Introduction

Today, the release of heavy metals such as palladium, copper, etc., in the aquatic environment due to industrial effluents, soil washing, metal extraction, plating facilities, petroleum products, factories, and manufacturing industries has been enormously increased [1]. The increase of mentioned toxic pollutants in the environment has created severe global issues [2], because of the adsorption of such pollutants by living organisms like plants and animals, obviously the most harmful to fish and other aquatic living beings [3]. Heavy metals are indestructible; as a result, their biological accumulation in the human body by the food chain threatens public health [4, 5].

Palladium and copper metals are the primary heavy metals pollutants in the environment because of their high application in significant quantities. In this respect, the World Health Organization (WHO), reported them as possible carcinogen elements [6]. In addition, disorders in the circulatory and nervous systems and severe damage to the liver and kidneys are some of the reported perilous side effects of such heavy metals in the environment [7]. Moreover, some other advanced studies proved that the side effects of these pollutants on children are more than the adults [8, 9].

A study on the death of 86 copper smelting workers in a metal smelter by Mr. Lars Gerhardsson shows the enormous dangers of these pollutants [10].

To date, several methods have been reported for removing such pollutants, including the use of EDTA-functionalized bamboo activated carbon (BAC) [11], anodic oxidation and electrocoagulation [12], composite PVDF membrane for microfiltration [13], the chemical composition of various clay minerals [14], nitrogen-functionalized carbon nanotubes [15], modified polymer resins [16], silica-gel supported with hyperbranched polyamidoamine [17], sodium dodecyl sulfate-containing polyurethane foam [18], walnut shell loaded semi-IPN based polyacrylic acid and polyvinyl alcohol bio-adsorbent [19].

The application of polymer adsorbents is increasing due to the ease of fabrication, affordability, and use of plastic waste [20, 21]. Among the methods mentioned above, polymer adsorbent foams have been considered one of the most effective materials for heavy metal removal. Polymer foam is formed by trapping gas pores in liquid or solid polymers. Closed-cell and open-cell are two kinds of solid polymer foam. In the first type, the closed-cell foam, the gas form discrete bubble-shaped pores surrounded by the solid material. The bubble-shaped pores are connected by the second type, the open-cell foam [22, 23]. Water can efficiently flow throughout the structure, and air can be displaced in this type. Water purification filter foam uses an open-cell foam [24, 25]. One of the most promising polymers used is polylactide, based on renewable biological sources such as corn starch, tapioca roots, chips, sugarcane, etc. and it was selected in this study. Amino polylactide foam (APLAF) was obtained by synthesizing amino polylactide as a biodegradable polymer containing adsorbent functional groups. APLAF was tested to get an excellent adsorbent with a suitable molecular cell structure, thermal resistance, and the ability to retain various heavy metal contaminants such as palladium and copper in its structures. The pores of amino polylactide foam (APLAF) contain nitrogen groups, and these groups can efficiently adsorb polluting metal ions such as copper(II) and palladium(II).

Experimental

Materials and Equipment

The l-lactide was purchased from Purac and stored under an N2 atmosphere. 2,6-Pyridinedimethanol, glutaryl chloride, triethylamine (Et3N), Tin octanoate [Sn(Oct)2], and solvents such as toluene, n-hexane, dichloromethane (CH2Cl2), deuterodichloromethane (CD2Cl2), chloroform (CHCl3) were purchased from Aldrich (United States). ATR-FTIR spectra were collected using an IR-Affinity spectrophotometer (Shimadzu) in the range of 400–4000 cm− 1 equipped with an ATR Golden Gate accessory with a single reflection diamond. NMR Varian Mercury 400 collected 1H-NMR spectra using CD2Cl2 as solvent. Images were gathered using an Optical Microscope model Nikon Diaphot 300, passing bright-field with a lamp Nikon PSM-2120. The images were obtained from a CCD Digital Sight DS-U Nikon. The micronization process was realized using a micronized machine homogenizer high-pressure (Model 110 S; Newton, Microfluidics, MA, USA) and a Y cell. The thermal behavior of prepared samples was studied with thermogravimetric (TG/DTG/DTA) analysis, using a Mettler Toledo Star TGA/SDTA 851e Module between 30 and 600 °C under a nitrogen atmosphere with a Thermal heating rate of 10 °C/min.

