Polyethyleneimine impregnated alginate capsule as a high capacity sorbent for the recovery of monovalent and trivalent gold

For the first time, a polyethyleneimine-impregnated alginate capsule (PEIIAC) with a high adsorption capacity is developed for the recovery of monovalent and trivalent gold from an acidic solution. The strategy results in a new type of adsorbent, polyethyleneimine impregnated alginate capsule (PEIIAC) with a core–shell structure having a large number of amine groups as cationic binding site, facilitating maximum uptake of anionic auric chloride. The maximum uptake of PEIIAC was 3078 and 929 mg/g for Au (III) and Au (I), respectively, are recordable compared to other reported adsorbents to date. The as-prepared material was executed to check the sorption efficacy for Au (III) and Au (I) in the pH range of 1–12. With an increment in pH, the uptake capacity for Au (III) increased, while the uptake capacity for Au (I) decreased. The FTIR, XRD, and XPS studies revealed that the gold adsorption mechanism includes ionic interactions and reduction, wherein the amine, hydroxyl, and carboxyl groups are involved. The capsule showed a higher adsorption efficiency than other reported sorbents, making the material applicable in acidic solutions for the recovery of Au (I) and Au (III).


Materials and methods
Materials. Sodium alginate (Showa Chemical Industry Co., Ltd., Japan) as an anionic biopolymer was used to make the capsules. Molecular weight of Sodium carboxymethyl cellulose (CMC) with high viscosity was approximately 700 kDa, which was used in the capsule. CMC (Sigma-Aldrich, Korea) was used as an anionic biopolymer to disperse PEI. Branched PEI as a cationic polymer (average M W : 750,000 by light scattering and concentration: 50 wt % in H 2 O) was purchased from Habjung Moolsan Co. Ltd. (Korea). Calcium chloride (CaCl 2 .2H 2 O, Samchun Pure Chemical Co., Ltd., Korea) was used as a divalent cross-linker to prepare the alginate capsules. Gold solutions were prepared by dissolving hydrogen tetrachloroaurate (III) hydrate (HAuCl 4 .3.6 H 2 O, Kojima Chemicals Co., Ltd., Japan) in 0.1 M HCl and potassium dicyanoaurate (I) K [Au (CN) 2 ], Kojima Chemicals Co., Ltd., Japan) in double-distilled water (DW). All analytical-grade chemicals were used for the experiments.
Preparation of the PEIIAC composite capsules, Au-alginate, Au-PEI and Au-CMC. The preparation of the PEIIAC composite capsule is shown in Fig. 1. 3 g of CMC and 5 g of polyethyleneimine were separately dissolved in 50 mL of 2% calcium chloride solution with magnetic stirring for 24 h. Subsequently, they were mixed just before the formation of the capsule and stirred with a magnetic stirrer to obtain a homogeneous mixture. The viscous mixture of CMC and PEI in CaCl 2 was added to 200 mL of 0.4% sodium alginate solution through a syringe. Capsules were created when the Ca 2+ ions of the drops came into contact with the alginate. Sample was prepared to study and compare the interaction of Au (III) and Au (I) with alginate, CMC, and PEI. 50 mL of PEI, CMC, and alginate solutions were prepared separately by dissolving 1 g of each molecule in DW under continuous magnetic stirring for 3 h. After then, 50 ml of 1000 ppm of Au (III) and Au (I) solutions were separately mixed with before mentioned solutions. Later, 100 ml of each solution were kept in a shaking incubator with 120 rpm and 25 °C for 24 h. After then the viscous samples solutions were frozen at − 40 °C for 3 h to get solid, which was freeze-dried for 1 week. The resultant sample was used for FTIR analysis.
