Metalloselective adsorption of Au(III), Pd(II) and Pt(IV) from acidic solutions by a thiourea-modified Cr-based metal-organic framework

Abstract

The development of effective adsorbents is crucial for the sustainable recovery of noble metals from secondary resources. In this study, MIL-101(Cr)-Tu, a Cr-based metal‒organic framework (MOF) modified with thiourea, was successfully prepared by double-phase encapsulation followed by post-synthetic modification. At 298 K, the largest adsorption capacities of Au(III), Pd(II), and Pt(IV) by MIL-101(Cr)-Tu were (1230.83 ± 8.72), (330.41 ± 7.29), and (315.58 ± 13.67) mg·g−1, respectively. The uptakes of Au(III), Pd(II), and Pt(IV) increased with temperature. The adsorption data fit well to both the Langmuir and pseudo-second-order kinetic models, indicating that the adsorption process conformed to monolayer adsorption and chemisorption. Mechanistic analysis revealed that the amino and thiourea groups facilitated the adsorption of noble metals through coordination and electrostatic attraction. Notably, Au(III), Pd(II), and Pt(IV) interacted strongly with the thiourea groups owing to their “soft–soft” interactions. Moreover, Au(III) was reduced to Au(I) and Au(0), Pt(IV) was reduced to Pt(II), and the amino groups were oxidized to NO2. The adsorption of Pd(II) did not involve redox reactions. Additionally, MIL-101(Cr)-Tu exhibited strong selectivity for noble metals and excellent reusability, demonstrating its great potential for the extraction of noble metals from secondary resources.

1. Introduction

Noble metals possess unique physical and chemical properties, including high chemical stability, excellent electrical and thermal conductivities, high ductility, and corrosion resistance, making them widely used in the production of currency, jewelry, catalysts, and electronics [1–3]. However, noble metals are nonrenewable resources, and their supply cannot meet the increasing demand, leading to global shortages. Moreover, advancements in technology and improvements in living standards have generated significant quantities of electronic waste (e-waste), including cell phones and computers. The concentration of noble metals in e-waste often surpasses that in natural ores. By 2016, e-waste contains over 6800 t of gold, accounting for 16% of the global reserves [4]. Unfortunately, only approximately 20% of e-waste is effectively recycled, with the majority remaining in landfills, resulting in a waste of resources and a significant threat to the ecosystem and human health. Therefore, the extraction of noble metals from e-waste is essential for ensuring a sustainable source of these vital resources and for environmental protection.

Hydrometallurgy is widely used to recover noble metals from e-waste. This process involves the dissolution of disassembled and crushed e-waste in strong acids [5] to form acid-stable complex anions, such as AuCl₄⁻, PdCl₄²⁻, and PtCl₆²⁻. These metal ions are concentrated and recovered using various physical and chemical techniques, such as ion exchange [6], membrane separation [7], solvent extraction [8], and adsorption [9]. Adsorption is widely recognized as an efficient and environmentally benign method to recover noble metals, owing to its simplicity, high selectivity, and operational flexibility. Compared to conventional techniques, such as precipitation or solvent extraction, adsorption offers lower energy consumption and better adaptability for the recovery of noble metals from industrial effluents [10–11]. Commonly used adsorbents such as activated carbon [12], resin [13], and lignin [14] offer wide availability, low cost, and high porosity. However, shortcomings such as limited adsorption capacity, low selectivity, and regeneration challenges seriously hinder their application and promotion in industrial practice.

Metal–organic frameworks (MOFs) are inorganic–organic hybrid porous materials composed of metal nodes (metal ions or metal clusters) and organic linkers [15–17]. Unlike traditional porous materials such as activated carbon, MOFs exhibit distinct structural advantages that are critical for selective adsorption applications, such as tunable pore sizes and versatile chemical properties, enabling precise control over the accessibility of noble metal ions and selective coordination and redox interactions. As a result, MOFs have been widely investigated in fields such as drug delivery [18], energy storage [19], wastewater treatment [20], and sensing [21]. Recently, MOFs have attracted considerable interest as adsorbents for the recovery of noble metals. For instance, Chang et al. [22] prepared UiO-66-NH2, which effectively adsorbed 50wt% of Au(III) within 6 min and achieved a maximum adsorption amount of 650 mg‧g−1 at 298 K, exhibiting significant advantages over traditional adsorbents. Lim et al. [23] synthesized amine-functionalized MIL-101(Cr) (MIL-101(Cr)-NH2) from its nitro-functionalized precursor MIL-101(Cr)-NO2 and employed it for the adsorption of Pd(II) and Pt(IV) anions from acidic aqueous solutio