Iron-Oxide Mineralogy of a Mollisol from Argentina: A Study By Selective-Dissolution Techniques, X-Ray Diffraction, and Mössbauer Spectroscopy

Abstract

Abstract--Selective-dissolution techniques by ammonium oxalate (OX), dithionite-citrate-bicarbonate (DCB), and dithionite-ethylenediaminetetraacetic acid (D-EDTA), and X ray diffraction and M6ssbauer spectroscopy were used to identify and characterize iron oxides and oxyhydroxides in the <2-mm, <50-p,m, and <2-p~m size fractions of a Mollisol from Bahia Blanca, Argentina. Iron compounds are present at low concentrations in mixtures with quartz, Na-rich feldspar, illite, interstratified illite-montmorillonite, and traces of kaolinite. Total Fe and AI content increases as soil particle size decreases, from 4.3 and 13.3 wt. % in the <2-mm size fraction to 8.5 and 22.8 wt. % in the clay fraction (<2 Ixm), respectively. No more than 25-30% of the total Fe is associated with the crystalline and the amorphous Fe oxides. Weakly ferromagnetic hematite and goethite were identified in the different fractions. These phases have small particle sizes and/or low crystallinity. They may also have A1 for Fe substitutions. Crystalline magnetite or maghemite is rare. These Fe-rich phases are probably coating coarser particles.The efficiency of Fe removal is highest for the D-EDTA treatment and least efficient for the OX method, for all fractions. The opposite is true for A1 removal. Poorly crystalline hematite and goethite, which are soluble in oxalate, are only present in the coarser fractions. Poorly crystalline and crystalline hematite and goethite, which are soluble in DCB and EDTA, are present in coarser fractions, but do not occur in the clay fraction. DCB treatment probably dissolves A1 in the 2:1 type phyllosilicates occurring in this soil, whereas D-EDTA dissolves Fe in the hydroxy interlayers of the srnectite minerals or in the silicate phases.

Speciation and distribution of P associated with Fe and Al oxides in aggregate-sized fraction of an arable soil

“…This is probably because organic matter in soil inhibits Al oxides crystallization (Borggaard et al, 1990). There is currently ongoing discussion in the literature (Acebal et al, 2000;Borggaard et al, 1990;Ryan and Gschwend, 1991) with respect to the extent to which c-Al is extracted by DCB and what reasons for such an incomplete extraction of this Al are. Only 12-20 % of total P (c-P, the difference between P DCB and P ox , Table 2) was released with crystalline Fe and Al oxides.…”

Sorption of Zn(II) and Cu(II) by four Argentinean soils as affected by pH, oxides, organic matter and clay content

“…and Mg 2? as the main exchangeable cations (Acebal et al 2000). Soils were classified following the Soil Taxonomy system (USDA 1999) as three Mollisols and one Entisol: Typic Argiudoll (A 1 ), Pachic Argiudoll (A 2 ), Petrocalcic Paleustol (A 3 ), and Typic Ustipsament (A 4 ).…”

“…Fe oxides (hematite and goethite) are present at low concentrations occurring with varying degrees of crystallinity and Al substitution, Al 3? occurs in Al-substituted sites in the soil iron oxides, and Mn oxides are scarce and very sensitive to pH changes (Acebal et al 2000).…”

“…Soils from this region principally contain hematite and goethite with varying degrees of crystallinity and Alsubstitution. Maghemite and magnetite are negligible, especially in the fine fractions of the soil (Acebal et al 2000). The linear correlations (R 2 = 0.80) between S w value of the \2-lm fractions (Table 1) and the surface calculated for same fractions, taking into account the mineralogical contents of illite, feldspar, hematite, goethite and smectite (Table 3) and the respective mineral-S w values (67, 85, 100, 68, and 472 m 2 /g, respectively; Taubaso et al 2004;Rueda et al 1992), is indicative of negligible interactions among different soil components (Torres Sánchez et al 2001).…”

The Transfer of Cu, Zn, Mn and Fe between Soils and &lt;i&gt;Allium&lt;/i&gt; Plants (Garlic and Onion), and Tomato in the Southwest of the Buenos Aires Province, Argentina

“…Soils of this region are characterized as sandy soils with low organic matter, containing mainly quartz, Narich feldspars and plagioclase in the sand fraction, and illite, interstratified illite-smectite, quartz, Na-rich feldspars and mica in the silt and clay fraction, being Ca 2+ and Mg 2+ the main exchangeable cations [11,12]. Soils were classified following the Soil Taxonomy system [13] into four Aridisols (Typic Camborthid: A 1 , A 2 , B 1 and B 2 ) and three Entisols (Typic Ustipsament: C, D, and E).…”