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.

The potential for constructed wetland mechanisms to treat alkaline bauxite residue leachate: carbonation and precipitate characterisation.

Leachates emanating from bauxite residue disposal areas are alkaline and require neutralisation prior to discharge. The use of passive technologies such as constructed wetlands has received increasing interest as possible treatments for alkaline leachates, including bauxite residues. Mechanisms proposed for wetland effectiveness have included calcite precipitation but it is not clear if such a pathway is feasible in the relatively low Ca residue leachates. Carbonation of Ca-spiked residue leachate treatments was conducted to observe rates of pH decrease and precipitate formation. For all treatments, carbonation effectively decreased pH to ca. 10.5 which remained stable following aeration. Decreases in Al content of 83-93% were also observed. Precipitates retrieved from carbonation experiments and from a constructed wetland trial were characterised using XRD, SEM, XPS and EDX. Calcium carbonates formed in Ca-spiked treatments and dawsonite precipitation occur in the absence of Ca. Rinsing of precipitates removes surface calcium indicating soluble forms adsorbed on precipitates. The results demonstrate that carbonation of bauxite residue leachate is an important component of passive treatments and neutralisation.

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.

Geochemical behavior of ultramafic waste rocks with carbon sequestration potential: a case study of the Dumont Nickel Project, Amos, Québec.

The geochemical behavior of ultramafic waste rocks and the effect of carbon sequestration by these waste rocks on the water drainage quality were investigated using laboratory-scale kinetic column tests on samples from the Dumont Nickel Project (RNC Minerals, QC, Canada). The test results demonstrated that atmospheric CO2 dissolution induced the weathering of serpentine and brucite within the ultramafic rocks, generating high concentrations of Mg and HCO3- with pH values ranging between 9 and 10 in the leachates that promote the precipitation of secondary Mg carbonates. These alkaline pH values appear to have prevented the mobilization of many metals; Fe, Ni, Cu, and Zn were found at negligible concentrations in the leachates. Posttesting characterization using chemical analyses, diffuse reflectance infrared Fourier transform (DRIFT), and scanning electron microscope (SEM) observations confirmed the precipitation of secondary hydrated Mg carbonates as predicted by thermodynamic calculations. The formation of secondary Mg carbonates induced cementation of the waste particles, resulting in the development of a hardpan.

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.

Potential for use of industrial waste materials as filter media for removal of Al, Mo, As, V and Ga from alkaline drainage in constructed wetlands--adsorption studies.

The potential to remove Al, Mo, V, As and Ga from alkaline (pH 8.0-8.6) drainage originating from seawater neutralized bauxite processing residue storage areas using constructed wetland technology was studied in a laboratory study. Bauxite processing residue sand, bauxite, alum water treatment sludge and blast furnace slag were investigated as potential active filter materials. Al was shown to precipitate as Al(OH)3 in the pH range 7.0-8.0 in aqueous solution and 6.0-8.5 in the presence of silica sand particles that provided a surface for nucleation. For V As Mo and Ga, adsorption to the surfaces of the adsorbents decreased greatly at elevated pH values (>pH 6-9). Water treatment sludge and bauxite had a greater ability to adsorb V, As and Mo at high pH (As and V at pH 7-9 and Mo at pH 5-7) than processing sand and slag. Adsorption isotherm data for As and V onto all four adsorbent than processing sand and slag. Adsorption isotherm data for As and V onto all four adsorbent materials fitted equally well to the Langmuir and Freundlich equations but for Ga, and to a lesser extent Mo, the Freundlich equation gave higher R(2) values. For all four ions, the maximum adsorption capacity (Langmuir value qmax) was greatest for water treatment sludge. Bauxite adsorbed more Mo, Ga and V than residue sand or slag. The pseudo-second order equation gave a better fit to the experimental kinetic data than the pseudo-first order model suggesting that chemisorption rather than diffusion/exchange was the rate limiting step to adsorption. It was concluded that water treatment sludge and bauxite were the most effective adsorbents and that for effective removal of the target ions the pH of the drainage water needs to be decreased to 6.0-7.0.