Synthesis of pure nickel(III) oxide nanoparticles at room temperature for Cr(VI) ion removal

Abstract

Ni₂O₃ nanoparticles of various sizes (∼25.8 to 49.7 nm), obtained by a facile oxidation process using Ni(NO₃)₂·6H₂O, NaOH and sodium hypochlorite as precursor materials at various temperatures (0°, 25°, 50° and 70 °C), are found to remove toxic Cr(VI) from aqueous solution (20 g L⁻¹). The structure, morphology, surface charge and chemical compositions of the synthesized samples were characterized by XRD, TEM, zeta potential and EDX respectively. Adsorption capacity is found to be strongly dependent on the size and surface heterogeneity of the synthesized particles and a plausible mechanism for such significant adsorption efficacy is attributed to the sorbate–sorbent electrostatic interaction and shielding of Cr(VI) ions. The adsorption mechanism fits with the Langmuir isotherm model with maximum 60% Cr(VI) removal capacity (20.768 mg g⁻¹ (calculated) and 20.408 mg g⁻¹ (predicted from isotherms)) corresponding to Ni₂O₃ nanoparticles, prepared at 70 °C in 3 hours at room temperature. Thermodynamic parameters, obtained from fitting, demonstrate that the adsorption process being endothermic in nature follows a pseudo-second-order kinetic model. The spontaneity of the adsorption process gets reduced with increasing particle size. pH of the solution is observed to have a remarkable effect on the adsorption, giving maximum adsorption at pH = 6.

---

Introduction

The presence of various heavy metals like Cr(VI), As(III), U(VI) etc. in aquatic environments causes major concern for aquatic life and human health due to their severe toxicity.¹ Among them, Cr(VI) exists in the effluents of various industries like electroplating, pigments etc. and is identified to be one of the most fatal species due to its carcinogenic effects on human beings and its abundance in drinking water.² Out of two different forms of chromium, the hexavalent (Cr(VI)) and trivalent form (Cr(III)) in aqueous medium, Cr(III) precipitates in its hydroxide and oxide forms, therefore it doesn't migrate in the solution and is easily separable. On the other hand, Cr(VI) being highly soluble in water is observed to be highly toxic due to its high redox potential it and causes cancer, kidney damage, gastrointestinal disorders etc.^3,4In this context, it may be stated that US Environmental Protection Agency (EPA) and World Health Organization (WHO) have set maximum allowed level of Cr(VI) concentration in drinking water 100 μg L⁻¹ and 50 μg L⁻¹ respectively.⁵Till date, among different readily available methods like chemical precipitation, solvent extraction, membrane separation, ion exchange, adsorption etc. to remove Cr(VI), adsorption is found to be the most suited one that is being successfully used to remove from its respective aqueous solutions.^6,7 The most commercially used material for this purpose is the activated carbon or mesoporous carbon due to their high surface porosity.^8,9Apart from them, bio-adsorbents are often used as adsorbents in waste water treatment.^10–13However, they have weak mechanical strength, poor separation capacity and hence they are not very effective in this field. Recently, alternative materials including various oxide based nanocomposites and nanostructure have been synthesized as potential adsorbent of heavy metal ions.^14,15 Thus, with the advancement of technology, newer materials with improved adsorption capacities are being developed. In this context, it has to be mentioned that metal oxides are known to have high adsorption capacity of toxic metals in nano-dimensions. Iron and aluminium oxide have been identified to adsorb toxic substances from waste water effectively due to their high surface adsorption ability.^16–18 Among different transitional materials, developed so far, NiO has proven itself as an another promising material that shows potential application in battery electrodes, super-capacitors, smart windows, catalysis etc.^19–22 Nickel, being a transitional metal, possesses variable oxidation states, but most of them are found to be abundant in normal conditions.^23,24 In this context, it has to be mentioned that pseudo hydrate of Ni(III) oxide was reported but their anhydrous form was not observed due to easy transformation of them into NiO on heating.²⁵ On the other hand, it was established that these higher oxides may possess many interesting properties if obtained in anhydrous form.

