Atomic force microscopy insights into Pb2⁺-Mediated benzohydroxamic acid adsorption and bubble attachment on cassiterite surfaces

Cassiterite (SnO 2 ), a strategically important tin-bearing mineral for the electronics and renewable energy sectors, remains challenging to recover by flotation because its hydrated and weakly reactive surface limits the direct adsorption of hydroxamic acid collectors. Herein, the Pb 2 ⁺-mediated interaction between benzohydroxamic acid (BHA) and cassiterite was investigated by integrating micro-flotation, surface spectroscopy, QCM-D adsorption monitoring, AFM imaging, functionalized-tip force spectroscopy, and bubble-probe force measurements. Compared with BHA alone and the conventional sequential Pb 2 ⁺/BHA addition strategy, the pre-mixed Pb–BHA system achieved the highest cassiterite recovery of 96.89% at pH 8.0. FTIR and XPS analyses indicated changes in surface chemical states consistent with Pb 2 ⁺-mediated BHA adsorption on cassiterite. Consistently, QCM-D results showed that pre-formed Pb–BHA species formed a compact and weakly dissipative adsorption layer on SnO 2 , while AFM imaging revealed a more uniform and continuous BHA-related surface coverage in the presence of Pb2⁺. Functionalized-tip AFM further provided direct force evidence for this enhanced interaction, with the Pb–4,4′-DBHA/cassiterite system exhibiting a much higher adhesion energy of 2.63 mJ/m2 than the 4,4′-DBHA/cassiterite and 4,4′-DBHA/Pb2⁺-activated cassiterite systems, which showed adhesion energies of 0.83 and 0.84 mJ/m2, respectively. Bubble-probe AFM measurements conducted in 100 mM NaCl demonstrated that reagent adsorption strengthened short-range hydrophobic attraction and destabilized the confined water film, increasing the critical film thickness for bubble attachment from 8.4 nm on reagent-free cassiterite to 11.5 nm after BHA treatment and 11.8 nm after Pb 2 ⁺–BHA treatment. These results indicate that Pb 2 ⁺ improves cassiterite flotation not simply by surface activation, but by acting as a coordination bridge that couples BHA adsorption, hydration-layer weakening, interfacial adhesion, and bubble attachment. This work provides direct nanoscale evidence for activator–collector synergy and offers guidance for the rational design of efficient oxide-mineral flotation reagent systems.