Rapid and selective E-waste gold extraction from aqua regia by dual redox active-sites of skeleton modulated poly(organo)phosphazenes

Abstract

The sustainable recovery of gold from electronic waste (E-waste) remains a significant challenge due to lack of multiple redox active-sites in the existing adsorbents, their high cost, and poor stability in extreme acidic conditions. Herein, we rationally designed the skeleton engineering of poly(organo)phosphazene via substitution reaction of hexachlorocyclotriphosphazene (HCCP) with p-phenylenediamine (DA), 1,4-benzenedithiol (DT), and 4-aminothiophenol (AT), producing poly(p-phenylenediamine-co-cylotriphosphazene) (PDA) with N-redox active-sites, poly(1,4-benzenedithiol-co-cylotriphosphazene) (PDT) containing S-redox active-sites, and poly(4-aminothiophenol-co-cylotriphosphazene) (PAT) holding dual N-S-redox active-sites. The PDA, PDT, and PAT in the N-bromosuccinimide/pyridine (NBS/Py) leaching solution demonstrated extraction efficiency of 83.7 % Au³⁺, 45.5 % Cu^2+, 44 % Ni²⁺, and 83.6 % Au³⁺, 47.4 % Cu²⁺, 48 % Ni²⁺, and 83.1 % Au³⁺, 48.5 % Cu²⁺, 48.8 % Ni²⁺. While, in the aqua regia, these adsorbents showed extraction efficiency of 70.0 % Au³⁺, 14.0 % Cu²⁺, 13.8 % Ni²⁺, and 92.3 % Au³⁺, 6.4 % Cu²⁺, 9.8 % Ni²⁺, and 95.6 % Au³⁺, 13.0 % Cu²⁺, 10.3 % Ni²⁺. In particular, the PAT with the dual N-S-redox active sites achieved maximal Au³⁺ extraction efficiency (99.2 %), which is 1.05 and 1.04 times higher than PDA and PDT within 15 min, superb uptakes of 2000 mg/g, excellent selectivity (99.9 %) in a mixture of eight metal ions (Al, Co, Cu, K, Mg, Na, Ni, Mn), 100 % removal capacity and high stability even in pure aqua regia leaching solution. Gold ions adsorbed on these adsorbents, spontaneously reduced, experienced nucleation, and grew into single crystalline nanoplates along (111) planes as well as nanolayers composed of Au-single-atoms and Au-nanoclusters. The density function theory (DFT) calculations and variation in binding energies of N and S indicated that due to the synergy of dual N-S-redox active-sites, the PAT showed enhanced performances and excellent selectivity. For commercial applications, the as-synthesized PDA, PDT, and PAT were highly stable in pure aqua regia leaching solution, showing precisely selective and rapid gold extraction (70 %, 92 %, 96 %, respectively) than N- NBS/Py leaching systems (83 %) with traces of other competing metals. This study proves that skeleton-modulated poly(organo)phosphazene greatly facilitates gold extraction for industrial-scale applications.