Bauxite-processing Residue Research Articles

Contaminant mobility and carbon sequestration downstream of the Ajka (Hungary) red mud spill: The effects of gypsum dosing

A number of emergency pollution management measures were enacted after the accidental release of caustic bauxite processing residue that occurred in Ajka, western Hungary in October, 2010. These centred on acid and gypsum dosing to reduce pH and minimise mobility of oxyanion contaminants mobile at high pH. This study assessed the effectiveness of gypsum dosing on contaminant mobility and carbon sequestration through assessment of red mud and gypsum-affected fluvial sediments via elemental analysis and stable isotope analysis. There was a modest uptake of contaminants (notably As, Cr, and Mn) on secondary carbonate-dominated deposits in reaches subjected to gypsum dosing. C and O stable isotope ratios of carbonate precipitates formed as a result of gypsum dosing were used to quantify the importance of the neutralisation process in sequestering atmospheric carbon dioxide. This process was particularly pronounced at sites most affected by gypsum addition, where up to 36% of carbonate-C appears to be derived from atmospheric in-gassing of CO2. The site is discussed as a large scale analogue for potential remedial approaches and carbon sequestration technologies that could be applied to red mud slurries and other hyperalkaline wastes. The results of this work have substantial implications for the aluminium production industry in which 3–4% of the direct CO2 emissions may be offset by carbonate precipitation. Furthermore, carbonation by gypsum addition may be important for contaminant remediation, also providing a physical stabilisation strategy for the numerous historic stockpiles of red mud.

Ecotoxicity of fluvial sediments downstream of the Ajka red mud spill, Hungary

An integrated assessment of biological activity and ecotoxicity of fluvial sediments in the Marcal river catchment (3078 km(2)), western Hungary, is presented following the accidental spill of bauxite processing residue (red mud) in Ajka. Red mud contaminated sediments are characterised by elevated pH, elevated trace element concentrations (e.g. As, Co, Cr, V), high exchangeable Na, and induce an adverse effect on test species across a range of trophic levels. While background contamination of the river system is highlighted by adverse effects on some test species at sites unaffected by red mud, the most pronounced toxic effects apparent in Vibrio fischeri bioluminescence inhibition, Lemna minor bioassay and Sinapis alba root and shoot growth occur at red mud depositional hotspots in the lower Torna Creek and upper Marcal. Heterocypris incongruens bioassays show no clear patterns, although the most red mud-rich sites do exert an adverse effect. Red mud does however appear to induce an increase in the density of aerobic and facultative anaerobic bacterial communities when compared with unaffected sediments and reference sites. Given the volume of material released in the spill, it is encouraging that the signal of the red mud on aquatic biota is visible at a relatively small number of sites. Gypsum-affected samples appear to induce an adverse effect in some bioassays (Sinapis alba and Heterocypris incongruens), which may be a feature of fine grain size, limited nutrient supply and greater availability of trace contaminants in the channel reaches that are subject to intense gypsum dosing. Implications for monitoring and management of the spill are discussed.

Dispersal and Attenuation of Trace Contaminants Downstream of the Ajka Bauxite Residue (Red Mud) Depository Failure, Hungary

This paper identifies the spatial extent of bauxite processing residue (red mud)-derived contaminants and modes of transport within the Marcal and Rába river systems after the dike failure at Ajka, western Hungary. The geochemical signature of the red mud is apparent throughout the 3076 km² Marcal system principally with elevated Al, V, As, and Mo. Elevated concentrations of Cr, Ga, and Ni are also observed within 2 km of the source areas in aqueous and particulate phases where hyperalkalinity (pH < 13.1) is apparent. Although the concentrations of some trace elements exceed aquatic life standards in waters (e.g., V, As) and fluvial sediments (As, Cr, Ni, V), the spatial extent of these is limited to the Torna Creek and part of the upper Marcal. Source samples show a bimodal particle size distribution (peaks at 0.7 and 1.3 μm) which lends the material to ready fluvial transport. Where elevated concentrations are found in fluvial sediments, sequential extraction suggests the bulk of the As, Cr, Ni, and V are associated with residual (aqua-regia/HF digest) phases and unlikely to be mobile in the environment. However, at some depositional hotspots, association of As, Cr, and V with weak acid-extractable phases is observed.

