Experimental investigation of titanium dioxide as an adsorbent for removal of Congo red (CR) from aqueous solution, equilibrium and kinetics modeling

The adsorption of Congo red onto titanium dioxide (TiO 2 ) material has been investigated at batch conditions. The effects of contact time (0 – 60 min), initial pH (3 – 11), agitation speed (100 – 500 rpm), temperature (298 – 343 K), adsorbent dosage (0.5 – 2 g/L), and Congo red concentration (5 – 15 mg/L) on the Congo red adsorption by TiO 2 have been studied. The kinetic parameters, rate constants, and equilibrium adsorption capacities were calculated and discussed for each kinetic model. The adsorption of Congo red onto TiO 2 is well described by the pseudo-second order equation. The adsorption isotherm follows the Langmuir model, providing a better ﬁ t of the equilibrium data. The batch adsorption experiments were carried out to optimize the physical parameters on the Congo red removal ef ﬁ ciency. It has been found that 152 mg/g at 25 (cid:1) C is removed. The thermodynamic parameters indicate the spontaneous and endothermic nature of the adsorption process with activation energy (Ea) of (cid:3) 64.193 kJ/mol. The positive value of the entropy ( Δ S (cid:1) ) clearly shows that the randomness is decreased at the solid – solution interface during the Congo red adsorption onto TiO 2 , indicating that some structural exchange may occur among the active sites of the adsorbent and the ions. thermodynamic study showed the spontaneous and endothermic nature of the adsorption.


INTRODUCTION
Dye production plants and many other industries which utilize dyes are increasing globally by the day with advancements in technology (Hameed & Daud ). The effluents from textile, leather, food processing, dyeing, cosmetics, paper, and dye manufacturing industries are important sources of dye pollution (Harrache et al. a, b). Many dyes and their breakdown products may be toxic for living organisms, in particular, Congo red (Abbas & Trari a, b). Therefore, decolorizations of dyes are important aspects of wastewater treatment before discharge.
It is difficult to remove the dyes from the effluent, because many dyes are carcinogenic and usually yield toxic organic compounds when they biodegrade. In addition, exposure of aquatic species to dye is known to be disastrous as it reduces the dissolved oxygen; hence, there is the need to adopt a holistic approach in removing dyes from industrial effluent before it is discharged into the environment. There are more than 100,000 types of dye commercially available, with over 7 × 10 5 tons of dyestuff produced annually (Harrache et al. a, b). Many treatment processes have been employed for the removal of dyes from industrial effluents. These treatment techniques include ultra filtration obtained from rubber seed coat, palm seed coat, and apricot stone (Abbas ) were investigated for the removal of a wide variety of impurities from water and wastewater. In general, these carbons will be as efficient in the adsorption of both organics and inorganics as the commercial activated carbons. Commercial activated carbons are sophisticated in the sense that they are designed for a variety of applications.
If low cost non-conventional sources are used to prepare activated carbons for a specific purpose, then they will be economical for wastewater treatment. The objective of this study was to investigate the feasibility of using carbonized coir pith for the removal of Congo red, a toxic dye, from wastewater by adsorption method. This study investigated the potential use of titanium dioxide (TiO 2 ) as an alternative adsorbent for removal of Congo red from wastewater. The effect of factors such as adsorbent dosage, dye concentration, pH, and temperature were experimentally studied to evaluate the adsorption capacity, kinetics, and equilibrium.

Adsorbate
The anionic dye used as adsorbate was Congo red bought from Nizochem Laboratory. Congo red has the molecular formula and weight of C 32 H 22 N 6 Na 2 O 6 S 2 and 696.66 g/mol, respectively (Table 1). H 2 SO 4 and NaOH were used to adjust the pH of solution. 100 mg/L of dye solution was prepared by adding 0.1 g of Congo red in 1,000 mL of distilled water, and solutions required for the experimental study were prepared by diluting the CR stock solution to various initial adsorbate concentrations.

