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

Dye production plants and many other industries which utilize dyes are increasing globally by the day with advancements in technology (Hameed & Daud 2008). The effluents from textile, leather, food processing, dyeing, cosmetics, paper, and dye manufacturing industries are important sources of dye pollution (Harrache et al. 2019a, 2019b). Many dyes and their breakdown products may be toxic for living organisms, in particular, Congo red (Abbas & Trari 2020a, 2020b). 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 × 10⁵ tons of dyestuff produced annually (Harrache et al. 2019a, 2019b). Many treatment processes have been employed for the removal of dyes from industrial effluents. These treatment techniques include ultra filtration (Lakdioui et al. 2017), ion exchange membrane (Ran et al. 2017), electrochemical degradation (Morsi et al. 2011), photo catalytic degradation (Abbas & Trari 2020a, 2020b), and adsorption process (Abbas et al. 2016). Among these methods, adsorption of dye onto activated carbon has been the most viable technique (Abbas et al. 2019). The growth in industrial and technological advancement globally brought with it the introduction of pollutants of diverse nature into water bodies. Such pollutants include dyes, organic matters, and heavy metals. Their presence in industrial effluents or drinking water is a public health problem due to their absorption and possible accumulation in organisms. Water pollution regulations require textile dye industries to reduce substantially the amount of color in their effluents. Adsorption, as a waste water treatment process, exploits the ability of some solids to concentrate certain substances from solution onto their surface. The most commonly used adsorbent for treatment of textile effluents is activated carbon. However, as the adsorption capacities of the above adsorbents are not large, new absorbents are still under development. Activated carbons obtained from rubber seed coat, palm seed coat, and apricot stone (Abbas 2020) 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 (TiO₂) 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.

The anionic dye used as adsorbate was Congo red bought from Nizochem Laboratory. Congo red has the molecular formula and weight of C₃₂H₂₂N₆Na₂O₆S₂ and 696.66 g/mol, respectively (Table 1). H₂SO₄ 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.

The TiO₂ 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.

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⁻¹·cm⁻¹, respectively. We also note that the pH influences the behavior of the Congo red, mainly, in an acidic environment (pH = 2.5), 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⁻¹.cm⁻¹. 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.

The study of the adsorption of Congo red on TiO₂ 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 TiO₂. The analysis by UV/visible spectrophotometry will allow the residual concentrations of each substrate to be determined for the samples taken at different times. Thus, determination of the equilibrium time for the Congo red on the support, will lead to the calculation of the maximum adsorption capacity, which shows that the speed of adsorption is rapid at the start of the process and becomes increasingly slower over time, to finally achieve balance. The adsorption equilibrium time determined is 45 min for Congo red. From the results reported in Figure 3, we note that the increase in the initial concentration of the substrate leads to an increase in the amount adsorbed for the Congo red. The equilibrium time averages 45 min, but for practical reasons, the adsorption experiments are run up to 60 min. With raising the initial Congo red concentration (5–15 mg/L), adsorption increases from 4.5 to 12.5 mg/g. From these results, we can deduce that the adsorption of Congo red onto TiO₂ is done in three stages: fast adsorption of Congo red due to the presence of free sites on the adsorbent surface which translates the linear increase of the adsorption capacity over time; reduction of the adsorption rate, reflected by a small increase in the adsorption capacity attributed to the decrease in the quantity of Congo red in solution and the number of available unoccupied sites. Stability of the adsorption capacity is observed, probably due to the total occupation of adsorption sites: the establishment of the level therefore reflects this stage. The adsorption capacity of Congo red increases over time to reach a maximum after 45 min and thereafter tends toward a constant value indicating that no more Congo red ions are removed from the solution.

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. (2005). 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.

