Synthesis of a Novel Polymer Adsorbent and Its Adsorption of Pb (II) and Cu (II) Ions in Water

Abstract

This study successfully synthesized a polymer adsorbent TOC containing S functional groups through thiophene and oxalyl chloride. The TOC was characterized by IR, molecular weight determination, EDS, SEM, and TG. For Pb (II), the maximum adsorption capacity of TOC can reach 122.7 mg/g (0.593 mmol/g) at pH = 4, 250 min. After adsorption, it can be desorbed with 1.00 mol/L HCl, and the adsorbent can be reused 10 times; For Cu (II), the maximum adsorption capacity of TOC can reach 95.9 mg/g (1.498 mmol/g) at pH = 6, 180 min. After adsorption, it can be desorbed with 0.50 mol/L HNO₃, and the adsorbent can be reused 11 times. The adsorption process of TOC on Pb (II) and Cu (II) conforms to the Freundlich model of multi-layer adsorption, and the chemical adsorption process is controlled by pseudo-second-order kinetic reactions.

1. Introduction

In recent years, with the rapid development of industry, a large amount of heavy metal wastewater containing Pb (II) and Cu (II) has been illegally discharged into rivers and other water bodies. Heavy metal ions have characteristics such as toxicity, not degrading in living organisms, and enrichment. They enter the human body through biological enrichment in drinking water or food chains, seriously endangering human health [1,2,3]. At present, the main methods for treating Pb (II) and Cu (II) in water bodies include chemical precipitation, oxidation reduction, membrane separation, adsorption, and biological methods [4,5,6]. The adsorption method is an effective and excellent method for treating heavy metal wastewater. Compared with other methods, it has the characteristics of simplicity, flexibility, high efficiency, not generating secondary pollutants, and low cost [7,8].

During the adsorption process, heavy metal ions in the aqueous solution combine with the active sites on the surface of the solid adsorbent and are transferred to the adsorbent, thereby achieving the goal of removing heavy metal ions from wastewater. The adsorption process mainly consists of three steps: first, heavy metal ions diffuse from the solution to the surface of the adsorbent, then are adsorbed on the surface of the adsorbent, and finally diffuse within the adsorbent particles. The adsorption mechanism can be divided into physical interactions (van der Waals forces) and chemical interactions (functional group coordination, ion exchange, and redox) based on the selected adsorbent. The effect of physical adsorbents on heavy metal ions is very weak and limited, while adsorbents mainly based on chemical reactions have excellent removal performance for heavy metal ions, especially functional group coordination, which is widely used [9,10].

The coordination atoms of commonly used functional groups are mainly N, O, S, and P. Polymer adsorbents can be classified into four types: polymers containing N groups, O groups, S groups, and P groups [11,12,13].

N atoms have lone pair electrons that can form chemical bonds or undergo specific interactions with specific adsorbates, thereby achieving selective adsorption of target metal ions. The commonly used functional groups for polymer adsorbents containing N functional groups include amino groups (-NH₂) and some nitrogen-containing heterocycles, such as pyridine [14,15]. Xu et al. [16] functionalized poly (p-phenylenediamine) with ethylenediamine to prepare a nanoscale polymer adsorbent containing amine groups. The very small nanostructure makes the adsorption kinetics of the adsorbent for Hg (II) very rapid, reaching adsorption equilibrium in just 15 min. The adsorption process conforms to the quasi-second-order kinetic model and Langmuir model. Liu et al. [17] prepared two acid resistant pyridine amine-based polymers, PMAA-PD and PMAD-PD, through a series of reactions. The PMAA-PD adsorbent can separate Ni²⁺ from acidic coexisting solutions of Co²⁺ and Ni²⁺ to produce cobalt with a purity of up to 99.99%. PMAD-PD adsorbent exhibits good adsorption capacity for various metal ions such as Cu (II), Ni (II), Co (II), Zn (II), and Cd (II) under acidic conditions. Zou et al. [18] successfully modified the tricyclic pyridine side group on the framework of chlorinated methylstyrene through a series of synthetic reactions. The adsorbent has good adsorption capacity for Cu (II), Ni (II), and Pb (II) in aqueous solution, with maximum adsorption capacities of 5.02, 3.38, and 1.27 mmol/g, respectively. At the same time, this adsorbent also has good reusability and can be used multiple times. The adsorbent after adsorbing Cu (II) can also serve as a catalyst for the degradation of bisphenol A.

