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Adsorptive Removal of Phosphate Ions Using Leached Sea Nodule Residue Generated by the Reduction–Roasting Ammoniacal Leaching Process

The present study was an attempt to find an alternative use for the leached residue generated after the selective extraction of Cu(II), Ni(II) and Co(II) ions during the reduction–roasting ammoniacal leaching of Indian Ocean sea nodules, so that the latter may be used as an adsorbent for the removal of phosphate ions from aqueous media. Physicochemical characterization revealed that the leached residue was a complex mixture of oxides of mainly manganese and iron along with MnCO3. Adsorption studies of the water-washed leached residue (WSNR), varying the pH, phosphate ion concentration, WSNR dosage, heat treatment and the presence of interfering anions in solution, indicated that phosphate (P) ion uptake increased with increasing heat-treatment temperature and pH, to attain a maximum value at 400 °C and a pH value of ca. 5.0, but thereafter decreased on increasing the heat-treatment temperature and pH further. The adsorption data were best fitted by the Freundlich isotherm model. Desorption studies showed that < 90% of the adsorbed phosphate ions could be desorbed from WSNR after stirring at pH ≥ 10.0 for 2 h. The results obtained could be useful in a consideration of the use of WSNR as an adsorbent for the removal of phosphate ions from contaminated water bodies.