According to the reviewed literature, the preparation of the pure Ni(III) oxide (Ni₂O₃) was not feasible by standard synthesis procedures like sol–gel, hydrothermal, solvothermal etc. No systematic study was performed on this material so far. In this study, we have successfully prepared pure Ni₂O₃ by an environment friendly, facile, chemical precipitation method at room temperature. The synthesis was carried out in a range of temperature to study the size effect on its Cr(VI) removal efficiency from waste water.

Experimental

Preparation of pure Ni₂O₃ nanoparticles

All the reagents were procured from MERCK India Pvt. Ltd. and were used without further purification. Pure Ni₂O₃ nanoparticles were synthesized by sustained oxidation of nickel precursors. In a typical experiment, 1.00 g of Ni(NO₃)₂·6H₂O was dissolved in 20 ml deionized water under constant stirring. In another beaker, 1.60 g NaOH was dissolved in 15 ml of sodium hypochlorite solution having 4% active chlorine. The alkaline solution of hypochlorite was added dropwise under constant stirring to the solution containing nickel precursor. The stirring was continued for half an hour to obtain black precipitate that formed as flocculates almost immediately after adding the hypochlorite solution. The precipitate was collected by filtration and dried to form grayish black powder of hydrated nickel(III) oxide (Ni₂O₃·xH₂O). This powder was then treated with 20 ml sodium hypochlorite solution under constant stirring for an hour to form a suspension. The suspension was left idle till the effervescence ceases and a dark black precipitate forms at the bottom of the beaker. This precipitate was then further washed with a small amount of hypochlorite solution, centrifuged at 20 000 rpm, collected and dried over hot air oven to obtain crystals of pure Ni(III) oxide nanoparticles. In order to investigate the temperature effect on particle size, the synthesis reaction was carried out at four different temperatures viz. freezing temperature (0 °C), room temperature (RT) (25 °C), 50 °C and 70 °C respectively.

Characterization

Crystallinity and phase purity of the synthesized samples were investigated by Ultima-III, Rigaku X-ray diffractometer (Cu Kα radiation, λ = 1.5404 Å). Chemical analysis of the synthesized samples were carried out using energy dispersive attached with field emission scanning electron microscope (S – 4800, Hitachi). Transmission electron microscope (TEM, JEM – 2100, JEOL) was used to investigate the microstructure and morphology of the synthesized samples. Surface potential was measured using Zetasizer NS Nano.

Adsorption of Cr(VI) by pure Ni₂O₃ nanoparticles

A batch adsorption procedure was applied to study the sorption of Cr(VI) ions by Ni₂O₃ nano-particles as proposed by Yao et al.²³ Dichromate stock solution was prepared by dissolving 0.40 gm of K₂Cr₂O₇ in 20 ml of deionized water. 20.00 mg of the powdered Ni₂O₃ sample was dispersed into the stock solution under constant stirring at room temperature. In order to investigate the effect of contact time, adsorption kinetics, adsorption process was carried out at seven different time intervals viz. 15 minutes, 30 minutes, 45 minutes, 1 hour, 1.5 hours, 2 hours and 3 hours. Each solution was centrifuged twice at 20 000 rpm for 10 minutes to remove any Ni₂O₃ particle and the residual solution was collected in glass vials for further analysis. Amount of adsorbed chromium was calculated using standard calibration curve as proposed by Ansari et al.¹¹ Pure Ni₂O₃ particles were collected after centrifugation to investigate any change in phase. It has been found that the crystal structure of the particles after the adsorption remains unchanged. The liquid samples collected after adsorption is subjected to serial dilution procedure to 10⁻⁴ M for ultraviolet spectroscopy between 800 nm and 200 nm (Perkin Elmer). After determining initial and final concentrations of the solution, adsorption percentage and adsorption capacity (Q_e) were calculated using the relations in eqn (1) and (2):¹⁰andwhere C_o and C_e represent the initial and final concentration of Cr(VI) in g L⁻¹ ‘V’ and ‘m’ are the volume of the testing solution (in ml) and mass of the sorbent (in mg) respectively.

