Synthesis of biosurfactant‐coated magnesium oxide nanoparticles for methylene blue removal and selective Pb²⁺ sensing

Abstract

Dyes and lead (Pb²⁺) are toxic compounds that can contaminant water. In this study, magnesium oxide (MgO) nanoparticles (NPs) prepared using clove, i.e. Syzygium aromaticum extract [clove extract (CE)] were used for methylene blue (MB) removal and Pb²⁺ ion sensing in aqueous solution. Maximum 90% MB removal was achieved using MgO NPs. The MB adsorption on MgO NPs surface followed second‐order kinetics and Langmuir isotherm. MB dye was adsorbed as a monolayer on the surface of MgO NPs with maximum adsorption capacity, 5555 mg g⁻¹. MgO NPs were also able to selectively detect lead (Pb²⁺) in 1 nM–200 µM range with 24 µM (3σ) limit of detection. So, CE prepared MgO NPs are useful for MB dye adsorption and metal ion sensing applications.

1 Introduction

Soil and water pollution due to dyes is dangerous to living organisms. Dyes in addition to being a toxic chemical possess a colour that inhibits penetration of water by light. Thus dye inhibits the growth of living organisms present in water bodies. Methylene blue (MB) is a dye released from various industries [1, 2]. MB can induce confusion, agitation, altered mental status, tachycardia, agitation, dystonia and abnormal eye movements, radiculomyelopathy, endophthalmitis and neurological sequelae in humans [3, 4, 5, 6, 7, 8]. Furthermore, MB has an adverse effect on an animal reproductive system like reduction in the sperm motility and hindering the growth of human luteal cell [9, 10, 11].

Various methods have been reported for the removal of dyes from aqueous solution. Bacteria namely, Agrobacterium radiobacter, Bacillus species, Sphingomonas paucimobilis and Aeromonas hydrophila have been used for the degradation of crystal violet and malachite green dyes [12].

A nanoparticle (NP)‐based strategy has also been tested for dye removal. CuS nanorods loaded on activated carbon have been used for simultaneous ultrasound‐assisted adsorption of malachite green and Pb²⁺ ions from aqueous solution [13]. Likewise, CuS/ZnS‐activated carbon nanocomposites have been reported for simultaneous adsorption of Congo red, Phloxine B and fast green dyes [14]. Ni‐doped ferric oxy‐hydroxide FeO(OH) nanowire loaded activated carbon has been used for the adsorption of Safranin‐O and Indigo Carmine dyes [15]. Cherry tree wood derived activated carbon functionalised with zinc hydroxide NPs has also been used for the removal of MB dye [16]. Similarly, apple tree wood derived activated carbon was loaded with ZnS:Ni NPs for the removal of MB and Janus Green B dyes [17]. Undoped and Au‐doped ZnO‐nanorods loaded on activated carbon have been reported for removal of MB and auramine O [18]. An attempt has also been made to create a model system to predict the dye removal efficiency of nanomaterials. In such a study response surface methodology, artificial neural network and radial basis function neural network techniques were applied to predict the dye adsorption efficiency of nanomaterials adsorbate. The efficiency of MB and Malachite green adsorption on Mn–CuS/ZnS nanocomposite‐loaded activated carbon was tested as a model system for future studies [19].

Lead (Pb) is a non‐biodegradable toxic metal naturally present in the earth. It is released to the environment in a huge amount from industries related to printing, construction, automobile, paint, agriculture, plastic, and electronics [20, 21]. Lead is toxic to plants, micro‐organisms, and animals including humans. Pb has toxic effects on the spleen, kidney, circulatory, nervous and reproductive system of animals. Pb can lead to the death of animals and birds [22, 23, 24, 25]. So, sensing of Pb is important for human safety. NPs have earlier been reported for removal of MB and sensing of Pb²⁺ from aqueous solutions. However, most of these NPs have been obtained by chemical methods using harmful chemicals or harsh conditions [2, 26, 27, 28, 29, 30, 31, 32, 33, 34]. Further, synthesis of some of these NPs like silver, gold, and platinum is costly due to the high cost of precursor salts, reducing agents and stabilisers used [35, 36, 37]. Among various methods of biological synthesis, plants extract mediated synthesis of NPs is faster, more cost‐effective and easy as compared with microorganisms. Furthermore, it is easy to scale up for the industrial scale production of NPs [38].

So, there is an urgent need for a greener cost‐effective route of NP synthesis. Clove bud is a rich source of medicinally important polyphenols. Phenol moieties from clove bud have been reported to possess antioxidant, antimicrobial, antiviral, anticancer, antidiabetes, hypolipidemic, hepatoprotective and gastroprotective activities [39, 40, 41, 42, 43]. So, covering the surface of MgO NPs with medicinally important CE moieties will add value to MgO NPs. In this study, CE prepared MgO NPs were used for MB dye adsorption and Pb²⁺ sensing (Scheme 1).

2 Experimental

2.1 Materials

Magnesium acetate tetrahydrate, MB dye and other chemicals used in this study were of analytical grade. The chemicals were purchased from Merck and Sigma‐Aldrich. Syzygium aromaticum L. (clove) flower buds were procured from the local market (Chandigarh, India).