Characterization of the capsules, PEI, CMC and alginate before and after adsorption of gold. The prepared capsules were characterized before and after the adsorption of gold. Light microscopy was used to study the physical appearance of capsules kept in a Petri dish containing water. Dry capsules were powdered and used for FTIR, DRX and XPS analysis. The functional groups of the capsules (before and after adsorption) were identified using FT-IR spectroscopy in the range of 4000-400 cm −1 , a PerkinElmer spectrophotometer (Spectrum GX, FTIR System), and a KBr disk. The mixture containing 1/8″ of the solid sample and 0.25-0.50 teaspoons pf KBr was grinded with pestle to prepare KBr disk. The crystallinity of the pristine and gold-loaded capsules was determined using an X-ray diffractometer (XRD, X'pert powder, PANalytical, The Netherlands). The surface chemistry of the capsule before and after the sorption of gold was studied using X-ray photoelectron spectroscopy (XPS).The analysis of atomic valence states of N, Au, Cl and O was carried out with the help of an AXIS-NOVA spectrometer (Kratos Analytical, Ltd., UK) with monochromatic Al Kα as the X-ray source (1486.71 eV of photons).

Dry weight experiment.
Experimental calculations were conducted based on the dry weight of the sorbent to understand the actual uptake of metals. Wet capsules were used in the experiment, considering the probability of blockage of the active sites in the dry capsules. A dry weight experiment was conducted to determine the water content of PEIIAC. 0.4 g of wet weight of capsules were kept in a freeze drier to sublimate the water of the sorbent for 72 h. The freeze dryer equipment (FD-Series, South Korea) are used at − 40 °C and 96.30 mTorr. Dry weight percentage was calculated using the following equation: Adsorption experiments. In adsorption studies, the effect of pH, isotherms, and kinetics experiments were conducted by keeping 0.4 g of wet capsules in 30 ml of a gold solution in 50 ml falcon tubes and placed in a multi-shaking incubator at 25 °C for 24 h under 120 rpm speed. In pH effect, NaOH and HCl of different molar concentrations were used to maintain required pH 1-12 values. Initial concentration of 1000 ppm of both Au (III) and Au (I) were used to carry out pH edge. An inductively coupled plasma-atomic emission spectrometer (ICP-AES, ICPS-7510 Shimadzu, Japan) was used to calculate the remaining gold in the supernatant solution after dilutions. Gold solutions of 50-5500 mg/L were used for the isotherm experiments. The kinetics experiments were conducted at various time ranges. The adsorption capacity of the capsules was calculated using the following equation: (1) Dry weight ratio(%) = Adsorbent dry weight Adsorbent wet weight × 100

Results and discussion
Effect of pH. The surface functional group of adsorbents and chemical speciation of adsorbate affected by the pH of the solution. So, it is crucial to study the effect of pH at all range on the adsorption of ionic species competing with H + and OH − ions in an aqueous solution. The adsorption trend of Au (III) and Au (I) onto PEIIAC at all pH ranges are shown in Fig. 2a,b, respectively. In the case of Au (III), sorption of it increases from 1 to 14 of pH due to adsorption and precipitation. According to gold speciation, [AuCl 4 ] − exists at pH 1-4 and Au (OH) 3 at pH 4-14 33 . Precipitation of Au (III) starts from pH4. The high adsorption at pH 4-14 is not due to adsorption but it is due to precipitation of gold in the form of Au (OH) 3 . The capsules become positively charged due to protonation in an acidic solution resulting in electrostatic attractions between [AuCl 4 ] − and the capsules and anion exchange between [AuCl 4 ] − and Cl − . So, the optimum pH for Au (III) recovery was 1-4 which is supported by literature data [34][35][36][37][38][39] . Recovery of Au (I) increased with the decrease in pH for the capsule. At pH 1-7, Au (I) precipitation is possible in the form of Au (0). So, it looks very high adsorption value at pH1 which is actually due to the precipitation of gold. At pH 7-14, adsorption is low since capsules become negatively charged resulting in repulsion with anionic species. The high stability and elastic nature of the capsule is due to crosslinking with Ca 2+ and intermolecular hydrogen bonding between -COOH groups at low pH 40 . Adsorption mechanism concerning PEIIAC and [AuCl 4 ] − can be expressed as Eq. (3).

