Effects of pH on chromate(VI) adsorption by Spirulina platensis biomass: batch tests and FT-IR studies

Raw and methylated biomass of Spirulina platensis was employed in chromate batch adsorption tests at pH range 1-7. The acid conditions seemed to favour the removal of chromium (Cr) with a yield of 87.0 and 97.6% by using raw and methylated biomass, respectively. However, the chromate and total chromium determination, carried out in the same sample, evidenced that a fraction of the initial chromate present in solution was reduced to Cr(III). This was ascribed to the presence of reducing groups on the biomass surface, active in the acid medium. The data showed that the methylated biomass was able to operate an effective Cr(VI) removal only. In fact, the biomass treatment allowed a lowering of the amount of negative functional groups, making the biomass surface available to bind the anions. The real best efficiency of Cr(VI) removal (83.5%) was obtained by methylated biomass of S. platensis at pH about 7.0. The nature of the biomass/chromate interactions was investigated by Fourier transform infrared spectroscopy (FT-IR) analysis. The bands ascribing to the adsorbed Cr(VI) species were well evident in the spectra of the biomass after adsorption, confirming this experimental finding.