Highly Efficient and Selective Capture of Pb(II) by New Crosslinked Melamine-Based Polymethyl Methacrylate for Water Treatment

Abstract

Owing to the recent developments in the polymer’s properties and application, the demands for designing different structure of polymers are greater than ever. Crosslinked polymers (CPs) are a type of porous materials that have a variety of potential applications. Because of simple methods of modification of polymethyl methacrylate (PMMA), the crosslinked PMMA considers the most commonly polymeric adsorbents. A new crosslinked melamine-based polymethyl methacrylate (C-PMMA/Mel) was prepared via a polycondensation reaction between PMMA and melamine used as a crosslinking agent. Different characterization methods were carried out to investigate the molecular structures, thermal stability, and morphology. C-PMMA/Mel was applied for the adsorption behavior toward different metal cations and detected a selective to Pb(II). The evaluation of the new polymers as adsorbent against Pb(II) ion was studied using the contact time, adsorbent dose, initial concentration, and effects of pH. The adsorption efficiency of heavy metals was improved in the presence of melamine in polymeric matrix. The C-PMMA/Mel has high efficacy in the removal of ~94% of Pb at pH 6 for one hour. Noticeably, the adsorption performance of C-PMMA/Mel perfectly suited with Freundlich isotherm and the pseudo-second-order kinetic model. Additionally, the new materials showed no obvious loss in Pb(II) removal after 7 cycles.

1. Introduction

Today, water pollution has become a key issue of world environmental concern [1]. Heavy metal ion pollution is considered to be the most hazardous substance in wastewater while they are not biodegradable and due to food chain will be collected in living organisms (including humans), causing serious environmental pollution and many diseases. It is threatening the ecosystem that can cause and pose damage and risk to other organisms. This prompted the scientists to further develop more new technologies in the areas of environmental remediation and protection [2]. Water pollution with Pb(II) is a serious concern in the field of health and increasing environmental problems due to its extremely high toxicity even at low concentrations. The main industrial sources of lead heavy metals are the industry of tanning, electroplating, textile, and leather. Lead must be considerably separated from wastewater before it is distributed into the environment or before it needs to be reformed in a less toxic form [3, 4].

Different methods primarily used to remove Pb(II) involve ion exchange, reverse osmosis, chemical precipitation, and electrodeposition [1, 5]. In very dilute solutions, these methods are not only unsuccessful for water remediation but also to be very high cost for removal of Pb(II). Therefore, many approaches have been reported to develop more effective techniques for removing lead from wastewater, for instance, liquid extraction, membrane cleaners, and adsorption [3, 6–8].

However, the most cost-effective processes and safest and direct approaches to efficiently eliminate heavy metals from wastewater are the solid adsorbents that depend on the properties of the adsorbents used. Several materials are considered as adsorbents and are divided into natural and synthetic adsorbents, involving inorganic and organic [9]. Nevertheless, these adsorbents have the disadvantage of low capacity and long equilibrium time. Therefore, higher capacity sewing material in less time is considered a water treatment challenge. Recently, there has been enhanced interest using polymeric materials for water treatment from heavy metals due to the ability to create an active motif on their chains to enhance the capability for better adsorption [10–12].

The important class of porous materials is crosslinked polymers (CPs) that can be applied in adsorption applications because of low density, high surface area, and large stability [13]. CPs also consist of inexpensive components and are highly stable and scalable. In addition, these materials have excellent thermal and chemical stability and can be prepared on a large amount [14–16]. Crosslinked polymethyl methacrylate is considered as one of the very important polymeric adsorbents due to simple modification methods, and the chemical modifications of PMMA can improve the physical and chemical properties with different functionalities. A few studies have been conducted using crosslinked PMMA as an absorbent material for environmental applications [17–19].

