A hybrid method for simultaneous removal of hexavalent chromium and fluoride from drinking water in plug-flow mode

An efficient, affordable, and environment-friendly method for removing toxic hexavalent and trivalent chromium ions from groundwater to comply with the WHO guideline values for drinking is still a great challenge. Chromium sometimes coexists with toxic fluoride in groundwater. Here we present a hybrid method based on reduction-precipitation-adsorption involving treatment with ferrous iron and phosphoric acid followed by treatment in a crushed limestone plug-flow filter that removes 99.5% of both Cr6+ as well as Cr3+ simultaneously with F- from the initial concentrations of as high as 10.0ppm to below their respective WHO guideline values for drinking water. The removal of Cr6+ is much better than previously reported by any method. Lab-scale piloting indicates the consistency of the results without needing regeneration, replenishment, or replacement of the limestone for a lifetime. The species precipitated during the treatment have been predicted using the PHREEQC model and then confirmed by characterization of the residue using Fourier Transform Infrared Spectroscopy (FT-IR), Energy Dispersive X-ray (EDX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. The present environment-friendly method removes Cr6+ or Cr3+ alone and simultaneously with fluoride very efficiently at a meager cost of USD 0.068 and 0.063 per m3 of treated water.