Environmental behaviour of iron and steel slags in coastal settings.

Iron and steel slags have a long history of both disposal and beneficial use in the coastal zone. Despite the large volumes of slag deposited, comprehensive assessments of potential risks associated with metal(loid) leaching from iron and steel by-products are rare for coastal systems. This study provides a national-scale overview of the 14 known slag deposits in the coastal environment of Great Britain (those within 100 m of the mean high-water mark), comprising geochemical characterisation and leaching test data (using both low and high ionic strength waters) to assess potential leaching risks. The seaward facing length of slag deposits totalled at least 76 km, and are predominantly composed of blast furnace (iron-making) slags from the early to mid-20th Century. Some of these form tidal barriers and formal coastal defence structures, but larger deposits are associated with historical coastal disposal in many former areas of iron and steel production, notably the Cumbrian coast of England. Slag deposits are dominated by melilite phases (e.g. gehlenite), with evidence of secondary mineral formation (e.g. gypsum, calcite) indicative of weathering. Leaching tests typically show lower element (e.g. Ba, V, Cr, Fe) release under seawater leaching scenarios compared to deionised water, largely ascribable to the pH buffering provided by the former. Only Mn and Mo showed elevated leaching concentrations in seawater treatments, though at modest levels (<3 mg/L and 0.01 mg/L, respectively). No significant leaching of potentially ecotoxic elements such as Cr and V (mean leachate concentrations <0.006 mg/L for both) were apparent in seawater, which micro-X-Ray Absorption Near Edge Structure (µXANES) analysis show are both present in slags in low valence (and low toxicity) forms. Although there may be physical hazards posed by extensive erosion of deposits in high-energy coastlines, the data suggest seawater leaching of coastal iron and steel slags in the UK is likely to pose minimal environmental risk.

Quantifying CO2 Removal at Enhanced Weathering Sites: a Multiproxy Approach.

Enhanced weathering is a carbon dioxide (CO2) mitigation strategy that promises large scale atmospheric CO2 removal. The main challenge associated with enhanced weathering is monitoring, reporting, and verifying (MRV) the amount of carbon removed as a result of enhanced weathering reactions. Here, we study a CO2 mineralization site in Consett, Co. Durham, UK, where steel slags have been weathered in a landscaped deposit for over 40 years. We provide new radiocarbon, δ13C, 87Sr/86Sr, and major element data in waters, calcite precipitates, and soils to quantify the rate of carbon removal. We demonstrate that measuring the radiocarbon activity of CaCO3 deposited in waters draining the slag deposit provides a robust constraint on the carbon source being sequestered (80% from the atmosphere, 2σ = 8%) and use downstream alkalinity measurements to determine the proportion of carbon exported to the ocean. The main phases dissolving in the slag are hydroxide minerals (e.g., portlandite) with minor contributions (<3%) from silicate minerals. We propose a novel method for quantifying carbon removal rates at enhanced weathering sites, which is a function of the radiocarbon-apportioned sources of carbon being sequestered, and the proportion of carbon being exported from the catchment to the oceans.

Comparative assessment of marine weathering of ROP-derived biopolymers against conventional plastics.

Bio-based plastics were designed to replace single-use plastics and to cause less post-consumer environmental damage. This paper assesses the weathering of four bio-based polymers created by ring opening polymerization (ROP) promoted by a previously reported Ti-based catalyst, to detect any problems before production was scaled up. Samples were aged in seawater to identify degradation products and monitor structural changes. Surfaces evidenced degradation and a range of leaching products was observed. Aside from compounds used in the preparation of the plastics (i.e. residual monomers and benzyl alcohol), the degradation products included carboxylic acids (often found in plastic leachate), oxacyclohexadecan-2-one (potentially toxic to aquatic life) and triphenylmethane (potential carcinogen). Overall, there were fewer structural changes in the fossil fuel based polymer (PS) and in the commercially available bio-based plastic studied for comparison purposes than the lab based bio-based polymers.

Atmospheric Carbon Capture Performance of Legacy Iron and Steel Waste.

