锑（Sb）和砷（As）是地表环境中典型的有毒类金属元素，二者通常伴生共存，形成复合污染。随着采矿和冶炼等工业活动的加剧，Sb和As被加速释放到地表环境中产生富集，它们的地球化学行为影响着环境安全和人类健康。Sb是中国特色矿产资源，其储量和产量均为世界之最。湖南资江流域分布有多个大型Sb成矿带，如龙山锑金矿、锡矿山锑矿、古台山锑矿、渣滓溪锑矿和板溪锑矿等，受采矿活动影响，资江流域水体Sb和As污染问题突出。作为优先污染物，Sb和As的地球化学行为和环境效应受到世界的广泛关注。金属在从矿山支流汇入到资江干流的环境条件（径流量、沉积环境和污染源等）变化下，其浓度分布存在空间差异，但对Sb和As在这种环境条件变化下的系统研究仍然缺乏，对特定水环境中Sb和As的浓度特征和环境地球化学行为的研究至关重要。
本研究以资江水体为研究对象，将野外采样和室内环境化学实验以及现代仪器分析（如多接收器等离子体质谱）相结合的方法，系统研究了水体中Sb和As的时空分布特征、分配规律，沉积物中Sb同位素分馏特征，并对Sb和As的潜在生态风险和健康风险进行了评估；阐明了Sb和As的形态转化和动态释放路径，比较了不同环境条件中Sb和As的迁移过程。其研究成果补充了区域性Sb和As基础研究数据，揭示了Sb和As的环境地球化学过程，为流域尺度Sb和As的污染治理提供了科学参考，具有重要的科学意义。本研究的主要结果如下：
（1）赋存特征：资江干流水中Sb和As的平均浓度分别4.29 μg/L和2.75 μg/L，Sb浓度存在时空差异，而As未表现出明显空间差异；资江干流表层沉积物中Sb和As的平均浓度分别为34.19 mg/kg和33.61 mg/kg，对应孔隙水中Sb和As的平均浓度分别为13.26 μg/L和42.78 μg/L，具有较大的空间变异性，柘溪大坝导致了Sb和As在中游积累和下游减少；矿区支流水中Sb和As浓度受Sb开采和冶炼活动的强烈调节在河流中呈随距离衰减的趋势，其浓度总体上均高于资江干流；然而在矿区支流沉积物中，除锡矿山涟溪河中的Sb和As浓度明显高于资江干流外，其余支流沉积物和孔隙水中Sb和As浓度持平或低于资江干流浓度，各支流沉积物中Sb和As浓度并未随距资江距离减小而减小，这与较强的水动力形成的冲刷环境有关；沿矿区—资江干流—洞庭湖流向的五个沉积柱状中Sb有明显的随距离衰减的趋势，Sb和As在同一个柱状样上的垂向分布不完全相同。总体上，矿区支流水中Sb和As的浓度高于资江干流，而沉积物（除涟溪河）中的Sb和As浓度相当，并受锡矿山支流汇入和柘溪大坝调节沉积物淤积的影响。
（2）分配规律：水中Sb和As主要以溶解态存在，Sb和As的沉积物—水分配系数分别为2.95–4.18和2.53–4.01，沉积物中Sb和As更容易与固相结合，并主要存在于残渣态中，Sb中残渣态所占比例≥ 92.39%，高于As中残渣态所占比例（≥ 65.52%），As的生物可利用态比例均高于Sb。表层沉积物中Sb和As在固相和孔隙水相中的分配主要受沉积物中Ca和非晶形氧化Fe/Al的影响。柱状样中的Sb与环境因子关系不大，而As受有机质和Al-Fe-Mn氧化物的影响；沉积物中Sb和As的不同形态浓度受其总浓度控制，保持着动态平衡关系，主要以线性与非线性组合的路径发生动态转移，人为活动可以加剧残渣态Sb和As转化为其他活性组分的进程，Sb的迁移性小于As；资江干流沉积物ε123Sb值小于矿区支流，沿河流Sb的简单稀释以及沉积物中Fe-Al氧化物的吸附，不会导致沉积物中明显的Sb同位素分馏，Sb是沿河流保守运输的。不同区域ε123Sb值的差异使Sb同位素组成成为追踪水文系统中Sb来源和研究生物地球化学过程的潜在有利工具。应尽可能减少破坏沉积物稳定性的人为活动（如采砂等），以避免Sb尤其是As释放增加其迁移性，造成更严重的环境效应。
（3）风险评估：资江流域的Sb比As的污染风险高，水中基本不存在污染，沉积物中的污染普遍存在。基于总浓度评估方法，沉积物中Sb的生态风险高于As，但基于形态的评估结果显示As的生态风险高于Sb，这是因为大部分Sb存在于残渣态中导致基于总浓度评估的Sb生态风险高估，而As较高的可利用形态使其基于化学形态评估的As生态风险较高。总体上资江中游和涟溪河沉积物中Sb和As存在较高污染和生态风险，这与锡矿山采矿活动向涟溪河输入人为Sb和As以及远距离运输到资江干流中有关。地表水中Sb和As健康风险总体较低，但沉积物暴露加剧了资江中总体的As的健康风险，儿童健康风险高于成人。资江中游和锡矿山沉积物较高的风险应该受到重点关注，基于总浓度和形态方法评估的沉积物中Sb和As的生态风险水平相反，说明多指标综合分析对增加流域Sb和As风险评估的可靠性十分必要。
本研究在以下三个方面取得了一些创新成果：（1）流域尺度上河流中Sb和As的分布和长距离迁移；（2）河流沉积物中的Sb同位素组成和分馏特征；（3）水体中Sb和As的综合风险评估。本研究不仅加深了对水环境中元素时空变化及其影响因素的认识，还弥补了区域Sb同位素基础数据的不足，丰富了Sb同位素指纹图谱数据库，更为流域Sb和As风险评估的可靠性提供了保证。

Antimony (Sb) and arsenic (As) are typical toxic metalloid elements in the surface environment, and they usually coexist together to form compound pollution. With the intensification of industrial activities such as mining and smelting, Sb and As are released into the surface environment to produce enrichment, and their geochemical behavior affects environmental safety and human health. Sb is a mineral resource with Chinese characteristics, and its reserves and output are the largest in the world. There are many superlarge Sb deposit zone located in the Zijiang River (ZR) Basin, such as the Longshan Sb-Au deposit, Xikuangshan Sb deposit, Gutaishan Sb deposit, Zhazixi Sb deposit, and Banxi Sb deposit. Due to the influence of mining activities, the pollution of Sb and As in the water bodies of the ZR Basin is prominent. As priority pollutants, the geochemical behavior and environmental effects of Sb and As have received extensive attention worldwide. There are spatial differences in the concentration distribution of metals under the changing environmental conditions (runoff, sedimentary environment, pollution sources, etc.) from the mine tributaries to the mainstream of ZR. However, systematic studies of Sb and As under such changing environmental conditions are still lacking, and it is crucial to study the concentration characteristics and the environmental geochemical behavior of Sb and As in specific water environments.
