# Adsorption of Chromium Ions from Aqueous Solutions by Synthesized Nanoparticles

Authors: Senthilkumar Kandasamy; Sampathkumar Velusamy; Pradeep Thirumoorthy; Mageshkumar Periyasamy; SenthilkumarVeerasamy; K. M. Gopalakrishnan; U. Sathish; Vallarasu Kiramani; Fathima Darras Gracy Antrini; Selvakumar Periyasamy
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

In the present manuscript, an attempt has been made to remove chromium metal ions from synthetic effluent using adsorption process. The synthesized titanium dioxide nanoparticles were used as adsorbents. Adsorption studies were performed in batch process. Various characterization of synthesized nanoparticles such as XRD analysis, optical properties of nanoparticles using UV-visible absorption spectroscopy, concentration of chemical bonds, and atomic arrangement using (FTIR) have been performed and analyzed. The dependency of adsorption percentage of metal ions and equilibrium amount of metal adsorbed with respect to pH, adsorbent dosage, initial concentration, and temperature are studied. Mechanisms of metal ion adsorption process explained by various adsorption isotherms and adsorption kinetic models. The criteria for statistical significance of correlation coefficient (R2) for fitting the experimental data to the various isotherms were tested and analyzed. The experimental results reveal that synthesized titanium dioxide nanoparticles could be used as adsorbents in order to remove chromium ions present in industrial wastewater.


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## Body

## 1. Introduction

Heavy metals may defined as metal with a density greater than 4 gm/cm3 or metals with a high atomic weight or metals commonly used in industry and toxic to man and other organisms. These include arsenic (As), silver (Ag), chromium (Cr), mercury (Hg), and platinum group of metal [1]. The various anthropogenic activities that introduce the heavy metals in the environment are mining of ores, municipal waste, burning, and agricultural activities [2, 3]. The released toxic metals are having more dangerous to concentrate in ecosystems and habitats. The source of heavy metals is mostly by industrials, i.e., metallurgical industries, electroplating industries, metal finishing industries, tanneries, chemical manufacturing industries, and battery manufacturing industries [4, 5].The presence of lead and chromium compounds causes more toxic to the environment. Various processes of sources of chromium and lead metals are mining, tanning, cement, textile and dyeing, electroplating industries, steel, photographic material, and paints. Heavy metals removed from wastewater using various technologies like ion exchange process, flocculation, precipitation process, membrane filtration, RO, and phytoremediation are the conventional methods, and the merits and demerits of each method have extensively reviewed [6, 7]. The conventional process has demerits such as increasing the disposal problems due to the large amount sludge production, and treated wastewater is contaminated and very difficult to treat the toxic industrial wastewater because of high biological demand, total solids, and huge amount of weakly biodegradable and toxic matters. The literature reveals that adsorption process gives more efficiency, low cost, and user-friendly [8].In recent research, [8] states that environmental related problems are reduced by using nanotechnology principles, and it has emerging and promising techniques for eliminating pollutants from industrial effluent. Nanomaterial preparation in nanoscale is the basic in the nanotechnology method. Nanoscale materials have distinctive characteristics, on account of their tiny size, and they possess high surface area. These characteristics improve the surface assimilation capability of the nanoparticles [9]. Due to their large area of surface, nanoparticles contain bigger interaction with chemical species [10, 11]. The current paper concentrates on synthesis of titanium oxide nanoparticles and the removal of chromium ions from synthetic wastewater using synthesized titanium oxide nanoparticles. Adsorption studies were performed in batch process. The pH was varied from 2 to 12 and for the different adsorbent dosage (0.2 to 1 gm/100 ml), the experiment was carried out. The removal of chromium was examined by changing the initial concentration between 50 ppm and 150 ppm. The procedure was repeated for 40°C and 50°C.


## 2. Materials and Methods

### 2.1. Synthesis of Nanoparticles

Titanium dioxide (TiO2) nanoparticles have prepared by using chemical precipitation method and mechanical grinding [12]. Titanium chloride was mixed with 10% sulfuric acid placed in an ice-water bath with constant stirring, because it produce dense fumes of titanium oxysulfate, then temperature was maintained at 60°C, for 1 hour, and the addition of ammonia forms sudden precipitate. Whatman filter paper was used for filtering the precipitate. Precipitate was heated at 500°C in a muffle furnace. Finally, product was recovered as TiO2 particles. It has been treated in a ball mill at 100 rpm for more than 4 hours, in order to avoid the contamination. Balls and container should be treated with deionized water.


