Electrocoagulation (EC) with an aluminum electrode arrangement as anode-cathode was applied to denitrify groundwater and electrooxidation (EO) was examined as a post-treatment step to remove the produced by-products. Initially, EC experiments were performed under batch operating mode using artificially-polluted tap water to investigate the effects of initial pH (5.5, 7.5, 8.5), initial NO3--N concentration (25, 35, 45, 55 mg L-1) and applied current density (10, 20 mA cm-2) on process efficiency. The effect of initial solution pH on ammonium cation concentration was also investigated as their generation (as a by-product) is the main drawback preventing wide-scale application of these treatment processes. Experimental results revealed high nitrate removal percentages (up to 96.3%) for initial pH 7.5 and all initial concentrations and current densities, while the final ammonium concentrations ranged between 5.3 and 9.2 mg NH4+-N L-1 (for initial NO3--N of 25 mg L-1). Therefore, EO was examined to oxidize the ammonium cations to nitrogen gas on iridium oxide coated titanium electrodes (IrO2/Ti) anode surface. The effects of cathode material (aluminum, stainless steel), total current density and anode surface area (3.3-30 mA cm-2 and 12-36 cm2, respectively) were investigated, and lead to NH4+-N percentage removals of between 25% (10 mA cm-2, 12 cm2) and 100% (30 mA cm-2, 24 cm2) for an initial NH4+-N concentration of 10 mg L-1. The optimum EC (20 mA cm-2, natural initial pH 7.5-7.8) and EO parameters (30 mA cm-2, 24 cm2 surface area anode, Al cathode) were combined into a hybrid system to treat two real nitrate-polluted groundwaters with initial NO3--N concentrations of 25 and 75 mg L-1. Results revealed that the proposed hybrid treatment system can be used to efficiently remove nitrate from groundwaters.
Keywords: Ammonium removal; Cathode effect; Denitrification; Electrochemical oxidation; Nitrite removal; Real nitrate-polluted groundwater.
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