Rapid quantification of humic components in concentrated humate fertilizer solutions by FTIR spectroscopy

PurposeThe use of humic substances is under thorough discussion of state-of-the-art agricultural science. They are marketed mostly as concentrated aqueous solutions of potassium or sodium humates, which are produced by alkali extraction of raw organic matter such as lignite, leonardite, or peat. Due to the presence of clay minerals in the raw materials, humate solutions are characterized with a substantial content of silicates. At the same time, rapid quantification techniques for selective determination of humic components and silicates in humate solutions are missing. The aim of this work was to develop an IR spectroscopic technique for rapid quantification of humic substances (HSs) and silicates in concentrated humate solutions with the minimum sample preparation, which could be used for quality control of humic fertilizers.Materials and methodsSodium humate from Sigma-Aldrich and two potassium humate fertilizers available on the market were used for the experiments. For FTIR measurements, thin-layer open-cell (DialPath) transmission and attenuated total reflectance (ATR) accessories were used. The secondary focus of this work was the use of a compact portable IR spectrometer, which can be used in the field. Total carbon analyzer and ICP-AES for determination of silicon and aluminum contents were used.Results and discussionFTIR spectra were registered for both dry samples and aqueous solutions of the humates. The most intense bands in IR spectra of HS were characterized with linear concentration dependences in the range of concentrations of 2-200gL(-1) of HS. The most sensitive band was shown to be 1560cm(-1). It corresponded to carboxyl groups (COO-) of humates (limits of detection [recalculated to carbon] for transmission and ATR modes are 3 and 1gL(-1), respectively). The band at 1015cm(-1) was attributed to silicate. It did not overlap with the bands of organic constituents and could be used for silicate quantification. The proposed technique can identify different trademarks of the fertilizers by the amount of both HS and silicate.ConclusionsRapid determination of humate and silicate components comprising three samples of humic fertilizers was proposed without isolation of the analytes from solution.