foil to protect the reaction mixture from sunlight. At pre-determined time intervals (0.5, 8, 16, 32, 64, 96, and 128 min), the reaction was terminated by an addition of 2-propanol (2 mL), and the sorbent was immediately separated by centrifugation (4000 r/min for 7 min). The supernatant was decanted and transferred to a 50 mL volumetric flask, and then the sorbent was re-dispersed in 4 mL of methanol and centrifuged again. The extraction of the sorbent with methanol was repeated three times. All the supernatants were combined into one volumetric flask, made up to the mark with methanol, and analyzed immediately by liquid chromatography (HPLC), gas chromatography with mass-spectrometric detection (GC- MS), and UV/Vis spectrometry. All degradation experi-ments were conducted at the laboratory temperature of 295.15 K in an air conditioned box. In each series of measurements, several types of quality control experi-ments were performed: procedure blank experiments with the sorbent and solvents without the presence of pesticide, and 2–3 experiments with various concentrations of pesticide in working solutions without the presence of sorbent. Using a spiked sample, the recovery of 4-nitrophenol as the main degradation product of para-thion methyl was examined.

1.4 Analytical methods and their main performance characteristics

Concentrations of parathion methyl and 4-nitrophenol were determined by liquid chromatography using the LaChrom HPLC system (Merck/Hitachi). The system

consisted of a L-7100 pump, L-7400 variable wavelength UV/Vis detector operating at 230 nm, Rheodyne 7725i injection valve with 20 µL sampling loop, and the Luna column (Phenomenex, Torrance, CA, USA) 150×4.6 mm, packed with PFP stationary phase, 5 µm. The composi-tion of mobile phase was methanol (HPLC-grade, Lab-scan, Dublin, Ireland)/water 80/20 (v/v), at a flow rate 1 mL/min. Similar conditions were used to determine other pesticides and their degradation products. An ex-ception was the simultaneous determination of chlor-pyrifos and 3,5,6-trichloro pyridinol, when the Hibar col-umn (Merck) 125×4 mm, packed with Purospher STAR, RP 18e, 5 µm, stationary phase, was used together with the mobile phase consisting of methanol/aqueous acetic acid (1%). Under the given conditions, the limits of de-tection were 0.26 and 0.12 mg/L for parathion methyl and 4-nitrophenol, respectively. The standard deviations of repeatability were 0.086 and 0.041 mg/L for parathion methyl and 4-nitrophenol, respectively, at a concentra-tion level 1 mg/L. The relative standard deviations (RSD) of the entire degradation test were estimated from a se-ries of duplicate measurements (n=9, degradation time 128 min) and expressed in terms of the parathion methyl disappearance (RSD=10.8%) and 4-nitrophenol produc-tion (RSD=14.9%). Analogous procedures were devel-oped for other pesticides and their hydrolytic products.

A gas chromatograph Varian GC 3800 coupled with an ion trap mass spectrometer (Varian 4000) and a fused silica capillary column (VF-5; 20 m×0.25 mm×0.25 μm), all from Varian (Varian Inc., Palo Alto, USA), were used to confirm the identity of the target analytes.

2 Results and discussion

2.1 Characterization of the reactive sorbents

The cerium carbonate was precipitated from an aque-ous solution of cerous salts in the form of a white amor-phous precipitate that, after several hours of aging, con-verted to a finely crystalline product easily separable from the mother liquor by sedimentation or filtration. The equivalent content of cerium dioxide in the dried (383.15 K) carbonate precursor was 85.6% according to the weight of calcine at 1173.15 K. As can be seen from the SEM images in Fig. 1, the precipitate consisted of clusters of thin plates of irregular shapes with a charac-teristic diameter of several micrometers (Fig. 1(a)). During calcination, the plates were broken down into smaller submicron particles (debris) that remained assembled in larger clusters (Fig. 1(b)). With the increasing tempera-ture of calcination, the submicron particles began to sinter to more compact solid aggregates (Fig. 1(c)).

