The Advantages Of Lipid Capsule

Published Date: 02 Nov 2017

Pratap Kotte a, Sung Wook Won a and Yeoung-Sang Yun a,b,*

a Department of BIN Fusion Technology, Chonbuk National University, Jeonbuk 561-756, Republic of Korea

b School of Chemical Engineering, Chonbuk National University, Jeonbuk 561-756, Republic of Korea

*Corresponding author: Prof. Yeoung-Sang Yun. E mail address: [email protected] . Tel.: +82 63 270 2308; fax: +82 63 270 2306.

Abstract

The role of phospholipids on the sorption of precious metals was examined using lipid capsule as a novel sorbent in Au(III), Pt(IV) and Pd(II) mixture, having similar properties and vary in their standard reduction potential values. Phospholipids in the capsule worked as a reducing agent for the sorption of precious metals. The electrostatic forces and covalent forces usually responsible for sorption process were very low in the present study. The lipid capsule behaved in the a manner of model reductive coupled sorbents. The selective sorption of Au(III) with sorption capacity of 336.5 mg g-1 among the Pt(IV) and Pd(II) ions was due to its high reduction potential value. However the sorption of Pd(II) and Pt(IV) exists very less compared to the Au(III) sorption. The reduction of gold(III) to metallic gold (Au(0)) was observed and conveyed the reduction mechanism for the sorption through XPS and TEM studies. The regeneration of the adsorbent for recycling made the selective sorption more affordable and applicable. The study suggests the lipid content in the capsule is responsible for the sorption of easily reduciable metal ions.

Key words: L-phosphatidylcholine; Reducing property; Gold; Adsorption

Introduction

The adsorption of precious metals has great demand now days due to the special properties possessed by them in different areas of scientific field. Precious metals are particularly used in the catalytic and electronic industries. The great demands for the precious metals are not compensated by the natural sources occurring on earth crust makes them high valuable. This generated the need to recover the precious metals from alternative sources, secondary sources.[1] Several conventional methods such as solvent extraction, precipitation, adsorption and ion exchange have been followed to recover the precious metals from their primary/secondary sources.[2] In the past few decades, adsorption using biomass gained more advantage than the existing methods for precious metals.[3] Potential benefits of biosorption process include low economical range operation, easy to recover metals, ease of operation, regeneration of biosorbent, least quantities of sludge (chemical/biological) disposal and having high efficiency.[4] Biosorption mainly focuses on the bacterial, fungal, plant or modified forms of these materials. Particularly the properties of biosorption of bacterial biosorbents were broadly discussed in the literature.[5-8]

Composition of the cell wall plays an important role for estimation of bacterial biosorption. The anionic functional groups like peptidoglycan, teichoic acids and phospholipids, lipopolysaccharides of Gram-positive and Gram-negative bacteria respectively are mainly responsible for the anionic character and the metal-binding ability of the cell wall.[9] Similarly extracellular polymeric substances (EPS) can also bind heavy metals, however their availability depends on the bacterial species and growth conditions.[9] The composition of EPS mainly includes a complex mixture of macromolecular biopolymers such as polysaccharides, proteins, lipids or humic substances.[10,11] Metabolism-independent processes like physical and chemical adsorption, electrostatic interaction, ion exchange, complexation, chelation and micro precipitation were responsible for biosorption.[12,13]

The assignment of definite role to the individual functional groups responsible in metal binding has not been predictable despite the huge efforts made by researchers. Different metals nonspecifically sorbs on microorganisms with different biosorption mechanisms operating together in particular microorganism made it difficult.[14] Their studies also suggested that polymers on cell wall might play important role in conferring selectivity in case of binding of the precious metals, gold and silver. Therefore the selection of fungal cultures based on their cell wall composition by manipulating certain factors like growth conditions or physic-chemical treatments to the biomass might be advised to fabricate special biosorbents having high efficiency for recovery of specific precious metals. Lipids are known to be defined as organic geochemical molecules usually insoluble in polar solvents but soluble in nonpolar solvents. The lipids work as transporters for the passage of materials across the cell wall i.e. from extra cellular to intracellular vice versa. Mixture of components such as waxes, or resins, n-alkan-ones, n-alkanols, sterols, n-fatty acids, n-alkenes and n-alkanes were mainly identified in lipids.[15] As per the previous literature on adsorption the presence of lipid in the sorbent regulates the adsorption characteristics. Guangwei and Rice [16] reported that there is strong competition for the hydrophobic sorption sites between lipids and sorbents which showed significant impact on sorption and desorption hysteresis. The lipid removal from natural organic matter (NOM) increased both the sorption isotherms nonlinearly.[17]

