The influence of iron oxides on phosphate adsorption by soil

Soils from Denmark and Tanzania were extracted with ammonium acetate (controls), EDTA to dissolve amorphous iron oxides, and dithionite-EDTA (DE) to dissolve crystalline iron oxides. The phosphate adsorption capacities of the extracted soils were taken as the maximum quantity of phosphate adsorbed computed from the Langmuir equation. The decreases in the phosphate adsorption capacity following EDTA extraction and DE extraction were attributed to the removal of iron oxides. Close correlations (P<0.001) were found (i) between EDTA-extractable iron (amorphous iron oxides) and the decrease in phosphate adsorption capacity following EDTA extraction, and (ii) between the difference between DE-extractable iron and EDTA-extractable iron (crystalline iron oxides) and the further decrease in phosphate adsorption capacity following DE extraction. The phosphate adsorption capacity, estimated to be approximately 2.5 μmol P m^?2, was in good agreement with the capacity of various synthetic iron oxides. The calculated phosphate adsorption capacity of soil iron oxides, obtained from the contents and specific surfaces of amorphous and crystalline iron oxides together with the phosphate adsorption capacity per m² for synthetic iron oxides, compared favourably with the measured phosphate adsorption capacity.     

Influence of iron oxides on the non-specific anion (chloride) adsorption by soil

Soils from Denmark and Tanzania were extracted with ammonium acetate (controls), EDTA to dissolve amorphous iron oxides, and dithionite-EDTA (DE) to dissolve crystalline iron oxides. The amounts of chloride adsorbed by the extracted soils from 1 m NaCl at pH 5 and pH 7 were determined. The differences (ΔCl) between chloride adsorption at pH 5 and pH 7, attributed to variably charged groups, decreased when iron oxides were removed by EDTA and DE extraction. Close correlations (P>0.001), with negligible intercepts, were found (i)between EDTA-extractable iron (amorphous iron oxides) and the decrease in ΔCl following EDTA extraction, and (ii) between the difference between DE-extracted iron and EDTA-extractable iron (crystalline iron oxides) and the further decrease in ΔCL following DE extraction. The difference between ΔCl for acetate-extracted and DE-extracted samples was calculated from the contents and specific surfaces of amorphous and crystalline iron oxides, together with ΔCl per m² for synthetic iron oxides. Calculated and measured values were in very good agreement, indicating that soil iron oxides, in relation to chloride adsorption, may be treated as if they consist of only two fractions.     

Influence of iron oxides on cobalt adsorption by soils

Eleven soils from Denmark and Tanzania were extracted with ammonium acetate (controls), EDTA, and dithionite-EDTA (DE) to fractionate iron (and manganese) oxides. The amounts of cobalt adsorbed and acidity desorbed by the extracted soils as well as by two synthetic iron oxides were determined, using 0.85 mM cobalt in 0.2 m NaNO₃.
No significant correlations were found between cobalt adsorption and the contents of extractable manganese or organic matter, presumably because of their low contents. The major part of the cobalt adsorption (by DE-extracted samples) was due to the clay fraction and was associated with the release of approximately one proton per adsorbed Co. The remaining cobalt adsorption was attributed to the iron oxides. This portion of adsorbed cobalt was well described by considering soil iron oxides composed of only two fractions, an EDTA-extractable fraction of high reactivity and a less reactive fraction corresponding to the difference between DE-extractable iron and EDTA-extractable iron. Approximately 1.7 protons were released per Co adsorbed by these iron oxide fractions and by the synthetic iron oxides.
The amounts of cobalt adsorbed by the soil iron oxides were well predicted from the contents and specific surfaces of the two iron oxide fractions together with the specific cobalt adsorption of synthetic iron oxides.     

