Martin V. Fey

Spontaneous vegetation encroachment upon bauxite residue (red mud) as an indicator and facilitator of in situ remediation processes.

The spontaneous colonization of a bauxite residue (alumina refining tailings) deposit by local vegetation in Linden, Guyana, over 30 years, indicates that natural weathering processes can ameliorate tailings to the extent that it can support vegetation. Samples were collected from vegetated and unvegetated areas to investigate the relationships between bauxite residue properties and vegetation cover. Compared to unvegetated areas, bauxite residue in vegetated areas had lower pH (mean pH 7.9 vs 10.9), lower alkalinity (mean titratable alkalinity 0.4 vs 1.4 mol H(+) kg(-1)), lower electrical conductivity (mean EC 0.3 vs 2.1 mS cm(-1)), lower total Al (mean Al2O3 19.8 vs 25.8% wt) and Na (mean Na2O 0.9 vs 3.7% wt), and less sodalite and calcite. Accumulation of N, NH4(+), and organic C occurred under vegetation, demonstrating the capacity for plants to modify residue to suit their requirements as a soil-like growth medium. Aeolian redistribution of coarse grained tailings appeared to support vegetation establishment by providing a thin zone of enhanced drainage at the surface. Natural pedogenic processes may be supplemented by irrigation, enhanced drainage, and incorporation of sand and organic matter at other tailings deposits to accelerate the remediation process and achieve similar results in a shorter time frame.

Fly ash as a permeable cap for tailings management: pedogenesis in bauxite residue tailings

PurposeClosure of tailings facilities typically involves either a ‘cap and store’ or ‘direct revegetation’ approach. Both have been used in the management of bauxite residue (alumina refining tailings), with mixed results. This study evaluated the merit of an intermediate approach, using a permeable cap, and examined the pedogenic trajectory of the Technosol.Materials and methodsChemical, mineralogical and physical properties of samples from a Brazilian bauxite residue deposit, which had been capped with fly ash 14xa0years prior and supported a vegetation cover, were compared to evaluate soil formation and pedogenic trajectory of the developing Technosol according to the World Reference Base for Soil Resources. Samples were collected at three points along an elevation gradient, and from 0 to 150xa0cm below the surface.Results and discussionRainfall leaching was identified as the most important pedogenic process occurring in the tailings, lowering salinity and pH. The Technosol classification was poorly suited to describe the soil materials within the study site because two wastes (fly ash and bauxite residue) were co-disposed in discrete layers.ConclusionsThe permeability of the fly ash cap is key to soil development in these tailings: it provides a suitable medium for plant growth whilst still allowing contact between the tailings and the surrounding environment. The introduction of a novel prefix qualifier, ordic, would enable more accurate description of layered Technosols. The Technosol at this site is likely to develop towards an Andosol or Ferralsol.

Evaluation of soil analytical methods for the characterization of alkaline Technosols: I. Moisture content, pH, and electrical conductivity

PurposeApplying standard soil analytical methods to novel soil materials, such as tailings or soils with unusual properties, should be done with caution and with special consideration of potential interferents and possible pretreatments. The aim of this study was to investigate the effects of common variations in methods on calculated total moisture content, pH, and electrical conductivity (EC) of saline alkaline soil materials.Materials and methodsBauxite residue (an alkaline, saline–sodic Technosolic material) as well as two saline alkaline soils from coastal and lacustrine environments were dried under various temperatures and atmospheres, and then analyzed for pH and EC at various soil–solution ratios over time.Results and discussionCalculated moisture content of all samples increased with drying temperature. Dehydration of gypsum elevated calculated moisture content. Decreases in soil–solution ratio decreased suspension EC and pH in highly alkaline samples. The pH and EC of soil/water suspensions generally rose with equilibration time for bauxite residue; stable values were attained within 24–120xa0h. Atmospheric carbonation substantially decreased the pH of samples dried at lower temperatures.ConclusionsVariations in temperature, time, and atmosphere during drying of highly alkaline and saline soil materials influenced calculated moisture content as well as chemical properties such as pH and EC. A drying temperature of 40xa0°C and drying to constant weight is recommended to minimize these effects. Soil–solution ratio, equilibration time, and sample preparation conditions influenced observed pH and EC, and should be standardized if attempting to compare results between studies.