Legacy iron (Fe) and steel wastes have been identified as a significant source of silicate minerals, which can undergo carbonation reactions and thus sequester carbon dioxide (CO2). In reactor experiments, i.e., at elevated temperatures, pressures, or CO2 concentrations, these wastes have high silicate to carbonate conversion rates. However, what is less understood is whether a more "passive" approach to carbonation can work, i.e., whether a traditional slag emplacement method (heaped and then buried) promotes or hinders CO2 sequestration. In this paper, the results of characterization of material retrieved from a first of its kind drilling program on a historical blast furnace slag heap at Consett, U.K., are reported. The mineralogy of the slag material was near uniform, consisting mainly of melilite group minerals with only minor amounts of carbonate minerals detected. Further analysis established that total carbon levels were on average only 0.4% while average calcium (Ca) levels exceeded 30%. It was calculated that only ∼3% of the CO2 sequestration potential of the >30 Mt slag heap has been utilized. It is suggested that limited water and gas interaction and the mineralogy and particle size of the slag are the main factors that have hindered carbonation reactions in the slag heap.

Constructed wetlands for steel slag leachate management: Partitioning of arsenic, chromium, and vanadium in waters, sediments, and plants.

Constructed wetlands can treat highly alkaline leachate resulting from the weathering of steel slag before reuse (e.g. as aggregate) or during disposal in repositories and legacy sites. This study aimed to assess how metal(loid)s soluble at high pH, such as arsenic (As), chromium (Cr), and vanadium (V) are removed in constructed wetlands and how they accumulate in the sediments and the plants (Phragmites australis, common reed). The results show that reedbeds were very effective at removing calcium (98%), aluminium (81%), barium (98%), chromium (90%), gallium (80%), nickel (98%), and zinc (98%), and lowering pH and alkalinity. No statistical difference was found for As and V between leachate influent and wetland samples, showing that these metal(loid)s were not efficiently removed. As, Cr, and V were significantly higher in the reedbed sediments than in a reference site. However, sediment concentrations are not at levels that would pose a concern regarding reuse for agricultural purposes (average values of 39 ±â€¯26 mg kg-1 for As, 108 ±â€¯15 mg kg-1 for Cr, and 231 ±â€¯34 mg kg-1 for V). Also, there is no significant uptake of metals by the aboveground portions of the reeds compared to reference conditions. Results show statistically significant enrichment in metal(loid)s in rhizomes and also a seasonal effect on the Cr concentrations. The data suggest minimal risk of oxyanion-forming element uptake and cycling in wetlands receiving alkaline steel slag.

Leaching behaviour of co-disposed steel making wastes: Effects of aeration on leachate chemistry and vanadium mobilisation.

Steelmaking wastes stored in landfill, such as slag and spent refractory liners, are often enriched in toxic trace metals (including V). These may become mobile in highly alkaline leachate generated during weathering. Fresh steelmaking waste was characterised using XRD, XRF, and SEM-EDX. Batch leaching tests were performed under aerated, air-excluded and acidified conditions to determine the impact of atmospheric CO2 and acid addition on leachate chemistry. Phases commonly associated with slag including dicalcium silicate, dicalcium aluminoferrite, a wüstite-like solid solution and free lime were identified, as well as a second group of phases including periclase, corundum and graphite which are representative of refractory liners. During air-excluded leaching, dissolution of free lime and dicalcium silicate results in a high pH, high Ca leachate in which the V concentration is low due to the constraint imposed by Ca3(VO4)2 solubility limits. Under aerated conditions, carbonation lowers the leachate pH and provides a sink for aqueous Ca, allowing higher concentrations of V to accumulate. Below pH 10, leachate is dominated by periclase dissolution and secondary phases including monohydrocalcite and dolomite are precipitated. Storage of waste under saturated conditions that exclude atmospheric CO2 would therefore provide the optimal environment to minimise V leaching during weathering.

Behaviour and fate of vanadium during the aerobic neutralisation of hyperalkaline slag leachate.

Vanadium is a toxic metal present in alkaline leachates produced during the weathering of steel slags. Slag leaching can therefore have deleterious effects on local watercourses due to metal toxicity, the effects of the high pH (9-12.5) and rapid carbonation (leading to smothering of benthic communities). We studied the fate and behaviour of V in slag leachate both through field observations of a heavily affected stream (Howden Burn, Consett UK) and in controlled laboratory experiments where slag leachates were neutralised by CO2 ingassing from air. V was found to be removed from leachates downstream from the Howden Burn source contemporaneously with a fall in pH, Ca, Al and Fe concentrations. In the neutralisation experiments pH reduced from 12 → 8, and limited quantities of V were incorporated into precipitated CaCO3. The presence of kaolinite clay (i.e. SiOH and AlOH surfaces) during neutralisation experiments had no measureable effect on V uptake in the alkaline to circumneutral pH range. XANES analysis showed that V was present in precipitates recovered from experiments as adsorbed or incorporated V(V) indicating its likely presence in leachates as the vanadate oxyanion (HVO42-). Nano-scale particles of 2-line ferrihydrite also formed in the neutralised leachates potentially providing an additional sorption surface for V uptake. Indeed, removal of V from leachates was significantly enhanced by the addition of goethite (i.e. FeOOH surfaces) to experiments. EXAFS analysis of recovered goethite samples showed HVO42- was adsorbed by the formation of strong inner-sphere complexes, facilitating V removal from solution at pH < 10. Results show that carbonate formation leads to V removal from leachates during leachate neutralisation, and the presence of both naturally occurring and neoformed Fe (oxy)hydroxides provide a potent sink for V in slag leachates, preventing the spread of V in the environment.