The commonly used functional groups for organic synthetic polymer adsorbents containing O functional groups are carboxyl (-COOH) and hydroxyl (-OH) [19,20]. Zhang et al. [21] prepared a polymer adsorbent rich in carboxyl and hydroxyl groups by surface-initiated atom transfer radical polymerization to react chloromethylated styrene skeleton with glycidyl methacrylate, and then modified the polymer with salicylic acid. When the pH value of the solution is four, the maximum adsorption capacities of the adsorbent for Ni (II) and Cu (II) are 138.52 mg/g and 111.21 mg/g, respectively. The adsorption process follows the quasi-second-order kinetic model and Langmuir model. In addition, the adsorbent also exhibits good desorption rate and reusability.

S atoms contain lone-pair electrons and can form coordination bonds with many metal ions, exhibiting strong adsorption capacity for specific substances. The commonly used functional groups for organic synthetic polymer adsorbents containing S functional groups are thiol (-SH) and thiophene [22,23]. Albakri et al. [24] prepared a polymer adsorbent containing two sulfur-containing functional groups, thiol and thiophene, through the condensation reaction of thiol and thiophene using methanol as a crosslinking agent. This adsorbent has good thermal stability and can remain stable above 300 °C. The removal ability of the adsorbent for Hg (II) in an aqueous solution and methylmercury in a hydrocarbon medium (decane/toluene mixture) was studied through a series of adsorption experiments. The results showed that the adsorbent had good removal ability for both Hg (II) and methylmercury. In an aqueous solution with a Hg (II) concentration of 100 mg/L, the removal rate of Hg (II) was close to 100%.

P atom has an empty d orbital and can form coordination bonds with many metal ions, thus exhibiting good adsorption properties for heavy metal ions. However, the synthesis process of P-containing polymers is often relatively complex, requiring specific reaction conditions, catalysts, and raw materials, and involves multiple synthesis steps, resulting in high production costs and hindering large-scale industrial production. The commonly used functional groups for organic synthetic polymer adsorbents containing P functional groups are phosphonic acid groups. Al et al. [25] used diallylaminomethylphosphonic acid and 1,1,4,4-tetraallyl piperazinium as raw materials to polymerize the two, and then treated the reacted polymer with NaOH to prepare a polymer adsorbent rich in phosphonic acid groups. This adsorbent has good adsorption effects on both Pb (II) and Cu (II) in aqueous solutions. The adsorbent has a higher adsorption capacity for Pb (II) and faster adsorption kinetics for Cu (II). Research has shown that the adsorption process of adsorbents for two types of heavy metal ions follows both quasi-second-order kinetic models and Langmuir models.

Overall, organic synthetic polymer adsorbents containing P functional groups have complex raw material structures, high preparation costs, and limited applications. Organic synthetic polymer adsorbents containing N functional groups or O functional groups or S functional groups generally have a low coordination atom ratio, which reduces the adsorption capacity, Moreover, the designed functional groups have a single function, only forming coordination compounds to adsorb heavy metal ions.

Usually, strong interactions such as hydrogen bonding, coordination reactions, and charge attraction are formed between functional groups containing “N”,”O”, “S”, and heavy metal ions. So, these functional groups are widely grafted into polymer adsorbents, effectively removing heavy metal ions from aqueous solutions.

The polymer adsorbent TOC containing S and O functional groups was successfully synthesized by thiophene and oxalyl chloride in this article (TOC is the spelling of the first letter of thiophene and oxalyl chloride, representing the polymer they form). As can be seen from Scheme 1, the high content of coordination atoms S and O in the polymer helps to enhance the adsorption capacity of TOC. At the same time, it can be seen that thiophene and carbonyl groups can form large π bonds through π-π conjugation, reducing the energy of the polymer and enhancing its stability.

TOC not only has advantages in structural design, but also has a simple synthesis method and low cost, which will play an important role in the treatment of industrial wastewater containing lead ions and copper ions in suburban areas.