Sorption kinetics. Sorption kinetics is important in industrial applications for process design and opera-
tion. The pseudo-first-order 41 and pseudo-second-order kinetic equations were used to study the kinetics of Au (III) and Au (I) adsorption on PEIAC which are expressed as below: where: q 1 and q 2 are amounts of metal ions adsorbed at equilibrium, q t is amount of metal ions adsorbed at time t, k 1 and k 2 are rates constant for Pseudo-first-order adsorption and pseudo-second-order adsorption respectively. According to regression coefficient (R 2 ) and constant parameters as shown in Table 1, experimental data were best fitted for pseudo-second-order kinetic model in case of both oxidation state of gold 42 . According to kinetics  www.nature.com/scientificreports/ graph shown in Fig. 3a,b, adsorption increases initially with time due to the presence of more sorption sites on capsules and reached equilibrium state within 8 h in the case of Au (III) and 4 h in the case of Au (I). There is repulsion between free carboxylate anion of alginate and anionic auric chloride and aurocyanide. Carboxylate anion of alginate are crosslinked with calcium ion to form calcium alginate shell. Compacted calcium alginate shell of the capsule formed after crosslinking might decrease rate of gold diffusion. Continuous reduction and lowering diffusion of gold with the shell might be the reason for very low kinetics. Based on the regression coefficient (R 2 ) value, data are well fitted with the pseudo-first-order (R 2 = 0.9727) and pseudo second-order kinetic model (R 2 = 0.9587) model for 8 h as shown in the Fig. 3b (2nd figure). Reduction of aurocyanide might be the reason for the increase in adsorption kinetics after 8 h. Gold is known to be easily reduced with PEI 3 and gold reduction was also confirmed in the study (Fig. 8c). However, the pseudo-first order and pseudo-second order kinetic models do not cover the reduction. That is likely why a larger deviation occurs after 8 h. For a comparative study of Au (III) and Au (I) kinetics at pH2, experimental data of capsules' sorption kinetics of Au (I) were fitted with the pseudo-first-order and pseudo-second-order models even though the fit was low (R2 < 0.78).
Sorption isotherm. Au (III) and Au (I) adsorption mechanism on PEIIACs were described by using Freundlich 43 and Langmuir models which are applicable for heterogeneous and homogeneous surface adsorption respectively. These models can be expressed as below: www.nature.com/scientificreports/ where, q e is the equilibrium amount of adsorbed metal (mg/g), q m is the maximum uptake (mg/g), b is the Langmuir equilibrium constant (L/mg), C f is the final concentration (mg/L), k F is the Freundlich constant (mg/g) (L/g) 1/n , and n is the Freundlich exponent. Isotherms inform maximum uptake of gold from solution at a constant temperature. Adsorption of both gold increased with increasing initial concentration till the equilibrium state was achieved as shown in Fig. 4a,b, respectively. According to the highest regression coefficient (R 2 ) value shown in Table 2, the experimental data were best fitted for Langmuir isotherms than Freundlich isotherms in the case of Au (III) and Au (I). The maximum uptake of Au (III) and Au (I) were predicted by the Langmuir model as 3077.56 ± 226.14 mg/g and 928.80 ± 77.71 mg/g respectively. The capsules have well adsorption capacity compared with other materials as shown in Table 3.   Table 3. Comparison of the maximum uptake capacity for different adsorbents.

