A review of enhanced adsorption removal of odor contaminants with low ppm concentration levels: the key to technological breakthrough as well as challenges

The industrial production processes often produce different concentrations and types of odorous pollutants. Most odors have a low odor threshold, and the human sense of smell can still have a strong, unpleasant odor even at low ppb concentrations. The main challenges in low ppm concentration odor purification are short contact time, high air volume, low equilibrium adsorption capacity, and easy physical desorption. For the first time, this work reviews the technical paths how to purify four typical types of low concentrations of odors such as H₂S, NH₃, CH₃SH, and CH₃SCH₃ from low ppm concentration levels to low ppb, with the view of the odor sources, the development of treatment technology, international permissible emission standards, and the recent status of adsorbent materials. To begin, Citespace software is employed to analyze the progress, hotspots, and technology trends in the field of odor pollutant research over the past 28 years and the factors that affect removal efficiency of low-concentration odorous pollutants are discussed in detail. Then, taking activated carbon, molecular sieve, and metal-organic frameworks as target adsorbents, how to strengthen the integrated ways of physical adsorption and chemical adsorption of these adsorbents are suggested starting from the synergistic effects of modifications for pore structure, surface chemical functional groups, and complexation and redox reactions of metal ions. As a practice, the application cases of purifying low-concentration odorous pollutants by the adsorption are briefly introduced. Finally, the challenges of developing novel adsorption materials and technologies to purify low-concentration odorous pollutants toward lower than odor threshold are presented.