Selectivity In Platinum and Gold Biosorption in Aqueous Media Using the Dead Fungus Aspergillus Niger

Research on biosorption in aqueous systems by microorganisms such as bacteria, algae or fungi is of considerable interest, for example, for the capture of metals in complex or multi-element aqueous solutions.Batch experiments were carried out with binary synthetic solutions in the biomass-metals system, to establish the biosorption selectivity of platinum and gold in the presence of the metals copper, calcium, titanium, aluminium, and iron. During the experiments, one gram of dead Aspergillus niger biomass was placed in contact for 120 minutes with a binary solution of 20ppm of gold or platinum with known concentrations (greater than 50ppm) of one of the other metals, at pH 4. The variables monitored were pH, electrical conductivity and Oxidation-Reduction Potential (ORP). The experimental results show that platinum was selectively and completely bio adsorbed by the fungus before the rest of the metals in the solution, including gold. In the case of binary solutions with gold, although gold is preferentially adsorbed, there is some competition in biosorption in the presence of the other metals. From the biomass-platinum-gold solution contact experiments, the calculation of the bio adsorption kinetic parameters indicate that the mechanism is controlled by physisorption

Keywords:Biomass; Binary solution; Biosorption; Gold; Platinum; Aspergillus niger

Gold and platinum are known as precious or noble metals, due to their high stability due to the physical and chemical properties they present [1]. These metals are obtained from minerals in the form of secondary products or by recovering some waste that contains them. From minerals, they have been conventionally recovered by acid leaching based on their chemical properties, mainly due to the coordination of their complexes in a chloride medium and the alteration they present in their oxidation states [2]; subsequently, the use of conventional techniques allows obtaining it in the form of precipitates, however they present certain limitations such as high cost and energy consumption, low efficiency, high contamination and sludge generation [3]. Biosorption is a cost-effective technique in which various biomaterials, including microorganisms, algae, and plant byproducts are used for the removal of metals in complex aqueous solutions, such as multielement media [4,5].

In a multielement system there is ion competition, and biosorption for a specific metal ion decreases, due to the available adsorption sites on the surface of the adsorbent [3]. In an aqueous phase, during multielement biosorption of metal ions, these ions are transported simultaneously through the fluid phase and competitively bind to the substrate; this biosorption process with respect to a bio adsorbent, develops in three stages [6]:
a. Diffusion of the contaminant ion towards the outer surface of the adsorbent. Overcoming the resistance to mass transfer offered by the double-layer interface between the main phase and the outer surface of the adsorbent gives rise to the phenomenon of surface diffusion.
b. Ions diffuse from the outer surface of the adsorbent into its internal pores; that is, the internal porosity of the biomass is directly related to diffusion in the pores, although in some cases diffusion is omitted and the process takes place on the surface of the biosorbent.
c. Once the internal walls of the pores are saturated with metal ions (physisorption), chemisorption occurs in certain cases at the substrate interaction sites. The mesoporous nature of bio adsorbents is one of the parameters that has a dominant influence on the general adsorption behaviour; that is, if the internal porosity of the packed bed is compared with the external void, and this is significant, the diffusivity in the pores improves the biosorption kinetics on the internal walls of the pores.

Regarding the kinetic study, the biosorption rate, the external mass transfer, the mass transfer within the particle and in general the biosorption phenomena, establish the mechanism that controls the process [7]. Using different models, adsorption processes can be evaluated and described using mathematical equations; currently, there are a variety of mathematical models that determine the kinetic parameters during the bio adsorption phenomenon in a solid-liquid system, the most common being the first-order model and the pseudo-second-order model. The first order or Lagergren equation is based on the adsorption capacity of the solid (adsorbent), where each metal ion is assigned an adsorption site on the adsorbent material [8].