### 2.2. Characterization of Nanoparticles

#### 2.2.1. XRD Results

The XRD analysis can be used to find peak intensity and full width [13]. The XRD results pointed out the presence of nanoscale particles in the synthesized materials. By using Debye-Scherrer formula, diameter of synthesized nanoparticle was obtained. Figure 1 and Table 1 show the results of XRD analysis in detailed.Figure 1
XRD results for TiO2.Table 1
XRD results for TiO2.
Φ (°2Th)ϕ/2FWHM (°2Th)FWHM (3.14/180)λ (Å)Size (Å)Nanometer (nm)10.0725.0360.0980.00171.542651.24265.1213.4686.73410.1470.00251.54624.6662.4627.05713.5280.3930.00681.54368.8936.8827.64313.8210.1470.00251.541811.80181.1823.15711.5780.1470.00251.541021.03102.10Average129.55


#### 2.2.2. UV Results

The nanoparticles size considered being a very important part in changing the whole properties of materials, and hence determination of size of semiconducting nanoparticles becomes very important to investigate the material properties. To examine the optical properties of nanoparticles, UV-visible absorption spectroscopy is used. For TiO2 band, emissions were observed at different wavelengths such as 403, 421, 425, 441, 446, 460, 466, 484, and 528 nm. The experimental observation reveals that in the absorption spectrum of Fe2O3, the absorption maximum (at 550 nm) progressively reduced upon UV irradiation, with no change of maximum peak of absorption [14].


#### 2.2.3. FTIR Results

In Fourier transform infrared spectroscopy, intensity frequency is measured using intensity-time output subjected to a Fourier transform, and it converts to an infrared spectrum. Concentration of chemical bonds and atomic arrangement presence can be identified by using FTIR, where transmission percentage and wave number are the output. The absorption band signals analyzed from 400 to 1000 cm-1 correspond to Ti–O–Ti vibration. Figure 2 shows the FTIR results of nanoparticles. The band with respect to Fe-O stretched manner of Fe2O3 is seen at 576 cm-1 [15].Figure 2
FTIR Result for TiO2.


## 2.1. Synthesis of Nanoparticles

Titanium dioxide (TiO2) nanoparticles have prepared by using chemical precipitation method and mechanical grinding [12]. Titanium chloride was mixed with 10% sulfuric acid placed in an ice-water bath with constant stirring, because it produce dense fumes of titanium oxysulfate, then temperature was maintained at 60°C, for 1 hour, and the addition of ammonia forms sudden precipitate. Whatman filter paper was used for filtering the precipitate. Precipitate was heated at 500°C in a muffle furnace. Finally, product was recovered as TiO2 particles. It has been treated in a ball mill at 100 rpm for more than 4 hours, in order to avoid the contamination. Balls and container should be treated with deionized water.


## 2.2. Characterization of Nanoparticles

### 2.2.1. XRD Results

The XRD analysis can be used to find peak intensity and full width [13]. The XRD results pointed out the presence of nanoscale particles in the synthesized materials. By using Debye-Scherrer formula, diameter of synthesized nanoparticle was obtained. Figure 1 and Table 1 show the results of XRD analysis in detailed.Figure 1
XRD results for TiO2.Table 1
XRD results for TiO2.
Φ (°2Th)ϕ/2FWHM (°2Th)FWHM (3.14/180)λ (Å)Size (Å)Nanometer (nm)10.0725.0360.0980.00171.542651.24265.1213.4686.73410.1470.00251.54624.6662.4627.05713.5280.3930.00681.54368.8936.8827.64313.8210.1470.00251.541811.80181.1823.15711.5780.1470.00251.541021.03102.10Average129.55


### 2.2.2. UV Results

The nanoparticles size considered being a very important part in changing the whole properties of materials, and hence determination of size of semiconducting nanoparticles becomes very important to investigate the material properties. To examine the optical properties of nanoparticles, UV-visible absorption spectroscopy is used. For TiO2 band, emissions were observed at different wavelengths such as 403, 421, 425, 441, 446, 460, 466, 484, and 528 nm. The experimental observation reveals that in the absorption spectrum of Fe2O3, the absorption maximum (at 550 nm) progressively reduced upon UV irradiation, with no change of maximum peak of absorption [14].


### 2.2.3. FTIR Results

In Fourier transform infrared spectroscopy, intensity frequency is measured using intensity-time output subjected to a Fourier transform, and it converts to an infrared spectrum. Concentration of chemical bonds and atomic arrangement presence can be identified by using FTIR, where transmission percentage and wave number are the output. The absorption band signals analyzed from 400 to 1000 cm-1 correspond to Ti–O–Ti vibration. Figure 2 shows the FTIR results of nanoparticles. The band with respect to Fe-O stretched manner of Fe2O3 is seen at 576 cm-1 [15].Figure 2
FTIR Result for TiO2.