Bauxite Processing Residue: A Critical Review of Its Formation, Properties, Storage, and Revegetation

Bauxite is processed in alumina refineries by the Bayer process in which Al-containing minerals are dissolved in hot NaOH. The insoluble solids (bauxite processing residue mud and sand) are washed, sometimes partially neutralized (using CO2 or seawater treatment), and deposited in impoundments surrounding the refinery using either wet (15–30% solids) or dry (50–65% solids) disposal techniques. Revegetation strategies for impoundments include constructing a soil cap over the mud deposit, amending the mud with residue sand and other materials or revegetating the sand fraction that surrounds and covers the mud deposit. Major limitations to plant growth in residues include salinity, sodicity, alkalinity, Al toxicity, and deficiencies of macro- and micronutrients. Physical properties are also problematic since residue mud consolidates to form a solid mass that waterlogs easily or dries to form a massive structure, while sand has a very low water holding capacity. Gypsum amendment reduces pH, displaces Na with Ca and tends to improve porosity. Organic amendments supply nutrients, increase CEC, and improve physical conditions while inorganic fertilizers supply essential nutrients. An integrated approach using all of the previously mentioned amendments as well as saline sodic-tolerant plant species is likely to increase the success of revegetation compared to use of only 1 or 2 techniques. There are very few reports on microbial activity in residue but since it is heat and chemically treated, activity is likely to be initially very low. Organic residue addition provides a substrate for heterotrophic microbes and provide the beginnings for a below-ground living ecosystem. Inoculation of plants with appropriate mycorrhiza and legumes with Rhizobium might also increase plant establishment and growth. These aspects deserve future examination. Although much research has concentrated on the establishment phase of revegetation, little consideration has been given to the subsequent growth and self-propagation of the introduced species, changes in residue properties during weathering and leaching of alkalinity from residue over time. These facets need careful study before definitive statements can be made regarding the sustainability of various revegetation strategies.

Influence of arbuscular mycorrhizal fungi on the growth and nutrient status of bermudagrass grown in alkaline bauxite processing residue

A nursery experiment was conducted to evaluate the potential role of arbuscular mycorrhizal (AM) fungi in encouraging the vegetation cover on bauxite residue (red mud) sites. An alkali tolerant bermudagrass (Cynodon dactylon) adapted to local conditions were grown in red mud with different amendments with and without AM fungi to assess mycorrhizal effects on plant growth, mineral nutrition, metal uptake and neutralization of bauxite residue. Inoculation of AM fungi significantly increased the plant growth, nutrient uptake and reduced Fe, Al accumulation in plant tissue and also improved the soil physico-chemical and biochemical properties. Gypsum and sludge amended treatments inoculated with AM fungi had maximum biomass, nutrient uptake and reduced accumulation of metals. The neutralization of red mud was significant in presence of AM fungi than control. The experiment provided evidence for the potential use of bermudagrass in combination with AM fungi for ecological restoration of bauxite residue sites.

Zinc forms in compost and red mud-amended bauxite residue sand

Purpose Re-vegetation is the preferred long-term practice for managing Alcoa’s bauxite-processing residue storage areas. Residue sand is the primary growth medium for rehabilitation; however, it is largely nutrient deficient. Although addition of organic and inorganic amendments can provide short-term supply of plant-available nutrients, but quickly exhausted, thus long-term deficiencies are often observed. The rapid transformation of added zinc into non-available pools is predicted as the main factor limiting vegetation performance.