2. Experimental

2.1. Instrumentation and Methods

Flame atomic adsorption spectrometric (FAAS) measurements were carried out on a Perkin Elmer Zeeman 1100 B spectrometer (Uberlingen, Germany) with an air/acetylene flame; Fourier transmission infrared spectra (FT-IR, 4000~300 cm⁻¹) in KBr were recorded on a Nicolet Nexus 470 FT-IR spectrometer (Nicolet, Scotia, NY, USA); Recorded on a micrographs of the adsorbents were obtained at 5.0 kV on a supra 40vp field emission scanning electron microscopy (FEI, Chino, CA, USA); microwave reactor: MKX-H1C1A, Qingdao makewave innovation technology Co., Ltd. (Qingdao, China); high temperature gel chromatograph, equipment model: GPCV2000, equipment factory number: WAT223010, equipment of Warers (Milford, MA, USA); thermogravimetric analysis (TGA) was performed using the NETZSCH STA409PC thermogravimetric analyzer from the German company NETZSCH Instruments (BY, German), with a temperature range of 25–800 °C and a heating rate of 10 °C/min.

2.2. Materials

Thiophene, oxalyl chloride, Cu(NO₃)₂, Pb(OAc)₂ were supplied by Sinopharm Chemical Reagent Co., Ltd. (BeiJing, China), Concentrations of heavy metal solutions were controlled at 1.00 g L⁻¹ in deionized water and were diluted subsequently to different concentration for their next use.

2.3. Synthesis of TOC

We placed a 250 mL three necked flask equipped with a constant pressure dropping funnel and a condenser tube (with a drying tube) in a microwave reactor. Under nitrogen protection, magnetic stirring, and ice bath conditions, we added 50 mL of dichloromethane, 6.76 g of anhydrous AlCl₃ (50.80 mmol), and 3.38 g of oxalyl chloride (25.40 mmol) to a three necked flask in sequence, and stirred evenly. Slowly, we added 2.20 g of thiophene (25.4 mmol) within 20 min, adjusted the microwave power of 200 W in the microwave reactor while adding thiophene, and irradiated for 60 min. Then, we continued stirring for 2 h. We removed the ice water bath and heat to reflux for 3 h. A large amount of yellow brown solid precipitates were filtered, washed, and dried to obtain the product.

2.4. Batch Procedure

First, the maximum adsorption capacity was measured by 0.10 g of TOC with 50.00 mL of various concentrations, respectively, of single-target metal ion solutions. In order to reach the “saturation”, the single-target metal ion concentration was increased until the plateau values (adsorption capacity values) were obtained.

Second, standard 0.10 M hydrochloric acid and 0.10 M sodium hydroxide solutions were used for pH adjustment. The effect of pH on the static removal efficiency of single-target metal ions were examined, respectively, by 0.01 g of TOC with 50.00 mL of sample solutions, containing 50.00 mg L⁻¹ of single-target metal ions under different pH conditions.

Third, the contacting times of single-target metal ions onto TOC were also examined, respectively, by 0.01 g of TOC with 50.00 mL of sample solutions containing 50.00 mg L⁻¹ of single-target metal ions.

In the above batch experiments, the mixtures were dispersed by ultrasonic for 10 min at room temperature. The solution was collected for metal ions concentration measurements. TOC were washed thoroughly with deionized water to neutralize them. The concentrations of metal ions were determined by FAAS. In order to obtain reproducible experimental results, the adsorption experiments were carried out at least 3 times.

The adsorption capacity was calculated as the following equations:

$$Q=\left(C_0-C_e\right)V/W$$

where Q is the adsorption capacity (mg/g); $C_0$ and $C_e$ are the initial and final concentrations of metal ions in the solution (mg/L); V is the solution volume (mL), and W is the dry weight of the adsorbent (g).

3. Results and Discussion

3.1. Characterization

3.1.1. Infrared Spectral Analysis of TOC

Figure 1 shows the infrared spectrum of TOC. The stretching vibration of C=O on oxalyl chloride at 1690 cm⁻¹ in the figure. The absorption peaks at 693, 1438, and 3336 cm⁻¹ are the characteristic absorption peaks of S, C=C, and the attached H on the thiophene ring, respectively.

3.1.2. Molecular Weight Determination of TOC

Table 1. Molecular weights of polymer (soluble part in THF).