## 2.2.1. XRD Results

The XRD analysis can be used to find peak intensity and full width [13]. The XRD results pointed out the presence of nanoscale particles in the synthesized materials. By using Debye-Scherrer formula, diameter of synthesized nanoparticle was obtained. Figure 1 and Table 1 show the results of XRD analysis in detailed.Figure 1
XRD results for TiO2.Table 1
XRD results for TiO2.
Φ (°2Th)ϕ/2FWHM (°2Th)FWHM (3.14/180)λ (Å)Size (Å)Nanometer (nm)10.0725.0360.0980.00171.542651.24265.1213.4686.73410.1470.00251.54624.6662.4627.05713.5280.3930.00681.54368.8936.8827.64313.8210.1470.00251.541811.80181.1823.15711.5780.1470.00251.541021.03102.10Average129.55


## 2.2.2. UV Results

The nanoparticles size considered being a very important part in changing the whole properties of materials, and hence determination of size of semiconducting nanoparticles becomes very important to investigate the material properties. To examine the optical properties of nanoparticles, UV-visible absorption spectroscopy is used. For TiO2 band, emissions were observed at different wavelengths such as 403, 421, 425, 441, 446, 460, 466, 484, and 528 nm. The experimental observation reveals that in the absorption spectrum of Fe2O3, the absorption maximum (at 550 nm) progressively reduced upon UV irradiation, with no change of maximum peak of absorption [14].


## 2.2.3. FTIR Results

In Fourier transform infrared spectroscopy, intensity frequency is measured using intensity-time output subjected to a Fourier transform, and it converts to an infrared spectrum. Concentration of chemical bonds and atomic arrangement presence can be identified by using FTIR, where transmission percentage and wave number are the output. The absorption band signals analyzed from 400 to 1000 cm-1 correspond to Ti–O–Ti vibration. Figure 2 shows the FTIR results of nanoparticles. The band with respect to Fe-O stretched manner of Fe2O3 is seen at 576 cm-1 [15].Figure 2
FTIR Result for TiO2.


## 3. Results

### 3.1. Adsorption Studies

#### 3.1.1. General Procedure

Adsorption studies were performed in batch process. Synthetic solution was prepared in order to optimize various parameters. The diluted solution was prepared from concentrated stock solution using distilled water with different concentrations from 50 to 150 ppm. The experimental study was performed with 100 ppm solution. The pH was varied from 2 to 12. For the different adsorbent dosages (0.2 to 1 gm/100 ml), the experiment was carried out. The removal of chromium was examined by changing the initial concentration between 50 ppm and 150 ppm. The same process was repeated for 40°C and 50°C [16].The % adsorption of metal ions was calculated by eq. (1).
(1)%Adsorption=C0–Cr/C0×10,whereC0 is concentration of Cr ions (ppm) at initial, and Cr is metal ion residual concentration in the filtrate after shaking for a definite time period (mg/l).The metal uptake at a particular timeqt was determined by the following equation.
(2)qt=C0–Cr/m∗V.Equilibrium amount of metal adsorbed,qe, was found by
(3)qe=C0–Ce/m∗V,wherem and V represent adsorbent mass and volume of sample, respectively. Ce is the Cr concentration at equilibrium.


### 3.2. Chromium Removal Using Titanium Dioxide

#### 3.2.1. Influence of pH

Chromium removed using titanium dioxide as an adsorbent from 2 to 12 was studied for analyzing the influence of pH. The change of pH affects the adsorptive process through dissociation of functional groups on the adsorbent surface active sites. The highest chromium removed level was observed on lower pH 6 and further increasing pH, there will be no change in the removal of chromium. Figure3 shows influencing of the pH factor of Cr removal percentage [17].Figure 3
Influence of pH on % of chromium removal.


#### 3.2.2. Influence of Adsorbent Dosage

Adsorption experiment was carried out at varying adsorbent dose (0.2-1 gm/100 ml) while pH (6) was kept constant for Cr. The increase in adsorbent dosage leads to an increase of metal removal that was observed and was shown in Figure4. As there is more surface area for the adsorption process, the more metal ions get adsorbed onto the surface of the titanium dioxide [18].Figure 4
Influence of adsorbent dosage on removal of Cr.


#### 3.2.3. Influence of Initial Concentration

At room temperature, the effect of initial Cr ion concentration of effluent on Cr removal has been evaluated using titanium dioxide by keeping the optimized value unchanged. These optimized values are maintained constant for the further studies. Initial concentration was increased while rate of adsorption increases. But after the process reached equilibrium, i.e., after 75 min, there was no increase in the metal removal. At 100 ppm, the removal attained equilibrium. Figure5 shows that the removal of metal ions on titanium dioxide powder depends on initial concentration of effluent. Moreover, with increasing Cr concentration, the boundary layer increases diffusion of Cr ions resulted maximum sorption by TiO2 [18, 19].Figure 5
Influence of initial concentration on Cr removal.


#### 3.2.4. Influence of Temperature

Batch studies were conducted at the optimal conditions and different temperatures. The temperature varied from 30°C to 50°C by keeping all the optimized parameters unchanged. The adsorption of Cr on TiO2 was noted by varying temperature. Figure 6 shows that there is no appreciable increase in percentage removal with increasing temperature [19].Figure 6
Influence of temperature on Cr removal.


### 3.3. Kinetics Studies on Adsorption

In this study, adsorption mechanism examined using the rate data was analyzed using the suitable kinetic model. The adsorption dynamics illustrated by metal ion rate uptake on stability time and adsorbents were controlled by rate. Different kinetic models were examined using experimental data (different initial concentration and temperatures). The adsorption dynamics of the metal ions on nanoadsorbent were checked with the different models as described below [20].