2.2 Synthesis and characterisation of MgO NPs

2.2.1 CE preparation

The CE was prepared with a slight modification of earlier method [44, 45]. Briefly, 2 g powdered clove was put in a flask containing 50 ml of double distilled water (DDW) and boiled for 2 min. The mixture was cooled and centrifuged at 7500 rpm for 10 min. After centrifugation, 25 ml of the CE was obtained. The clear supernatant was labelled as clove extract (CE) and was stored at 4 °C for further experiments. 25 ml aqueous CE when lyophilised contains 414 mg dry content.

2.2.2 Synthesis of MgO NPs

To prepare MgO NPs, 25 ml of metal ions, i.e. the aqueous solution of magnesium acetate tetrahydrate was mixed with 25 ml of CE, i.e. 1:1 metal ion to CE ratio. The reaction mixture was incubated at room temperature, i.e. 25 °C for 30 min unless specified. NPs formed were separated by centrifugation and washed with DDW followed by washing with ethanol. Purified NPs were dried using a rota‐evaporator. 35–40 mg MgO NPs were obtained in a single batch and were stored in a vacuum desiccator. To obtain MgO NPs of different sizes, physiochemical factors namely, metal ion concentration, CE concentration, metal ion volume ratio and CE volume ratio were varied. All reaction mixtures were incubated at 25°C unless specified. Metal ion concentration was varied from 0.1 to 4 M. 2 ml of varying metal ion concentration was incubated at room temperature with 2 ml CE (Table 1). To see the effect of CE concentration on MgO NPs synthesis, 2 ml of 1 M metal ion was incubated with varying concentration of CE, i.e. 0.25, 0.50, 1, 1.5, 1.75 and 2 ml. The final volume of CE was maintained at 2 ml by adding DDW to avoid the effect of CE to metal ion volume ratio. 2 ml CE concentration contain undiluted CE. The effect of metal ion volume ratio on MgO NPs synthesis was evaluated by using fix 1 M metal ion concentration and varying the metal ion volume (Table 1). The CE volume was varied from 0.25–4 ml, by keeping metal ion volume 2 ml (Table 1).

Table 1.

Effect of various physiochemical factors on the size of MgO NPs

Metal ion concentration, M	Metal ion volume, ml	CE volume, ml	Size of NPs, nm
effect of metal ion concentration on the size of MgO NPs
0.1	2	2	346 ± 15
0.5	2	2	350 ± 10
1	2	2	380 ± 21
2	2	2	477 ± 73
4	2	2	499 ± 25
effect of CE concentration on the size of MgO NPs
1	2	2 (0.25CE + 1.75W)	613 ± 55
1	2	2 (0.5CE + 1.5W)	604 ± 11
1	2	2 (1.0CE + 1.0W)	590 ± 127
1	2	2 (1.5CE + 0.5W)	652 ± 11
1	2	2(1.75CE + 0.25W)	608 ± 75
1	2	2 (undiluted CE)	380 ± 21
effect of metal ion volume on the size of MgO NPs
1	1	2	648 ± 11
1	2	2	380 ± 21
1	4	2	582 ± 34
1	5	2	587 ± 68
1	6	2	590 ± 27
effect of CE volume ratio on the size of MgO NPs
1	2	0.25	514 ± 74
1	2	0.5	477 ± 49
1	2	1	464 ± 31
1	2	2	380 ± 21
1	2	4	456 ± 12

2.2.3 Characterisation of MgO NPs

The synthesis of MgO NPs was confirmed using ultraviolet (UV)‐visible spectroscopy analysis at a preselected time interval, i.e. 0, 5, 15, 30, 45 and 60 min. As most of the MgO NPs synthesis was over in 30 min, reaction mixtures at varying physiochemical conditions were screened for MgO NPs synthesis at 30 min of reaction. The reaction mixtures were centrifuged after 30 min of reaction incubation at 7500 rpm for 10 min to separate MgO NPs from unreacted reactants. The MgO NPs obtained after centrifugation were dispersed in DDW and again centrifuged to obtain purified MgO NPs. The initial size characterisation of MgO NPs was carried out using dynamic light scattering (DLS) technique (Malvern Nano‐S90 zetasizer).

More precise size and shape characterisation of MgO NPs was performed using a field‐emission scanning electron microscope (FESEM, Hitachi, Sv8010, 15 kV) and a transmission electron microscope (TEM, Hitachi, H‐7500, 120 kV). Size and structure of MgO NPs were confirmed using X‐ray diffraction (XRD) technique (Panalytical D/Max‐2500). MgO NPs were subjected to Fourier transform infrared (FTIR) analysis (Perkin Elmer Spectrum 400) to identify the CE moieties involved in NPs synthesis. The spectrum was recorded in the range 400–4000 cm⁻¹. The thermal stability of MgO NPs was carried out at 20–1000°C using thermal gravimetric analysis (TGA) technique (SDT Q600 thermal analysis).

2.2.4 Liquid chromatography–mass spectrometry (LC–MS) characterisation of CE

For LC–MS analysis of CE, earlier reported method was used [46]. Lyophilised CE (5 mg) was dissolved in 5 ml DDW and filtered through a 0.2 µM syringe filter prior to high performance liquid chromatography (HPLC) injection. Waters 2795 HPLC instruments equipped with a 100 µl syringe auto sampler was used for CE analysis. The mobile phase consists of water (A) and acetonitrile (B), both containing 1% acetic acid. Gradient elution of the mobile phase was applied, 5–70% (B) over 0–30 min, 70–100% (B) over 30–35 min, held for 5 min. The flow rate was 1 ml min⁻¹ and injection volume was 10 µl.