Owing to the features of melamine like nitrogen content, high surface area, tunable basic functionalities, great porosity, and relatively low-cost starting materials remain an interesting field of research in polymer chemistry [20, 21]. Melamine-based polymeric materials have attracted considerable attention used as an absorbent material to make novel potential applications [22–26]. This study designed to use the (triamine) melamine as a crosslinker for PMMA to prepare new crosslinked PMMA, termed C-PMMA/Mel using polycondensation methods. Then, the new materials were applied as adsorbents towards heavy metals for water treatment. This attempt opens the research gate toward outstanding adsorbents with good properties that can suffer the limitations stated above. Overall, this research presents a new approach for constructing low-cost adsorbents and high efficiency to remove heavy metals, and it can be applied for environmental treatment.

2. Experimental

2.1. Materials

Poly (methyl methacrylate) (PMMA, $M_{wt} = 300 k$ ) has been obtained from Alfa Aesar. Dimethyl sulfoxide (DMSO 99%) and melamine (97.5%) were purchased from BDH laboratory reagents. Zn(NO₃)₂·6H₂O (99%), Cd(NO₃)₂·4H₂O (99%), Pb(NO₃)₂·6H₂O (99%), and MnCl₂·4H₂O (98%) were supplied by Fisher chemicals; NiCl₂·6H₂O (97%) was supplied by BDH chemicals; Ba(NO₃)₂·4H₂O (99%) was provided by Ward’s Natural Science. Cr(NO₃)₃·9H₂O (97%) was provided by Panreac and NaOH and HCl (35%) supplied from LOBA Chemie. All chemicals were used without further purifications.

2.2. Synthesis of C-PMMA/Mel

An evacuation-argon-backfill cycle was applied three-necked flask connected with a condenser and magnetic stirrer. So, 1 g of PMMA and different amounts of melamine (10, 20, and 30%) were added into 30 mL DMSO and heated to 175°C for three days under argon flow. Then, the product was collected by filtration after cooling and washed with methanol, THF, and dichloromethane to give a $yield = 85 %$ .

2.3. Adsorption Study

To study the adsorption of polymer, a standard solution (20 mg/L) of different metal nitrates/chlorides was prepared. MnCl₂·4H₂O, Pb(NO₃)₂·6H₂O, NiCl₂·6H₂O, Cd(NO₃)₂·4H₂O, Cr(NO₃)₃·9H₂O, Ba(NO₃)₂·4H₂O, and Zn(NO₃)₂·6H₂O were dissolved in deionized water (DI) under continuous mixing. 20 mg of C-PMMA/Mel was added into 100 mL of the standard solution and stirred for one hour followed by filtration. The concentrations of metals were measured by ICP using adsorption capacity $q_{e}$ (mg/g), $q_{t}$ (mg/g), and the removal efficiency $R %$ calculated according to the following equations. $\begin{matrix} (1) & q_{e} = \frac{(C_{i} - C_{f}) V}{m}, \\ (2) & q_{t} = \frac{(C_{i} - C_{t}) V}{m}, \\ (3) & R % = \frac{C_{i} - C_{f}}{C_{i}} \times 100, \end{matrix}$ where the amount of adsorbent represents as $m$ (g), $C_{i}$ is the initial concentrations of metal cations (mg/L), $C_{t}$ is the concentrations of metal cations after time, and $C_{f}$ is the final concentrations of metal cations (mg/L). $V$ is the volume (L) of the solution. To study the effect of the adsorbent efficiency to remove the heavy metals, different factors were utilized such as effect of different metals, pH value, contact time, initial concentration of Pb²⁺, and adsorbent dosage.

2.3.1. pH Effect

At different pH values 3, 4, 5, 6, and 7, a standard solution 20 mg/L Pb(II) was prepared. To adjust the pH of the metal solution, 1 M NaOH and 1 M HCl were used. Then, at each pH value, 20 mg of C-PMMA/Mel2 was added into 100 mL of Pb(II) solution for 1 h.

2.3.2. Effect of Adsorbent Dose

Different adsorbent dose 10, 20, 30, and 40 mg of C-PMMA/Mel2 was weighted and added to 100 mL of 20 mg/L Pb(II) solution for examining the adsorption of Pb(II) ions at $pH = 6$ .

