Preparation and Exploration of Spherical Fe-C Micro-Electrolysis Materials for the Removal of Crystal Violet

Abstract

In this paper, the spherical Fe-C micro-electrolysis materials (Fe-C MEM) were prepared using iron powder, activated carbon powder, corn straw, and bentonite as the raw materials. The preparation conditions optimized by single factor test showed Fe-C MEM had a high crystal violet removal and strength under 1:1 of Fe/C ratio, 2% corn straw content, 25% bentonite content, and 900 °C sintering temperature. The porous Fe-C MEM had a high specific surface area of 108.069 m²/g with an even distribution of zero-valent iron and carbon. The maximum removal capacity of CV by Fe-C MEM was 105.48 mg/g at 25 °C. The CV removal was a spontaneous endothermic process. The mechanism of CV removal by Fe-C MEM was adsorption combined with degradation. Fe-C MEM has a good performance in dye wastewater treatment.

1. Introduction

Dyes are widely used in textile, leather, plastic, and other fields because of their bright colors and strong adhesion properties. However, most of the dyes are toxic, non-biodegradable, and harmful to the ecosystem and human beings [1]. Crystal violet (CV), a commercial dye, bears a triphenyl methane group in its structure and can persist in the natural environment [2]. Considering the recalcitrance and carcinogenesis, it is urgent to develop an efficient and low-cost method for the removal of CV.

Many technologies, such as precipitation, coagulation, adsorption, catalytic degradation, electrolytic oxidation, etc., can be used in dye wastewater treatment [1]. The technologies are often limited by incomplete removal, secondary pollution, or high operating costs. Adsorption is an effective method of dye removal. However, the high cost of the efficient adsorbents limits the commercial application. In recent years, the Fe-C micro-electrolysis materials (Fe-C MEM) have shown potential application in the field because of their high effectiveness and low cost.

The Fe-C MEM contains carbon and iron, which can combine adsorption (carbon) and ozonation (zero-valent iron (ZVI)) in the process. In addition, the Fe-C MEM uses iron as the cathode, carbon as the anode, and wastewater as the electrolyte to form the galvanic cell reaction. The formation of galvanic cells between Fe and C, a kind of micro-electrolysis reaction, could promote the degradation and removal of the dye wastewater [3,4]. Accordingly, iron filings using the internal carbon to form galvanic cells could be used directly. However, the lack of carbon and the formation of iron compounds covering the surface make the treatment of low efficiency [5]. Then, fly ash, carbon, slag, sponge iron, etc., are mixed [6,7,8]. The addition of carbon can improve the dispersion of iron and make the material more effective. Then, granular Fe-C MEM are produced for easy separation in the treatment [8]. Zhang et al. used sintering ferric-carbon ceramic coupled with a modified biological aerated filter in wastewater treatment. The ferric-carbon ceramic enhanced phosphorus and ciprofloxacin removal in the process [9]. To further improve the efficiency, porous Fe-C MEM are produced by adding pore former. Ammonium salt works by generating ammonia during sintering. However, the emission of irritant gas and the cost of salts are not conducive to large-scale production and utilization. The addition of biomass also works because the pyrolysis of biomass generates H₂O, CO₂, CO, H₂, etc., and the reducing gases contribute to the reduction of ferric oxides to ZVI [10]. Some researchers focus on the formation of ZVI in the process because ZVI is more reactive than ferric oxides. The addition of red mud, iron ore tailings, ferric iron, and reduction to ZVI is studied [4,9]. In summary, the granular porous Fe-C MEM are promising in dye wastewater treatment. Most research focuses on the reduction of iron and high efficiency of degradation (Fenton treatment, ozone oxidation) in wastewater treatment [3,9,11].

The properties, easy production, low cost, and high efficiency are the keys to Fe-C MEM in commercial applications. Therefore, iron powder, activated carbon powder, corn straw, and bentonite were chosen to explore the production of porous Fe-C MEM by biomass pyrolysis. The effects of Fe-C ratio, sintering temperature, and the addition of corn straw and bentonite were optimized for high efficiency in CV removal. Then, a detailed study of CV removal by Fe-C MEM was conducted, and the mechanism of CV removal was explored.

2. Materials and Methods

2.1. Materials

Bentonite was obtained from Mingxi mineral product plant (Chuzhou, China). Corn straw was obtained from the farmland in the suburb of Qingdao, Shandong Province. CV was obtained from Dengke Chemical Reagent Co., Ltd. (Tianjin, China). Iron powder and activated carbon powder were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were of analytical grade. Deionized water was prepared and used in this work.