The XRD analysis identified a single crystalline phase in the calcination products obtained in the temperature range of 473.15–1273.15 K cerium dioxide with its char-


Fig. 1 SEM images

(a) Cerium carbonate; (b) CeO2 annealed at 773.15 K; (c) CeO2 annealed at 1273.15 K (Left column—SE, right column—BSE) acteristic face-centered cubic fluorite-type structure. With increasing calcination temperature, the diffraction peaks narrowed, witnessing that the crystallites grew and acquired a more ordered structure[26]. The dependence of the crystallite sizes on the calcination temperature is shown in Fig. 2; the sizes of the crystallites increased substantially between ca. 773.15 and 1073.15 K. A cor-responding decrease of the specific surface area is shown in Fig. 2 with the most substantial change between 673.15 and 973.15 K.

Chemical changes on heating were characterized by XPS. It was deduced that the O/Ce ratio decreases with

Fig. 2 Dependencies of the specific surface area and crystallite

sizes on the calcination temperature

increasing calcination temperature, and the abundance of O–Ce3+ and the hydroxyl groups decreases at higher cal-cination temperatures. The results show O/Ce over- stoichiometry at temperatures below 673.15 K, probably due to the –OH groups bonded to the ceria surface, a common phenomenon observed with metal oxides ob-tained by calcining precursors from wet syntheses.

The infrared spectra of the carbonate precursor and of the calcines confirmed the presence of structural OH groups; the corresponding absorption bands at 3000– 3500 cm–1 were weakened with increasing calcination temperature, but they did not disappear completely, in-dicating their persistence. A total dehydration and de-carbonation was observed in the temperature range 1073.15–1273.15 K. The Raman spectra (Fig. 3) of sor-bents are dominated by F2g symmetrical stretching vibra-tion mode in fluorite structure of CeO2 near 463 cm–1 for coarse crystalline oxide. The band asymmetry and broadening in the spectra of samples calcined at tem-peratures lower than 973.15 K are a joint consequence of phonon confinement (important for particles <10 nm) and oxygen non-stoichiometry[27–29]. The presence of oxygen non-stoichiometry is also responsible for a band centered at about 590 cm–1 [29], which continuously de-creased with growing temperature up to 773.15 K and then it was not discernible. This change was coincident with the decrease in Ce3+ content observed by XPS. That band near 863.15 K was much stronger in ceria speci-mens containing trivalent lanthanides, which introduced thermally stable O vacancies, as demonstrated by the band intensity in 5% La-doped CeO2 and CeO2 from CEROX (Fig. 3, inset B).

Fig. 3 Raman spectra of CeO2 with growing calcination tem-

perature (Inset A: detailed view of the inten-sity-normalised main Raman band showing narrowing and shift with calcination temperature. Inset B: detailed view of the region of vibrations related to O-vacancies (presence of Ce3+ or other trivalent cations in CeO2 lat-tice), 5% La-doped CeO2 and CeO2 obtained from CEROX (R) for comparison. Gray arrow indicates the impact of heating, numbers at spectra are calcination temperatures (K))


2.2 Degradation of parathion methyl in non-polar solvent (heptane)

Using the procedure described in the experimental sec-tion, we measured the time dependencies of the degrada-tion of parathion methyl with ceria-based reactive sor-bents prepared at various temperatures ranging from 473.15 to 1273.15 K. Examples of the degradation curves are shown in Fig. 4. Using the cerium dioxide prepared at 773.15 K, the pesticide disappeared almost completely in less than 0.5 min and 4-nitrophenol (4-NP) was identi-fied as the main degradation product. No other reaction product (e.g., paraoxon methyl) was positively detected at any stage of the degradation on any reactive sorbent. Changes in the 4-NP concentration with time are also shown in Fig. 4. The removal of parathion methyl and the formation of 4-NP was also visualized on UV spectra measured at various stages of the degradation reaction.