#### 3.3.1. Kinetic Studies

(1) Pseudo 1st Order. The kinetics of adsorption may be explained by a pseudo 1st order equation as mentioned below (eq. (4)).
(4)logqe−qt=logqe–k1t/2.303,whereqe and qt are adsorbed metal ions (mg/g) at equilibrium and time, and k1 is the rate constant (min-1).The plot of (log (qe−qt) vs. t) and the suitability of equation imply 1st order process. The experiments were conducted at various temperatures with different concentrations. Using eq. (4), the theoretical qe values of Cr removal have been calculated, and the results revealed that this adsorption system is pseudo first-order.(2) Pseudo 2nd Order. The linearized pseudo 2nd order is mentioned in eq. (5).
(5)tqt=1k2qe2+1qt∗t,wherek2 is the rate constant (g/mg.min), and k2 and qe can be found that a good agreement with experimental data and confirm with 2nd order kinetics for different temperatures and initial concentrations. For second order kinetics, obtained regression coefficients are in the range of acceptable value, and it is found that this model suits for adsorption of Cr ions [21, 22].(3) Fractional Power Model. The linear fractional power function equation is given by eq. (6).
(6)lnqt=lnk+μlnt.The values ofμ and k can be found using the above linear relationship. For different temperatures and concentrations, the model was checked to fit. Figure 7 indicates that the power function model described the time-dependent. The kinetic of Cr ion adsorption can be satisfactory described by the power function model. Power model describes rates of adsorption. However, the regression coefficient R2 was very high for a Cr ion which indicates that power function is the best model to correlate kinetic data [23].Figure 7
Determination of constants using the fractional power model.(4) Intraparticle Diffusion. It is represented by the following eq. (7).
(7)qt=klntt1/2,wherek is the rate constant (mg/g.min).Adsorption mechanism is difficult but in the initial stage, intraparticle diffusion is important. In this case, linear portions observed at different temperatures and concentrations were shown in Figure8. When intraparticle diffusion occurs, the slopes of these linear portions can be distinct by adsorption rate and rate parameter in this area. At first, minimum time period, it was assumed that ions transferred through nanoadsorbent to external surface. Behind surface saturation, until equilibrium was reached, ions are entered through intraparticle diffusion to nanoadsorbent, pore, and interior surface diffusion, and it is by second straight. This model well suits for Cr removal [24].Figure 8
Determination of constants using intraparticle diffusion.(5) Elovich Equation. It is mentioned by eq. (8) that
(8)qt=1β∗lnαβ+1β∗lnt,whereα is the primary sorption rate (mg/g.min), and ß is the surface area coverage (g/mg).Elovich equation explains about chemical adsorption on heterogeneous adsorbents, but for adsorbate–adsorbent interaction, this equation does not recommend any specific mechanism. The experiment was conducted for various temperatures, and different concentrations were shown that this kinetic data not fit Cr removal process, because of the low regression coefficient value [25]. Table 2 represents the consolidated values of various parameters of the different models at various temperatures.Table 2
Cr removal for different concentrations and temperatures.
ConcentrationTemperatureModelsParameters50 ppm100 ppm150 ppm30°C40°C50°CPseudo first orderR20.850.960.970.970.950.95K0.020.0260.020.030.030.04Qe1.611.851.891.912.072.090.990.980.99Pseudo second orderR20.860.99K0.020.0160.010.030.020.02Qe3.510.430.370.440.460.42Fractional powerR20.990.950.990.950.930.95K0.860.460.860.460.490.45N0.941.852.111.851.711.74Intraparticle diffusionR20.720.850.940.850.810.83K0.460.330.460.330.340.32ElovichR20.550.970.980.970.960.96


### 3.4. Adsorption Isotherm

Equilibrium isotherm was represented by a sorption isotherm and characterized by some of the constants. The adsorbent sorption was established when the sorbate concentration in the bulk solution was in dynamic balance at sorbent interface. The statistical significance of the correlation coefficient (R2) was the criteria by which the fitting of the data to the various isotherms was tested and tabulated in Table 3. In order to measure the nanoadsorbent affinity for the metal examined, i.e., Cr, the following isotherm models were used [25].Table 3
Constants of isotherm and kinetics studies for TiO2.
ConcentrationTemperatureModelsParameters50 ppm100 ppm150 ppm30°C40°C50°CFreundlich isothermR20.95630.05660.966310.91620.9993Kf0.37523.785850.720.02170.03130.0214N5.49054.7058292.630.00540.12230.028Langmuir isothermR20.999910.99990.87820.86870.8733Temkin isothermR20.96730.92910.94260.99090.98950.9894a52.5772.922140.9296.50592.57598.766b12.80712.04424.1131.25830.40531.867