In this study, the ZR water body was selected as the research object, and the spatial and temporal distribution and allocation of Sb and As in water bodies, as well as the characteristics of Sb isotope fractionation in sediments, were systematically studied by combining field sampling, indoor environmental chemical experiments, and modern instrumental analysis (such as multicollector‒inductively coupled plasma‒mass spectrometry), and the potential ecological and health risks of Sb and As were evaluated. The fraction transformation and dynamic release paths of Sb and As were clarified, and the migration processes of Sb and As in different environmental conditions were compared. The research results supplemented the regional basic research data of Sb and As, revealed the environmental geochemical processes of Sb and As, and provided a scientific reference for the pollution control of Sb and As at the basin scale, which has important scientific significance. The main results of this study are as follows:
(1) Occurrence characteristics: The average concentrations of Sb and As in the waters from the mainstream of ZR were 4.29 μg/L and 2.75 μg/L, respectively, and the concentration of Sb showed spatial and temporal differences, while As did not show significant spatial differences. The average concentrations of Sb and As in surface sediments of the mainstream of ZR were 34.19 mg/kg and 33.61 mg/kg, respectively, corresponding to the average concentrations of Sb and As in pore water of 13.26 μg/L and 42.78 μg/L, respectively, with large spatial variability, and the Zhexi Dam had led to the accumulation of Sb and As in the midstream and the decrease in the downstream. The concentrations of Sb and As in the waters from the tributaries of the mining area were strongly regulated by the Sb mining and smelting activities, showing a trend of attenuation with distance in the river, and its concentrations were generally higher than those in the mainstream of ZR. However, in the sediment of tributaries in the mining area, except for the Sb and As concentrations in the Lianxi River in Xikuangshan, which were significantly higher than those in the mainstream of ZR, the Sb and As concentrations in the sediment and pore water of other tributaries were either equal to or lower than those in the mainstream of ZR. The Sb and As concentrations in the sediment of each tributary did not decrease with the decrease in distance from ZR, which was related to the scouring environment formed by strong hydrodynamic forces. There was a clear trend of attenuation of Sb in the five sedimentary profiles with distance along the flow direction of the mining area-ZR-Dongting Lake. The vertical distribution of Sb and As in the same sedimentary profile was not completely the same. In summary, the concentrations of Sb and As in the waters of the tributaries of the mining area were higher than that of the mainstream of ZR, while the concentrations of Sb and As in sediments (except for the Lianxi River) were equivalent and were influenced by the inflow of Xikangshan tributaries and the regulation of sediment deposition the Zhexi dam.