Usually the extraction process of lipid fraction from biomass was done by treating the biosorbent with benzene. And the biosorption of chromium (Eo=1.33) from electroplating effluent reduced after extracting lipid from yeast biosorbent S. cerevisiae.[18] Therefore the reduction in the sorption capacity reveals the contribution of lipid fraction present on the cell wall for the biosorption of chromium. The study of Pethkar and Paknikar [19] on fungal biomass, helped in elucidating the mechanism of preferential metal biosorption and manipulating surface groups responsible for improved biosorption of Au and Ag (easily reducible metals) than other metals. On the contrary, according to the experimental conclusions of Guibaud et al.[20] using extra cellular polymers extracted from sludge, there is no correlation between lipids content and their affinity for heavy metals (having less reduction potentials) like Cd, Cu and Pb. When lipid fraction was extracted from Aspergillus niger, slight reduction in biosorption of lead, cadmium and copper were observed.[21] Harsh extraction conditions may be change in the structural condition or lipid extraction were derived to be the possible (not sure about one) factors behind the reduced sorption. According to Kenney and Fein [22] and Ueshima et al. [23], the removal of EPS from some of four kinds of bacterial species affected neither the proton reactivity nor the sorption behavior of Cd. The fact was explained to be the proton and metal binding sites present in the EPS molecules responsible for the binding of proton or metal were also present in similar manner on the cell walls of bacteria.[22,23]

The above discussion leads to confuse the readers on the role of lipids in sorption process. As different researchers presented their opinion on lipids role based on the sorption of metals they selected. The role of lipids particularly phospholipids in the adsorption process which were major building blocks of cell wall was expected to be high. Because phospholipids have hydrophobic and hydrophilic parts and they can form in the form of vesicles/ micelles depending on the medium and concentrations. The advantage of these formations can be applied for the accountability of selective property. Here under the present work focused on the generalization of all the statements by performing a model sorption studies. The precious metals Au(III), Pt(IV) and Pd(II) were selected which have similar reactive properties and separation of them is also a difficult task. The standard reduction potential values of the selected metals are different so they can be reduced differentially depending on the strength of reducing agent. We also considered and studied the selective property of the lipids in the sorption process. The reducing nature of the lipids made selective sorption to sorb easily reducible gold when it is present along with platinum and palladium. The underlined reduction mechanism was suggested for the sorption of precious metals using the lipids as sorbents. The generalization of the all above confusing statements about the role of lipids on sorption was proposed. The present study suggests the selective property can be applied to a sorbent based on lipids content depending on the target metal to be sorbed on biosorbents among the different metal ion mixtures by considering the reduction potentials of metal ions.

Results and discussion

pH studies

The pH influence on the sorption of metal ions on lipid capsule (LC) is shown in Figure 1. Alteration in the solution pH causes variation in metal speciation, degradation of biomass and protonation/deprotonation of surface groups might be responsible for the biosorption.[24] The adsorption percentage was maximum at high acidic pH for all the three metal ions, gold, platinum and palladium while low for palladium and platinum at basic pH. The decrease in the sorption percentage for the gold was almost negligible while a drastic change was observed for the other two metal ions, platinum and palladium. The sorption of Pd(II) and Pt(IV) followed regular fashion with the biosorbents and ionic exchange resins where the electrostatic attraction and ion exchange mechanism play an important role on the sorption process. The process of electrostatic and ion exchange mechanisms were clearly explained for the sorption of platinum group metals on sorbents of modified chitosan.[25,26] However the sorption behavior of gold exhibited unusual trend in the present study. The pattern of sorption behavior with change in concentration is different than the usual sorption processes previously described for gold. These results indicated that there is some other factor dominating the sorption of gold on the LCs other than electrostatic attraction forces between the sorbent and metal ions. In acidic solutions the precious metals tends to exist in the form of anionic complexes where they have high stability. At high acidic pH, the sorption of all the precious metals was similar to the trend of electrostatic and ion exchange mechanisms. The order of sorption capacities of three metals were gold >> palladium > platinum for the fixed quantity of sorbent. On the other hand the result was deviated from the usual sorption process in two ways. First the sorption of gold was pH independent and second the sorption capacity of palladium is greater than platinum. One interesting phenomenon observed was the change in color of the capsules to the dark wine red color after the completion of sorption process. The plausible explanation might be the reducing power of sorbent additionally gave high contribution for the sorption process than electrostatic interactions leads to higher sorptions of all metal ions in the order of their reduction potential values. Similar type of color change and reduction of metal ions was observed on sorbents and reported by Park et al.[27] The proposed reduction mechanism was confirmed and explained by the studies of TEM images and XPS spectra of the gold nanoparticles formed during the process of sorption.

