Fluoride removal in a fixed bed packed with granular calcite

A fixed bed reactor filled with calcite particles (mean strokes diameter, 0.20 mm) was constructed and the reactor was used to treat synthetic wastewater containing HF, NaF, NH4F or Na2SiF6 under various conditions. Calcium ions (Ca2+) were found to exist in the treated effluent and the molar Ca2+ concentration ([Ca2+]) was related to the molar ratio of H+ to F− ([H+]0/[F−]0) in influent as log10[Ca2+]=0.020[H+]0/[F−]0−4.24. The effluent F− concentration was determined by the concentration of effluent Ca2+ as [F−]2[Ca2+]0.94=3.98×10−10 under an SV of 1.0 to 5.0 h−1 and 25°C. This equation was similar with that of the solubility product of CaF2 and could be applied to wastewaters containing HF, NaF, NH4F or Na2SiF6 under an influent F− concentration ranging from 25 to 3000 mg l−1. The effluent F− was also found to be related to effluent pH as log10[F−]=0.51pH−2.91. The effluent F− was about 6 mg l−1 at an effluent Ca2+ of 80 mg l−1 or effluent pH of 6.8 under the above conditions. The effluent F− increased from 6 to 12 mg l−1 and effective calcite conversion rate decreased from ca. 60 to 40% when SV was increased from 1.6 to 16.0 h−1 at a [H+]0/[F−]0 of 0.75. Significant increase of effluent F− as well as decrease of effective calcite conversion rate was observed when reaction temperature was decreased to 8.5°C at an SV of 7 h−1 and [H+]0/[F−]0 of 0.75. The performance of the reactor was affected by the existence of SO42− to some extent and significantly inhibited by PO43−. A molar ratio of PO43− to F− higher than 0.05 stopped F− removal at a PO43− of 15 mg l−1.