The time dependencies of the parathion methyl degra-dation were measured for all the samples prepared at temperatures in the range of 473.15–1273.15 K, and the

experimental data were fitted to the pseudo-first-order kinetic equation[30]; qt = q1 exp(–k1t) + q∞ (2)

The q are dimensionless fractions of the reactant with respect to its initial amount: qt represents a residual quan-tity of the parathion methyl at time t; q1 is the fraction of parathion methyl degraded during the experiment; q∞ is the residual fraction of parathion methyl at the end of the reaction, if the destructive capacity of the reactive sor-bent was not sufficient for complete degradation; and k1 is the degradation rate constant. The model parameters were estimated using a non-linear analysis (Table 1). As can be seen, the reactive sorbent annealed at 773.15 K had the best degradation efficiency, with a reactant half- life of 0.10 min and the final conversion level at 98%. Others reactive sorbents showed a lower final conversion of parathion methyl and a longer half-life of the reactant. The degradation of parathion methyl gave rise to 4-NP as the main degradation product; the time dependencies for this process were measured and evaluated in a similar fashion using the following equation:

Fig. 4 Kinetics of the parathion methyl degradation on cerium dioxide prepared by calcination at various temperatures (solvent: heptane)


qt

*=q1*[1–exp(–k1

*t)] (3) In this equation, qt

* represents the relative amount of 4-NP produced by degradation of parathion methyl at time t, q1

* represents the maximal relative amount of 4- NP produced during degradation of parathion methyl under the given conditions, k1

* is a rate constant of the formation of 4-NP, and t is the reaction time. The model parameters are listed in Table 2. Again, the reactive sor-bent annealed at 773.15 K exhibited the best degradation efficiency; the formation of 4-NP was very fast, with a rate constant of 7.23 min–1 and a half-life of 0.10 min.

To directly compare the degradation efficiency of the CeO2 powders annealed at different temperatures, the degradation test with a reaction time of 128 min was performed with all of the examined sorbents; Fig. 5(a) summarizes the results expressed in terms of the per-centages of disappearance of parathion methyl and for-mation of 4-NP. The highest degradation efficiency was achieved with the cerium dioxide prepared at 773.15 K; however, all samples prepared at temperatures lower than ca. 973.15 K were capable to decompose parathion methyl with sufficient efficiency. The degradation effi-

Table 1 Parameters of the pseudo-first order kinetic model for the degradation of parathion methyl using ce-rium dioxides prepared at various temperatures

Kinetic parameters of pseudo-first

order decay

Quality model

parameters

Annealing

tempera-

turea)/K q∞±SE q1 ± SE k1± SE/min–1 t1/2/min R2 SEE

473.15 0.41±0.03 0.53±0.04 0.047±0.009 14.7

0.9775 0.0398

573.15 0.47±0.04 0.43±0.0.7 0.093±0.042 7.45 0.8872 0.0771

673.15 0.45±0.05 0.42±0.08 0.142±0.084 4.88 0.8472 0.0898

773.15 0.02±0.01 0.98±0.01 6.72±0.54 0.10 0.9998 0.0051

873.15 0.43±0.03 0.49±0.05 0.107±0.032 6.48 0.8676 0.0570

973.15 0.62±0.03 0.31±0.04 0.159±0.076 4.36 0.9057 0.0497a) Kinetic dependencies for the samples annealed at temperatures 1073.15 –

1273.15 K were not evaluated, as the degradation of pesticide was very low.

SE—standard error of the estimated parameter; SEE—standard error of the

estimate

Table 2 Parameters of the kinetic model describing an in-crease of the 4-nitrophenol concentration in the presence of cerium dioxides

Kinetic parameters of first

order growth

Quality model

parameters

Annealing

temperaturea)/

K q1*±SE k1±SE/min–1 t1/2/min R2 SEE

473.15 0.38±0.02 0.065±0.015 10.7

0.9399 0.0434

573.15 0.40±0.03 0.060±0.016 11.6 0.9233 0.0512

673.15 0.40±0.03 0.104±0.040 6.66 0.8472 0.0696

773.15 0.85±0.01 7,23±1.52 0.10 0.9988 0.0117

873.15 0.43±0.03 0.112±0.040 6.30 0.8676 0.0697

973.15 0.21±0.02 0.078±0.023 8.89 0.9118 0.0286a) Kinetics for the samples annealed at temperatures 1073.15–1273.15 K was

