Adsorption of malathion on mesoporous monetite obtained by mechanochemical treatment of brushite

Abstract

Mesoporous monetite (CaHPO₄), obtained by mechanochemical treatment of previously synthesized brushite (CaHPO₄·2H₂O), was used as efficient adsorbent for the organic pesticide malathion. The structure of brushite was confirmed by Raman spectroscopy. The phase transformation process was investigated by X-ray powder diffraction (XRD) and Fourier transformation infra-red spectroscopy (FTIR). The microstructure and morphology were determined by scanning electron microscopy (SEM) and the nitrogen adsorption–desorption method. It was found that five minutes of milling induces brushite–monetite phase transformation. Adsorption of malathion from aqueous solutions showed that this pesticide can be successfully adsorbed on surface of this material.

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Introduction

Dicalcium phosphate cements such as brushite (CaHPO₄·2H₂O, DCPD) and monetite (CaHPO₄, DCPA) have been produced for more than two decades.^1,2 Due to their non-toxicity and low cost, these materials have a large number of applications such as: drug delivery, orthopedics, cancer therapy, biosensors, biological matrixes, etc.^1,3–7 Brushite can also be used as a major component of toothpaste due to its abrasive properties, in wastewater treatment as a remediation medium, heavy metal remover, nutrient supplement and as a stabilizer for plastics.^6,8–11 Monetite is used in the food industry as an acidity regulator, anti-caking agent, dough modifier and food additive, and as a reactant phase for the solid-state synthesis of other calcium-phosphate materials.^4,12 These materials have made a significant contribution to the modern health care industry because they can be used as moldable or injectable pastes to fill bone cavities, defects or discontinuities.^5,13 Brushite is a starting material in the preparation of other calcium-phosphate materials.¹¹ Literature data show that different methods can be used for synthesizing these materials.^{1,6,7,11,14–22} The most common are: chemical precipitation method,^11,21 hydrothermal method,²⁰ diffusion method²² and micro emulsion method.²³ Monetite can be obtained by sol–gel method,²⁴ induction heating deposition,²⁵ hydrothermal method,^26,27 micro emulsion method.²⁸ Usual procedures for the production of monetite from brushite considers two-steps thermal decomposition at 220 °C.^9,10,29

Mechanochemical treatment is a method which is usually used for the physical and chemical changes modification and processing of ceramic materials. Mechanical activation involves the dispersion of solids and their plastic deformation, causing the generation of defects which accelerate the migration and increase the number of contacts between the particles providing chemical interaction.³⁰

Malathion is a pesticide that is widely used in agriculture and in public health pest control programs such as mosquito eradication. In the USA, it is the most commonly used organophosphate insecticide. Malathion itself is of low toxicity; however, absorption or ingestion into the human body readily results in its metabolism to malaoxon, which is substantially more toxic. Studies of the effects of long-term exposure to oral ingestion of malaoxon in rats showed that malaoxon is 61 times more toxic than malathion. So far, various adsorbents have been applied for removal of malathion from waste waters, some of them are: activated carbon,³¹ rice husk and activated rice husk,³² bagasse fly-ash from sugar industry waste,³³ clays and organ clays.³⁴ Based on literature data and authors knowledge monetite was not previously used for adsorption of malathion.

In this paper, we described a new, simple, cost effective procedure for the synthesis of mesoporous monetite by using mechanochemical treatment of brushite. To our knowledge, there are no literature data about synthesis of monetite by this method. Brushite was synthesized by low-cost, room temperature precipitation method. The optimal time for the mechanochemical treatment was determined by using X-ray diffraction and nitrogen adsorption–desorption method. Mesoporous monetite, obtained after 5 min of milling, was used as adsorbent for the malathion.

Experimental

Preparation of brushite (DCPD) and monetite (DCPA) materials

Brushite was synthesized by modified precipitation method, already described in literature data.²¹ Briefly, brushite (sample B) was synthesized by drop wise addition of 100 ml, 0.2 M calcium-acetate solution ((CH₃COO)₂Ca·H₂O, p.a. quality, Sigma-Aldrich) to a 100 ml, 0.2 M sodium dihydrogen phosphate solution (NaH₂PO₄·H₂O, p.a. quality, Sigma-Aldrich). Solutions were stirred magnetically, at 60 °C, for one hour. After this procedure we obtained white precipitate which was filtered and rinsed with distilled water. The solid product was dried over night at 40 °C.