SELECTIVE EXTRACTION OF AMORPHOUS IRON OXIDES BY EDTA FROM SOILS FROM DENMARK AND TANZANIA

Soils from Denmark and Tanzania were extracted with EDTA solutions of different concentrations and pH. After extraction for 3 months there was no significant (95% level) further increase in amounts of iron (and aluminium, calcium, and magnesium) during longer extraction periods. X-ray diffraction showed no change of the crystalline minerals caused by the extraction, which is believed to be specific for amorphous iron oxides. The EDTA method may thus serve as a reference method for the determination of amorphous iron oxides in soils. Although the difference between EDTA-extractable iron and that extracted during 2 h by ammonium oxalate at pH 3.0 in the dark may be high, the ammonium oxalate method is considered to give a fast and often fair estimate of amorphous iron oxides.     

Adsorption of cobalt by soil iron oxides at low solution concentration

Abstract

Eleven selected soils from Denmark and Tanzania were treated with ammonium acetate (controls), EDTA, and dithionite‐EDTA (DE) to fractionate iron and (manganese) oxides. The amounts of cobalt adsorbed were determined from a 3 μM equilibrium cobalt solution, corresponding to the cobalt level in natural soil solutions using sodium nitrate (0.2 M) to suppress non‐specific adsorption, by the extracted soils as well as by two synthetic iron oxides.

No significant correlations were found between cobalt adsorption and the contents of organic matter and extractable manganese, presumably due to their low contents in the soils investigated. Close correlations were, however, found between the amounts of cobalt adsorbed, especially fractions thereof, and the contents of iron oxides.

The amounts of cobalt adsorbed by the DE‐ex‐tracted soils, void of iron (and manganese) oxides, were attributed to the clay silicates. The remaining cobalt adsorption, i.e. the difference between cobalt adsorbed by acetate‐extracted and DE‐extracted samples, was attributed to the iron oxides. This portion of adsorbed cobalt was well described by considering soil iron oxides composed of only two fractions, an EDTA‐extractable fraction of high reactivity and a less reactive fraction corresponding to the difference between DE‐extractable iron and EDTA‐extractable iron.

The amounts of cobalt adsorbed by the soil iron oxides were well predicted from the contents and specific surface of the two iron oxide fractions in soil together with the amount of cobalt adsorbed by the synthetic iron oxides.     

Dissolution of Poorly Crystalline Iron Oxides in Soils by EDTA and Oxalate

Poorly crystalline iron oxides in soils are often estimated by 2 hours oxalate extraction at pH 3 and less often by 3–7 months EDTA extraction at pH 7.5–10.5. Calculated solubility products (Ksp) of iron oxides in equilibrium with EDTA and oxalate showed EDTA to dissolve only iron oxides with Ksp > 10^?40-10^?41 at pH > 10, whereas at pH 3 oxalate (and EDTA) should theoretically dissolve all iron oxides. The different pHs could largely account for the great difference in extraction speed between the two methods. Although EDTA and oxalate seem to act by surface complexation, where the adsorbed ligand by attenuating lattice Fe-O bonds causes iron detachment, the mechanisms are considered to be different. Possibly EDTA forms tetranuclear surface complexes, which are considered to inhibit dissolution of well crystallized but not poorly crystallized iron oxides due to differences in bond strengths. Oxalate forming binuclear and mononuclear surface complexes can probably also act as an electron bridge between iron(II) in solution and surface iron(III) leading to iron(II) catalyzed dissolution of iron oxides. This mechanism is obviously of particular importance in the dissolution of magnetite and maghemite. Despite the great theoretical differences the published methods with EDTA and oxalate dissolve comparable amounts of iron from many soils and the dissolved iron corresponds to poorly crystalline (highly reactive) iron oxides, mainly ferrihydrite.     