Results and discussion

Crystal structure determination by X-ray diffraction

The X-ray diffraction pattern of the pure dried powder samples are represented in Fig. 1. Diffraction peaks, obtained at 27.34°, 31.66°, 45.41°, 56.38° and 66.12° are readily be indexed with (101), (002), (111), (202) and (004) plane of hexagonal Ni₂O₃ (JCPDS Card number 14-0481, CAS Number 1314-06-3). Absence of any peak other than Ni₂O₃ rules out the presence of any impurity phase and confirms the phase purity of Ni₂O₃.

Fig. 1 X-ray diffraction pattern of pure Ni₂O₃ nanoparticles.

The particle sizes of the synthesized samples, calculated using the Scherrer's formula.²⁶ are listed in Table 1. It is noticed that the particle size increases on increasing the synthesis temperature.

Table 1 Particle sizes of Ni₂O₃ nanoparticles based on the synthesis temperatures

Temperature of synthesis	Particle size (in nm)
0 °C	25.8
RT (25 °C)	34.2
50 °C	42.6
70 °C	49.7

---

Eqn (3) that represents the variation of particles size with synthesized temperature can be used to calculate the activation energy (E_a) of formation of pure Ni₂O₃ nanoparticles.²⁷

where, ‘r’ and ‘k’ represent the size of the particle synthesized at absolute temperature ‘T’ and Boltzmann's constant respectively. From slope of the ln r vs. 1/T curve (shown in Fig. 2), E_a is calculated to be ∼75.49 meV.

Fig. 2 Arrhenius plot for calculation of the activation energy of formation of pure Ni₂O₃ nanoparticles.

Synthesis mechanism of pure Ni₂O₃nanoparticles

Synthesis of pure Ni₂O₃ involves a rapid competitive oxidation of the nickel precursor. The whole reaction may be sub-divided into two broad divisions viz., active chlorination of the nickel precursor resulting in the preferential active oxygen attachment to the nickel ion that results in the formation of hydrated nickel(III) oxide in water medium and basic pH. The second step involves forced and prolonged oxidation of the so formed hydrated oxide to form pure anhydrous Ni₂O₃ nanoparticles. The detailed steps may be predicted as follows:

Firstly, dissociation of nickel precursor (Ni(NO₃)₂·6H₂O) takes place into Ni²⁺ and NO₃⁻ ions in the presence of water. A standard alkaline solution of sodium hypochlorite (having 4% active chlorine) that is introduced to the prepared solution acts as a source of active oxygen. In presence of H⁺ ions (which is present in the solution in abundance), NaOCl dissociates to form one nascent oxygen and HCl. The alkaline solution of NaOCl (as used in this experiment) neutralizes the so formed HCl and prevents the formation of NiCl₂ as an unwanted product.

The nascent oxygen, also known as reactive oxygen having one lone electron, would try to acquire one more electron to stabilize its atomic configuration. As the nascent oxygen is formed within the solution, it readily oxidizes the Ni²⁺ species by accepting one electron from the ions to form Ni³⁺ ions. The ionic reaction can be written as below.

Nickel ions, seated above H⁺ and Na⁺ in the electrochemical series, preferentially gets oxidized by the [O] or O˙ species present in the solution. Thus, after this reaction, a pseudo hydrated oxide of trivalent nickel is formed. This is due to the competitive attachment of [O] and OH⁻ to the nickel cation. The detailed ion movement is schematically shown below:

The second step involves forced and prolonged oxidation of the Ni–OH bonds so formed to convert it to Ni=O thereby forming anhydrous Ni₂O₃. The detailed mechanism is shown below:

The nascent oxygen attacks the slight positive charge centre of H⁺ to remove it from the hydrated structure. This results the formation of the unstable dangling bond due to the cleavage of the O–H bond which immediately stabilizes by forming a double bond with nickel. Same mechanism is followed by the other Ni–OH bonds. As a result, pure Ni₂O₃ phase is obtained. In order to identify the ratio of Ni : O, chemical analysis using EDAX spectroscopy was carried out and the analysis shows the ratio to be 2 : 3 (Fig. S1 in ESI†).

