Photochemically Enhanced Selective Adsorption of Gold Ions on Tannin-Coated Porous Polymer Microspheres

Abstract

Metal recovery from electronic waste and industrial wastewater has attracted increasing attention to recycle precious metals and inhibit the emission of hazardous heavy metals. However, the selective recovery of precious metals with a large quantity is still very challenging because wastewater contains a variety of different cations while precious metal ions are relatively scarce. Here, we introduce a simple method to selectively increase the adsorption of gold ions using tannin-coated porous polymer microspheres through photochemical reduction. Mesoporous poly­(ethylene glycol dimethacrylate-co-acrylonitrile) microspheres with an average pore diameter of 13.8 nm were synthesized and used as an adsorbent matrix. Tannic acid (TA) was deposited onto the internal pores of the polymer matrix by simple immersion in an aqueous milieu. TA coatings increased the maximum number of adsorbed gold ions by 1.3 times because of the well-known metal ion chelation of TA. Under light illumination, the maximum number of adsorbed gold ions dramatically increased by 6.1 times. We examined two distinct mechanisms presumably involved in the enhanced adsorption: the photooxidation of TA and plasmon-induced hot electrons. Moreover, TA-coated microspheres exhibited remarkable selectivity for gold ions among competing metal ions commonly found in waste resources. This work suggests that the photochemically activated TA can serve as an excellent adsorbent for the selective and efficient recovery of gold ions from wastewater.

Direct Z-Scheme Tannin–TiO2 Heterostructure for Photocatalytic Gold Ion Recovery from Electronic Waste

“…This result demonstrates that the hydroxyl groups in TA are efficient at attracting and adsorbing the gold ions, as demonstrated in our previous work. 24,27 On the other hand, the k 2 values, which indicate the amount of adsorbent required to adsorb 1 mol of metal ions/min, decreased by a factor of 7.0 for TA−TiO 2 (1-sun) (0.08 g mmol −1 min −1 ) in comparison with TA−TiO 2 (dark) (0.56 g mmol −1 min −1 ). This result suggests that light leads to improved adsorption rates of gold ions, probably by reducing the gold ions, which is discussed in the following section.…”

“…25,26 Our recent work demonstrated that the photochemical activation of TA-coated polymer microspheres can facilitate the adsorption of metal ions. 27 However, the photochemical activity of TA was measured on an inert polymer substrate. This single-component photochemical reaction has limitations, such as undesirable backward reactions and reduction of light absorption by redox mediators.…”

Cross‐Linked Polyphosphazene Hollow Nanosphere‐Derived N/P‐Doped Porous Carbon with Single Nonprecious Metal Atoms for the Oxygen Reduction Reaction

“…[6] Because of its biocompatibility,o xidation resistance,a ntibacterial and anti-inflammation properties,a nd strong affinity with biomacromolecules such as DNA, collagen, gelatin and thrombin, TA has been employed to prepare numerous biomaterials. [7] Thehigh affinity between TA and biomacromolecules is attributed to multiple hydrogen bonds formed between the phenolic hydroxyl-rich moieties and target objects. [8] Additionally,w ith five galloyl groups and five catechol groups on each molecule,T Ac an coordinate with different kinds of metal ions to form metal-organic coordination polymers (MOCs).…”

Selective Gold Recovery from Homogenous Aqueous Solutions Containing Gold and Platinum Ions by Aromatic Amino Acid-Containing Peptides

“…However, it requires specialized equipment and generates hydrogen as a byproduct at the cathode surface under dilute conditions. Furthermore, electronic waste generally contains dilute noble metal ions together with metal ion contaminants [ 5 ]. New noble metal recovery processes should meet the following challenging criteria: (i) high metal element selectivity, (ii) one-pot operation, (iii) easy separation and (iv) a monophasic reaction in (homogenous) aqueous solution under dilute conditions (i.e., a sustainable chemical process).…”