Gel Permeation Chromatography (GPC) was carried out with an HPLC 1525 Waters Binary system, using a column calibrated with polystyrene (PS) up to 400,000 g mol−1 and a refraction index detector (RI). SEM images acquisition, ionic imaging, cross-sectioning, and EDX analysis were performed using a TESCAN GAIA 3 FIB/SEM microscope equipped with an EDX Octane Elect Super EDX detector. The microscope hosts a 30 kV Triglav electron column and a Cobra-Focused Gallium Ion Beam column.

Synthesis of Amino Polylactic Acid (APLA)

APLA was synthesized using the following procedure via Ring-Opening Polymerization (ROP) of l-lactide [26,27,28,29]. Firstly, l-lactide (0.69 mol, 100 g), 2,6-pyridinedimethanol (0.17 mol, 24.13 g), toluene (100 mL), and tin octanoate [Sn(Oct)2] (0.124 g dissolved in 1 mL toluene) [30], were placed in a three-necked round-bottom flask (250 mL) equipped with a mechanical stirrer, an oil bath, thermometer, condenser, and inert gas N2 atmosphere. The flask was heated up to 135 oC under a nitrogen atmosphere and moderate stirring for 12 h (300 rpm). Afterward, the flask was cooled to 0 °C with an ice bath, and triethylamine (Et3N, 0.5 mL) was added with a syringe, followed by glutaryl chloride (0.130 mol, 22.06 g) using a dropping funnel. The white smoke formed after adding glutaryl chloride indicates the release of triethylamine chloride and the completion of the reaction. The reaction was continued for 30 min, then the temperature was raised to 135 °C with a heating rate of 1 °C/min. The reaction was continued for 3 h at this temperature; next, the flask was cooled to 50 °C, and volatiles was removed under reduced pressure (400 mm/Hg) for 10 min. To remove all unreacted monomers and the catalyst, 100 mL of dichloromethane solvent was added to the residue and stirred to complete solubilization. The solution was placed into a 1000 mL glass beaker, and 200 mL of n-hexane was added to obtain a white residue. The residue was collected through filtration and dried in a vacuum oven at room temperature for 12 h. (Fig. 1 and Scheme 1).

ATR-FTIR (Fig. 5D): 3405, 3373 (OH and NH), 2998 (CH2), 2949 (CH3), 1746 cm− 1 (C=O), 1082 cm− 1 (C–O). 1H-NMR (CD2Cl2), δ (ppm): 7.71 (dd, 1 H, pyridine ring), 7.25 (d, 2 H, pyridine ring), 5.22 (s, 2 H, CH2), 5.15 (q, 1 H, O–CH–C=O), 4.34 (q, 1 H, –CH–OH), 2.32 (t, 4 H, CH–C=O), 1.70–1.61 (m, 4 H, –CH*2–CH2–C = O), 1.54 (s, 3 H, –CH3).

Fig. 1
figure 1

Apparatus for the synthesis of amino polylactic acid (APLA)

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Scheme 1
scheme 1

Synthesis of amino polylactic acid (APLA)

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Synthesis of Amino Polylactide foam

For the first time, a CH2Cl2 suspension of amino APLA was micronized. Amino polylactide (150 g) was placed in a 1000 mL glass beaker and dissolved in 160 mL of dichloromethane using a magnetic stirrer rotating at 300 rpm, then distilled water (150 mL) was added. The color of the solution changed from clear to milky yellow, while spherical particles were visible to the naked eye in the solution. The micronization process was performed for 20 min by injecting the solution into the micronization machine. Samples were collected and they were dried, and particle sizes were detected using an optical microscope. The solution was placed into a 1000 mL beaker containing a giant magnet and a syringe of n-hexane (200 mL) was added. The solution was stirred at 300 rpm, and the solution became biphasic after 5 min, and spherical particles were formed. The liquid phase was separated from the emulsion phase. The emulsion phase was collected and dried in a vacuum oven at a pressure of −760 mm/Hg, obtaining the amino polylactide foam (APLAF). A Scheme of the procedure is reported in Fig. 2.

The thermal properties of APLAF were studied using thermogravimetric (TG/DTG/DTA) analysis, while the surface morphology of APLAF was tested with SEM analysis.

