Improved analysis of dissolved organic nitrogen in water via electrodialysis pretreatment

This study evaluated electrodialysis (ED) for direct, accurate, and precise dissolved organic nitrogen (DON) analysis in water. Unlike conventional methods that calculate DON as the difference between total dissolved nitrogen (TDN) and dissolved inorganic nitrogen (DIN), we designed a compact ED reactor as a pretreatment tool that completely separates DIN from DON in water and then measures DON by equating DON to TDN. The experiments confirmed that the ED pretreatment process can achieve 99% removal of all three major DIN species (i.e., ammonia, nitrite, and nitrate) and an average recovery rate of 88% for an array of model DON compounds of varying characteristics (e.g., urea, amino acids, tripeptide, protein, and humic substances). Variations in nitrogen removal and recovery might be explained by a combined effect of molecular weight, acid dissociation ability (pK(a)), aromaticity, and ED reactor configurations. For model solutions with DIN/DON ratios varying from 1 to 10 mg-N/mg-N, the relative standard deviations in DON concentrations were considerably lower with ED pretreatment (<10%) than without pretreatment (47%). A survey of seven field samples, including lake water, tap water, and treated wastewater, also demonstrated the benefits of using ED pretreatment as compared with a conventional DON analysis method. Overall, this study provides evidence and mechanistic insight for a new DON detection method that uses ED pretreatment. The ED unit is robust for separating DIN and DON, and thus it may facilitate more frequent detection of DON and ultimately enhances understanding of DON issues in the environmental studies.