#### 3.4.1. Isotherm Studies on Cr Using Tio2

(1) Langmuir Adsorption Isotherm. At a constant temperature, adsorbent surface was represented by Langmuir adsorption isotherm. It is represented by (9).
(9)Ceqe=1Kdqm+1qmCe,whereqe is the Cr adsorbed level (mg/g), Ce is the aqueous phase metal concentration (mg/l), Kd is the sorption equilibrium constant, and qm is the monolayer capacity(mg/g).For different temperatures and different concentrations, the equation is checked to fit the Cr removal using titanium dioxide. The value ofqm and K attained from linear graph [25].(2) Freundlich Isotherm. The important assumption of Freundlich isotherm is adsorption on heterogeneous surface by multilayer adsorption. The linear form of the equation is
(10)logqe=logkf+1nlogCe.The nomenclature is as follows:qe is the adsorption capacity, Ce is the final concentration, and n is the empirical constant.From the experimental data, various temperatures and concentrations, the plots lnqe vs. ln Ce and Freundlich coefficients n and Kf are obtained. From the data, it was observed that this adsorption process has not followed Freundlich isotherm [24].(3) Temkin Isotherm. It assumed that due to interactions between adsorbent and adsorbate, adsorption heat of molecules in the adsorbed layer diminishes.It is given by eq. (11).
(11)X=a+blnC,whereC is the concentration of adsorbate (mg/l), X is the metal adsorbed (mg/g), and a and b are constants.For different temperatures and concentrations, the given equation was checked for the fitness [26].(4) Isotherm Study Table.


## 3.1. Adsorption Studies

### 3.1.1. General Procedure

Adsorption studies were performed in batch process. Synthetic solution was prepared in order to optimize various parameters. The diluted solution was prepared from concentrated stock solution using distilled water with different concentrations from 50 to 150 ppm. The experimental study was performed with 100 ppm solution. The pH was varied from 2 to 12. For the different adsorbent dosages (0.2 to 1 gm/100 ml), the experiment was carried out. The removal of chromium was examined by changing the initial concentration between 50 ppm and 150 ppm. The same process was repeated for 40°C and 50°C [16].The % adsorption of metal ions was calculated by eq. (1).
(1)%Adsorption=C0–Cr/C0×10,whereC0 is concentration of Cr ions (ppm) at initial, and Cr is metal ion residual concentration in the filtrate after shaking for a definite time period (mg/l).The metal uptake at a particular timeqt was determined by the following equation.
(2)qt=C0–Cr/m∗V.Equilibrium amount of metal adsorbed,qe, was found by
(3)qe=C0–Ce/m∗V,wherem and V represent adsorbent mass and volume of sample, respectively. Ce is the Cr concentration at equilibrium.


## 3.1.1. General Procedure

Adsorption studies were performed in batch process. Synthetic solution was prepared in order to optimize various parameters. The diluted solution was prepared from concentrated stock solution using distilled water with different concentrations from 50 to 150 ppm. The experimental study was performed with 100 ppm solution. The pH was varied from 2 to 12. For the different adsorbent dosages (0.2 to 1 gm/100 ml), the experiment was carried out. The removal of chromium was examined by changing the initial concentration between 50 ppm and 150 ppm. The same process was repeated for 40°C and 50°C [16].The % adsorption of metal ions was calculated by eq. (1).
(1)%Adsorption=C0–Cr/C0×10,whereC0 is concentration of Cr ions (ppm) at initial, and Cr is metal ion residual concentration in the filtrate after shaking for a definite time period (mg/l).The metal uptake at a particular timeqt was determined by the following equation.
(2)qt=C0–Cr/m∗V.Equilibrium amount of metal adsorbed,qe, was found by
(3)qe=C0–Ce/m∗V,wherem and V represent adsorbent mass and volume of sample, respectively. Ce is the Cr concentration at equilibrium.


## 3.2. Chromium Removal Using Titanium Dioxide

### 3.2.1. Influence of pH

Chromium removed using titanium dioxide as an adsorbent from 2 to 12 was studied for analyzing the influence of pH. The change of pH affects the adsorptive process through dissociation of functional groups on the adsorbent surface active sites. The highest chromium removed level was observed on lower pH 6 and further increasing pH, there will be no change in the removal of chromium. Figure3 shows influencing of the pH factor of Cr removal percentage [17].Figure 3
Influence of pH on % of chromium removal.


### 3.2.2. Influence of Adsorbent Dosage

Adsorption experiment was carried out at varying adsorbent dose (0.2-1 gm/100 ml) while pH (6) was kept constant for Cr. The increase in adsorbent dosage leads to an increase of metal removal that was observed and was shown in Figure4. As there is more surface area for the adsorption process, the more metal ions get adsorbed onto the surface of the titanium dioxide [18].Figure 4
Influence of adsorbent dosage on removal of Cr.


### 3.2.3. Influence of Initial Concentration

At room temperature, the effect of initial Cr ion concentration of effluent on Cr removal has been evaluated using titanium dioxide by keeping the optimized value unchanged. These optimized values are maintained constant for the further studies. Initial concentration was increased while rate of adsorption increases. But after the process reached equilibrium, i.e., after 75 min, there was no increase in the metal removal. At 100 ppm, the removal attained equilibrium. Figure5 shows that the removal of metal ions on titanium dioxide powder depends on initial concentration of effluent. Moreover, with increasing Cr concentration, the boundary layer increases diffusion of Cr ions resulted maximum sorption by TiO2 [18, 19].Figure 5
Influence of initial concentration on Cr removal.