Fig. 3(a) and (b) represent the transmission electron microscope (TEM) images that reveal uniform distribution of particles. A histogram for the particle size distribution, represented in the inset of Fig. 3(a), indicates the narrow size distribution of the particles.

Fig. 3 (a) TEM micrograph of pure Ni₂O₃ nanoparticles showing particle size distribution (b) TEM micrograph of pure Ni₂O₃ nanoparticles showing particle size.

Removal of Cr(VI) ions from aqueous solution

The batch experiments for Cr(VI) ion removal were carried out for Ni₂O₃ corresponding to four different particle size as performed by Liu et al.¹⁴ The temperature (25 °C), pH (∼6) and concentration of Cr₂O₇²⁻ and Ni₂O₃ of the solution were kept constant. When Cr(VI) is absorbed as chromate i.e. in alkaline medium, it possesses a peak in the range of 460 nm to 340 nm. The amount of removed Cr(VI) was calculated using the standard calibration curve obtained for spectrophotometric determination of Cr(VI).²⁸ It was found that Cr(VI)removal efficiency is different for samples having different particle size. Cr(VI) removal capabilities of the synthesized nanoparticles was examined with respect to volume of the stock solution, pH of the solution, adsorbent dose, adsorbate dose, shaker speed and are discussed in the ESI.†

Mechanism of Cr(VI) ions adsorption from aqueous solution

To understand the basic mechanism of Cr(VI) removal, it is essential to study the behaviour of the adsorbate and the adsorbent in presence of a polar solvent (in this case, water). In acidic medium having pH < 6, Cr(VI) exists as HCrO₄⁻ while in neutral pH, it exists as CrO₄⁻ (chromate ion). The chromate ion possesses tetrahedral geometry with partial positive chromium centre surrounded by four oxygen atoms having two lone pairs that stabilize the structure. The experiment was carried out close to the neutral pH where the chromate ions are predominant.² Chromate ions, due to the presence of two lone pairs, attract the positive species towards itself thereby making the Cr(VI) more positive. Thus, eventually, Cr(VI) becomes loosely bound to its surrounding oxygen. Ni₂O₃ on the other hand, has a lone pair of electron and four bonded pairs of electrons. This lone pair of electron acts as a negative charge centre. This negative centre attracts the δ⁺ Cr(VI) centre of the loosely bonded chromate ion towards the particle surface. This leads to the adsorption of the Cr(VI) ion onto the particle surface. Each particle can pick up one Cr(VI) molecule at a time. However, from Fig. 4, it is found that close to about 60% Cr(VI) ions are removed from the solution. Weight percent of Cr in K₂Cr₂O₇ is ∼33% of the total weight of the molecule. So, out of 400.00 mg K₂Cr₂O₇ used for the experiment, only 132.00 mg Cr is available for active adsorption. 20.00 mg adsorbent can take up 20.00 mg of Cr(VI). The adsorption of the rest ∼50.00 mg Cr(VI) corresponding to the highest adsorption result may be attributed to the surface heterogeneity and cationic vacancies on the surface that leads to the formation of more active centers for adsorption of Cr(VI).

Fig. 4 Variation of adsorption percentage with respect to contact time for the particles prepared at different temperatures.

Effect of contact time on Cr(VI) adsorption by Ni₂O₃ nanoparticles

The adsorption of Cr(VI) from solution onto pure Ni₂O₃ nanoparticles is found to be highly time dependent. It has been observed that the effective adsorption of Cr(VI) ions increases with increase in contact time. The adsorption percentages (adsorption spectra are shown in ESI, Fig. S7–S10†), calculated using eqn (1), are plotted with respect to the contact time for the four different particles (shown in Fig. 4). It can be inferred that maximum Cr(VI) ion adsorption percentage of ∼59.62% is achieved by the particles prepared at 70 °C while the others are below 50%.

The zeta potential measurement reveals that the surfaces of Ni₂O₃ nanoparticles are negatively charged (Table 2) that provides ease adsorption of the positively charged metal ions on the surface of the particles. Cr(VI) ions being positively charged are attracted towards a negatively charged surface and effectively get adsorbed. The surface charge developed due to the dispersion of nanoparticles in a solution may be another important factor governing the adsorption. Spectra are presented in the electronic supplementary information (Fig. S11†).

