The synthesized titanium dioxide nanoparticles (TiO2-NPs) were used as adsorbent to remove reactive black 5 (RB 5) in aqueous solution. Various factors affecting adsorption of RB 5 aqueous solutions such as pH, initial concentration, contact time, dose of nanoparticles, and temperature were analyzed at fixed solid/solution ratio. Langmuir and Freundlich isotherms were used as model adsorption equilibrium data. Langmuir isotherm was found to be the most adequate model. The pseudo-first-order, pseudo-second-order, and intraparticle diffusion models were used to describe the adsorption kinetics. The experimental data was fitted to pseudo-second-order kinetics. The thermodynamic parameters such as Gibbs-free energy, enthalpy, and entropy changes were determined. These parameters indicated the endothermic and spontaneity nature of the adsorption. The results demonstrated the fact that the TiO2-NPs are promising adsorbent for the removal of RB 5 from aqueous solutions.

1. Introduction

In order to remove dyes from wastewaters, there are various methods of development, including chemical oxidation [1], biodegradation [2], electrocoagulation [3], photodegradation [4], solvent extraction [5], ultrafiltration [6], and adsorption [7]. The adsorption technique is the most favorable method for the removal of dyes because of its simple design, easy operation, and relatively simple regeneration [8]. Wastewater industries may contain a variety of organic compounds and toxic substances that exhibit toxic effects for microbial populations and can be toxic and carcinogenic for animals [9].

Azo reactive dyes, which have two azo groups, represent about half of all reactive dyes such as RB 5 (see Figure 1). These types of dyes are known to be toxic, carcinogenic, and mutagenic. Their removal from the environment can result in nonaesthetic pollution. Moreover, these dyes are not easily degraded by conventional aerobic wastewater treatment due to their recalcitrance [10]. Many important sources of environment contamination are synthesis of dye, leather, cosmetics, papers, food processing, pulp mill, pharmaceuticals, and plastics industries [11].

Equilibrium isotherms and kinetics with the presence of methylene blue adsorption onto activated carbon which have been the focus of various studies were used to be prepared from various agricultural wastes such as bamboo [12], coconut husk [13], vetiver roots [14], peach stones [15], rattan sawdust [16], durian shell [17], oil palm fiber [18], coffee husk [19], ground nut shell [20], waste apricot, olive stones [21], hazelnut shell [22], and corncob [23]. Several methods which treat dye wastewater with various biological, chemical, and physical technologies [24, 25] prove to be very useful for environmental purification. Previous studies conducted on the adsorption rate of benzoic acid on cetyl pyridine bromide-modified bentonites fitted a pseudo-second-order kinetics model well ( = 0.999). The results were analyzed according to Henry, Langmuir, and Freundlich isotherm model equations. TiO2 has been extensively studied as one of the superior candidates of semiconductors due to its cheapness, photostability, chemical inertness, nontoxicity, and strong photocatalytic activity [26]. Many methods, such as adsorption, chemical precipitation, ion exchange, membrane processes, biological degradation, chemical oxidation, and solvent extraction have been employed to remove organic pollutants from aqueous solutions [27]. It is well known that an increase in a plentiful of the surface quality of the TiO2-NPs is due to the increase of the overall crystallinity and crystal size of nanoparticles which can improve the anchoring geometry of the dye on their surfaces and faster adsorption [28].