Adsorbent and species q m (mg/g) References
L-cysteine impregnated alginate capsules 1.51 18 Cross-linked chestnut pellicle 2100 44 Ca-alginate beads 1.47 45 Wattle tannin gel 8000 46 Thiourea modified alginate powder 6.40 47 Chemically www.nature.com/scientificreports/ Characterization and possible adsorption mechanism. Visual observation of the capsules before and after adsorption of Au 3+ and Au + , through the microscope is shown in Fig. 5a-c. The average diameter of the nearly spherical capsule was 2.2 ± 0.1 mm. Diameter of several capsules were measured and expressed as average value with standard error. The uncertainty of the diameter measurement of the capsule was 2.2 ± 0.1 mm. After adsorption of gold, the capsules were slightly expanded as shown in Fig. 5b,c. Expansion of the capsule after Au (I) adsorption is higher compared to Au (III) sorption. Furthermore, the capsule has a white mixture of PEI and CMC surrounded by calcium alginate shells as shown in Fig. 5a. When the viscous mixture of PEI, CMC, and CaCl 2 is dropped into the alginate solution, Ca + + ions of the mixture diffuse out, meet alginate, and form alginate gel on the surface of the drop via ionic crosslinking. This leads to a core-shell structure with PEI and CMC in the core and alginate in the shell. To confirm calcium alginate shell formation, FTIR spectra of PEIIAC, CMC, Alginate and PEI were compared as shown in Fig. 6d. The IR band position of the capsule is significantly different compared to those of sodium alginate, PEI, and CMC. The peak of PEIIAC at 3200-3600 cm −1 is narrower than those of sodium alginate, PEI, and CMC 3 . The difference is likely due to the involvement of hydroxyl and carboxylate groups in crosslinking with calcium ions during alginate shell formation. As a result, narrow bands of calcium alginate appear due to decreasing the number of O-H groups involved in hydrogen bonding. The shifting of the peak due to carboxylate ion is because of displacement of the sodium ion with calcium ion in sodium alginate.
To study the interaction of Au (III) and Au (I) with alginate, CMC, and PEI, FTIR analysis of the polymers before and after adsorption of gold was carried out as shown in Fig. 6a-c. The difference in spectra and absorbance band before and after adsorption indicate the interaction of respective molecules with gold. Different spectra of Au (I)-polymer and Au (III)-polymer indicate different interaction mechanisms. The appearance of nitrile peak in Au (I)-PEI (2142 cm −1 ) and Au (I)-CMC (2160 cm −1 ) confirms the existence of aurocyanide without reduction. In Fig. 6a,c, tapering of O-H peak and appearance of carbonyl peak at 1727 cm −1 after Au (III) adsorption might be due to the reduction of Au (III) with a hydroxyl group 3 . As shown in Fig. 6b, the narrowing of the N-H peak at 3200-3600 cm −1 and decrease in absorbance at 1000-1700 cm −1 after adsorption of gold indicate electrostatic, chelation, and redox interaction. Different spectra of Au (III)-PEI and Au (I)-PEI indicate different actions of PEI with them. PEI does electrostatic, chelation, and redox interaction toward Au (III) while only electrostatic and chelation action on Au (I).
To understand the adsorption mechanism, impregnation of PEI inside capsule and surface functional group, the capsules before and after adsorption of Au 3+ were characterized by FTIR spectroscopy which is shown in Fig. 7a. It is important for the identification of functional groups that are mainly responsible for adsorption. The functional group and fingerprint region of spectra show characteristics peak of the functional group present in individual PEI, CMC, and Alginate with a slight shift due to interaction during fabrication. The change in peak value after the adsorption of gold indicates interaction between adsorbate and adsorbent through the active site. The O-H and N-H bond stretching vibration of the capsules at 3416 cm −1 changes to 3440 cm −1 after adsorption of ionic gold 3 . The change in peak value pointed out the interaction between adsorbate and adsorbent through ionic chelating interaction. The broad peak indicates the presence of a large number of hydroxyl and primary amino groups in the composite which reduces after adsorption due to their participation of them in ion exchange and reduction 54 56 .The C-H bending at 877 cm −1 