Effect of amendment of bauxite processing sand with organic materials on its chemical, physical and microbial properties

The effects of addition of a range of organic amendments (biosolids, spent mushroom compost, green waste compost and green waste-derived biochar), at two rates, on some key chemical, physical and microbial properties of bauxite-processing residue sand were studied in a laboratory incubation study. Levels of exchangeable cations were not greatly affected by additions of amendments but extractable P was increased significantly by mushroom and green waste composts and massively (i.e. from 11.8 to 966 mg P kg −1) by biosolids applications. Levels of extractable NO 3 −–N were also greatly elevated by biosolids additions and there was a concomitant decrease in pH. Addition of all amendments decreased bulk density and increased mesoporosity, available water holding capacity and water retention at field capacity (−10 kPa), with the higher rate having a greater effect. Addition of biosolids, mushroom compost and green waste compost all increased soluble organic C, microbial biomass C, basal respiration and the activities of β-glucosidase, L-asparaginase and alkali phosphatase enzymes. The germination index of watercress grown in the materials was greatly reduced by biosolids application and this was attributed to the combined effects of a high EC and high concentrations of extractable P and NO 3 −. It was concluded that the increases in water storage and retention and microbial activity induced by additions of the composts is likely to improve the properties of bauxite-processing residue sand as a growth medium but that allowing time for soluble salts, originating from the organic amendments, to leach out may be an important consideration before sowing seeds.

Surface charge characteristics and sorption properties of bauxite-processing residue sand

Bauxite-processing residue sand (BRS) is the primary growth medium used to rehabilitate Alcoa’s residue storage areas (RSAs) in south-west Western Australia. This material is typically coarse-textured, highly saline, highly alkaline, extremely sodic, and deficient in plant nutrients. To develop appropriate fertiliser strategies for optimising rehabilitation performance, a fundamental understanding of the surface charge and nutrient retention properties of BRS is essential. The contribution of permanent (σp) and variable (σv) charge to the overall magnitude and sign of the surface charge, and ammonium (NH4) and phosphorus (P) sorption, as a function of pH were studied. Samples of BRS were obtained from Alcoa’s Kwinana (KW), Pinjarra (PJ), and Wagerup (WG) Refineries. Each sample exhibited predominantly variable charge (σv ≈ 8–12 cmol/kg at pH 12), and negligible permanent negative charge (σp ≈ 0.2 cmol/kg). The point of zero net charge (PZNC) was observed at pH 6.96, 6.89, and 5.98 for the KW, PJ, and WG samples, respectively. These values are consistent with those reported for soils dominated by Fe and Al oxides and hydroxides but containing negligible organic matter. Solution and adsorbed NH4 decreased with increasing pH (pH 7–11) for BRS. It was suggested that ammonia volatilisation was a major loss pathway for NH4 applied to BRS. Phosphorus sorption decreased with increasing pH for each BRS. It was suggested that the presence of competing anions (i.e. carbonate) and increasing negative surface charge density were the major causes for this behaviour. The results from this study have major implications for the selection of suitable types of fertilisers (particularly nitrogen) for rehabilitating alkaline BRS.

Behaviour and dynamics of di-ammonium phosphate in bauxite processing residue sand in Western Australia—II. Phosphorus fractions and availability