Nanomaterials have a higher distortion of surface structure than bulk materials due to their inherent lattice strain. As a result, the surface modifications of TiO2-NPs are more beneficial for adsorption than that of bulk TiO2 [30]. Many methods have been employed to fabricate TiO2-NPs. One of these methods is the metal organic chemical vapor deposition (MOCVD), which is considered a promising technique for producing nanoparticles because of its relative low cost and simplicity of the process [31]. The as-prepared of anatase TiO2 hollow spheres have a higher surface area; therefore, they showed much higher adsorption and photoreactivity than TiO2 nanoparticles. In addition, some of hallow spheres which use a sulfonated polystyrene template have very good adsorption and photocatalytic activity [32]. TiO2 is a very important functional metal oxide material with direct band gap of 3.2 eV at room temperature [33], large specific surface area, and aspect ratio. One-dimensional TiO2 nanostructures have wide applications in optical, electronic, and photocatalytic fields [34]. The synthesized nanoparticles are attractive for further improvement of the reactivity of TiO2 as a catalyst. Compared with the other TiO2 powders, these TiO2 nanoparticles have several advantages, such as amorphous form, fine particle size with more uniform distribution, and high dispersion in polar solvent, stronger interfacial adsorption, and being environmentally friendly. In the adsorption results, not only the surface area factor should be considered, but also the TiO2 high ion-exchange ability and the dye cationic character [35]. In our work, 80% of the RB 5 was adsorbed on the surface TiO2 after one hour of dark stirring conditions. This result is clear evidence of the high RB 5 adsorption capability of adsorbent. Identical adsorbability values were reported in the literature [36]. TiO2 can be seen as an efficient and cost-effective adsorbent for the removal of pollutants from real wastewaters. The main advantages of the synthesized catalyst for the removal of RB 5 from water and wastewater include a high adsorption rate, capacity, and efficacy, as well as a suitable equilibration time. Furthermore, TiO2-NPs are available as a no-cost waste and can be used without modifications. Thus, TiO2-NPs adsorption is environmentally friendly and achieves treatment goals in a simple and low-cost manner. The adsorption emphasizes the engineering applications that govern the actual water purification process, including the fabrication of TiO2-based adsorbents, process optimization, and economic consideration [37].

Recently, advanced oxidation processes (AOPs) have considerable interest due to their utility in complete elimination of dyes. Most reactive species are generating through AOPs such as hydroxyl radicals that oxidize a broad range of organic pollutants quickly and nonselectively [38, 39]. It is worth mentioning here that the previous studies have not explored this synthesized TiO2-NPs; thus, this study is considered the first attempt to assess the suitability of this catalyst for engineering applications. In particular, this study aims to determine whether this catalyst delivers better performance in adsorption. It is expected to play a prominent role as an effective adsorbent in competing of the purification processes and economical ways.

2. Experimental Procedure

2.1. Materials

The supplier of reactive black 5 was textile factory (Hilla, Iraq), with formula of C26H21N5Na4O19S6, molecular weight of 991.82 g mol−1, and maximum light absorbed wavelength of 597 nm. The synthesized TiO2-NPs were prepared using sol gel method (from 99.98% absolute EtOH (GCC) and 99.99% TiCl4 (Fluka)).

2.2. Adsorption Experiments

The equilibrium isotherm of RB 5 adsorption on TiO2-NPs was determined by performing adsorption tests in 250 mL conical flasks. 100 mL of RB 5 dye solutions with different initial concentrations (30, 40, 50, 60, 70, and 80 mg L−1) was placed in each flask. The normal pH of the solutions was 5.5. The prepared TiO2-NPs of 0.175 g were added to each flask and kept in a shaker of 190 rpm at 30°C for 1 hour to reach equilibrium. The suspensions were sampled at regular intervals. 4 cm3 of the reaction mixture was collected and centrifuged for 15 min. The supernatant was carefully removed by a syringe with a long pliable needle. This is necessary to remove the fine particles of the catalyst. The supernatant concentrations of RB 5 were analyzed by UV-visible spectrophotometer (PG instruments Ltd., Japan, UV-160A) at maximum wave lengths of 597 nm. The adsorbed amount of RB 5 at equilibrium (mg g−1) was calculated by the following expression: where and (mg L−1) are the initial and equilibrium concentrations of RB 5 solution, (L) is the volume of solution, and (g) is the weight of synthesized TiO2-NPs used.

A typical preparation procedure is exemplified later in the paper.

