molecules Article The Mechanism of Adsorption of Rh(III) Bromide Complex Ions on Activated Carbon Marek Wojnicki 1,* , Andrzej Krawontka 1, Konrad Wojtaszek 1, Katarzyna Skibi ´nska 1 , Edit Csapó 2,3 , Zbigniew P˛edzich 4 , Agnieszka Podborska 5 and Przemysław Kwolek 6 1 Faculty of Non-Ferrous Metals, AGH University of Science and Technology, Mickiewicza Ave. 30, 30-059 Krakow, Poland; [email protected] (A.K.); [email protected] (K.W.); [email protected] (K.S.) 2 MTA-SZTE Biomimetic Systems Research Group, University of Szeged, H-6720 Dóm tér 8, 6720 Szeged, Hungary; [email protected] 3 Interdisciplinary Excellence Centre, Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich B. tér 1, H-6720 Szeged, Hungary 4 Faculty of Materials Science and Ceramics, AGH University of Science and Technology, al. A. Mickiewicza 30, 30-059 Krakow, Poland; [email protected] 5 Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, al. A. Mickiewicza 30, 30-059 Krakow, Poland; [email protected] 6 Department of Materials Science, Faculty of Mechanical Engineering and Aeronautics, Rzeszow University of Technology, 35-959 Rzeszow, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-126-174-126; Fax: +48-126-332-316 Abstract: In the paper, the mechanism of the process of the Rh(III) ions adsorption on activated carbon ORGANOSORB 10—AA was investigated. It was shown, that the process is reversible, Citation: Wojnicki, M.; Krawontka, i.e., stripping of Rh(III) ions from activated carbon to the solution is also possible. This opens the A.; Wojtaszek, K.; Skibi´nska,K.; possibility of industrial recovery of Rh (III) ions from highly dilute aqueous solutions. The activation Csapó, E.; P˛edzich,Z.; Podborska, A.; energies for the forward and backward reaction were determined These are equal to c.a. 7 and Kwolek, P. The Mechanism of 0 kJ/mol. respectively. Unfortunately, the efficiency of this process was low. Obtained maximum Adsorption of Rh(III) Bromide load of Rh(III) was equal to 1.13 mg per 1 g of activated carbon. Complex Ions on Activated Carbon. Molecules 2021, 26, 3862. https:// Keywords: activated carbon; rhodium adsorption; recycling; recovery; sorption doi.org/10.3390/molecules26133862 Academic Editor: Sebastian Schwaminger 1. Introduction Received: 16 April 2021 Rhodium, Rh, is a silvery-white noble metal belonging to the platinum group metals, Accepted: 12 June 2021 PGM. It is highly corrosion-resistant, and, similarly to iridium and contrary to platinum, Published: 24 June 2021 palladium and osmium does not react with aqua regia. Rhodium is an extremely rare metal. Its concentration in the earth’s crust equals 10−1 ppb. Usually, it is found together with Publisher’s Note: MDPI stays neutral other PGMs. Metallic rhodium is a by-product of the extraction of nickel and copper from with regard to jurisdictional claims in their ores [1]. The vast majority of the world’s rhodium resources is probably located in published maps and institutional affil- Bushveld Complex in the Republic of South Africa, RSA. This country is also the main iations. supplier of this metal. In 2019 it provided 18 tons of Rh which accounts for c.a. 90% of the world production [2]. In 2020 lower quantity of rhodium was produced due to some technical problems of the biggest Rh-supplier, Anglo American Platinum (Amplats), and a very strict lockdown implemented to prevent COVID-19 transmission among miners in Copyright: © 2021 by the authors. RSA [3]. This affected the price of the metal (vide infra). Other Rh deposits are located in Licensee MDPI, Basel, Switzerland. Canada, the USA, Colombia, and Russian Federation [4]. This article is an open access article Rhodium is added to platinum to improve its hardness and reduce the thermal expan- distributed under the terms and sion coefficient. Pt-Rh alloys are used for fabrication of crucibles for laboratory furnaces, conditions of the Creative Commons spark plug electrodes, thermocouples, also in jewelry, glass industry, electronics, and Attribution (CC BY) license (https:// electroplating. [1]. The most important, however, is fabrication of catalysts working at creativecommons.org/licenses/by/ the elevated temperature such as catalytic converters for the automotive industry. This 4.0/). Molecules 2021, 26, 3862. https://doi.org/10.3390/molecules26133862 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 3862 2 of 18 consumes around 80% of rhodium supplies. [5,6]. Due to more and more strict environ- mental regulations regarding NOx emission and high demand for gasoline vehicles in developing countries, such as the People Republic of China, consumption of rhodium increases gradually, but the supplies not. Thus, the price of Rh increased rapidly during last year (Figure1). Consequently, this stimulates the development of recycling processes. Recycling has already become an important source of rhodium supplies. As long ago as in 2010, rhodium recycling