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富营养化下长江口邻近海域沉积物-水界面砷的迁移转化过程
张玉婷
学位类型硕士
导师段丽琴
2022-05-10
学位授予单位中国科学院大学
学位授予地点中国科学院海洋研究所
关键词砷,沉积物-水界面,迁移转化,富营养化,长江口邻近海域
摘要

  砷(As)因其生物毒性和生物累积效应,能够对水体造成严重且持久的生态风险。富营养化和缺氧条件会刺激沉积物-水界面As的迁移转化,进而促使As的沉积物-上覆水界面扩散通量增加。近年来,长江口邻近海域的富营养化和夏季底层海水低缺氧持续加强,势必会造成沉积物-水界面As释放的增加,但目前缺乏对长江口邻近海域沉积物-水界面As迁移转化过程的研究。因此本文聚焦于富营养化下长江口邻近海域沉积物-水界面砷的迁移转化过程这一科学问题,以富营养化严重的夏季长江口邻近海域4个典型站位为研究对象,系统分析了间隙水和固相沉积物中As及相关化学参数的剖面变化,结合微生物参数(As功能基因丰度)的剖面分布,解析了长江口邻近海域沉积物-水界面As迁移转化过程的化学调控机制和微生物调控机制。同时,通过分析模拟富营养化和低氧培养实验上覆水和间隙水中As形态的时间变化规律,结合外海结果,探讨了富营养化和缺氧对沉积物-水界面As迁移转化的影响机制。本论文获得的主要结果如下:

1. 基于间隙水和固相沉积物中As及相关参数的剖面变化特征及耦合关系,发现长江口邻近海域沉积物水界面As的迁移转化与Fe-Mn-S的氧化还原密切相关,底层水氧化还原条件和pH影响着Fe-Mn-SAs的化学调控过程,As的界面扩散通量主要受沉积物基质和底层水氧化还原条件影响。

  长江口邻近海域沉积物-水界面As的迁移转化与Fe-Mn-S氧化还原存在耦合。间隙水总As剖面变化与间隙水Fe2+或/和Mn2+剖面变化趋势基本一致,与中上层As的铁锰氧化物赋存形态AsF2(铁锰氧化物强吸附态)以及AsF3(无定形铁锰氧化物结合态)呈镜像关系,表明中上层As的释放来自负As铁锰氧化物还原、移除是由于负As铁锰氧化物的再生。沉积物中下层间隙水总As的剖面变化与As的硫化物赋存形态AsF5(无定形雌黄及黄铁矿结合态砷)呈镜像关系,即中下层间隙水As的移除可能主要与该形态的生成有关。

  Fe-Mn-S对As在沉积物-水界面迁移转化的化学调控受底层水氧化还原条件和pH的影响。底层水相对低氧的站位A5-5和A13-2,其间隙水Fe2+和Mn2+高值区较宽,说明低氧条件更利于固相As随铁锰氧化物还原释放。各站位沉积物活性As赋存形态之间的相关性表明相对低氧站位AsF1、AsF2、AsF3形态的溶解和次生过程同步,而在相对有氧站位的沉积物中,溶解自AsF1、AsF2的间隙水As主要再吸附于晶质负砷铁锰氧化物AsF4上。底层水pH(pHbw)大于8的站位A7-4,间隙水As的释放受负As锰氧化物还原主导,这可能是较高的pHbw(>8)有利于间隙水As吸附于锰氧化物上;而pHbw<8的站位A1-3和A5-5,其间隙水总As的释放受负As铁氧化物还原主导。

  沉积物-水界面As的扩散通量受底层水氧化还原条件和沉积物基质的影响。4个站位的沉积物-水界面总As扩散通量在1.18×10-7~2.07×10-7 µM/(m2·s)之间,均为正值,表明长江口邻近海域的沉积物是As源。除站位A1-3外的3个站位,其沉积物-水界面总As扩散通量随着DObw浓度降低而增加,表明低氧条件有利于As的内源性释放;但DObw较高的站位A1-3,其As界面扩散通量也较高,这是由于沉积物基质影响着砷的内源性释放。

2. 基于间隙水As形态、固相As赋存形态和As功能基因丰度剖面变化的对应关系,发现长江口邻近海域沉积物-水界面中As的还原释放与arrA基因(As5+异化还原基因)和arsC基因(As5+细胞质还原基因)表达有关,间隙水As形态的变化与aioA基因(As3+呼吸氧化基因)表达有关,上述基因的表达受氧化还原条件、PO43-浓度的影响。