Fig. 2
figure 2

 A Scheme of micronization of APLA and preparation of APLAF

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Polymer Adsorption Experiment

The efficiency of Pd(II) and Cu(II) ions adsorption in a water solution by amino poly lactide foam (Fig. 3) was tested at pH = 2, 4, 6, 7, 7.5, and 9.4. Measurements were conducted at varying contact times from 1 to 2 h using suspension with different copper and palladium ions concentrations. Stock suspension of copper and palladium ions with a 1000 mg/mL concentration was prepared. A solution containing concentrations of Pd(II) and Cu(II) metal ions of 10, 12, and 15 mg/L was obtained. APLAF adsorbent (20, 25, 50, 75, and 100 mg/L) was added to the solutions. At the test' end, the solution′s ion concentration was evaluated. The % removal of metal ions was calculated using the following equation [31].

$${\text{\% Removal = }}\frac{{\left( {{\text{Ci }} - {\text{ Cf}}} \right)}}{{{\text{Ci}}}}{\text{ }} \times {\text{100}}$$

Ci is the initial concentration of the ion in the solution (mg/L) and Cf is the final concentration of the ion in the solution (mg/L).

Fig. 3
figure 3

Structure of metal ions adsorbent

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Preparation of Amino Polylactide foam (APLAF) Films

The film was prepared by adding 5 g of a solution of micronized APLAF solution on a hand-made aluminium foil disc (diameter 5 cm) and drying it in ambient conditions for 48 h; then, it was dried for 6 h at 60 °C and followed by vacuum drying at room temperature for 12 h obtaining a dried film (typically about 1.0 mm thick) [32]. The film was stored under a vacuum in a desiccator at room temperature. A sample of the prepared film is reported in Fig. 4.

Fig. 4
figure 4

Preparation of APLAF films

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Results and Discussion

Amino polylactide foam (APLAF) was synthesized in three steps: (i) Synthesis of amino polylactide (APLA), (ii) APLA micronization process, (iii) Film formation by precipitation from a solution of APLA.

APLA was synthesized by ring-opening living polymerization of L-lactide in the presence of tin octanoate [Sn(Oct)2] as a catalyst. Even if it contains traces of stannous alkoxides, the final polymer was approved for biomedical and industrial applications by the Food and Drug Administration (FDA) [33, 34]. The reaction between tin octanoate and 2,6-pyridinedimethanol resulted in the catalyst/initiator that initiates the ROP of the lactide obtaining the APLA polymer. The reduction in the concentration of the 2,6-pyridinedimethanol initiator, relative to the l-lactide monomer, also choosing a molar ratio of amino lactide oligomer synthesized and glutaryl chloride added increased the molecular weight of the product. In this way, the desired molecular weight was reached. In the micronization step, a yellow APLA emulsion was formed starting from a solution in dichloromethane through the water addition. In the third step, by adding n-hexane to the micronized APLA emulsion, the microparticles adhered one each other, which was evident to the naked eye. They were settled at the bottom of the container, collected, and dried as reported in the experiment obtaining the desired film of APLAF.

ATR-FTIR Analysis

The ATR-FTIR of APLA is reported in Fig. 5D. For comparison, the ATR-FTIR spectra of l-lactide (Fig. 5A), 2,6-pyridinedimethanol (Fig. 5B), and glutaryl chloride (Fig. 5C) are also reported in this Figure. The adsorption of APLA may be attributed as follow: the broad peak from 3337 cm− 1 is due to the O–H groups indicating the completion of the reaction; the peak at 1746 cm−1 is due to the carbonyl stretching vibrations; in the peaks at 3002, 2949, and 2880 cm− 1 are attributed to the asymmetric and symmetric C–H stretching vibration of methyl and methylene groups, and the peak between 1600 and 1450 is due to C=C stretching of the aromatic ring, while the peaks at 1185 and 1082 cm− 1 are due to C–O–C and C–O stretching vibration.

Fig. 5
figure 5

ATR-FTIR spectra of APLA

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1H-NMR Analysis

One of the most efficient methods for determining polymer structure is 1 H-NMR spectroscopy. The spectrum was recorded at room temperature, and TMS was employed as the internal standard, with CD2Cl2 as a solvent; the 1 H-NMR spectrum of APLA is shown in Fig. 6, and the number of hydrogen and the number of branches of hydrogens corresponding to each peak are specified using the Latin alphabet both in the Figure and the text, singlet peak with s, doublet peak with d, triplet peak with t, quartet peak with qa, and multiplte peak with m. The resonance for each peak is shown in ppm. At 7.25 (dd, 2 H, a-α-pyridinedimethanol), 7.18 (dd, 4 H, b-β-pyridinedimethanol), 5.56 (s, 2 H, e –OH), 5.21 (s, 8 H, c-CH2–C5H3N), 5.15 (qa, 10 H, d –O–CH–C = O ), 4.34 (qa, 2 H, f –CH–OH), 2.35 (t, 4 H, g-CH2–C = O), 1.73 (m, 2 H, i-CH2–CH2–CH2), 1.56 (d, 30 H, h-CH3), 1.48 (d, 6 H, j-CH–CH3).