not evaluated due to very slow degradation of pesticide. SE—standard error

of the estimated parameter; SEE—standard error of the estimate

ciency decreased steeply with increasing calcination-temperature, especially above ca. 773.15 K. This occurred in parallel with the decrease of the specific surface area, which is generally related to a decrease in removal effi-ciency[31]. The 773.15 K calcine was more reactive than the 473.15–673.15 K calcines, despite the latter’s larger specific surface area, and the decrease in reactivity at 873.15 and 973.15 K was much smaller than expected based on their markedly decreased specific surface areas. The changes in surface chemistry and oxide stoichiome-try with increasing annealing temperature, as demon-strated by the XPS analysis and Raman spectroscopy, played a decisive role in the degradation process, as will be discussed later. Obviously, the specific reactivity (re-action rate related to specific surface area) was best for the 773.15–973.15 K calcines, which approached most closely the ceria stoichiometry; i.e., which had the lowest Ce3+ concentration and the lowest O/Ce surface over- stoichiometry.

As described above, the kinetics of the parathion methyl removal and 4-NP formation were fitted as inde-pendent processes. However, it is clear that the degree of

Fig. 5 Comparison of the degradation efficiency of the oxides annealed at various temperatures (a), and cerium oxides with varying

concentrations of dopants expressed as the respective oxides (b) (REG—cerium dioxide prepared from Cerox 1670)


pesticide degradation closely correlated with the 4-NP formation, and a plausible correlation was also found between the corresponding rate constants and the half-times. A mass balance taking into account the (sum of the) concentrations of parathion methyl and 4-NP gave an overall recovery above 80% (mostly above 90%). Thus, 4-NP may be considered the main product of the parathion methyl degradation. Other (side) products (if any) occur in minor quantities; their presence was not positively proven but should not be excluded.

In nature cerium is usually accompanied by other rare earth elements. Cerium compounds (concentrates) con-taining other light lanthanides, such as La, Pr, and Nd, are more easily accessible (and cheaper) than pure ones. The effect of other lanthanides on the degradation effi-ciency of cerium dioxide was examined for samples containing 1%, 3%, or 5% lanthanum, praseodymium, or neodymium (expressed in the form of the respective oxides) calcined at 773.15 K. Cerium dioxide prepared from the polishing powder Cerox 1670 was also tested; it

contained approximately 13% La3O3 and some minor impurities (Ca and Si, below 1%). The XRD patterns of all the samples confirmed the presence of a single crys-talline phase corresponding to the fluorite-like structure of cerium dioxide. The absence of additional peaks indi-cated the formation of a single phase of the Ce1–xLnxO2–y type (Ln=La, Pr, or Nd) over the full doping range[32]. The corresponding enhanced concentration of O vacan-cies was confirmed by Raman spectroscopy. Specific surfaces of the doped samples ranged from 85 to 117 m2/g, whereas the crystalline sizes varied from 13.1 to 16.4 nm; no correlations with the dopant concentrations were proven. Representative examples of the degradation curves for the doped ceria are shown in Fig. 6, parame-ters of the kinetic model are listed in Tables 3 and 4. Compared with the pure ceria prepared by annealing at 773.15 K without the addition of dopants, the presence of dopants resulted in reduced degradation efficiency; the same holds true for cerium dioxide prepared from Cerox 1670. The negative role of trivalent lanthanide ions in the

Fig. 6 Kinetics of the parathion methyl degradation and effect of dopants (Concentrations of dopants expressed in the respective ox-

ides, all samples calcined at 773.15 K)


Table 3 Parameters of the kinetic model for the degradation

of parathion methyl using doped cerium dioxides and cerium dioxide prepared from Cerox 1670

