Quantitative measurement of fluoride in acid-alkali leaching solution of fluoride contaminated soil, qualitatively in chemically treated soil and their thermal properties

The fluoride-contaminated soil underwent various analyses, including NMR (19F, 27Al, 29Si), thermal analysis (TG-DSC and CP), ATR-IR, and UV-Vis-NIR spectrum. A fluoride meter measured the fluoride ion in the acid and alkali-treated soil filtrate. Entropy was calculated for the original, acid and alkali-treated soil. The calculated entropy of all three soil types is approximately 7.45 $$J{g}^{-1}{K}^{-1}$$ . The results showed that the maximum CP of the acid-treated soil at ~310K, 2.37 $$J{g}^{-1}{K}^{-1},$$ is higher than that of the original soil at ~326K, 2.11 $$J{g}^{-1}{K}^{-1}$$ . This is due to increased hydrated molecules and decreased fluoride compounds in the acid-treated soil. According to NMR analysis, the composition of all the examined soil is similar to various ZSM-5F, HY, H-ZSM-5/ NH4-ZSM-5 FLD, C3A/ C12A7, AlOyN4-y(0≤y≤4), and AHFS washed ZSM-5 zeolite types. It was found that treating the soil with acid followed by thudding filtrate with alkali is more effective in removing the presence of fluoride in the filtrate. After filtering the precipitate, the fluoride ion is reduced from 266 ppm at pH 11.08 to ~2.0 ppm at 7.08 pH. It is also found that the fluoride ion is suppressed as fluoride compounds, {NaF, NH4F, (NaF)n, and NaHF2}, through the use of an ammonia solution.