Concurrent removal of hardness and fluoride in water by monopolar electrocoagulation

Water treatment based on electrocoagulation (EC) is attractive since required chemicals and colloids are produced in-situ. However, optimisation of EC operation parameters is necessary to enhance its efficiency. We optimised EC cell parameters by the response surface method (RSM). The optimal removal efficiencies of hardness (63%) and fluoride (97%) were achieved at 1.98 kW h/m3. With the removal of divalent cations, some anionic species concurrently remove via an energetically feasible route to adjust the charge balance. When simulated water is used (450 mg/L TDS, 580 mg/L CaCO3, 10 mg/L fluorides and pH 6.50), 83% hardness and 99% fluoride are removed with 0.69 kW h/m2 energy consumption. The chemical species in the solution matrix, particularly SO42, significantly affect the hardness and fluoride removal efficiencies. The contaminated EC sludge resulted from feed water is characterised by spectroscopic methods to probe hardness and fluoride removal mechanisms. In the presence of Mg2+, F- interacts with Al-sludge sites forming≡MgF−OH. When Ca2+ and F- are present, both ≡CaF−OH and ≡CaF are formed. In Ca2+, Mg2+ and F- treated Al-sludge dominates CaF−OH and ≡CaF over ≡MgF−OH.