There exists a large difference in the sorption of gold and other platinum group metals ions Pt and Pd from pH 3 to separate gold from the remaining metal solutions. The pH 3.5 was chosen as the optimum pH for the selective separation of gold from the other two metal solutions using the LC and performed the isothermal studies.

XPS and TEM images in support of the suggested mechanism

Mechanism in sorption

The performed sorption experiments showed the selective sorption of gold in the presence of mixture of platinum, palladium and gold on the capsule made up of lipid. Therefore the study of fact behind the selective sorption was essential to know the factors attributed for the selectivity to further fabrication of selective sorbents from the mixtures. Mechanism study presents responsible nature of forces for the sorption and the other factors for the prevention of other metals staying far from sorbent. The present study, the high standard reduction potential of the gold metal makes it to reduce easily in the presence of easily electron providing reagents environment. The less reduction potential values of the platinum and palladium made them difficult to reduce in the presence of gold (easily reducible) intern showed less sorption of these ions than the gold. Formations of the gold nanoparticles were assumed to be due to the lipid present in the sorbent, capsule. The responsibility of phospholipids in making the nanoparticles from their chloride metal solutions was previously reported where the lipids play reducing as well as capping properties of the formed nanoparticles.[28-31] The underlying reduction process can be shown as bellow.

Oxidation: Lipid (R-CH(OH)-CH2OH)  Lipid (R-CH(OH)-COOH)

Reduction: Au(III) +2e-  Au(I) + e-  Au(0) or Au(III) +3e-  Au(0)

The proposed mechanism of reduction of gold was confirmed by the TEM and XPS studies.

TEM images

The sorption process of different metal ions particularly exclusive gold on the surface of LC was achieved through the experimental results from the mixed metal solution of platinum, palladium and gold. The sorption process was assumed or proposed to be due to the reduction of gold ions by the sorbent. If so the reduced gold from Au(III) to Au(I) or Au(0) should be occurred which may be the nanoparticles. The result of the TEM images reinforces the process of reduction process by the observation of gold nanoparticles on the surface of the sorbent. These results are presented in Figure 2, which shows the presence of Au nanoparticles. By the careful observation of the Figure 2, the different shapes such as spherical, triangular and truncated of Au nanoparticles of different size ranging from 10 to 120 nm were observed. The literature reports also suggested the responsibility of the surfactants and biomolecules responsible for the formation of nanoparticles from gold solutions.[32,33] This is very clear and perfect evidence of the proposed reduction mechanism.

XPS studies

An XPS spectrum of gold present on the sorbent, LC is presented in Figure 3. The observation of the binding energies clearly indicates the presence of gold in the metallic form (Au0). The XPS analysis of the Au 4f region indicates the presence of the oxidation states of the gold in according to the binding energies.[34-38] The binding energies were slightly shifted to lower side than the bulk gold value of 84 eV for the gold deposited on the sorbent. The binding energy of Au 4f values was found to be 83.7 and 87.5 eV for 4f7/2 and 4f5/2 respectively. It can be assigned to metallic form of gold or Au(0) oxidation state respectively.[35-38] The exhibited error value of -0.2 eV in accordance with the 83.9 and 87.7 eV binding energies of pure metallic gold for 4f7/2 and 4f5/2 may be due to the interferences of other elements on the sorbent, LC. As we carried out the experiments with solution of Au(III) along with Pd(II) and Pt(IV) for the sorption experiments. The absence of the peak corresponding to the Au(III) in XPS spectrum implies and conforms the reduction of the gold ions on the surface of LC.

