Kinetic Model for Simultaneous Adsorption/Photodegradation Process of Alizarin Red S in Water Solution by Nano-TiO₂ under Visible Light

Abstract

The simultaneous adsorption and visible light photodegradation of Alizarin Red S in water solutions were studied in real time mode by using nano-TiO₂, such as Anatase and Aeroxide P-25, supported on polypropylene strips. Kinetic results of the overall process were compared with those obtained from separated steps of adsorption and photodegradation previously studied; kinetic advantages were evidenced with the simultaneous approach. From the study of different dye concentrations, a kinetic model has been proposed which describes the overall process. This model considered two consecutive processes: The adsorption of dye on TiO₂ surface and its photodegradation. The obtained results were in good agreement with experimental data and can predict the profiles of free dye, dye adsorbed on TiO₂ and photoproduct concentrations during the total process.

1. Introduction

Water pollution is one of the greatest problems that the world is facing today as it leads to numerous fatal diseases and it is responsible for the death of over 14,000 people every day [1]. It occurs when pollutants are discharged into water bodies without adequate treatment to remove harmful constituents. There are many sources of water pollution and several pathways through which pollutants can move [2]. Textile dyes are an important class of synthetic organic compounds which are found in water bodies coming from different sources and they represent an environmental danger [3]. In the textile industry, dyes are lost during the dyeing process; the discharge of dyes into the water is unpleasant, not only because of their color, but also because many released dyes are toxic, carcinogenic or mutagenic to life forms [4].

One of the most important families of dyes are anthraquinone dyes. Common madder (Rubia tinctorum L. Rubiaceae) produces anthraquinone dyes in its roots; one of them is Alizarin [5]. Another important example of anthraquinone derivative is Alizarin Red S (ARS), a sodium salt of Alizarin. ARS is a common water soluble anthraquinone dye extensively employed for cotton and silk manufacturing, this dye is also used in clinical practices and in geology [6]. People with skin allergies are more susceptible to its hazardous effects. Its acute toxicity leads to skin, eyes, lungs, mucous membranes and gastro-intestinal tract irritation. In chronic conditions, it leads to dermatitis [7]. Because of its synthetic origin and its complex structure of aromatic rings, ARS is difficult to remove by general chemical, physical and biological processes [8]. Wide ranges of technologies have been developed to remove this dye from wastewaters by applying catalytic and photochemical methods [9].

In recent years, the semiconductor photocatalytic process has shown great results as a low-cost, environmentally friendly and sustainable treatment technology in the removal of persistent organic compounds and microorganisms in water and wastewater [10]. Heterogeneous photocatalysis is one of the most promising technologies to remove organic pollutants from water and air [11]. TiO₂ is the most widely applied photocatalyst [12] that has been used as active in several applications such as CO₂ reduction [13,14], hydrogen production [15], air depuration [16] and wastewater treatment with total conversion of organic compounds into carbon dioxide [17,18,19]. The large use of TiO₂ as photocatalyst derived from its high efficiency in the decomposition of organic pollutants, its non-toxicity, biological and chemical stability, its low cost and its transparency to visible light [20].

The general mechanism of TiO₂ photocatalytic degradation of dyes by visible light (shown in Figure 1) suggests that the excitation of the adsorbed dye takes place in appropriate singlet or triplet states. The excitation of the adsorbed dye is followed by electron injection onto the conduction band of TiO₂, whereas the dye is converted into cationic dye radical that undergoes degradation to produce mineralized products as follows [21]:

$$Dye+Ti{O}_2\to DyeTi{O}_2$$

$$DyeTi{O}_2+h\nu \to Dy{e}^{·+}+Ti{O}_2\left(e^{-}\right)$$

$$Ti{O}_2\left(e^{-}\right)+O_2\to Ti{O}_2+O_2^{·-}$$

$$Dy{e}^{·+}+O{H}^{.}\to mineralized products$$

In this study, in order to optimize and clarify the total photocatalytic mechanism of ARS as target pollutant, two commercial types of nano-TiO₂ such as Anatase and Aeroxide P-25 coated on polypropylene supports (PP@TiO₂) were used, and the simultaneous adsorption and visible light photodegradation of dye from water solution were studied. The obtained kinetic results were discussed and compared with those derived from the previous study regarding separated steps of adsorption and photodegradation [22], and a kinetic model which described the overall process in an adequate way has been proposed.

2. Results

In order to obtain a kinetic model for the total adsorption and photocatalytic degradation of ARS under visible light in water solutions, adsorption and photodegradation steps were simultaneously studied in the same experimental conditions by changing ARS water concentrations. Figure 2 shows the change of absorbance profiles for both TiO₂ Anatase [PP@TiO₂]_A (a) and TiO₂ Aeroxide P-25 [PP@TiO₂]_P-25 (b) in a typical single-step process.