The production of alumina involves its extraction from bauxite ore using sodium hydroxide under high temperature and pressure. This process yields a large amount of residue wastes, which are difficult to revegetate due to their inherent hostile properties--high alkalinity and sodicity, poor water retention and low nutrient availability. Although phosphorus (P) is a key element limiting successful ecosystem restoration, little information is available on the availability and dynamics of P in rehabilitated bauxite-processing residue sand (BRS). The major aim of this experiment was to quantify P availability and behaviour as affected by pH, source of BRS and di-ammonium phosphate (DAP) application rate. This incubation experiment was undertaken using three sources of BRS, three DAP application rates (low, without addition of DAP; medium, 15.07 mg P and 13.63 mg N of DAP per jar, 100 g BRS; and high, 30.15 mg P and 27.26 mg N per jar, 100 g BRS), and four BRS pH treatments (4, 7, 9 and 11 (original)). The moisture content was adjusted to 55% water holding capacity and each BRS sample was incubated at 25 degrees C for a period of 119 days. After this period, Colwell P and 0.1 M H(2)SO(4) extractable P in BRS were determined. In addition, P sequential fractionation was carried out and the concentration of P in each pool was measured. A significant proportion (37% recovered in Colwell P and 48% in 0.1 M H(2)SO(4) extraction) of P added as DAP in BRS are available for plant use. The pH did not significantly affect 0.1 M H(2)SO(4) extractable P, while concentrations of Colwell P in the higher initial pH treatments (pH 7, 9 and 11) were greater than in the pH 4 treatments. The labile fractions (sum of NH(4)Cl (AP), bicarbonate and first sodium hydroxide extractable P (N(I)P)) consisted of 58-64% and 70-72% of total P in the medium and high DAP rate treatments, respectively. This indicates that most P added as DAP remained labile or moderately labile in BRS, either in solution, or in adsorbed forms on the surface of more crystalline P compounds, sesquioxides and carbonate, or associated with amorphous and some crystalline Al and Fe hydrous oxides. In addition, differences in the hydrochloric acid extractable P and the residual-P fractions among the treatments with and without DAP addition were relative small comparing with other P pools (e.g., NaOH extractable P pools), further indicating the limited capacity of BRS for fixing P added in Ca-P and other most recalcitrant forms. P availability in the original BRS without addition of DAP was very low, mostly in recalcitrant form. It has been clearly demonstrated that significant proportions of P added as DAP could remain labile or moderately labile for plant use during the rehabilitation of bauxite-processing residue disposal areas. There was limited capacity of BRS for fixing P in more recalcitrant forms (e.g., Ca-P and residual-P). Concentrations of most P pools in BRS increased with the DAP application rate. The impact of the pH treatment on P availability varied with the type of P pools and the DAP rate. It is recommended that the development of appropriate techniques for more accurate estimation of P availability in BRS and the quantification of the potential leaching loss of P in BRS are needed for the accurate understanding of P availability and dynamics in BRS. In addition, application of organic matters (e.g., biosolids and biochar, etc.) to BRS may be considered for improving P availability and buffering capacity.

Behaviour and dynamics of di-ammonium phosphate in bauxite processing residue sand in Western Australia—I. NH3 volatilisation and residual nitrogen availability

Australia is the largest producer of bauxite in the world, with an annual output of approximately 62 million metric dry tons in 2007. For every tonne of alumina, about 2 tonnes of highly alkaline and highly saline bauxite-processing residue are produced. In Western Australia, Alcoa World Alumina, Australia (Alcoa) produces approximately 15 MT of residue annually from its refineries (Kwinana, Pinjarra and Wagerup). The bauxite-processing residue sand (BRS) fraction represents the primary material for rehabilitating Alcoa's residue disposal areas (RDAs). However, the inherently hostile characteristics (high alkalinity, high salinity and poor nutrient availability) of BRS pose severe limitations for establishing sustainable plant cover systems. Alcoa currently applies 2.7 t ha(-1) of di-ammonium phosphate ((NH(4))(2)HPO(4); DAP)-based fertiliser as a part of rehabilitation of the outer residue sand embankments of its RDAs. Limited information on the behaviour of the dominant components of this inorganic fertiliser in highly alkaline BRS is currently available, despite the known effects of pH on ammonium (NH(4)) and phosphorus (P) behaviour. The aim of this study was to quantify the effects of pH on NH(3) volatilisation and residual nitrogen (N) in BRS following DAP applications. The sponge-trapping and KCl-extraction method was used for determining NH(3) volatilisation from surface-applied DAP in samples of BRS collected from each of Alcoa's three Western Australia Refineries (Kwinana, Pinjarra, Wagerup) under various pH conditions (pH 4, 7, 9 and 11). Following cessation of volatilisation, the residual N was extracted from BRS using 2 M KCl and concentrations of NH (4) (+) -N and NO (3) (-) -N were determined by flow injection analysis. The quantities of NH(3) volatilised increased dramatically as the pH increased from 4 to 11. Much of the N lost as NH(3) (up to 95.2%) occurred within a short period (24 h to 7 days), particularly for the pH 9 and 11 treatments. Concentrations of residual NH (4) (+) -N recovered in DAP-treated BRS at the end of the experiment decreased with increasing pH. This finding was consistent with increasing loss of N via volatilisation as pH increased. The concentration of NO (3) (-) -N was very low due to no nitrification in BRS. The pH was a key driver for NH(3) volatilisation from DAP-treated BRS and primarily controlled N dynamics in BRS. Results indicate that NH(4) not adsorbed by BRS was highly susceptible to volatilisation. The likely lack of nitrifying bacteria did not allow conversion of ammonium to nitrate, thereby further exacerbating the potential for loss via volatilisation It was demonstrated that the pH is the key factor controlling the loss of inorganic N from BRS. Although volatilisation was considerably lower at pH 4, achieving this pH reduction in the field is not possible at present. Findings from this study highlight the need to better understand which forms of N fertiliser are most suitable for use in highly alkaline BRS. Although pH reduction is the most likely means of stopping NH(3) volatilisation in BRS, it is economically and operationally unfeasible to add sufficient acidity for adequately lowering pH in the BRS for revegetation. More attention on forms of fertilisers more suitable to highly alkaline, microbially inert soil conditions appears to be warranted.