2.2. Preparation of Fe-C MEM

Iron powder, activated carbon powder, corn straw, and bentonite were used to prepare the Fe-C MEM. The iron powder was passed through a 200-mesh sieve. The corn straw was ground into a size of 100 mesh. The raw materials were added to the blender and mixed at different ratios with deionized water. The mixture was pressed into spherical particles with a diameter of 8 mm. Then the particles were dried at 105 °C in a vacuum drying oven and sintered in an atmosphere sintering furnace with a heating rate of 10 °C for 1 h. The particles were covered with activated carbon powder to isolate oxygen during sintering. Finally, the prepared Fe-C MEM was stored in the sealing pocket. The preparation process is shown in Figure 1.

2.3. Optimization of Fe-C MEM

The impacts, such as Fe/C ratio (2:1–14:1), corn straw content (0–5%), bentonite content (15–35%), and the sintering temperature (550 °C–1000 °C) were explored to optimize the properties of Fe-C MEM. The properties included CV removal efficiency and strength of Fe-C MEM.

The experiment of CV removal was conducted in a shaker at 120 rpm, 25 °C, and 0.5 g Fe-C MEM was added in 250 mL conical flasks containing 100 mL CV solution with a concentration of 1000 mg/L. The concentration of CV was examined at λ_max = 585 nm using a UV–Vis spectrophotometer (L8 Plus, INESA). CV removal was calculated as follows.

$$CV removal={\displaystyle \frac{\left(C_{0 }-C_t\right)V}{m}}\times 100\%$$

(1)

where C₀ and C_t were the concentration of CV solution at beginning and time t in the adsorption. V was the volume of CV solution, and m was the weight of Fe-C MEM.

The strength of Fe-C MEM was evaluated by weight loss. Fe-C MEM was shaken in 50 mL deionized water at 200 rpm and 25 °C for 60 min. Then Fe-C MEM was washed and heated at 105 °C to constant quantity in a vacuum oven. Weight loss was calculated as follows.

$$Weight loss={\displaystyle \frac{\left(m-m_2\right)}{m_2}}\times 100\%$$

(2)

where m₂ was the weight of Fe-C MEM after being heated.

2.4. CV Removal by Fe-C MEM

To evaluate the CV removal efficiency of Fe-C MEM, batch experiments were carried out. A 0.2 g amount of Fe-C MEM was added in 100 mL CV solution in a shaker at 120 rpm for 72 h. The initial concentration of CV, pH of the solution, removal time, and temperature were studied. The experiment was triplicated.

Regeneration of Fe-C MEM after CV adsorption was conducted by sintering Fe-C MEM at 900 °C again in the atmosphere sintering furnace for 1 h. Then, the sintered Fe-C MEM were used for CV adsorption.

2.5. Characterization

Scanning electron microscopy (SEM, JSM-6610 LV, JEOL, Japan) was used to analyze the surface morphology. X-ray diffraction (XRD, XRD-6100, Shimadzu, Japan) was used to analyze iron in the material. Fourier infrared spectrometer (FTIR, IRAffinity-1S, Shimadzu, Japan) was used to analyze the chemical functional groups on the surface of the materials. The pore size distribution and specific surface area of the iron–carbon micro-electrolytic materials were analyzed by BET (ASAP 2020, Micromeritics, USA).

3. Results

3.1. Optimization of Fe-C MEM Production

The impacts, such as Fe/C ratio, corn straw content, bentonite content, and the sintering temperature, were explored to optimize the properties of Fe-C MEM. The results are shown in Figure 2. In addition to bentonite content, Fe/C ratio, corn straw content, and the sintering temperature show similar variations in CV removal and weight loss of Fe-C MEM. The removal capacity of CV increases first and then decreases, and the weight loss of Fe-C MEM increases. According to the reports, dyes could be adsorbed or degraded by Fe-C MEM [7,12,13]. Therefore, the carbon content and galvanic cells formed in the solution might contribute to CV removal. The removal capacity of CV is the highest at Fe/C 1:1. Then it decreases with the reduction of activated carbon in Fe-C MEM, indicating the synergistic effect of carbon and iron reaches the highest at Fe/C 1:1.

