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

六价铬是一种特殊类型的铬，是一种已知的致癌物质和常见的环境污染物。在超级基金所在地的无机污染物中，铬的重要性排名第五。六价铬通过工业排放和某些岩石的风化进入我们的环境。该研究项目通过应用一种新的方法，使用专门的、高精度的铬稳定同位素测量方法，促进了对现代和过去环境中铬地球化学的理解。这种方法的目标包括：(1)对铬进行指纹识别，以区分和跟踪环境中的铬来源；(2)确定有毒的、流动的、六价铬何时转化为固定的、无毒的、三价铬。这种反应发生在地球历史的早期，这项研究有助于确定古代地球系统如何以及何时从低氧状态转变为富氧环境，从而促进了复杂生命的发展。这一发现将推动铬同位素测量的实际应用，以改善环境中铬污染的管理。为了促进多样化的科学队伍，该项目为在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。该研究小组为一个为少数民族服务的学区提供了地球化学内容，包括一个中学女生地球科学夏令营。这项研究回答了关于铬稳定同位素变异的根本驱动因素的两个突出问题。现代水体和古岩石的铬同位素测量表明，氧化风化释放的六价铬相对于母岩中的三价铬具有系统的重同位素富集性。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，也没有被很好地理解。这在我们对现代和古代环境中铬同位素变化的理解上存在着基本的知识鸿沟。该项目针对两个主要问题：固相三价铬在近中性pH下的氧化是否会产生显著的铬同位素位移，如果是，铬氧化引起的同位素分馏的程度是否取决于诸如pH、氧水平和被氧化的含铬固体的类型等地球化学变量？该项目通过一系列新颖的实验室实验来回答这些问题，在这些实验中，铬以三种不同的固体形式存在，天然沉积物中的铬在一系列相关条件下被自然环境中主要的铬氧化剂锰氧化物氧化。该奖项反映了NSF的法定使命，并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，被认为值得支持。

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.