Antiscalant Effects on pH-Mediated Electrochemical Precipitation of Scaling Ions in Brackish Reverse Osmosis Concentrates

Brackish groundwater (BGW) desalination is promising for producing freshwater, but the disposal of reverse osmosis concentrates (ROCs) poses environmental concerns. One strategy to minimize the volume of ROC is to remove scaling ions─a primary factor limiting water recovery due to membrane scaling─for the additional desalination of the treated ROC in a secondary reverse osmosis (RO) unit. While promising, electrochemical precipitation remains poorly understood in the presence of antiscalants, which are commonly added to the RO feed to inhibit precipitation. This study investigates the effects of antiscalants and their concentrations on a pH-mediated electrochemical process to remove scaling ions in the BGW ROC, using nitrilotris(methylenephosphonic) acid (NTMP) as a model antiscalant. The use of the hydrogen evolution reaction increased the catholyte pH, promoting the precipitation of Ca²⁺and Mg²⁺and reaching up to a hardness reduction of 93%. The inhibitory effect of NTMP on the formation of CaCO₃is more pronounced at low current densities, altered precipitate morphologies, and increased the energy consumption of precipitation. The inhibitory effect was observed both at the cathode surface and in the bulk solution and was also compared with that in chemical precipitation. These new findings highlight the capability of electrochemical precipitation as a viable method for brine management toward zero liquid discharge.