Corn straw underwent rapid thermal cracking and released a large amount of reducing gases, such as CO and CH₄, when the temperature reached 200 °C [10]. The escape of gases formed the connected pores in Fe-C MEM, increasing the specific surface area of the material, thus improving the removal capacity of CV. Excessive addition of corn straw results in an increase in carbon content, leading to a decrease in CV removal. The results are in agreement with those shown in Figure 2a.

The effects of sintering temperature on CV removal by Fe-C MEM are shown in Figure 2c. Fe-C MEM have the highest removal capacity of CV at 900 °C. As reported, the pyrolysis of corn straw finished below 800 °C, and the reduction of ferric oxides usually happened above 900 °C [10]. The synergistic effect promotes the formation of porous material with reactive ZVI. When the sintering temperature is 1000 °C, ferric oxides will react with SiO₂ in the clay, resulting in the reduction of reactive ZVI and the blockage of the pores [14].

Bentonite works as a binder Fe-C MEM. Therefore, CV removal decreases with the increasing bentonite content while the strength of Fe-C MEM increases. Except for bentonite, the addition of iron powder and corn straw leads to a decrease in the strength of Fe-C MEM.

In summary, Fe-C MEM was produced at a 1:1 Fe/C ratio, 2% of corn straw content, 25% of bentonite content, and 900 °C.

3.2. Characterization of Fe-C MEM

The properties of Fe-C MEM under optimal conditions were analyzed by SEM, XRD, FTIR, and BET.

The XRD patterns show the characteristic peaks at 2θ of 44.6° and 65°, which are in good agreement with the standard data of ZVI (PDF #06-0696) (Figure 3a). The strong diffraction peaks of Fe indicate a good reduction and protection of ZVI in the process of preparation and sintering. There is also a strong diffraction peak of SiO₂ (PDF #27-0605), which might come from the calcined bentonite in the XRD pattern. The FTIR spectra are shown in Figure 3b. The peak at 3400 cm⁻¹ is assigned to OH stretching vibration. The two peaks at 1420 cm⁻¹ and 1640 cm⁻¹ are assigned to COOH [15,16]. The spectra indicate that the carbon in Fe-C MEM have a large amount of carboxyl and hydroxyl groups, which might be beneficial for CV removal [17].

The specific surface area of Fe-C MEM is 108.069 m²/g, and the average pore size is 5.630 nm. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the nitrogen isothermal adsorption–desorption curve of the material belongs to type IV, with a lag loop in the middle, indicating that the pores in the Fe-C MEM are mainly mesoporous. The result is consistent with that in pore size distribution (Figure 3c). The results indicate the Fe-C MEM have a large specific surface area and well-developed pore structure.

Figure 4 shows the morphology and Fe, C distribution of Fe-C MEM. The pores are obvious in Figure 4a, which might be attributed to the evaporation of water, pyrolysis of corn straw, and the addition of activated carbon. According to the reports, pyrolysis of corn straw generated CO, H₂, CH₄, and biochar, making Fe-C MEM porous [10]. The release of reducing gas could efficiently reduce ferric oxide to obtain ZVI. EDX-mapping (Figure 4b) shows Fe and C distributed evenly in the material. The existence of ZVI and carbon could form a galvanic cell, facilitating CV removal.

3.3. CV Removal by Fe-C MEM

The effects of the initial concentration of CV, time, temperature, and pH on CV removal by Fe-C MEM are shown in Figure 5. At the initial stage, the CV removal is fast. Then, the removal rate gradually decreases until reaching the final equilibrium. CV removal increases with the increasing initial concentration. At an initial concentration of 300 mg/L, the removal capacity of CV is 45.57 mg/g, while it changes to 105.48 mg/g when the initial concentration is 1000 mg/L. As the initial concentration increases, the collision between CV molecules and the active site on the surface of Fe-C MEM increases. Meanwhile, the increase in the initial concentration also increases the driving force for CV molecules to move toward the surface of the material, overcoming certain mass transfer resistance [18,19].

The effect of temperature on CV removal by Fe-C MEM is shown in Figure 5b. CV removal increases when the temperature increases from 25 °C to 45 °C, indicating that the increase in temperature facilitates CV removal. Because the thermal motion speed of CV molecules becomes faster when the temperature increases, the probability of collision between CV and the surface of Fe-C MEM increases, promoting CV removal [20]. According to the reports, the internal pore size and specific surface area of the porous materials could increase under high-temperature conditions while the hydrated radius of CV decreases. Therefore, CV removal could be promoted in high temperatures [21].

