Recovering Zinc and Iron from Waste Tire-Derived Pyrolysis Carbon Black to Prepare Layered Metal Hydroxide Composites for Efficient Adsorption of Dye Methyl Orange

Abstract

The pyrolysis carbon black (CBp) from waste tires contains zinc, iron, and other metal elements, which have high recycling value. This study proposes a simple method of recovering zinc and iron from waste tire-derived CBp to synthesize hydrotalcite-type adsorbents for the treatment of anodic dye wastewater. Firstly, zinc-aluminum hydrotalcite (LDH) and zinc-iron aluminum hydrotalcite (FeLDH) were obtained by leaching the zinc and iron ions from CBp with an acid solution. As compared with LDH, FeLDH shows increased laminate metal ion arrangement density and layer spacing. By calcining the LDH and FeLDH at 500 °C, zinc aluminum oxides (LDO) and zinc iron aluminum oxides (FeLDO) were then prepared and applied for the adsorption of dye methyl orange (MO). The results demonstrate that the maximum adsorption capacity of LDO and FeLDO are 304.9 and 609.8 mg g⁻¹ at pH of 4.0, respectively. The adsorption processes of both LDO and FeLDO are consistent with the Langmuir adsorption isotherm and the proposed second-order kinetic model. The adsorption regeneration performance and adsorption mechanism of LDO and FeLDO were also investigated in detail. Regeneration experiments show that after three cycles, the removal rate of MO by LDO remains above 80%, while that of FeLDO only remains around 64% in the first cycle after regeneration. This work would provide a new pathway to realize the high-value metal recycling of waste tire-derived CBp and solve the contamination of dye wastewater.

1. Introduction

In the contemporary era of globalization, the automotive industry is advancing at breakneck speed, and transportation has become busier. This remarkable development has spurred a dramatic surge in tire consumption [1]. The annual global output of waste tires has been escalating steadily, giving rise to progressively severe environmental pollution challenges [2,3]. The treatment of waste tires is very complicated, and only 50–60% of them can be recycled [4]. How to deal with waste tires efficiently has become an environmental problem that needs to be solved urgently. At present, the most common method of waste tire treatment is pyrolysis [5,6,7], and the products are mainly pyrolysis gas [8], pyrolysis oil [9], and pyrolysis carbon black (CBp) [10].

Among them, as a kind of inorganic functional material, CBp has shown different application prospects in many fields [11,12,13]. However, the practical application of CBp is subject to many limitations due to the wide particle size distribution, low surface activity, and high ash content of 17–20 wt%, which is mainly composed of Si, O, Zn, Ca, Fe, and other elements [14,15]. Therefore, different modification treatments were carried out to improve the utilization value of CBp [16,17]. Among them, acid treatment was the most common approach to remove the ash to obtain high purity CBp [18,19]. However, most of the current research focuses on the purification process of CBp, and few investigators have paid attention to the treatment of waste acid leaching solution, which contains abundant metal ions with high recovery value. There are several reports alone on the recovery of zinc in CBp to prepare ZnO and zinc metal [20,21]. If high-value elements in the ash of CBp can be further recycled and synthesized into other compounds with diverse structures, it is anticipated that this will unlock more extensive application prospects for CBp, thereby augmenting its comprehensive utilization value.

In addition, dye wastewater has emerged as a grave menace to the environment and human health because of the high content of organic toxicity pollutants [22,23]. Therefore, researchers have developed many technologies to treat dye wastewater in order to reduce environmental harm and ecological toxicity [24,25]. Among them, the adsorption method for dye wastewater treatment is most attractive because of its simple design and low cost [26]. Layered double hydroxides (LDHs) are a category of materials with two-dimensional nanostructures that have been widely used for dye MO removal [27,28,29]. LDH can be further calcined to produce calcined-layered double oxide (LDO) with a larger specific surface area and greater anion exchange capacity [30]. LDO can be rebuilt into its original layered structure by rehydration or by adsorption of anions from aqueous solution, which is referred to as the memory effect [31]. Zhou et al. used the calcined Zn-Al hydrotalcite to adsorb phosphate and the maximum adsorption capacity could reach 232 mg g⁻¹ [32]. Ni et al. investigated the adsorption of the dye MO on the calcined lamellar double hydroxide. The equilibrium adsorption capacity of MO was 181.9 mg g⁻¹ with 100 mg L⁻¹ of MO at 298 K and pH of 6.0 [33]. The flower-like NiAl hydrotalcite prepared by Hassani et al. has an adsorption capacity of up to 500.6 mg g⁻¹, higher than that of conventional LDH for the removal of dye MO [34].