2.3.3. Effect of Contact Time

In 100 mL of 20 mg/L Pb(II) solution, a sequence of time 10, 30, 60, 90, and 120 min was tested to study Pb(II) removal using 20 mg adsorbent in each at $pH = 6$ .

2.3.4. Effect of Initial Concentration

A series of concentration of Pb²⁺ solutions 5, 10, 20, 50, 100, and 200 mg/L were prepared to study Pb(II) removal in 100 mL at $pH = 6$ and 20 mg adsorbent for 1 h.

2.3.5. Reusability of C-PMMA/Mel2

In the desorption experiment, 50 mL HCl (0.5 M) and 50 mL ethanol were used for desorption method to renew the adsorbent from the Pb(II)@ C-PMMA/Mel complex. After stirring for 3 h at 25°C, the sample was rinsed using DI water then recycled using the adsorption–desorption process. The adsorption–desorption cycle was repeated 7 times.

2.4. Instrumentation

Thermogravimetric analysis (TGA) was performed on TGA4724 with a heating rate of 10°C/min between 25 and 500°C under N₂ atmosphere. Fourier transform infrared spectra (FTIR) with a Nicolet Magna 6700 FT spectrometer were conducted in a wavenumber region (400-4000 cm⁻¹). X-ray diffraction (XRD) patterns were studied using a Bruker D8 Advance with Cu Kα radiation (wavelength 1.5418 Å) at 40 kV and 40 mA. The patterns were collected between $2 θ$ of 10° and 60°, and the scan speed was 1.5 degree/min. Scanning electron microscopy (SEM) imaging was performed with a FEI TENEO VS microscope equipped with an EDAX detector. Polymer sample was mounted on the aluminium stub using adhesive carbon tape and sputter coated with 3 nm iridium to avoid sample charging during imaging. Inductively coupled plasma optical emission spectroscopy (ICP-AES) on a PerkinElmer Optima 7000 DV was utilized to determine the metal contents.

3. Results and Discussion

3.1. Structural Investigation

As listed in Table 1, PMMA was chemically reacted with different ratios from aromatic triamines (melamine) as a crosslinking agent to create C-PMMA/Mel(1–3) by the polycondensation technique in the presence of DMSO as a solvent at 175°C for 72 h. Figure 1 illustrates the schematic representation of the crosslinking process to prepare C-PMMA/Mel.

Table 1

Chemical compositions of PMMA and melamine for C-PMMA/Mel(1–3).

Sample	$w / w %$ PMMA	$w / w %$ melamine
C-PMMA/Mel1	90	10
C-PMMA/Mel2	80	20
C-PMMA/Mel3	70	30

Figure 1

Synthetic routes to prepare C-PMMA/Mel.

FTIR spectroscopy investigates the connections between PMMA and the melamine as crosslinking agent for the new polymer from the range 600 to 4000 cm^-1 as displayed in Figure 2(a). The spectrum of linear PMMA showed that the stretching vibrations (C–H) in CH₃ and CH₂ have absorption peaks at 2993 and 2948 cm^-1, respectively [27]. The signal of carbonyl group (C=O) stretching vibration appeared as a sharp peak around 1725 cm^-1. Peaks at 1242 and 1149 cm^-1 are attributed to the stretching vibration (C-C-O) of –O–CH₃. The peaks of primary amino groups in the melamine appeared at wavelength 3442-3360 cm^-1. FTIR spectra of C-PMMA/Mel2 demonstrate the formation of amide bonds by the presence of bands at 3365 and 1188 cm^-1 assigned to the stretching vibrations and deformation vibrations of secondary amine (NH) [17, 19].

Figure 2

(a) FTIR spectra of the C-PMMA/Mel2. (b) XRD pattern of pure PMMA and the synthesized C-PMMA/Mel2.

(b)

Figure 2(b) reveals the XRD patterns of the pure PMMA and the synthesized C-PMMA/Mel2. The diffractograms of XRD were analyzed in the $2 θ$ range from 10° to 60°. The broadened peaks at 43°, 30°, and 15° are assigned for pure PMMA. The peak of C-PMMA/Mel2 crosslinked polymer was found to be a broad peak and absence of any sharp peaks observed related to melamine. According to the high level of crosslinking interaction between PMMA and melamine, the XRD diffractogram of new polymer differs from pure melamine [28] and the loss of the melamine sharp peaks.

