Nitrogen compounds removal from brackish water by electrodialysis at fixed electric potential and dynamic current density operations.

Nitrogen (N) compounds can occur in water resources from natural and anthropogenic activities. It is ideal that these contaminants be removed before water consumption. As water quality has been affected by increased salinity and pH variation, more advanced and robust technologies such as electrodialysis (ED) can be considered for simultaneous desalination and pollutant removal. In this context, the removal of N-species (NO3-, NO2-, NH4+, and CH4N2O) from brackish water by ED was investigated for different feed water quality, considering increased salinity (0 - 10g/L NaCl) and pH variation (3 - 11), under limit current density (LCD) at fixed electric potential condition. The applied electric potential (5 - 25V) under, at, and over the LCD at fixed electric potential and dynamic current density (DCD), as a percentage of LCD (0.4 - 1.2), were analyzed to improve the process. In addition, energy efficiency in the form of specific energy consumption (SEC) and current efficiency (CE) were assessed for ED at fixed electric potential and DCD. The results showed that, at extreme pH of the feed water, the removal of NO2- and NH4+ can be affected, while NO3-was the most stable compound with pH variation. An increase in feed water salinity just slightly impacted the removal of N-compounds, due to the similar characteristics of the ions in the water. The increase in electric potential at fixed electric potential or DCD increased the removal and molar flux of N-compounds. However, operating over the LCD increased the SEC of the ED process while changes in removal were not significant. DCD procedures resulted in higher CE and shorter run time of the experiments. Therefore, ED proved to be a suitable treatment technique to produce fresh water due to the selective removal of the studied ions, especially at 15V (fixed electrical potential) and 0.8 LCD (DCD) related to removal, molar flux, and run time to achieve guidelines.

Nitrate, arsenic and fluoride removal by electrodialysis from brackish groundwater.

Nitrate, arsenic and fluoride are some of the most hazardous elements contaminating groundwater resources. In this work, the impact of operative (flowrate, electricpotential) and water quality (salinity, contaminant feed concentration, pH) parameters on brackish water decontamination was investigated using a batch electrodialysis (ED) system. Electrodialysis at low electric potentials (5 V) was more selective toward monovalent ions, at higher potentials (>15 V) removal of all ions increased and selectivity approached one, meaning removal of all ions. Changing the flowrate from 30 to 70 L/h, increased nitrate and fluoride removal slightly, while arsenic(V) removal was maximum at 50 L/h. Rising salinity delayed removal of ions with low ionic mobility and diffusivity (i.e. fluoride, arsenic(V)). Increased feed concentration of contaminants had no impact on removal values. pH variations did not impact the nitrate, fluoride and salinity removal, yet arsenic(V) removal was greatly pH dependent. This was explained in part by lower diffusivity and higher hydration number of bi- and trivalent species of arsenic(V) at basic pH. The results of this work showed the significance of ionic characteristics (diffusivity, ionic mobility, hydration number) in ED. Nitrate concentrations satisfied guideline threshold in all experiments with concentrations below 50 mg/L. Lowest arsenic(V) concentration was 35 µg/L at the highest electric potential, 25 V. Using ionic characteristics makes separation of different ions possible, providing new opportunities for ED in environmentally friendly processes (e.g. resource recovery and zero liquid discharge).