Regeneration of spent mercury catalyst for the treatment of dye wastewater by the microwave and ultrasonic spray-assisted method

2.1 Materials

The SMCs used in this work were obtained from a chemical plant for producing PVC (Huize, Yunnan, China). The mercury catalyst supported activated carbon was prepared by coal and used as raw material, which presented columnar particles. All chemical reagents, such as HCl, NaCl and HNO₃ were purchased from Tianjin Reagent Chemicals Co. Ltd., Tianjin, China. All materials were used in this study without further purification. The chemical properties of MO and CR are shown in Table 1.

Table 1:

The properties of MO and CR.

Dye	Parameters	Character/value
MO	Chemical structure
	Chemical formula	CHNSONa
	Molecular weight	327.33 g/mol
CR	Chemical structure
	Chemical formula	C32H22N6Na2O6S2
	Molecular weight	696.68 g/mol

2.2 Regeneration of SMC

Experiments were carried out in a self-made microwave furnace (Key Laboratory of Unconventional Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan, China). The frequency of microwave oven was 2.5 GHz and the power intensity was 1000 W. A certain amount of SMC was placed into microwave oven, in which N₂ with a purity of 99.9% was discharged at 100 l/h, and the flow rate of ultrasound spray was 30 l/h. During the regeneration experiments, different parameters of microwave oven were set to get different samples. The obtained RAC was placed inside a vacuum drying oven at 85°C for 3 h and then crushed These were then stored in sealed bags for further characterization. To prevent harmful gas leakage, the solution adsorption was used to treat the exhaust gas. The experimental device diagram was showed in Figure 1.

Figure 1:

The diagram of the experimental setup.

2.3 Characterization of SMC and RAC

Samples of SMC and RAC were characterized by various techniques as described below. The yield (Y) of RAC was calculated using the following formula:

where W₁ and W₂ are the weights of RAC and SMC, respectively. The MB adsorption capacity was determined using the Standard Testing Methods of PR China (GB/T 7702.6-2008) [27]. The pore and textural properties were characterized at 77 K (Quant chrome, Autosorb-1-C). Before the gas adsorption test, the samples were degassed at 300°C for 10 h under vacuum condition. The surface area was calculated using the Brunauer-Emmett-Teller (BET) equation, and the pore size distribution was analyzed using the Density Functional Theory (DFT) model. The total pore volumes were evaluated at the relative pressure of 0.99, which was the equivalent liquid volume of nitrogen. The surface texture and morphology was observed by a SEM (Phenom-World-ProX, the Netherlands), which equipped with EDS (Phenom-World-ProX, the Netherlands) under an acceleration voltage of 15 KV. FTIR was used to observe the functional groups by using a NicoletiS10 spectrophotometer (Thermo Fisher Scientific, USA). The FTIR spectra were obtained between 4000 and 400 cm⁻¹.

2.4 Determination of mercury content

The mercury content was determined using the Standard Testing Methods of PR China (GB/T31530-2015) [18]. About 1 g SMC and RAC was placed into a 250-ml Erlenmeyer flask, to which 15 ml of HCl, 5 ml of HNO₃ and 20 ml of 100 mg/l NaCl solution. The solution was heated to boiling for 15 min and then cooled to room temperature, which was diluted with distilled water to 500 ml. The blank experiment was also carried out. The mercury concentration in the filtrate was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Scientific iCAP 7200).

2.5 Adsorption study

The adsorption capacity of RAC was determined by using MO and CR as organic pollutants, which carried were out in a thermostatic oscillator that the temperature ranges between 303 K and 323 K. The MO and CR solution were prepared by diluting the 1000 mg/l MO and CR solution with distilled water. One hundred milliliter MO solution and CR solution were added into 250-ml Erlenmeyer flasks with 0.05 g RAC, respectively. The remaining concentration of MO and CR solution were tested by UV-vis spectrophotometer (Shimadzu UV2600). To obtain the equilibrium concentration, Erlenmeyer flasks were placed in the thermostatic oscillator with shaking speed of 250 rpm for 5 h. The equilibrium concentration was used in the following tests. The equilibrium adsorption capacity (q_e, mg/g) is calculated according to Equation (2)

where C₀ and C_e are the initial concentration and equilibrium concentration (mg/l), respectively, V is the volume of the dye solution (ml) and m is the mass of RAC (g).

3.1 Effect of regeneration temperature on MB value and yield of RAC

Regeneration temperature was a significant parameter. The effect of regeneration temperature on MB value and yield of RAC was investigated when the temperature was in the range from 500°C to 900°C and regeneration time was 60 min. As shown in Figure 2, the overall trend of the MB value increased and the yield declined. The MB value reached to a maximum of 210 mg/g when the regeneration temperature was 900°C. This can be explained as follows: the small part of the pores were constructed when the regeneration temperature was low. The MB value increased along with the increase of temperature. It was clearly revealed that some substances in SMC were volatilized and decomposed under nitrogen atmosphere and ultrasonic spray and more pores formed. Thus, the pore channel of activated carbon can be restored with microwave heating and ultrasonic spray. A temperature of 900°C was selected and used for the following experiments.

