长江河口潮滩沉积物中砷的迁移转化机制研究
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摘要
砷(As)作为一种典型有毒元素,被认为是首要控制的污染物之一,其在不同环境介质中生物地球化学行为和生物可利用性成为当前科学研究热点之一。有报道长江河口砷已受到人类活动的影响,因此选择长江河口滨岸盐沼湿地(潮滩)这一特殊的海陆交互作用地带为研究对象,探讨砷在长江河口滨岸盐沼湿地上覆水体、表层沉积物及不同植被根际沉积物中的迁移转化,试图寻找沉积物中的砷在潮汐和植被等因素影响下的迁移转化规律。本工作重点分析了长江河口东滩湿地砷的来源、上覆水体溶解态砷及互花米草根际沉积物孔隙水中溶解态砷的季节变化,并采用改进的砷五步连续提取法,探讨了潮滩表层沉积物和三种主要植被芦苇、互花米草和海三棱藨草根际(<40cm)沉积物中砷的赋存相态,尤其是互花米草根际沉积物中不同结合态砷的季节变化和主要影响因子,以及这些变化对潮滩生态系统的潜在环境影响。结合室内模拟实验探讨了在不断变化的氧化还原条件下,沉积物中砷迁移转化过程与铁、硫迁移转化过程的关系,这进一步验证了野外实地调查。基于以上的研究分析,主要得出以下几点结论：
(1)长江河口水体溶解态砷含量很低(<10μg/L),不会对水生生物产生危害作用。相当一部分溶解态砷可能形成悬浮颗粒态砷,随径流向海输送,或在一定条件下随泥沙在潮滩上沉积下来。河口水体颗粒态砷含量高低,在一定程度上能够反映河口砷向海输送状况,目前流域人类活动对砷向海输送的影响可能是微弱的。
(2)沉积物中<16μm粒径的细颗粒物与总砷的显著正相关关系,表明了<16μm粒径的细颗粒物空间分异决定了沉积物中总砷的空间分异。表层沉积物和根际沉积物中不同结合态砷含量及占总砷的比例,大小顺序依次为：残渣态>非晶形铝、铁、锰氧化物结合态(非晶形结合态)>晶形铝、铁、锰氧化物结合态(晶形结合态)>强交换态>弱交换态,这表明潮滩表层沉积物及根际沉积物中的砷,以非活性砷为主(残渣态),富集于<16μm粒径的颗粒物中。
(3)根际沉积物孔隙水中溶解态砷含量往往大于10μg/L,呈现出明显时空变化,而且与溶解态铁含量变化有较好的正相关关系,这主要源于潮汐、植物生长及微生物作用的时空变化。春冬季节,沉积物中氧化条件往往增强,而还原条件减弱,导致孔隙水中溶解态砷被铁/锰氧化物所吸附,而迁移能力降低。夏秋季节,沉积物中还原条件往往增强,导致砷的迁移能力增强,释放进入孔隙水。采用修正的菲克第一定律估算表明,潮滩沉积物中砷向上覆水体扩散通量,可能因互花米草的入侵而显著增大。
(4)根际沉积物中不同结合态砷含量及占总砷的比例,随季节变化而变化。非晶形、晶形结合态及残渣态砷随季节变化较大,而弱交换态和强交换态砷随季节变化较小,这些变化的主要影响因素是潮滩不断变化的氧化还原条件。当还原条件不断增强时,非晶形态砷可能向溶解态、弱交换态、强交换态、晶形结合态和残渣态砷转化,这样的情况往往发生在夏秋季节；当还原条件不断减弱,而氧化条件不断增强时,溶解态砷和强交换态可能向非晶形结合态砷转化,这样的情况往往发生在春冬季节。