Evaluation of soil analytical methods for the characterization of alkaline Technosols: II. Amorphous constituents and carbonates

PurposeThe aims of this study were to identify potential sources of error in common methods for determination of amorphous oxide concentrations and carbonate concentrations, as applied to a Technosolic material (bauxite residue), and where possible, suggest improvements to the methods.Materials and methodsAn acid ammonium oxalate (AAO) extraction was applied to fresh and weathered bauxite residues, at soil to solution ratios varying from 1:100 to 1:800. Two methods for carbonate concentration were compared: the ‘weight loss’ method, and the ‘difference in total C’ method. These were applied to six weathered bauxite residue samples, with CaCO3 concentrations ranging from 0.1–2xa0% weight.Results and discussionChemically extractable amorphous content was underreported in bauxite residue at the standard 1:100 extraction ratio, likely due to Al and Si saturation of the oxalate complex. A 1:400 soil: AAO ratio extracted the highest amount of amorphous material. Some crystalline minerals such as sodalite, inherited from the Technosolic parent material, were soluble in the acid ammonium oxalate extractant. The difference in total C method was more precise than the weight loss method for the determination of carbonate concentration in bauxite residues.ConclusionsThe high amorphous content of bauxite residues requires a wider soil to solution ratio (1:400) for acid ammonium oxalate extraction than is used for typical soil materials (1:100). The difference in total C method is recommended for the routine analysis of field samples where small variations in carbonate concentration need to be detected.

Petroduric and "petrosepiolitic" horizons in soils of Namaqualand, South Africa

Indurated, light-coloured sepiocrete horizons have been found in Namaqualand Calcisols and Durisols. These horizons resembled calcrete but were non- to only mildly calcareous, resisted slaking in acid and alkali, and often broke with a conchoidal fracture. The presence of elevated quantities of sepiolite in the bulk-soil was confirmed by XRD analysis. The degree of induration in some these horizons suggested cementation by silica, and so in this paper the slaking properties, bulk chemistry, mineralogy and micromorphology of these horizons are compared with the typical silica-cemented, reddish-brown petroduric/duripan (dorbank) encountered in the region. Sepiocrete horizons are chemically, mineralogically and morphologically distinct from the petrocalcic and petroduric horizons with which they are commonly associated. Micromorphology of the petroduric horizons revealed prominent illuviation in the form of oriented clay parallel to grains and crescent coatings on voids, a red matrix due to iron oxides, and translucent, isotropic amorphous silica coatings on grains and voids. In the sepiocrete horizons, sepiolite appeared as a matrix of interlocking, sub-parallel fibres while the amorphous material was localised. The amorphous material was silica-rich with prominent aluminium and lesser magnesium; light brown under plane polarised light; not completely isotropic and had a lower birefringence than the sepiolite. The calcite was usually micritic, but also appeared as loose granules and as elongate crystals in a sepiolite matrix. The presence of the laminar Si-Al -rich areas on the sections suggested at the least localised duric properties and so mutual reinforcement of sepiolite and silica is possible. However, the sepiocrete horizons did not meet the slaking requirements of the petroduric (dorbank) horizons and are distinct in appearance to the typical petroduric horizons in the region. They contained more MgO than the region’s typical petroduric, and too little SiO 2 to be silcrete. While thexa0-crete terminology provides a useful expression of the cemented nature of the horizon, in order to fit existing soil classification and description schemes the terms sepiolitic and petrosepiolitic (in the same sense as calcic and petrocalcic) are proposed and defined. The term sepiolitic would be useful in the adjectival form in petrocalcic or petroduric horizons where sepiolite is significant but not the primary cement. The genesis of the petrosepiolitic horizons is likely to be essentially similar to that of petrocalcic and petroduric horizons, except for chemical differences in the matrix solutions from which secondary minerals were precipitated, dictated by the pH and evaporative evolution of the soil solution.Indurated, light-coloured sepiocrete horizons have been found in Namaqualand Calcisols and Durisols. These horizons resembled calcrete but were non- to only mildly calcareous, resisted slaking in acid and alkali, and often broke with a conchoidal fracture. The presence of elevated quantities of sepiolite in the bulk-soil was confirmed by XRD analysis. The degree of induration in some these horizons suggested cementation by silica, and so in this paper the slaking properties, bulk chemistry, mineralogy and micromorphology of these horizons are compared with the typical silica-cemented, reddish-brown petroduric/duripan (dorbank) encountered in the region. Sepiocrete horizons are chemically, mineralogically and morphologically distinct from the petrocalcic and petroduric horizons with which they are commonly associated. Micromorphology of the petroduric horizons revealed prominent illuviation in the form of oriented clay parallel to grains and crescent coatings on voids, a red matrix due to iron oxides, and translucent, isotropic amorphous silica coatings on grains and voids. In the sepiocrete horizons, sepiolite appeared as a matrix of interlocking, sub-parallel fibres while the amorphous material was localised. The amorphous material was silica-rich with prominent aluminium and lesser magnesium; light brown under plane polarised light; not completely isotropic and had a lower birefringence than the sepiolite. The calcite was usually micritic, but also appeared as loose granules and as elongate crystals in a sepiolite matrix. The presence of the laminar Si-Al -rich areas on the sections suggested at the least localised duric properties and so mutual reinforcement of sepiolite and silica is possible. However, the sepiocrete horizons did not meet the slaking requirements of the petroduric (dorbank) horizons and are distinct in appearance to the typical petroduric horizons in the region. They contained more MgO than the regions typical petroduric, and too little SiO2 to be silcrete. While the -crete terminology provides a useful expression of the cemented nature of the horizon, in order to fit existing soil classification and description schemes the terms sepiolitic and petrosepiolitic (in the same sense as calcic and petrocalcic) are proposed and defined. The term sepiolitic would be useful in the adjectival form in petrocalcic or petroduric horizons where sepiolite is significant but not the primary cement. The genesis of the petrosepiolitic horizons is likely to be essentially similar to that of petrocalcic and petroduric horizons, except for chemical differences in the matrix solutions from which secondary minerals were precipitated, dictated by the pH and evaporative evolution of the soil solution.