Figure 2:

The effect of regeneration temperature on MB value and yield of RAC.

3.2 Effect of regeneration time on MB value and yield of RAC

The effects of regeneration time on MB value and yield of RAC was investigated under a regeneration temperature of 900°C. As shown in Figure 3, the MB value increased firstly and then decreased with the increase of regeneration time. This can be attributed to the fact that the increase of regeneration time was beneficial to the volatilization of the adsorbed substances in SMC. However, with the extension of regeneration time, the MB value decreased gradually. Exceeding a certain time, the pores of RAC would be damaged and the surface will be burned. Considering comprehensively, 60 min was selected as the regeneration time. According to the above analysis, the optimum conditions of regeneration experiments included 900°C of regeneration temperature and 60 min of regeneration time.

Figure 3:

The effect of regeneration time on MB value and yield of RAC.

3.3 Textural characteristics and mercury content

Figure 4 shows the N₂ adsorption-desorption isotherms of the SMC and RAC samples. According to the IUPAC classification, both isotherms can be classified to be a type IV [28]. When the relative pressure was between 0.4 and 0.9, both samples exhibited hysteresis loops. The N₂ adsorption-desorption isotherm of RAC presented a wider hysteresis loops than that of SMC due to the capillary condensation occurred. The pore structure details and mercury content of two samples were listed in Table 2. The obtained RAC possessed a specific surface area (801.6 m²/g) and total pore volume that was comparatively higher to SMC. It can be explained that the combination of microwave heating and ultrasonic spray was beneficial to develop new gaps. The average pore size of RAC was estimated to be 1.08 nm, which was lower than before regeneration, indicating that microporous morphology of RAC. Furthermore, the mercury contents of SMC and RAC were 23.70 and 0.46 mg/l. It can be explained that the mercuric chloride easily sublimated and evaporated at a high temperature, and most of the mercury was taken away under the atmosphere of N₂ and ultrasonic spray. Through experimental data analysis, the microwave and ultrasonic spray-assisted regeneration of SMC can effectively develop a new pore structure.

Figure 4:

The N₂ adsorption isotherm of SMC and RAC.

3.4 SEM and EDS analysis

The SEM was carried out to evaluate the surface morphology and texture of materials [29]. As can be seen in Figure 5A, the surface of SMC was relatively rough and irregular and there was no apparent pore structure. After regeneration (Figure 5B), the surface of RAC appeared obvious pore structure and the impurities on the surface of the activated carbon reduced. As can be seen from the EDS results (Figure 6), there was almost no mercury on the surface of RAC, proving that the mercury was effectively removed. This result is consistent with the determination of mercury content. Therefore, microwave heating and ultrasonic spray-assisted regeneration of activated carbon was an effective method.

Figure 5:

The SEM micrographs of SMC (A), RAC (B).

Figure 6:

The EDS images of SMC (A), RAC (B).

3.5 FTIR analysis

The FTIR technique was used to examine the important functional groups existed on the surface of SMC and RAC (Figure 7). As can be seen from Figure 7, the FTIR spectra of the two samples are roughly similar [30]. Both samples had a same clear high intense peak at 3433 cm⁻¹, and this may be water and existed O-H and N-H stretching vibration, respectively. The peak around 2924 cm⁻¹ corresponded to the stretching vibration of the C-H group and caused by CO₂ in the air. The band fund at 2852 cm⁻¹ can be assumed as the C-H group. The absorption peak at about 2340 cm⁻¹ was the asymmetric expansion vibration absorption of carbon dioxide. The peak at 1636 cm⁻¹ can be attributed to the aromatic C=C bond. The symmetric bending of CH₃ was detected in the peak with approximate maximum around 1457 cm⁻¹ [31]. The peak at about 1100 cm⁻¹ was attributed to the C-O bond. The peak at about 400–800 cm⁻¹ corresponded to vibrations of iron and zinc compounds [32]. It can be seen that SMC has a stronger vibrational peak at 1100 and 1636 cm⁻¹, which can indirectly prove that the functional groups on RAC have been weakened and some substances have been effectively removed.

Figure 7:

The FTIR spectra of SMC and RAC.

3.6 Adsorption kinetics

To check the mechanism of adsorption, the kinetic experiments were performed using 0.05 g RAC with 100 ml MO and CR solution at 313 K. The removal efficiencies of the MO and CR solution with different initial concentrations were investigated. The results are presented in Figure 8. It can be found that the removal efficiency of MO and CR solution almost reached equilibrium at 100 min. Three models of pseudo first-order, pseudo second-order and intra-particle diffusion were employed to obtain the important parameters, respectively. The correlation coefficients (R²) were used to evaluate the conformity between the experimental data and the model predicted values.