Herein, we propose a simple method to recover zinc and iron from waste tire-derived CBp to synthesize hydrotalcite-type adsorbents for anodic dye wastewater treatment. Firstly, the waste acid solution from leaching CBp was recovered to prepare LDH and FeLDH for providing metal ions. Then, by calcining LDH and FeLDH at 500 °C, LDO and FeLDO, as efficient absorbents for dye methyl orange adsorption, were prepared. The structural characteristics of LDH/FeLDH and LDO/FeLDO were evaluated and the dye MO adsorption by LDO/FeLDO under different conditions was systematically assessed. The analysis shows that LDO and FeLDO exhibit high MO removal efficiencies at an initial pH of 4.0. The maximum adsorption capacity of LDO and FeLDO adsorbents can reach 304.9 and 609.8 mg g⁻¹ at 298 K, respectively. This work would provide a new pathway to realize high-value metal recycling of waste tire-derived CBp and solve the contamination of dye wastewater.

2. Results and Discussion

2.1. Determination of Leaching Conditions and Material Characterization

In order to systematically investigate the effect of acid treatment conditions on the leaching efficiency of zinc ions, different leaching conditions (including the type of acid, acid concentration, reaction temperature, and time) were investigated (Figure S1). The experimental results show that the optimal leaching agent is 2 M HCl and the reaction temperature and time are determined to be 55 °C for 6 h by taking into account the leaching rate of zinc ions and energy consumption.

The XRD diffraction spectra of the as-prepared LDH and FeLDH are shown in Figure 1a. The sharp and symmetric diffraction peaks and smooth baseline in the plots indicate that both samples have high crystallinity and excellent crystal shape. The typical diffraction peaks of (003), (006), and (012) of the hydrotalcite material are clearly observed for both the LDH and FeLDH samples, and symmetrical bimodal peaks are present at around 60°, which are consistent with the stand card PDF#48-1021, indicating a typical hydrotalcite structure [35]. The cellular parameters of the two samples derived from the calculation of the data related to the XRD diffraction peaks are shown in

Table 1. Cellular parameters of LDH and FeLDH.

Sample	2θ003	2θ110	d003 (Å)	d110 (Å)	c (Å)	a (Å)
LDH	11.54	60.13	7.701	1.536	23.10	3.07
FeLDH	11.02	59.59	8.029	1.550	24.09	3.10

. The cell parameter a reflects the arrangement density of metal ions on the sample laminates and is calculated according to the equation a = 2d₁₁₀. Compared with LDH, the cell parameter a of FeLDH increases from 3.07 to 3.10 Å, indicating the increase in the density of the atomic arrangement of the laminae. This may be due to the insertion of iron into the laminates in the form of ions, resulting in an increase in the charge density of the laminates [36,37,38].

The cellular parameter c is calculated according to the crystal plane spacing of (003) (c = 3d₀₀₃), which reflects the change in the spacing between the layers of the samples [39]. The cellular parameter c of FeLDH is 24.09 Å, higher than that of LDH (23.10 Å), demonstrating that the addition of Fe increases the spacing of the layers. This may be due to the increased density of positive charges on the laminates, resulting in increased Coulomb forces between adjacent positively charged laminates, which in turn increases the layer spacing. From the XRD spectra, it is inferred that a small amount of Fe is inserted in between the zinc-aluminum-layered double-metal hydroxide laminates to form zinc-aluminum-iron-layered metal hydroxide.

The XPS full spectra of LDH and FeLDH and the 2p spectra of Fe in FeLDH are shown in Figure 1b,c. It can be seen from Figure 1b that there are Fe 2p spectra in the XPS full spectra of FeLDH, but the intensity of the 2p peaks of Fe is low, probably due to its low content (Table S1). The Fe 2p spectrum (Figure 1c) shows two typical peaks at binding energies of 711.1 and 724.4 eV, corresponding to the spin splitting of Fe (III)2p^3/2 and Fe (III)2p^1/2, respectively, suggesting that Fe exists in the sample in the form of trivalent iron [40]. Therefore, it is presumed that Fe is inserted into LDH lamellae in the form of Fe³⁺ to form zinc-aluminum-iron layered metal hydroxides. This result is consistent with that of XRD.