From tailings to soil: long-term effects of amendments on progress and trajectory of soil formation and in situ remediation in bauxite residue

PurposeBauxite residue is an alkaline, saline-sodic byproduct of the Bayer process for alumina production. In situ remediation of bauxite residue is a cost-effective management strategy that transforms the residue into a soil-like medium; however, the effects of applied amendments on trajectories and progress of soil formation over the medium-long term are unclear. Here, we investigated how in situ remediation amendments guided the progress and trajectories of soil formation in bauxite residue over 20xa0years.Materials and methodsChemical and mineralogical properties of samples from a field site in Corpus Christi, Texas, were analyzed to compare the effects of three different amendments (sewage sludge, yard waste, and topsoil) on progress and trajectories of soil formation in bauxite residue. Bauxite residue was deposited ca. 40xa0years prior to sampling; amendments were applied at varying frequencies for 20xa0years prior to sampling.Results and discussionSewage sludge was the most effective amendment for decreasing pH, EC, and total alkalinity of bauxite residue and increasing plant nutrients and exchangeable cations. Overall soil development is in an early stage; however, incipient soil horizons were identified from differences in chemical and mineralogical properties with depth. Although amendment type influenced the progress of soil formation, it did not appear to influence the overall trajectory of soil formation processes. Soil derived from the bauxite residue parent material is likely to progress from a spolic Technosol to a technic Cambisol and finally either a Ferralsol or Luvisol depending on organic matter dynamics.ConclusionsLong-term impacts of amendments were consistent with those observed in previous short-term studies, with some outcomes improving further over the long term (e.g., Na+ leaching) and were realized to a depth enabling maintenance of a vegetation cover. These outcomes provide strong support for the use of in situ remediation as an alternative to soil capping that enables transformation of bauxite residue to a soil capable of supporting a self-sustaining ecosystem.

Petroduric and ‘petrosepiolitic’ horizons in soils of namaqualand, South Africa | Horizontes petrodúricos e “petrosepiolíticos” em solos de Namaqualand, África do Sul | Horizontes petrodúricos y “petrosepiolíticos” en suelos de Namaqualand, Suráfrica