### 3.2.4. Influence of Temperature

Batch studies were conducted at the optimal conditions and different temperatures. The temperature varied from 30°C to 50°C by keeping all the optimized parameters unchanged. The adsorption of Cr on TiO2 was noted by varying temperature. Figure 6 shows that there is no appreciable increase in percentage removal with increasing temperature [19].Figure 6
Influence of temperature on Cr removal.


## 3.2.1. Influence of pH

Chromium removed using titanium dioxide as an adsorbent from 2 to 12 was studied for analyzing the influence of pH. The change of pH affects the adsorptive process through dissociation of functional groups on the adsorbent surface active sites. The highest chromium removed level was observed on lower pH 6 and further increasing pH, there will be no change in the removal of chromium. Figure3 shows influencing of the pH factor of Cr removal percentage [17].Figure 3
Influence of pH on % of chromium removal.


## 3.2.2. Influence of Adsorbent Dosage

Adsorption experiment was carried out at varying adsorbent dose (0.2-1 gm/100 ml) while pH (6) was kept constant for Cr. The increase in adsorbent dosage leads to an increase of metal removal that was observed and was shown in Figure4. As there is more surface area for the adsorption process, the more metal ions get adsorbed onto the surface of the titanium dioxide [18].Figure 4
Influence of adsorbent dosage on removal of Cr.


## 3.2.3. Influence of Initial Concentration

At room temperature, the effect of initial Cr ion concentration of effluent on Cr removal has been evaluated using titanium dioxide by keeping the optimized value unchanged. These optimized values are maintained constant for the further studies. Initial concentration was increased while rate of adsorption increases. But after the process reached equilibrium, i.e., after 75 min, there was no increase in the metal removal. At 100 ppm, the removal attained equilibrium. Figure5 shows that the removal of metal ions on titanium dioxide powder depends on initial concentration of effluent. Moreover, with increasing Cr concentration, the boundary layer increases diffusion of Cr ions resulted maximum sorption by TiO2 [18, 19].Figure 5
Influence of initial concentration on Cr removal.


## 3.2.4. Influence of Temperature

Batch studies were conducted at the optimal conditions and different temperatures. The temperature varied from 30°C to 50°C by keeping all the optimized parameters unchanged. The adsorption of Cr on TiO2 was noted by varying temperature. Figure 6 shows that there is no appreciable increase in percentage removal with increasing temperature [19].Figure 6
Influence of temperature on Cr removal.


## 3.3. Kinetics Studies on Adsorption

In this study, adsorption mechanism examined using the rate data was analyzed using the suitable kinetic model. The adsorption dynamics illustrated by metal ion rate uptake on stability time and adsorbents were controlled by rate. Different kinetic models were examined using experimental data (different initial concentration and temperatures). The adsorption dynamics of the metal ions on nanoadsorbent were checked with the different models as described below [20].


### 3.3.1. Kinetic Studies

(1) Pseudo 1st Order. The kinetics of adsorption may be explained by a pseudo 1st order equation as mentioned below (eq. (4)).
(4)logqe−qt=logqe–k1t/2.303,whereqe and qt are adsorbed metal ions (mg/g) at equilibrium and time, and k1 is the rate constant (min-1).The plot of (log (qe−qt) vs. t) and the suitability of equation imply 1st order process. The experiments were conducted at various temperatures with different concentrations. Using eq. (4), the theoretical qe values of Cr removal have been calculated, and the results revealed that this adsorption system is pseudo first-order.(2) Pseudo 2nd Order. The linearized pseudo 2nd order is mentioned in eq. (5).
(5)tqt=1k2qe2+1qt∗t,wherek2 is the rate constant (g/mg.min), and k2 and qe can be found that a good agreement with experimental data and confirm with 2nd order kinetics for different temperatures and initial concentrations. For second order kinetics, obtained regression coefficients are in the range of acceptable value, and it is found that this model suits for adsorption of Cr ions [21, 22].(3) Fractional Power Model. The linear fractional power function equation is given by eq. (6).
(6)lnqt=lnk+μlnt.The values ofμ and k can be found using the above linear relationship. For different temperatures and concentrations, the model was checked to fit. Figure 7 indicates that the power function model described the time-dependent. The kinetic of Cr ion adsorption can be satisfactory described by the power function model. Power model describes rates of adsorption. However, the regression coefficient R2 was very high for a Cr ion which indicates that power function is the best model to correlate kinetic data [23].Figure 7
Determination of constants using the fractional power model.(4) Intraparticle Diffusion. It is represented by the following eq. (7).
(7)qt=klntt1/2,wherek is the rate constant (mg/g.min).Adsorption mechanism is difficult but in the initial stage, intraparticle diffusion is important. In this case, linear portions observed at different temperatures and concentrations were shown in Figure8. When intraparticle diffusion occurs, the slopes of these linear portions can be distinct by adsorption rate and rate parameter in this area. At first, minimum time period, it was assumed that ions transferred through nanoadsorbent to external surface. Behind surface saturation, until equilibrium was reached, ions are entered through intraparticle diffusion to nanoadsorbent, pore, and interior surface diffusion, and it is by second straight. This model well suits for Cr removal [24].Figure 8
Determination of constants using intraparticle diffusion.(5) Elovich Equation. It is mentioned by eq. (8) that
(8)qt=1β∗lnαβ+1β∗lnt,whereα is the primary sorption rate (mg/g.min), and ß is the surface area coverage (g/mg).Elovich equation explains about chemical adsorption on heterogeneous adsorbents, but for adsorbate–adsorbent interaction, this equation does not recommend any specific mechanism. The experiment was conducted for various temperatures, and different concentrations were shown that this kinetic data not fit Cr removal process, because of the low regression coefficient value [25]. Table 2 represents the consolidated values of various parameters of the different models at various temperatures.Table 2
Cr removal for different concentrations and temperatures.
ConcentrationTemperatureModelsParameters50 ppm100 ppm150 ppm30°C40°C50°CPseudo first orderR20.850.960.970.970.950.95K0.020.0260.020.030.030.04Qe1.611.851.891.912.072.090.990.980.99Pseudo second orderR20.860.99K0.020.0160.010.030.020.02Qe3.510.430.370.440.460.42Fractional powerR20.990.950.990.950.930.95K0.860.460.860.460.490.45N0.941.852.111.851.711.74Intraparticle diffusionR20.720.850.940.850.810.83K0.460.330.460.330.340.32ElovichR20.550.970.980.970.960.96