2.3. Thermodynamic Experiments

Batch adsorption experiments were performed using 0.175 g of the synthesized TiO2-NPs with 100 mL of RB 5 aqueous solutions in 250 cm3 conical flasks. Concentration, pH, and temperature check were determined. The sample was shaken at 120 rpm in a shaking water bath (Memmert GmbH+Co.,KG, Germany). After 60 min desired contact time, suspension was separated by centrifugation. The supernatant solutions for color removal were analyzed by using an UV-vis spectrophotometer (PG instruments Ltd., Japan, UV-160A). The adsorbed amount of RB 5 at equilibrium (mg g−1) was calculated.

3. Results and Discussion

3.1. Adsorption

Adsorption isotherms are important for the description of how adsorbate (RB 5) interacts with an adsorbent (TiO2-NPs) and are also critical in optimizing the use of adsorbent. Thus, the correlation of experimental equilibrium data using either a theoretical or empirical equations is essential for adsorption data interpretation and prediction. Several mathematical models can be used to describe experimental data of adsorption isotherms. Five famous isotherm equations, namely, the Langmuir, Freundlich, pseudo-first-order, pseudo-second-order, and intraparticle diffusion model, were applied to fit the experimental equilibrium isotherm data of RB 5 adsorption on the prepared TiO2-NPs.

3.2. Fundamental Parameters in Adsorption
3.2.1. Effect of Contact Time

The effect of contact time on adsorption capacity of TiO2-NPs for RB 5 at different initial concentrations of RB 5 is shown in Figure 2. The results indicate that the adsorbed amount of RB 5 increases with the increase of contact time. The adsorption approximately reached equilibrium in about 1 hour. The maximum adsorbed amount of 16.87, 22.43, 27.38, 31.33, 33.76, and 36.67 mg g−1 was obtained at 30, 40, 50, 60, 70, and 80 mg L−1 initial RB 5 concentration, respectively; one hour contact time; normal pH value 5.5; and 1.75 g L−1 adsorbent dose. These results also show that rapid increase in adsorbed amount of RB 5 was achieved during the first 10 minutes. Similar results for the removal of hazardous contaminants from wastewater were reported [40]. The fast adsorption at the initial stage may be due to the higher driving force making fast transfer of RB 5 to the surface of TiO2-NPs, the availability of the uncovered surface area, and the remaining active sites on the adsorbent [41].

3.2.2. Effect of Initial Concentration of RB 5 Dye

Six different concentrations of 30, 40, 50, 60, 70, and 80 mg L−1 for RB 5 were selected to investigate the effect of initial concentration of dye onto the synthesized TiO2-NPs. The amounts of dye molecules adsorbed at equilibrium and pH 5.5 are graphed in Figure 3. With the initial increase of the concentration of RB 5 from 30 to 80 mg L−1, the removal of dye molecules adsorbed by adsorbent decreases from 98.04 to 80.21% after one hour of adsorption time. These results correspond to the adsorption of nitrate from aqueous solution using modified rice husk [42].

3.2.3. Effect of Adsorbent Dosage

The effect of adsorbent dosage on the adsorption of RB 5 is shown in Figure 4. The percent removal increases from 40.06 to 99.16% by increasing the adsorbent dosage from 0.1 to 0.35 g after one hour of adsorption time. It is apparent from this figure that, by increasing the catalyst amount, the adsorption efficiency increases, but adsorption density and the amount adsorbed per unit mass decrease. It is easily understood that the number of available adsorption sites increases by increasing the adsorbent amount, but the drop in adsorption capacity is basically due to the sites remaining unsaturated during the adsorption process. If the active sites are available, the pollutant left in the system will continuously be adsorbed. In other words, the increase with TiO2-NPs dosage of the amount of dye adsorbed was caused by the availability of more surface area of the TiO2-NPs. Direct evidence of TiO2-NPs entanglement was not clear and unexpected. However, similar observations can be found in the literature [4346].

