Understanding the Dissolution and Passivation of an Aluminum Electrode during Electrocoagulation of Groundwater Using Neutron and X-ray Reflectometry

An aluminum (Al)-based electrocoagulation (EC) system can effectively remove dissolved silica and hardness in groundwater. The effectiveness of Al-EC in terms of pollutant removal, Faradaic efficiency, and energy consumption depends on the interfacial electrolysis or passivation of the electrode in water. Thus, understanding the electrolysis reaction at the liquid/electrode interface during operation is important for sustainable EC deployment. A continuous flow-through Al-EC system was tested with various groundwater simulants, i.e., chloride (Cl^-)-based, sulfate (SO₄^2-)-based, and mixed solutions. High pollutant removal with low energy consumption was observed in Cl^--based groundwater treatment, while low pollutant removal with high energy consumption was observed in SO₄^2--based groundwater. For example, the required energy per unit mass of Al dosing in SO₄^2--based groundwater is three times higher than that in Cl^--based groundwater at 10 mA/cm². However, increasing the Cl^- concentration significantly reduces this energy demand. In SO₄^2--based groundwater, the silicate removal efficiency drops from 85.1% to 24.0% compared to that for Cl^--based groundwater, while Mg²⁺ and Ca²⁺ removal efficiencies decrease to 0.6% from 15.8% and 5.7% from 44.8%, respectively. To better understand this EC performance, we used in situ neutron reflectometry (NR) to examine the interfacial dynamics of Al dissolution and passivation at a 100 nm scale occurring on the surface of the sacrificial Al electrodes during EC. Ex situ X-ray reflectometry (XRR) was also used to support the in situ NR results. Both NR and XRR results revealed that Al dissolution is influenced by the presence of Cl^- in the simulants, while a passivating layer forms on the electrode in a SO₄^2--based solution. In the Cl^--based solution, anodic Al dissolution occurred locally and inhomogeneously across the surface of the Al anode film, resulting in a localized thickness reduction over time. In the SO₄^2--based solution, no apparent dissolution of the Al anode was identified. Instead, Al underwent oxidation, forming an amorphous Al₂O₃ surface layer within the Al electrode film that increased in thickness over time. In the mixed solution, both anodic Al dissolution and surface Al₂O₃ layer formation occurred, indicating that Al dissolution and surface Al₂O₃ layer formation are attributable to the Cl^- and SO₄^2- ions, respectively.