Effect of Salinity on the Denitrification of Tunisian Groundwater by Batch Electrocoagulation Process

In Tunisia, in order to meet water needs, there has been an overexploitation of aquifers. This has led to the degradation of groundwater quality. Therefore, groundwater became loaded with salts (up to 2000 mg L−1) and nitrate, which exceed 200 mg L−1 in the region of Bizerte. Electrocoagulation (EC) in the batch mode was proposed as an alternative method for the denitrification of groundwater polluted with nitrate (148 mg L−1) and chloride (from 0 to 2000 mg L−1). Nitrate removal yield decreased with the increase in Cl− concentration. It decreased from 100 to 63.5% after 160 min of EC when chloride concentration varied from 0 to 2000 mg L−1. This can be explained by the electrochemical behavior of Cl− which, by anodic electro-oxidation, is converted into hypochlorite ions and hypochlorous acid. These oxidants reacted with nitrite and ammonium to regenerate nitrate. In addition, hypochlorite reacted with NH3 to be converted into N2. However, this is not the case with 2000 mg L−1 Cl− where ammonium content reached 20 mg L−1. This is due to pH effect, which remained lower than 9 at high Cl− concentration, so that NH3 is converted into NH4+ which cannot be oxidized directly into N2. The water pH must be adjusted to 9.25 to reach the pKa of NH4+/NH3 to enhance N2 selectivity.