3.2.4. Effect of pH

The adsorption of the RB 5 onto an adsorbent generally varies with pH because pH changes the radius of hydrolyzed cation and the charge of the adsorbent surface. Therefore, in this study, the adsorption of RB 5 dye onto the prepared TiO2-NPs in our lab was studied as a function of pH. The pH values of RB 5 solutions were adjusted as 2.0, 4.0, 6.0, and 8.0. The relationship between initial pH and the amounts of dye adsorbed on the TiO2-NPs for the initial solution concentration of 70 mg L−1 at 30°C and a contact time of 60 min is illustrated in Figure 5. pH change affects the adsorption quantity of organic pollutants and the ways of adsorption on the surface of catalyst. As a result, the adsorption efficiency will greatly be influenced by pH changes which can be explained with the protonation or unprotonation of the functional groups on the surface of TiO2 as well as the dye. When initial pH values of solutions are increased from 2.0 to 8.0, the amounts of adsorbed dye per unit mass of adsorbent are changed. For example, the amounts of dye molecules adsorbed dye per adsorbent decrease from 20.32 to 15.08 mg g−1 when the pH value increases from 2.0 to 4.0. As seen in this figure, pH 6 is a value for the maximum adsorption of RB 5. A rising in the pH closes to 6.0 gives the maximum adsorption capacity. In this point, pH was called the zero point charge (Z.P.C). Hussein [47] reported that the zero point charge for commercial TiO2 (Degussa P25) is equal to 6.25. Also, when the pH values of solutions were continuously increased from 6.0 to 8.0, the amounts of dye molecules adsorbed per unit adsorbent decrease from 23.95 to 18.11 mg g−1. These results indicate that the adsorbed amount of RB 5 was strongly dependent on pH of solution because the reaction takes place on the surface of synthesized catalyst. The concept could be explained as follows: the increase of the pH solution makes the surface of catalyst negatively charged by adsorbed hydroxyl ions and the decrease of the pH solution makes it positively charged by adsorbed hydrogen ions. Both the acidic and basic media leave an inverse impact on the adsorption efficiency because of the change of electrostatic forces between surface catalyst and dye molecules.

3.2.5. Effect of Temperature

The uptake of dye solution was increased with the rise in temperature from 5 to 30°C as shown in Figure 6. Equilibrium time was found to be reached in 60 min, at pH 5.5, and 50 mg L−1 of dye solution. The adsorption kinetics depends on the surface area of the adsorbent. The adsorption which increased with temperature indicates that the mobility of dye molecules increased with temperature, as did the number of dye molecules that interact with the active sites at TiO2-NPs. Moreover, the viscosity of dye solution reduces with the rise in temperature, increasing the rate of diffusion of dye molecules. These results also showed that the adsorption process was endothermic and spontaneous in nature. These results are in agreement with application of acidic treated pumice for the removal of azo dye from aqueous solution [48].

3.3. Analysis of Adsorption Kinetics

Numerous adsorption processes have been studied during the past 25 years. The diffusion control, mass transfer, chemical reactions, and particle diffusion are different kinds of mechanisms related to adsorption processes. The pseudo-first-order kinetic model, pseudo-second-order kinetic model, and intraparticle diffusion model were used for testing dynamic experimental data at 0.175 g of adsorbent and the different initial concentrations of RB 5 were 30, 40, 50, 60, 70, and 80 mg L−1 in the normal pH 5.5. The pseudo-first-order kinetic model of Lagergren is given as [49] where and (mg g−1) are the amount of RB 5 adsorbed at equilibrium and at time (min), respectively, and (min−1) is the adsorption rate constant.

The pseudo-second-order kinetic model can be expressed as [50] where (g mg−1 min−1) is the rate constant of second-order equation.

The initial adsorbent rate (mg g−1 min−1) can be determined from and values using the following equation: The intraparticle diffusion model can be described as [51] where is the amount of dye adsorbed at time (min) and is the intraparticle diffusion rate constant (mg g−1 min−1/2).

Moreover, the validity of these models was determined by calculating the standard deviation (R.S.D%) using where subscripts and refer to the experimental and calculated data and is the number of data points.

