Application of pretreatment methods for reliable dissolved organic nitrogen analysis in water - A review

Dissolved organic nitrogen (DON) is ubiquitously present in the environment, and its occurrence plays important roles in nitrogen cycling and water safety issues. However, analytical techniques for DON are currently not well established. The DON data obtained from conventional methods by subtracting dissolved inorganic nitrogen (DIN, including nitrate, nitrite, and ammonia) from total dissolved nitrogen (TDN) are subject to aggregated analytical errors and therefore can be low in precision and accuracy. To minimize the subtraction-induced errors, in recent years scientists have examined several pretreatment methods to achieve either direct detection or more reliable analysis of DON. The key challenge is to maximize the extents of DIN removal and DON recovery simultaneously. This review summarizes these pretreatment methods within four categories: (1) membrane methods, (2) adsorptive methods, (3) catalysis methods, and (4) preconcentration methods. By identifying the advantages and disadvantages of each method, this review may facilitate future development of DON detection methods and make DON analysis more rapid, direct, accurate, and precise. According to available information, many studies have endorsed the dialysis method, which is thus favored, except that its processing time needs to be dramatically shortened. Adsorptive methods using ion exchange resins, chromatographic columns, or nonionic adsorbents have been investigated in a few cases with promising results but mixed information regarding the DON loss. Therefore, their use remains controversial and may require optimization and systematic verification in the future. In one study nanofiltration achieved substantial (69-92%) removal of DIN and certain levels (18-22%) of DON loss within 4 hr; this also should be confirmed by more studies. Electrodialysis and reverse osmosis, although not evaluated previously, are known to be robust for removing DIN species and isolating DOM (DOM); therefore, they may merit exploration as alternative methods. In contrast, catalytic reduction of DIN was proven to be inappropriate for DON analysis owing to the high level of DON loss. Lyophilization and rotary evaporation were previously assumed to preserve all solutes, but some evidences suggest that they may cause considerable changes in the DOM content and integrity. Overall, these methods have laid solid groundwork for further studies, and users need to be aware of the limits and potential interferences of these methods in practice.