Polymer	Mn × 10−3	Mw × 10−3	PDI
TOC	16.42	27.68	1.69

shows the molecular weight of TOC, with number-average molecular weight (Mn), weight-average molecular weight (Mw), and molecular weight distribution (PDI) of 16,420, 27,680, and 1.69, respectively. Mn reflects the average molecular weight of polymer molecules, while Mw focuses more on the contribution of high molecular weight components to the overall molecular weight. Through the analysis of Mn and Mw data, it can be seen that the polymer has formed, but the molecular weight is not very large. It is likely that the observed low molecular weight of the product is directly related to the precipitation of the polymer from the reaction medium as the degree of polycondensation (molecular weight) increases. PDI = Mw/Mn = 1.69 indicates that there is not much difference in the molecular weight between larger and smaller molecules in TOC, and the polymer molecular weight is relatively uniform [26].

3.1.3. EDS Analysis of TOC

Figure 2 shows the EDS spectrum of TOC. From the figure, it can be seen that TOC only contains three elements: C, O, and S, indicating that thiophene and acetyl chloride have successfully synthesized polymer TOC. From the perspective of Wt% and At%, there is not much difference in the mass fractions of O and S elements, with atomic percentages close to 2:1. These data basically conform to the 2:1 ratio relationship between O and S atoms in the polymer TOC structure.

3.1.4. Scanning Electron Microscopy Analysis of TOC

Figure 3 is a scanning electron microscope image of TOC, which shows that TOC has a porous and loose morphology. This may be due to the large volume of thiophene monomer, which may hinder the tight packing of polymer chains and form pores after the condensation reaction of thiophene and acetyl chloride. The morphology of TOC increases the contact area with heavy metal ions in aqueous solution, which can effectively enhance the adsorption of heavy metal ions.

3.1.5. Thermogravimetric (TG) Analysis

Figure 4 is a thermogravimetric diagram of TOC. From the figure, it can be seen that the thermogravimetric curve of TOC reaches a basic constant weight state after undergoing a significant weight loss process. Between 0 and 180 °C, there is approximately 2% weight loss, mainly caused by the evaporation of adsorbed water in the sample. Between 180 and 450 °C, the weight loss of the sample is about 95%, and the reason for this weight loss is related to the thermal decomposition and oxidation of the polymer. After 450 °C, there is basically only a small amount of residue left in TOC, and the weight change is minimal and tends to stabilize.

The weight of the TOC adsorbent sample did not show significant changes before 180 °C, indicating that the TOC adsorbent has good thermal stability.

3.2. Study on Adsorption Performance

3.2.1. Saturated Adsorption Capacity

It can be clearly seen from Figure 5 that the adsorption capacity of TOC increases with the initial concentration of the solution, and tends to saturate at high concentrations. The maximum adsorption capacities of TOC for Pb (II) and Cu (II) were calculated to be 122.7 mg/g (0.593 mmol/g) and 95.9 mg/g (1.498 mmol/g), respectively. TOC has a large adsorption capacity, which may be related to the porous surface of TOC, fully exposed coordination sites, and the ability to maximize contact with adsorbed ions for the most effective adsorption of these ions. The maximum number of ions adsorbed by TOC of the same mass for Cu (II) is greater than that for Pb (II), which may be related to the small particle radius of Cu (II) resulting in a larger number of particles accommodated in the same space.

3.2.2. Influence of pH Value

In Figure 6, the pH value of the solution has a significant impact on the performance of adsorbents in adsorbing metal ions, as it can affect the protonation degree of polar functional groups on the surface of the adsorbent, thereby affecting the surface charge of the adsorbent and the types of heavy metal ions. Appropriate solution pH can not only reduce the interference of environmental factors, but also increase the adsorption capacity of adsorbents [27].

The ionic form of lead ions varies at different pH values. When the pH is low, lead ions are the main form of Pb (II), when 2 < pH < 6, Pb (II) and Pb (OH)⁺ are the main forms of existence. When pH > 6, Pb (II) hydrolyzes to form Pb(OH)₂ precipitate. Similarly, copper ions also exist in different ionic forms at different pHs. When the pH is low, copper ions are the main form of Cu (II), and when the pH > 6, Cu(OH)₂ precipitation occurs in the solution. After repeated experimental testing, it was determined that TOC had the best adsorption effect on lead ions and copper ions when the pH was 4 and 6, respectively [25].