In adsorption process, there are two criteria, namely, the regression coefficients and predicted values that assess the validity of the order of adsorption [52]. The validities of these three kinetic models for all concentrations are checked and depicted in Figures 7, 8, and 9. The values of the parameters, correlation coefficient, and standard deviation obtained from these three kinetic models are all listed in Table 1. Among these, Figure 8 shows a good agreement with pseudo-second-order kinetic model. Table 1 presents the coefficients and R.S.D% of the pseudo-first- and second-order adsorption kinetic models and the intraparticle diffusion model. The values of for pseudo-second-order kinetic model are extremely high (all greater than 0.9989), but R.S.D values of the pseudo-second-order are smaller than those of the pseudo-first-order and the intraparticle diffusion model. Hence, this study suggests that the pseudo-second-order model better represents the adsorption kinetics. Consequently, the description of adsorption process could be the best by the pseudo-second-order kinetic. This also implies that the rate-limiting step may be the chemical adsorption. A similar phenomenon has been observed in the literature [53, 54]. The values of the pseudo-second-order rate constant, , were found to decrease from 1.32 × 10−2 to 1.2 × 10−3 g mg−1 min−1, for an increase in the concentration solution of dye solution from 30 to 80 g L−1.

Table 2 shows the values of adsorption rate constant at the following concentrations (30, 40, 50, and 60 mg L−1), was found to increase from 19.142, 9.736, 1.611, and 0.435 to 130.857, 43.238, 9.859, and 1.370 L g−1, for an increase in the solution temperature of 278.14 to 303.14 K, respectively.

3.4. Analysis of Adsorption Isotherm

The relationship between the amount of RB 5 dye adsorbed onto the adsorbent surface and the remaining RB 5 concentration in the aqueous phase at equilibrium can be observed by the equilibrium adsorption isotherm analysis as shown in the investigation of the effect of initial concentration of dye. This relationship which is shown in Figure 10 indicates that the adsorption capacity RB 5 dye on the surface of TiO2-NPs increases with the equilibrium concentration of dye solution, progressively reaching saturation of the adsorbent. Adsorption isotherm curve indicates that adsorption phenomenon is represented by isotherms of type L which represent a monolayer adsorption until the saturation of active sites. The higher dye adsorption value reveals that the synthesized TiO2-NPs are well interconnected and the electrons are efficiently transported through the particles. This is consistent with the work reported by Kathirvel et al. [55].

The study employed the Langmuir and Freundlich models to describe the equilibrium adsorption. The expression of the Freundlich model [56] is In logarithmic form, where is the amount of RB 5 adsorbed at equilibrium time (mg g−1) and is the equilibrium concentration of the dye solution (mg L−1). and n are Freundlich isotherm constants which indicate capacity and intensity of the adsorption, respectively.

Freundlich [ = 0.9578] is not as adequate as Langmuir model [ = 0.9724]. The values of and were calculated from the slope and intercept of the plot versus (Figure 11). The values of and obtained are shown in Table 3. From value physical adsorption is unfavorable because the value of is not in the range 1 < < 10 [57].

The Langmuir isotherm is expressed as [58] where (mg g−1) is the maximum amount of RB 5 per unit weight of TiO2-NPs to form complete monolayer coverage on the surface bound at high equilibrium RB 5 concentration and is Langmuir constant related to the affinity of binding sites (L mg−1). represents a particle limiting adsorption capacity when the surface is fully covered with dye molecules and assists in the comparison of adsorption performance. and are calculated from the slope and intercept of the straight line of the plot of 1/ versus 1/ as shown in Figure 12.

Parameters of the Langmuir and Freundlich isotherms are computed in Table 4. Langmuir isotherm fits quite well with the experimental adsorption data correlation coefficient (), whereas the low shows poor agreement of the Freundlich isotherm with the experimental data. Calculated maximum capacities () are close to maximum capacities obtained at equilibrium (Table 3).