(5)根际沉积物中AVS (Acid Volatile Sulfide)含量的时空变化,很好地反映了沉积物中氧化还原条件的时空变化,其变化的原因主要是周期性潮水、植被生长及微生物作用的时空变化。植物生长季节,其根系产生了大量新鲜有机质,加之潮滩处于湿季阶段,随有机质降解的增强,铁/锰氧化物还原溶解作用增强,硫酸盐异化还原作用增强,同时砷的迁移能力增强。植物凋落季节,潮滩处于干季阶段,有机质降解减弱,沉积物中氧化作用增强,铁/锰被氧化,砷被铁/锰氧化物所吸附,而迁移能力降低,以上砷的迁移转化过程因植被种类的不同而有所差异,AVS分析表明互花米草对于其根际沉积物中硫酸盐异化还原有较为明显的影响,这意味着入侵物种互花米草可能改变了潮滩湿地沉积物中原有的硫酸盐异化还原循环过程,进而对潮滩沉积物中砷的迁移转化产生一定的影响。
(6)模拟实验结果表明,在氧化还原条件不断变化过程中,铁的迁移转化对于砷的迁移转化有着十分重要的影响。当氧化条件由于新鲜有机质的降解转变为还原条件,并不断增强时,铁/锰氧化物一部分还原溶解,被吸附的砷,由于失去吸附主体而迁移能力增强；乙酸钠可提取铁和砷的显著正相关关系,表明还原条件下非晶形铁氧化物向铁的次生矿物及晶形氧化物(如：绿锈、磁铁矿、铁的硫化物、FeCO3等)转化,这一过程中有相当一部分溶解态砷被铁的次生矿物所吸附,而仍然保留在沉积物中；在强还原条件下(互花米草根际),溶解态砷可能与硫化物生成砷的硫化物沉淀,从而使其迁移能力降低,但潮滩沉积物中存在大量的活性铁,活性铁硫摩尔比值Fe：S>1,这意味着活性铁还原会产生大量的Fe(Ⅱ),其可能与硫酸盐还原生成的S(Ⅱ)优先结合,生成铁的硫化物,这会阻止砷硫化物的生成,而进一步促使砷的迁移能力增强。模拟实验结果有力支持了野外调查结果和推断。
总之,模拟实验和野外调查结果表明,长江河口潮滩沉积物中的生物地球化学氧化还原反应十分迅速(几十分钟、几小时到几天),这使得沉积物中的氧化还原条件不断发生快速变化,导致孔隙水中砷的含量及砷释放通量等都发生快速而明显的时空变化。在潮汐和植被等因素共同作用下,潮滩沉积物中砷的迁移转化主要受控于铁的迁移转化,而且因季节和不同植被带的变化而变化。在夏秋季节,砷的迁移能力往往增强,沉积物孔隙水中砷的含量明显增高,可能会对底栖生物产生一定的毒害。在冬春季节,砷的迁移能力明显降低,以上这些过程均受到入侵物种互花米草的显著影响,即互花米草可能改变了潮滩湿地沉积物中砷的迁移转化,进而对潮滩沉积物中砷原有的生物地球化学循环过程产生了显著影响。
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.
(1)长江河口水体溶解态砷含量很低(<10μg/L),不会对水生生物产生危害作用。相当一部分溶解态砷可能形成悬浮颗粒态砷,随径流向海输送,或在一定条件下随泥沙在潮滩上沉积下来。河口水体颗粒态砷含量高低,在一定程度上能够反映河口砷向海输送状况,目前流域人类活动对砷向海输送的影响可能是微弱的。
(2)沉积物中<16μm粒径的细颗粒物与总砷的显著正相关关系,表明了<16μm粒径的细颗粒物空间分异决定了沉积物中总砷的空间分异。表层沉积物和根际沉积物中不同结合态砷含量及占总砷的比例,大小顺序依次为：残渣态>非晶形铝、铁、锰氧化物结合态(非晶形结合态)>晶形铝、铁、锰氧化物结合态(晶形结合态)>强交换态>弱交换态,这表明潮滩表层沉积物及根际沉积物中的砷,以非活性砷为主(残渣态),富集于<16μm粒径的颗粒物中。
(3)根际沉积物孔隙水中溶解态砷含量往往大于10μg/L,呈现出明显时空变化,而且与溶解态铁含量变化有较好的正相关关系,这主要源于潮汐、植物生长及微生物作用的时空变化。春冬季节,沉积物中氧化条件往往增强,而还原条件减弱,导致孔隙水中溶解态砷被铁/锰氧化物所吸附,而迁移能力降低。夏秋季节,沉积物中还原条件往往增强,导致砷的迁移能力增强,释放进入孔隙水。采用修正的菲克第一定律估算表明,潮滩沉积物中砷向上覆水体扩散通量,可能因互花米草的入侵而显著增大。