SELECTIVE EXTRACTION OF AMORPHOUS IRON OXIDE BY EDTA FROM A MIXTURE OF AMORPHOUS IRON OXIDE, GOETHITE, AND HEMATITE

Iron (III) was extracted by EDTA and ammonium oxalate from a model substance consisting of amorphous iron oxide, goethite, and hematite precipitated in the presence of quartz sand. Even by varying the EDTA concentration between 0.02 and 0.1 M, pH between 4.40 and 6.00, the solid:solution ratio between I:25 and I:250, and using extraction times up to go days, it was found that EDTA was able to extract only a limited amount of iron. In contrast, 0.2 M ammonium oxalate at pH 3.0 is able to dissolve all the iron compounds if the extraction time is sufficient. Nevertheless, the amount of EDTA-extractable iron is equal to the amount of iron extracted after 4–5 hours with ammonium oxalateat pH 3.0. From X-ray analysis, DTA curves, a solubility product determination, and a kinetic investigation, it is concluded that the EDTA-extractable fraction consists of X-ray amorphous iron oxide, less soluble than polymeric iron hydroxide, and presumably only one compound. Therefore, it is concluded that it may be possible by means of EDTA to carry out a selective extraction of X-ray amorphous iron oxides mixed with goethite and hematite.     

Molybdenum Determination in Mehlich‐1 and Mehlich‐3 Soil Test Extracts and Molybdenum Adsorption in Brazilian Soils

Abstract

The extractant Mehlich‐1 is routinely used in Brazil for determination of soil nutrients, whereas Mehlich‐3 has been suggested as a promising extractor for soil fertility evaluation. Both were used for extraction of molybdenum (Mo) in Brazilian soils with Mo dosage by the KI+H₂O₂ method. The Langmuir and Freundlich isotherms were used to study soil Mo adsorption. Mehlich‐1 extracted more Mo than Mehlich‐3 in soils with high contents of organic matter, clay, and iron (Fe) oxides. Mehlich‐3 and Mehlich‐1 extractions correlated positively and significantly with amorphous Fe oxides, crystalline Fe oxides, and organic matter. Molybdenum recovering rates correlated to crystalline Fe oxides and clay contents but not to organic matter, pH, and Mo adsorption capacity. Amorphous and crystalline Fe oxides, clay, and organic matter were responsible for most of the Mo adsorption. The Langmuir isotherm described better the Mo adsorption to soil amorphous Fe oxides and organic matter than the Freundlich isotherm.     

铁氧化物与土壤表面电荷性质的关系

The relationship between iron oxides and surface charge characteristics in variable charge soils (latosol and red earth) was studied in following three ways.(1)Remove free iron oxides (Fed) and amorphous iron oxides (Feo) from the soils with sodium dithionite and acid ammonium oxalate solution respectively.(2) Add 2% glucose (on the basis of air-dry soil weight) to soils and incubate under submerged condition to activate iron oxides,and then the mixtures are dehydrated and air-dried to age iron oxides.(3) Precipitate various crystalline forms of iron oxides onto kaolinite.The results showed that free iron oxides (Fed) were the chief carrier of variable positive charges.Of which crystalline iron oxides (Fed-Feo) presented mainly as discrete particles in the soils and could only play a role of the carrier of positive charges,and did little influence on negative charges.Whereas the amorphous iron oxides (Feo),which presented mainly fas a coating with a large specific surface area,not only had positive charges,but also blocked the negative charge sites in soils.Submerged incubation activated iron oxides in the soils,and increased the amount of amorphous iron oxides and the degree of activation of iron oxide,which resulted in the increase of positive and negative charges of soils.Dehydration and air-dry aged iron oxides in soils and decreased the amount of amorphous iron oxides and the degree of activation of iron oxide,and also led to the decrease of positive and negative charges.Both the submerged incubation and the dehydration and air-dry had no significant influence on net charges.Precipitation of iron oxides onto kaolinite markedly increased positive charges and decreased negative charges.Amorphous iron oxide having a larger surface area contributed more positive charge sites and blocked more negative charge sites in kaolinite than crystalline goethite.     