Furthermore, the essential characteristic of the Langmuir isotherm can be expressed by a dimensionless separation factor called equilibrium parameter [59] . It is also evaluated in this study and is determined from the relation where is the Langmuir constant (mL g−1) and is the initial dye concentration (mg L−1). Parameter indicates the shape of isotherm as shown in Table 4.

value between 0 and 1 indicates a favorable adsorption. The values of between 0 and 1 indicate a favorable chemical adsorption. The results show that, for Langmuir isotherm, the value of is found to be 0.042605, suggesting that the prepared TiO2-NPs are favorable for adsorption of RB 5 under the conditions used in this study.

The fact that the Langmuir isotherm fits the experimental data very well may be due to homogenous distribution of active sites on the TiO2-NPs surface, since the Langmuir equation assumes that the surface of catalyst is homogeneous [60].

3.5. Analysis of Adsorption Thermodynamic

The thermodynamic parameters should be properly evaluated because they provide in depth information regarding the inherent energetic changes associated with adsorption. Free energy of adsorption (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) changes were calculated in this study to predict the process of adsorption. Determination of the thermodynamic parameters is independent of the Langmuir equilibrium adsorption constant, . The van’t Hoff equation is used to evaluate the variation of equilibrium adsorption constant with temperature [61]. The integrated form of this equation is given as Gibbs free energy change of adsorption () was calculated as −6.815, −8.166, −9.009, −10.610, −11.346 and −12.264 k J mol−1 at 278.14, 283.14, 288.14, 293.14, 298.14 and 303.14 K, respectively onto the synthesized TiO2-NPs at concentration was 30 mg L−1. The negative values indicated that the adsorption of RB 5 onto TiO2-NPs was thermodynamically feasible and spontaneous. The enthalpy (ΔH°) and entropy () changes were determined as 54.248 k J mol−1 and 26.509 × 103 J mol−1 K−1 from versus plot (Figure 13). The positive value of confirmed the endothermic character of the adsorption process. The positive values of also revealed the increase of randomness at the solid-liquid interface during the adsorption of RB 5 onto the TiO2-NPs. The low value of indicated that no remarkable change on entropy occurs. Similar results on RB 5 as compared to other materials [62] indicated that the adsorption of RB 5 was feasible, spontaneous, and endothermic. Thus results show that the adsorbents can be used for the treatment of aqueous solutions as an alternative low-cost adsorbent.

4. Conclusions

The synthesized TiO2-NPs are a well-known adsorbent that can be used to remove azo dyes such as RB 5. Among the kinetic models, the pseudo-second-order kinetic model was considered the best to explain the behavior of the adsorption process because the average of pseudo-second-order is the highest among other models (pseudo-first-order average = 0.9739, pseudo-second-order average = 0.98545, and intraparticle diffusion average = 0.9304). The extent of RB 5 adsorption on TiO2 increases along with an increase of initial RB 5 concentration. Freundlich and Langmuir isotherm models have been found to be suitable for the description of adsorption. The synthesized catalyst has been found to have a Langmuir monolayer adsorption capacity of 88.495 mg g−1 at normal pH 5.5 and 30°C. The Langmuir model fitted the experimental data better than Freundlich model indicating that the adsorption tends to monolayer adsorption. The dimensionless separation factor confirms that the adsorption process certainly involves chemical adsorption. The pH of the zero point charge (pHzpc) for the adsorption process was determined to be 6. The results demonstrated that the prepared TiO2-NPs are a promising adsorbent for the removal of RB 5 dye from aqueous solutions. From the results of thermodynamic parameters, the negative values indicated that the adsorption of RB 5 onto TiO2-NPs was thermodynamically feasible and spontaneous. The kinetic studies showed that the contact time was suitable for technological applications. Consequently, fundamentally the outlook is promising that the prepared TiO2-NPs have higher potential of removing the most significant amounts of azo dyes from aqueous solutions. Moreover, the adsorbate and adsorbent ratio provides an economical way to produce expensive semiconductor material support and it is convenient for the detoxification of pollutants.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.


This paper was supported by Advanced Lab of Physical Chemistry, University of Babylon, Hilla, Iraq. The authors also acknowledge Dr. Fadhil Mohsen’s technical assistance and the support they received from the Iraqi Ministry of Science and Technology.