The dry grinding method in vibrating mill was used for decreasing grain sizes of synthesized brushite powder, as well as brushite–monetite phase transformation. The material was treated using Fritsch Pulverisette Analysette Laborette, type 09003, no. 155, 380 vibratory mill. Brushite sample was ground ambient atmosphere, at different times (2.5, 5.0, 7.5, 10.0 and 12.5 min) with speed of 750 rpm (B1, B2, B3, B4 and B5 sample, respectively).

Powder characterization

The synthesized starting brushite (B) material was analysed by Raman spectroscopy. The polycrystalline sample was prepared using pressed pellet technique. Raman spectrum was acquired in the range of 100–1500 cm⁻¹ using a Jobin Yvon T-64000 spectrometer equipped with 3 gratings with 1800 grooves per mm and a liquid-nitrogen cooled CCD detector. Ar⁺/Kr⁺ ion laser line with wavelength λ = 514.5 nm was used for the excitation. Low laser power was applied in order to prevent thermal degradation of the sample. Spectrum was acquired at room temperature.

In order to observe the brushite–monetite phase transformation, the starting B sample, as well as samples obtained after different periods of milling time, were analyzed by X-ray powder diffraction (XRD) method, using Siemens D500 X-ray diffractometer with Cu Kα radiation and Ni filter. The range of 10–60° 2θ was used for all powders with a scanning step size of 0.02, and scanning time of 0.5 s per step. The phases were identified using JCPDS database for brushite (record no. 09-0077) and monetite (record no. 75-1520). Mean crystallite sizes were calculated for the initial sample (B) and for the samples treated for 5.0 (B2) and 12.5 (B5) minutes by using Scherrer's equation.³⁵ The internal strain of samples B, B2 and B5 was calculated using the Williams–Hall method.³⁶

For the Fourier transform infrared (FTIR) spectrometry, samples B, B2 and B5 were powdered finely and dispersed evenly in anhydrous potassium bromide (KBr) pellets (1 : 100). Spectra were taken in transmission mode using a BOMEM (Hartmann and Braun; MB-Series) spectrometer, in the mid infra red (MIR) regions (4000–400 cm⁻¹).

The surface morphology of B, B2 and B5 samples were investigated by using a JEOL JSM-6610LV scanning electron microscope.

Adsorption–desorption isotherms of N₂, at −196 °C, were measured for B2 sample using the gravimetric McBain method. The specific surface area, S_BET, pore size distribution, mesopore including external surface area, S_meso, and micropore volume, V_mic, for the sample was calculated from the isotherms. Pore size distribution was estimated by applying BJH method³⁷ to the desorption branch of isotherms and mesopore surface and micropore volume were estimated using the high resolution α_s plot method.^38–40 Micropore surface, S_mic, was calculated by subtracting S_meso from S_BET.

Adsorption of malathion

After characterization of obtained materials, monetite sample B2 was chosen for the adsorption of malathion. B2 was dispersed in deionized water and equal amount of malathion solution (Pestanal, Sigma Aldrich, Denmark) was added to give the final concentration of the material amounting to 10 mg cm⁻³. The concentration of pesticides was set to either 2.5 × 10⁻³ or 5 × 10⁻⁴ mol dm⁻³. The vessel containing the mixture of adsorbent and malathion was placed on a laboratory shaker (Orbital Shaker-Incubator ES-20, Grant-Bio) and left overnight at room temperature. After that, the mixture was centrifuged for 10 min at 14 500 rpm. Supernatant was filtered though the nylon filter membrane and the concentration of malathion was determined using Ultra Performance Liquid Chromatography (UPLC) analysis, while their physiological effects were determined by measurements of acetylcholinesterase (AChE) activity. Control experiments were performed in identical way but without adsorbent. The performance of the adsorbent was quantified as the malathion uptake, defined as:where C₀ and C stand for the malathion concentration before and after the adsorption experiment, respectively. This actually gives the percentage of adsorbed malathion from the solution.

Waters ACQUITY UPLC system coupled with a tunable UV detector controlled by the Empower software was used. Chromatographic separations were run on an ACQUITY UPLC™ BEH C₁₈ column with the dimensions 1.7 μm, 100 mm × 2.1 mm (Waters). The analyses of malathion solutions were done under isocratic conditions with mobile phase consisting of 60% acetonitrile and 40% water (v/v). The eluent flow rate was 0.25 ml min⁻¹ and the injection volume was 10 μL. Under these experimental conditions the retention time of malathion is 2.6 min.