The removal capacity of CV increases in the pH ranged from 3 to 11, shown in Figure 5c. CV is positive in an acidic solution. The abundant H⁺ would compete for the adsorption site and hinder the removal of CV. And the basic condition could promote CV removal. Meanwhile, the chromogenic group of CV is destroyed under strong alkaline conditions, suggesting a higher CV removal in the study.

Since Fe-C MEM contains activated carbon and biochar, which have a large amount of hydroxyl and carboxyl functional groups, CV could be adsorbed on the porous material [4,17]. Therefore, the results were fitted with the pseudo-first-order and pseudo-second-order adsorption kinetics models (Equations (3) and (4) [22,23], the Langmuir and Freundlich isotherm (Equations (5) and (7) [24,25] to study the CV adsorption mechanism on Fe-C MEM. The equations are as follows:

$$log\left(q_e-q_t\right)=log{q}_e-{\displaystyle \frac{k_{1}t}{2.303}}$$

(3)

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_e}}$$

(4)

$${\displaystyle \frac{C_e}{q_e}}={\displaystyle \frac{1}{Q_{0}b}}+{\displaystyle \frac{C_e}{Q_0}}$$

(5)

$$R_L={\displaystyle \frac{1}{1+b{C}_0}}$$

(6)

$$ln{q}_e=ln{K}_F+{\displaystyle \frac{1}{n}}ln{C}_e$$

(7)

where q_e (mg/g) and C_e (mg/L) are the equilibrium adsorption capacity and the equilibrium concentrations of the solution, respectively. q_t (mg/g) and C_t (mg/L) are the adsorption capacity and concentration of solution at time t, respectively. k₁ and k₂ are the pseudo-first-order and pseudo-second-order adsorption rate constant, respectively. Q₀ (mg/g) is the maximum adsorption capacity, and b is the equilibrium adsorption constant. K_F and n are the Freundlich constants.

The results are shown in

Table 1. Corresponding parameters of kinetics models.

C0 (mg/L)	qe (mg/g)	Pseudo-First-Order Model		Pseudo-Second-Order Model
		k1 (h−1)	R2	k2 (h−1)	R2
300	46.99	0.127	0.938	0.0206	0.995
500	88.03	0.667	0.942	0.108	0.996
1000	105.48	0.846	0.9453	0.0091	0.997

and

Table 2. Corresponding parameters of Langmuir and Freundlich models.

T (°C)	Langmuir				Freundlich
	R2	qm (mg/g)	b (L/mg)	RL		R2	Kf	1/n
25 °C	0.9978	105.47	0.008	0.05–0.27		0.988	23.68	0.4934
35 °C	0.9892	117.03	0.007	0.07–0.21		0.9833	29.74	0.3206
45 °C	0.9945	129.98	0.004	0.05–0.17		0.982	35.82	0.4428

. The correlation coefficient R² of the pseudo-second-order model is higher than that of the pseudo-first-order model, indicating that the CV adsorption process on Fe-C MEM is consistent with the pseudo-second-order model. The equilibrium adsorption isotherm determines the adsorption affinity of CV on Fe-C MEM. The correlation coefficient R² of the Langmuir model is higher than that of the Freundlich model. In addition, the maximum adsorption capacity q_e calculated by the Langmuir model is 105.47 mg/g, which is very close to the practical adsorption capacity of 105.48 mg/g. The calculated R_L in the Langmuir model is between 0 and 1, indicating that Fe-C MEM have a good effect on CV removal [26]. In summary, the fitted results suggest that CV adsorption on Fe-C MEM is monolayer absorption.

Meanwhile, the thermodynamic parameters, including the enthalpy change (ΔH, kJ/mol), Gibbs free energy change (ΔG, kJ/mol), and entropy change (ΔS, J/(mol·K)) for CV removal were calculated. The equations are as follows [27]:

$$\mathsf{\Delta }\mathrm{G}=-RT\mathit{ln} K_L$$

(8)

$$\mathit{ln}{K}_L=-{\displaystyle \frac{\mathsf{\Delta }\mathrm{G}}{RT}}=-{\displaystyle \frac{\mathsf{\Delta }\mathrm{H}}{RT}}+{\displaystyle \frac{\mathsf{\Delta }\mathrm{S}}{R}}$$

(9)

where K_L is the equilibrium constant. R is the universal gas constant (8.314 J/(mol·K)). The values of ΔH and ΔS were calculated from the plot of ln K_L versus 1/T.

