In China, thousands of tons of highly toxic hexavalent Cr slags are produced annually. The existing remediation technologies face challenges due to the facile oxidation, "re-yellowing" effects, and contamination from iron additives, hindering the achievement of sustained governance and resource recycling for Cr pollution. This study conducted microbial mineralization experiments on soil at actual Cr-contaminated sites, delving into the stability, recyclability, and reaction mechanisms of mineralization products. The results indicate that this method can efficiently reduce Cr⁶⁺ and Fe³⁺ in Cr slag to form precipitates. After 60 d extensive air exposure, the mineralized products exhibited little reoxidation or re-decomposition, and demonstrated weak magnetism, indicating a potential for recovery by magnetic separation. This paper was to investigate a low-cost, efficient in-situ, and long-term governance solution for treatment of Cr pollution on-site and facilitation of the recovery and reuse of Cr resources.

Cr-contaminated soil samples were collected in Henan province, China. The pH value, total Cr, and total Fe concentrations of the soil were measured. Shewanella oneidensis MR-1 (S. MR-1) was cultured and acclimated to high concentrations of Cr⁶⁺ and alkaline conditions to enhance its reduction capabilities. In the microbial reduction experiments, S. MR-1 was added into Cr-contaminated soil at different concentrations of Cr⁶⁺, Cr³⁺, and Fe²⁺.

The microbial morphology and surface elemental composition of the samples were determined by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). The microstructure of the samples was analyzed by transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDS). The concentrations of Cr⁶⁺ and Fe²⁺ were measured by ultraviolet-visible spectrophotometry (UV-Vis). The magnetic properties of the eaction products were characterized by vibrating sample Magnetometry (VSM). The Cr valence states before and after remediation were analyzed by X-ray photoelectron spectroscopy (XPS). The concentrations of Cr and Fe were determined by inductively coupled plasma emission spectroscopy (ICP).

The results show that S. MR-1 significantly reduces Cr⁶⁺ in the contaminated soi and the Cr⁶⁺ concentration decreases to detectable limits for 4 d treatment. Concurrently, the concentration of Cr³⁺ initially increases and then stabilizes, indicating the formation of stable Cr precipitates. The Fe²⁺ concentration increases after most of Cr⁶⁺ reduces, showing the simultaneous reduction of Fe³⁺ in the soil.

Based on the evaluation of the stability of the remediation products for 60 d, the treated soil has a minimal release of Cr³⁺ and no reoxidation to Cr⁶⁺, even when exposed to environmental conditions. This stability is attributed to the formation of stable mineral phases, which are confirmed by VSM and XPS.

The microbial reduction process results in the consumption of soil organic matter, which favors preventing the formation of soluble Cr³⁺ complexes that can be reoxidized. The mineralization control process of Cr-contaminated soil mediated by S. MR-1 can be divided into three stages, i.e., S. MR-1 decomposes soil organic matter, produces OH^-, creates an alkaline environment, and transfers the obtained electrons to the solution and Cr⁶⁺ and Fe³⁺ in the soil, generating Cr³⁺ and Fe²⁺; Microorganisms inhibit the binding of Cr³⁺ with organic matter, engage in ion exchange with the environment, and Fe³⁺, Cr³⁺, and Fe²⁺ are adsorbed on the cell surface or enter the cell body, forming stable minerals under the control of microorganisms, which belongs to the microbial-controlled mineralization mechanism. Microbial secretion of extracellular polymeric substances (EPS) regulates the surrounding environment to achieve mineral precipitation conditions, inducing the mineralization of Cr³⁺, Fe³⁺, and Fe²⁺, which belongs to the microbial-induced mineralization mechanism. The minerals generated in the solution due to their higher density than water deposit at the bottom of the solution, forming a black mineral layer; and S. MR-1 continuously decomposes organic matter in the soil, reducing excess Fe³⁺ to produce Fe²⁺ and maintaining a reducing environment.

This study introduced the use of the dissimilatory metal-reducing bacterium S. MR-1 for the treatment of substantial iron in the existing Cr-contaminated sites. The introduced microorganisms utilized soil organic matters to generate an alkaline environment favorable for mineralization. Within a short time frame, they reduced all Cr⁶⁺ to Cr³⁺ and some Fe³⁺ to Fe²⁺, thereby facilitating the controlled formation of stable phases of Cr³⁺, Fe³⁺, and Fe²⁺, intracellularly and extracellularly. The microbial treatment products exhibited the structural and valence state stability under prolonged oxidative conditions, preventing the release of Cr ions into the environment. Also, the product had a degree of magnetism, offering a cost-effective solution for subsequent resource recovery. This method could overcome the limitations of conventional microbial in-situ remediation technologies (i.e., extended treatment durations, intolerance to high Cr concentrations, and poor product stability). This work could address the challenge of unstable and easily re-oxidized products at Cr-contaminated sites, providing an economically viable solution for Cr pollution remediation in diverse environments, i.e., surface water, groundwater, and soil with varying pollution levels. Moreover, this work could give a basic foundation for low-cost Cr resource recovery.