Increased resolution in high-performance liquid chromatograph spectra of high-molecular-weight organic components of Bayer liquors

A high-performance liquid chromatographic (HPLC) method has been developed to study the chemistry of higher-molecular-weight components (>1200 Da) of humic materials in an alumina refinery (Bayer process). The optimum separation was achieved using a Nova-Pak C18 column at a flow rate of 0.5 mL/min. A step gradient was developed using tetrabutylammonium hydrogen sulfate (PIC A) as the ion-pairing reagent and acetonitrile. Extraction of the humic material with a series of solvents differing on the basis of polarity yielded solvent extracts, which were then characterized and correlated with the optimum HPLC separation. The carboxylic-rich components concentrated in the material that eluted first, as expected for reverse-phase separations, thus confirming the validity of the methodology. To account for the solubilities and the absence of a continuum in the elution of the Bayer humics, it is suggested that the organic matter is associated in solution as micelles or other clusters.