Batch mode adsorption studies
The effects of experimental parameters such as the initial Congo red concentration (5-15 mg/L), pH (2-14), adsorbent dosage (0.5-2 g/L), agitation speed (100-500 rpm), and temperature ( The amount of Congo red ions adsorbed by activated carbon q t (mg/g) is calculated by using the following equation: where C o is the initial Congo red concentration and C t the Congo red concentration (mg/L) at any time, V the volume of solution (L), and m the mass of TiO 2 (g).

Characterization of TiO 2
The TiO 2 used in this study was a commercial TiO 2 purchased from Ahlstrom (France). It consists of PC500 Titania from Millennium Inorganic Chemicals with a specific surface area 400 m 2 /g and mean crystallite sizes (5-10 nm). The pH is a parameter that determines the surface properties of solids and the state of the pollutant as a function of its pKa and characterizes the water to be treated. In general, when a compound is partially ionized or carrying charged functions, it is necessary to consider the electrostatic interactions that occur with TiO 2 .
Indeed, according to the zero charge point (pHpzc), the surface charge of the solid depends on pH. Thus, for TiO 2 , the surface is positively charged below pHpzc (¼6.5), and negatively charged above pHpzc (Abbas et al. ). The reactions of the surface of the TiO 2 are as follows: TiOH þ H þ →TiOH þ 2 pH < 6:5 TiOH þ HO À →TiO À þ H 2 O pH> 6:5 The TiO 2 surface is positively charged in acidic solution related to the fixation of protons and negatively in basic medium. The surface charge influences the dye adsorption and, therefore, can promote or limit the adsorption.

Spectrum of Congo red in aqueous solution at different pH
The spectrum of the Congo red obtained at pH 7.70 shows the existence of three absorption bands, of unequal intensity and located successively at: 340, 494, and 596 nm ( Figure 1).
The molar absorption coefficients of these bands are of the order of 15.200 and 21.200, and 29.75 L·mol À1 ·cm À1 , respectively. We also note that the pH influences the behavior of the Congo red, mainly, in an acidic environment where there was a change in the color of the solution, changing from red to purplish blue. Under these conditions, we also noted a widening and a displacement of the most intense band of the spectrum from 500 to 570 nm and a significant reduction in the absorption coefficient which goes to a value of 10,400 L·mol À1 .cm À1 . To determine the pKa of the acid-base couple of the Congo red, the optical density (OD) was plotted as a function of the pH in the same way as above. To determine the pKa of the acid-base couple of Congo red, the OD was plotted as a function of pH. According to the results reported in Figure 2, the pKa value is 3.6.

Influence of the initial concentration of Congo red on its retention on TiO 2
The study of the adsorption of Congo red on TiO 2 obviously involves determining the contact time, which corresponds to the adsorption equilibrium or a state of saturation of the support by the substrate. This consists of bringing into contact 10 mg/L of Congo red with 0.1 g/100 mL of TiO 2 . The analysis by UV/visible spectrophotometry will allow the residual concentrations of each substrate to be determined  In this case, these results clearly indicate that if the concentration of Congo red in the solution is high, there will be more molecules which will diffuse towards the surface of the sites of the particles of the support, resulting in a significant increase in retention. Similar results were also observed by Tsai et al. (). We also note that the time required to reach the maximum saturation level is longer for Congo red. This phenomenon is even better perceived for the highest initial concentrations.
Influence of the adsorbent dosage on the retention of Congo red increases. This will therefore lead to better retention.

Influence of pH on the retention of Congo red on TiO 2
The pH is an important factor for any study in adsorption. It can condition both the surface charge of the adsorbent and the structure of the adsorbate. This parameter also characterizes the waters and its value will depend on the origin of the effluent. In our study, we followed the effect of pH on the absorption of the dye for an initial concentration of Influence of the agitation speed on the retention of Congo red on TiO 2 The results of the variation in the speed of agitation on the retention of Congo red are shown in Figure 6. From this representation, it can be seen that the retention capacity of the dye Congo red increases slightly as a function of the speed of agitation. These results can be explained by the fact that the

Influence of temperature on the retention of Congo red on TiO 2
Experience has shown that temperature has two major effects on the adsorption process, as shown in Figure 7.
The first, linked to an increase in temperature, promotes the diffusion of molecules through the outer boundary layer and the internal pores of the particles of the adsorbent (decrease in the viscosity) while the second, always linked to the increase in temperature, can affect the adsorption capacity. This, therefore, leads to the quantities of Congo red indicating that the increase in temperature promotes the retention of the dye (Al-qodah ).

