Chromium isotope fractionation during oxidation of trivalent chromium by birnessite at near-neutral pH

水钠锰矿在接近中性 pH 条件下氧化三价铬过程中的铬同位素分馏

• 批准号: 2301484
• 负责人: Thomas Johnson
• 金额: $ 24.9万
• 依托单位: University of Illinois at Urbana-Champaign
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-06-15 至 2025-05-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2301484&HistoricalAwards=false
• 关键词: Chromium; isotope; fractionation; during; oxidation

A specific type of chromium, hexavalent chromium, is a known carcinogen and common environmental contaminant. Chromium ranks fifth in importance among inorganic contaminants at superfund sites. Hexavalent chromium enters our environment from industrial releases and weathering of certain rocks. The research project advances understanding of chromium geochemistry in modern and past environments by applying a novel approach that uses specialized, highly precise measurements of the stable isotopes of chromium. The objectives of this approach include (1) “fingerprinting” chromium to distinguish and track chromium sources in the environment and (2) determining when toxic, mobile, hexavalent chromium converts to immobile, non-toxic, trivalent chromium. Such reactions occurred early in Earth’s history and the research helps identify how and when ancient earth systems shifted from a low oxygen state to an oxygen-rich environment that fostered development of complex life. The findings will advance practical use of chromium isotope measurements to improve management of chromium contamination in the environment. To promote a diverse science workforce, the project provides environmental geochemical training for graduate and undergraduate students underrepresented in the geosciences. The research team provides geochemical content for outreach to a minority-serving school district, including a middle school girls geosciences summer camp.The research answers two outstanding questions regarding the fundamental drivers of chromium stable isotope variation. Chromium isotope measurements in modern waters and ancient rocks suggest that hexavalent chromium released by oxidative weathering is systematically enriched in heavy isotopes relative to trivalent Cr in the parent rock. This implies that oxidation of chromium favors the heavier isotopes, but the processes behind this phenomenon are paradoxical and not well understood. This presents a fundamental knowledge gap in our understanding of Cr isotope variations in both modern and ancient environments. The project targets two major questions: Does oxidation of solid-phase trivalent chromium at near-neutral pH generate significant chromium isotope shifts, and if so, does the magnitude of isotopic fractionation induced by chromium oxidation depend on geochemical variables such as pH, oxygen levels, and the type of chromium-bearing solids being oxidized? The project is answering these questions via a series of novel laboratory experiments wherein chromium present in three different solid forms, and in natural sediments, is oxidized under a range of relevant conditions by manganese oxides, the dominant chromium oxidant in natural settings.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

铬的一种特殊类型，六价铬，是一种已知的致癌物和常见的环境污染物，在超级基金场地的无机污染物中，铬排名第五。该研究项目促进了对六价铬的了解。通过应用一种新颖的方法，对铬的稳定同位素进行专门、高精度的测量，来研究现代和过去环境中的铬地球化学。该方法的目标包括。 (1) 对铬进行“指纹识别”，以区分和追踪环境中的铬来源；(2) 确定有毒的、可移动的六价铬何时转化为不可移动的、无毒的三价铬。此类反应发生在地球历史的早期，这项研究有助于。确定地球古代系统如何以及何时从低氧状态转变为促进复杂生命发展的富氧环境，这些发现将推动铬同位素测量的实际应用，以改善铬的管理。为了促进科学队伍的多元化，该项目为地球科学领域代表性不足的研究生和本科生提供环境地球化学培训，以向少数族裔服务的学区（包括中学女生地球科学）提供地球化学内容。该研究回答了有关铬稳定同位素变化的基本驱动因素的两个悬而未决的问题。现代水域和古代岩石中的铬同位素测量表明，氧化风化释放的六价铬是系统性的。相对于母岩中的三价铬，重同位素含量较高，这意味着铬的氧化有利于较重的同位素，但这种现象背后的过程是自相矛盾的，而且我们对铬同位素变化的理解存在基本的知识差距。该项目针对两个主要问题：在接近中性 pH 条件下固相三价铬的氧化是否会产生显着的铬同位素变化？那么，铬氧化引起的同位素分馏的程度是否取决于地球化学变量，例如 pH 值、氧含量以及被氧化的含铬固体的类型，尽管存在铬，但该项目正在通过一系列新颖的实验室实验来回答这些问题？三种不同固体形式和天然沉积物中的铬在一系列相关条件下被锰氧化物（自然环境中主要的铬氧化剂）氧化。该奖项反映了通过使用基金会的智力价值和更广泛的影响审查标准进行评估，NSF 的法定使命被认为值得支持。