Potential of a Layered Double Hydroxide Mineral to Reduce Fluoride Concentrations in Drinking or Waste Waters

摘要

Water treatment methods based on sorption or ion exchange reactions are usually applied to reduce F concentrations to within the range of 0.5 to 4 mg/L recommended for drinking waters or discharged wastewaters. Layered double hydroxide minerals, or anionic clays, contain anions in the interlayers that can be easily exchanged. In the current study, the exchange of F~- for OH~- ions in the structure of hydrocalumite (Ca_4Al_2(OH)_(12)(OH)_2·6H_2O) was investigated as a function of initial F concentration. Competition between F and other anions including Cl", SO_4~(2-) and CO_3~(2-) for anionic sites in hydrocalumite was also examined. After half an hour of reaction with OH-hydrocalumite at a watensolid ratio of 15:1, fluoride concentrations were reduced from 5 mg/L to 1 mg/L, and decreased further to 0.1 mg/L after 7 days. At initial F~- concentrations between 340 and 3400 mg/L, 94% or more of the F was removed by reaction with hydrocalumite at the same watensolid ratio. Under these conditions, final solution concentrations ranged from 18 to 208 mg/L. The maximum measured uptake capacity for F~- by hydrocalumite was 45 000 mg/kg. Using starting concentrations of 0.005 M for each of F~-, Cl~-, CO_3~(2-) and SO_4~(2-), reaction with OH-hydrocalumite reduced anion concentrations by 50, 10, 22 and 300 times, respectively after 7 days of reaction. The selectivity of the anions by OH-hydrocalumite was SO_4~(2-) > F~- ≈ CO_3~(2-) > Cl~-. Although all four anions were incorporated into hydrocalumite, the concentration of F~- was still reduced to the maximum contaminant limit (MCL) for drinking water of 4 mg/L within 1 hour.