  长江口邻近海域沉积物中携带aioA基因的微生物介导了间隙水As3+氧化为As5+;表层以深沉积物中携带arrA基因和arsC基因的微生物介导了固相As(V)还原为As3+。许多深度的沉积物中较高的aioA基因丰度与较高的As5+浓度相对应,表明携带aioA基因的砷呼吸氧化菌介导了As3+的氧化。4个站位arrA基因丰度的剖面变化与间隙水As3+浓度或移除As3+的主要形态-AsF5含量剖面变化趋势基本一致,站位A13-2的arsC基因丰度剖面变化与间隙水As3+浓度变化基本一致,这是由于携带arrA基因或arsC基因的As5+还原菌介导了固相As(V)还原释放As3+

  功能基因arrAarsC的表达因沉积物还原性的增强和底层水PO43-浓度的增强而增强,aioA基因在表层氧化沉积物处表达。在DObw浓度较低的站位A5-5和A13-2,间隙水总As和As3+浓度变化与arrA基因的丰度变化基本一致;在DObw较高的站位A1-3和A7-4,arrA基因丰度剖面变化与AsF5形态的基本一致,表明沉积物还原性越强,间隙水As越倾向于通过微生物介导固相As(V)直接还原释放。DObw相近的A5-5和A13-2,其中底层水PO43-浓度较高的站位A13-2,微生物As(V)的直接还原对间隙水As释放贡献更大,arrA基因和arsC基因与间隙水As3+变化更一致,表明PO43-浓度升高有利于间隙水As的微生物直接还原释放。aioA基因在表层沉积物中的表达可能是由于携带aioA基因的细菌属于好氧型As氧化菌,在含氧沉积物中介导As3+氧化为As5+

3. 基于加藻培养实验和缺氧灭菌下添加硫酸盐还原菌的培养实验,及与外海调查的对比研究,推测富营养化引发赤潮的海域,藻类耗氧降解促进了负As铁锰氧化物还原以及As功能基因的表达,缺氧条件下硫酸盐还原菌直接介导固相As(V)的还原释放,从而导致砷的内源性释放增加。

  藻类耗氧降解造成As与Fe、Mn的还原释放耦合更加强烈。培养实验加藻组(AD组)的沉积物-水界面总As的扩散通量为4.68×10-4 mg/(cm2·d),而空白对照组(Control组)的总As扩散通量为-2.31×10-3 mg/(cm2·d),表明藻类耗氧降解增加了As的内源性释放,使沉积物从As汇转变为As源。AD组的间隙水总As浓度峰值与Fe2+、Mn2+浓度峰值对应,表明藻类耗氧降解使As与Fe、Mn的还原释放耦合更加强烈,与外海调查中低氧条件有利于负As铁锰氧化物还原对应。

  藻类耗氧降解增强了沉积物As功能基因的表达;缺氧条件下附加硫酸盐还原菌能促进沉积物中As的还原释放。AD组沉积物各深度处的间隙水As3+浓度增长率(第30天相比第0天)均为正值,对应AD组相比Control组arrA基因和arsC基因丰度的增加,表明在藻类耗氧降解的条件下,As5+还原基因的表达增强。AD组中上层间隙水二甲基砷(DMA)浓度增长率(第30天相比第0天)为正值,且对应该深度arsM基因丰度相比Control组的增加,表明在藻类耗氧降解的条件下,arsM基因的表达增强。在培养实验SRB组(缺氧灭菌条件下加入硫酸盐还原菌)中,表层沉积物间隙水As3+浓度增加,峰值处可达24.16 mg/L,远高于AD组峰值(11.74 mg/L),表明缺氧条件下硫酸盐还原菌能够介导沉积物中固相As(V)的还原。SRB组的总As扩散通量为1.96×10-3 mg/(cm2·d),即砷的内源性释放增加。

其他摘要

Due to its biological toxicity and bioaccumulation effect, As can cause serious and lasting marine ecological risks. Eutrophication and anoxic conditions can stimulate the migration and transformation of As at the sediment-water interface(SWI), and then induce the increase of As diffusion flux at the SWI. In recent years, the continuous strengthening of eutrophication and seasonal hypoxia in the Yangtze River Estuary results in the increase of As release at the SWI, however, there is a lack of study on the migration and transformation process of As at the SWI in the Yangtze River Estuary. Therefore, this paper focuses on the As migration and transformation at the SWI of the Yangtze River Estuary under eutrophication. Four typical sites in the Yangtze River Estuary in summer with serious eutrophication were selected to study the profile changes of As and related chemical parameters in porewater and solid phases. Combined with the profile variations of As functional gene abundances, the chemical and microbial regulation mechanisms of As migration and transformation at the SWI in the Yangtze River Estuary were discussed. Meanwhile, the temporal variations of As species in the overlying water and porewater in eutrophication and hypoxia cultures were studied. Combined with the results in-situ in culture, the influence of eutrophication and hypoxia on the migration and transformation of As at the SWI were ascertained. The main results of this paper are as follows:

1. Based on the profile variations and the coupling relationships of As and related parameters in porewater and solid phases, it was found that the migration and transformation of As at the SWI of the Yangtze River Estuary was closely related to the redox of Fe-Mn-S, and the redox conditions and pH of bottom water affected the chemical regulation of As by Fe-Mn-S. The diffusion flux of As at the SWI was affected by redox conditions and sediment matrix of the bottom waters.

The migration and transformation of As at the SWI of the Yangtze River Estuary was coupled with Fe-Mn-S redox. The change of total As profile of porewater was basically consistent with the change trend of Fe2+ or / and Mn2+ concentration profile of porewater, which was in a mirror relationship with the occurrence forms of iron and manganese oxides AsF2(iron and manganese oxides strongly adsorbed As) and AsF3(amorphous iron and manganese oxides binding state) of As in the middle and upper layers, indicating that the release of As in the middle and upper layers came from the reduction of iron and manganese oxides that carrying As, and the removal of As was due to the regeneration of iron and manganese oxides that carrying As. The profile changes of total As in the porewater in the middle and lower layers were in a mirror relationship with the occurrence forms of As sulfide AsF5(amorphous orpiment and pyrite binding As), that is, the removal of As in the porewater in the middle and lower layers might be mainly related to the formation of this form.

The chemical regulation of Fe-Mn-S on the migration and transformation of As at the SWI was affected by the redox conditions and pH of the bottom water. Sites A5-5 and A13-2 with relatively low oxygen in bottom water had a wide range of high values of Fe2+ and Mn2+ in porewater, indicating that hypoxia was more conducive to the reduction and release of solid phase As with iron and manganese oxides. The correlation between the occurrence forms of active As in sediments of each site showed that the dissolution and secondary processes of AsF1, AsF2 and AsF3 in the relatively hypoxic site were synchronous, while the porewater dissolved from AsF1 and AsF2 in the relatively aerobic site was mainly re-adsorbed on the crystalline iron and manganese oxides that carrying As— AsF4. The pH of bottom water(pHbw) at site A7-4 was greater than 8, its release of As was dominated by manganese oxides that carrying As, which might be that higher pHbw(>8) was conducive to the adsorption of porewater As on manganese oxides; However, for sites A1-3 and A5-5 with pHbw<8, its release of As was dominated by iron oxides that carrying As.

The diffusion flux of As at the SWI was affected by redox conditions of the bottom waters and sediment matrix. The total As concentration in the overlying water of the four sites was less than that in the porewater, and the diffusion flux of total As at the SWI was 1.18×10-7~2.07×10-7 µM/(m2·s), and all were positive, indicating that the sediments in the Yangtze River Estuary were As source. Three sites except A1-3, the total As diffusion flux at the SWI increased with the decrease of DObw concentration, indicating that hypoxic conditions were conducive to increasing the endogenous release of As. However, the total As interfacial diffusion flux of A1-3 with highest DObw was high, because the sediment matrix also affected the endogenous release of As.

2. Based on the corresponding relationship of profiles between porewater As species, solid As fractions and As functional genes, it was found that the reductive release of As at the SWI of the Yangtze River Estuary was related to the expression of As5+ dissimilatory reduction gene arrA and As5+ cytoplasmic reduction gene arsC, and the porewater As species change was related to the expression of As3+ respiratory oxidation gene aioA. The expression of these genes was affected by redox conditions and PO43- concentrations.

The microorganisms carrying aioA gene in the surface oxidized sediments of the Yangtze River Estuary mediated the oxidation of As3+ in porewater, and the microorganisms carrying arrA gene and arsC gene in sediments deeper than the surface mediated the reduction of solid phase As(V) to As3+. The higher aioA gene abundance of certain depth of sediments corresponded to the higher As5+ concentration, indicating that As respiratory oxidizing bacteria carrying aioA gene in surface sediments mediated the oxidation of As3+. The profile change of arrA gene abundance at four sites was basically consistent with the profile change of As3+ concentration in porewater or the content of AsF5—the main form of removing As3+, and the profile change of arsC gene abundance at site A13-2 was basically consistent with the change of As3+ concentration in porewater, because As5+ reducing bacteria carrying arrA gene or arsC gene mediated the reduction and release of As3+ from solid phase As(V).