Recovery of Al, Cr and V from steel slag by bioleaching: Batch and column experiments.

Steel slag is a major by-product of the steel industry and a potential resource of technology critical elements. For this study, a basic oxygen furnace (BOF) steel slag was tested for bacterial leaching and recovery of aluminium (Al), chromium (Cr), and vanadium (V). Mixed acidophilic bacteria were adapted to the steel slag up to 5% (w/v). In the batch tests, Al, Cr, and V were bioleached significantly more from steel slag than in control treatments. No statistical difference was observed arising from the duration of the leaching (3 vs 6 d) in the batch tests. Al and Cr concentrations in the leachate were higher for the smaller particle size of the steel slag (<75 µm), but no difference was observed for V. In the column tests, no statistical difference was found for pH, Al, Cr and V between the live culture (one-step bioleaching) and the supernatant (two-step bioleaching). The results show that the culture supernatant can be effectively used in an upscaled industrial application for metal recovery. If bioleaching is used in the 170-250 million tonnes of steel slag produced per year globally, significant recoveries of metals (100% of Al, 84% of Cr and 8% of V) can be achieved, depending on the slag composition. The removal and recovery percentages of metals from the leachate with Amberlite®IRA-400 are relatively modest (<67% and <5%, respectively), due to the high concentration of competing ions (SO42-, PO43-) in the culture medium. Other ion exchange resins can be better suited for the leachate or methods such as selective precipitation could improve the performance of the resin. Further research is needed to minimise interference and maximise metal recovery.

Hydration of dicalcium silicate and diffusion through neo-formed calcium-silicate-hydrates at weathered surfaces control the long-term leaching behaviour of basic oxygen furnace (BOF) steelmaking slag.

Alkalinity generation and toxic trace metal (such as vanadium) leaching from basic oxygen furnace (BOF) steel slag particles must be properly understood and managed by pre-conditioning if beneficial reuse of slag is to be maximised. Water leaching under aerated conditions was investigated using fresh BOF slag at three different particle sizes (0.5-1.0, 2-5 and 10 × 10 × 20 mm blocks) and a 6-month pre-weathered block. There were several distinct leaching stages observed over time associated with different phases controlling the solution chemistry: (1) free-lime (CaO) dissolution (days 0-2); (2) dicalcium silicate (Ca2SiO4) dissolution (days 2-14) and (3) Ca-Si-H and CaCO3 formation and subsequent dissolution (days 14-73). Experiments with the smallest size fraction resulted in the highest Ca, Si and V concentrations, highlighting the role of surface area in controlling initial leaching. After ~2 weeks, the solution Ca/Si ratio (0.7-0.9) evolved to equal those found within a Ca-Si-H phase that replaced dicalcium silicate and free-lime phases in a 30- to 150-µm altered surface region. V release was a two-stage process; initially, V was released by dicalcium silicate dissolution, but V also isomorphically substituted for Si into the neo-formed Ca-Si-H in the alteration zone. Therefore, on longer timescales, the release of V to solution was primarily controlled by considerably slower Ca-Si-H dissolution rates, which decreased the rate of V release by an order of magnitude. Overall, the results indicate that the BOF slag leaching mechanism evolves from a situation initially dominated by rapid hydration and dissolution of primary dicalcium silicate/free-lime phases, to a slow diffusion limited process controlled by the solubility of secondary Ca-Si-H and CaCO3 phases that replace and cover more reactive primary slag phases at particle surfaces.

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.

Hydraulic and biotic impacts on neutralisation of high-pH waters.