The TG curves of LDH and FeLDH are shown in Figure 1d. Both thermogravimetric curves are consistent with the weight loss pattern of hydrotalcite-like materials. With the temperature increasing from room temperature to 230 °C, the first weight loss occurs for the two samples, mainly attributed to the evaporation of physically adsorbed water on the surface of the hydrotalcite and interlayer adsorbed water [41]. The second weight loss occurs in the range of 230–500 °C, mainly due to the decomposition of carbonates and hydroxides in the structure of hydrotalcite [42].

SEM images of pure LDH, LDH, and FeLDH prepared with CBp-derived zinc ions are shown in Figure 2. It can be seen that the pure LDH exhibits a dispersed lamellar hexagonal structure with thicker edges, while the prepared LDH and FeLDH samples show finer and thinner lamellar structures, which is beneficial to providing more active sites during the adsorption process. The difference in morphology may be attributed to the presence of other impurity ions (e.g., calcium, copper, etc.) in the acid-leaching solution that interfered with the uniform growth and dispersion of the hydrotalcite crystals.

According to reports in the literature, LDO has a higher specific surface area and better adsorption properties than LDH [43]. Figure S2 demonstrates the effect of calcination temperature on the adsorption of MO by LDH and FeLDH. The results show that the adsorption capacity and removal rate of the samples first increase and then decrease with the increase in calcination temperature and reach the maximum value at 500 °C. Therefore, LDO and FeLDO were prepared by calcinating LDH and FeLDH at 500 °C and then characterized.

The SEM images of LDO and FeLDO are shown in Figure S3. It is observed that the lamellar morphology of hydrotalcite disappears and irregular lumps are formed after calcinating at 500 °C. The morphology of FeLDO seems to be finer than that of LDO, meaning that the specific surface area of FeLDO may be larger. The EDS elemental distribution and contents of LDO and FeLDO are exhibited in Figure S4 and Table S2. Before calcination, the carbon contents in LDH and FeLDH are 37.3% and 38.7%, respectively (Table S1). However, the C element in LDO and FeLDO disappears after calcination, mainly due to the decomposition of CO₃²⁻ into CO₂ escaping at high temperatures. The relative amounts of Zn and Al elements in both LDO and FeLDO are roughly 3:1, which agrees with the standard card PDF#48-1021 [Zn_0.71Al_0.29(OH)₂ (CO₃)_0.145]. The Fe element in FeLDO is about 3.5%. Additionally, both samples contain a tiny amount of elemental Cl, which is due to the addition of AlCl₃·9H₂O during the synthesis.

The XRD diffractograms of the calcined samples are shown in Figure 3a. The diffraction peaks appearing at 31.6°, 34.5°, and 36.3° correspond to (100), (002), and (001) crystal faces of ZnO (standard card PDF#99-0111), indicating that the structure of the hydrotalcite transforms into an oxide after calcination. The absence of diffraction peaks of alumina and iron oxide may be due to the fact that alumina is structurally amorphous and the content of iron oxide is very low. Figure 3b–d shows the N₂ adsorption–desorption isotherms and pore size distributions of LDO and FeLDO, respectively. From the shape of the curves, it can be inferred that the adsorption of both LDO and FeLDO belongs to the H3-type hysteresis line, indicating that the adsorbent has slit-like pores formed by the aggregation of plate-like particles. The pore sizes of the two adsorbents are mainly distributed in the range of 2–35 nm, which belong to the mesoporous structure [44]. As compared with LDO, FeLDO shows a micropore structure at 0–2 nm. The calculated specific surface area and pore characteristics of the LDO and FeLDO samples are shown in Table S3. The total pore volumes of LDO and FeLDO are 0.30 and 0.29 cm³g⁻¹, respectively, showing no significant difference. The specific surface areas of LDO and FeLDO are 80.85 m²g⁻¹ and 101.06 m²g⁻¹, respectively. The larger specific area and the presence of micropores may be the reasons for the subsequent higher adsorption capacity of FeLDO as compared with LDO.