The decrease of ARS solution concentrations versus time during the overall adsorption/photodecomposition process under visible light, for both [PP@TiO₂]_A (a) and [PP@TiO₂]_P-25 (b) and for all examined ARS concentrations, is shown in Figure 3.

However, even if the absorbance profiles during time (Figure 2) for both [PP@TiO₂]_A and [PP@TiO₂]_P-25 during the process were similar, the decrease of ARS concentration during irradiation time (Figure 3), shows that the effect of initial ARS concentrations on adsorption/photodegradation rate by using [PP@TiO₂]_A was different respect to that of [PP@TiO₂]_P-25. In addition, by using [PP@TiO₂]_A, the reaction reached completion for each initial concentration of ARS while, in the same time, by using [PP@TiO₂]_P-25 the reaction was not complete with ARS concentrations greater than 4.38 × 10⁻⁵ mol·L⁻¹.

The obtained results do not show immediate evidence in the calculation of kinetic constants because two different processes occur at the same time and correlation is never possible by using first order, second order or Langmuir-Hinshelwood kinetic models [23,24,25,26]. It is important to note that, in these simultaneous processes, obtained without a previous adsorption-desorption equilibrium in dark conditions, knowledge of the kinetic parameters of separated processes is necessary. From this consideration, the transformation of ARS to adsorbed ARS (ARS_TiO2) and to photodegradated products (ARS_PH) could be treated in terms of two consecutive processes. The first is represented by the decrease of ARS concentration in the solution due to adsorption on PP@TiO₂ surface. The absorption of ARS is proved that occurs with a process described by the first order kinetic constant k₁ expressed by the equation$\ln [(q_e-q_t)/q_t]=-k_{1}t$, where q_t is the amount of adsorbed dye at time t and q_e is the equilibrium concentration [22].

The second process is represented by photodegradation of ARS, process that occurs with a rate described by the first order kinetic constant k₂ expressed by the equation $\text{ln}\left(C/C_0\right)=-k_{2}t$ [22,23] where C₀ is the initial concentration of ARS and C the concentration of dye at t time.

$$\begin{array}{c}\text{ARS}+{\text{TiO}}_2\to {\text{ARS}}_{\text{TiO}2}\end{array}$$

k₁ 

$$\begin{array}{c}{\text{ARS}}_{\text{TiO}2}+\text{h}\mathsf{\nu }\to {\text{ARS}}_{PH}\end{array}$$

k₂ 

The rate at which ARS decreased and the formation rates of ARS_TiO2 and ARS_PH can be described as follows:

$$-\mathrm{d}\left[\text{ARS}\right]/\mathrm{d}t=k_1\left[\text{ARS}\right]$$

(1)

$$\mathrm{d}\left[{\text{ARS}}_{\text{TiO}2}\right]/\mathrm{d}t=k_1\left[\text{ARS}\right]-k_2\left[{\text{ARS}}_{\text{TiO}2}\right]$$

(2)

$$\mathrm{d}\left[{\text{ARS}}_{PH}\right]/dt=k_2\left[{\text{ARS}}_{\text{TiO}2}\right]$$

(3)

The integration of Equation (1) gives:

$${\left[\text{ARS}\right]}_t={\left[\text{ARS}\right]}_0{\mathrm{e}}^{-k_{1}t}$$

(4)

where [ARS] = [ARS]₀ at time 0, and [ARS] = [ARS]_t at time t.

By substituting Equation (4) into Equation (2), a linear differential equation can be obtained:

$$\mathrm{d}\left[{\text{ARS}}_{\text{TiO}2}\right]/\mathrm{d}t=k_1{\left[\text{ARS}\right]}_0{\mathrm{e}}^{-k_{1}t}-k_2\left[{\text{ARS}}_{\text{TiO}2}\right]$$

(5)

that, after integration, can be written as:

$$[{\text{ARS}}_{\text{TiO}2}]=\frac{k_1}{k_2-k_1}\left({\mathrm{e}}^{-k_{1}t}-{\mathrm{e}}^{-k_{2}t}\right){\left[\text{ARS}\right]}_0$$

(6)

At any moment during the process, ${\left[\text{ARS}\right]}_0=\left[\text{ARS}\right]+\left[{\text{ARS}}_{\text{TiO}2}\right]+[{\text{ARS}}_{PH}]$ so, at time t,

$$\left[{\text{ARS}}_{PH}\right]={\left[\text{ARS}\right]}_0-{\left[\text{ARS}\right]}_t-{[{\text{ARS}}_{\text{TiO}2}]}_t$$

Substituting [ARS]_t and $\left[{\text{ARS}}_{\text{TiO}2}\right]$_t with Equations (4) and (6) may be obtained the follow equation:

$$\left[{\text{ARS}}_{PH}\right]=\left\{1+\frac{k_1{e}^{-k_{2}t}-k_{2 }{e}^{-k_{1}t}}{k_2-k_1}\right\}{\left[\text{ARS}\right]}_0$$

(7)

In order to prove the validity of this model it is necessary to use the values of k₁ and k₂ constants, previous described considering both the two separate processes (adsorption and photodegradation). These values are reported in

Table 1. k₁ and k₂ for [PP@TiO₂]_A and [PP@TiO₂]_P-25.