The management of alkaline (pH11-12.5) leachate is an important issue associated with the conditioning, afteruse or disposal of steel slags. Passive in-gassing of atmospheric CO2 is a low cost option for reducing Ca(OH)2 alkalinity, as Ca(OH)2 is neutralised by carbonic acid to produce CaCO3. The relative effectiveness of such treatment can be affected by both the system geometry (i.e. stepped cascades versus settlement ponds) and biological colonization. Sterilized mesocosm experiments run over periods of 20days showed that, due to more water mixing and enhanced CO2 dissolution at the weirs, the cascade systems (pH11.2→9.6) are more effective than settlement ponds (pH11.2→11.0) for lowering leachate alkalinity in all the tested conditions. The presence of an active microbial biofilm resulted in significantly more pH reduction in ponds (pH11.2→9.5), but had a small impact on the cascade systems (pH11.2→9.4). The pH variation in biofilm colonized systems shows a diurnal cycle of 1 to 1.5pH units due to CO2 uptake and release associated with respiration and photosynthesis. The results demonstrate that, where gradient permits, aeration via stepped cascades are the best option for neutralisation of steel slag leachates, and where feasible, the development of biofilm communities can also help reduce alkalinity.

Mechanism of Vanadium Leaching during Surface Weathering of Basic Oxygen Furnace Steel Slag Blocks: A Microfocus X-ray Absorption Spectroscopy and Electron Microscopy Study.

Basic oxygen furnace (BOF) steelmaking slag is enriched in potentially toxic V which may become mobilized in high pH leachate during weathering. BOF slag was weathered under aerated and air-excluded conditions for 6 months prior to SEM/EDS and µXANES analysis to determine V host phases and speciation in both primary and secondary phases. Leached blocks show development of an altered region in which free lime and dicalcium silicate phases were absent and Ca-Si-H was precipitated (CaCO3 was also present under aerated conditions). µXANES analyses show that V was released to solution as V(V) during dicalcium silicate dissolution and some V was incorporated into neo-formed Ca-Si-H. Higher V concentrations were observed in leachate under aerated conditions than in the air-excluded leaching experiment. Aqueous V concentrations were controlled by Ca3(VO4)2 solubility, which demonstrate an inverse relationship between Ca and V concentrations. Under air-excluded conditions Ca concentrations were controlled by dicalcium silicate dissolution and Ca-Si-H precipitation, leading to relatively high Ca and correspondingly low V concentrations. Formation of CaCO3 under aerated conditions provided a sink for aqueous Ca, allowing higher V concentrations limited by kinetic dissolution rates of dicalcium silicate. Thus, V release may be slowed by the precipitation of secondary phases in the altered region, improving the prospects for slag reuse.

Removal and recovery of vanadium from alkaline steel slag leachates with anion exchange resins.

Leachable vanadium (V) from steel production residues poses a potential environmental hazard due to its mobility and toxicity under the highly alkaline pH conditions that characterise these leachates. This work aims to test the efficiency of anion exchange resins for vanadium removal and recovery from steel slag leachates at a representative average pH of 11.5. Kinetic studies were performed to understand the vanadium sorption process. The sorption kinetics were consistent with a pseudo-first order kinetic model. The isotherm data cannot differentiate between the Langmuir and Freundlich models. The maximum adsorption capacity (Langmuir value qmax) was 27 mg V g-1 resin. In column anion exchange, breakthrough was only 14% of the influent concentration after passing 90 L of steel slag leachate with 2 mg L-1 V through the column. When eluting the column 57-72% of vanadium was recovered from the resin with 2 M NaOH. Trials on the reuse of the anion exchange resin showed it could be reused 20 times without loss of efficacy, and on average 69% of V was recovered during regeneration. The results document for the first time the use of anion exchange resins to remove vanadium from steel slag leachate. As an environmental contaminant, removal of V from leachates may be an obligation for long-term management requirements of steel slag repositories. Vanadium removal coupled with the recovery can potentially be used to offset long-term legacy treatment costs.

Vanadium removal and recovery from bauxite residue leachates by ion exchange.