Kinetic parameters of pseudo-first

order decay

Quality model

parameters Reactive

adsorbent q∞±SE q1±SE k1±SE/min–1 t1/2/min R2 SEE

CePr-1% 0.09±0.02 0.91±0.06 2.98±0.57 0.23

0.9798 0.0543

CePr-3% 0.24±0.03 0.77±0.08 2.40±0.66 0.29 0.9547 0.0698

CePr-5% 0.47±0.03 0.53±0.07 2.30±0.85 0.30 0.9198 0.0654

CeNd-1% 0.12±0.02 0.88±0.05 4.24±0.86 0.16 0.9868 0.0421

CeNd-3% 0.12±0.03 0.88±0.07 2.87±0.65 0.24 0.9711 0.0629

CeNd-5% 0.40±0.03 0.60±0.07 1.10±0.42 0.63 0.9503 0.0621

CeLa-1% 0.13±0.03 0.87±0.08 3.66±1.14 0.19 0.9590 0.0746

CeLa-3% 0.28±0.03 0.72±0.07 2.71±0.78 0.26 0.9525 0.0671

CeLa-5% 0.44±0.03 0.56±0.07 2.72±0.98 0.25 0.9276 0.0649

REG 0.38±0.03 0.62±0.08 2.10±0.74 0.33 0.9271 0.0738


estimate

Table 4 Parameters of the kinetic model describing an in-crease of the 4-nitrophenol concentration in the presence of doped cerium dioxides and cerium di-oxide prepared from Cerox 1670

Kinetic parameters of

first-order growth

Quality model

parameters Reactive ad-

sorbent q1

* ±SE k1±SE/min–1 t1/2/min R2 SEE

CePr-1% 0.85±0.03 1.96±0.46 0.35

0.9561 0.0777

CePr-3% 0.77±0.03 2.06±0.63 0.34 0.9279 0.0904

CePr-5% 0.51±0.02 2.324±0.72 0.30 0.9267 0.0595

CeNd-1% 0.85±0.03 2.09±0.52 0.33 0.9499 0.0824

CeNd-3% 0.71±0.03 1.86±0.45 0.37 0.9533 0.0670

CeNd-5% 0.43±0.03 2.06±0.85 0.34 0.8754 0.0682

CeLa-1% 0.80±0.03 3.33±1.07 0.21 0.9401 0.0831

CeLa-3% 0.73±0.02 2.28±0.45 0.30 0.9686 0.0551

CeLa-5% 0.53±0.02 2.15±0.42 0.32 0.9685 0.0400

REG 0.46±0.02 2.38±0.69 0.29 0.9354 0.0502


estimate

ceria structure is related to the negative role of O vacan-cies in calcines <773.15 K. Nevertheless, the worsening of the degradation effect was not dramatic, and it is

therefore expected that high-purity cerium compounds are not necessary to produce ceria-based reactive sor-bents. Effective degradation agents could be prepared even from spent polishing sludges[33]. Similar conclu-sions can be drawn from a direct comparison of the deg-radation efficiencies of the doped cerium dioxides for the reaction time 128 min (Fig. 5(b)).

2.3 Degradation mechanism, role of solvent

The phosphorus atom bears a positive charge in the molecule of the organophosphate pesticide, and thus it becomes highly electrophilic and reactive towards nu-cleophiles. Therefore, organophosphate pesticides are susceptible to hydrolysis, at a rate dependent on the chemical structure of the pesticide and the reaction con-ditions (pH, in the first place in aqueous solutions)[34]. The degradation half-life times may be as long as several weeks under ambient conditions, but may be substan-tially affected (shortened) in the presence of suspended particles (montmorillonite), homogeneous catalysts (iron salts)[35], or metal oxides[36]. We observed that the degra-dation of parathion methyl using cerium dioxide as a re-active sorbent in a non-aqueous environment proceeded much faster, within ~102 minutes, compared to ~102 hours or more in aqueous solutions.

In nonpolar media, parathion methyl gives 4-NP as the main (sole) degradation product. It has been hypothe-sized that nucleophilic substitution (SN2) is the main mechanism responsible for cleavage of the P-O-aryl bond in the pesticide molecule[37,38]. The –OH groups on the surface of the sorbent play a crucial role in the deg-radation process[38], acting as a very strong nucleophile toward P atoms[37]. Analogous to hydrolytic reactions in aqueous solutions[39], the possible reaction pathway in-volves the nucleophilic attack on the central phosphorus atom with a subsequent liberation of 4-NP (leaving group) through a transition state (depicted in Fig. 7). It was ex-pected that the pesticide degradation as the SN2 nucleo-philic substitution is promoted in aprotic solvents, whereas protic solvents may exhibit an adverse effect on the degradation process. To verify this hypothesis, the degradation of parathion methyl on CeO2 was investi-