3.2. Thermal Study

As demonstrated in Figure 3, the thermal stability of pure PMMA and C-PMMA/Mel(1–3) crosslinked polymers was investigated by TGA. The TGA curve of pure PMMA reveals that the degradation occurs mainly in one step ranged between 250°C and 345°C. The result shows a considerable increase in the thermal presentation of polymer network C-PMMA/Mel(1–3) compared to the pure PMMA, and the crosslinking influenced on the stability of polymer. $T_{50}$ , $T_{30}$ , and $T_{10}$ are the temperature of percentages of decomposition that explain the mass loss of 50%, 30%, and 10%, respectively, as listed in Table 2. $T_{50}$ , $T_{30}$ , and $T_{10}$ of C-PMMA/Mel(1–3) were higher than those of pure PMMA. It is observed that C-PMMA/Mel2 has the highest thermal stability than C-PMMA/Mel1 and C-PMMA/Mel3.

Figure 3

TGA micrograph of pure PMMA and C-PMMA/Mel(1–3).

Table 2

Thermal properties of pure PMMA and C-PMMA/Mel(1–3).

Sample	$T_{10}$	$T_{30}$	$T_{50}$
Pure PMMA	345	315	300
C-PMMA/Mel1	360	340	330
C-PMMA/Mel2	405	385	365
C-PMMA/Mel3	380	360	345

3.3. Morphological Investigation

Scanning electron microscope (SEM) measurements were typically applied to assess the surface of C-PMMA/Mel as shown in Figure 4. Pure PMMA was displayed as a smooth surface [29] while the pure melamine shows an irregular stone-like structure with a size of ten micrometers [28]. As an example, the SEM image of C-PMMA/Mel2 crosslinked polymer shows in micrometer range a roughness surface with pores with irregular tiny particles. With the addition of melamine as a crosslinker, the surface morphology of PMMA significantly changed from smooth to amorphous character with increased porosity and deformation of the circular holes confirming the excellent interaction between melamine and PMMA and the crosslinking process effectively occurred.

Figure 4

SEM micrographs for C-PMMA/Mel2 at different magnifications.

3.4. Metal Adsorption

Table 3 shows the adsorption capacities (mg/g) of C-PMMA/Mel(1–3) toward different metal cations and C-PMMA/Mel(1–3) has very good uptake with superior selectivity towards Pb(II) cation as displayed in Figure 5. The significant Pb(II) adsorption capacity observed by C-PMMA/Mel(1–3) is a major phenomenon may be due to the following: (i) amino groups can very effectively bind to the Pb(II) cation, since they act as coordinating ligands [26]; (ii) the specific surface shape and random structures are very useful for the adsorption of heavy metal ions [30]. According to the highest adsorption capacity of C-PMMA/Mel2 toward Pb(II), C-PMMA/Mel2 and Pb(II) cation were selected for the following adsorption experiments.

Table 3

Adsorption capacities $q_{e}$ (mg/g) of C-PMMA/Mel(1–3) and removal efficiency ( $R %$ ) toward different metal ions at initial $pH = 6$ and adsorbent dose of 20 mg.

Sample	Metal ion	$q_{e}$ (mg/g)	Removal efficiency ( $R %$ )
C-PMMA/Mel1	Pb(II)	43.479	71.96
	Cr(III)	2.337	32.53
	Mn(II)	4.3485	12.79
	Ba(II)	8.1255	10.76
	Cd(II)	10.6115	21.84
	Ni(II)	3.592	16.61
	Zn(II)	6.665	20.17

C-PMMA/Mel2	Pb(II)	88.67	93.96
	Cr(III)	2.49	24.27
	Mn(II)	6.78	19.97
	Ba(II)	17.68	23.42
	Cd(II)	11.75	24.19
	Ni(II)	4.68	21.64
	Zn(II)	7.04	21.30

C-PMMA/Mel3	Pb(II)	38.1725	63.17
	Cr(III)	2.402	23.41
	Mn(II)	5.0525	14.86
	Ba(II)	10.916	14.46
	Cd(II)	9.4	19.35
	Ni(II)	3.926	18.16
	Zn(II)	6.2945	19.05

Figure 5

Schematic illustrations of the newly synthesized C-PMMA/Mel on the removal of Pb(II).

