Wet mechanochemistry redirects heavy-metal fate from surface adsorption to lattice mineralization

Zinc and cadmium are prototypical heavy metals whose aqueous sequestration is often limited to metastable surface adsorption or precipitation, leaving their ultimate solid-phase fate poorly controlled. Here, we establish a wet mechanochemical reaction field using inexpensive microscale zero-valent iron as the reactive scaffold. Through triphasic bead-iron-water collisions and friction, a mechanochemical reaction field is established, accelerating Zn/Cd removal rates by 6.98-fold and 10.85-fold, respectively, relative to stirred systems, while enhancing metal-to-iron molar ratios per unit Fe by 9.1-fold and 14.6-fold. Kinetic gains and elevated loadings extend to a broad spectrum of metals spanning electrode potentials. In 20-hour continuous-flow operation, the MCR sustains effluent Zn below 2.0 mg·L^-1 and Cd below 0.1 mg·L^-1. Multiscale characterization reveals that Zn(II)/Cd(II) transform via an interfacial complexation, Fe-O framework rearrangement, long-range ordering pathway into spinel ferrites dominated by ZnFe₂O₄/CdFe₂O₄. The resulting solids exhibit cumulative leaching rates below 1.1% across wide pH/E_h windows and in the presence of EDTA, with acid washing yielding high-crystallinity ZnFe₂O₄/CdFe₂O₄. These results show that wet mechanochemistry can redirect heavy-metal fate from surface-bound intermediates to lattice-mineralized ferrites, establishing an aqueous route to stable sequestration - yielding recoverable ZnFe₂O₄ mineral products while converting Cd into a stabilized, low-leaching CdFe₂O₄ phase for controlled management.