Sustained Bauxite Residue Rehabilitation with Gypsum and Organic Matter 16 years after Initial Treatment.

Bauxite residue is a high volume byproduct of alumina manufacture which is commonly disposed of in purpose-built bauxite residue disposal areas (BRDAs). Natural waters interacting with bauxite residue are characteristically highly alkaline, and have elevated concentrations of Na, Al, and other trace metals. Rehabilitation of BRDAs is therefore often costly and resource/infrastructure intensive. Data is presented from three neighboring plots of bauxite residue that was deposited 20 years ago. One plot was amended 16 years ago with process sand, organic matter, gypsum, and seeded (fully treated), another plot was amended 16 years ago with process sand, organic matter, and seeded (partially treated), and a third plot was left untreated. These surface treatments lower alkalinity and salinity, and thus produce a substrate more suitable for biological colonisation from seeding. The reduction of pH leads to much lower Al, V, and As mobility in the actively treated residue and the beneficial effects of treatment extend passively 20-30 cm below the depth of the original amendment. These positive rehabilitation effects are maintained after 2 decades due to the presence of an active and resilient biological community. This treatment may provide a lower cost solution to BRDA end of use closure plans and orphaned BRDA rehabilitation.

Leaching of copper and nickel in soil-water systems contaminated by bauxite residue (red mud) from Ajka, Hungary: the importance of soil organic matter.

Red mud is a highly alkaline (pH >12) waste product from bauxite ore processing. The red mud spill at Ajka, Hungary, in 2010 released 1 million m(3) of caustic red mud into the surrounding area with devastating results. Aerobic and anaerobic batch experiments and solid phase extraction techniques were used to assess the impact of red mud addition on the mobility of Cu and Ni in soils from near the Ajka spill site. Red mud addition increases aqueous dissolved organic carbon (DOC) concentrations due to soil alkalisation, and this led to increased mobility of Cu and Ni complexed to organic matter. With Ajka soils, more Cu was mobilised by contact with red mud than Ni, despite a higher overall Ni concentration in the solid phase. This is most probably because Cu has a higher affinity to form complexes with organic matter than Ni. In aerobic experiments, contact with the atmosphere reduced soil pH via carbonation reactions, and this reduced organic matter dissolution and thereby lowered Cu/Ni mobility. These data show that the mixing of red mud into organic rich soils is an area of concern, as there is a potential to mobilise Cu and Ni as organically bound complexes, via soil alkalisation. This could be especially problematic in locations where anaerobic conditions can prevail, such as wetland areas contaminated by the spill.

Gypsum addition to soils contaminated by red mud: implications for aluminium, arsenic, molybdenum and vanadium solubility.

Red mud is highly alkaline (pH 13), saline and can contain elevated concentrations of several potentially toxic elements (e.g. Al, As, Mo and V). Release of up to 1 million m(3) of bauxite residue (red mud) suspension from the Ajka repository, western Hungary, caused large-scale contamination of downstream rivers and floodplains. There is now concern about the potential leaching of toxic metal(loid)s from the red mud as some have enhanced solubility at high pH. This study investigated the impact of red mud addition to three different Hungarian soils with respect to trace element solubility and soil geochemistry. The effectiveness of gypsum amendment for the rehabilitation of red mud-contaminated soils was also examined. Red mud addition to soils caused a pH increase, proportional to red mud addition, of up to 4 pH units (e.g. pH 7 â†’ 11). Increasing red mud addition also led to significant increases in salinity, dissolved organic carbon and aqueous trace element concentrations. However, the response was highly soil specific and one of the soils tested buffered pH to around pH 8.5 even with the highest red mud loading tested (33 % w/w); experiments using this soil also had much lower aqueous Al, As and V concentrations. Gypsum addition to soil/red mud mixtures, even at relatively low concentrations (1 % w/w), was sufficient to buffer experimental pH to 7.5-8.5. This effect was attributed to the reaction of Ca(2+) supplied by the gypsum with OH(-) and carbonate from the red mud to precipitate calcite. The lowered pH enhanced trace element sorption and largely inhibited the release of Al, As and V. Mo concentrations, however, were largely unaffected by gypsum induced pH buffering due to the greater solubility of Mo (as molybdate) at circumneutral pH. Gypsum addition also leads to significantly higher porewater salinities, and column experiments demonstrated that this increase in total dissolved solids persisted even after 25 pore volume replacements. Gypsum addition could therefore provide a cheaper alternative to recovery (dig and dump) for the treatment of red mud-affected soils. The observed inhibition of trace metal release within red mud-affected soils was relatively insensitive to either the percentage of red mud or gypsum present, making the treatment easy to apply. However, there is risk that over-application of gypsum could lead to detrimental long-term increases in soil salinity.

Performance of three resin-based materials for treating uranium-contaminated groundwater within a PRB.

Three materials that are designed to treat uranium-contaminated water were investigated. These are a cation exchange resin, IRN 77; an anion exchange resin, Varion AP; and a recently developed material called PANSIL (quartz sand coated with 2% amidoxime resin by weight). The reaction rate, capacity, and effective pH range of the three materials are reported. The capacity and conditional distribution coefficient in neutral, uranyl-contaminated synthetic groundwater containing carbonate are also reported. The suitability of each material for treating uranium-contaminated groundwater using a permeable reactive barrier (PRB) approach is then discussed. All three materials react rapidly in the pH range 5-7, reaching equilibrium in less than 4h at approximately 23 degrees C. The unconditioned cation exchange resin removed 8 g UO2 2+ per kg of resin from neutral synthetic groundwater containing 30 mg/l of UO2 2+, but a lower capacity is anticipated in groundwater with either higher ionic strength or lower UO2 2 concentrations. It operates by first acidifying the solution, then sorbing UO2 2, and can release UO2 2 when its buffering capacity has been exhausted. The anion exchange resin is very effective at removing anionic uranyl carbonate species from solutions with a pH above 5, with good specificity. Up to 50 g/kg of uranium is removed from contaminated groundwater at neutral pH. PANSIL is effective at sequestering cationic and neutral uranyl species from solutions in the pH range 4.5-7.5, with very good specificity. The capacity of PANSIL is pH-dependent, increasing from about 0.4 g/kg at pH 4.5, to about 1 g/kg at pH 6, and 1.5 g/kg around pH 7.5. In neutral groundwater containing carbonate, both the anion exchange resin and PANSIL exhibit conditional distribution coefficients exceeding 1470 ml/g, which is about an order of magnitude higher than comparable reactive barrier materials reported in the literature.

Development of a functionalized polymer-coated silica for the removal of uranium from groundwater.

A new active material for the treatment of uranium-contaminated groundwater using permeable reactive barriers has been developed. This material, called PANSIL, is an example of a tailored ligand system that selectively removes a contaminant from solution. The active medium in PANSIL is a polyacryloamidoxime resin derived from polyacrylonitrile, which is deposited from solution onto the surface of quartz sand to form a thin film coating. PANSIL is highly effective at sequestering UO2(2+) from solution when the pH is between about 5 and 8 and can preferentially sequester UO2(2+) from solutions that are typical of the groundwater from a mine tailings site, due to the stability of the polyacryloamidoxime uranyl complex formed. Uranium sequestration capacity will depend on the surface area of the sand that is resin coated, but in the batch of PANSIL tested (<2% resin by weight), it exceeds 4000 mg of UO2 per kg of PANSIL at pH 4.5 when the dissolved UO2(2+) concentration is greaterthan 300 mg/L. PANSIL largely retains the permeability and strength of the sand employed and therefore has suitable engineering properties for permeable reactive barrier applications.