Bauxite residue is an important by-product of the alumina industry, and current management practices do not allow their full valorisation, especially with regard to the recovery of critical metals. This work aims to test the efficiency of ion exchange resins for vanadium (V) removal and recovery from bauxite residue leachates at alkaline pH (11.5 and 13). As an environmental pollutant, removal of V from leachates may be an obligation of bauxite residue disposal areas (BRDA) long-term management requirements. Vanadium removal from the leachate can be coupled with the recovery, and potentially can be used to offset long-term legacy treatment costs in legacy sites. Kinetics studies were performed to understand the adsorption process. The rate kinetics for the V adsorption was consistent with the pseudo-first-order kinetic model, with a higher adsorption rate for pH 11.5 (1.2 min-1). Adsorption isotherm data fitted better to Freundlich equations than to the Langmuir model. The maximum adsorption capacity (Langmuir value q max) was greatest for pH 13 (9.8 mg V g-1 resin). In column tests, breakthrough was reached at 70 bed volumes with the red mud leachate at pH 13, while no breakthrough was achieved with the effluent at pH 11.5. In regeneration, 42 and 76 % of V were eluted from the resin with 2 M NaOH from the red mud leachate at pH 13 and 11.5, respectively. Further optimization will be needed to upscale the treatment.

Long-term evolution of highly alkaline steel slag drainage waters.

The disposal of slag generated by the steel industry can have negative consequences upon the surrounding aquatic environment by the generation of high pH waters, leaching of potentially problematic trace metals, and rapid rates of calcite precipitation which smother benthic habitats. A 36-year dataset was collated from the long-term ambient monitoring of physicochemical parameters and elemental concentrations of samples from two steel slag leachate-affected watercourses in northern England. Waters were typified by elevated pH (>10), high alkalinity, and were rich in dissolved metals (e.g. calcium (Ca), aluminium (Al), and zinc (Zn)). Long-term trend analysis was performed upon pH, alkalinity, and Ca concentration which, in addition to Ca flux calculations, were used to highlight the longevity of pollution arising as a result of the dumping and subsequent leaching of steel slags. Declines in calcium and alkalinity have been modest over the monitoring period and not accompanied by significant declines in water pH. If the monotonic trends of decline in alkalinity and calcium continue in the largest of the receiving streams, it will be in the region of 50-80 years before calcite precipitation would be expected to be close to baseline levels, where ecological impacts would be negligible.

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.

Red mud a byproduct of aluminum production contains soluble vanadium that causes genotoxic and cytotoxic effects in higher plants.

Red mud (RM) is a byproduct of aluminum production; worldwide between 70 and 120 million tons is produced annually. We analyzed RM which was released in the course of the Kolontar disaster in Hungary into the environment in acute and genotoxicity experiments with plants which are widely used for environmental monitoring. We detected induction of micronuclei which reflect chromosomal damage in tetrads of Tradescantia and in root cells of Allium as well as retardation of root growth with contaminated soils and leachates. Chemical analyses showed that RM contains metals, in particular high concentrations of vanadium. Follow-up experiments indicated that vanadate causes the effects in the plants. This compound causes also in humans DNA damage and positive results were obtained in carcinogenicity studies. Since it was found also in RM from other production sites our findings indicate that its release in the environment is a global problem which should be studied in more detail. CAPSULE ABSTRACT: Our findings indicate that the red mud causes genotoxic effect in plants probably due to the presence of vanadate which is contained at high concentrations in the residue.

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.

Behavior of aluminum, arsenic, and vanadium during the neutralization of red mud leachate by HCl, gypsum, or seawater.

Red mud leachate (pH 13) collected from Ajka, Hungary is neutralized to < pH 10 by HCl, gypsum, or seawater addition. During acid neutralization >99% Al is removed from solution during the formation of an amorphous boehmite-like precipitate and dawsonite. Minor amounts of As (24%) are also removed from solution via surface adsorption of As onto the Al oxyhydroxides. Gypsum addition to red mud leachate results in the precipitation of calcite, both in experiments and in field samples recovered from rivers treated with gypsum after the October 2010 red mud spill. Calcite precipitation results in 86% Al and 81% As removal from solution, and both are nonexchangeable with 0.1 mol L(-1) phosphate solution. Contrary to As associated with neoformed Al oxyhydroxides, EXAFS analysis of the calcite precipitates revealed only isolated arsenate tetrahedra with no evidence for surface adsorption or incorporation into the calcite structure, possibly as a result of very rapid As scavenging by the calcite precipitate. Seawater neutralization also resulted in carbonate precipitation, with >99% Al and 74% As removed from solution during the formation of a poorly ordered hydrotalcite phase and via surface adsorption to the neoformed precipitates, respectively. Half the bound As could be remobilized by phosphate addition, indicating that As was weakly bound, possibly in the hydrotalcite interlayer. Only 5-16% V was removed from solution during neutralization, demonstrating a lack of interaction with any of the neoformed precipitates. High V concentrations are therefore likely to be an intractable problem during the treatment of red mud leachates.