Mobilisation of arsenic from bauxite residue (red mud) affected soils: effect of pH and redox conditions

The tailings dam breach at the Ajka alumina plant, western Hungary in 2010 introduced ~1 million m3 of red mud suspension into the surrounding area. Red mud (fine fraction bauxite residue) has a characteristically alkaline pH and contains several potentially toxic elements, including arsenic. Aerobic and anaerobic batch experiments were prepared using soils from near Ajka in order to investigate the effects of red mud addition on soil biogeochemistry and arsenic mobility in soil–water experiments representative of land affected by the red mud spill. XAS analysis showed that As was present in the red mud as As(V) in the form of arsenate. The remobilisation of red mud associated arsenate was highly pH dependent and the addition of phosphate to red mud suspensions greatly enhanced As release to solution. In aerobic batch experiments, where red mud was mixed with soils, As release to solution was highly dependent on pH. Carbonation of these alkaline solutions by dissolution of atmospheric CO2 reduced pH, which resulted in a decrease of aqueous As concentrations over time. However, this did not result in complete removal of aqueous As in any of the experiments. Carbonation did not occur in anaerobic experiments and pH remained high. Aqueous As concentrations initially increased in all the anaerobic red mud amended experiments, and then remained relatively constant as the systems became more reducing, both XANES and HPLC–ICP-MS showed that no As reduction processes occurred and that only As(V) species were present. These experiments show that there is the potential for increased As mobility in soil–water systems affected by red mud addition under both aerobic and anaerobic conditions

Status of contaminated marine sediments in four Nordic countries: assessments, regulations, and remediation approaches

Purpose Since the 1960s, pollution of the marine environment has been observed around the world, and a general awareness of the potential environmental hazards from contaminants arose. This general awareness was followed by implementation of environmental regulations at the national and international levels regarding industrial discharges, and forcing the development of management programs to improve the environmental quality. This overview focuses on the status of assessments, regulations, and remediation actions of contaminated marine sediments in four North European countries: Sweden, Norway, Finland, and Denmark. Results and discussion We found that the extent of assessment surveys and the implementation of sediment remediation actions vary between the countries, with Norway the most advanced. Norway has initiated national pilot remediation tests, followed by full-scale site remediation by dredging and in situ capping in several projects along the coast. Sweden is following Norway’s example, although with fewer completed projects. In Finland, the tradition of maintenance dredging is still very strong although the main purpose has not been cleanup actions. Accordingly, in Denmark, dredging is primarily used to maintain sailing depths. All four countries have implemented the Water Framework Directive in addition to national regulations that provide the authority for imposing and implementing sediment remediation measures. However, only Norway has implemented a national strategy on remediation of contaminated sediments. Conclusions This overview highlights the need for enhanced communication and exchange of experiences between the four Nordic countries of concern with regard to sediment remediation strategy and management

Status of contaminated marine sediments in four Nordic countries : assessments, regulations, and remediation approaches

Purpose Since the 1960s, pollution of the marine environment has been observed around the world, and a general awareness of the potential environmental hazards from contaminants arose. This general awareness was followed by implementation of environmental regulations at the national and international levels regarding industrial discharges, and forcing the development of management programs to improve the environmental quality. This overview focuses on the status of assessments, regulations, and remediation actions of contaminated marine sediments in four North European countries: Sweden, Norway, Finland, and Denmark. Results and discussion We found that the extent of assessment surveys and the implementation of sediment remediation actions vary between the countries, with Norway the most advanced. Norway has initiated national pilot remediation tests, followed by full-scale site remediation by dredging and in situ capping in several projects along the coast. Sweden is following Norway's example, although with fewer completed projects. In Finland, the tradition of maintenance dredging is still very strong although the main purpose has not been cleanup actions. Accordingly, in Denmark, dredging is primarily used to maintain sailing depths. All four countries have implemented the Water Framework Directive in addition to national regulations that provide the authority for imposing and implementing sediment remediation measures. However, only Norway has implemented a national strategy on remediation of contaminated sediments. Conclusions This overview highlights the need for enhanced communication and exchange of experiences between the four Nordic countries of concern with regard to sediment remediation strategy and management

Magnetic resonance imaging measurements evidence weak dispersion in homogeneous porous media

International audienceWe measure the dispersion coefficient through homogeneous bead or sand packings at different flow rates from direct MRI visualizations of the transport characteristics of a pulse of paramagnetic nanoparticles. Through 2D imaging we observe homogeneous dispersion inside the sample, but we show that entrance effects may induce significant radial heterogeneities, which would affect the interpretation of breakthrough curve. Another MRI approach then provides quantitative measurements of the evolution in time of the longitudinal particle distribution in the sample. These data can be analyzed to deduce the coefficient of dispersion independently of entrance effects. The values obtained for this " effective " dispersion coefficient are almost ten times lower than the commonly admitted values

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 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 → 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 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