Adsorption and Post Adsorption Behavior of Schwertmannite with Various Oxyanions

Abstract

The adsorption and post adsorption behavior of schwertmannite with various oxyanions were investigated for clean-up contaminated water with hazardous oxyanions and safe disposal of spent schwertmannite. The result of adsorption experiments showed that the maximum capacities of oxyanions adsorption onto schwertmannite are 1.023, 0.934, 0.723 and 0.313 mmol/g for arsenate, phosphate, chromate and selenate, respectively. Based on the differences in the adsorption capacities, the selectivity of oxyanion adsorption on schwertmannite decreases as the order: arsenate ≥ phosphate > chromate >> selenate. Change in the Zeta potential after adsorption by arsenate, phosphate and chromate were very different from those after adsorption by selenate and of the original schwertmannite. This difference implies that the adsorption mechanism on schwertmannite with arsenate, phosphate and chromate is different from that with selenate and sulfate. Arsenate, phosphate, and chromate ions form inner-sphere complexes with the surface of schwertmannite, while selenate and sulfate ions form outer-sphere complexes with the surface of schwertmannite. Based on a comparison with anion adsorption, strong base anions form inner-sphere complexes, which induce a strong adsorption with schwertmannite as well as it is conducive to high adsorption capacity. From the results of aging experiments, schwertmannite with sulfate and selenate changed to a more stable phase, goethite, in a short time, whereas there is no change in the XRD patterns of schwertmannite with arsenate and phosphate after 30 days. The stability of schwertmannite after the adsorption increased in the following order: sulfate ≅ selenate  chromate < phosphate ≅ arsenate. The solubility of schwertmannite with different oxyanions was calculated according to solid solution theory. The solubility of schwertmannite decreased after adsorption of oxyanions with high selectivity. It is concluded that oxyanions with high selectivity can stabilize schwertmannite by decreasing the solubility of the schwertmannite after adsorption of the oxyanions.

Comparison of Cr(VI) Adsorption Using Synthetic Schwertmannite Obtained by Fe3+ Hydrolysis and Fe2+ Oxidation: Kinetics, Isotherms and Adsorption Mechanism

“…As reported by several authors, Cr(VI) adsorption on schwertmannite can be mainly attributed to ion exchange with SO 4 2 − groups [ 22 , 25 , 52 , 71 ]. Due to the same charge and a similar radius of SO 4 2 − (0.23 nm) and CrO 4 2 − (0.24 nm) ions, the replacement of sulphate groups with chromate groups occurs without changing the crystal structure of the schwertmannite [ 52 ].…”

“…Khamphila and co-workers found that the adsorption of Cr(VI) on easily accessible active sites during the fast initial stage is likely to be ion exchange with SO 4 2 – groups. In contrast, the second stage involves the slow diffusion of Cr(VI) from the surface sites into the interior of the adsorbent and the formation of complexes [ 25 ]. According to Zhang and co-workers, two types of surface complexes can form on the schwertmannite surface: monodentate (≡FeOCrO 3 − ) and bidentate (≡Fe 2 O 2 CrO 2 ) [ 52 ].…”

“…Schwertmannite occurs naturally, but for pollutant removal, synthetic schwertmannite is usually used [ 23 ]. Schwertmannite sorbents can remove a variety of contaminants including fluorides, toxic metal ions, oxyanions, and organic pollutants [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. Schwertmannite is an excellent oxyanion adsorbent as it is characterised by positive surface potential and a large surface area of 100–200 m 2 ·g −1 , and additionally its crystallographic structure allows it to incorporate oxyanions to its crystal structure [ 22 ].…”

Schwertmannite: occurrence, properties, synthesis and application in environmental remediation

“…oxyanions are absorbed by the 2 nd mechanism. The adsorption and post adsorption behaviour of Sch towards various oxyanions were investigated by Khamphila et al 103 The results implied that the . One is to directly stabilize it and the other is to oxidate it rst and then chemically stabilizes it.…”

The Microbiology of Metal Mine Waste: Bioremediation Applications and Implications for Planetary Health

“…1.8). These secondary Fe minerals sequester metals, removing them from solution (Ahoranta, 2016;Khamphila et al, 2017;Shu et al, 2019). Burgos et al (2012) compared the sequestration of metals by schwertmannite formed at low pH (2.5 -4.0) by biological Fe(II) oxidation to the chemical formation of ferrihydrite [Fe 10 O 14 (OH) 2 ] at higher pH (4.4 -8.4).…”