The effect of pH on Pb²⁺ removal is shown in Figure 6(a). The adsorption capacity increases while pH increases from pH 3 to 6 and then decreases at $pH = 7$ . As a result, the addition of an acid that protonates the nitrogen atom of C-PMMA/Mel2 creates a positively charged surface that electrostatically repels metal ions [31]. If the pH is increased, the surface charge of the polymer converted into negative due to the nitrogen atom is totally deprotonated. Therefore, the adsorption capacities of Pb(II) were considerably elevated due to the electrostatic attractions between ions of different charges [32]. However, when pH exceeds 6, OH- interacts with Pb(II) cations, resulting in the production of PbOH⁺ and Pb(OH)₂. Under these conditions, the mechanism of Pb(II) removal becomes more difficult, and it becomes difficult to distinguish the precipitation and adsorption of Pb(II) in solution [33]. Thus, at pH 6 the maximum adsorption capability for Pb²⁺ removal was found 88.67 mg/g by C-PMMA/Mel2 crosslinked polymer which was then used in the following adsorption studies.

Figure 6

Impact of (a) pH, (b) initial concentration of Pb²⁺ metal ions, and (c) C-PMMA/Mel2 dose of Pb(II) adsorption.

(b)

(c)

Different adsorbent dosages from10 to 40 mg were applied as shown in Figure 6(b). The maximum adsorption capacity was 84.5 mg/g using adsorbent dose 20 mg used in further adsorption experiments. The influence of different initial concentrations of Pb(II) on adsorption capability of C-PMMA/Mel2 for Pb²⁺ is revealed in Figure 6(c) (20 mg of adsorbent, pH 6.0). Clearly, $q_{e}$ increased from 80 to 450 mg/g while the concentration raised from 0 to 200 mg/L. In order to study Pb(II) removal at low concentration, 20 mg/L was used in further adsorption experiments.

3.5. Adsorption Kinetics

To study the adsorption ability of a polymer adsorbent, the kinetic method was carried out by immersing 20 mg of C-PMMA/Mel2 into 100 mL of a Pb²⁺ solution (20 mg/L, pH 6.0) at 25°C.

As found in Figure 7(a), the adsorption of Pb(II) rapidly happened in the first 60 minutes, and the equilibrium was reached after 120 minutes. The adsorption behavior of C-PMMA/Mel2 is most likely due to its porous network architecture, and the pseudo-first-order model (PFO) and the pseudo-second-order model (PSO) were carried out to analyze the adsorption dynamic in Figures 7(b) and 7(c).

Figure 7

(a) Impact of contact time on removal of Pb(II). (b) Pseudo-first-order. (c) Pseudo-second-order.

(b)

(c)

Pseudo-first-order model: $\begin{matrix} (4) & \log (q_{e} - q_{t}) = \log (q_{e}) - \frac{K_{1} t}{2.303} . \end{matrix}$

Pseudo-second-order model: $\begin{matrix} (5) & \frac{t}{q_{t}} = \frac{t}{q_{e}} + \frac{1}{K_{2} q_{e}^{2}}, \end{matrix}$ where $q_{e}, q_{t}$ (mg/g) are the adsorption capacity at equilibrium and the adsorption capacity at time (min), respectively. $k_{1}$ (min^-1) and $k_{2}$ (g/mg·min) are the rate constants in PFO and PSO models, respectively. Consistent with the PFO and PSO models, the linear plots of $\ln (q_{e} - q_{t})$ or $(t / q_{t})$ against time (min) are illustrated in Figures 7(b) and 7(c). By comparing the correlation coefficient ( $R^{2}$ ) for PFO and PSO, the value of the PFO model (0.63934) was lower than that of the PSO model (0.99777), demonstrating that the adsorption process of Pb(II) was controlled by chemisorption mechanism [34].