Physiological effects – measurement of AChE activity

Physiological effects of organophosphate pesticides, including malathion, in terms of their (neuro)toxicity, are defined by measurements of AChE activity. AChE activity was assayed according to Ellman's procedure.⁴¹ The in vitro experiments were performed by exposure of 2.5 IU commercially purified AChE from electric eel to malathion solutions obtained in adsorption experiments at 37 °C in 50 mM PB pH 8.0 (final volume 0.650 ml). The enzymatic reaction was started by addition of acetylcholine-iodide (AChI) in combination with DTNB as a chromogenic reagent, and allowed to proceed for 8 min until stopped by 10% sodium dodecyl sulfate (SDS). The product of enzymatic reaction, thiocholine, reacts with DTNB and forms 5-thio-2-nitrobenzoate, whose optical adsorption was measured at 412 nm. It should be noted that in these measurements the enzyme concentration was constant and set to give an optimal spectrophotometric signal. Physiological effects were quantified as AChE inhibition given as:where A₀ and A stand for the AChE activity in the absence of OP and the one measured after the exposure to malathion.

Results and discussion

Raman spectrum of brushite (B)

It is well known that structure of Brushite consists of compact corrugated sheets consisting of parallel chains perpendicular to the crystallographic plane (010), in which calcium ions are coordinated by six oxygen atoms of the anions and by two oxygen atoms of the water molecules.⁴² The Raman spectrum of brushite (B) in the 100–1500 cm⁻¹ region is displayed on Fig. 1.

Fig. 1 Raman spectra of synthesized brushite material (B).

The most intense Raman band for brushite occurs at 988 cm⁻¹ from a P–O symmetrical stretching mode (ν₂) in PO₄³⁻ ion in the structure. The band (ν₆) which occurs on 1086 cm⁻¹ and 1059 cm⁻¹ originates from triply degenerate P–O stretching mode. A Raman band found at 873 cm⁻¹ is assigned to the P–O(H) symmetric stretching mode (ν₃). The bands positioned at 586 cm⁻¹ and 535 cm⁻¹ belong to ν₄ and ν₇ vibration of O–P–O(H) bending mode. The bands of water molecule are situated at 415 cm⁻¹ and 375 cm⁻¹ and it belongs to double degenerated ν₈ vibration in water molecule. The bands on 204 cm⁻¹, 175 cm⁻¹ and 141 cm⁻¹ belong to lattice modes. These results are fully correlated with literature data.^43,44 Based on Raman spectrum of synthesized brushite we can conclude that we have obtained pure material.

XRD analysis

Fig. 2 shows the X-ray powder diffraction patterns for the brushite (B) and samples obtained after different periods of mechanochemical treatment. XRD of sample B confirmed single phase brushite with monoclinic symmetry, space group Ia, which is in fully agreement with results obtained by Raman spectroscopy. Mechanochemical treatment of brushite, at different periods of time, leads to the decreasing of particle size (wider peaks with lower intensities). It is obvious that, after 5 min of milling (sample B2), the brushite–monetite phase transition is finished.

Fig. 2 XRD patterns of brushite (B) and samples treated for: 2.5 minutes (B1), 5 minutes (B2), 7.5 minutes (B3), 10.0 minutes (B4) and 12.5 minutes (B5).

By using decomposition and refinement fitting method in the Powder Cell program,⁴⁵ we obtained that sample B2 mainly consists of monetite (99.4%) with traces of brushite (0.6%). The main phase in this sample is monetite with triclinic symmetry, which crystallizes in space group P1̄. Powder patterns of samples B3, B4 and B5 which were treated for longer time presents also monetite phase. Based on these results, and in order to investigate the influence of milling time, for the further characterization we have chosen samples B, B2 and B5.

Crystallite sizes of samples B, B2 and B5 were calculated using Scherrer equation,³⁵ and the internal strain of samples were calculated using following eqn (3):³⁶

B_total cos θ = kλ/D + 4Δd/sin θ (3)

where β_total is the full width half maximum of the XRD peak, λ is the incident X-ray wavelength, k is shape factor, θ is the diffraction angle, D is crystallite size and Δd is the difference of the d spacing corresponding to a typical peak.

Unit cell parameters, crystallite sizes and lattice strains of samples B, B2 and B5 are shown in Table 1.

Table 1 Unit cell parameters, crystallite sizes and lattice strain of samples: B, B2 and B5

Space group symbol	Ia	P1̄	P1̄
Lattice parameters (Å)	Sample B	Sample B2	Sample B5
a	6.3645(4)	6.3903(4)	6.9401(4)
b	5.1968(4)	6.624(4)	6.652(4)
c	5.8153(4)	6.997(4)	7.048(4)
α	90	96.2461(3)	96.251(3)
β	118.548(3)	103.884(3)	103.689(3)
γ	90	88.431(3)	88.232(3)
Vol. (Å^3)	494.073(3)	308.817(3)	314.275(3)
Crystallite size (nm)	106.30(2)	98.54(2)	88.19(2)
Strain (‰)	0.003	0.664	0.893

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Micro structural parameters indicate that decreasing of crystallite sizes leads to increasing of lattice strain parameters. It is well known that milling process leads to destruction of grains. At the same time, the number of structural defects and dislocations increase. Consequently, unit cell parameters (a, b, c) also increase. The degree of introduced changes into the structure is proportional to the milling time. Unit cell volume of sample B5 is increased compared to the volume of sample B2 for 1.92%_vol. It is evident that the loss of water molecules from the brushite structure and the transformation to dehydrated monetite phase is achieved after 5 minutes of milling.

