Overcoming Diffusion Mass Transfer Barriers by Surface Electro-Precipitation (SEP)

The economically and environmentally sustainable recovery of dilute (<100 mg L–1) and ultradilute (<1 mg L–1) metal ions from complex aqueous matrices is one of the grand challenges in inorganic separation science and technology. Existing separation methods fail in this task due to their inherently slow recovery rates, stemming from either the slow diffusion of dilute elements toward the separating surface or the slow agglomeration of colloidal precipitates. This work reports a novel electrochemical–chemical phenomenon observed during surface electro-precipitation (SEP) that can surmount these mass transport limitations in an essentially green manner, without using reagents and at low energy and low material costs. Using a porous carbon electrode, we demonstrate that SEP can reversibly uptake dilute and ultradilute Pb2+ at least 2 orders of magnitude faster than diffusion-controlled electrodeposition/electrowinning and adsorption. Paradoxically, the rate of SEP increases with a decrease in adsorbent dosage. We put forward a semiquantitative model that explains these extraordinary results by the rapid precipitation of basic lead carbonates at the cathode–solution interface. This discovery suggests a shift in the existing paradigm of accelerating mass transport at low concentrations, opening the door to recovering, rather than just removing, valuable and toxic elements from abundant depleted resources and contaminated water.