Kinetic and Equilibrium Analysis of Tartrazine Photocatalytic Degradation Using Iron-Doped Biochar from Theobroma cacao L. Husk via Microwave-Assisted Pyrolysis ^†

Abstract

The MAP process involved placing acid-pretreated biomass (CPH) in a domestic microwave oven at 600 W for 15 min. This was followed by a doping process with iron salts (+2, +3) to obtain BCCPH-Fe. Characterization of BCCPH-Fe was carried out using surface analysis (BET), TGA analysis and FTIR. Subsequently, the photodegradation process was performed using three different light sources (solar, UV 254 nm and UV 356 nm), with tartrazine as the adsorbate. The effect of pH on photodegradation was studied, and the percentage of degradation was evaluated through equilibrium and kinetic studies. The amount of BCCPH-Fe, tartrazine concentration, and exposure time to the light source were also evaluated. The best conditions for the photodegradation process were: light source was 254 nm, pH of 5, 1 g of BCCPH-Fe over 100 mL of tartrazine, 25 ppm tartrazine concentration, and 40 h exposure time. Under these conditions, a 93.45% removal of tartrazine was achieved. The experimental data of the adsorption equilibrium best fit the Langmuir-Hinshelwood model, while the adsorption kinetics best fit the pseudo-first-order model. The apparent kinetic constant was 0.04053 [h⁻¹], and the correlation coefficient was 0.98667. In conclusion, photodegradation using BCCPH-Fe can be an effective method for the removal of tartrazine from wastewater, offering a sustainable alternative to traditional methods.

1. Introduction

Synthetic dye pollution in water bodies poses a significant threat to aquatic ecosystems [1]. This study proposes an innovative and sustainable solution: the use of BCCPH-Fe as a photocatalyst for the degradation of tartrazine, a common azo dye [2].

Biochar, a porous carbonaceous material, offers high adsorption capacity and chemical stability [3]. Doping it with iron introduces new active sites that favor the generation of free radicals during photocatalysis, accelerating dye degradation [4]. This strategy combines the advantages of biochar (sustainability, low cost and adsorption capacity) with the catalytic activity of iron, offering a promising alternative to conventional photocatalysts [5].

2. Materials and Methods

2.1. Obtaining BCCPH-Fe

All chemical reagents (iron salt) and solvents were purchased from commercial sources and used as received without further purification. A carbonaceous biomaterial was developed from CPH, this material was treated via acid-washed method with a solution of 0.5 M HCl, at a ratio of 0.75 g of CPH over 5 mL of HCl solution with a stirring velocity of 3000 rpm. CPHW underwent microwave-assisted pyrolysis at 600 W for 15 min to produce BCCPH, and subsequent iron doping in conditions of 180 °C for 1 h with a stirring velocity of 800 rpm, using a solution of ferric and ferrous sulfate on a ratio of 1:2 BCCPH over iron salts, obtaining BCCPH-Fe.

2.2. Equipments and Characterization

The obtained BCCPH-Fe was characterized using BET, TGA, and FTIR analyses. For BET analysis, the BCCPH-Fe was ground and dried at 100 °C for 20 min. Subsequently, the Micromeritics AutoChem II apparatus was employed, with the ground BCCPH-Fe placed in a measurement tube containing quartz wool. Liquid nitrogen and warm water were used as specified in the methodology. The TGA analysis was conducted using a METTLER TOLEDO TGA 1 Star System. The temperature was initially ramped to 100 °C and held for 10 min, followed by an increase to a target temperature of 700 °C, all performed in a nitrogen atmosphere. This temperature was maintained for 20 min in an atmospheric environment. For FTIR analysis, the PerkinElmer Spectrum Two was utilized, employing the ATR methodology.

2.3. Experiments and Studies

The BCCPH-Fe was assessed for its ability to photodegrade tartrazine under various conditions, including light sources (solar, UV 254 nm, and UV 356 nm), pH levels (2, 3, 5, 7, 9, and 12), BCCPH-Fe dosages (0.1, 0.5, 1, 1.5, and 2 g), tartrazine concentrations (5, 10, 20, 25, and 30 ppm), and reaction times (8, 16, 24, 32, 40, 48, and 72 h). The optimal conditions identified were applied in the subsequent phase of the study.

3. Results and Discussion

3.1. BCCPH-Fe

Initially, the CPH underwent an acid pretreatment to remove impurities and enhance its porosity. This pretreatment decomposes the lignocellulosic components of the biomass, facilitating its subsequent conversion into biochar.

Following this, a microwave-assisted pyrolysis (MAP) process was conducted to produce biochar. During this stage, the biomass decomposes into biochar, gases, and tar. The BCCPH was then doped with iron, which involved incorporating iron particles into its structure. This doping process increases the mass of the material and imparts new catalytic properties.