[ARS]0 (mol·L−1)	[PP@TiO2]A		[ARS]0 (mol·L−1)	[PP@TiO2]P-25
	k × 102 (min−1)			k × 102 (min−1)
	k1	k2		k1	k2
2.87 × 10−5	2.57 ± 0.02	1.45 ± 0.02	2.92 × 10−5	2.07 ± 0.02 *	1.59 ± 0.02
4.38 × 10−5	2.54 ± 0.03	2.11 ± 0.03 *	4.38 × 10−5	2.83 ± 0.03 *	1.55 ± 0.03 *
5.84 × 10−5	2.27 ± 0.03	1.23 ± 0.02 *	5.84 × 10−5	3.34 ± 0.02 *	0.67 ± 0.02 *
7.76 × 10−5	2.62 ± 0.02	0.67 ± 0.03	8.21 × 10−5	4.31 ± 0.04	0.34 ± 0.02

*: Reference [22].

.

By applying the Equation (7) obtained from the model to a simultaneous process of adsorption/photodegradation catalyzed by [PP@TiO₂]_A, experimental results showed a good correlation with theoretical values of [ARS]_t at any time, calculated by Equation (4), by using ARS concentrations up to 5.84 × 10⁻⁵ mol·L⁻¹ In Figure 4 we report, as an example, the validation model applied in the photodegradation of ARS at 5.84 × 10⁻⁵ mol·L⁻¹ by [PP@TiO₂]_A.

As it can be seen in Figure 4, while [ARS]_t decreases to zero, the concentration of intermediate [ARS_TiO2] calculated with the Equation (6) rises to a maximum, and then falls until zero, while the concentration of [ARS_PH] calculated from Equation (7) rises from zero towards [ARS]₀. These results show that the applied model can explain the experimental data and predict the evolution of process by calculation of [ARS_TiO2] and [ARS_PH].

When the Equation (7) of the obtained model is applied to [PP@TiO₂]_P-25, experimental results show high correlation with ARS concentrations up to 4.38 × 10⁻⁵ mol·L⁻¹ as it can be seen in Figure 5, while deviations of the model occur at higher concentrations.

To explain the deviations of model for this photocatalyst it is very important to consider that, in the consecutive processes, the rate-determining step is the slowest step and it controls the overall rate of the process [27]. Therefore, in this study, the comparison of experimental results with those obtained from relative kinetic constants of two separate processes k₁ and k₂ (Table 1) it is necessary. It is possible to note that, for [PP@TiO₂]_A the values of k₁ are of the same order of k₂ up to ARS concentration of 5.84 × 10⁻⁵ mol·L⁻¹, while, when k₁/k₂ ratio is greater than 2, deviations from the model occur.

The same consideration applied on [PP@TiO₂]_P-25 gives a similar interpretation at ARS concentration up to 4.38 × 10⁻⁵ mol·L⁻¹, i.e., the model is consistent only when k₁ and k₂ are of the same order, while deviations occur when k₁/k₂ ratio is greater than 2. Values of k₁/k₂ ratio for both [PP@TiO₂]_A and [PP@TiO₂]_P-25 are reported in

Table 2. k₁/k₂ ratio for [PP@TiO₂]_A and [PP@TiO₂]_P-25.

[PP@TiO2]A		[PP@TiO2]P-25
[ARS]0 (mol·L−1)	k1/k2	[ARS]0 (mol·L−1)	k1/k2
2.87 × 10−5	1.8	2.92 × 10−5	1.3
4.38 × 10−5	1.2	4.38 × 10−5	1.8
5.84 × 10−5	1.9	5.84 × 10−5	5.0
7.76 × 10−5	4.0	8.21 × 10−5	12.7

.

The slow step of these consecutive processes is therefore related to that of photodegradation process. The major influence of this, observed in the case of [PP@TiO₂]_P-25 and also clearly demonstrated in Figure 3b, is probably due to different adsorption behavior of [PP@TiO₂]_P-25 with respect to [PP@TiO₂]_A. In fact, in the case of [PP@TiO₂]_P-25, the absorption process is in accordance with the Langmuir model by which all dye molecules incorporated into the film have similar adsorption energy. In this case, the maximum ARS adsorption corresponds to a saturated layer of dye molecules on the TiO₂ surface that cannot contribute to an additional incorporation of other molecules. In contrast, the multilayer adsorption process according to the Freundlich isotherm occurs on [PP@TiO₂]_A [22].