Fig. 7 Proposed pathway for the degradation of parathion methyl on CeO2


Fig. 8 Degradation of various pesticides on CeO2 in acetonitrile

(a) Dichlofenthion; (b) Fenchlorphos; (c) Prothiofos; (d) Chlorpyrifos

Table 5 Degradation of selected organophospates in ace-tonitrile (pesticides) and nonane (nerve agents) using cerium dioxide annealed at 500 °C

Parent compound

Main degradation product

Degree of conversion

after 128 min /%

Reactionhalf-time/

min

Parathion methyl 4-nitrophenol 93.2±2.3 0.17

Fenchlorfos 2,4,6-trichlorphenol 81.1±4.1 12.6

Prothiophos 2,4-dichlorophenol 45.2±4.6 30.1

Chlorpyrifos 3,5,6-trichloropyridinol 18.2±2.7 99.0

Dichlofenthion 2,4-dichlorophenol 41.7±3.3 10.3

Somana) ND 100 (10 min) ND

VXb) ND 100 (30 min) ND a) O-Pinacolyl methylphosphonofluoridate; b) O-ethyl S-[2-(diisopropylamino)

ethyl] methylphosphonothioate; ND – not determined

gated in several solvents that differed in their polarities and hydrogen-bonding abilities. The pesticide degrada-tion proceeded well not only in nonpolar solvents such as heptane, but also in typical polar solvents such as ace-tonitrile or acetone. Methanol, on the other hand, with a polarity similar to acetonitrile, exhibited an adverse effect on pesticide degradation. Although the role of the solvent is not fully understood yet, it was clearly shown that very diverse solvents (polar vs. non-polar) are utilizable as reaction media for pesticide destruction. One can choose between “oil-compatible” and “water-compatible” sol-vents with comparable effects on the degradation effi-ciency, which would be advantageous for the proper se-lection of a remediation strategy.

2.4 Degradation of other organophosphates

The proposed procedure is also applicable to the degra-dation of other organophosphate pesticides; the degrada-tion curves for dichlofenthion, fenchlorphos, prothiofos, and chlorpyrifos are shown in Fig. 8. In all cases, the pheno-lic compounds were identified as degradation products in accordance with the reaction mechanisms suggested above. It was also confirmed that CeO2 is very affective in degra-dation of dangerous nerve agents of organophosphate type—soman and VX agent. Main parameters of the deg-radation processes are presented in Table 5.

3 Conclusions Ceria was a perspective reactive sorbent for the de-

struction of parathion methyl and other organophosphate pesticides. In suitable non-aqueous solvents (heptane, acetonitrile, acetone), the ceria-promoted cleavage of the C–O–aryl bond occurred in a matter of 101–102 min, compared to days/hours in aqueous environments. The degradation efficiency of the ceria could be tuned by the temperature of the calcination of cerium carbonate, which controlled the product stoichiometry. Trivalent lanthanide impurities somewhat worsened the sorbent performance, but not to a fatal degree; this allowed the use of inexpensive (technical-grade) ceria with some impurities as a sufficiently effective reactive sorbent. The fast reaction was feasible in both non-polar and polar sol-vents (however, not in protic solvents producing H-


bonds). These findings could lead to the future use of ceria-based reactive sorbents for practical applications. It would hence be possible to develop both “oil-compati-ble” and “water-compatible” degradation systems (al-most equally efficient), which would be advantageous for the selection of a suitable remediation strategy. Prelimi-nary results indicated that the ceria-based sorbents were able to decompose very quickly also some chemical weapons of the organophosphate type.

Acknowledgments: Additional financial support was obtained from the Internal Student Grant Agency (Grant No. 4410115004701) of the University of Jan Evangelista Purkyně in Ústí nad Labem. Dr. Miroslav Skoumal and Dr. Karel Ma-zanec from the Military Research Institute, Brno, are thanked for measurements of degradation curves for nerve agents.

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cerium dioxide as a new reactive sorbent for fast degradation of parathion methyl and some other...

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