Adsorption kinetic study
The kinetic study is important since it describes the uptake rate of adsorbate, and controls the residual time of the whole process. Several models have been proposed to study the mechanisms controlling the adsorption. In this study, the experimental data of Congo red adsorption are examined using a pseudo-first and pseudo-second order kinetic model.
The pseudo-first order equation (Lagergren ) is given as: The pseudo-second order model (Ho & McKay ) is expressed as: where q t (mg/g) is the amount of Congo red adsorbed on TiO 2 at the time t (min). K 1 (min À1 ) and K 2 (g/mg·min) are the pseudo-first order and pseudo-second order kinetics constants, respectively. The slope and intercept of the plots ln (q eq t ) versus t and t/q e versus t were used to determine the first-order rate constants K 1 and q e and second-order rate constants K 2 and q e , respectively.
The rate constants predict the uptakes and the corresponding correlation coefficients for TiO 2 are summarized in Table 2 where the surface of the adsorbent is heterogeneous (Juang & Chen ); the linear form is given by:  where α (mg/g·min) is the initial adsorption rate, and β (mg/g) the relationship between the degree of surface coverage and the activation energy involved in the chemisorption.

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M. Abbas | Removal of Congo red onto titanium dioxide Journal of Water Reuse and Desalination | in press | 2020 Corrected Proof

Intra-particle diffusion equation
The possibility of intra-particle diffusion during the transportation of adsorbate from solution to the particles' surface was also investigated by using the intra-particle diffusion model given by the following formula (Weber & Morris ): where K in is the intra-particle diffusion rate constant (mg/g min 1/2 ), q t the amount of Congo red adsorbed at time t and C (mg/g) the intercept. A plot of q t versus t 1/2 enables determination of both K in and C. Figure 11 present a multi-linearity correlation, which indicates that two steps occur during the Congo red adsorption. The mechanism of adsorption is complex but the intra-particle diffusion is important in the early stages. The first linear portions could be due to intra-particle diffusion effects. The slopes of the linear parts are defined as rate parameters, characteristic of the adsorption rate in the region where the intra-particle diffusion occurs. Initially, and within a short-time period, it is postulated that Congo red is transported to the adsorbent external surface through the film diffusion with a high rate. After saturation of the surface, the Congo red ions enter inside adsorbent by intraparticle diffusion through the pores and internal surface diffusion until equilibrium is reached, which is represented by the second straight lines. The constants of the different models deduced after modeling are grouped in Table 2.

Adsorption equilibrium isotherms
To assess the performance of adsorbent, different equations and isotherms exist, out of which, the Langmuir (),