The expression of arrA gene and arsC gene was enhanced by the enhancement of sediment reducibility and the concentration of PO43- in bottom water, while aioA gene was expressed in oxidized sediments. At sites A5-5 and A13-2 with lower DObw concentration in bottom water, the changes of total As and As3+ concentration in porewater are basically consistent with the abundance of arrA gene. At sites A1-3 and A7-4 with higher DObw, the profile changes of arrA gene abundance were basically consistent with the morphology of AsF5, indicating that the stronger the reduction of sediments, the stronger the expression of arrA gene, the more porewater As tended to be released by microorganisms mediated direct reduction of solid phase As(V). A5-5 and A13-2 had similar DObw, in A13-2 with higher PO43- concentration in bottom water, the direct reduction of microbial As(V) contributed more to the release of porewater As, and the changes of arrA gene and arsC gene were more consistent with those of porewater As3+, indicating that the higher the PO43- concentration, the more likely the porewater As was to be released by the direct reduction by microorganisms carrying arrA gene and arsC gene. The expression of aioA gene in surface sediments might be due to the fact that the bacteria carrying aioA gene belong to aerobic As oxidizing bacteria, which mediated the oxidation of As3+ to As5+ in oxidized sediments.

3. Based on the culture experiments with adding algae and sulfate reducing bacteria after sterilization and results in situ, it was speculated that in the eutrophication sea areas, algae degradation promoted the endogenous release of As, the reduction of iron and manganese oxides that carrying As and the expression of As functional genes. Sulfate reducing bacteria directly mediated the reductive release of solid phase As(V) under anoxic conditions, the endogenous release of arsenic increased concomitantly.

Algal degradation and oxygen consumption increased the endogenous release of As, resulting in stronger coupling between As and Fe-Mn. The diffusion flux of total As at the SWI of the culture experiment algae adding group(AD group) was 4.68×10-4 mg/(cm2·d), while the total As diffusion flux at the SWI of the blank control group(Control group) was estimated to be -2.31×10-3 mg/(cm2·d), indicating that the oxygen consumption and degradation of algae increased the endogenous release of As and transformed the sediments from As sink to As source. The peak value of total As concentration in porewater was consistent with the peak value of Fe2+ and Mn2+ in AD group, indicating that the oxygen consumption and degradation of algae made the coupling between As and Fe-Mn stronger, which corresponded to the fact that hypoxic conditions in the offshore investigation were conducive to increasing the reduction of iron and manganese oxides that carrying As.

Algae degradation and oxygen consumption enhanced the expression of As functional genes; algae degradation and additional sulfate reducing bacteria under hypoxia could promote the reduction and release of As in sediments. The growth rate of As3+ concentration in porewater at each depth of sediment in AD group(day 30 compared with day 0) was positive, and compared with Control group, the abundance of arsC and arrA gene in AD group increased, indicating that the expression of As5+ reducing gene was enhanced under the condition of algae degradation and oxygen consumption. The profile change of porewater dimethyl As(DMA) concentration growth rate(day 30 compared with day 0) in AD group was positive in middle and upper layers, consistent with the increase of arsM gene(As3+ methylation gene) abundance compared with Control group, indicating that the expression of arsM gene was enhanced under the condition of algae degradation and oxygen consumption. In SRB group(the microsystem added with sulfate reducing bacteria after sterilization), the concentration of As3+ in porewater of surface sediments increased, and the peak value reached 24.16 mg/L, which was much higher than the peak value of AD group(11.74 mg/L), indicating that sulfate reducing bacteria may mediated the reduction of solid phase As(V) in sediments under anoxic conditions, the total As diffusion flux at the SWI of SRB group was estimated to be 1.96×10-3 mg/(cm2·d), that is, the endogenous release of As increased.

语种中文
文献类型学位论文
条目标识符http://ir.qdio.ac.cn/handle/337002/178305
专题海洋生态与环境科学重点实验室
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张玉婷. 富营养化下长江口邻近海域沉积物-水界面砷的迁移转化过程[D]. 中国科学院海洋研究所. 中国科学院大学,2022.
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