Fig. 6
figure 6

1H-NMR spectrum of APLA

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The Molecular Weight of APLA

Molecular weight is an essential parameter of polymeric materials, and this data is reflected in the density of the polymer. The absence of a by-product (similar to the water) in the ROP improves the control of the MW and avoids reverse reactions and unwanted hydrolysis. In this study, selecting the molar ratio among reactants (lactide, 2,6-pyridinedimethanol, and glutaryl chloride) was possible to change the MW of the polymer. The molar ratio of the reagent employed in the syntheses was reported in Table 1; Fig. 7, indicating that with an increased molar ratio of oligomer and glutaryl chloride, the density of the amino polylactide foam (APLAF) also increases.

Table 1 MW and density of the synthesized polymeric materials (films) (The molar ratio of l-lactide to 2,6-pyridinedimethanol for synthesis oligomer is 4:1)
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Fig. 7
figure 7

The molecular weight curves of APLAF I (A), APLAF II (B), APLAF III (C), and APLAF IV (D)

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Optical Microscope Images of Micronized Powder Particles of APLA

APLA emulsion solution was prepared and introduced in the micronization apparatus, separated through sedimentation, and the micronized particles dried. The optical microscope images reported in Fig. 8 show that the emulsion and micronization process was successfully done, and spherical micronized particles in the range of 50 microns in diameter were obtained.

Fig. 8
figure 8

A and B Optical microscope images of micronized particles of APLA, and B is an enlargement of part of (A)

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The optical microscope images of APLAF showed bubble-shaped gas pores (Fig. 9) with different size bubbles. Liquid regions separated the gas bubbles. The micronized spherical particles were formed during drying when water and dichloromethane solvent was removed from the solid particles. If the bubbles are in the case of a press or a closed-door mould, they will be in better order, when drying occurs. Therefore, in the surface image of the foam, bubbles can be seen sticking together and have been in the labyrinth of filaments polylactide.

Fig. 9
figure 9

Optical microscope images particles of APLAF

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Thermal Properties

TG /DTG /DTA determinations were run to study the thermal properties of APLAF I, APLAF IV, and amino polylactide foam with Pd and Cu containing APLAF. In the TG curves, the weight loss (W) of the sample relative to temperature (T) under a 60 mL/min rate of inert gas (nitrogen) and 10 °C/min rates of increased temperature from lab temperature to 700 °C in terms of temperature (T) and weight loss on the y-axis in descending order and the weight gain in the ascending form will be. The sample weight change rate as a function of temperature (T) is shown in the DTG curve. Exothermic events cause a curve above the baseline, while endothermic events cause a curve down the baseline in the DTA. As shown in Fig. 10, the melting point is the first change in the DTA curve, as an endothermic curve down the baseline. Figure 10, parts A and B show APLAF without metal adsorption, and Fig. 10 C and 10D, respectively, show APLAF with Pd and Cu adsorption. The melting point increases from 140.8 to 149.5 °C when the MW of the polymer increases, as shown in APLAF 1 (Fig. 10A) and APLAF IV (Fig. 10B). The adsorption of Pd and Cu on APLAF causes an expansion of the melting point up to 204.8 and 209.4 °C, respectively as shown in Fig. 10, parts C, and D. The DTG curve shows that maximum weight changes occur at higher temperatures with increasing molecular mass and Pd and Cu metal adsorption. Figure 10, part A, 260.9 °C, part B, 285.8 °C, part C, 349.4 °C; and part D, 363.2 °C show the first maximum peak, while the residual weight of the polymer after heating at the highest temperature is shown in the TG curve. As reported in Fig. 9, the residual weight% of APLAF at 699 °C increased from 2% in part (A) to 12% in part (D). As a result, with increasing molecular weight and Pd and Cu metal adsorption, the thermal resistance of APLAF increases.