3.6. Adsorption Isotherms

To estimate the interaction properties between adsorbent and adsorbate, isotherm models have been extensively utilized [35]. The equations of Langmuir and Freundlich models are presented as follows: $\begin{matrix} (6) & \frac{C_{e}}{q_{e}} = \frac{C_{e}}{q_{m}} + \frac{1}{{b q}_{m}}, \\ (7) & \log q_{e} = \log k_{F} + \frac{1}{n} \log C_{e}, \end{matrix}$ where $q_{e}$ (mg/g) symbolizes the equilibrium sorption capacity, $q_{m}$ (mg/g) reveals the maximum adsorption capacity, $b$ and $K_{F}$ (L/mg) are the Langmuir and Freundlich constants, respectively, and the equilibrium concentration in solution is $C_{e}$ (mg/L). Figures 8(a) and 8(b) demonstrate the Langmuir and Freundlich schemes for C-PMMA/Mel2 on Pb(II) adsorption. Noticeably, the $R^{2}$ value (0.77132) for Freundlich model is higher than the $R^{2}$ value (0.40618) of Langmuir model. Therefore, the adsorption methods of C-PMMA/Mel2 for Pb(II) were clearer defined by the Freundlich model, representing the multilayer adsorption of Pb(II) onto C-PMMA/Mel2 [36].

Figure 8

(a) Langmuir model and (b) Freundlich model for Pb²⁺ adsorption.

(b)

Various adsorbents with the new adsorbent in this study are listed in Table 4 for comparing the adsorption capacities towards Pb(II). Thus, the current adsorbent has great adsorption capacities, and it can be effectively used for removal of Pb(II) from wastewater.

Table 4

Comparison between various adsorbents on the adsorption capacities $q_{e}$ (mg/g) for Pb(II) removal.

Adsorbent	$q_{e}$ (mg/g)	Ref.
PMMA-g-Alg/Fe₃O₄	62.50	[37]
PEI-immobilized PMMA microspheres	28.32	[38]
Melamine-vanillin polymer (MVP)	8.53	[25]
Melamine- pyridine polyaminal network (MA-Py)	53.13	[26]
C-PMMA/Mel2	88.67	This study

To study the effectivity of the treatment method, adsorbent recycling was applied. As shown in Figure 9, the polymer can be recycled a maximum of three times without losing its efficiency after performing the adsorption and desorption method seven times. The removal efficiency after three recycling runs at optimum conditions was 94%, 88%, and 74%, respectively.

Figure 9

Adsorption–desorption recycles of Pb(II) ( $m = 20 mg$ , $V = 100 mL$ , $pH = 6$ , and $initial concentration of Pb (II) = 20 mg / L$ ).

4. Conclusions

A new crosslinked melamine-based PMMA has been synthesized, and the results demonstrated a selective absorbent property against Pb(II) from aqueous solutions. Different techniques such as XRD, FTIR, TGA, and SEM were utilized to characterize the new material. The structural study confirmed the chemical interaction between PMMA and melamine, while the morphological analysis proved the formation of crosslinked C-PMMA/Mel, followed by the evaluation of the new crosslinked as adsorbent for Pb(II) using different pH value and the adsorbent dose. As a result, the new crosslinked polymer has high adsorbing efficiency for Pb(II) removal at one hour with removal efficiency equaled 94% at pH value equaled 6. This attempt opens the door toward outstanding adsorbents with good properties and low cost for removing heavy metals, and it can be applied for environmental treatment.

Footnotes

Data Availability

No data were used to support this study.

Conflicts of Interest

The author declares that there are no conflicts of interest.

Acknowledgments

This research work was funded by Institutional Fund Projects under grant no. IFPDP-244-22. Therefore, the author gratefully acknowledges the technical and financial support from the Ministry of Education and Deanship of Scientific Research (DSR), King Abdulaziz University (KAU), Jeddah, Saudi Arabia.

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