2.3 Adsorption of MB dye using MgO NPs

Initially, 15 mg l⁻¹ aqueous anionic (solochromo black T, methyl red) and cationic dyes (rhodamine 6G, MB) solution were used for dye adsorption experiments using 400 mg l⁻¹ MgO NPs. All adsorption experiments were carried out under atmospheric pressure at 25°C. For all adsorption studies, aqueous dye solution was stirred with MgO NPs for 120 min unless specified. Subsequently, this mixture was centrifuged at 8000 rpm for 15 min to separate the unabsorbed dye. The supernatant was subjected to UV‐visible spectroscopic analysis to find out the amount of unabsorbed dye. To quantify the MB dye removal, absorbance was recorded at 664 nm, i.e. characteristic absorption wavelength of MB. All the solutions used in this study were prepared using DDW unless specified. The dye concentrations were determined using the calibration curve of MB.

To study the effect of contact time, 20 mg MgO NPs were properly mixed with 50 ml of MB solution (15 mg l⁻¹) using a magnetic stirrer for 120 min. The mixture was screened for MB removal at fixed time intervals, 0, 5, 15, 30, 45, 60, 90 and 120 min. To check the influence of pH, 15 mg l⁻¹ MB solution was mixed with 400 mg l⁻¹ of MgO NPs at pH range 3–11. To see the effect of MgO NPs amount, dye solutions (15 mg l⁻¹) were stirred with 200, 400, 600, 800, 1200 and 1600 mg l⁻¹ of MgO NPs. To test the effect of dye concentration on the rate of adsorption, 15 mg l⁻¹ MB were mixed separately with 400 mg l⁻¹, MgO NPs.

The amount of MB adsorbed per unit MgO NPs was calculated using (1). The percentage removal of MB using MgO NPs was estimated using the following equations [47]:

$$q_{\mathrm{e}}=\frac{\left(C_0-C_{\mathrm{e}}\right)V}{M},$$ (1)

$$\mathrm{Percentage}\mathrm{removal}=\frac{C_0-C_{\mathrm{e}}}{C_0}\times 100\text{\%}.$$ (2)

Here q _e is the amount of dye adsorbed on the surface of MgO NPs (mg g⁻¹), C ₀ is the initial MB concentration (mg l⁻¹), C _e is the equilibrium concentration of MB in solution (mg l⁻¹), V is the volume of experimental solution (ml), and M is the mass of MgO NPs (mg).

2.3.1 Mechanism of MB adsorption on MgO NP adsorption isotherms

Langmuir and Freundlich adsorption isotherm models were used to investigate the mechanism of adsorption of MB on the surface of MgO NPs.

2.4 MgO NPs as a fluorescence sensor for detecting toxic Pb²⁺ ions

100 µM aqueous solutions of acetates of Pb²⁺, Ba²⁺, Cd²⁺, Co³⁺, Cr³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺ Na⁺, Ni²⁺ and Zn²⁺ were prepared. MgO NPs (5 mg) were dispersed in 15 ml of DDW and this concentration was used throughout the fluorescence sensing study. 1 ml of each metal acetate solution was added to 1 ml of MgO NPs solution. Samples were subjected to excitation at a wavelength of 390 nm. The emission peak for MgO NPs appeared at 495 nm.

Likewise, to check the linearity of the MgO nanosensor, 1 ml Pb acetate aqueous solution of different concentrations, 1 nM–200 µM were added separately to 1 ml MgO NPs aqueous solution. Fluorescence spectra were recorded at 0 min (just after mixing) of Pb acetate with MgO NPs. The linearity of different concentrations of Pb²⁺ was calculated from a fluorescence intensity value of 495 nm. To test the interference of other metal ions with the MgO NP nanosensor, 200 µM of each of cations namely Ba²⁺, Cd²⁺, Co³⁺, Cr³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺ and Zn²⁺ was added to the test sample in 1:1 volume ratio, i.e. 1 ml 100 µM of Pb acetate in 1 ml MgO NPs solution.

