Spectroscopic characterization of the reduction and removal of chromium (VI) by tropical peat and humin

► The reduction of Cr(VI) to Cr(III) was characterized by multiple techniques. ► Use of peat or its fraction as adsorbents for the reduction/remotion of Cr(VI). ► Removal capacity of chromium was increased at lower pH. ► Removal efficiencies of chromium increased with the reduction of Cr(VI) to Cr(III). The reduction of Cr(VI) to Cr(III) in aqueous solution, and the subsequent removal of Cr(III) using in natura peat or its humin fraction, was characterized by electron paramagnetic resonance (EPR), ultraviolet–visible (UV–Vis) spectroscopy and X-ray photoelectron (XPS) spectroscopy. EPR spectroscopy g-factors ranging from 2.0029 to 2.0030 indicated the presence of organic free radicals (OFR) associated with carbon atoms. The spin densities of the samples increased with pH in the range pH 2.0–pH 6.0. An XPS spectroscopy peak at 579.0eV was attributed to Cr(VI), and peaks at 576.8eV and 677.3eV to two different chemical forms of Cr(III). Removal of chromium increased with the reduction of Cr(VI) to Cr(III), and removal rates after 50h were 13% (peat) and 15% (humin) at pH 6.0, and 70% (peat) and 80% (humin) at pH 2.0. The greater efficiency of the humin fraction in the reduction/removal process is discussed in terms of the chemical structures of these materials.