Where e is the concentration of solute removed at equilibrium per amount of adsorbent (mg/g); qt is the concentration of solute removed at time t per amount of adsorbent (mg/g), k_L is the rate constant at pseudo first order equilibrium (min^-1), and t the solidliquid contact time (min). The pseudo-second order model is based on the assumption that the adsorbate adheres to two different active sites of the biomass [8,9]; it is expressed by the following equation:

Where q_t is the concentration of adsorbed metal (mg/g); t is the solid-liquid contact time (min); q_e is the equilibrium biosorption capacity (mg/g); and k₂ the equation constant (g/mg min).

This work studies the recovery of precious metals Au and Pt by biosorption from binary synthetic solutions that also contain some of the metals copper, calcium, titanium, aluminum, and iron. Biosorption is carried out by contact with dead biomass of Aspergillus niger. A pregnant solution produced by the leaching of a gold mineral is simulated.

Materials and methods

Dead biomass of Aspergillus niger was used; gold, platinum, calcium, magnesium, iron, titanium, and aluminium standards (Sigma-Aldrich) were used.

Cultivation and reproduction of the biomass strain

The Aspergillus niger strain was grown on Potato Dextrose Agar (PDA) and placed in a SHL incubator (LABV model 1525) for 72 hours at 26 °C. The fungus was identified by macroscopic and microscopic monitoring; once the strain was isolated and purified, it was grown in nutrient medium (PDA), incubated at 30 °C for 72 hours at 150rpm, and to obtain dead biomass it was dried in a FELISA oven between 60-70 °C for 12 hours; finally, it was crushed in a porcelain mortar and stored in a glass container with a lid in the desiccator for contact experiments.

Biosorption experiments

Batch contact experiments were performed for 20mg/l of gold and platinum separately, in binary systems with known concentrations (greater than 50mg/l) of the metallic elements Ca, Fe, Al, Cu, and Ti to establish the selectivity in biosorption. These experiments were carried out using synthetic solutions. 25ml of each of the synthetic binary solutions were placed in contact with 1 gram of dead Aspergillus nigerbiomass in Falcon tubes for 120 minutes at pH 4, and the pH, conductivity and Oxidation-Reduction Potential (ORP) were monitored. Aliquots were collected every 20 minutes during the first hour and another aliquot at 40 minutes in the second hour. The collected samples and the residual solution were analyzed by atomic absorption technique (EAA – PinAacle 900H Perkin Elmer).

The adsorption capacity was evaluated from the following expression:

Where q_e is the amount adsorbed at equilibrium (mg/g), C_o is the initial concentration of metal ions (mg/l), C_e is the concentration of metal ions at equilibrium (mg/l), V is the volume of the aqueous phase (l) and m is the amount of adsorbent used (g) [10].

Binary systems of Pt with Ca, Cu, Fe, Ti, or Al

The results obtained for the different binary systems with platinum are shown in Figure 1 & 2; in all cases, the removal percentage for platinum was higher than the rest of the metals analyzed, even though the concentration of platinum (20ppm) was lower compared to the concentrations of the rest of the metals (greater than 50ppm). The above behaviour is due to the fact that platinum ions have a lower ionic radius (52pm), and higher ionization energy (870KJ/mol) [11], such that the dead biomass of Aspergillus niger showed preference for platinum ions. In the case of the Pt-Ca binary system, the removal for Pt was 100%, because platinum has a lower ionic radius (52pm) than calcium (118) and greater electronegativity (Pt=2.2, and Ca=1.0). The lower the ionic radius, the greater the amount of metal that will accommodate itself in the active sites present in the biomass, and the greater the electronegativity, the more easily it adheres to the functional groups present on the surface of the biomass.

Figure 1:Percentage of metal removal in binary systems: Pt-Fe, Pt-Ca and Pt-Cu.

Figure 2:Percentage of metal removal in binary systems: Pt-Al, and Pt-Ti.