3 Results and discussion

3.1 UV‐visible and morphological characterisation of MgO NPs

The change in colour of aqueous metal ion solution on the addition of CE is a visible confirmation of MgO NPs synthesis. The colour of aqueous metal ion solution changed from transparent to dark brown on the addition of CE. Physiochemical factors were varied to obtain MgO NPs of various sizes (Table 1). The MgO NPs absorb in the UV‐visible range and have characteristic absorption at around 275 nm. The intensity of this peak was found to increase with an increase in the MgO NPs concentration. The effect of various physiochemical factors on the synthesis and DLS size of MgO NPs was studied (Table 1). Careful interpretation of DLS results revealed that 1 M metal ion and 1:1 metal ion to CE volume ratio were optimum conditions for the MgO NPs synthesis using CE as reducing and stabilising agents (Tables 1). UV‐visible spectra of the standard reaction mixture (1 M metal ion and CE in 1:1 ratio) revealed that the synthesis of MgO NPs was completed in 30 min. Further incubation of the reaction mixture has no effect on the MgO NPs absorption intensity. Furthermore, the MgO NPs have −31 mV, surface charge. MgO NPs prepared using these optimum conditions were also subjected to FESEM and energy dispersive X‐ray (EDX). Mg and O peaks in the EDX spectrum of the NPs confirmed that the NPs have a magnesium oxide chemical composition (Fig. 1 a). FESEM and TEM analysis revealed that the average size of MgO NPs was 3.5 ± 1.88 nm (Fig. 1 b) and 2.8 ± 1.4 nm (Fig. 1 c), respectively. The size of MgO NPs was also calculated from the XRD curve using the Scherrer equation and was found to be 4.3 nm (Fig. 1 d) [47]. So, overall MgO NPs have average size ∼2–5 nm. The peaks in the XRD curve were broad due to the coating of MgO NPs with amorphous biosurfactant moieties from CE and small size of MgO NPs. TGA analysis revealed two sharp peaks at 75 and 280°C in the TGA curves of MgO NPs. These peaks were due to the degradation of CE moieties surrounding MgO NPs (Figs. 2 a and b). MgO NPs were stable up to the maximum tested temperature, 1000°C. The degradation temperature of MgO NPs has been reported to be more than 2000 °C [48]. So, the XRD and TGA analysis indicated that MgO NPs are covered by different surfactant moieties from CE. These moieties were identified using FTIR spectroscopy. CE acts as a reducing and stabilising agent during the synthesis of MgO NPs. Careful interpretation of various peaks in the FTIR spectrum of MgO NPs gave an idea of stabilising moieties (Fig. 2 c). FTIR peaks around 477 and 665 (Mg‐O bond), 693, 754 (O‐H bending) and 830 (metal oxygen bond) cm⁻¹ were due to MgO NPs [49, 50, 51, 52]. However, peaks around 1060 (C–O), 1105, 1201 (C–C bond, CH₂ group or C–O stretching), 1371 (C–O vibrations, CH₃ bending), 1427, 1494 (CH₂ group bending, aromatic C=C), 1577, 1625 (aromatic C=C bond or C=O stretching) and 3397 (OH bond) cm⁻¹ were observed mainly due to the presence of phenolic moieties in the CE like eugenol, isoeugenol, eugenol acetate clovinol, caryophyllene and humulene moieties and their derivatives [53, 54, 55, 56, 57, 58, 59, 60]. So, medicinally important moieties of the clove were responsible for the synthesis of MgO NPs.

Fig. 1.

Characterisation of CE prepared MgO NPs

(a) EDX spectrum confirms that characterised NPs were of Mg and O chemical composition, (b) FESEM image with size distribution pattern of MgO NPs, (c) TEM image with size distribution pattern of MgO NPs (2.8 ± 1.4 nm), (d) XRD pattern of MgO NPs. Broad peaks confirm small size of MgO NPs and surface covering of MgO NPs by CE moieties

Fig. 2.

TGA and FTIR analyses of MgO NPs

(a) TGA curve of MgO NPs, (b) Differential TGA curve of MgO NPs. Peaks at 75 and 280°C were due to the degradation of surface stabiliser molecules, (c) FTIR spectrum of MgO NPs. FTIR peaks around 1060, 1105, 1201, 1371, 1427, 1494, 1577, 1625 and 3397 cm⁻¹ were mainly due to the presence of phenolic moieties of CE. So, MgO NPs were covered with polyphenolic moieties

3.2 LC–MS characterisation of CE

LC–MS spectra analysis revealed that various phenolic moieties present in the CE were responsible for synthesis of MgO NPs. The phenolic moieties were mainly represented by M /Z peaks at 144.1 (l,5‐naphthyridin‐3‐amine), 203.02 (humulene/caryophyllene/copaene/cadinene), 205.04 (eugenin/eugenol acetate/eugenyl acetate), 256.96 (3‐iodo‐l,5‐naphthyridine), 300.96 (quercetin), 337.0981 (hexahydroxydiphenic acid), 338.97 (caffeoyl‐d‐glucose/coumaroylquinic acid), 355.1 (biflorin/5‐caffeoylquinic acid), 360.97 (clovamide(dihydroxy‐trans‐cinnamoyl)‐3‐(3,4‐dihydroxyphenyl)‐L‐alanine).

3.3.1 Adsorption of MB dye on MgO NPs

There was no adsorption of methyl red on the MgO NPs surface, while the solochrome was adsorbed with 10% efficiency. This may be due to the charge dependent interaction of MgO NPs with O⁻ moieties of the dyes (Fig. 3 a) [61]. The anionic dyes being negatively charged may not prefer to get adsorbed on the surface of negatively charged MgO NPs. However, the cationic dyes carrying positive charge were attracted towards O⁻ moieties on the surface of MgO NPs (Fig. 3 b). Thus, the cationic dyes were better adsorbed on the surface of MgO NPs. Among cationic dyes, MB was adsorbed with 90% efficiency, while the rhodamine 6G was absorbed with only 40% efficiency. The better adsorption of MB than rhodamine 6G may be due to the difference in the structure of two dyes. The structure of MB is less bulky than the Rhodamine 6G. So, this may be a possible reason for the better adsorption of MB on the surface of MgO NPs. A similar structure‐dependent difference in the dye adsorption has also been reported earlier [62, 63]. Thus MB dye was selected for the detailed adsorption studies.

Fig. 3.

