Fluoride removal in a fixed bed packed with granular calcite

Abstract

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, NH 4F or Na 2SiF 6 under various conditions. Calcium ions (Ca 2+) were found to exist in the treated effluent and the molar Ca 2+ concentration ([Ca 2+]) was related to the molar ratio of H + to F − ([H +] 0/[F −] 0) in influent as log 10[Ca 2+]=0.020[H +] 0/[F −] 0−4.24. The effluent F − concentration was determined by the concentration of effluent Ca 2+ as [F −] 2[Ca 2+] 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 CaF 2 and could be applied to wastewaters containing HF, NaF, NH 4F or Na 2SiF 6 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 log 10[F −]=0.51pH−2.91. The effluent F − was about 6 mg l −1 at an effluent Ca 2+ 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 SO 4 2− to some extent and significantly inhibited by PO 4 3−. A molar ratio of PO 4 3− to F − higher than 0.05 stopped F − removal at a PO 4 3− of 15 mg l −1.