For the Pt-Fe, Pt-Al and Pt-Ti systems, the results show a competition for the binding sites in the biomass, this behaviour is due to their properties, such as oxidation number and electronegativity; For iron, aluminium and titanium the oxidation number they present is 3+, while platinum has an oxidation number of 2+, however, it has greater electronegativity than the elements in question, which makes it more selective for the dead biomass of Aspergillus niger, while for the Pt-Cu system there is also competition for the active sites of the biomass, which is attributed to the electronegativity of both copper (1.9) and platinum (2.2), since there is no great difference between the two, however, platinum is preferentially adsorbed due to its lower ionic radius (ionic radius of Cu equal to 96pm).

Binary systems of Au with Ca, Cu, Fe, Ti, or Al

For binary systems with gold (see Figure 3 & 4), the removal percentage was higher for gold in all binary systems than for the rest of the elements analyzed (Ca, Cu, Fe, Ti and Al). From the above figures, although in most cases gold absorption does not reach 100%, the rest of the metals are adsorbed in a lesser proportion and it can be established that the dead biomass of Aspergillus niger is selective for gold; as with binary solutions with platinum, the selectivity and extent of biosorption of metals will depend on the differences between the electronegativity, ionic radius and valence values of the elements.

Figure 3:Removal percentage of metals in the binary systems: Au-Fe, Au-Ca, and Au-Cu.

Figure 4:Removal percentage of metals in the binary systems: Au-Ti, and Au-Al.

Binary system Au- Pt

The results of the Au-Pt binary system are shown in Figure 5, it is observed that both metals compete for the binding sites of the fungal biomass. This is because gold and platinum are found together in the same period of the periodic table and their electronegativity is similar (Au=2.4; Pt=2.2); however, the ionic radius is greater for gold (138pm) and smaller for platinum (52pm) so the dead biomass of Aspergillus niger is even more selective for platinum than for gold; that is, more platinum ions settle on the surface of the biomass.

Figure 5:Percentage removal of the Pt-Au binary system on the surface of the fungus.

The results of the biosorption kinetics of platinum and gold at pH 4 are presented in the same Figure 5. An increase in the biosorption rate is distinguished for platinum, due to the high availability of active sites, reaching the highest percentage of removal at 20 minutes and maintaining it up to 120 minutes. In the case of gold, an increase in the speed of removal of the metallic ion is noted in the first twenty minutes, which is attributed to the availability of active sites; continuing, after forty minutes there is a decrease in the speed of the biosorption process, since the main available sites are occupied; after sixty minutes there is an increase in the biosorption of the metal, reaching maximum biosorption at 100 minutes, and the maximum percentage of removal. Based on the biosorption kinetics data of the Pt-Au system, a mathematical analysis was carried out in order to identify the kinetic model that best describes the biosorption process using the following models: pseudo-first order (Lagergren) and pseudo-second order.

The determination of the parameters of interest and the adjustment to the models was carried out by means of non-linear regression analysis with the Statistica 7.0® software, shown in Figure 6. The model that best describes the experimental adsorption data is the pseudo-first order model or Lagergren equation; for both cases, the correlation coefficients for platinum and gold biosorption are higher than for the other two models. Table 1 shows the biosorption kinetics parameters obtained from the fits to the kinetics models tested in this study.

Figure 6:Kinetic models for the biosorption of the platinum-gold binary system. (a and b, respectively)..

Table 1:Equilibrium retention capacity (qe), adsorption rate constant (k) and correlation coefficient (R²) of the Pt-Au system. The value of k=0.5984g/mg.min for platinum, and K=0.1192g/mg.min for gold, indicates the biosorption rate in the pseudo-first order model.

