Biogenic amorphous ferric hydroxide as adsorbent for vanadium removal in drinking water production
Daniel Mahringer, Sami S. Zerelli, Urs Dippon, Aki Sebastian Ruhl
Abstract
Vanadium as toxic heavy metal is a drinking water relevant contaminant. However, there is a lack in treatment processes to meet regulatory requirements (e.g. 4 μg l−1 in Germany). This study introduces a novel treatment process — the vanadium adsorption onto biogenic amorphous ferric hydroxide (AFH). Basic mechanisms of adsorption onto AFH are described and compared to granular ferric hydroxide (GFH). Adsorption kinetics and pH dependent isotherms in drinking and ultrapure water, parametrization via the empirical Freundlich and Langmuir models, and bond type and strength assessments via sequential extraction are presented. AFH was generated in pilot waterworks in which Fe(II) and oxygen were dosed and subsequently Fe(II) microbiologically oxidized and precipitated in the filter bed. The backwash-water was collected and used for adsorption experiments. Sequential extraction was executed with vanadium loaded AFH produced in the pilot plant. AFH is identified as alternative adsorbent to GFH with similar affinity and capacity. The isotherms cover a concentration range from 10 μg l−1 to 4 mg l−1 and the Freundlich model showed a better fit with the experimental data than the Langmuir model. A bidentate mononuclear inner sphere complex is assumed for vanadium adsorption onto AFH, while a bidentate binuclear inner sphere complex is expected for GFH. Sequential extraction showed a strong bond between AFH and vanadium, which was only mobilized by the last extraction step the dissolution of iron particles. A treatment process – adsorption onto biogenic AFH – is suitable for effective vanadium removal and should be further investigated for technical implementation.