The photocatalytic reduction of NO3− to N2 with ilmenite (FeTiO3): Effects of groundwater matrix

Abstract

This work analyzes the role of natural groundwater, as well as the effect of HCO3−, Ca2+, Mg2+, K+, SO42- and Cl− concentrations, upon the photocatalytic nitrate reduction using ilmenite as catalyst and oxalic acid as hole scavenger. The nitrate removal and the selectivity towards N2 are significantly limited compared to previous experiments using ultrapure water matrix. Calcium (Ca2+), bicarbonate (HCO3−) as well as pH are claimed as the major controlling factors related to the process yield. Thus, Ca2+ promotes the formation of insoluble oxalate microcrystals, reducing the amount of hole scavenger available. The presence of HCO3− leads to a steeply increase in the pH value, favoring the adsorption onto the ilmenite surface of ions OH−instead of NO3−, NO2− and C2O42. The aforementioned issues are overcome by working with C2O42−/NO3− ratio well above the stoichiometric one, that also maintains the pH value in an acid range. A completed depletion of the starting NO3−, the no detection of either NO2− or NH4+ in the aqueous phase, and a selectivity towards N2 above 95% were achieved using two times the stoichiometric dose.