2.2. Adsorption Performance of LDO and FeLDO

The pH of the solution exerts a significant influence on both the surface charge of the adsorbent and the degree of ionization of the adsorbate. As a result, it plays a crucial role in the adsorption process [45,46]. Therefore, the effect of pH on the adsorption of MO by LDO and FeLDO was tested and the results are shown in Figure 4a. For FeLDO, the equilibrium adsorption capacity (q_e) increases from 127.6 to 481.2 mg g⁻¹ with increasing pH from 2.0 to 4.0, peaking at a pH of 4.0. When the pH is further increased, q_e gradually reduces and decreases to lower than 100 mg g⁻¹ at a pH of 12, mainly because of the competition between the excess OH⁻ and MO anions in the solution. The same pattern is observed for LDO. When the pH value is less than 4.0, the q_e values of LDO and LDO are relatively low, which may be due to the dissolution of some adsorbents at low pH values. Therefore, an initial pH of 4.0 was chosen for the subsequent experiments.

Figure 4b,c illustrates the influence of absorbent dosage on the MO adsorption at pH = 4.0. The q_e of MO decreases with increasing dosage of adsorbent, while the removal rate of MO increases, which is attributed to the higher adsorbent amount providing a larger surface area and more adsorption sites [47]. The removal rate is over 93.8% and approaches 100% when the amount of FeLDO is increased to10 mg and beyond, meaning all the MO molecules in the solution are adsorbed. Compared with FeLDO, the removal rate of MO increases with increasing LDO dosage, and the removal rate of MO achieves 100% at an adsorbent dosage of 30 mg. In contrast, FeLDO only requires 10 mg to achieve complete adsorption under the same condition. This result also indicates that the adsorption capacity of FeLDO is superior to that of LDO.

Figure 4d illustrates the impact of the contact time on the adsorption process of MO. In the initial stage, from 0 to 20 min, the q_t of MO by LDO exhibits a rapid increase. Subsequently, in the time range of 20 to 100 min, the adsorption rate gradually decelerates. Eventually, after 2 h, the adsorption capacity remains steady without further change, meaning the adsorption reaches equilibrium. The influence trend of contact time on the MO adsorption by FeLDO is in the same manner as LDO, except the initial phase of the q_t rapid increase is in a shorter range of 0–10 min. This phenomenon occurs because the adsorption sites on the adsorbent surface are initially more abundant, leading to a relatively high adsorption rate. However, after a certain period of contact, MO anions start to occupy parts of the surface of the adsorbent. In consequence, the residual vacant adsorption sites are difficult to take up due to the repulsive force between the adsorbed MO anions and the free MO anions in the solution. This process causes the adsorption rate to gradually slow down until it eventually reaches equilibrium [48]. The q_e of MO by LDO is up to 292.3 mg g⁻¹ at an initial MO concentration of 100 mg L⁻¹, while the qe of FeLDO reaches 458.4 mg g⁻¹, indicating that the adsorption ability of FeLDO is much higher than that of LDO.

The variations in adsorption capacity and removal rate of LDO and FeLDO at different concentrations of MO are shown in Figure 4e,f. For LDO samples, the q_e exhibits an upward trend as the initial concentration of MO increases and reaches a maximum of 301.1 mg g⁻¹ when the initial concentration reaches 120 mg L⁻¹. However, the q_e decreases as the initial MO concentration is further increased, indicating that the adsorption limit of LDO is reached. For FeLDO samples, the equilibrium adsorption capacity shows the same trend increase for different initial MO concentrations as LDO. Notably, at an initial MO concentration of 120 mg L⁻¹, FeLDO achieves a q_e of 492.7 mg g⁻¹, much higher than LDO (301.1 mg g⁻¹).

2.3. Adsorption Kinetics

For the purpose of determining and explaining the adsorption control steps and adsorption kinetic mechanism of MO on LDO and FeLDO, the effects of MO concentration and contact time on the adsorption capacity of MO by LDO and FeLDO were systematically investigated (Figure 5a,d). The kinetic models were fitted according to the equations shown in SI. The fitted curves of the proposed first- and second-order kinetic models for MO adsorption by LDO and FeLDO are shown in Figure 5b–f, respectively. For both LDO and FeLDO absorbents, the correlation coefficients R² of the proposed secondary kinetic models calculated by linear fitting of the relevant physical quantities are all higher than 0.99 and significantly higher than those of the proposed primary kinetic adsorption models. Therefore, the adsorption processes of both absorbents are more consistent with the pseudo-second-order kinetic model, indicating that the rate at which MO molecules occupy the adsorption sites has a direct proportional relationship with the square of the number of unoccupied sites and the adsorption process is dominated by chemisorption [49,50,51].