3.2.3. Effect of Contacting Time on the Adsorption Capacity

From Figure 7, it can be seen that the adsorption capacity of TOC increases with the increase of adsorption time, and its adsorption capacity continues to rise. Adsorption equilibrium can be reached within 250 min, with high adsorption efficiency. This may be related to the porous surface morphology of TOC, which increases the surface area of TOC, exposes adsorption sites more fully, and accelerates the adsorption of heavy metal ions. The adsorption time of Cu (II) by TOC is faster than that of Pb (II), which may be related to the smaller particle radius of Cu (II) and its faster movement speed in porous polymers. In order to fully react the adsorbent with heavy metal ions, the time for TOC adsorption of Pb (II) is set to 250 min, and the time for adsorption of Cu (II) is set to 180 min.

3.2.4. Selection of Eluents

The desorption of Pb (II) or Cu (II) adsorbents used acid solutions of different concentrations. The results are shown in

Table 2. Recovery (%) of heavy metal ions using different eluents.

Eluents	Recovery (%)
	Pb (II)	Cu (II)
0.50 mol/L HNO3	78.95	99.61
1.00 mol/L HNO3	86.56	99.58
0.50 mol/L H2SO4	80.35	88.23
1.00 mol/L H2SO4	90.23	90.51
0.50 mol/L HCl	93.51	87.15
1.00 mol/L HCl	99.04	92.53

. At room temperature, 1.00 mol/L HCl and 0.50 mol/L HNO₃ have the best desorption effect on Pb (II) and Cu (II). Therefore, this experiment chose 1.00 mol/L HCl to desorb Pb (II) and 0.50 mol/L HNO₃ to desorb Cu (II).

3.2.5. Reusability of Adsorbents

The effect of repeated use of the adsorbent on the adsorption capacity was tested, and the results are shown in Figure 8. During the 12 cycles of adsorption and elution, the maximum adsorption capacity decreased with increasing usage, which may be due to a small amount of residual heavy metal ions occupying the adsorption sites, hindering the results of readsorption. From the results in the figure, it can be seen that, for Pb (II), after 10 repeated uses, the adsorption capacity of TOC is 111 mg/g; for Cu (II), after 11 repeated uses, the adsorption capacity of TOC was 90 mg/g, and the reuse efficiencies were 90.46% and 93.85%, respectively. This indicates that TOC has excellent reusability, which may be related to its good structural stability. The number of repeated uses of Pb (II) by TOC is slightly smaller than that of Cu (II), which may be related to the larger particle radius of Pb (II), which makes it difficult to elute, and the higher density of lead ions, which damages the adsorbent to a greater extent during elution.

3.2.6. Adsorption Isotherm Model

The adsorption behavior of TOC is described using Langmuir and Freundlich adsorption isotherm equations, and the adsorption processes of Pb (II) and Cu (II) by TOC were fitted using Langmuir and Freundlich adsorption isotherm models, respectively. The adsorption isotherm equation is as follows [28].

$${\displaystyle \frac{C_e}{Q_e}}={\displaystyle \frac{1}{Q_{\mathrm{m}\mathrm{a}\mathrm{x}}b}}+{\displaystyle \frac{C_e}{Q_{\mathrm{m}\mathrm{a}\mathrm{x}}}}$$

$$\ln Q_e=\ln k_F+{\displaystyle \frac{1}{n}}\ln C_e$$

In the formula, Q_e is the equilibrium adsorption capacity (mg/g), C_e is the concentration of metal ions after adsorption reaches equilibrium (mg/L), Q_max is the saturation adsorption capacity (mg/g), and b, k_F, and n are constants.

The fitting results are shown in

Table 3. Isotherm parameters for the adsorption of Pb (II) and Cu (II) by TOC.

Metal Ions	Langmuir Isotherm			Freundlich Isotherm
	Qmax (mg/g)	b (L/mg)	R2	KF	n	R2
Pb(II)	89.68	0.4503	0.6796	34.762	5.32	0.9912
Cu(II)	70.52	0.4079	0.8217	23.562	3.75	0.9851

. The adsorption equilibrium of Pb (II) and Cu (II) ions by TOC can be well described by the Freundlich model, because Freundlich (R² = 0.9912, 0.9851) can better fit the experimental data than Langmuir (R² = 0.6796, 0.8217). At the same time, it was found that the theoretical values of saturated adsorption capacity, calculated according to the Langmuir model, were 89.68 and 71.52 mg/g, which were significantly different from the experimental values of 122.7 and 95.9 mg/g. This also indicates that the Langmuir model cannot be used to fit the experimental data.