Table 2 Zeta potential of pure Ni₂O₃ nanoparticles

Particle preparation temperature	Zeta potential (in mV)
0 °C	−26.6
RT (25 °C)	−20.8
50 °C	−21.2
70 °C	−20.1

---

From Table 2, it is observed that all the four particles have a negative surface charge. The smallest particle possesses the highest negative charge followed by the particles prepared at 50 °C which in turn follows the particles prepared respectively at room temperature and 70 °C. Thus, initially, adsorption is high in case of the particles prepared at 0 °C and 50 °C that is evident from Fig. 4. Smaller particles having more surface area possesses high shielding effect due to same concentration of H⁺ ions in the solution. Therefore, due to less shielding effect by H⁺ ions for particles with largest size show the highest Cr(VI) adsorption ability.²⁹Fig. 5 shows schematically the probable difference in shielding density of H⁺ ions in case of two particles of different size.

Fig. 5 Comparison of shielding effect for particles of different size.

Kinetics of Cr(VI) adsorption by pure Ni₂O₃ nanoparticles

In order to understand the appropriate kinetic models for sorption of Cr(IV), the experimentally obtained data of Cr(VI) removal process were simulated using pseudo first order and pseudo second order kinetic model. Pseudo first order model (shown in Fig. 6) that often describes the sorption process is expressed as follows:

ln(q_e − q_t) = ln q_e − k₁t (4)

Fig. 6 Pseudo first order kinetics curve fitting for the sorption mechanism by particles prepared at (a) 0 °C (b) RT (25 °C) (c) 50 °C (d) 70 °C.

The pseudo second order kinetic model (Fig. 7) that relates the whole sorption process including sorption and internal particle diffusion and can be written as:

where, q_e and q_t are the sorption amounts (mg g⁻¹) at equilibrium time and at time‘t’ respectively; k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹) represent the kinetic rate constants corresponding to pseudo-first-order and pseudo-second-order respectively.²⁹ Obtained kinetic parameters are listed in Table 3. It is observed from the r² values that the sorption of Cr(VI) by pure Ni₂O₃ nanocrystals follows the pseudo second order kinetic model. Moreover, the value of q_e, calculated from the second order kinetic model, matches well with its experimental value. This also implies that the absorption is dominated by chemisorption or strong surface complexation rather than mass transport.³⁰

Fig. 7 Pseudo second order kinetics curve fitting for the sorption mechanism by particles prepared at (a) 0 °C (b) RT (25 °C) (c) 50 °C (d) 70 °C.

Table 3 Kinetics result for adsorption of Cr(VI) by pure Ni₂O₃ nanoparticles

Synthesis temperature (in °C)	Pseudo first order model			Pseudo second order model
	k 1 (min^−1)	q e (mg g^−1)	r ^2	k 2 (g mg^−1 min^−1)	q e (mg g^−1)	r ^2
0 °C	0.011	13.90	0.899	0.087	3.35	0.860
25 °C (RT)	0.01	14.21	0.8995	0.056	5.38	0.989
50 °C	0.006	5.95	0.705	0.042	9.90	0.977
70 °C	0.011	8.80	0.569	0.018	20.00	0.973

---

Adsorption isotherms of Cr(VI) by pure Ni₂O₃ nanoparticles

Isotherm study being an important tool to analyse the adsorption capacity of the adsorbent indicates the distribution of adsorbed molecules between the solid and the liquid phase when the adsorption reaches equilibrium. To understand the adsorption behaviour of the adsorbent, two isotherm models were tested viz. Freundlich and Langmuir isotherms.^30,31 The Freundlich isotherm is expressed as follows:³⁰

It is considered that the particle surface is heterogeneous that is expressed by the degree of heterogeneity ‘n’ in the eqn (6) The Freundlich plots for adsorption of Cr(VI) by pure Ni₂O₃ nanocrystals are presented in Fig. 8. The value of the Freundlich constant ‘k_f’ and heterogeneity constant ‘n’, obtained from ln q_evs. ln C_e plot, are listed in Table 4.