Adsorption of MB dye on MgO NPs surface

(a) Curve showing per cent dye removal of MgO NPs (400 mg l⁻¹) incubated with 15 mg l⁻¹ aqueous solution of anionic (methyl red, solochrome) and cationic (rhodamine 6G, MB) dyes. MB dye has shown maximum 80% adsorption under these conditions, (b) Possible interaction of MB dye with MgO NPs

pH plays an important role in the stability of dye in aqueous medium. UV‐visible analysis of MB dye solution revealed that MB dye has shown good stability in aqueous medium at pH range 3–11. There was not much difference in the UV‐visible absorption intensity of MB at 664 nm in this pH range. However, slight decrease in the characteristic absorption was observed at pH 12. pH also influences the rate of adsorption of dye on NPs. pH affects the interactions between MB binding sites present on the MgO NPs surface and free MB [64, 65, 66]. There was constant increase in the dye adsorption with increase in the pH up to 11. Maximum MB adsorption was obtained at pH 11. Thereafter, a decrease in the dye adsorption was noticed at pH=12 (Fig. 4 a).

Fig. 4.

Graphs showing adsorption of MB on MgO NPs at different reaction conditions

(a) Effect of pH and contact time, (b) Effect of pH, (c) Effect of initial dye concentration on adsorption efficiency (%), (d) MB removal (%) at pH 11 as a function of incubation temperature

So, highly alkaline pH was favourable for the adsorption of MB on MgO NPs (Fig. 4 b). In this study, percentage removal of MB dye was calculated using (2). The removal of MB at different pH values also depends upon the contact time, i.e. time after MB dye incubation with MgO NPs. At all pH values, the dye removal efficiency increased with increase in the contact time from 0 to 60 min (Figs. 4 a and b). Contact time affects the availability of reactive sites to MB. At initial stages, the rate of adsorption was higher due to the availability of more reactive sites on the surface area of MgO NPs. Further increase in the contact time causes maximum immobilisation of MB onto MgO NPs [67, 68]. Hence thereafter no increase in the rate of MB adsorption was observed with increase in contact time up to 120 min. Similar results have been observed earlier [66].

MB dye concentration also affected the rate of adsorption of MB on MgO NPs. MB adsorption percentage increased as the MB concentration was increased from 5–15 mg l⁻¹ (Fig. 4 c). This increase in MB adsorption may be due to the availability of appropriate amount of MB to the MB binding sites of MgO NPs. However, the rate of adsorption decreased with further increase in the MB concentration up to 20 mg l⁻¹. This may be due to the reason that 15 mg l⁻¹ is the equilibrium concentration for MB adsorption on MgO NPs. At this concentration, the ratio of MB to binding sites is approximately equal. Higher MB concentration, 20 mg l⁻¹ may be hindering the interaction of MB with binding sites of MgO NPs due to molecular crowding [66, 68, 69, 70, 71, 72, 73].

The MB adsorption efficiency was also influenced by incubation temperature and best adsorption was observed at 25 °C (Fig. 4 d). The amount of MgO NPs also affected the rate of MB removal. There was a constant increase in the MB dye adsorption with an increase in MgO NPs from 200–400 mg l⁻¹. This may be due to the availability of more binding sites at 400 mg l⁻¹ concentration. However, the MB adsorption decreased constantly with further increase in MgO NPs amount up to 1600 mg l⁻¹. This may be due to the destabilisation of MgO NPs on the addition of its higher amount in fixed volume of water [66, 69]. Thus 400 mg l⁻¹ was optimum MgO NPs concentration for the adsorption of 15 mg l⁻¹ MB at pH 11 (Fig. 5 a).

Fig. 5.

Curve showing effect of amount of MgO NPs and incubation time on MB adsorption

(a) Curve showing the effect of MgO NPs amount on MB adsorption, (b) Curve showing a decrease in MB concentration with time due to the adsorption on the MgO NPs surface. More than 50% of MB adsorption was observed in ∼10 min and is supported by half‐life value (11.1 min) of second‐order reaction

3.3.2 Adsorption kinetics

The adsorption kinetics data reveal the progress of MB adsorption on the surface of MgO. For kinetic analysis, the variation of concentration of MB was plotted against time. The MB concentration decreased in solution due to the adsorption of increasing amount of MB on MgO NPs (Fig. 5 b). This graph clearly indicates that the adsorption process occurred rapidly. Almost 80% of MB dye was adsorbed on MgO NPs within 15 min. Maximum 90% removal of MB was achieved in 60 min contact time. There was no increase in adsorption with further increase in contact time up to 120 min.

The integrated forms of first‐order rate equation (3) and second‐order rate equation (4) were used to interpret the kinetic data [71, 74]

$$\ln \frac{c_0}{c_{\mathrm{e}}}=k_{1}t,$$ (3)

$$\frac{1}{C_{\mathrm{e}}}-\frac{1}{C_0}=k_{2}t.$$ (4)

where k ₁ is the first‐order rate constants and k ₂ is the second‐order rate constant. t designate contact time in minutes. The linearity plots of the first‐ and second‐order kinetics of MB adsorption on MgO NPs at different temperatures as a function of time are shown in Figs. 6 a and b, respectively. Figs. 6 c and d represent the linearity plots showing first‐ and second‐order kinetics of adsorption of different MB concentrations on the MgO NPs surface at 25°C, respectively.

Fig. 6.

