Mobility And Transformation Of Arsenic In Salt-marsh Sediments Of The Yangtze River Estuary, China

Arsenic (As) is now regarded as one of the most serious contaminants as a typical noxious element, which toxicity is growing concered by scientists and governments all over the world widely. As biogeochemistry behaviors (mobility and transformation) and bioavailabality in a wide variety of natural environment systems are becoming one of important issues in environment pollution researches. Biogeochemical redox processes are important for controlling the behaviors and bioavailabality of As in the waters and sediments.There are large areas of salt marsh in the intertidal zone of the Yangtze River Estuary, China. Studies have been undertaken and have shown that As in the estuarine sediments has been significantly impacted by anthropogenic activity and there was As pollution within the intertidal zone. As in surface water, pore water, surface sediments and the rhizosphere sediments were investigated in the salt marsh of Dongtan wetland of the Yangtze River Estuary. The motivation of this study is to:(i) examine the aqueous and solid phases of As in sediments of the different rhizospheres, and (ii) evaluate the influence of temperature, tide and plants, especially the effects of Spartina alterniflora on the dynamics of As in salt marsh sediments. Our researches mainly focus on the resources of As in sediments; the seasonal changing of dissolved As in surface water and pore water; the seasonal changing of solid phases of As in sediments, particularly in the rehizophere sediments, and the potential effects of As biogeochemical behavior on the salt marsh ecosystem. Furthermore, the geochemical conditions and processes were investigated by the microcosm experiments in changing redox conditions (flooding conditions) to examine As dynamics that occurred during changing redox conditions and validate our field observations. Based on our results we get some conclusions as follows:(1) The concentrations of dissolved As in surface water is lower than10μg/L that is safe for aquatic organisms living in the Yangtze River Estuary. Dissolved As absorbed by phases of Fe(III) and Mn (IV, III)(hydr)oxides is important, forming suspended particulate, and tansports form esruray to sea or sediments with the tide at intertidal zones. The concentrations of total As in suspended sediment can reflect the potential quantity of As transport from the river to the sea. The transport of As form the esruray to the sea could be influenced by huaman activity slightly.(2) Pearson correlation analysis showed that there is a significant positive correlation between fine-grain particulate (<16μm) and total As, implying the spatial variations of fine-grain particulate controlls the spatial variations of total As in salt sediments. The adaptable sequential extraction procedures for arsenic in the surface and the rhizophere sediments showed that the decreasing order of the As-bearing solid fractions appeare to be residual phases> amorphous and poorly-crystalline (hydro)oxides of Fe, Mn and Al> well-crystallized (hydro)oxides of Fe, Mn and Al> specifically-sorbed> non-specifically sorbed. This indicates that the residual phases are primary As-bearing solid phases characterized by grain-size effect.(3) The dissolved As in pore water were significantly higher than that in surface water. The profiles of dissolved As and Fe in pore water showed highly spatial and seasonal variation among the different sites. Dissolved As and Fe concentrations in April and August are higher than that in December, and a positive correlation was found between of them. This indicates that As become mobilizing during sediment reduction with the development of anoxic condition in summer and autumn. Possibly it is due to that the reductive dissolution the host of Fe(III) and Mn(IV, III)(hydr)oxides that results in mobilization of As. However, when subsurface conditions changed from anoxic to progressively more oxidizing, dissolved As is removed from pore water by adsorption onto or co-precipitation with (hydr)oxides of Fe(Ⅲ) and Mn(Ⅳ, Ⅲ) again, resulting in immobilization of As. Diffusive fluxes were calculated by a modification of Fick’s first law, revealing that the sediment-water exchange of As could be significantly mediated by S. alterniflora. (4) The percentage of As-bearing solid fractions to total As in the rhizophere sediments exhibited highly seasonal variation. The seasonal variation in the As-bearing phases of amorphous and poorly-crystalline and well-crystallized (hydro)oxides of Fe, Mn and Al, and residual phases were more significant compared to As-bearing phases of specifically-sorbed and non-specifically sorbed, duo to seasonal changes of redox conditions in salt-marsh sediemts. When subsurface conditions changed from oxidized to progressively more reduced, the transformation of poorly-crystalline phasaes to aqueous, non-specifically sorbed, specifically-sorbed sorbed, well-crystallized and residual phases occured, especially during summer and autumn. On the countary, when subsurface conditions changed from reduced to progressively more oxidized, the transformation of aqueous, non-specifically sorbed, specifically-sorbed sorbed to poorly-crystalline phasaes occured, especially during spring and winter.