Molecular Characterization of Dissolved Organic Matter in Coal Coking Wastewater by FT-ICR MS

Efficient removal of dissolved organic matter (DOM) is an important task for wastewater treatment. However, the molecular composition of DOM and its transformation during the treatment are still unclear. In this paper, the organic components in the stream of a coal coking wastewater treatment process were studied. The wastewater was extracted by dichloromethane (CH₂Cl₂)，the extract was named as ‘oil phase’. Then the raffinate was extracted by solid-phase extraction, the extract was named as ‘water phase’. The ratios of total organic carbon contents (TOC) in the oil and water phase samples were generally consistent during the treatment process. The extracted DOM was characterized by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Eight class species were as assigned from the mass spectra, namely N₁O_1-7, N₂O_1-6, N₁O₅, N₁O_2-6S₁, N₂O_3-5S₁, O_2-8, O_2-6S₁, O_4-5S₂. The class categories which contain multiple oxygen atoms (Ox) and both multiple oxygen and sulfur atoms (O_xS_y) were dominant in the oil phase. The relative abundance of each species was decreased sharply after the first aerobic biological process, which was consistent with the TOC rapid decline in the sample. O_2-8 compounds existed in all wastewater samples and dominated in the final sample, indicating that these compounds were the most stubborn pollutants which were difficult to remove in all treatment processes. It is speculated that these compounds may come from some humic-acids which have complex structure and multiple oxygen atoms naturally existing in natural water. The composition of O₃ class species showed two patterns among the wastewater treatment process, compounds with high double bond equivalent (DBE) were dominant in the first four processes, while a proliferation of low DBE components was observed in the last three samples. It indicated that those low DBE components were generated by the oxidation of other substances, such as O₁ and O₂ species. The water phase organic matter had an extremely complex molecular composition and a total of 27 different compound class species were identified in a single mass spectrum. O_x, O_xS_1-2 and N_xO_yS_1-2 were the major class categories in water phase. On the whole, O_xS_1-2 was much easier than the other two categories to be removed during the wastewater treatment process. In the O_xS_1-2 class category, O₄S₁ had a much higher relative abundance than other species. It decreased sharply among the process, but it still accounted for a large proportion in the final process, which implied the O₄S₁ species had strong resistance to removal. The relative abundances of each species in N_xO_yS_1-2 had consistent composition and transformation, and could be effectively removed after the wastewater treatment process. The composition transformation trend of O₆S₁ was similar to N₁O₆S₁, those compounds with low DBE value (512) and low carbon number (1020) were dominated after the aerobic biological process section and remained in the final process. The study of DOM in coal coking wastewater treatment from the molecular level by FT-ICR MS will contribute to understand the transformation law of pollutant compounds. However, limited by the mass spectrometry, only the molecular composition information can be obtained. The molecular structure information, especially for the functional groups, is hard to be obtained. In order to research the transformations law of pollutant compounds for further, the structural information needs to be deduced combining the molecular composition, the precursor and the polarity of wastewater pollutants. Developing more efficient and advanced separation and enrichment methods is another way to research the molecular structure information. By the further work, a fully understand of the pollutant composition will be obtained, and a more effective wastewater treatment process will be developed.