Fractionation of humic acid upon adsorption to goethite: Batch and column studies

• We study adsorption of humic acid (HA) to goethite in batch and column experiments.
• HA adsorption on goethite is a fast process.
• HA with higher aromaticity were firstly adsorbed and then replaced by smaller ones.
• Desorption of HA from goethite-coated sand was a very slow process.

The fractionation and replacement of humic acid (HA) during its interactions with goethite (or goethite-coated sand) were conducted using batch and column experiments. The weight-average molecular weight (Mw) of HA in solutions was determined by a high performance size exclusion chromatography (HPSEC). The E2/E3 (absorbance ratio at 250 nm and 365 nm) and specific ultraviolet absorbance (SUVA) of HA at 280 nm were measured with a UV–visible spectrophotometer and a total organic carbon (TOC) analyzer, respectively. The results indicated that HA adsorption on goethite was a fast process, and HA fractions with higher Mw and lower aromaticity were preferentially adsorbed to goethite. Kinetic experiments demonstrated that HA fractions with relatively higher Mw and aromaticity were firstly adsorbed onto the surface of goethite, and then they were replaced by lower ones in solutions, which were also confirmed in column experiments. The relative concentrations of HA in the effluent of columns increased slowly before 12 pore volume (PV), then increased sharply, and reached over 90% after 50 PV. Desorption of HA from goethite-coated sand was a very slow process. HA fractions with larger size (or Mw) or lower aromaticity are easily desorbed. As a consequence, goethite had a high capacity for natural organic matter (NOM) adsorption, which would significantly affect the environmental fate of NOM and related contaminants in natural systems, and also be a potential adsorbent for NOM removal in the water treatment.

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