The intra-particle diffusion model was further employed to assess the diffusion mechanism and forecast the rate-determining step. (Figure 5g,h). For LDO and FeLDO, three sets of q-t curves corresponding to different initial concentrations of MO were selected for fitting, respectively. It is clearly observed that all the curves for both adsorbents exhibit a multi-linear characteristic, implying that the adsorption process encompasses different stages. The adsorption slopes for both LDO and FeLDO are stage 1 > stage 2 > stage 3. The initial phase with a steep slope represents the external mass-transfer stage. During this period, most of the MO molecules are adsorbed onto the external surface of the adsorbent. When the adsorption sites on the outer surface of the adsorbent are occupied, the second stage of the adsorption process begins. At this point, MO molecules penetrate into the adsorbent pores and adsorb onto the inner surface. As the concentration of MO diminishes, the intra-particle diffusion gradually decelerates and eventually enters the equilibrium stage. Since the slope of exterior transfer (stage 1) is higher than that of internal transfer (stage 2 and stage 3), it is suggested that internal transfer may be the rate-limiting step and the adsorption rate is controlled by particle diffusion [52,53]. Figure 5i shows the comparison of the slopes of the three adsorption stages at 100 mg L⁻¹ of MO; it is evident from the three stages that the slope of FeLDO is steeper than that of LDO, indicating that FeLDO adsorbs more rapidly and requires less time to reach the equilibrium state.

2.4. Adsorption Thermodynamics

The investigation of equilibrium adsorption isotherms possesses significant importance in elucidating the interaction between MO and adsorbents and in providing theoretical basis that support the design and regulation of the adsorption process [54,55]. The effect of different temperatures on the adsorption of MO by LDO and FeLDO is shown in Figure S5. The q_e of MO shows an upward trend as the temperature rises, suggesting that the adsorption process is an endothermic reaction and higher temperatures are conducive to the adsorption of MO by both LDO and FeLDO.

In order to accurately determine the maximum adsorption capacity (q_m) of the adsorbent, we further fitted the adsorption isotherm data using the Langmuir, Freundlich, and Dubinin–Radushkevich models, which were obtained by the equations expressed as follows.

$$\mathrm{Langmuir}: {\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}}={\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{m}}·b}}+{\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{m}}}},$$

(1)

$$\mathrm{Freundlich}:{ \lg \mathrm{q}}_e=\lg {\mathrm{K}}_{\mathrm{F}}+{\displaystyle \frac{1}{\mathrm{n}}}{\lg \mathrm{C}}_{\mathrm{e}},$$

(2)

$$\mathrm{Dubinin}–\mathrm{Radushkevich}: \ln {\mathrm{q}}_e=\ln {\mathrm{q}}_{\mathrm{DR}}-\mathrm{\beta }·{\mathrm{\epsilon }}^2,$$

(3)

where q_e (mg g⁻¹) represents the amount of adsorbed MO, q_m (mg g⁻¹) denotes the monolayer adsorption capacity, and Ce (mg L⁻¹) refers to the equilibrium concentration of MO in the solution. b (L mg⁻¹) represents the Langmuir constant, which is associated with the affinity of the binding site. K_F (mg g⁻¹) and n (factorless) are the relevant parameters of the Freundlich isotherm. ε, in the Dubinin–Radushkevich model, is the Polanyi potential and equal to RT ln(1 + 1/Ce). β is a constant and the average adsorption energy E (J mol⁻¹) = 1/$\sqrt{2\mathrm{\beta }}$.

The relevant model fitting profiles for LDO and FeLDO are shown in Figure 6a–f and the obtained correlation parameters are shown in

Table 2. Adsorption isotherm fitting data for LDO and FeLDO adsorption of MO at 298, 323, and 348 K.