Figure 8:

The removal efficiencies of the MO (A) and CR (B) solution with different initial concentrations at 313 K.

The pseudo first-order can be expressed as follows (3) [33]:

where q_t was the adsorption capacity at time t (mg/g), the k₁ is the rate constant [g/(mg⋅min)].

The pseudo second-order model can be illustrated according the following equation (4) [34]:

where k₂ is the rate constant [g/(mg⋅min)].

The intra-particle diffusion model can be defined by the following equation (5) [35]:

where k_dif is the intra-particle diffusion rate constant (mg/(g⋅min^1/2), and c is the intercept (mg/g) relating to the thickness of boundary layer.

The obtained kinetic parameters were given in Table 3. As can be seen, the values of R² for three kinetic models are satisfactory. However, the adsorption process of MO and CR can be described with pseudo second-order due to the values of R² were close to 1 and the calculated adsorption values were consistent with the experimental data. The pseudo second-order indicated that the chemisorption played an important role in the rate limiting step [36], where the dye molecules attached to the surface of RAC by the transferring or sharing of the electron [37]. The adsorption was a complex process. Although the experimental data fitted well with pseudo second-order, the results obtained from this model could not predicate the diffusion process. Furthermore, two steps were studied. The first stage mainly depended on the surface pore structures of RAC. The adsorption appeared to be easing in the second stage due to the intraparticle diffusion was a rate-limiting process. In general, the fitted line passed through origin, which indicated that the pore diffusion controlled the adsorption processes. As can be seen the data in Table 3, both lines did not pass through the origin. This explains that the transfer rates in two stages are not consistent. It confirmed that the intraparticle diffusion was not the only rate-limiting step. Similar results have been reported in previous literatures [38]. The pseudo second-order for MO and CR adsorption was showed in Figure 9.

Figure 9:

The experimental data fitting analysis by pseudo second-order model for MO and CR dye solution.

3.7 Adsorption isotherm

The adsorption equilibrium isotherm can reveal the status of the adsorbate and adsorbent at a confirmed temperature. The isotherm studies were conducted with the MO and CR solution with initial concentration of 200–400 mg/l and 100–300 mg/l. The adsorption process was conducted at the temperature of 303 K, 313 K and 323 K. Three isotherm equations were considered to study the mechanism and calculate the maximum adsorption capacity.

The Langmuir isotherm is applied in the following form (6) [39]:

where q is the maximum adsorption capacity of RAC (mg/g) and K_L is the constant of Langmuir isotherm related to energy of adsorption.

The Freundlich isotherm is represented as (7) [40] follows:

where K and n are the constants of the Freundlich isotherm.

The Tempkin isotherm is given as the following equation (8):

where a and b are the constants of Tempkin isotherm.

The calculated parameters according to three models are listed in Table 4. From the results, we can see that the Langmuir models fitted the results slightly better than the other two models and the values of R² are greater than 0.98. Accordingly, the Langmuir isotherm can be used to describe the adsorption behavior to MO and CR dye solution onto RAC. The Langmuir model suggests that the adsorption can be attributed to the monolayer coverage and the surface is uniform [41]. However, the data of Freundlich and Temkin models did not fit well with actual experimental data due to its low R². The data were fitted with Langmuir isotherm as shown in Figure 10.

Figure 10:

The experimental data fitting analysis by Langmuir isotherm for MO (A) and CR (B) dye solution at 303 K, 313 K, 323 K.

Table 5 summarizes the maximum adsorption capacity of the MO and CR dye solution onto different adsorbents with previous studies. From Table 5, it can be established that the RAC regenerated from the SMC exhibited much higher adsorption capacity, and the maximum MO and CR adsorption capacities were 529 and 301 mg/g, confirming that the microwave and ultrasonic spray-assisted method was beneficial to the regeneration.

3.8 Thermodynamic studies

The thermodynamic studies of the adsorption of MO and CR onto RAC were carried out to reveal the possible mechanism [50]. The thermodynamic parameters were estimated from the Langmuir constants (K_L). The change in Gibbs free energy (ΔG°), enthalpy energy (ΔH°) and the entropy change (ΔS°) are calculated from the following equations (9–11):

where R is the gas constant (8.314 J/mol/K), and T is the temperature in Kelvin (K).

The slopes and intercepts of the plot of lnK_L versus 1/T were employed to calculate the values of ΔS^θ and ΔH^θ (Figure 11). The thermodynamic parameters are listed in Table 6. It can be observed that the values of ΔG^θ were negative for all temperatures and became more negative as the temperature increased, indicating that the process was endothermic and spontaneous [51]. It also explained that the high temperature would promote the extent of adsorption by decreasing the viscosity of the solution, which accelerate the mass transfer rate [52]. Moreover, the values of ΔH^θ and ΔS^θ were positive for RAC, suggesting that the endothermic nature of the adsorption and an increase in randomness at the solid-liquid interface [53].

Figure 11:

The thermodynamic study of adsorption of MO (A) and CR (B) dye solution.