Fourier-transform infrared (FTIR) results

The FTIR spectra of brushite (B) and monetite (B2 and B5) are reported in Fig. 3. The obtained spectra are in correlations with literature data.^46–49 FTIR spectrum of brushite (B) shows characteristic bands for this material. Bands that occur around 3544, 3497 and 3167 cm⁻¹ present O–H stretching of lattice water molecules. Band at around 2924 cm⁻¹ belongs to (P)O–H stretching vibration mode. At 1649 cm⁻¹ are vibration of H–O–H bending of lattice molecules, mode at 1213 cm⁻¹ presents P–O–H in-plane bending mode. Bands at 1140, 1061, 982 cm⁻¹ present P–O stretching mode. Vibration mode at 876 cm⁻¹ presents P–O(H) stretching mode. Vibration mode at 784 cm⁻¹ presents P–O–H out-of-plane bending mode. Water vibrations mode are at 678 cm⁻¹. Bands which occurs at 570 and 523 cm⁻¹ belong to O–P–O(H) bending modes. The obtained data for brushite are in good agreement with results obtained by Raman spectroscopy and XRD. In the FTIR spectra of B2 and B5 samples with monetite structure (s.g. P1̄) three kinds of hydrogen bonds could be noticed, but vibration behavior of them is more complex than in brushite (Fig. 3). Tortet, et al. are clearly defined tree types of hydrogen bonds which occur in FTIR spectrum of monetite and, based on this data, it is possible to prove disorder introduced by the different splitting and bond lengths in structure.⁴⁶ In monetite structure, band that occurs at 3437 cm⁻¹ belongs to O–H stretching of residual free water. Bands around 2928 and 2855 cm⁻¹ correspond to (P)O–H stretching mode. Broad band at 1628 cm⁻¹ belongs to H–O–H bending vibration mode. Shoulder at 1133 and 1066 cm⁻¹ presents P–O stretching mode. Band at 891 cm⁻¹ belongs to P–O(H) stretching vibration mode. Single band at 570 cm⁻¹ belongs to O–P–O(H) bending mode.⁴⁶

Fig. 3 FTIR spectra of initial brushite sample (B), sample milled for 5.0 minutes (B2) and sample milled for 12.5 minutes (B5).

Obtained results confirmed phase transition and purity of obtained monetite material after 5.0 minutes of milling (sample B2). This structure is retained after prolonged time of mechanochemical treatment (sample B5).

Scanning electron microscopy (SEM)

SEM micrographs of B, B2 and B5 samples are shown at Fig. 4. SEM image of brushite (B) shows well-defined grains (Fig. 4a). It can be observed that the grains have monoclinic plate like morphology and size about 70 μm. After five minutes of milling the degree of crystallinity decreases and the grains are broken into smaller pieces, sample B2 (Fig. 4b). The morphology of the powder shows the bulk structure was composed of cemented, agglomerated individual particles. Evidently, the grains are not uniformly milled. SEM micrograph of B5 sample (Fig. 4c) shows very small grains which are also cemented to form agglomerates with smaller dimensions than in sample B2.

Fig. 4 SEM micrographs of: (a) initial (B) brushite sample (b) sample milled for 5.0 minutes (B2) and (c) sample milled for 12.5 minutes (B5).

According to collected results we can conclude that optimal time of mechanochemical treatment for the achieving of the brushite–monetite phase transition and optimal morphological characteristic of the material is 5 min. Prolonged milling time causes the agglomeration of particles.

Nitrogen adsorption–desorption measurements

Sample B2 was analyzed by using nitrogen adsorption measurements. Samples at prolonged time of mechanochemical treatment were not characterized by nitrogen adsorption since after 5 min of mechanochemical treatment the monetite structure of the samples is achieved. Specific surface of the starting brushite is very low and results are not presented here. Nitrogen adsorption isotherms for the B2 sample, as the amount of N₂ adsorbed as function of relative pressure at −196 °C, are shown in Fig. 5 (inset).