Monitoring pH, electrical conductivity, and ORP in the biosorption process of the Pt-Au system

Figure 7 shows the pH variations during the biosorption experiments. This variation is insignificant and remains practically constant during the 120 minutes of contact. In the case of electrical conductivity, it is observed that it decreases as the contact time passes, due to the decrease of ions in solution as the bio adsorption proceeds. Figure 8 shows the ORP variations for the platinum-gold system. According to the Pourbaix diagrams for the Au-H₂O-Cl and Pt-H₂O-Cl systems, the obtained ORP values indicate that the experiments are being carried out in the stability zone of the AuCl₄ and PtCl₄ ionic species, concluding that there is no formation of precipitates and that the gold and platinum ionic species will then be adsorbed on the appropriate active sites on the biomass surface.

Figure 7:Variation of pH and electrical conductivity in the Pt-Au system.

Figure 8:ORP variations during solution-fungus contact, at pH 4 for the Pt-Au binary system. Pourbaix diagrams of the Au-H₂O-Cl, and Pt-H₂O-Cl.systems.

From contact experiments between dead Aspergillus niger mass and binary synthetic liquid mixtures composed of platinum or gold mixed separately with each of the metals Ca, Fe, Al, Cu, and Ti, at pH 4, the following conclusions are derived: For binary systems with platinum, this metal was bio absorbed first in the case of calcium, iron and aluminium; however, there is a competition in biosorption with copper and titanium, where the extent of biosorption depends on the ionic radius and electronegativity of the metals.

In binary systems with gold, although gold is preferentially adsorbed, there is more competition than with platinum solutions. For the platinum-gold mixture, the adsorption selectivity is for platinum with 100% recovery, while gold was recovered at 50%, apparently limited by the amount of biomass or active sites available for biosorption.

Gabriela Tapia-Quiroz deeply thanks the UMSNH for the support she received during her doctoral studies and for carrying out this research work.

1. Tobón Mazo MJ (2018) Geochemistry and mineralogy of platinum group elements (PGE) in the nickel-bearing laterites of Cerro Matoso and Planeta Rica, Colombia. National University of Colombia, Faculty of Mines, Department of Materials and Minerals, Medellín, Colombia.
2. Doyle FM (1988) Developments in hydrometallurgy. JOM 40(4): 32-38.
3. Abdulaziz M, Musayev S (2017) Multicomponent biosorption of heavy metals from aqueous solutions: A review. Pol J Environ Stud 26(4): 1433-1441.
4. Flórez EC (2020) Sorption model for the removal of copper and lead from leachates of sanitary landfills.
5. Gouda SA, Taha A (2023) Biosorption of heavy metals as a new alternative method for wastewater treatment: A Review. Egypt J Aquat Biol Fish 27(2): 135-153.
6. Basu A, Ali SS, Hossain SS, Asif M (2022) A review of the dynamic mathematical modelling of heavy metal removal with the biosorption process. Processes 10(6): 1154.
7. Moafi HF, Ansari R, Ostovar F (2016) Ag₂O/Sawdust nanocomposite as an efficient adsorbent for removal of hexavalent chromium ions from aqueous solutions. J Mater Environ Sci 7(6): 2051-2068.
8. Febrianto J, Kosasih AN, Sunarso J, Ju Y, Indraswati N, et al. (2009) Equilibrium and kinetic studies in adsorption of heavy metals using biosorbent: A summary of recent studies. Journal of Hazardous Materials 162(2-3): 616-645.
9. Ho YS (2006) Review of second-order models for adsorption systems. J Hazard Mater 136(3): 681-689.
10. Najam R, Andrabi SMA (2016) Removal of Cu(II), Zn(II) and Cd(II) ions from aqueous solutions by adsorption on walnut shell-equilibrium and thermodynamic studies: treatment of effluents from electroplating industry. Desalination Water Treatment 57(56): 27363-27373.
11. Fagundes-Klen MR, Ferri P, Martins TD, Tavares CRG, Silva EA (2007) Equilibrium study of the binary mixture of cadmium-zinc ions biosorption by the Sargassum filipendula species using adsorption isotherms models and neural network. Biochem Eng J 34(2): 136-146.