of the capsule is mainly due to the aliphatic -CH 2 -group of Alginate, CMC, and PEI, which changes to 881 cm −1 after sorption indicating the change in the electronic environment. Furthermore, the peak at 2928 cm −1 belongs to aliphatic C-H stretching and bending vibration which reduces to 2927 cm −1 after adsorption pointing adsorption of adsorbate onto adsorbent 57 . The shift in FTIR peak after adsorption of gold indicates adsorption of gold with capsule through -NH 2 , -OH, -O-and COOH functional groups, which was not sufficient for confirmation of the reduction of gold inside the capsule. Therefore, XRD was carried out to know metallic gold formation after sorption.  The multipurpose X-ray diffraction (XRD) was conducted to know the crystallinity of the capsules and the mechanism of gold adsorption. As shown in Fig. 7b, the XRD patterns of the capsule before and after adsorption of trivalent gold are different, indicating that the formation of metallic gold after adsorption and reduction. The formation of metallic gold indicates that the possible mechanism was adsorption followed by reduction. The XRD pattern of capsules after adsorption of trivalent gold displayed sharp peaks of Au 0 at 38.1, 44.3, 64.5, 77.5, 81.6, corresponding to the Bragg's reflections of (111), (200), (220), (311) and (222), respectively 58 . The results indicate the formation of crystalline metallic gold nanoparticle having face-centered cubic lattice inside the capsule, justifying that ionic Au (III) was adsorbed in PEIIAC followed by reduction. Also, the intense peak at (111) plane supports the formation of reduced gold. Thus, XRD justified the hypothesis that the redox-active PEIAC can reduce trivalent gold of auric chloride to metallic gold at acidic solutions (pH1). However, there is no significant difference in the XRD pattern of the capsule before and after adsorption of monovalent gold in aurocyanide as shown in Fig. 7b. This indicates that there is no reduction of monovalent gold from aurocyanide after adsorption with PEIIAC at the acidic solution.
The XPS was carried out to reveal the adsorption and reduction mechanism of gold with the capsules. The XPS of the capsules before and after the adsorption of gold are displayed in Fig. 8a,b. In the N1s spectra of the capsule, before sorption of gold as shown in Fig. 8a, the peaks at 399.68 eV, 398.10 eV, and 397.12 eV are attributed to RNH 2 , R 2 NH and R 3 N, respectively 59 . After sorption as shown in Fig. 8b, the peaks of N1s spectra appeared at 402.41 eV, 401.61 eV, 400.91 eV and 399.71 eV related to RNH 3 + , R 2 NH 2 + , R 3 NH + and -NO 2 , respectively 3 . It indicated that the positively charged amines formed after protonation in a strongly acidic solution of gold were the main driving force for the adsorption of anionic AuCl 4 − through electrostatic attraction. The appearance of the peak due to -NO 2 depicted the oxidation of the amino group during the reduction of gold. Also, Fig. 8c of Au 4f. spectra conformed metallic and ionic gold inside the capsule. The peak at 88.41 eV and 84.71 eV were attributed to Au 0 , whereas the peaks at 87.61 eV and 91.30 eV were due to Au 3+ . Previous studies also supported the adsorption and reduction of gold inside PEI-Alginate composites. From FTIR, XRD, and XPS data, possible adsorption and reduction mechanism of gold with the capsule can be shown as the following equations: Ionic interaction followed by reduction

Conclusions
A capsule with a high adsorption capacity was prepared using a simple method for gold recovery. The mechanism analysis showed the ionic attraction of AuCl 4 − with cationic amines and reduction of Au (III) to Au (0) with primary amines. The pH edge experiment revealed an increase in Au (III) sorption but a decrease in Au (I) sorption with increasing the pH. The maximum Au (III) adsorption capacity of the capsule was 3078 mg/g, which was approximately three times that of Au (I). The equilibrium state was attained at 8 h for Au (III) and 4 h for Au (I). The experimental data fit well with the pseudo-second-order kinetic models and Freundlich isotherm models. Thus, the capsule could be evaluated as a high-efficiency adsorbent for the recovery of gold from an acidic solution.

Data availability
All data generated or analyzed during this study are included in this published article.