Isothermal studies

The experimental isotherm data were obtained by measuring the amount of metal ions adsorbed in the LC (Figure 4). The concave curve plots observed between the metal ion concentration in the solution and the concentration sorbed on the sorbent shows the progressive saturation of the solid biomass as the initial solute concentration increases. This L-shaped plateau curves refers a typical feature of Langmuir curves. The efficiency of any sorption process was determined by the sorption isotherms. Therefore The data obtained for the adsorption of Pt(IV), Pd(II) and Au(III) on LC plotted with two-dimensional nonlinear Langmuir isotherm model. The equations were presented for all the models in Supporting Information Section 1. For the monolayer biosorption, the LC showed a maximum Au uptake of 366.5 mg g-1 at optimum pH 3.5 and ambient temperature was consistent with the experimental result. Similarly the maximum uptake of Pt(IV) and Pd(II) on LC were found to be 20.5 and 51.9 mg g-1, respectively. The decreasing order of maximum sorption uptakes of the studied three metal ions on LC were therefore predicted to be Au(III) >> Pd(II) > Pt(IV). The Au(III) sorption uptake was 7.1 and 17.9 fold in excess than the sorption uptakes of Pd(II) and Pt(IV). Intern Pd(II) uptake was 2.5 times greater than that of Pt(IV). Therefore the adsorbent greatly separated Au from the remaining two metal ions Pt and Pd. The order of decrease in sorption from gold to Pt was consistent with the order their standard reduction potential values. Hence this sorption uptake result could be directly related to the values of their corresponding reduction potentials without any hesitation (Supporting Information Section 2). The similar kind of sorption results in case of precious metal sorption, the order of maximum uptakes followed Au(III) >> Pd(II) > Pt(IV) were reported,[39,40] where the reduction of the Au(III) to metallic form was responsible and confirmed. On the other hand high separation factor for Au uptakes to the remaining ions with huge sorption capacities were observed if reduction plays its role in sorption.[41] On the contrary the sorption capacities of precious metals in absence of reduction of gold follows the other order and depends on equilibrium pH and shows less separation factor for gold with other metal ions and not very high uptakes for gold.[25]

The corresponding isothermal parameters and coefficient of determinations (R2) are tabulated in Table 1. The observation of the results of R2 of the metal ions the gold shows to be the reliable one with more than 0.99 values where as it was very poor in case of Pt and Pd. The Freundlich isotherm model was also applied to sorption data to verify the heterogeniety factor.[42] The R2 values were 0.99 and 0.94 for Langmuir and Freundlich models, respectively, which indicates that the two adsorption model equations reasonably fit well, but the Langmuir isotherm provided better fit than the Freundlich isotherm. The better fit obtained by the Langmuir isotherm model provided the sorption of metal ions on the surface of LC was monolayer sorption. The dimensionless equilibrium parameter RL, an essential characteristic of Langmuir isotherm was calculated to identify the favorability of the adsorption system and the experimental result was within the favorable limit (0<RL<1) at various initial concentrations studied in the present work.[42] Another supporting information for the reductive sorption of Au(III) can be drawn from the Langmuir constant in the Langmuir sorption. The values of constant b obtained in the Langmuir isotherm modeling of the metal ions sorbed on LC were 0.20, 0.22 and 0.33 for Au(III), Pd(II) and Pt(IV), respectively. These were almost similar values which can be related to the affinity of metal ions and sorbate (electrostatic or ion exchange interactions). But the experimental sorption of Au(III) was far greater than Pt(IV) and Pd(II) deliberately introduces and supports the other responsible reductive sorption. The present paper showed the sorption of Au(III) by LC was comparable and higher, which could be good enough to say the adsorption power of biosorbent is best in the present study than the reported sorbents.

Kinetic Studies

The kinetic studies are important key steps to design and control the sorption efficiency process for practical applications. It also enables better understanding of the path way of sorption reactions as well as mechanism. By measuring solute uptake it determines the residence time between the interface of solid, sorbate and liquid in which sorbent has being used.[43] Therefore the sorption of Au(III), Pt(IV) and Pd(II) were measured and plotted against different time intervals (Figure 5). The kinetic experimental data revealed the adsorption process attained maximum value in 3 h in case of Pt(IV) and Pd(II) while it took longer time around 5 h for the Au(III). And the sorption result of Au(III) was extremely higher than that of the other two metal ions could be due to reductive sorption of Au(III) on LC. Therefore the sorbent LC can be used as a selective sorbent for the removal of Au(III) from the mixture of Au(III), Pt(IV) and Pd(II). The traditional nonlinear kinetic models, pseudo-first and pseudo-second order were applied to the kinetic adsorption experimental data and presented in Figure 5. The results of kinetic parameters i.e. rate constants, corresponding correlation coefficients, equilibrium uptakes for different metal ions under study were calculated and were summarized in Table 2. The sorption of Au(III) was found to be 52.5 mg g-1 by pseudo-second order model was 1.8 and 3.0 times higher than that of Pd(II) and Pt(IV), respectively. The values calculated by pseudo-second-order model were more nearer to the experimental values of the kinetic data. According to the data presented in table 2, the coefficient of determination, R2 values of pseudo-second order model were almost equal but very little greater values than that of pseudo-first order model, so the sorption process was better explained by the pseudo-second order model than pseudo-first order model.[30] Moreover the second order rate constant was similar in case of pseudo-second order model about 0.4 g mg-1 min-1. Usually the second order kinetic model mainly stresses on the chemical sorption but not on the mass transfer in solution. Therefore the sorption of Au(III) on LC was confirmed by the reduction mechanism between adsorbent and the adsorbate.