Fig. 8 Freundlich isotherm plots for the sorption mechanism by particles prepared at (a) 0 °C (b) RT (25 °C) (c) 50 °C (d) 70 °C.

Table 4 Adsorption isotherm parameters obtained for pure Ni₂O₃ nanoparticles prepared at different temperatures

Synthesis temperature (in °C)	Freundlich isotherm			Langmuir isotherm
	n (degree of heterogeneity)	k f (Freundlich constant)	r ^2	b (in L mg^−1)	q m (in mg g^−1)	r ^2
0 °C	0.255	1.026	0.935	0.964	18.519	0.996
25 °C (RT)	0.234	1.063	0.872	1.000	18.868	0.999
50 °C	0.378	1.282	0.945	1.314	18.925	0.989
70 °C	0.400	1.192	0.745	0.875	20.408	0.999

---

The Langmuir isotherm that has been developed on the basis of one site occupancy by one pollutant species on the homogenous surface of the adsorbent is expressed by the following equation:

On simplifying the above relation, the following linear relation is obtained.

Fig. 9 shows the Langmuir plots for the pure Ni₂O₃ nanoparticles. On plotting 1/q_evs. 1/C_e curves, Langmuir binding constants (b) and maximum adsorption (q_m) are calculated (tabulated in Table 4).

Fig. 9 Langmuir isotherm plots for the sorption mechanism by particles prepared at (a) 0 °C (b) RT (25 °C) (c) 50 °C (d) 70 °C.

High value of r² signifies that the Langmuir isotherm fits well with the Cr(VI) adsorption data compare to Freundlich isotherm. From the value of q_m, it is observed that the binding constant is least for the particles prepared at 70 °C. This explains the initial adsorption of Cr(VI) for the first 15 minutes when the adsorption rate is lowest for the particles at 70 °C as noticed from Fig. 4. Moreover, from Table 2, it is noticed that the spontaneity of the reaction for particles prepared at 70 °C is the least. However, for particles having smaller size, the effective area for adsorption is less. As the particle size increases, more and more active adsorption sites are available. As a result, more number of Cr(VI) can be adsorbed in a single layer. As the adsorption follows Langmuir isotherm, it forms a monolayer of the pollutant species surrounding the nanoparticle. In such a case, if there is a tendency of formation of a second layer, the Cr(VI) ions of the second layer is repelled by the already existing layer. Therefore, the adsorption in this case is proportional to the surface exposed for adsorption. Hence, the particle having the highest size possesses highest adsorption ability.

The Freundlich isotherm results also show that the degree of surface heterogeneity of the particles prepared at 70 °C is the most. This means that these particles have got more heterogeneous sites which act as additional active sites for Cr(VI) ion adsorption. Thus, this model also supports the particles prepared at 70 °C to be the highest adsorber though they are the largest particle size having lowest spontaneity of reaction. The comparison of the adsorption capacity of Ni₂O₃ with other materials has been thoroughly discussed in the ESI.†

It has been observed from above discussion that the adsorption and the associated degradation are significantly dependent on the particle size. Therefore, in order to get more insight into the above mentioned mechanism, we have thermodynamically computed the free energy (G₀) of the sorption reaction using the following relations:^31,32

andwhere, ‘k_d’, ‘T’ and ‘R’ are the distribution coefficient, temperature (in K) and universal gas constant (8.314 J mol⁻¹ K⁻¹) respectively. Therefore, a correlation has been drawn between the particle size and distribution coefficient to relate enthalpy, entropy and Gibb's free energy of the adsorption process on the basis of formation of a homogeneous single layer of Cr(VI) ion around the particle. Considering radius of a single particle to be ‘r’, the effective surfaces for adsorption increase with decrease in size. As adsorption is a surface phenomenon, the volume atoms and their contribution can be neglected. Thus, surface available for adsorption is 4πr². Therefore, from the results of adsorption, it can be considered that amount of Cr(VI) adsorbed in directly proportional to the adsorbent surface and may be written as: Q_eα r², which in turn implies Q_e = s × r², where ‘s’ is the surface constant that is equal to 4π × ‘number of effective surface per molecule of adsorbent’. The number of effective surface per molecule is a property of a molecule in any polar solvent and hence, it is constant for a particular molecule. Again, adsorption of any ionic species on the surface of the nanoparticles is supposed to be proportional to the potential corresponding to surface charge in the absence any shielding effect (Q_eα ξ). In the presence of any electrical double layer that causes shielding, Q_e would be inversely proportional to σ (Q_eα).³² However, for smaller particles, the shielding effect that forms an electrical double layer which effectively prevents adsorption. Therefore using eqn (9) and (10), it may be written,