干湿交替过程中水稻土铁形态和磷吸附解吸的变化

采用室内培养试验 ,经过连续 3次淹水 落干处理 ,研究了干湿交替过程中土壤中氧化铁形态的变化以及对土壤磷吸附和解吸的影响。结果表明 ,淹水使土壤中结晶态氧化铁含量明显减少 ,无定形氧化铁和土壤对磷的吸附量急剧增加 ,磷解吸下降 ;落干则使之发生相反的变化。土壤中的无定形氧化铁含量与土壤对磷的吸附之间存在着密切的相关关系。因此 ,淹水 落干过程中无定形氧化铁的变化是影响水稻田磷有效性的一个主导因子。     

三种铁氧化物的磷吸附解吸特性以及与磷吸附饱和度的关系

采用三种人工合成铁氧化物(针铁矿、赤铁矿和水铁矿)比较了结晶态和无定形铁氧化物对磷的吸附—解吸特性以及与磷吸附饱和度的关系。结果表明,三种铁氧化物的磷吸附特性均可用Langumir方程来描述,相关系数均大于0.9,达到极显著水平。从磷最大吸附量(Qm)、吸附反应常数(K)和最大缓冲容量(MBC)三项吸附参数综合考虑,水铁矿(无定形)对磷的吸附无论在容量还是强度方面均比结晶态铁氧化物针铁矿和赤铁矿大得多。水铁矿吸附的磷比针铁矿和赤铁矿所吸附的磷更难解吸;水铁矿的大量活性表面并没有表现出增加磷释放的作用。磷吸附饱和度有望作为评价土壤或铁氧化物磷吸附—解吸的强度和容量因子的一个综合指标。     

Changes in microstructure of soils following extraction of organically bonded metals and organic matter

The influence of selective removal of organically bonded metals and organic matter on soil microstructure was investigated. Two samples of soils with different mineralogical, chemical and mechanical composition were treated with acetylacetone in both polar and non-polar solvents to dissolve amorphous organic iron and aluminium, and with hydrogen peroxide to destroy organic matter. Transmission electron micrographs of ultrathin sections and scanning electron micrographs of <5?μm fractions of the extracted soils showed distinct changes of microstructure of clays after successive removal of cementing agents. Although untreated soils showed flocculent or honeycomb structure, soils with organic matter and organically bonded metals removed showed turbostratic domain structure with stepped clusters. The changes in microstructure of soils following extraction were confirmed by determination of pore-size distribution and total cumulative volume of pores using the mercury porosimetry method. In addition, the surface area of the extracted soils was determined by water adsorption. The results showed that organically bonded iron and aluminium and organic matter distinctly influence the fabric of microstructure as flocculating agents.     

COMPARISON OF AQUEOUS ACETYLACETONE AND POTASSIUM PYROPHOSPHATE SOLUTIONS FOR SELECTIVE EXTRACTION OF ORGANIC-BOUND Fe FROM SOILS

Seven samples of iron oxyhydroxides, characterised by crystallinity and surface area (monolayer water content), were examined for solubility in aqueous acetylacetone (0.68 M), potassium pyrophosphate pH 10 (0.1 M) and acid ammonium oxalate pH 3 (0.2 M). Solubilities in acetylacetone and oxalate were dependent on surface area, being 30 per cent Fe or more for non-crystalline oxides in 40 h. Solubility in pyrophosphate was 2 per cent Fe or less in 40 h even when surface area was 300 m²/g. Pyrophosphate solution was more suitable than aqueous acetylacetone for selective extraction of iron-organic complexes from soils which contain amorphous or poorly crystalline iron oxides.     

The forms of cobalt in some Scottish soils as determined by extraction and isotopic exchange

The results of fractionation and correlation studies provided evidence that cobalt in soils is associated predominantly with the soil oxide fraction, particularly the manganese oxides. Only a small proportion of the total cobalt present in soils was extracted by acetic acid, EDTA, pyrophosphate or hydroxylamine. Cobalt extracted with these reagents was considered to be derived principally from easily reducible manganese oxides, although the origin of the cobalt extracted by acetic acid in particular was not well defined. The bulk of the cobalt present in soils appeared to be occluded by more highly crystalline oxide materials or was present in the structures of primary and secondary minerals. Labile cobalt in soil was assessed by extraction with acetic acid and EDTA and by determination of isotopically exchangeable cobalt. The amounts of cobalt extracted by both EDTA and acetic acid were highly dependent on the length of extraction period and on the temperature of extraction. Neither of these reagents appeared likely to give good estimates of the quantity or intensity factors of soil cobalt supply to plants as defined by the isotopic exchange determinations.     