Indurated, light-coloured sepiocrete horizons have been found in Namaqualand Calcisols and Durisols. These horizons resembled calcrete but were non- to only mildly calcareous, resisted slaking in acid and alkali, and often broke with a conchoidal fracture. The presence of elevated quantities of sepiolite in the bulk-soil was confirmed by XRD analysis. The degree of induration in some these horizons suggested cementation by silica, and so in this paper the slaking properties, bulk chemistry, mineralogy and micromorphology of these horizons are compared with the typical silica-cemented, reddish-brown petroduric/duripan (dorbank) encountered in the region. Sepiocrete horizons are chemically, mineralogically and morphologically distinct from the petrocalcic and petroduric horizons with which they are commonly associated. Micromorphology of the petroduric horizons revealed prominent illuviation in the form of oriented clay parallel to grains and crescent coatings on voids, a red matrix due to iron oxides, and translucent, isotropic amorphous silica coatings on grains and voids. In the sepiocrete horizons, sepiolite appeared as a matrix of interlocking, sub-parallel fibres while the amorphous material was localised. The amorphous material was silica-rich with prominent aluminium and lesser magnesium; light brown under plane polarised light; not completely isotropic and had a lower birefringence than the sepiolite. The calcite was usually micritic, but also appeared as loose granules and as elongate crystals in a sepiolite matrix. The presence of the laminar Si-Al -rich areas on the sections suggested at the least localised duric properties and so mutual reinforcement of sepiolite and silica is possible. However, the sepiocrete horizons did not meet the slaking requirements of the petroduric (dorbank) horizons and are distinct in appearance to the typical petroduric horizons in the region. They contained more MgO than the region’s typical petroduric, and too little SiO 2 to be silcrete. While thexa0-crete terminology provides a useful expression of the cemented nature of the horizon, in order to fit existing soil classification and description schemes the terms sepiolitic and petrosepiolitic (in the same sense as calcic and petrocalcic) are proposed and defined. The term sepiolitic would be useful in the adjectival form in petrocalcic or petroduric horizons where sepiolite is significant but not the primary cement. The genesis of the petrosepiolitic horizons is likely to be essentially similar to that of petrocalcic and petroduric horizons, except for chemical differences in the matrix solutions from which secondary minerals were precipitated, dictated by the pH and evaporative evolution of the soil solution.Indurated, light-coloured sepiocrete horizons have been found in Namaqualand Calcisols and Durisols. These horizons resembled calcrete but were non- to only mildly calcareous, resisted slaking in acid and alkali, and often broke with a conchoidal fracture. The presence of elevated quantities of sepiolite in the bulk-soil was confirmed by XRD analysis. The degree of induration in some these horizons suggested cementation by silica, and so in this paper the slaking properties, bulk chemistry, mineralogy and micromorphology of these horizons are compared with the typical silica-cemented, reddish-brown petroduric/duripan (dorbank) encountered in the region. Sepiocrete horizons are chemically, mineralogically and morphologically distinct from the petrocalcic and petroduric horizons with which they are commonly associated. Micromorphology of the petroduric horizons revealed prominent illuviation in the form of oriented clay parallel to grains and crescent coatings on voids, a red matrix due to iron oxides, and translucent, isotropic amorphous silica coatings on grains and voids. In the sepiocrete horizons, sepiolite appeared as a matrix of interlocking, sub-parallel fibres while the amorphous material was localised. The amorphous material was silica-rich with prominent aluminium and lesser magnesium; light brown under plane polarised light; not completely isotropic and had a lower birefringence than the sepiolite. The calcite was usually micritic, but also appeared as loose granules and as elongate crystals in a sepiolite matrix. The presence of the laminar Si-Al -rich areas on the sections suggested at the least localised duric properties and so mutual reinforcement of sepiolite and silica is possible. However, the sepiocrete horizons did not meet the slaking requirements of the petroduric (dorbank) horizons and are distinct in appearance to the typical petroduric horizons in the region. They contained more MgO than the regions typical petroduric, and too little SiO2 to be silcrete. While the -crete terminology provides a useful expression of the cemented nature of the horizon, in order to fit existing soil classification and description schemes the terms sepiolitic and petrosepiolitic (in the same sense as calcic and petrocalcic) are proposed and defined. The term sepiolitic would be useful in the adjectival form in petrocalcic or petroduric horizons where sepiolite is significant but not the primary cement. The genesis of the petrosepiolitic horizons is likely to be essentially similar to that of petrocalcic and petroduric horizons, except for chemical differences in the matrix solutions from which secondary minerals were precipitated, dictated by the pH and evaporative evolution of the soil solution.

Horizontes petrodúricos y "petrosepiolíticos" en suelos de Namaqualand, Suráfrica .

