From microscale redox cycling to urban dust: A three-stage Cu isotope fractionation model for arid tailings and the role in tracing Cu in street dust.

Cu-isotope compositions, supported by XAS speciation, reveal a three-stage cyclic model of copper mobility in historical mine tailings from Nacozari de García (northwestern Mexico): (1) sulfide oxidation and AMD generation, (2) capillary transport with redox-driven isotope filtering, and (3) evaporative precipitation of efflorescent salts. A key finding is the persistence of Cu(I) in the inner tailings materials. XAS analysis shows that 58-65% of Cu at depth remains Cu(I) despite the overall oxidized environment, indicating localized reducing microenvironments where Cu(II) reduction and secondary sulfide precipitation drive extreme 65Cu depletion (down to -2.78‰). These microscale processes, combined with large-scale Cu mobilization, shape the bulk isotopic signature. Efflorescent salts act as a short-lived, highly soluble Cu reservoir. Their isotopic compositions are systematically less negative than the underlying matrix, reflecting fractionation during capillary rise and surface precipitation, and capturing the surface expression of the three-stage cycle. This offset reflects the combined effect of large-scale Cu mobilization and microscale redox processes that further modify the residual isotopic signature. The Cu isotope composition of street dust (-0.10 to +0.30‰) indicates that mining-derived Cu coexists with an additional non-exhaust vehicular Cu source, including efflorescence salts formed on worn lead-acid batteries, whose Cu isotope composition is characterized here for the first time.