According to the Freundlich model data in Table 3, the n values for the adsorption of Pb (II) and Cu (II) by TOC are 5.32 and 3.75, respectively. Therefore, 1/n is between 0 and 1, indicating that adsorption is easy to occur, which is consistent with the faster adsorption rate of TOC. The fitting results are related to the structure of TOC. Due to the stable, porous, and layered structure of TOC, heavy metal ions will dissociate in aqueous solution and fully contact with the adsorption sites hidden deep inside the block solid to form complexes. Therefore, the process of TOC adsorbing heavy metal ions is a multi-layer adsorption process.

3.2.7. Adsorption Kinetics Model

For the adsorption behavior of TOC, pseudo-first-order kinetic models and pseudo-second-order kinetic models are used for fitting. The pseudo-first-order kinetic model describes [29]:

$$\ln \left(Q_{eq}-Q_t\right)=\ln Q_{eq}-k_{1}t$$

In the equation, Q_t is the adsorption capacity at time t (mg/g), Q_eq is the equilibrium adsorption capacity (mg/g), and k₁ (min⁻¹) is the pseudo-first-order rate constant.

The pseudo-second-order kinetic model is as follows:

$${\displaystyle \frac{t}{Q_t}}={\displaystyle \frac{1}{k_2{Q}_{eq}^2}}+{\displaystyle \frac{t}{Q_{eq}}}$$

In the equation, Q_t is the adsorption capacity at time t (mg/g), Q_eq is the equilibrium adsorption capacity (mg/g), and k₂ is the pseudo-second-order rate constant g/(mg·min).

As shown in

Table 4. Kinetic parameters for the adsorption of Pb (II) and Cu (II) by TOC.

Metal Ions	Pseudo-First-Order Kinetic			Pseudo-Second-Order Kinetic
	Qmax (mg·g−1)	K1 (min−1)	R2	Qmax (mg·g−1)	K2 (g·mg−1·min−1)	R2
Pb(II)	61.61	0.2165	0.8924	125.27	0.01657	0.9879
Cu(II)	69.57	0.6321	0.6952	98.75	0.00628	0.9827

, the pseudo-second-order kinetic models (R² = 0.9879, 0.9827) of TOC for Pb (II) and Cu (II) ions can better fit the adsorption kinetics data than pseudo-first-order kinetic models (R² = 0.8924, 0.6952), and the predicted equilibrium adsorption amounts of 125.27, 98.75 mg/g and experimental values of 122.7, 95.9 mg/g using the pseudo-second-order kinetic models are also relatively close. This indicates that the adsorption process of Pb (II) and Cu (II) ions by TOC follows a chemical adsorption process controlled by pseudo-second-order kinetic reactions. The fitting results indicate that the adsorption process of Pb (II) and Cu (II) ions by TOC is achieved through the formation of coordination bonds between the functional groups of TOC and heavy metal ions, which is consistent with experimental analysis and results.

4. Conclusions

(1)

In this article, the polymer adsorbent TOC containing an S functional group was successfully synthesized by thiophene and oxalyl chloride. The successful synthesis of TOC was confirmed by IR, molecular weight determination, EDS, SEM, and TG.

(2)

For Pb (II), the maximum adsorption capacity of TOC can reach 122.7 mg/g (0.593 mmol/g) at pH = 4, 250 min. After adsorption, it can be desorbed with 1.00 mol/L HCl, and the adsorbent can be reused 10 times; for Cu (II), the maximum adsorption capacity of TOC can reach 95.9 mg/g (1.498 mmol/g) at pH = 6, 180 min. After adsorption, it can be desorbed with 0.50 mol/L HNO₃, and the adsorbent can be reused 11 times. The adsorption process of TOC on Pb (II) and Cu (II) conforms to the Freundlich model of multi-layer adsorption, and the chemical adsorption process is controlled by pseudo-second-order kinetic reactions.

(3)

Adsorbents of the same quality adsorb more substances of Cu (II) than Pb (II), in a shorter time and with more repeated use. The reason may be because Cu (II) has a smaller radius, can accommodate more in a limited space, moves faster, and is easier to desorb.