Trace humic and fulvic acid determination in natural water by cloud point extraction/preconcentration using non-ionic and cationic surfactants with FI-UV detection

Abstract

A preconcentration and determination method for humic and fulvic acids at trace levels in natural water samples was developed. Cloud point extraction was successfully employed for the preconcentration of humic acid (HA) and fulvic acid (FA) prior to the determination by using a flow injection (FI) system coupled to a spectrophotometric UV-Vis detector. The quantitative extraction of HA and FA within the pH range 1–12 was obtained by neutralization of the anionic charge on the humic substances with a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB). This generated a hydrophobic species that was subsequently incorporated (solubilized) into the micelles of a non-ionic surfactant polyethylene glycol, tert-octylphenyl ether (Triton X-114). The FI method for HA and FA determination was developed by injection of 100 µl of the extracted surfactant-rich phase using an HPLC pump with spectrophotometric detection at 350 nm. A 50 ml sample solution preconcentration allowed an enrichment factor of 167. The limit of detection (LOD) obtained under the optimal conditions was 5 µg l⁻¹. The precision for ten replicate determinations at 0.2 mg l⁻¹ HA was 3.1% relative standard deviation (RSD), calculated from the peak heights. The calibration using the preconcentration system for HA and FA was linear with a correlation coefficient (r²) of 0.9997 at levels near the detection limits up to at least 1 mg l⁻¹. The method was successfully applied to the determination of HA and FA in natural water samples (river water).