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Coal fire sponges (CFSs) are a type of sponge-like contaminated soil bulge common in coal fire areas. However, the impacts of CFSs on the local environment are not yet understood. Thus, this study investigated soil samples from CFSs in the Wuda coalfield, Inner Mongolia, China, focusing on the acidity, sulfate, and fluorine content. The results showed that the CFSs were highly acidic, with an average pH of 0.76, and contained high levels of SO 4 2− (257.29 × 103 μg/g), total fluorine (TF, 2011.6 μg/g), and water-soluble fluorine (WF, 118.94 μg/g), significantly exceeding those in the regional background soil and indicating that CFSs are a point source of heavy pollution. Soils in the 8000 m2 reclamation zone showed elevated acidity and high SO 4 2− (129.6 × 103 μg/g), TF (1237.8 μg/g), and WF (43.05 μg/g) levels, which was likely the result of the weathering and dissemination of CFS. The CFS samples were rich in hydrogen fluoride, releasing 202.05 ppb of it when heated to 40 °C. Correlation analysis indicated that the acid sulfate soils in CFSs are likely caused by HSO 4 −/SO 4 2−. Time-of-flight secondary ion mass spectrometry detected four characteristic ions (F−, H 3 O+, H 2 SO 4 +, and HSO 4 −) in all micro-domains of each sample, indicating that ionic fluorine compounds and sulfuric acid hydrate were found in the CFS samples. Sulfate minerals detected in CFSs included CaSO 4 , Fe 2 (SO 4) 3 , CdSO 4 , NH 4 HSO 4 , and Na 2 SO 4. Thus, the results identified CFSs as a transmission channel for contamination, with erosional surface soils as the carrier, for the first time. CFSs pose a serious threat of contamination, albeit over limited areas. [Display omitted] • Coal fire sponges (CFSs) are found globally in soft soil above burning coal seams. • Samples compared CFSs with the local coalfield area and two reference sites. • High acidity, sulfate, and fluorine levels were detected in all CFS samples. • CFSs were confirmed as a source of environmental contamination. • Contamination from air deposition is wider, but CFS intensity is higher. [ABSTRACT FROM AUTHOR]

A total of 69 soil samples and 80 Bassia dasyphylla (sandwort) leaf and stem samples from Gobi desert land in the Wuda coalfield, Inner Mongolia, were analyzed. The results show that the average fluorine content in the soil of the coalfield area, the periphery of the coalfield area, and the Wuda urban area was 1250, 757, and 552 µg/g, respectively, which is higher than the background value. This suggests that the coal seam fire burning there, leads directly to fluorine pollution within a limited area. The average fluorine content in the leaves and stems of Bassia dasyphylla sampled immediately around the sites of the ongoing coal fire was 532 and 388 µg/g, respectively, while the concentration in leaf and stem samples collected in areas remote from the fire, but within the field, was 146 and 107 µg/g, respectively. This implies that this historic coal fire, which has been burning for the past 50 years, is responsible for a general deposition of fluorine in the local area. This result suggests that coal seam fires may also be a novel source of fluorine pollution in some urban areas. The scenario of urban areas being adjacent to coal seam fires is not limited to Wuda but is relatively common in northern China, and elsewhere. Whether or not there are other cities currently under the influence of coal seam fires merits further investigation. [ABSTRACT FROM AUTHOR]