## 3.3.1. Kinetic Studies

(1) Pseudo 1st Order. The kinetics of adsorption may be explained by a pseudo 1st order equation as mentioned below (eq. (4)).
(4)logqe−qt=logqe–k1t/2.303,whereqe and qt are adsorbed metal ions (mg/g) at equilibrium and time, and k1 is the rate constant (min-1).The plot of (log (qe−qt) vs. t) and the suitability of equation imply 1st order process. The experiments were conducted at various temperatures with different concentrations. Using eq. (4), the theoretical qe values of Cr removal have been calculated, and the results revealed that this adsorption system is pseudo first-order.(2) Pseudo 2nd Order. The linearized pseudo 2nd order is mentioned in eq. (5).
(5)tqt=1k2qe2+1qt∗t,wherek2 is the rate constant (g/mg.min), and k2 and qe can be found that a good agreement with experimental data and confirm with 2nd order kinetics for different temperatures and initial concentrations. For second order kinetics, obtained regression coefficients are in the range of acceptable value, and it is found that this model suits for adsorption of Cr ions [21, 22].(3) Fractional Power Model. The linear fractional power function equation is given by eq. (6).
(6)lnqt=lnk+μlnt.The values ofμ and k can be found using the above linear relationship. For different temperatures and concentrations, the model was checked to fit. Figure 7 indicates that the power function model described the time-dependent. The kinetic of Cr ion adsorption can be satisfactory described by the power function model. Power model describes rates of adsorption. However, the regression coefficient R2 was very high for a Cr ion which indicates that power function is the best model to correlate kinetic data [23].Figure 7
Determination of constants using the fractional power model.(4) Intraparticle Diffusion. It is represented by the following eq. (7).
(7)qt=klntt1/2,wherek is the rate constant (mg/g.min).Adsorption mechanism is difficult but in the initial stage, intraparticle diffusion is important. In this case, linear portions observed at different temperatures and concentrations were shown in Figure8. When intraparticle diffusion occurs, the slopes of these linear portions can be distinct by adsorption rate and rate parameter in this area. At first, minimum time period, it was assumed that ions transferred through nanoadsorbent to external surface. Behind surface saturation, until equilibrium was reached, ions are entered through intraparticle diffusion to nanoadsorbent, pore, and interior surface diffusion, and it is by second straight. This model well suits for Cr removal [24].Figure 8
Determination of constants using intraparticle diffusion.(5) Elovich Equation. It is mentioned by eq. (8) that
(8)qt=1β∗lnαβ+1β∗lnt,whereα is the primary sorption rate (mg/g.min), and ß is the surface area coverage (g/mg).Elovich equation explains about chemical adsorption on heterogeneous adsorbents, but for adsorbate–adsorbent interaction, this equation does not recommend any specific mechanism. The experiment was conducted for various temperatures, and different concentrations were shown that this kinetic data not fit Cr removal process, because of the low regression coefficient value [25]. Table 2 represents the consolidated values of various parameters of the different models at various temperatures.Table 2
Cr removal for different concentrations and temperatures.
ConcentrationTemperatureModelsParameters50 ppm100 ppm150 ppm30°C40°C50°CPseudo first orderR20.850.960.970.970.950.95K0.020.0260.020.030.030.04Qe1.611.851.891.912.072.090.990.980.99Pseudo second orderR20.860.99K0.020.0160.010.030.020.02Qe3.510.430.370.440.460.42Fractional powerR20.990.950.990.950.930.95K0.860.460.860.460.490.45N0.941.852.111.851.711.74Intraparticle diffusionR20.720.850.940.850.810.83K0.460.330.460.330.340.32ElovichR20.550.970.980.970.960.96