Linear plots of kinetics of MB adsorption on the surface of MgO NPs

(a) First‐order plot of MB adsorption at different temperatures, (b) Second‐order plot of MB adsorption at different temperatures. (c) First‐order plot MB adsorption at various MB concentrations, (d) Second‐order plot of MB adsorption at various MB concentrations. R ² values confirm that the adsorption of MB on MgO NPs surface follow second‐order kinetics data

The half‐life of adsorption for first‐ and second‐order reaction is shown in Table 2. The results of linear regression were used to calculate the value of the rate constant. The kinetic parameters have been obtained at different temperatures and different initial dye concentration as given in Table 2. Both first‐ and second‐order rate constants were obtained at different temperatures (10, 25, 40, 55 °C) and initial dye concentrations (5, 10, 15, 20 ppm). Half‐life periods were calculated for both first‐ and second‐order kinetics under same conditions. The values of regression constant (R ²) were found to be higher in the case of second‐order reaction at all temperatures and initial MB concentrations than first‐order reaction (Table 2). So, adsorption of MB on the MgO NPs surface follows second‐order kinetics. Furthermore, the half‐life period value obtained using second‐order rate equation (11.1 min) was more close to experimental observations (Fig. 5 b) than the value obtained using first‐order rate equation (69.3 min).

Table 2.

Rate constant (K ₁), R ² and half‐life (t _1/2) values for first and second order of MB adsorption on MgO NPs surface

Temperature, °C	First order			Second order
K 1	R 2	t 1/2, min	K 2	R 2	t 1/2, min
10	0.03	0.91	23.1	0.008	0.99	8.3
25	0.01	0.94	69.3	0.006	0.98	11.1
40	0.04	0.86	17.3	0.001	0.94	66.6
55	0.01	0.84	69.3	0.001	0.88	66.6
initial MB concentration, ppm
5	0.002	0.89	346.5	0.001	0.97	200
10	0.003	0.94	231	0.007	0.97	14.3
15	0.01	0.94	69.3	0.006	0.98	11.1
20	0.001	0.95	693	0.002	0.99	25

3.3.3 Mechanism of MB adsorption by MgO NPs

To investigate the mechanism of adsorption of MB on the surface of MgO NPs, Langmuir and Freundlich adsorption isotherm models were used [67, 73, 75, 76, 77, 78]. Langmuir equation can be described as

$$\frac{C_{\mathrm{e}}}{q_{\mathrm{e}}}=\frac{1}{Q_{0}b}+\frac{C_{\mathrm{e}}}{Q_0},$$ (5)

where C _e designate final MB concentration after adsorption has occurred and equilibrium is attained (mg of MgO NPs per litre of MB solution), q _e is the adsorbent phase concentration after equilibrium designating amount of dye adsorbed (MB in mg)/(MgO NPs in g), Q ₀ is the adsorption capacity in the form of a monolayer (mg g⁻¹), b is the Langmuir constant and its designated capacity of adsorption (l mg⁻¹). A plot of C _e /q _e versus C _e gives a straight line (Fig. 7 a). Here 1/Q ₀ and 1/Q ₀ b correspond to slope and intercept, respectively [73, 74].

Fig. 7.

BET analysis of MgO NPs

(a) Langmuir and (b) Freundlich adsorption isotherm for adsorption of MB onto MgO NPs. R ² value clearly indicated that MB adsorption occurs in the form of monolayer

To check the nature of the MB adsorption process on MgO NPs, the value of dimensionless equilibrium parameter known as a separation factor (R _L) is required [73, 74]. R _L can be calculated as shown as

$$R_{\mathrm{L}}=\frac{1}{1+b{c}_0},$$ (6)

where C ₀ is the initial concentration of MB used. The value of R _L indicates whether the adsorption is favourable or not. The value of R _L in the present study was 0.067. This proves that the adsorption of MB on MgO NPs was a favourable process (Table 3) [29, 79, 80].

Table 3a.

Langmuir and Freundlich isotherm analysis of MB adsorption on MgO NPs surface

Langmuir isotherm
R 2	Slope	Intercept	Q 0	B	R L
0.99	0.00018	0.0097	5555	0.75	0.067

Table 3b.

Freundlich isotherm
R 2	Slope	Intercept	N	K f
0.88	1.88	4.86	0.53	70794.58

Freundlich Isotherm analysis provides information about the multilayer adsorption. The linear form of Freundlich equation is shown in [67, 79, 80, 81] as

$$\log q_{\mathrm{e}}=\log K_{\mathrm{f}}+\frac{1}{n}\log C_{\mathrm{e}},$$ (7)

where K _f is the Freundlich constant. K _f indicates the adsorption capacity of the MgO NPs. n indicates the favourability of the adsorption process. The Freundlich plot plotted between, ln q _e and ln C _e, is shown in Fig. 7 b. The slope gives the values of n, while the intercept represent K _f.

Adsorption isotherm constants (b, K _f and n) were obtained by carrying out the adsorption process at 25°C using 15 ppm initial dye concentration with different adsorbent doses as given in Table 3.

Higher values of regression constant in Langmuir isotherm than in Freundlich clearly indicated that the adsorption of MB on the MgO NPs followed the Langmuir model. Hence, the MB adsorption was in the form of a monolayer on the surface of MgO NPs (Table 3). The adsorption of MB in the form of the monolayer may be due to very small size of the MgO NPs [66, 80]. The R _L value of Langmuir adsorption isotherm was within 0–1 that indicated the adsorption of MB was a favourable process. Maximum adsorption capacity Q ₀ was obtained from intercept and was found to be 5555 mg g⁻¹.

