Research on Speciation and Bioavailability of Chromium in Environment and Analysis Techniques: A Review

Chromium (Cr) pollution has become a globally-concerned environmental issue, seriously threatening ecosystems and human health. In the natural environment, chromium mainly exists in the forms of trivalent Cr(Ⅲ) and hexavalent Cr(Ⅵ), and its speciation transformation is affected by environmental physicochemical properties such as humidity, pH, redox potential, and microorganisms. The research progress on chromium speciation analysis and bioavailability in the natural environment in the past five years is reviewed, focusing on the valence and speciation changes of chromium, chromium speciation analysis techniques, chromium bioavailability, and its evaluation methods. There are various classification methods for chromium chemical forms in different disciplines, such as the Tessier method and the BCR method. These methods enhance the understanding of chromium from different perspectives and provide a basis for assessing chromium pollution risks and remediation. However, the practical application needs to be selected according to the purpose and scenario. Furthermore, advanced detection methods such as diffusive gradient in thin-films (DGT), X-ray absorption fine structure spectroscopy (XAFS), machine learning, and gene sequencing are summarized, which are based on a deeper understanding of the factors affecting Cr valence. Novel materials, such as graphene oxide, are introduced for their application in sensors. These new methods and materials have significantly improved the selectivity and sensitivity for the detection of Cr(Ⅲ) and Cr(Ⅵ), achieving detection limits of 0.073μmol/L and 0.36μmol/L, respectively. Such advancements facilitate a better understanding of the bioavailability of Cr. Future research should focus on the migration mechanisms of chromium in different environmental media and their target toxicity mechanisms, and develop more convenient, rapid, and cost-effective detection technologies to meet the demands of environmental monitoring. These efforts will provide more comprehensive and precise solutions for chromium pollution environmental risk assessment, ecological restoration, and health risk management.