Freundlich (), Temkin (Temkin & Pyzhev ), and
Elovich (Ghaedj et al. ) isotherms were used and are presented in Figure 12. In addition, the isotherm models were applied at optimal conditions of the parameters. The Langmuir model is the best known and most widely applied, and it is represented by the non-linear and linear forms: Figure 11 | Application of intra-particle diffusion for the adsorption of Congo red on TiO 2 .
where C e is the equilibrium concentration (mg/L), q max the monolayer adsorption capacity (mg/g), and K L the constant related to the free adsorption energy (L/mg). The applicability to the adsorption is compared by evaluating the statistic RSE values at 25 C. The smaller RSE values obtained for the models indicate a better fitting. The essential features of the Langmuir are where C o is the initial concentration of the adsorbate in solution. R L indicates the type of isotherm: irreversible (R L ¼ 0), favorable (0 < R L < 1), linear (R L ¼ 1), or unfavorable (R L > 1). In this study, the R L values are smaller than 1, thus confirming that the adsorption is favored in both cases as well as the applicability of the Langmuir isotherm.
The Freundlich isotherm is valid for non-ideal adsorption on heterogeneous surfaces as well as multilayer sorption.
The constant K F characterizes the adsorption capacity of the adsorbent (L/g) and n an empirical constant related to the magnitude of the adsorption driving force. Therefore, a plot lnq e versus lnC e enables the determination of both the constant K F and n. The Temkin isotherm describes the behavior of adsorption systems on heterogeneous surfaces, and is applied in the following form: The adsorption data are analyzed according to Equation where K E (L/mg) is the Elovich constant at equilibrium, q max (mg/g) the maximum adsorption capacity, q e (mg/g) the adsorption capacity at equilibrium, and C e (g/L) the concentration of the adsorbate at equilibrium. Both the equilibrium constant and maximum capacity are calculated from the plot of ln (q e /C e ) versus q e . The constants of the different models deduced after modeling are grouped in Table 3.

Error analysis
An error is required to evaluate the fit of an isotherm equation to the experimental equilibrium data obtained. In this study, the linear and non-linear (Langmuir, Freundlich, Temkin, and Elovich isotherm models) coefficients of determination (R 2 ) and RSE (X 2 ) (residual sum of errors) test were performed for all the isotherm models. The RSE test statistic is basically the sum of the squares of the differences between the experimental data and the data obtained by calculating from models, with each squared difference divided by the corresponding data calculated using the models. This can be represented mathematically as: (q e,exp À q e,cal ) 2 q e,cal where q e , cal is the equilibrium capacity obtained by calculating from the model (mg/g) and q e,exp is the experimental data of the equilibrium capacity (mg/g).

Thermodynamic properties' modeling studies
The thermodynamic properties were investigated to determine whether the adsorption process occurred spontaneously. The thermodynamic parameters, namely, standard enthalpy (ΔH , kJ/mol), standard entropy (ΔS , J/mol K), and standard free energy (ΔG , kJ/mol), were calculated using the following equations (Li et al.

)
: ΔG ¼ ÀRT lnK d (13) where R is the gas constant (8.314 J/mol K), T (K) is temperature where K d is the distribution coefficient, q e (mg/g) is the quantity of Congo red adsorbed at equilibrium, and  Table 4.
The adsorption capacity of Congo red increases with raising temperature in the range 293-328 K.
A pre-exponential factor, E a the activation energy, and K 2 (g/mg·min) is the pseudo-second order kinetics constant at different temperatures. The energy (E a ¼ À64.193 kj/Mol) can be obtained by plotting lnk 2 against the reciprocal of the absolute temperature T (Figure 13).

Performance of the prepared ASAC
In order to have an idea about the efficiency of the TiO 2 , a comparison of basic dye adsorption of this work and other relevant studies is reported in Table 5. The adsorption capacity of the adsorbent q max is the parameter used for the comparison. One can conclude that the value of q max is in good agreement with those of most previous works, suggesting that Congo red could be easily adsorbed on TiO 2 used in this work.

CONCLUSION
This study has shown that TiO 2 can be employed as an effec-  that some structural exchange may occur among the active sites of the adsorbent and the ions.
The adsorption of Congo red ions by TiO 2 follows a pseudo-second order kinetic model, which relies on the assumption that chemisorptions may be the rate-limiting step. In chemisorption, the Congo red ions are attached to the adsorbent surface by forming a chemical bond and tend to find sites that maximize their coordination number with the surface. The value of q max is in good agreement with those of most previous works, suggesting that Congo red could be easily adsorbed on TiO 2 used in this work.
This study in a tiny batch gave rise to encouraging results, and we wish to achieve adsorption tests in column mode under the conditions applicable to the treatment of industrial effluents. The present investigation showed that TiO 2 is a potentially useful adsorbent for metals, acid, and basic dyes.