Uranium retention and possible redox transformation by Sphingomonas metalli-silicate mixtures in simulated mining environments

While the speciation and transport of uranium (U) in binary mineral-only or microbe-only systems have been well characterized, the coupled regulation of U(VI) retention and valence transformation at coexisting mineral-microbe interfaces remain poorly constrained. Therefore, this study investigated how Sphingomonas metalli (S. metalli), a prevalent bacterium in U mining areas, influences U(VI) sorption and redox behavior during its interaction with three representative silicate minerals with contrasting surface properties (chlorite, albite, and muscovite). Batch experiments conducted across a pH range of 5 - 10 show that a slight decrease in U sorption (up to 22%) was observed in the presence of bacteria, except for the chlorite under near-neutral conditions, with this effect becoming less pronounced under alkaline conditions (pH 8 - 10). This decrease could be due to bacteria-induced modification of mineral surfaces, e.g., mineral surface charge screening and site-blocking effects driven by EPS, and possible sorption competition between U and bacterial extracellular metabolites. Infrared spectroscopy analyses support such surface modification, showing that bacterial attachment perturbs Si-O and Al-O functional groups on mineral surfaces. In addition to sorption processes, X-ray photoelectron spectroscopy suggests that a minor reduced U (as U(IV)) component may form in bacterial and mineral-bacteria systems even under aerobic conditions. Collectively, these results highlight that S. metalli acts as an interfacial regulator in microbe-mineral-U systems, suppressing mineral surface retention and inducing limited U(VI) reduction. These findings improve understanding of U partitioning in microbially active mining environments and provide a basis for more realistic assessment of uranium mobility and environmental risk.