(2) Allocation rules: Sb and As in waters mainly existed in dissolved form, with sediment-pore water partition coefficients of 2.95–4.18 and 2.53–4.01, respectively, and Sb and As in sediments were more likely to combine with solids and mainly existed in the residual fraction. The proportion of residual fraction in Sb was ≥ 92.39%, which is higher than that in As (≥ 65.52%). The proportion of bioavailable concentration in As was higher than that in Sb. The distribution of Sb and As in the solid and pore water phases of surface sediments was mainly influenced by the presence of Ca and Fe/Al oxides in the sediments. The relationship between Sb in profile samples and environmental factors was not significant, while As was affected by organic matter and Al-Fe-Mn oxides. The concentrations of different fractions of Sb and As in sediment were controlled by their total concentrations, maintaining a dynamic equilibrium relationship. Dynamic transfer occurs mainly through linear and nonlinear combinations of pathways. Human activities can accelerate the process of transforming residual Sb and As into other active components, with Sb having a lower mobility than As. In the mainstream of ZR sediments, the ε123Sb value was lower than that of the tributaries in the mining area, and simple dilution of Sb along the river and adsorption of Fe-Al oxides in the sediment did not result in significant Sb isotopic fractionation in the sediment. Sb was transported conservatively along the river. The differences in ε123Sb values across regions make Sb isotopic composition a potentially useful tool for tracing the source of Sb in the water system and studying biogeochemical processes. Human activities that disrupt the stability of sediments (such as sand mining) should be minimized to avoid increased Sb, especially As released and its mobility, which can cause more severe environmental effects.
(3) Risk assessment: The risk of Sb contamination in the ZR basin was higher than that of As, and there was little contamination in the waters, while contamination in the sediment was universal. Based on the total concentration evaluation method, the ecological risk of Sb in sediment was higher than that of As, but the assessment based on the speciation showed that the ecological risk of As was higher than that of Sb, which was because most Sb existed in the residual fraction, leading to an overestimation of the ecological risk of Sb based on total concentration evaluation, while the high available concentration of As led to an overestimation of the ecological risk of As based only on chemical form evaluation. Overall, there was high contamination and ecological risk of Sb and As in the sediment of the midstream of ZR and Lianxi River, which is related to the mining activities in Xikuangshan being transported to Lianxi River and then transported to the mainstream of ZR at a long distance. The health risks from Sb and As in waters were generally low, but sediment exposure had increased the overall health risk of As in ZR, with children's health risk being higher than adults. The high risks in the sediments of the midstream of ZR and Xikuangshan should be focused on, and the ecological risk levels of Sb and As in sediments assessed based on total concentration and speciation methods were opposite, indicating that multi-index comprehensive analysis is necessary to increase the reliability of risk assessment for Sb and As in river basins.
This study has achieved some innovative results in the following three aspects: (1) the distribution and long-distance migration of Sb and As in rivers at the basin scale, (2) isotope composition and fractionation characteristics of Sb in river sediments, and (3) comprehensive risk assessment of Sb and As in water bodies. This study not only deepened the understanding of the spatio-temporal changes of elements in the water environment and their influencing factors, but also made up for the deficiency of regional Sb isotope basic data, enriched the Sb isotope fingerprint database, and provided a guarantee for the reliability of Sb and As risk assessment in the basin.