accounted for 7 tons of metal [7]. Even further increase of rhodium demand in the future can be speculated knowing that its organic complexes are promising light-emitting material in OLEDs [8,9]. Figure 1. Rhodium price evolution over the last decade [10]. Rhodium, due to its high corrosion resistance, can be dissolved in acidic solution only when it is in the form of rhodium(III) oxide, Rh2O3. Thus, the first step of the recycling process is roasting the host material of the catalytic converter in the air. Then rhodium oxide isdissolved, recovered and refined to obtain pure metal [7]. The recovery can be achieved using various methods e.g., precipitation with polyamines [11], solvent extraction, extraction with resins [12–14] and zinc cementation [15]. Very often, the recovery process determines the economic efficiency of the recycling of PGMs. Thus, it deserves appropriate scientific attention to study and develop the recovery methods. Besides, environmental factors must also be taken into account. The most important recovery methods, such as solvent extraction [16], ion exchange [12], cementation [17], electrowinning [18] and adsorption [19] are well known since years. Their applicability, however, strongly depends on the chemical composition of the leachate [13,20]. Thus, they should be adapted to the specific conditions of the recycling process. The literature on the problem of Rh extraction from aqueous solutions is scarce, possibly due to low rhodium prices over the last decades. Solvent extraction of Rh is possible using organic extractors [14,21,22]. This method, however, is still too expensive for widespread industrial use. Ion exchange, in turn, with coal or resin [23] as an absorbent is time-consuming [24]. Cementation, relatively cheap and rapid, is readily used in industry. This is simply a reduction process, with a metal less noble as rhodium as the reducing agent This is usually applied in the form of a fine powder. Unfortunately, the reducing agent becomes the contaminant of the extracted metal. Thus, Rh of higher purity is obtained when the reduction is performed in a homogeneous system, using formic acid, hydrazine hydrate, hydrazine sulfate [15], or ferrous ions as the reducing agent [25]. Electrowinning, in turn, is not suitable for highly diluted solutions [26]. Interestingly, the adsorption of rhodium ions was rarely studied. Rhodium chloride complex ions were adsorbed onto activated carbon [27] andTiO2. under AM1 simulated sunlight illumination [28]. In this work, we decided to explore the possibility of adsorption of Rh(III) bromide complex ions on activated carbon. This method was extensively studied for the recovery of noble metals, mainly gold [29,30] and platinum [31–33]. Bromide anions were selected as the complexing agent because the stability of the complex ion is lower when compared Molecules 2021, 26, 3862 3 of 18 to the chloride one [34,35]. This offers, at least theoretically, the possibility of adsorption and subsequent reduction of Rh(III) to the metallic form as it was observed in the case of Au(III) chloride complex ions [36–39]. Such a process is irreversible and offers a higher recovery yield when compared to the physical adsorption, where a certain equilibrium is established between the species adsorbed and remaining in the solution. The TD-DFT calculation method was used to calculate theoretical UV-Vis spectra of adsorbed species. The TD-DFT methods are often applied for IR, Raman, UV-Vis spectra calculations as well as for molecule structure optimization [40–43]. In the case of systematic studies, it may also help to understand the mechanism of the reaction. This is also the first step towards the systematic studies of adsorption of various complexes of noble metals onto activated carbon. The aim of such a study is a validation of a recently introduced adsorption isotherm which seems to be useful for modeling the industrially important processes of noble metals recovery from leachate. 2. Experimental Rh(III) bromide complex ions were prepared according to the following procedure. Firstly, 2.0152 g of metallic rhodium (Onyx Met, 99.97 wt.%) was placed in a round bottom flask and 40 mL of the concentrated, 95 wt.%, sulfuric(VI) acid of laboratory grade, together with 20 mL of deionized (DI) water (>18 MW, Polwater), were slowly added. The reactor was heated up to 340◦C in the sand bath, after c.a. 5 days the metal was dissolved and the brown solution was obtained. This was cooled to room temperature and diluted to 2 L with DI water. The color of the solution changed to yellow-brown. Secondly, sodium carbonate (Eurochem, laboratory-grade) was added dropwise to the Rh-containing solution to obtain rhodium(III) carbonate. Although no immediate precipitation was observed, the electrical conductivity of the solution was decreasing gradually. The process was stopped when the electrical conductivity started increasing.