So, careful analysis of kinetics and isotherm data obtained at operational conditions revealed that the adsorption of MB dye on the MgO NPs occurred in the form of the monolayer and follow second‐order reaction. So, the adsorption of MB on MgO NPs depends on both adsorbate and adsorbent.

3.3.4 Thermodynamics of MB adsorption

To elucidate the mechanism of MB adsorption various thermodynamic parameters related to adsorption were analysed. The change in the standard free energy (ΔG °) of MB adsorption was calculated using the following equation [75]:

$$\mathrm{\Delta }{G}^{\circ }=-2.303RT\ln K_{\mathrm{c}},$$ (8)

where R is the universal gas constant (8.314 J mol K⁻¹), T designates the temperature in Kelvin and K _c is the equilibrium thermodynamic adsorption constant. Under identical experimental conditions, if K _c is expressed in l mol⁻¹ it is equal to Langmuir equilibrium constant K _L, distribution coefficient K _d and q _e /c _e [73]. C _e and q _e can be related to the standard enthalpy change, i.e. ΔH ° (kJ mol⁻¹) and standard the entropy change, i.e. ΔS ° (kJ mol⁻¹ K⁻¹) and T (K) by the following equation:

$$\ln \frac{q_{\mathrm{e}}}{C_{\mathrm{e}}}=\frac{\mathrm{\Delta }{S}^{{}^{\circ }}}{R}+\frac{-\mathrm{\Delta }{H}^{\circ }}{R}.$$ (9)

By plotting q _e /C _e versus 1/T, the slope and intercept provided the value of ΔH ° and ΔS °. Assumption made is, the values of ΔH ° and ΔS ° are constants over the temperature range used in the study. So, ΔG ° can be calculated at different temperatures using the following equation [73]

$$\mathrm{\Delta }{G}^{\circ }=\mathrm{\Delta }{H}^{\circ }-T\mathrm{\Delta }{S}^{\circ }.$$ (10)

The negative values of ΔG ° and ΔH ° revealed that the adsorption of MB on MgO NPs is a spontaneous and exothermic process, respectively (Table 4). ΔG ° becomes less negative with increase in the temperature. This indicated that the low temperature favours MB adsorption. The overall negative value of ΔG ° confirms the feasibility of the adsorption process in spite of the negative value of ΔS °.

Table 4.

Thermodynamic parameters for the adsorption of MB onto MgO NPs

Temperature, K	▵G °, kJ mol−1	▵H °, kJ mol−1	▵S °, kJ mol−1
283	−8.12	−26.18	−0.063
298	−7.81
313	−6.20
328	−5.25

3.3.5 Brunauer–Emmett–Teller (BET) analysis

The surface area of MgO NPs was analysed using Quantachrome Instruments version 3.01. The total surface area of synthesised MgO NPs was obtained with reference to the BET multi‐point and single point methods using N₂ adsorption/desorption isotherm data (Fig. 8). The surface area of MgO NPs was found to be 41 m² g⁻¹. The adsorption capacity of MgO NPs (5555 mg g⁻¹) is better than some earlier reported NPs as shown in Table 5.

Fig. 8.

BET surface area plot of MgO NPs. MgO NPs have 41 m² g⁻¹ surface area and 5555 mg g⁻¹ adsorption capacity

Table 5.

Dye removal using nanomaterials

Nanomaterials	Dye	Adsorption capacity, mg/g	Dye removal, %	Reference
porous MgO	crystal red	2409	–	[82]
Ni NPs	crystal violet	0.45	48−92	[65]
	eosin yellow	0.62	39−94
	orange II A	0.27	21−91
agar based Fe/Cu and Fe/Pd	MB	875	70−90	[83]
	rhodamine B	780	68−81
MgO	MB	5555	98	present study

The stability and reusability efficiency of MgO NPs for MB removal were also analysed. Adsorption efficiency of 98, 88, 87.5 and 48% was obtained after second, third, fourth, fifth cycles of MgO NP regeneration, respectively. Thereafter the NPs were difficult to recover and as a result the efficiency of dye removal was reduced below 20%.

3.4.1 Fluorescence sensing of Pb²⁺ using MgO NPs

In addition to dye adsorption, MgO NPs also acted as a label free selective and sensitive Pb²⁺ fluorescence nanosensor. The Pb²⁺ sensing has been reported earlier under acidic conditions. The basic pH may leads to precipitation of Pb²⁺ in the form of metal hydroxide [84, 85]. So, the performance of MgO NPs was tested between pH 7 and pH 3. Our results are in agreement with previous reports that Pb²⁺ sensing is better in acidic pH. Pb²⁺ decreased the fluorescence intensity of MgO NPs at 495 nm. Other metal ions did not decrease the fluorescence intensity to much extent at pH 3. So, the selectivity of MgO NP nanosensor was pH dependent and best selectivity was observed at pH 3. Within pH range 7–4, the MgO NPs interacted significantly with all the tested metal ions including Pb²⁺. All metal ions either increased or decreased the fluorescence intensity of MgO NPs significantly at 495 nm at pH range 4–7.

