Iron oxidation and associated structural alterations in K-bearing minerals: How do they impact K phytoavailability in soils?

Potassium (K) is ubiquitous in soils and has therefore received much less attention in modern edaphology compared with nitrogen (N) and phosphorus (P). However, the need to elucidate the phytoavailability of native soil K has recently been re-emphasized due to the rising cost of K fertilizers. Although native soil K largely occurs in minerals in immobile forms, biotite—a trioctahedral mica containing iron (Fe) and magnesium (Mg) in the octahedral sheet—can release K more rapidly than other K-bearing minerals. Octahedral Fe in biotite, originally present as ferrous iron (Fe²⁺), is oxidized to ferric iron (Fe³⁺). This Fe oxidation is hypothesized to cause two opposing effects on K retention. If the oxidized Fe³⁺ remains in the trioctahedral structure, the reduced layer charge may weaken K retention in the interlayer. Conversely, if part of the oxidized Fe³⁺ is released from the octahedral sheet, the structure shifts from a trioctahedral to a dioctahedral type, which may strengthen interlayer K retention. Although both mechanisms have been proposed, no direct evidence has been provided to date. The objective of this study was to determine how Fe oxidation in biotite influences K retention in the interlayer.

Biotite (2–50 µm) was first treated with sodium (Na) tetraphenylborate solution to replace most interlayer K with Na. The Na-biotite was then reacted with H₂O₂ at molar ratios of 0, 0.1, 0.5, and 10 relative to structural Fe, resulting in Fe³⁺ proportions of 6%, 30%, 69%, and 92%, respectively. These oxidized Na-biotite samples were subsequently washed several times with KCl solution to refill the interlayer with K, yielding biotite samples with varying degrees of Fe oxidation. Their atomic arrangements were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). Iron speciation was examined using selective dissolution analysis and Mössbauer spectroscopy. The release rate of interlayer K from biotite was evaluated using a resin extraction method.

XRD 060 reflections clearly showed a gradual shift from tri- to dioctahedral structures with increasing Fe³⁺ proportions, which was also supported by shifts in the OH absorption bands in the FTIR spectra. Although we initially assumed that this alteration would strengthen interlayer K retention, the oxidized and dioctahedral biotite released K more rapidly than the less oxidized samples. The weaker K retention after Fe oxidation could not be explained solely by changes in the octahedral sheet structure. TEM analysis revealed that highly oxidized biotite exhibited partially expanded interlayer spaces, which were likely filled with Fe hydroxides.

We concluded that Fe oxidation not only modifies the octahedral sheet structure but also promotes the formation of Fe hydroxides within the interlayer, leading to weakened K retention and enhanced K release from biotite.