Methods for total organic halogen (TOX) analysis in water: Past, present, and future

Total organic halogen (TOX) is a surrogate bulk organic parameter indicating a variety of fluoridated, chlorinated, brominated, and iodinated organic compounds either formed in water/wastewater disinfection processes or present in the environment derived from natural products and anthropogenic activities. Since its introduction in the 1970s, TOX analytical methods have developed rapidly in parallel with the advances of disinfection byproducts (DBPs) studies. Thanks to the contributions of numerous researchers, tremendous progresses have been made to refine or revolutionize existing methods. However, unlike other bulk parameters, an extensive and routine use of TOX methods is still lacking currently, largely due to issues like high costs associated with instruments and consumables, tedious and trivial operations, undesirably high blank error, and unsatisfactory halogen recovery. In this review, we summarized a chronicle of advances for available TOX analytical methods, and compared several standard methods issued by global authorities to help better understand their specific requirements. Then, we discussed each step in the key procedures employed in the detection of adsorbable organic halogen (AOX)-specifically, the separation, conversion, and determination processes-to identify the important factors that deserve careful consideration and in-depth understanding. Finally, we proposed some research challenges and opportunities, such as developing online and automatic system for TOX analysis, exploring more robust methods to enable fluorinated compounds detection, and lowering the method detection limit of iodinated species at affordable cost. The results of our study may aid in better understanding and implementing existing AOX analytical methods for practitioners and meanwhile motivating more researchers to develop cheaper, more convenient, reliable, and automatic alternatives in the future.