## 3.4. Adsorption Isotherm

Equilibrium isotherm was represented by a sorption isotherm and characterized by some of the constants. The adsorbent sorption was established when the sorbate concentration in the bulk solution was in dynamic balance at sorbent interface. The statistical significance of the correlation coefficient (R2) was the criteria by which the fitting of the data to the various isotherms was tested and tabulated in Table 3. In order to measure the nanoadsorbent affinity for the metal examined, i.e., Cr, the following isotherm models were used [25].Table 3
Constants of isotherm and kinetics studies for TiO2.
ConcentrationTemperatureModelsParameters50 ppm100 ppm150 ppm30°C40°C50°CFreundlich isothermR20.95630.05660.966310.91620.9993Kf0.37523.785850.720.02170.03130.0214N5.49054.7058292.630.00540.12230.028Langmuir isothermR20.999910.99990.87820.86870.8733Temkin isothermR20.96730.92910.94260.99090.98950.9894a52.5772.922140.9296.50592.57598.766b12.80712.04424.1131.25830.40531.867


### 3.4.1. Isotherm Studies on Cr Using Tio2

(1) Langmuir Adsorption Isotherm. At a constant temperature, adsorbent surface was represented by Langmuir adsorption isotherm. It is represented by (9).
(9)Ceqe=1Kdqm+1qmCe,whereqe is the Cr adsorbed level (mg/g), Ce is the aqueous phase metal concentration (mg/l), Kd is the sorption equilibrium constant, and qm is the monolayer capacity(mg/g).For different temperatures and different concentrations, the equation is checked to fit the Cr removal using titanium dioxide. The value ofqm and K attained from linear graph [25].(2) Freundlich Isotherm. The important assumption of Freundlich isotherm is adsorption on heterogeneous surface by multilayer adsorption. The linear form of the equation is
(10)logqe=logkf+1nlogCe.The nomenclature is as follows:qe is the adsorption capacity, Ce is the final concentration, and n is the empirical constant.From the experimental data, various temperatures and concentrations, the plots lnqe vs. ln Ce and Freundlich coefficients n and Kf are obtained. From the data, it was observed that this adsorption process has not followed Freundlich isotherm [24].(3) Temkin Isotherm. It assumed that due to interactions between adsorbent and adsorbate, adsorption heat of molecules in the adsorbed layer diminishes.It is given by eq. (11).
(11)X=a+blnC,whereC is the concentration of adsorbate (mg/l), X is the metal adsorbed (mg/g), and a and b are constants.For different temperatures and concentrations, the given equation was checked for the fitness [26].(4) Isotherm Study Table.


## 3.4.1. Isotherm Studies on Cr Using Tio2

(1) Langmuir Adsorption Isotherm. At a constant temperature, adsorbent surface was represented by Langmuir adsorption isotherm. It is represented by (9).
(9)Ceqe=1Kdqm+1qmCe,whereqe is the Cr adsorbed level (mg/g), Ce is the aqueous phase metal concentration (mg/l), Kd is the sorption equilibrium constant, and qm is the monolayer capacity(mg/g).For different temperatures and different concentrations, the equation is checked to fit the Cr removal using titanium dioxide. The value ofqm and K attained from linear graph [25].(2) Freundlich Isotherm. The important assumption of Freundlich isotherm is adsorption on heterogeneous surface by multilayer adsorption. The linear form of the equation is
(10)logqe=logkf+1nlogCe.The nomenclature is as follows:qe is the adsorption capacity, Ce is the final concentration, and n is the empirical constant.From the experimental data, various temperatures and concentrations, the plots lnqe vs. ln Ce and Freundlich coefficients n and Kf are obtained. From the data, it was observed that this adsorption process has not followed Freundlich isotherm [24].(3) Temkin Isotherm. It assumed that due to interactions between adsorbent and adsorbate, adsorption heat of molecules in the adsorbed layer diminishes.It is given by eq. (11).
(11)X=a+blnC,whereC is the concentration of adsorbate (mg/l), X is the metal adsorbed (mg/g), and a and b are constants.For different temperatures and concentrations, the given equation was checked for the fitness [26].(4) Isotherm Study Table.


## 4. Conclusion

The experimental results indicated that heavy metals can be removed by nanomaterials as adsorbents with low concentration and adsorption capability. Mechanisms of adsorption process enlightened by various adsorption isotherms and adsorption kinetic models. The influence of various important variables on heavy metal removal was examined. The development of nanotechnology reduces the exploitation of resourceful adsorption materials. In future, a nanomaterial application in heavy metal removal in wastewater treatment is practically vivid.



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