(5) The concentrations of acid volatile sulfide (AVS) in the rhizophere sediments exhibited major spatial and seasonal variations among different sites, especially in the rhizosphere of S. alterniflora. This finding could possibly be explained by the fact that temperature change, tidal flooding and seasonal growth of cycling of salt-marsh plants, these are regarded as major controlling processes for these changes. During growing season of plants, labile organic matter degradation and sulfate-reducing bacteria (SRB) activity are enhanced and lead to initially mobilise more As. However, under sulfate-reduced conditions, the sulfide is produced by bacteria, which can reduce As(V) to As(Ⅲ) and may also enable formation of As sulfide or As-Fe sulfide phases, thus promoting dissolved As sequestrated within anoxic environments. These biogeochemical redox processes of As could be significant influenced by growth of cycling of plants. Fox example, more sulfate is reduced in the rhizosphere of S. alterniflora in April and August, suggesting that sulfate reduction is enhanced by S. alterniflora and likely to play an important role in As mobility and transformation. We can infer that the dynamics of As mobility and transformation in salt-marsh sediments could be significantly influenced by the rapid spread of S. alterniflora.(6) The results from our microcosm experiments revealed that As mobility and transformation during biogeochemical redox processes are controlled by the transformation of iron minerals. When conditions changed from oxidizing to progressively more anoxic due to labile organic degradation, the reductive dissolution (hydr)oxides of Fe(Ⅲ) and Mn(IV, Ⅲ) that results in mobilization of As. Our results also revealed that acetate-extractable fractions of As had a positive correlation with acetate-extractable Fe, suggesting that probably bacterial reductive dissolution of Fe(Ⅲ)(hydr)oxides produces large quantities of Fe(II) which promotes siderite, green rust and magnetite formation. These iron secondary mineral phases often immobilize As (Ⅲ) and As (V) by inner-sphere adsorption and promote sequestration of As into sediments. The enhanced sulfate reduction may lead to precipitate As in sulfide phases and keep them retention in sediments. However, the reactive Fe abounds, As could be hardly sequestered into sulfide precipitations. The precipitation of iron sulfides may remove sulfide from solution but not As if precipitation rates are fast which may not result in the accumulation of As sulfides, whose effects require further investigation.Generally, the results suggest that a clear seasonally variable redox conditions exists in subsurface sediments of salt marsh and the change of redox conditions are strongly rapid. Following the growth of plans, higher amounts of labile and reactive organic materials are provided, and these organic matters are decomposed and leaded to more reductive conditions in the sediments, suggesting that the degradation of labile and reactive organic materials could be key to understanding the seasonal variations of dissolved As and As-bearing solid fractions in salt-marsh sediments. The mobility and transformation of As are controlled by the transformation of iron minerals during the changing redox conditions. Furthermore, combined with the results of the mobility of As and accumulation of AVS in the sediments of microcosm incubation experiments during the changing redox conditions, we infer that when subsurface conditions change from oxidized to progressively more anoxic conditions, the reduction of reactive Fe (hydr)oxides could promote the mobility of As, leading to dissolved As increase drastically in pore water, and potentially influencing bentonic organism, particularly in rhizosphere of S. alterniflora during the warmer seasons. The rate of these reactions could be enhanced during summer and autumn, and more reducing conditions could release As from the sediments to the pore water and surface water, which could cause potentially toxic to benthic bio-communities and aquatic life in the salt-marsh of the Yangtze River Estuary.