Bright Light, Clean Water: A New Way to Tackle Cyanide

A team of researchers tried a fresh approach to clean up the nasty chemical cyanide that often ends up in mining wastewater. They mixed a special material called bismuth vanadate (BiVO₄) with ozone gas and exposed the mixture to light. The goal was to break down cyanide that is stuck inside metal‑cyanide complexes, which are common in the waste from silver mining. The study first created a lab version of the real wastewater. The mixture had an alkaline pH of 10. 5 and contained silver, copper, iron, lead, and zinc. When they looked at how cyanide behaves in this environment, they found that free cyanide and certain metal complexes stay stable at high pH. Lead and zinc, however, do not form strong cyanide bonds. Adding ozone alone or using light on BiVO₄ separately could loosen these complexes, but the combination of light, BiVO₄, and ozone worked best. The light activates BiVO₄, producing reactive species that attack cyanide molecules, while ozone also reacts in the bulk solution. Together they broke down both free and bound cyanide faster than either method alone.The researchers measured the reaction rate with a modified Langmuir–Hinshelwood equation, which takes into account both surface reactions on the catalyst and reactions in the liquid. In two full cycles, they removed all detectable cyanide. In a third cycle, some activity dropped because metals stuck to the BiVO₄ surface, but the system still removed cyanide more efficiently than using ozone alone. An analysis of the by‑products showed that instead of turning into cyanate as predicted, most of the degraded material ended up as ammonium. This suggests a different breakdown pathway when BiVO₄ is involved. Overall, the study shows that using light‑activated BiVO₄ with ozone can clean up cyanide‑laden mining wastewater while needing less oxidant. It offers a greener and more effective option for treating industrial waste.