Indurated, light-coloured sepiocrete horizons have been found in Namaqualand Calcisols and Durisols. These horizons resembled calcrete but were non- to only mildly calcareous, resisted slaking in acid and alkali, and often broke with a conchoidal fracture. The presence of elevated quantities of sepiolite in the bulk-soil was confirmed by XRD analysis. The degree of induration in some these horizons suggested cementation by silica, and so in this paper the slaking properties, bulk chemistry, mineralogy and micromorphology of these horizons are compared with the typical silica-cemented, reddish-brown petroduric/duripan (dorbank) encountered in the region. Sepiocrete horizons are chemically, mineralogically and morphologically distinct from the petrocalcic and petroduric horizons with which they are commonly associated. Micromorphology of the petroduric horizons revealed prominent illuviation in the form of oriented clay parallel to grains and crescent coatings on voids, a red matrix due to iron oxides, and translucent, isotropic amorphous silica coatings on grains and voids. In the sepiocrete horizons, sepiolite appeared as a matrix of interlocking, sub-parallel fibres while the amorphous material was localised. The amorphous material was silica-rich with prominent aluminium and lesser magnesium; light brown under plane polarised light; not completely isotropic and had a lower birefringence than the sepiolite. The calcite was usually micritic, but also appeared as loose granules and as elongate crystals in a sepiolite matrix. The presence of the laminar Si-Al -rich areas on the sections suggested at the least localised duric properties and so mutual reinforcement of sepiolite and silica is possible. However, the sepiocrete horizons did not meet the slaking requirements of the petroduric (dorbank) horizons and are distinct in appearance to the typical petroduric horizons in the region. They contained more MgO than the region’s typical petroduric, and too little SiO 2 to be silcrete. While thexa0-crete terminology provides a useful expression of the cemented nature of the horizon, in order to fit existing soil classification and description schemes the terms sepiolitic and petrosepiolitic (in the same sense as calcic and petrocalcic) are proposed and defined. The term sepiolitic would be useful in the adjectival form in petrocalcic or petroduric horizons where sepiolite is significant but not the primary cement. The genesis of the petrosepiolitic horizons is likely to be essentially similar to that of petrocalcic and petroduric horizons, except for chemical differences in the matrix solutions from which secondary minerals were precipitated, dictated by the pH and evaporative evolution of the soil solution.Indurated, light-coloured sepiocrete horizons have been found in Namaqualand Calcisols and Durisols. These horizons resembled calcrete but were non- to only mildly calcareous, resisted slaking in acid and alkali, and often broke with a conchoidal fracture. The presence of elevated quantities of sepiolite in the bulk-soil was confirmed by XRD analysis. The degree of induration in some these horizons suggested cementation by silica, and so in this paper the slaking properties, bulk chemistry, mineralogy and micromorphology of these horizons are compared with the typical silica-cemented, reddish-brown petroduric/duripan (dorbank) encountered in the region. Sepiocrete horizons are chemically, mineralogically and morphologically distinct from the petrocalcic and petroduric horizons with which they are commonly associated. Micromorphology of the petroduric horizons revealed prominent illuviation in the form of oriented clay parallel to grains and crescent coatings on voids, a red matrix due to iron oxides, and translucent, isotropic amorphous silica coatings on grains and voids. In the sepiocrete horizons, sepiolite appeared as a matrix of interlocking, sub-parallel fibres while the amorphous material was localised. The amorphous material was silica-rich with prominent aluminium and lesser magnesium; light brown under plane polarised light; not completely isotropic and had a lower birefringence than the sepiolite. The calcite was usually micritic, but also appeared as loose granules and as elongate crystals in a sepiolite matrix. The presence of the laminar Si-Al -rich areas on the sections suggested at the least localised duric properties and so mutual reinforcement of sepiolite and silica is possible. However, the sepiocrete horizons did not meet the slaking requirements of the petroduric (dorbank) horizons and are distinct in appearance to the typical petroduric horizons in the region. They contained more MgO than the regions typical petroduric, and too little SiO2 to be silcrete. While the -crete terminology provides a useful expression of the cemented nature of the horizon, in order to fit existing soil classification and description schemes the terms sepiolitic and petrosepiolitic (in the same sense as calcic and petrocalcic) are proposed and defined. The term sepiolitic would be useful in the adjectival form in petrocalcic or petroduric horizons where sepiolite is significant but not the primary cement. The genesis of the petrosepiolitic horizons is likely to be essentially similar to that of petrocalcic and petroduric horizons, except for chemical differences in the matrix solutions from which secondary minerals were precipitated, dictated by the pH and evaporative evolution of the soil solution.