Origin of extractable titanium and vanadium in the a horizons of scottish podzols

The A horizons of Scottish podzols often contain unusually high amounts of EDTA-extractable Ti and V. Five such soils developed on different parent materials were studied, total and EDTA-extractable Ti, V and Fe being determined in the samples and their particle size fractions. The fine sand accounts for 50–80% of the total and extractable amounts of all three elements in the total soil although, in general, both the total and extractable Ti and V concentrations increase with decreasing particle size. Thus the clays contain up to 420 ppm Ti and 27 ppm V extractable with EDTA. The main Ti minerals in the samples are anatase, rutile and ilmenite. Observations by scanning electron microscopy of the surfaces of the minerals show clear evidence of chemical, and possibly physical weathering, processes that may lead to the accumulation of Ti in fine fractions, but not to downward translocation. Vanadium is probably associated with iron oxides and ferromagnesian minerals, from which it is released by weathering and subsequently adsorbed by the clay fraction. Electron paramagnetic resonance spectra confirm that most of the V extracted from the samples and their clay fractions is in the vanadyl, (VO)²⁺, form.     

Pre-lysis washing improves DNA extraction from a forest soil

A pre-lysis buffer washing procedure was introduced to DNA extraction from a forest soil with high organic matter and iron oxide contents. Sodium phosphate of 0.1 M (pH 7.5) was used as a buffer to wash soil samples when subsequent lysis buffer was phosphate, and 20 mM EDTA (pH 7.5) was used when subsequent lysis buffer included EDTA. Initial experiments were not successful because the DNA extracts could not be amplified by polymerase chain reaction (PCR). The consideration of introducing a pre-lysis washing procedure was based on the idea that the washing should promote soil dispersion and homogeneity, decrease DNA adsorption by soil components (e.g. iron oxides), and remove covalent cations and those easily-dissolving organic compounds from the soil samples. Results revealed that humic substance content decreased by 31%, but DNA yield increased by 24% in the DNA extracts of the pre-lysis washing procedures, compared to the non-washing procedures. DNA extracted by the pre-washing procedure needed less purification for subsequent 18S and 16S rDNA PCR amplifications. It was recommended that the pre-lysis buffer washing should be used for DNA extraction from those difficult environmental samples, such as the forest soil with high contents of organic matter and iron oxides.     

Availability of soil boron fractions to M.26 apple rootstock

The availability of various boron (B) fractions in soil to M.26 apple (Malus spp.) rootstock was examined. The study was carried out in a greenhouse on soils with diverse chemical and physical properties. The following B fractions were determined: (i) B in soil solution, (ii) B non‐specifically adsorbed on soil surface, (iii) B specifically adsorbed on soil colloid surfaces, (iv) B occluded in Mn oxyhydroxides, (v) B occluded in noncrystalline aluminum (Al) and iron (Fe) oxides, (vi) B occluded in crystalline Al and Fe oxides, (vii) B fixed with soil silicates, and (viii) total soil B. In the studied soils there were: 0.07–0.17 mg kg^‐1 B in soil solution, 0.01–0.03 mg kg^‐1 B non‐specifically adsorbed on soil surface, 0.04–0.08 mg kg^‐1 B specifically adsorbed on soil colloid surfaces, 0.28–0.67 mg kg^‐1 B occluded in manganese (Mn) oxides, 4.03–17.22 mg kg^‐1 B occluded in noncrystalline Al and Fe oxides, 8.93–50.62 mg kg^‐1 B occluded in crystalline Al and Fe oxides, 12.2–42.5 mg kg^‐1 B fixed with soil silicate, and 52.9–82.2 mg kg^‐1 total B. Simple correlation analysis showed positive correlation between B contents in M.26 apple rootstocks and amounts of B in soil solution (r=0.77), B non‐specifically adsorbed on soil colloid surfaces (r=0.65), B specifically adsorbed on soil surface (r=0.76) and B occluded in Mn oxyhydroxides (r=0.77). No relation was found between plant B contents and amounts of B occluded in non‐crystalline and crystalline Al and Fe oxides, B fixed with soil silicates and total B. The results indicated that extraction of B by 0.1 M NH₂OH HCl solution adequately represented amounts of B in soil solution, B non‐specifically and specifically adsorbed on soil compound surfaces and B occluded in Mn oxyhydroxides to assess availability of B to apple trees.     