Micronutrient fractionation and plant availability in bauxite-processing residue sand

Bauxite-processing residue must be disposed of in specifically designed facilities for long-term management. Consideration of alkalinity, salinity, sodium content, and poor nutritional status is essential for successful rehabilitation of residue disposal areas (RDA). The aim of this study was to examine the availability and distribution of the micronutrients, B, Cu, Fe, Mn, and Zn, in (i) fresh bauxite-processing residue sand (particle size &gt;150 μm) with and without gypsum amendment, and (ii) aged residue sand from a 4-year-old rehabilitated RDA that had received past gypsum and fertiliser addition. Samples of fresh residue sand from India and Australia exhibited high alkalinity, high salinity, and sodicity. Gypsum addition significantly lowered pH, soluble Na, and alkalinity. Aged residue sand had low levels of all micronutrients, with low extractability for Zn and Mn followed by B, Cu, and Fe. Fractionation showed that 30–78% of Zn and Mn and 40–60% of B existed in non-available (residual) forms. The next most dominant fractions were the Fe and Mn oxide-bound and carbonate-bound fractions. Plant-available fractions (i.e. exchangeable and organically bound) contributed &lt;1% of the total concentration. Total concentration was found to be a reliable indicator for Zn, Cu, and B extractability but not for DTPA-extractable forms of Fe and Mn. Leaf analysis of vegetation grown on aged residue sand indicated deficiencies of Mn and B. Results demonstrated that bauxite-processing residue sand contained very low levels of B, Mn, and Zn and these concentrations may be limiting to plant growth. Distribution of micronutrients among chemical pools was significantly influenced by pH, organic carbon, exchangeable Na, and alkalinity of residue. Nutrient management strategies that account for the characteristics of residue sand need to be developed for residue rehabilitation. Importantly, strategies to limit the conversion of nutrients to non-available forms are required to minimise micronutrient disorders.

Reclamation of Fine Fraction Bauxite Processing Residue (Red Mud) Amended with Coarse Fraction Residue and Gypsum

Establishment of vegetation on residues produced from the bauxite refining process is a beneficial part of their environmental management. Of the two fractions produced in the refining, the coarse fraction has greater efficiency in the leaching of excess salts and alkalinity. However, these same properties can result in increased loss of nutrients and low water-holding capacity. The current study investigated the use of mixing coarse fraction residue with fine fraction residue, at two different application rates (10% and 25%), with and without the use of gypsum as an ameliorant, for re-vegetation of the residue with Trifolium pratense. Optimum plant growth was observed in treatments that had also received gypsum amendment, with higher plant biomass, Mn nutrition and lower Al and Fe concentration. However, use of process sand at the higher application rate (25%) promoted lower levels of soluble Al and Fe and exchangeable Na in the substrate and, consequently, lower plant uptake of Na. Results indicate that co-disposal of the coarse fraction sand at 25% w/w with fine fraction residue can improve the substrate and, therefore, plant uptake and growth. Further monitoring is recommended to determine the effect of the absence of gypsum and other nutrient sources on plant growth.