Samples	T(K)	Langmuir Isotherm			Freundlich Isotherm			Dubinin–Radushkevich Isotherm
		qm (mg g−1)	b (L mg−1)	R2	Kf (mg g−1)	n	R2	qDR (mg g−1)	E (J mol−1)	R2
LDO	298	304.88	0.53	0.998	94.93	3.85	0.907	252.77	146.93	0.394
	323	313.48	0.21	0.999	110.85	3.86	0.922	286.82	511.64	0.490
	348	389.11	0.19	0.991	158.72	5.11	0.973	314.22	100.76	0.661
FeLDO	298	609.76	0.40	0.991	417.14	13.13	0.871	532.84	440.21	0.501
	323	645.16	0.61	0.995	489.00	19.54	0.822	565.75	149.17	0.548
	348	689.66	2.70	0.999	539.11	14.69	0.987	619.62	141.26	0.633

. For both LDO and FeLDO, the correlation coefficients R² of the fitted Langmuir isotherms at different temperatures are all greater than 0.99 and higher than the other two adsorption models, indicating that the Langmuir adsorption model can explain the adsorption process of MO on LDO and FeLDO well. The Langmuir adsorption model indicates that the adsorption of MO takes place at specific homogeneous reaction sites on the absorbent surface and follows the monolayer adsorption process [56].

The q_m of the two adsorbents for MO was calculated from the Langmuir model slopes and shown in Figure 6g. It is observed that the q_m of both LDO and FeLDO increases with increasing temperature. The q_m of LDO and FeLDO for MO at 298 K are 304.9 and 609.8 mg g⁻¹, respectively.

Table 3. Comparison of optimum adsorption pH and maximum adsorption capacity of MO by different adsorbents.

Adsorbents	pH	qm (mg g−1)	References
ZnAl-LDO	6.0	181.9	[33]
ZnAlLDH/ZnO	/	276.6	[57]
Fe2O3/AC	3.0	362.0	[58]
Calcined MgNiAl-LDH	8.0	375.4	[59]
Chitosan/Al2O3/magnetite composite	6.0	416.0	[60]
MWCNTs/Fe3O4/polyaniline	4.5	446.3	[61]
NiCoLDH	/	497.0	[62]
NiAlLDH	3.0	500.6	[34]
MnOx@MgAlLDO	/	557.2	[63]
LDO	4.0	304.9	This study
FeLDO	4.0	609.8	This study

shows that the q_m of MO on LDO is higher than those reported by the literature for ZnAl-LDO (181.9 mg g⁻¹) [33] and ZnAlLDH/ZnO (276.6 mg g⁻¹) [57]. The adsorption capacity of MO on FeLDO exceeds that of most reported iron-oxide composites and other ternary hydrotalcite materials such as Fe₂O₃/AC (362.0 mg g⁻¹) [58], calcined MgNiAl-LDH (375.4 mg g⁻¹) [59], and Chitosan/Al₂O₃/magnetite composite (416.0 mg g⁻¹) [60]. This suggests that both LDO and FeLDO have promising prospects of application in MO removal from the aqueous phase.

The thermodynamics of adsorption processes is capable of furnishing in-depth information regarding the internal energy alterations linked to adsorption [64,65]. Figure 6h shows the plot of ln Kd versus 1/T calculated from the adsorption data of LDO and FeLDO adsorbed at different temperatures. According to Figure 6h and the equations in SI, the Gibbs free energy change (∆G), enthalpy change (∆H), and entropy change (∆S) were computed [66]. The relevant data are presented in

Table 4. Thermodynamic data for the adsorption of MO by LDO and FeLDO at 298, 323, and 348 K.

Samples	T (K)	ΔG (kJ mol−1)	ΔH (kJ mol−1)	ΔS (J (mol K)−1)
LDO	298	−4.02	4.47	28.44
	323	−4.71
	348	−5.44
FeLDO	298	−5.64	8.59	47.62
	323	−6.71
	348	−8.03

. For both LDO and FeLDO adsorbents, the ΔG is negative, while the ΔH is positive, indicating that the adsorption reaction of MO on LDO and FeLDO is spontaneous and endothermic [67]. Therefore, elevated temperature is beneficial to the adsorption of MO on LDO and FeLDO.

2.5. Recovery Properties of Adsorbents and the Adsorption Mechanism

From a practical perspective, the reusability of adsorbents stands as a crucial factor in their evaluation. Figure 7a,b depicts the removal rate of MO by the regenerated LDO and FeLDO after three cycles of regeneration. The results indicate that the MO removal rate of both adsorbents progressively declined as the number of cycles increased, which is chiefly attributed to the reduction in crystallinity or the collapse of the regenerated materials as suffering from multiple calcination processes during their structural reconstruction. After three cycles, the removal rate of MO by LDO maintains more than 80%, indicating its good recycling performance. The removal rate by FeLDO is maintained at about 64% in the first cycle after regeneration and decreases to about 45% after the third cycle. In order to investigate the reasons for the differences in the recovery performance of the two adsorbents, the adsorption mechanism of LDO and FeLDO was investigated.