The pH of zero charge for MgO NPs lies in acidic pH 4.5. Above pH 4.5 MgO NPs carry negative charge and hence chances of non‐specific positive (metal ions)–negative (MgO NPs) charge interaction increases. However, at pH 3, the interference from other metal ions was found to be minimum and maximum fluorescence quenching of only Pb²⁺ ions was observed (Fig. 9 a). The fluorescence intensity of MgO NPs was quenched linearly with increase in the Pb²⁺ concentration. Decrease in the fluorescence intensity of MgO NPs may be due to the specific interaction of Pb²⁺ ions with MgO NPs at pH 3, as other metal ions Cd²⁺, Co²⁺, Cr²⁺, Cu²⁺, Mn²⁺, Na⁺, Ni²⁺ and Zn²⁺ did not interact to this extent at this pH. The fluorescence quenching of MgO NPs by Pb²⁺ ions followed linearity in a broad range, 1 nM–200 µM (Figs. 9 b and c). Thus, MgO NPs have good sensitivity for the Pb²⁺ sensing in a wide concentration range. The detection limit of the MgO NP nanosensor was found to be 24 µM (3σ method). Other metal ions have very low interaction with MgO NPs in the presence of Pb²⁺. The interference due to other metal ions used even at double concentration to that of Pb²⁺ was very low and well within acceptable limit, i.e. change in fluorescence intensity was <10% (Fig. 9 d) [86]. This may be due to the preferential binding of MgO NPs with Pb²⁺ than other metal ions at pH 3.

Fig. 9.

Effect of different metal ions on the fluorescence intensity of MgO NPs at pH 3 (excitation wavelength, 390 nm and emission wavelength, 495 nm)

(a) Bar diagram showing fluorescence intensity represented as F 0 − F /F 0 of MgO NPs at 100 μM metal ion concentration, (b) Spectra showing the effect of 1 nM–200 μM Pb²⁺ on the fluorescence intensity of the MgO NPs, (c) Linearity graph showing the effect of different Pb²⁺ concentrations on the fluorescence intensity of MgO NPs, (d) Bar diagram showing the effect of various metal ions on the selectivity of the MgO NP‐based nanosensor at pH 3. The Pb²⁺ concentration was 100 μM. Interfering metal ions (200 μM) have no significant effect on the selectivity and sensitivity of MgO NP nanosensors

Further to check the applicability of the MgO nanosensor in different water sources, we tested the sensing of Pb²⁺ in tap and RO water and compared it with sensing conducted in DDW (Fig. 10). The data for Pb²⁺ sensing in DDW, tap water and RO water are shown in Table 6. So, MgO NPs prepared using CE are cost effective, eco‐friendly i.e. involve less use of toxic chemicals during sensing, no need of functionalisation using chemical entity, very low response time (0 min), low detection limit and good sensitivity (Table 7).

Fig. 10.

Diagram showing Pb²⁺ detection performance of the MgO NP nanosensor in DDW, tap water, and RO water. MgO NPs were able to detect Pb²⁺ in all three water samples

Table 6.

Comparison of Pb²⁺ sensing performance of MgO NP nanosensor in DDW with tap water and RO water

Pb2+ concentration (µM) in DDW	Detection in tap water in µM (recovery %)	Detection in RO water in µM (recovery %)
1	1.1 (110)	1 (100)
10	10.5 (105)	9.8 (98)
50	48 (96)	48.8 (97.6)
100	107 (107)	107 (107)

Table 7.

Comparative account of current studies with earlier reported methods. These studies have used chemical method for NPs synthesis, functionalisation, activation or detection

Nanomaterial used for sensing	Sensing method used	Modification of NPs	Response time, min	Linearity range, μM	Limit of detection, μM	Selectivity tested	Reference
MgO	fluorescence spectroscopy	No	0	0.001–200	24	yes	present study
gold	colorimetry	yes	−	−	400	no	[85]
gold	colorimetry	yes	>10	−	0.1	no	[86]
nickel	colorimetry	yes	−	0–150	100	no	[65]
gold	colorimetry	yes	−	0.1–2	0.24	no	[83]
ionic liquid‐cerium oxide NPs–carbon nanotubes composite	electrochemical	yes	60	0.01–0.1	0.005	yes	[87]
gold	fluorescence spectroscopy	yes	20	0.001–1	0.005	yes	[88]
gold	fluorescence spectroscopy	yes	8	0.05–1	0.004	yes	[89]
fluorescein‐labelled starch maleate NPs	fluorescence spectroscopy	yes	−	−	0.3	no	[90]

4 Conclusions

CE has the ability to synthesise MgO NPs of different sizes by acting as a reducing as well as stabilising agent. CE prepared MgO NPs can efficiently adsorb MB dye and maximum adsorption was obtained at pH 11. MgO NPs have a high surface area, 41 m² g⁻¹ that ultimately accounts for their high adsorption capacity, i.e. 5555 mg g⁻¹. The adsorption of MB follows Langmuir adsorption isotherm. So, the MB was adsorbed in the form of a monolayer due to the very small size of MgO NPs. Furthermore, the adsorption of MB followed second‐order reaction. The MgO NPs were also able to selectively sense most abundantly present toxic metal ion Pb²⁺ over 1 nM–200 µM range. Pb²⁺ sensing was pH dependent and pH 3 was best pH for nanosensing. Limit of detection (3σ) was found to be 24 µM and it can selectively detect Pb²⁺ even in the presence of other metal ions. So, green chemistry‐based CE mediated synthesis is a useful green method to obtain MgO NPs for dye removal and Pb²⁺ sensing application.