(4)根际沉积物中不同结合态砷含量及占总砷的比例,随季节变化而变化。非晶形、晶形结合态及残渣态砷随季节变化较大,而弱交换态和强交换态砷随季节变化较小,这些变化的主要影响因素是潮滩不断变化的氧化还原条件。当还原条件不断增强时,非晶形态砷可能向溶解态、弱交换态、强交换态、晶形结合态和残渣态砷转化,这样的情况往往发生在夏秋季节；当还原条件不断减弱,而氧化条件不断增强时,溶解态砷和强交换态可能向非晶形结合态砷转化,这样的情况往往发生在春冬季节。
(5)根际沉积物中AVS (Acid Volatile Sulfide)含量的时空变化,很好地反映了沉积物中氧化还原条件的时空变化,其变化的原因主要是周期性潮水、植被生长及微生物作用的时空变化。植物生长季节,其根系产生了大量新鲜有机质,加之潮滩处于湿季阶段,随有机质降解的增强,铁/锰氧化物还原溶解作用增强,硫酸盐异化还原作用增强,同时砷的迁移能力增强。植物凋落季节,潮滩处于干季阶段,有机质降解减弱,沉积物中氧化作用增强,铁/锰被氧化,砷被铁/锰氧化物所吸附,而迁移能力降低,以上砷的迁移转化过程因植被种类的不同而有所差异,AVS分析表明互花米草对于其根际沉积物中硫酸盐异化还原有较为明显的影响,这意味着入侵物种互花米草可能改变了潮滩湿地沉积物中原有的硫酸盐异化还原循环过程,进而对潮滩沉积物中砷的迁移转化产生一定的影响。
(6)模拟实验结果表明,在氧化还原条件不断变化过程中,铁的迁移转化对于砷的迁移转化有着十分重要的影响。当氧化条件由于新鲜有机质的降解转变为还原条件,并不断增强时,铁/锰氧化物一部分还原溶解,被吸附的砷,由于失去吸附主体而迁移能力增强；乙酸钠可提取铁和砷的显著正相关关系,表明还原条件下非晶形铁氧化物向铁的次生矿物及晶形氧化物(如：绿锈、磁铁矿、铁的硫化物、FeCO3等)转化,这一过程中有相当一部分溶解态砷被铁的次生矿物所吸附,而仍然保留在沉积物中；在强还原条件下(互花米草根际),溶解态砷可能与硫化物生成砷的硫化物沉淀,从而使其迁移能力降低,但潮滩沉积物中存在大量的活性铁,活性铁硫摩尔比值Fe：S>1,这意味着活性铁还原会产生大量的Fe(Ⅱ),其可能与硫酸盐还原生成的S(Ⅱ)优先结合,生成铁的硫化物,这会阻止砷硫化物的生成,而进一步促使砷的迁移能力增强。模拟实验结果有力支持了野外调查结果和推断。
总之,模拟实验和野外调查结果表明,长江河口潮滩沉积物中的生物地球化学氧化还原反应十分迅速(几十分钟、几小时到几天),这使得沉积物中的氧化还原条件不断发生快速变化,导致孔隙水中砷的含量及砷释放通量等都发生快速而明显的时空变化。在潮汐和植被等因素共同作用下,潮滩沉积物中砷的迁移转化主要受控于铁的迁移转化,而且因季节和不同植被带的变化而变化。在夏秋季节,砷的迁移能力往往增强,沉积物孔隙水中砷的含量明显增高,可能会对底栖生物产生一定的毒害。在冬春季节,砷的迁移能力明显降低,以上这些过程均受到入侵物种互花米草的显著影响,即互花米草可能改变了潮滩湿地沉积物中砷的迁移转化,进而对潮滩沉积物中砷原有的生物地球化学循环过程产生了显著影响。
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.
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