The thermodynamic parameters are shown in

Table 3. Thermodynamic parameters for CV removal on Fe-C MEM.

T (K)	△H (KJ/mol)	△S (J/mol·K)	△G (KJ/mol)
25 °C	24.12	124.13	−15.69
35 °C	24.12	124.13	−17.24
45 °C	24.12	124.13	−19.26

. The ΔH is positive, indicating that the CV adsorption is endothermic [27]. According to the reports, when ΔH < 20 kJ/mol, it is physical adsorption; when 80 < ΔH < 200 kJ/mol, it is chemical adsorption [28]. In this study, ΔH is 24.12 kJ/mol, indicating the chemisorption of CV on Fe-C MEM. Meanwhile, the positive ΔS suggests that the disorder of the solid–liquid system increases during the removal process, and the randomness at the solution interface increases. The negative ΔG decreases with the increasing temperature, indicating that CV removal is a spontaneous process. It is more favorable for the process at higher temperatures [29].

In addition to the adsorption, many microcurrent cells could be formed spontaneously in solution between carbon and ZVI and might contribute to CV removal on Fe-C MEM [13]. Therefore, the UV–vis spectra of CV during the decolorization process were analyzed. The results are illustrated in Figure 6. The characteristic bands of CV are located at 580 nm, 300 nm, and 246 nm, respectively. Then, the intensity of the bands decreases during decolorization. Two new bands at 340 nm and 373 nm appear 12 h later, indicating CV is degraded and new species are generated. In the process, ZVI acted as an anode and was oxidized to Fe²⁺. Then, Fenton-like chain reactions produced hydroxyl radicals (·OH) to destroy CV molecules. Carbon acted as a cathode and accepted and transferred the electrons [3,7,30].

Therefore, the removal mechanism of CV by Fe-C MEM was adsorption combined with degradation, as shown in Figure 7. CV molecules diffused from the solution to the surface of Fe-C MEM and were adsorbed. Part of the CV was degraded in the process.

Table 4. Comparison of Fe-C MEM with other adsorbents for CV removal.

Adsorbents	Qm (mg/g)	Reference
Palm Kernel Shell-Derived Biochar	24.45	[2]
Magnetic HKUST-1 MOF	70.42	[31]
Cu-chitosan nano-biocomposite particles	84.75	[32]
Aromatic polyamides (APIs)-1 APIs-2	285.71 303.03	[33]
Barium encapsulated alginate/carbon composites	50	[34]
Fe/sponge structure peanut shell carbon composite	143.25 (addition of H2O2)	[35]
Activated carbon embedded zero-valent iron	95	[12]
Fe-MEM	105.48	This study

presents the Fe-C MEM with other adsorbents for CV removal. The adsorption capacity of CV varied from 24.45 to 303.03 mg/g by different kinds of adsorbents. The Fe/C-based adsorbents showed similar adsorption capacity of CV, which was relatively high in the table. Considering the low cost, Fe-C MEM showed a great potential for CV removal in practice.

3.4. Reuse of Fe-C MEM

Fe-C MEM were reused three times, and the results are shown in Figure 8. The removal capacity of CV decreases a little bit after sintering. This might be due to the loss of reactive site on the surface of Fe-C MEM. The formation of ferric minerals covered the surface of ZVI and blocked the pores of Fe-C MEM, leading to a decrease in CV removal capacity. The CV removal efficiency is above 70% of the original after three times regeneration. Therefore, Fe-C MEM could be an effective and low-cost material for dye wastewater treatment.

4. Conclusions

The spherical Fe-C MEM was produced with iron powder, activated carbon powder, corn straw, and bentonite by sintering. In the process, corn straw worked as the pore former, and bentonite worked as the binder. The optimal condition for the production was a 1:1 Fe/C ratio, 2% of corn straw content, 25% of bentonite content, and 900 °C. The even distribution of Fe and C, abundant functional groups, and high specific surface area of 108.069 m²/g contributed to the high CV removal efficiency. CV was removed by adsorption and degradation by Fe-C MEM. The adsorption was endothermic and well fitted the pseudo-second-order kinetics model and Langmuir isotherm model. Fe-C MEM had a good reusability. Therefore, Fe-C MEM could be an effective and low-cost material for dye wastewater treatment.