The advantages of lipid capsule

The preparation of capsule with lipid gained advantage to the present study to know easily the mechanism and the responsibility of the lipids for the sorption process. The process of capsule formation made the sorbent to be present in the solid form and to provide large surface area due to its hollow shape intern helped for the high rate of sorption. As there exists confusion and unclear properties of extra cellular lipids on the sorption of metal ions, present study provides a clear result and understanding the lipid role. The role of lipids in the sorption process of metal ions depends on several factors, mainly the surrounding and other molecules along with lipid and secondly the type of metal ions to be sorbed. The presence of easily reduciable metals or the metals having high standard reduction potentials can be sorbed by the lipids easily in huge amounts.

Desorption and regeneration of Sorbent for recycling

The adsorbent gets its high end value if that has the reusable nature. Therefore the regeneration of the sorbent LC for economical and effective recycling process was carried out. The effective desorption agent to desorb all the adsorbed metal ions (Au>>Pd>Pt) on sorbent LC was a prior task. The batch method using various desorbing agents like HCl (0.5 and 2 M), thiourea (0.5, 1 and 2 M) and mixture of thiourea-HCl (1:1 ratio of 1M) were performed to recover the sorbed metal ions on the sorbate LC. The results were discussed in the Supporting Information Section 3. According to the results the thiourea-HCl mixture showed effective efficiency in removing all the sorbed metal ions, Au, Pd and Pt above 99% from the LC. As the desorption efficiency was good we went further to study the recycling process of sorption and desorption by following the optimum conditions discussed in the present study. The successive sorption and desorption results up to 5 cycles were presented in Figure 6. From the analysis data there was no significant change observed in the sorption and desorption process of metal ions on LC. Until 5 cycles the desorption efficiencies were more than 97%. This property offered the sorbent LC to be commercially adaptable in the sorption process of selective sorption of Au(III) from the mixtures of Au(III), Pt(IV) and Pd(II) solutions.

Materials and methods

Materials

Carboxymethyl cellulose (CMC), L-phosphatidylcholine, calcium chloride, sodium alginate and thiourea were supplied by Sigma-Aldrich Korea (Yongin, Korea). Hydrogen tetrachloroaurate(III) hydrate (HAuCl4·3.6H2O), hydrogen hexachloroplatinate(IV) hydrate (H2PtCl6·5.5H2O) and palladium(II) chloride (PdCl2) were purchased from Kojima Chemicals Co., Ltd. (Saitama, Japan). Hydrochloric acid was supplied by Junsei Chemical Co., Ltd. (Tokyo, Japan). The chemicals received were used without any further purification. Triply distilled water was used throughout the experiments, unless specified.

Preparation of lipid capsule

For the selective recovery of gold among other precious metals, the L-phosphatidylcholine was used as a model lipid and encapsulated in alginate hollow capsules. The mixture (50 mL) of 1.5% calcium chloride and 1.5% CMC was added with 4 g of lipid and was vigorously stirred on magnetic stirrer. The resulting viscous solution was passed through a needle to control the drop rate into the solution of 0.6% (w/v) sodium alginate where diffusion process responsible for the formation of hollow capsules. The formed capsules were washed with distilled water and transferred to 2% calcium chloride solution for 30 min in order to enhance the alginate shell strength. The resulting capsules were then washed with distilled water again several times. The capsule impregnated with L-phosphatidylcholine was named as Lipid capsule (LC) and used as an adsorbent in this study.

Selectivity testing

The selectivity experiments among Pt, Pd and Au were carried out by simple procedures in which the prepared LCs were agitated with the solutions containing equal concentration of the above precious metals with different scales. Similarly the experiments were done for different time intervals. The metal concentrations in the supernatant were measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES; ICPS-7500, Shimadzu, Japan).

