Experimental and Theoretical Investigation of Electrochemical Water Treatment Processes

Abstract

Two kinds of electrochemical methods for water treatment were investigated, namely electrocoagulation and bipolar membrane electrodialysis (BMED). Electrocoagulation with mild steel anode was investigated to remove dissolved silica from simulated high efficiency reverse osmosis (HERO) concentrate solutions, and was compared with traditional chemical coagulation with FeCl3. The recommended optimal initial pH value for electrocoagulation is 8. 76-89% silica removal was achieved with 4.0 mM iron dose in electrocoagulation, while a maximum of 64% removal was achieved by chemical coagulation with a dose of 4.0 mM FeCl3. BMED was used to produce acid and base from dilute sodium sulfate or sodium chloride salt solutions. Using single pass BMED, >75% current utilization was achieved producing acids and bases with concentrations of ~75% of the feed salt concentration. Factors affecting current utilization and limiting current density were investigated. The energy required to produce a mole of acid or based increased linearly with increasing current density. The energy costs for producing acids and bases were ~10 times lower than costs for purchasing bulk HCl and NaOH from local suppliers. A BMED stack was used in a zero-liquid-discharge (ZLD) system of water softening for regenerating ion exchange media and for promoting crystallization of hardness minerals in a fluidized bed crystallization reactor (FBCR). The overall closed-loop process eliminates the addition of extra chemicals and the creation of waste brine solutions. However, the key component in BMED – bipolar membranes (BPMs) are ill suited for water/wastewater treatments, due to low stability in alkaline solutions and high voltage drop at low current densities. The alkaline stability of the BPMs was improved by replacing the anion exchange layer with base-stable anion exchange membranes designed for alkaline fuel cells. In order to decrease the water splitting voltage, different electronically conductive materials and graphene oxide were tested as the interlayer catalyst of BPMs. Two methods of modeling were applied to study the structure of BPM and the mechanisms of water splitting in BPM. A 3-D point-charge method was used to model the 3-D interlayer of the BPM, where each functional group in both ion exchange layer (IEL) was treated as a point charge and different charge screening was applied. A one-dimensional continuum model of BPM was also applied to investigate ion concentration gradients in BPMs under reverse bias conditions.