For these reasons, the absorption behavior on [PP@TiO₂]_P-25 at ARS concentrations over 4.38 × 10⁻⁵ mol·L⁻¹, mostly influences the total process because the slow photodegradation step highly limits the adsorption step. In fact, the kinetic of only monolayer absorption on this support decelerates due to a slower photodegradation process that acts as a brake in the absorption process. Moreover, to confirm this, Figure 3b shows that, for ARS concentrations over 4.38 × 10⁻⁵ mol·L⁻¹, two trends are present; the first is mostly related to adsorption process and correlates with k₁, while the second is related to the adsorption/photodegradation process, which is conditioned to slower kinetic constant k₂ and therefore, in this case, the rate is reduced.

The decrease of ARS solution concentration during the overall process conducted under visible light, was successive compared with those obtained from separated steps (defined with a red separation line) of adsorption in dark conditions and photodegradation under visible light, as reported in the example of Figure 6 for both [PP@TiO₂]_A (a) and [PP@TiO₂]_P-25 (b).

As it may be observed in Figure 6, kinetic advantages were evidenced with the simultaneous approach showing that this method can be successfully used in water solution containing ARS.

3. Materials and Methods

3.1. Reagents and Materials

Photocatalysts were TiO₂ Anatase nano-powdered (size < 25 nm) and TiO₂ Aeroxide P-25 nano-powdered (size 21 nm), both supplied by Sigma Aldrich (Sant Louis, MO, USA). Photocatalyst support was constituted by polypropylene 2500 material obtained from 3M. Alizarin Red S, hydrochloric acid volumetric standard 1.0 N and acetyl acetone was bought from Sigma Aldrich (Sant Luis, MO, USA). Triton X-100 is purchased from Merck (Darmstadt, Hesse-Darmstadt, 64293, Germany). All of chemicals used were of analytical grade.

3.2. Methods

3.2.1. Photocatalysts Preparation

PP@TiO₂ photocatalysts strips were obtained as previously described in [22]. Briefly, two pastes of TiO₂ Anatase and TiO₂ Aeroxide P-25 were prepared form by a treatment with water, acetyl acetone and Triton X-100. These pastes were then supported on PP strips (of defined size with 2 cm of width and 10 cm of length) through dip coating technique, dried and clean with diluted hydrochloric acid to remove the excess of TiO₂ particles. The obtained surface was of 18 cm².

3.2.2. Operative Procedure for Kinetic Study

The photocatalytic activities were evaluated by ARS adsorption and photodegradation processes that were simultaneously investigated in water solutions at 25 °C and at acidic pH under the continuous action of visible light (tubular JD lamp, 80W, 1375 Lumen, Duralamp SpA, Florence, Italy) by using a glass thermostated photo reactor [22]. Nine PP@TiO₂ strips were immersed into ARS solutions at concentrations from 2.87 × 10⁻⁵ to 8.21 × 10⁻⁵ mol L⁻¹. Solutions were kept under constant air-equilibrated conditions before and during the irradiation. The overall process was monitored in real-time mode every 7 min by UV-Vis spectrophotometer (Cary 8454 Diode Array System spectrophotometer, Agilent Technologies Measurements, Agilent Technologies, Santa Clara, CA, USA), using a quartz cuvette in continuous flux (Hellma Analytics, 178.710-QS, light path 10 mm, Hellma Analytics, Müllheim, Germany) connected through a peristaltic pump Gilson miniplus 3 to the photo reactor. Previously, the adsorption (in dark conditions) and photodegradation (under visible light) processes were separately investigated at the same experimental conditions [22]. All spectrophotometric measurements were performed by measuring the absorption spectra of dye solutions; the decrease in concentration of the dye, calculated at 424 nm, was plotted as function of time.

4. Conclusions

We have studied simultaneously the adsorption of ARS on [PP@TiO₂] surface and its photodegradation in order to obtain a kinetic model usefully to allow the description of the total process. Obtained results were interpreted as two consecutive reactions and were in good agreement with the experimental data.

The obtained model can predict the profiles of free dye, ARS adsorbed on TiO₂ and photoproduct concentrations during the process only when the kinetic constant that relates to adsorption process is of the same order as that of photodegradation process. In fact, when k₁/k₂ ratio is greater than 2, deviations of model are observed because the rate of the second step limits the rate of total process. These deviations are more evident in the case of [PP@TiO₂]_P-25 probably because the absorption process occurs in a monolayer form according to the Langmuir model. The absorption process, in this case, decreases for the brake due to slower photodegradation process.

Finally, when the kinetic results of the overall processes were compared with those obtained from separated steps of adsorption and photodegradation kinetic, advantages were associated with the simultaneous approach, showing that this method is appropriate in the photodegradation of water solution containing ARS, suggesting the possibility of using it with other dyes.