Fig. 10
figure 10

Thermal properties of APLAF I (A), APLAF II (B), APLAF IV with Pd (C), and APLAF IV with Cu (D)

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Polymer Adsorption Tests

The optimal pH for the maximum adsorption of palladium(II) and copper(II) by APLAF was detected. The results (Fig. 11) show that the adsorption of Pd and Cu by APLAF is increased from pH = 2 to pH = 7.5, and the most suitable pH was pH = 7.5. Adsorption is low at low pH because N and O-containing moieties were protonated, and the active sites for metal adsorption were blocked. At pH > 7.5, the adsorption decreases since there are more OH-ions in the solution which causes the formation of Pd(OH)2 and Cu(OH)2, and these hydroxides were precipitated from the solution.

Fig. 11
figure 11

The curve of % metal ions adsorption of Pd(II) and Cu(II) in adsorption of various pH

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In Fig. 12, the metal ion adsorption vs. contact time is reported. The amount of metal ions adsorption mainly stayed the same with increasing time, and the optimum adsorption time was 45 min. At higher adsorption times, APLAF shows almost constant adsorption.

Fig. 12
figure 12

The curve of % metal ions adsorption Pd(II) and Cu(II) at various contact times

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The results of the adsorption of Pd and Cu ions starting with different concentrations of the ions are reported in Fig. 13. When the initial concentrations are lower than 15 mg/L, the APLAF adsorption rate is low. However, with the increase of the initial concentration from 10 to 100 mg/L, adsorption intensity is increased reaching the maximum at the initial concentration of 20 mg/L. Furthermore, the amount of adsorption has an insignificant increase from the concentration range of 20 to 100 mg/L and is almost constant. As the solution concentration increases, the active sites of APLAF are saturated, and the adsorption rate decreases.

Fig. 13
figure 13

APLAF adsorption (%) of Pd(II) and Cu(II) using various initial ions concentrations

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SEM Analysis

Scanning electron microscopy (SEM) is one of the best analytical methods for the evaluation of surface characteristics and microstructure up to nanometer dimensions. If equipped with an X-ray probe, it can also be used for chemical analysis; SEM detected usually has two types of signals. Secondary Electrons (SE) and Backscattered Electron (BSE). Secondary electrons (SE) originate from are surface or regions Close to the sample surface.

The origin of the BSE-type electrons is from a wide area in the interaction volume. The elastic collision of electrons with atoms causes a change in the path of electrons and creates BSE electrons. The number of returned electrons that reach the detector is proportional to their Z number (atomic number). This dependence of the BSE number on the atomic number by creating images that convey information about the sample’s composition.

Figure 14 shows picture A, the topography of the surface (Secondary Electrons, SE): it is clear, and without spots before the immersion test, but in Fig. 14B, the formation of black spots is shown after the immersion test indicating metal adsorption.

The SEM images of metal ions Pb(II) and Cu(II) adsorbed by the APLAF are correlated with the images of neat APLAF. The first (Fig. 14D) BSE images appear brighter than extraordinary APLAF (Fig. 14C), clearly showing that the polymer has adsorbed the metal.

Fig. 14
figure 14

SEM, SE, and BSE of APLAF before adsorbing metal ion (A and C), and SEM, SE, and BSE of APLAF after adsorbing metal (B and D)

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Conclusions

Today, since to the high volume of industrial production, the release of heavy metals from industrial effluents, factories, and metal industries, has increased the risk of over-emission of pollutants in the aquatic environment causing damage to the environment and human life. This study attempted to synthesize amino polylactide foam (APLAF) as a new environmentally friendly polymer containing adsorbent functional groups to help decontaminate water polluted with heavy metals. APLAF was successfully synthesized with a new multi-stage method (1) ring-opening polymerization ROP of lactide to obtain PLA, (2) Increase of amine functional group into polymer obtaining APLA, (3) production of emulsion micronization solution, (4) deposition and (5) moulding to obtain APLAF. ATR-FTIR and 1H-NMR analysis results show the successful synthesis of the desired compound. Images of particles of APLAF with an optical microscope show the well-performed APLAF micronization process. MW of APLAF was detected using GPC analysis, allowing a selection of the appropriate MW of the polymer and consequently obtaining the desired density of APLAF. Thermal properties of APLAF were determined by TG /DTG /DTA analysis showing an increase in the thermal stability of the polymer with increasing molecular weight. The TG /DTG /DTA diagrams also show that the thermal resistance has increased with the adsorption of Cu, and Pd, and the percentage of polymer degradation decreased. The adsorption showed an optimum pH of 7.5 for Pd(II) and Cu(II) metal ions, with the best adsorption of 45 min and an optimum initial concentration of 20.4 mg/L. The APLAF polymer can be essential in wastewater purification from heavy metal ions using bio-polymers.