Solar-driven photocatalytic reduction of copper(ii) to copper(i) and zerovalent copper (Cu(0)): a sustainable approach for solar recovery of copper on a pilot scale

Copper stands at the forefront of materials driving the global transition to renewable energy and is a valued material for various important applications. For the first time, this paper presents an environmentally sustainable approach for recovering metallic copper through photocatalytic processes on a pilot scale, avoiding the energy-intensive conventional techniques. The study is focused on the selective photocatalytic reduction of copper(ii) to either copper(i) or zerovalent copper (Cu(0)) based on the reaction conditions. This entire process does not involve strong acids or bases or any hazardous chemicals but needs only light and simple photocatalysts such as zinc oxide (ZnO) and poly(o-phenylenediamine)/zinc oxide (POPD/ZnO). A raceway pond reactor (RPR) was used to scale up the process in deionized water (DW), tap water (TW), and seawater (SW) using ZnO. Thermodynamic considerations were used to predict the reduction of Cu(ii) to Cu(i) {Cu(ii)/Cu(i) (+0.153 V)} and Cu(0){Cu(ii)/Cu(0) (+0.337 V), Cu(i)/Cu(0) (+0.521 V)}. Formic acid served as a sacrificial reagent, while chloride ions modulated the reaction pathways and products at pH 6.5. The copper speciation of Cu(ii), Cu(i), and Cu(0) was analyzed using X-ray diffraction (XRD), fluorescence spectroscopy (FS), laser-induced breakdown spectroscopy (LIBS), energy-dispersive X-ray spectroscopy (EDX), and flame atomic absorption spectroscopy (FAAS). The "first copper coin" was produced solely through 100% solar energy-driven photocatalysis. With an 80% recovery rate of Cu(0), our approach demonstrates a proof of concept for efficient copper recovery from wastewater, the mining industry, and e-waste. These findings offer valuable insights for further exploration of solar-driven metal recovery processes, underscoring the potential of solar energy in fostering sustainable industrial practices.