Iron oxides as weathering indicator and the origin of Luvisols from the Vistula glaciation region in Poland

Purpose

The aim of the research was to determine the effect of lithogenic and pedogenic processes on the formation of Luvisols from the area of Vistula glaciation on the base of profile distribution of iron oxides and total iron in relation to texture and physicochemical properties. The indices of weathering of the soil material in genetic horizons were calculated, and changes in the content and forms of iron oxides were evaluated.

Materials and methods

The predominant type of soil in the study area is Luvisols under agricultural use, formed from silt formations on loam. The analyses were made applying the following methods: grain size composition using the sieve method and hydrometer method, the interpretation of the results was performed according to the World Reference Base for Soil Resources classification, the pH of soils was measured with the potentiometric method, C-organic with the Walkley-Black dichromate method, the content of the following iron forms was determined (total iron (Fe_t) after the mineralization of soils in the mixture of HF and HClO₄ acids), free iron oxides were extracted using dithionite-citrate-bicarbonate method, and amorphous iron oxides after the ammonium oxalate extraction (using the Philips 9100PU apparatus). The clay mineralogy was estimated by X-ray diffraction analysis.

Results and discussion

It was observed that total iron enrichment occurs in argic horizons accompanied by iron depletion in luvic horizons, while the profile distribution of iron is similar to the distribution of clay. The (Fe_d/Fe_t) ratio indicates a low degree of weathering; the highest values were observed in argic (Bt) horizons, which confirms the effect of the process of pedogenesis on the value of that index. In the soils investigated, crystalline iron oxides generally dominate over the amorphous forms. The mineralogical composition of clay fraction separated from the upper part of soils was different as compared to the underlying material.

Conclusions

The results of the study showed that iron contents (together with the other indicators) and its forms can be used to distinguish soil layers of different origin. The depth distribution of Fe_d, Fe_o and Fe_t within soil profiles indicates that the soil material may be of different lithogenic origin in the studied pedons.

     

铁铝氧化物–层状硅酸盐矿物互作对Cr (Ⅵ)和As (Ⅴ)吸附的影响

以高岭石、蒙脱土、针铁矿和三水铝石四种单一典型土壤矿物以及针铁矿-蒙脱石和三水铝石-蒙脱石（质量比为1:1）两种代表性土壤矿物复合体为吸附材料，采用吸附平衡实验、能谱分析（(EDS）、红外光谱、扫描电镜、酸碱滴定和zeta电位测定等方法，研究了铁铝氧化物与层状硅酸盐矿物之间的相互作用对Cr(Ⅵ)和As(Ⅴ)吸附的影响及其机制。吸附平衡实验和EDS实验结果表明，两种复合体对Cr(Ⅵ)和As(Ⅴ)的吸附容量均小于其两种组成矿物单一体系吸附量的平均值，即铁铝氧化物与蒙脱石的互作降低了这些氧化物对Cr(Ⅵ)和As(Ⅴ)的吸附能力。表面性质表征结果表明，与蒙脱石复合后，针铁矿与三水铝石表面的正电荷均被完全中和，电荷符号发生反转。与理论值相比，三水铝石-蒙脱石复合体的表面位点总浓度无明显变化，比表面积减小。针铁矿-蒙脱石复合体的比表面积与理论值无明显差异，但矿物表面位点浓度减小，表面羟基红外吸收峰强度减弱。氧化物与层状硅酸盐矿物互作改变了矿物表面性质，这可能是导致氧化物对Cr(Ⅵ)和As(Ⅴ)的吸附能力降低的主要原因。当评估污染元素在土壤中有效性时应当考虑土壤固相组分间的互作对离子吸附的影响。