Usually, the mechanism of removing pollutants from water bodies by talc-based adsorbents is its unique memory effect [68]. When the mixed oxides obtained from calcining hydrotalcite at a specific temperature are immersed in an aqueous solution containing the corresponding anions, the structure can be reconstructed to the pre-calcination lamellar structure. In this study, FT-IR tests were first performed on LDO and FeLDO after the adsorption of MO (Figure 7c). It is observed that after adsorption, the new peaks occur at 1169, 1121, and 1032 cm⁻¹, which are assigned to the telescopic vibrations of -SO₂-O-, O=S=O, and -SO₃- in MO, respectively, and the peak at 1008 cm⁻¹ corresponds to the in-plane bending vibration of the benzene ring C-H of MO, indicating that MO is adsorbed on the adsorbents. In addition, the peak detected at 1369 cm⁻¹ can be explained by the stretching vibration of CO₃²⁻, demonstrating the reinsertion of CO₃²⁻ between the layers of the absorbents after adsorption.

The XRD diffraction peaks of LDO and FeLDO after MO adsorption are shown in Figure 7d. For the LDO sample, the typical diffraction peaks are in accordance with the standard card of zinc-aluminum hydrotalcite (PDF#48-1021), clearly indicating that the structure has reverted to the pre-adsorption lamellar form. Remarkably, the (003) peaks exhibit no displacement, which is a strong indication that the interlayer spacing remains unchanged. The zeta potential of the LDO before and after adsorption is shown in Figure S6. The zeta potential value is 50.7 mV before adsorption and decreases to 21.0 mV after MO adsorption, indicating that the MO anion binds to the LDO surface by electrostatic interaction, neutralizing part of the positive charge. Based on the FT-IR, XRD, and zeta potential observations, it is reasonable to speculate that LDO restores the LDH structure during the adsorption process and MO is adsorbed onto the laminate by the electrostatic interaction. The diagrams of the probable adsorption mechanism are illustrated in Figure 8.

For FeLDO, after the adsorption of MO, impurity peaks of ZnO emerge, indicating that FeLDO does not fully revert to the hydrotalcite structure. This incomplete structural restoration may account for its subpar subsequent recovery performance. Additionally, the (003) diffraction peaks of the hydrotalcite shift forward, suggesting an increase in the layer spacing after adsorption. Calculations reveal that the layer spacing expands from 0.80 nm to 1.21 nm, which is likely attributed to the insertion of MO anions into the interlayer region. It is hypothesized that the adsorption mechanism of FeLDO may be as follows: some MO anions are inserted into the interlayer, facilitating the restoration of the hydrotalcite structure, while some are adsorbed on the lamellae. This dual-mode adsorption process leads to FeLDO exhibiting better adsorption performance compared with LDO.

3. Materials and Methods

3.1. Materials and Reagents

All reagents were purchased from China. The CBp product was provided by Hubei Zhongshuo Environmental Protection Co. AlCl₃·9H₂O, urea, ammonia, EDTA-2Na, NaOH, and HCl (AR) were bought from Beijing Tongguang Fine Chemical Co. Ltd. (Beijing, China). Methyl orange was purchased from the Tianjin Fine Chemical Development Center(Tianjin, China).

3.2. Preparation of LDH/FeLDH and LDO/FeLDO

The leaching solution was collected for the subsequent synthesis reaction. EDTA titration was used to determine the zinc ion concentration in the leaching solution. The pH of the acid-leaching solution was adjusted to 7.0 by ammonia, and a small amount of reddish-brown flocculent precipitate appeared and was then filtered. AlCl₃·9H₂O solid (Zn: Al molar ratio 3:1) and urea (2 times of the total cation) were added to the filtrate, stirred to dissolve completely, and then transferred into a hydrothermal reactor and reacted at 110 °C for 5 h. The synthesized sediment was separated by centrifugation, washed to neutrality, and then dried at 60 °C for 12 h. The obtained white powder was marked as LDH. LDO was prepared by calcining LDH at 500 °C for 4 h. The preparation of FeLDH was similar to that of LDH except that AlCl₃ and urea were directly added to the leaching solution containing a small amount of reddish-brown suspended particles. The reddish-brown powder obtained was FeLDH, which was then calcined at 500 °C for 2 h to obtain FeLDO. In addition, pure ZnAl-LDH samples were prepared using pure ZnCl₂ as a raw material and analyzed in comparison with LDH materials synthesized with CBp-derived zinc ions. The detail experimental procedure is shown in Figure 9.

