minerals Article Recovery of Palladium and Gold from PGM Ore and Concentrate Leachates Using Fe 3 O 4 @SiO 2 @Mg-Al-LDH Nanocomposite Nkositetile Raphael Biata 1,2,3 , Silindokuhle Jakavula 1,2 , Richard Motlhaletsi Moutloali 1,3 and Philiswa Nosizo Nomngongo 1,2,3, *   Citation: Biata, N.R.; Jakavula, S.; Moutloali, R.M.; Nomngongo, P.N. Recovery of Palladium and Gold from PGM Ore and Concentrate Leachates Using Fe 3 O 4 @SiO 2 @Mg-Al-LDH Nanocomposite. Minerals 2021, 11, 917. https://doi.org/10.3390/ min11090917 Academic Editor: Zhiyong Gao Received: 12 July 2021 Accepted: 23 August 2021 Published: 25 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, Johannesburg 2028, South Africa; raphaelbiata@gmail.com (N.R.B.); jakavulasilindokuhle@yahoo.com (S.J.); moutloalirichard@gmail.com (R.M.M.) 2 Department of Science and Innovation (DSI)/National Research Foundation (NRF) South African Research Chair (SARChI), Nanotechnology for Water, University of Johannesburg, Doornfontein 2028, South Africa 3 Department of Science and Innovation (DSI)/Mintek Nanotechnology Innovation Centre, University of Johannesburg, Doornfontein 2028, South Africa * Correspondence: pnnomngongo@uj.ac.za Abstract: In this work, we developed a core–shell nanostructured magnetic composite by function- alizing layered double hydroxide (Mg-Al-LDH) microspheres with Fe 3 O 4 @SiO 2 , for the recovery of Au(III) and Pd(II). The magnetic Fe 3 O 4 nanoparticles provided effective magnetic separation of the adsorbent from aqueous solutions. While silica protected the Fe 3 O 4 nanoparticles, increased the adsorption sites and the stability of the material. Finally, Mg-Al-LDH was chosen because of its large anion sorption capacities which lead to the improved adsorption capacity of Fe 3 O 4 @SiO 2 @ Mg-Al-LDH nanocomposite. The morphology and structural composition of the nanocomposite were characterized using various analytical techniques. It was satisfactorily established that silica was coated on iron oxide and layered double hydroxide was immobilized on Fe 3 O 4 @SiO 2 . Parameters affecting adsorption of the composite towards Au(III) and Pd(II), such as effects of sample pH, mass of adsorbent, extraction time, eluent type and concentration were investigated using response methodology based on central composite design. Maximum adsorption capacities of Fe 3 O 4 @SiO 2 @ Mg-Al-LDH for Au(III) and Pd(II) were 289 mg g −1 and 313 mg g −1 , respectively. Under optimum conditions, the proposed method displayed good analytical performance suggesting that the adsor- bent is a good candidate for quantitative extraction of Au(III) and Pd(II) from secondary sources. Additionally, %recoveries ranging from 85%–99.6% were obtained revealing that Fe 3 O 4 @SiO 2 @ Mg-Al-LDH could selectively extract Au(III) and Pd(II) from leaching solutions of SARM 107 PGM ore and SARM 186 PGM concentrate. Keywords: F e3 O 4 @SiO 2 @Mg-Al-LDH; magnetic adsorption; PGM concentrates; gold; palladium; ore concentrates 1. Introduction The availability of precious metals, such as gold and palladium, is becoming very scarce and natural resources are depleting. This is due to their demand in numerous applications ranging from jewellery to automotive industries [1–6]. Due to their broad application, limited stock and the high price of precious metals make it vital to recycle and recover them from industrial waste [7]. Gold (Au) and palladium (Pd) metals are economi- cally important as investments and as currency. Recovering and recycling precious metals from waste have gained much attention in research industries [8,9]. Mohamed et al. [10], reported in their work that, there is about 300 g per ton of gold in computer motherboards and approximately 200 g in mobile phones [10]. Out of the top six PGMs, Pd plays a key role in manufacturing processes in industries such as fossil fuel refinery, automobile, Minerals 2021, 11, 917. https://doi.org/10.3390/min11090917 https://www.mdpi.com/journal/minerals