ln k_d = (ln s − ln C_e − ln σ + ln ξ) + 2 ln r (11)

This is essentially a linear relation. Thus, on plotting the straight line curve of ln k_dvs. ln r, the surface constant ‘s’ and in turn the number of active surfaces available for adsorption can be calculated. The plot is illustrated in Fig. 10.

Fig. 10 Relation between ln k_d and ln r.

Now, from the intercept, ln k⁰_d = − ln C_e curve, where k⁰_d may be considered as the distribution coefficient for a particle of unit dimension, it is possible to calculate that denotes the effective adsorption taking place per unit weight of the adsorbent per unit shielding potential which obstructs the effective adsorption using the following equation,

Again, using (9) and (11), Gibb's free energy ‘G₀’ (positivity or negativity will depend on values of ‘r’, ‘s’ and ‘C_e’ and hence is considered without the negative sign) can be calculated as follows:

Table 5 enlists the thermodynamic parameters calculated from the above results. It is observed that reaction spontaneity decreases as the particle size increases. Thus, it can be concluded from the results that smaller particles undergo a more spontaneous reaction as compared to the larger particles.

Table 5 Thermodynamic and surface parameters of adsorption reaction by pure Ni₂O₃ nanoparticles

Radius (in nm)	(in mg g^−1 mV^−1)	G 0 (in J mol^−1)
12.9	7.68 × 10^−5	−8457.30
17.1	1.09 × 10^−4	−7060.69
21.3	8.66 × 10^−5	−5972.40
24.85	8.73 × 10^−5	−5208.56

---

From the above result, it can be seen that adsorption coefficient is highest for the particles prepared at room temperature. This can be attributed to the extent of heterogeneity of the room temperature particles. However, due to the screening effect of the H⁺ ions, the effective adsorption decreases in case of the smaller particles. Thus, the 70 °C synthesized particles which are largest in size, has the highest effective adsorption.

Conclusions

Pure Ni₂O₃ nanoparticles are prepared by a mild, environment friendly, room temperature process. Particles of four different sizes are prepared varying from ∼25.8 nm to 49.7 nm by varying the synthesis temperature. The activation energy of formation of the Ni₂O₃ nanoparticles is calculated to be ∼75.49 meV. A relation between particle size and Gibb's free energy have been formulated to calculate the degree of spontaneity and number of active adsorption sites per unit area for particles with different particle size. The prepared particles are found to possess excellent Cr(VI) adsorption capacity adsorbing about ∼60% Cr(VI) in 3 hours from its aqueous solution. Thus, this material has a potential to replace the conventional adsorbents used commercially which has lower time efficiency of heavy metal removal. The material thus possesses huge environment friendly impact that will improve the hydrosphere condition drastically throughout the globe thereby addressing several major global threats possessed by the industries.

Acknowledgements

The authors (SB & SH) wish to thank the University Grant Commission, the Govt. of India for University with Potential for Excellence (UPE – II) scheme.