3.3. Adsorption Experiments

An MO solution of 2–10 mg L⁻¹ was prepared, the absorbance corresponding to different concentrations of MO was tested at 464 nm on the UV–visible spectrophotometer, and a standard curve of absorbance versus concentration was plotted (Figure S7).

In total, 10 mg of LDO and FeLDO was placed in 50 mL of 100 mg L⁻¹ MO solution separately, and the initial pH value was adjusted to 2.0–12.0 by 0.1 M HNO₃ or NaOH solution. The adsorption experiments were conducted in a temperature-controlled shaker (SHA-B, Changzhou Putian Instrument Manufacturing Co. LTD) with a rotation speed of 150 rpm. The supernatant was centrifuged and the remaining MO concentration was measured by the UV spectrophotometer. The adsorption capacity q_t (mg g⁻¹) and removal rate of MO were calculated according to the method shown in SI.

In order to investigate the adsorption kinetics, 30 mg of adsorbent was added to 200 mL of MO solution of different concentrations, and the changes in adsorption amount were measured at different times. The adsorption experiments were carried out at 298, 323, and 348 K in the tests of different concentrations of MO solution with the addition of 10 mg adsorbent for isothermal and thermodynamic studies.

The equations for determining the pseudo-first-order kinetic model and pseudo-second-order kinetic model are shown as follows:

$${\ln (\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}{)=\mathrm{lnq}}_{\mathrm{e}}-{\mathrm{k}}_1\mathrm{t},$$

(4)

$${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}}={\displaystyle \frac{1}{{\mathrm{k}}_2{{\mathrm{q}}_{\mathrm{e}}}^2}}+{\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}},$$

(5)

Here, q_e (mg g⁻¹) represents the equilibrium adsorption capacity, while q_t (mg g⁻¹) denotes the adsorbed capacity at t (min) time. Additionally, k₁ (min⁻¹) and k₂ (g mg⁻¹min⁻¹) are the rate constants for the proposed first and second-order kinetic models, respectively.

3.4. The Regeneration of LDO and FeLDO

The LDO and FeLDO after adsorption of MO were washed with deionized water until the filtrate was nearly colorless. Then, the LDO and FeLDO were regenerated by calcining at 500 °C in air for 2 h. The recovery effects were examined using the regenerated adsorbents to adsorb different concentrations of MO, and the corresponding removal rates were calculated.

4. Conclusions

In this manuscript, a novel method was put forward for the recycling and reuse of zinc and iron elements from waste tires-derived CBp, in which LDH and FeLDH were firstly synthesized and then calcined at 500 °C to obtain LDO and FeLDO, which were applied for the adsorption of dye MO. The structural characterization results indicate that Fe doping is beneficial to increasing laminate metal ion arrangement density and layer spacing. The influences of solution pH, calcination temperature, contact time, and reaction temperature on MO adsorption by LDO and FeLDO were systematically investigated. The results show that the q_m of LDO and FeLDO are 304.9 and 609.8 mg g⁻¹ at 298 K and pH = 4.0, respectively. The Langmuir adsorption isotherm model and the proposed second-order kinetic model are more adequate to depict the adsorption process of MO. It is verified that the removal of MO is governed by monolayer non-homogeneous adsorption and chemisorption control. After three cycles, the removal of MO by LDO remains above 80%, while that of FeLDO only remains around 64% in the first cycle. This study not only materializes the concept of “turning waste into useful materials” but also significantly presents a sustainable solution for both environmental protection and resource management. In the future, hydrotalcite materials prepared from the waste acid leaching solution of CBp can also be used in other applications such as catalysts and supercapacitors. In addition, other products (e.g., ZnO nanomaterials or zinc-based MOFs) can be synthesized by using zinc ions in the acid leaching solution, which is expected to further broaden the scope of high-value-added utilization of the waste CBp and to promote the extension and value added of the recycling industry chain of waste tires.