Fig. 5 Pore size distribution (PSD) of B2 sample. Inset: nitrogen adsorption isotherms, as the amount of N₂ adsorbed as function of relative pressure for B2 sample. Solid symbols – adsorption, open symbols – desorption.

According to the IUPAC classification⁵⁰ isotherm of B2 sample is of type IV and with a hysteresis loop which is associated with mesoporous materials. Specific surface area calculated by BET equation, S_BET, is 37 m² g⁻¹ for B2 sample. Pore size distribution (PSD) of B2 sample is shown in Fig. 5. Figure shows that sample is mesoporous, with bimodal PSD. The most of the pores radius lies between 2.0 and 12 nm.

α_s plot, obtained on the basis of the standard nitrogen adsorption isotherm is shown in Fig. 6. The straight line in the medium α_s region gives a mesoporous surface area including the contribution of external surface, S_meso, determined by its slope, and micropore volume, V_mic, is given by the intercept.

Fig. 6 α_s – plot for nitrogen adsorption isotherm of B2 sample.

Calculated porosity parameters confirm that sample B2, which was mechanochemically treated for 5 minutes, is completely mesoporous (S_meso = 37 m² g⁻¹).

After this set of experiments we can conclude that, after 5 min of the mechanochemical treatment of the previously synthesized brushite, a mesoporous monetite, with relatively high specific surface, can be obtained. Mesoporosity (pore size between 2 and 50 nm), extremely low cost of the production and non-toxicity make this material very promising for the adsorption of the organic pesticides.

Adsorption of malathion

The results of batch adsorption experiments unambiguously show that malathion adsorbs on monetite B2 with the uptake above 60% for rather high concentration of the order of 10⁻³ to 10⁻⁴ mol dm⁻³ (Table 2). When recalculated to adsorption capacity (defined as the mass of malathion per unit mass of adsorbent) this translates to 52 mg g⁻¹ (for higher concentration, Table 2) to 10 mg g⁻¹ (lower concentration). Measured adsorption capacities are sufficiently high to support practical use of this material having in mind its extremely low price. For example, adsorption capacities of graphene-based materials decay with adsorbent concentration and are found to be somewhat below 100 mg g⁻¹ for adsorbent concentration of 10 mg dm⁻³ (three orders of magnitude higher than in this work).⁵¹

Table 2 The results of malathion removal by monetite – B2 in batch adsorption experiments and the toxic effects of purified water samples^a,b

C0 (M)	C (M)	Malathion uptake (%)	AChE inhibition before the adsorption (% of control)	AChE inhibition after the adsorption (% of control)
a C0 – malathion concentration before the adsorption.b C – malathion concentration after the adsorption.
2.50 × 10^−3	0.93 × 10^−3	63 ± 6	84 ± 7	63 ± 7
5.00 × 10^−4	1.94 × 10^−4	61 ± 5	70 ± 3	49 ± 5

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For the initial concentrations of malathion used in the adsorption experiments AChE inhibition is induced, which translates into neurotoxic effects. After the adsorption of malathion on monetite B2 sample, AChE inhibition decreases. Moreover, under realistic conditions, the concentration of malathion is unlikely to be as high as 2.50 × 10⁻³ mol dm⁻³, the highest one we use in described experiments.

Besides desired textural properties which are necessary to provide adsorbent with the access to the surface sites, the underlying mechanism of malathion adsorption on monetite surface can be explained by the formation of surface complex between surface Ca²⁺ sites and malathion molecules.⁵² Considering molecular structure of malathion the interaction is likely to be mediated by P=S moiety or carboxyl moieties.

Having in mind these results, there is no doubt that these materials can be used for the purification of water. This is especially true having in mind rather high concentration of malathion used in these experiments.

Conclusions

Brushite (CaHPO₄·2H₂O, DCPD) was successfully obtained by modified precipitation method using calcium-acetate solution. Mechanochemical treatment was performed to induce the phase transition from brushite to monetite. Raman spectroscopy, XRD and FTIR techniques confirmed brushite–monetite phase transition. SEM and nitrogen adsorption method revealed that particle size decreases and specific surface increases during the mechanochemical treatment. Monetite sample, obtained after 5 min of the mechanochemical treatment, with specific surface S_BET = 37 m² g⁻¹, and mesoporous structure was chosen for the adsorption of malathion. It was shown that this material can be successfully used for the adsorption of this organic pesticide.

Acknowledgements

This project was financially supported by the Ministry of Education and Science of the Republic of Serbia (Project Number: 45012). The authors acknowledge to Dr Sonja Aškrabić and Dr Zorana Dohčević-Mitrović for Raman measurements and Dr Aleksandar Pačevski for SEM results.

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