References

1. R. Hana, W. Zoua, Z. Zhang, J. Shi and J. Yang, Journal of Hazardous Materials B, 2006, 137, 384.
2. A. Zhitkovich, Chem. Res. Toxicol., 2011, 24, 1617.
3. B. Qiu, H. Gu, X. Yan, J. Guo, Y. Wang, D. Sun, Q. Wang, M. Khan, X. Zhang, B. L. Weeks, D. P. Young, Z. Guo and S. Wei, J. Mater. Chem. A, 2014, 2, 17454.
4. H. Gu, S. B. Rapole, J. Sharma, Y. Huang, D. Cao, H. A. Colorado, Z. Luo, N. Haldolaarachchige, D. P. Young, B. Walters, S. Wei and Z. Guo, RSC Adv., 2012, 2, 11007.
5. B. Qiu, C. Xu, D. Sun, H. Wei, X. Zhang, J. Guo, Q. Wang, D. Rutman, Z. Guo and S. Wei, RSC Adv., 2014, 4, 29855.
6. S. Mellah, S. Chegrouche and M. Barkat, Hydrometallurgy, 2007, 85, 163.
7. H. Singh, S. L. Mishra and R. Vijayalakshmi, Hydrometallurgy, 2004, 73, 63.
8. V. P. Ravi, R. V. Jasra and T. S. G. Bhat, J. Chem. Technol. Biotechnol., 1998, 71, 173.
9. J. P. Chen and S. Wu, J. Chem. Technol. Biotechnol., 2000, 75, 791.
10. T. Yao, T. Cui, J. Wu, Q. Chen, S. Lu and K. Sun, Polym. Chem., 2011, 2, 2893.
11. R. Ansari and N. K. Fahim, React. Funct. Polym., 2007, 67, 367.
12. S. Bettini, R. Pagano, L. Valli and G. Giancane, Nanoscale, 2014, 6, 10113.
13. M. E. Grass, Y. Yue, S. E. Hab, R. M. Rioux, C. I. Teall, P. Yang and G. A. Somorjai, J. Phys. Chem. C, 2008, 112, 4797.
14. L. Tan, J. Wang, Q. Liu, Y. Sun, X. Jing, L. Liu, J. Liu and D. Song, New J. Chem., 2015, 39, 868.
15. J. Quinton, L. Thomsen and P. Dastoor, Surf. Interface Anal., 1997, 25, 931.
16. R. C. Vaishya and S. K. Gupta, J. Chem. Technol. Biotechnol., 2002, 78, 73.
17. C. Tiffreau, J. Lutzenkirchen and P. Behra, J. Colloid Interface Sci., 1995, 172, 82.
18. J. M. Tarascon and M. Armand, Nature, 2001, 414, 359.
19. S. H. Park, S. H. Kang, C. S. Johnson, K. Amine and M. M. Thackeray, Electrochem. Commun., 2007, 9, 262.
20. L. Wöhler and O. Balz, Z. Elektrochem., 1921, 27, 415.
21. H. Baubigny, Compt Mend., 1905, 87, 1082 , 1878; 141. 1232.
22. R. W. Cairns and E. Ott, J. Am. Chem. Soc., 1933, 55, 527.
23. T. Yao, T. Cui, J. Wu, Q. Chen, S. Lu and K. Sun, Polym. Chem., 2011, 2, 2893.
24. B. D. Cullity, Elements of X-Ray Diffraction, Addison-Wiley Publishing Company Inc., 1956, 56-10137.
25. S. J. Iyengar, M. Joy, C. K. Ghosh, S. Dey, R. K. Kotnala and S. Ghosh, RSC Adv., 2014, 4(110), 64919–64929.
26. A. Kamari, W. S. W. Ngah, M. Y. Chong and M. L. Cheah, Desalination, 2009, 249, 1180.
27. I. Langmuir, J. Am. Chem. Soc., 1918, 40, 1361.
28. J. X. Li, Z. Q. Guo, S. W. Zhang and X. K. Wang, Chem. Eng. J., 2011, 172, 892.
29. H. M. F. Freundlich, J. Phys. Chem., 1906, 57, 385.
30. W. T. Tsai, Y. M. Chang, C. W. Lai and C. C. Lo, J. Colloid Interface Sci., 2005, 289, 333.
31. M. Sprynskyy, B. Buszewski, A. P. Terzyk and J. Namiesnik, J. Colloid Interface Sci., 2006, 304, 21.
32. D. E. Yates, S. Levine and T. W. Healy, J. Chem. Soc., Faraday Trans., 1974, 1(70), 1752–1768.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05810d

---