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Influence of natural organic matter on the mobility of arsenic in aquatic systems, soils and sediments
(2008)
- The element As is today recognized as one of the most dangerous inorganic contaminants and threats for the world’s water resources. Arsenic is ubiquitious in the earth crust and humans are especially affected through As polluted drinking water supplies. The occurrence of high As groundwater concentrations is often caused by geogenic processes of As release from the solid phase and accumulation in the water phase. Many contaminated aquifers are also characterized by high concentrations of natural organic matter (NOM). Previous studies showed that NOM presence may affect As mobility, but we are lacking evidence about the reactions pathways and about the importance As-DOM interactions in the environment. We therefore focussed on studying reactions between NOM and As, including redox reactions, complexation, colloid formation and sorption competition in laboratory experiments. Moreover we also studied As behaviour in columns experiments and wetland soils rich in organic matter. Arsenic mobility strongly depends on its redox state. Dissolved organic matter was previously found to be redox active but its redox properties are only poorly understood. In laboratory experiments we therefore elucidated the electron transfer characteristics of different DOM samples. The results showed the high potential of humic substances to chemically reduce different Fe(III) complexes and oxidize H2S and metallic Zn. Reactions occurred over short periods of time with reaction rates in the range from 0.03 to 27 h-1. Under otherwise identical conditions rising DOC concentrations caused higher total electron transfer. This supports the assumption that functional groups of DOM, such as quinones, were indeed the redox active moieties involved in the redox reactions. The calculated electron transfer capacities (ETC) ranged from 0.07 to 6.2 mequiv (g C)-1. The wide range of observed reaction rates and ETC values could be related to the different redox potential of the inorganic reactants used. This suggests that DOM molecules contain redox active moieties with different redox potential and that they possibly represent a redox ladder with the capacity to buffer electrons over a wide range of redox conditions. Humic substances also influenced the As redox speciation as dissolved H3AsO4 was - either chemically or microbially- reduced to H3AsO3 in DOM solution. No oxidation of As(III) to As(V) was found in these experiments. The presence of organic matter thus changes the redox speciation of As as well as that of other environmentally relevant elements like Fe or S. This possibly also contributes to a higher mobility of As due to the presence of reduced As and Fe species. The formation of complexes on mineral surfaces is one of the most important immobilization processes for As in soils or sediments. DOM strongly interfered with this As sequestration mechanism due to aqueous and surface complexation reactions. Humic substances were found to prevent the precipitation and sedimentation of iron oxide minerals and promote the formation of DOM and Fe containing colloids at aqueous molar Fe/C ratios of up to 0.1. This impeded the co-precipitation and sedimentation of As with Fe mineral structures and increased the amount of mobile As. Arsenic and Fe content were correlated in the different particle size classes was, suggesting As binding to Fe e.g. in cation bridging complexes or DOM stabilized Fe oxide colloids. DOM sorption on synthetic goethite and natural soil and sediment samples also caused a release of As from these solid phases due to sorption competition for mineral binding sites. Especially the weakly adsorbed fraction of As in the natural samples was affected by this process. Both the formation of aqueous complexes or colloids and the sorption competition in the presence of DOM lead to higher As concentration in the water phase and demonstrate the potential of humic substances to increase As mobility. In the studied laboratory columns As redox transformation and complexation by DOM could not be identified. Instead As mobilization was dominated by microbial processes in these experiments. At DOM input concentrations between 5 and 100 mg L-1 the release of As occurred mainly due to the reductive dissolution of the Fe oxide sorbent phase during microbial respiration. The occurrence of sulfate reduction and the precipitation of sulfide minerals at the highest DOM concentrations did not represent a substantial immobilization mechanism. The studied wetland soils represent natural sinks for geogenic As. Fe oxides were the main As sorbents, which is surprising as both soils were temporarily water saturated and likely under reducing conditions. Moreover, the high porewater DOC concentrations and the high organic carbon content in the solid phase apparently did not interfere with As sorption on the iron phases in these soils. Chemical extractions also showed that smaller As fractions were associated with solid phase organic matter pool and with a not identified residual pool, likely sulfide minerals. However, as most As was bound to Fe oxides its fate was strongly affected by changing redox conditions. Fast As immobilization sorption occurred under dry conditions when Fe was oxidized and precipitated, while short-term mobilization of As and Fe in their reduced form was observed upon rewetting. These soils therefore are As sinks as long as oxic conditions are maintained but may turn into As sources when reducing conditions prevail for longer periods of time. Organic molecules influence the redox state and the complexation of As and are able to shift As partitioning in favour of the solute phase. Our results showed that especially the association of As with aqueous complexes and colloids has a strong potential to reduce As retention and increase As mobility. This has to be considered in future studies of As behaviour in aquifers, surface waters, soils or sediments rich in organic substances. Peatland soils were found to represent sinks for geogenic As, showing that the presence of organic matter not necessarily prevents As immobilization. It also depends on the biogeochemical conditions whether an organic matter rich system will accumulate or release As.