Sorption, kinetic and desorption experiments

50-mL polyethylene conical tubes were used to carry out batch adsorption experiments. The appropriate (30 mL in most cases) aliquots of aqueous metal mixture solution containing various concentrations of Pt(IV), Pd(II) and Au(III) were introduced into tubes. Weighed amount (0.05 g) of adsorbent was added and the tubes were agitated at 160 rpm at room temperature (25±1 ◦C) in a mechanical shaker for sufficient time to attain equilibrium. The residual concentration of the metal ions Pt, Pd and Au were measured using ICP-AES after filtration. The effects of Pt(IV), Pd(II) and Au(III) metal ion concentration, contact time, solution pH and temperature were investigated. Blank solutions were treated in the same way without adsorbent and were assumed as initial concentration while calculation. The uptake of the metal ions (q, mg g-1) was calculate from the following mass balance equation:

where Ci and Cf, are the initial and final concentrations of the sorbate (mg L-1), respectively. Vi and Vf are the initial and final volumes (L), respectively. M represents the adsorbent weight (g).

Kinetic studies were also conducted in a similar manner but the samples were collected for the analysis of metal ions sorbed on the adsorbent at predetermined time intervals.

Batch wise desorption studies were performed using different eluents such as 0.5 M thiourea (0.5 M T), 1 M thiourea (1 M T), 0.5 M HCl, 1 M HCl, 2 M HCl and the mixture of 1 M HCl and 1 M thiourea. After adsorption, the metal-sorbed adsorbents were washed with water several times and transferred into 50-mL polyethylene conical tubes. Each eluent (30 mL) was added and agitated at room temperature (25±1 ◦C) for 30 min in a mechanical shaker. The concentration of precious metal ions released from the LCs into aqueous phase was determined by analysis with ICP-AES. The regenerated capsules after desorption of metal ions were washed with water and subjected for the sorption experiments using sorption procedure. In the similar manner once again subjected to desorption to recover the metal ions and continued these consecutive processes until completion of 5 cycles.

TEM and XPS studies of Lipid capsules

The TEM image helps to recognize the presence of nanoparticles in the solution or on the surface of adsorbent, LC. After the sorption process the capsules were separated, grinded and treated with water. The observable solid layers were separated and the remaining solution was centrifuged. Added few drops of ethanol and one drop of this solution was dispersed on the carbon coated copper grid and dried prior to measurement. The TEM images were collected for the sample on the grid using HITACHI-JP/ H7600 (Japan) instrument operated at an accelerating voltage of 100 kv.

Similarly the X-ray photoelectron spectroscopy (XPS) measurements were carried out to find the state of metal ion on the surface of sorbent. AXIS-NOVA spectrometer (Kratos Analytical Ltd., UK) with a monochromatic Al Kα X-ray source (1486.71 eV of photons) was used to study the XPS data. The Au 4f7/2 photo peak with a binding energy of 84.0 eV was the reference peak for the calibration. A reduced power of 150 W and pressure less than 5.8x10-8 Torr in the analysis chamber were maintained during the measurement. Neutral C 1s peak at 285.1 eV was the reference to compensate the surface charging effects.

Conclusions

The present study shows a good relation between sorption capacity of metal ions and lipids. The responsibility of lipids for the sorption was successfully studied. The sorption capacity of easily reducible metals are greatly depends on the lipid percentage on the biomass. L-phosphatedylcholine was fabricated as capsule by impregnating in the alginate capsule which acts like a model reducing agent impregnated sorbent. The adsorbent achieved the good separation factor and shows high selectivity towards gold even in the presence of platinum and palladium. The sorption capacity of LC was found to be 366 mg g-1. The good sorption capacity of LC and separation factors made it as a selective adsorbent for gold from the platinum, palladium and gold mixtures. The reducing nature of lipids is responsible for the selective sorption of gold as the gold have high reduction potential value of 1.498 volts. The remaining metal ions have low reduction potentials consecutively the low sorption capacities. By the deep and thoughtful examination of the other researcher’s studies and our present study, we can clearly explains the responsibility of lipids in sorption for the metal ions having high reduction potentials. The selective sorption properties can be applied to the lipids if the mixtures of metal ions have different reduction potentials. The role of lipids present in biosorbents can be considered significant based on the target metal ion reduction potential values otherwise the electrochemical order of metal ions. This study also suggests the advantages of reduction agents in case of metal ions separations.