Nutrient status of vegetation grown in alkaline bauxite processing residue amended with gypsum and thermally dried sewage sludge - A two year field study

Although the treatments for overcoming the high pH and exchangeable sodium percentage (ESP) of bauxite residue are well known, there is little information on long-term nutrient management of vegetation after rehabilitation. The present study examined the chemical and physical amendment of fine fraction residue (red mud) at the Aughinish Alumina Ltd. Bayer Plant, Ireland followed by a two-year field investigation. Gypsum and sewage sludge were incorporated into the residue and amended mud sown with Lolium perenne and Holcus lanatus. Aerial portions were harvested and nutrient composition determined annually for the first two years growth. Amended substrate was low in manganese and magnesium. After year one herbage contained adequate calcium levels, but there were deficiencies for nitrogen, manganese, potassium and magnesium. Sodium levels were not considered excessive and levels declined further in year two. Levels for nitrogen, calcium, manganese, magnesium, phosphorous and potassium were also reduced in the second year. As levels were already deficient in year one the further decreases suggest severe nutrient shortage in the residue substrate. For long-term success of revegetation of bauxite residue, even after gypsum and organic amendment, the deficiencies of nutrients in the substrate must be overcome.

The liming effect of bauxite processing residue (red mud) on sandy soils

Bauxite residue (red mud) is produced in large amounts in alumina refineries as a waste product of the Bayer process. This material has the potential to be used as a soil liming agent due to its high pH and high acid neutralisation capacity. Soil incubation experiments compared red mud from several Australian and overseas refineries to estimate its liming effect relative to lime (CaCO3). Two acid (pH 4.3, 4.6) sandy soils were mixed with 8 rates of 17 red muds, 3 limes, and NaOH. Values of EC and pH were measured at 1, 4, and 16 weeks incubation. Each red mud produced a different buffering curve when added to soil, although the shapes of curves could be classified into 5 behavioural groups. The liming equivalent of red mud decreased with increasing target soil pH. For a target pH of 6 the lime equivalent of the red muds was 11–42% for soil 1 and 13–50% for soil 2. No single characteristic of red mud could be used to accurately predict the liming equivalent, as its liming effect is due to several, variable constituents (e.g. free caustic, sodalite, calcite). The pH reached for a certain rate of addition of red mud to soil can be estimated from a comparison of the buffering (titration) curves of both the soil and the red mud. In every case, CaCO3 had a much larger liming capacity than red mud and red mud may not be an economical alternative to lime except where other benefits are associated with its use (e.g. lower cost, decreased P leaching, reduced water repellence).

Reducing phosphorus leaching from sandy soils with red mud bauxite processing residues

This study used field lysimeters to investigate the reduction in the leaching of phosphorus (P) applied as superphosphate fertilizer from a very sandy Swan Coastal Plain soil treated with bauxite processing residue (red mud) neutralized with either waste gypsum from the phosphate industry or ferrous sulfate (copperas) from the titanium dioxide industry. Addition of 500 t ha-' red mud/gypsum or 200 t hap1 red mud/copperas were found to reduce the leaching of P to below 3 kg ha-l for application rates of 270 and 80 kgP ha-1, respectively. Water retention from these excessively well drained soils was increased by 14 and 50% by the addition of 200 and 2000 t ha-l red mud, respectively. The pH of the leachate for all rates of red mud/copperas application increased from approximately 4 to range between 7 and 7-5. The concentrations of Na and SO4 were about 8 and 17 g1-l, respectively, in the initial leachates collected from the 2000 t ha-' red mud treatment but declined to approximately 0.4 and 2.0g l-1 after 3 years of leaching. The Ca leaching appeared to be initially controlled by the solubility of the excess CaSO4 remaining after red mud neutralization, with concentrations ranging between 0.3 and 0.5 g l-1 before declining to approximately the levels for untreated soils of 0.01-0.06g l-1. The Na, So4 and Ca concentrations in the leachates from the 500 t ha-l red mud/copperas treated soil decreased to acceptable levels after 2 years. High total soluble salt (TSS) levels associated with high levels of residue application may affect pasture production in the years immediately following soil amendment