3.2. BCCPH-Fe Characterization

FTIR, BET, and TGA analyses provided valuable information about the structure and composition of the iron-doped biochar. FTIR analysis confirmed the successful incorporation of iron onto the biochar surface, evidenced by the appearance of characteristic peaks of iron compounds such as Fe₂O₃, Fe-O, and Fe-O-OH. Additionally, a significant interaction between iron and the functional groups of the biochar was observed, forming coordination complexes.

BET analysis revealed a decrease in the specific surface area and pore volume of the biochar after iron impregnation, suggesting that iron particles blocked part of the material’s porosity. However, the presence of iron within the porous structure could generate new active sites and modify the selectivity of the material.

TGA analysis showed an increase in the ash content of the iron-doped biochar, confirming the incorporation of the metal into the material’s structure and the formation of stable iron oxides at high temperatures.

3.3. Experiments

3.3.1. Equilibrium Studies

Figure 1a shows the experiments conducted with different light sources, demonstrating that ultraviolet C radiation (UV-C) at a wavelength of 254 nm was the most effective for dye degradation, achieving up to 75% removal. The high energy associated with UV-C facilitates photodegradation processes, efficiently breaking the dye’s chemical bonds. Consequently, 254 nm UV-C light was selected for the subsequent stages of the study. Figure 1b presents results indicating that the optimal pH range for tartrazine photodegradation is between 5 and 7, with a maximum degradation of 61%. A pH of 5 was chosen for further experiments due to its similarity to the natural pH of tartrazine solutions (5.5–6.2). This pH level promotes a negative charge on the biochar surface, enhancing the adsorption and subsequent degradation of the dye. Figure 1c evaluates the optimal amount of biochar for tartrazine degradation, revealing that 1 g of biochar provided the highest degradation efficiency (60%). Smaller amounts led to competition for active sites, while larger quantities resulted in saturation of the material. Finally, Figure 1d indicates that the optimal tartrazine concentration for the experiment was found to be 25 ppm. At this concentration, an equilibrium was reached between the adsorption of tartrazine molecules onto the active sites of the biochar and their subsequent degradation.

3.3.2. Kinetic Study

The kinetic study was determined applying the best conditions, where data on % removal of concentration with respect to time is shown in Figure 2.

From the Langmuir-Hinshelwood equation, we have:

$$r=-{\displaystyle \frac{dC}{dt}}=k\times {\displaystyle \frac{K\times C}{(1+K\times C)}}$$

(1)

The Langmuir-Hinshelwood equation is influenced by the dye concentration [6]. In this study, the concentration of tartrazine was set at 20 ppm (equivalent to $3.75\times {10}^{-5}{\displaystyle \frac{\mathrm{mol}}{\mathrm{L}}}$). As a result, the reaction kinetics follow a pseudo-first order equation.

$$r=-{\displaystyle \frac{dC}{dt}}=k_{ap}\times C$$

(2)

where $k_{ap}$ is the apparent kinetic rate constant, C is the final concentration and $C_0$ is the initial concentration. In it’s integrated form, we have:

$$\ln {\displaystyle \frac{C_0}{C}}=k_{ap}\times t$$

(3)

This equation is used to linearize the obtained data.

As seen in the Figure 3, the slope with a value of 0.04053 corresponds to the value of ${\mathrm{k}}_{ap}$. These data are accurate with respect to the mathematical study carried out due to its correlation coefficient ${\mathrm{R}}^2$ being 0.98667. The kinetic study determines that the photodegradation of tartrazine fits a Langmuir-Hinshelwood kinetic model, with a pseudo-first-order reaction.

3.3.3. Study with the Best Conditions

This experiment was conducted to analyze the conditions identified during the equilibrium and kinetic studies. The following parameters were used: light source (UV 254 nm), pH level (5), BCCPH-Fe dosage (1 g), tartrazine concentration (25 ppm), and duration (40 h). Although this tartrazine concentration is relatively high and the results indicated low degradation (see Figure 1c), it was selected to assess the maximum effect under the previously established conditions. The 40-hour duration was considered optimal in terms of time and resource efficiency (energy). This experiment demonstrated that it is possible to achieve 93.45% degradation of tartrazine.

4. Conclusions

The findings in this study suggest that BCCPH-Fe has significant potential as a sustainable and efficient photocatalyst for the treatment of wastewater contaminated with tartrazine. Choosing the right conditions, it can be used as an effective way to treat contaminated water from industries. Future studies could explore it’s applicability to other types of dyes and emerging contaminants, as well as investigate the long-term stability and re-usability of the material.

Additionally, optimizing the synthesis process and exploring different iron doping methods could further enhance the catalytic performance of the BCCPH-Fe.