Rare earth elements in water of the St. Lawrence River system

To meet the strong demand for green and digital technologies, the production of rare earth elements (REEs) has increased significantly, yet the fate of REEs (Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) in large rivers with multiple hydrological, geochemical, and anthropogenic gradients in their catchments remains poorly documented. Here we evaluated the spatial and temporal evolution of the concentrations, patterns, and speciation of REEs in surface waters along a 550 km transect of the St. Lawrence River (SLR) (Canada) and its tributaries from 2017 to 2020. Particulate and filtered (<0.45 µm) REEs concentrations increased downstream, suggesting that REEs found in the SLR predominantly originate from nearby tributaries, especially those draining the REE-rich Canadian Shield geological formation. Moreover, filtered concentrations were significantly higher in years that experienced intense spring freshets, indicating greater influence of runoff and surface soil processes on water chemistry of the SLR. Correlations with terrestrial-DOM, WHAM speciation modelling, and standardized patterns showed REEs speciation dominated by organic colloids, even in alkaline sites with low DOC and low terrestrial inputs. Strong positive correlations with Fe and Al suggested co-transport on organic matter and adsorption on oxyhydroxides. Compared to the influence of water chemistry and geology, the overall influence of anthropogenic inputs was generally low, although we observed influence of increased erosion on particulate REEs and we measured anthropogenic gadolinium (Gd) at the Montréal wastewater outlet. Our results provide an integrative understanding of the fate of REEs through a geochemical and landscape perspective in one of the largest rivers in North America.为应对绿色与数字技术的强劲需求，稀土元素（rare earth elements, REEs）的产量大幅增长，但目前针对流域兼具多重水文、地球化学与人为活动梯度的大型河流中稀土元素（钪Sc、钇Y、镧La、铈Ce、镨Pr、钕Nd、钷Pm、钐Sm、铕Eu、钆Gd、铽Tb、镝Dy、钬Ho、铒Er、铥Tm、镱Yb、镥Lu）的归趋，相关研究记载仍较为匮乏。本研究于2017至2020年间，沿加拿大圣劳伦斯河（St. Lawrence River, SLR）及其支流长达550公里的样带，对地表水中稀土元素的浓度、分布模式与赋存形态的时空演变特征展开系统评估。颗粒态与过滤态（<0.45 µm）稀土元素浓度均沿顺流方向升高，表明圣劳伦斯河中的稀土元素主要源自邻近支流，尤其是流经富含稀土的加拿大地盾地质构造的支流。此外，在春季洪水频发的年份，过滤态稀土元素浓度显著更高，这提示径流与地表土壤过程对圣劳伦斯河河水化学特征的影响更为显著。通过与陆地源溶解性有机质（terrestrial-DOM）、WHAM形态模拟模型以及标准化分布模式的相关性分析，本研究发现即便在溶解有机碳（Dissolved Organic Carbon, DOC）浓度较低且陆地输入较少的碱性水域，稀土元素的赋存形态仍以有机胶体为主。稀土元素与铁、铝元素呈显著正相关，表明其可通过有机质协同迁移，并吸附于羟基氧化物表面。相较于水化学与地质作用的影响，人为输入的整体影响总体较弱，但研究团队观测到侵蚀作用增强对颗粒态稀土元素的影响，并在蒙特利尔污水处理厂排放口检测到人为源钆（Gd）。本研究结果从地球化学与流域景观视角，为北美大型河流之一的圣劳伦斯河中稀土元素的归趋提供了综合认知。