Figure 4 represents the variation in the adsorbed amounts of the Congo red by varying the initial amount of the adsorbent while keeping the concentration of the dye constant in solution (10 mg/L). The results obtained show that the increase in the mass of the adsorbent in the reaction medium conversely influences the retention capacity and, consequently, the quantity of adsorbed dye. In other words, a reduction in the mass of the support results in a significant improvement in the fixing efficiency. This variation is attributed to an increase in the free surface area of the grains in our adsorbent for low ratios. Indeed, if the mass of the solid in the solution is more and more important, the number of adsorption sites will be too. Consequently, the probability of encountering ‘molecule-site’ also increases. This will therefore lead to better retention.

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 Congo red of 10 mg/L and also for a dose of the adsorbent in TiO₂ of 1 g/L. The study of the influence of pH will be considered only in basic medium where no modification on the absorption spectrum of our dye is observed. The influence of this parameter is studied experimentally, by adjusting the solutions to the desired pH values with NaOH. The curves in Figure 5 show that the increase in the pH of the medium causes a decrease in the amount of adsorbed dye in an alkaline medium. This reduction can be explained both by the chemical state of the TiO₂ surface and by the state of the organic molecule which is ionizable at this pH. It is well known that for pH values higher than that of the zero charge point (pHpzc), for example, at pH = 6.5, the surface becomes negatively charged for TiO₂ (Hu et al. 2003). However, for higher pH values, Congo red with two sulfonic groups, ionizes more easily and therefore becomes a soluble anion. Under these conditions, it is far from the surface negative of TiO₂, resulting in a decrease in its retention capacity.

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 increase acts favorably on the probability of contact of the substrate with the support, by creating a zone of turbulence. Furthermore, the reduction in the thickness of the boundary layer around the adsorbent particles is attributed to the increase in the phenomenon, known as ‘mixing’ (Weeks & Rabani 1966), thus promoting the process of adsorption. These results are in agreement with those of Ho & Chiang (2001).

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 2000).

The rate constants predict the uptakes and the corresponding correlation coefficients for TiO₂ are summarized in Table 2. For the pseudo-first order kinetic (Figure 8), the experimental data deviate from linearity, as evidenced from the low values of q_e and C_o and the model is inapplicable for the present system. By contrast, the correlation coefficient and q_e,_cal determined from the pseudo-second order kinetic model agree with the experimental data (Figure 9) and its applicability suggests that the adsorption of Congo red onto TiO₂ is based on chemical reaction (chemisorption), involving an exchange of electrons between adsorbent and adsorbate.

A pre-exponential factor, E_a the activation energy, and K₂ (g/mg·min) is the pseudo-second order kinetics constant at different temperatures. The energy (E_a = −64.193 kJ/mol) can be obtained by plotting lnk₂ against the reciprocal of the absolute temperature T (Figure 13).

In order to have an idea about the efficiency of the TiO₂, 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 TiO₂ used in this work.

This study has shown that TiO₂ can be employed as an effective adsorbent for the removal of Congo red from aqueous solution. The Freundlich and Langmuir isotherm models provided a better fit of the equilibrium adsorption data giving a maximum adsorption capacity of 152 mg/g at a temperature of 25 °C. The pseudo-second order model proved the best description of the kinetic data. The negative value of ΔG° and positive value of ΔH° indicate that the adsorption of Congo red onto TiO₂ is spontaneous and endothermic over the studied range of temperatures.

The positive value of ΔS° demonstrated clearly that the randomness increased at the solid–solution interface during the Congo red adsorption onto TiO₂, indicating that some structural exchange may occur among the active sites of the adsorbent and the ions.

The adsorption of Congo red ions by TiO₂ 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 TiO₂ 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 TiO₂ is a potentially useful adsorbent for metals, acid, and basic dyes.

Financial support for the work was provided by Boumerdes University, Science Faculty, Chemical Department. The author attests that there is no conflict of interest and financial, personal, or other relationships with other people, laboratories, or organizations worldwide.

Data cannot be made publicly available; readers should contact the corresponding author for details.