(464d) Neutron Characterization of Aluminum Electrodes Used in Electrocoagulation Pretreatment of Groundwater for Silica and Hardness Removal

Aluminum-based electrocoagulation pretreatment effectively removes dissolved silica and hardness from groundwater, which generate irreversible scale on the membrane surface and limit water recovery in membrane-based desalination applications. For the mechanism of the electrocoagulation process, it has been hypothesized that the anode (Al) is sacrificially dissolved in the groundwater to produce trivalent ions (2Al →2Al³⁺ +6e^-), while oxidation and cathodic reduction (6H₂O +6e^- →6OH^- +3H₂) occurs at the cathode. Subsequently, electrocoagulation occurs as dissolved aluminum ions from the anode react with the OH^- produced at the cathode to form M(OH)_n, which adsorbs the silica from the solution. In this study, we employ in situ neutron reflectometry (NR), ex situ X-ray reflectometry (XRR), and ex situ neutron imaging (NI) to achieve fundamental understanding of how the dissolution of Al anode and cathodic reduction of Al cathode affect the interfacial structure of Al anode and cathode. 100 nm thick Al film was deposited onto Si-wafer to use as anode and electrode. Also, three different variations of groundwater simulant, i.e., chlorine base (Cl), 50:50 Cl: SO₄ (sulfate) base, and SO₄ base, were prepared for the electrocoagulation. For the NI study, porous Al electrodes were employed. NR, XRR, and NI results indicated the formation of Al₂O₃ layer on the surface of Al anode surface prior to dissolution into groundwater. The surface Al₂O₃ layer grew in thickness with time and finally Al begun to dissolve into groundwater resulting in the reduction of Al anode thickness. The dissolution (thickness reduction) rate increased with increasing Cl base. In Al cathode, gradual thickness change was discerned, and indication of a hydroxide layer formation was identified. Results can help us improve the electrocoagulation process, and also reduce the energy requirements for water treatment.