IntroductionThe remediation of tailings sand requires improving both mechanical properties and the stabilization of heavy metals. Practical engineering applications must also consider cost and feasibility. Microbially Induced Carbonate Precipitation (MICP) is an innovative technique that uses microorganisms to enhance soil properties and immobilize heavy metals. However, the traditional MICP method using Bacillus pasteurii has several limitations: reduced activity in high concentrations of heavy metals, calcium carbonate buildup on the soil surface that clogs and limits solidification depth, risks of introducing invasive species, and high costs associated with bacterial cultivation and transport. These factors make it unsuitable for large-scale applications.In situ stimulation of indigenous urease-producing microorganisms in the soil offers a solution. This approach avoids species invasion, eliminates the need for bacterial cultivation and injection, reduces costs, and minimizes clogging. Developing a cost-effective and feasible in situ stimulation strategy for tailings sand solidification is essential for applying this technology in practical engineering.MethodsThe tailings sand used in this study was sourced from the tailings reservoir of Fujian Makeng Mining Co., Ltd., moderately contaminated with cadmium and slightly with zinc. An initial stimulation solution was designed to screen and enrich urease-producing microorganisms in the tailings sand, considering the necessary carbon source, nitrogen source, urea, and other growth factors. Since urease activity is key to the effectiveness of MICP solidification, the urease activity of tailings sand suspensions after in situ stimulation was used as the evaluation metric for optimization.The optimization process involved three steps: 1) Starting with the initial stimulation solution formula, single-factor experiments were conducted to vary carbon and nitrogen sources to identify those yielding the highest urease activity. 2) Using the selected optimal carbon and nitrogen sources, along with other components and pH, a Plackett-Burman (PB) design was employed to identify key factors affecting urease activity. Single-factor tests were then conducted to determine the optimal concentration range for each key factor. 3) A Central Composite Design (CCD) response surface analysis was performed, using urease activity as the response value, to calculate the optimal concentration of each key factor. This resulted in the most effective stimulation solution for tailings sand.Different concentrations of cementation solution and numbers of solidification rounds were tested to compare the effects of indigenous urease-producing bacteria and Bacillus pasteurii on tailings sand in terms of surface strength, cementation thickness, permeability, raindrop erosion resistance, heavy metal leachate toxicity, and heavy metal morphological changes. Microbial community analysis, combined with scanning electron microscopy (SEM) and X-ray diffraction (XRD), was conducted to explore how in situ stimulation enhances mechanical properties and immobilizes heavy metals.Results and discussion1) The optimal in situ activation formula for tailings sand includes sodium acetate at 35.85 mmol/L, ammonium sulfate at 53.59 mmol/L, urea at 481.0 mmol/L, yeast extract (YE) at 0.2 g/L, nickel chloride at 0.01 mmol/L, and a pH value of 8.70, achieving a urease activity 1.944 times higher than that before optimization. 2) The best cementation solution concentration for in situ solidification of tailings sand is 1 mol/L. In situ activation demonstrated better heavy metal immobilization, deeper solidification, and more uniformity compared to Bacillus pasteurii at the same round count. Although the mechanical properties of in situ-treated sand were lower after the same number of rounds, 15 rounds of in situ treatment achieved comparable properties to 10 rounds with Bacillus pasteurii. 3) The cost per round of in situ activation is only one-fifth that of Bacillus pasteurii. Achieving similar mechanical properties, the total cost of 15 rounds of in situ treatment is only one-third of 10 rounds with Bacillus pasteurii. In situ activation eliminates the need for bacterial culture or transportation and avoids the risk of introducing non-native species, providing superior economic and environmental benefits. 4) The optimized activation solution effectively enriched previously low-abundance urease-producing bacteria, making them the dominant group (70%-90% relative abundance) by inhibiting other microorganisms. This also increased the abundance of the urease accessory protein COG0830. 5) The final product of MICP is calcite. Compared to Bacillus pasteurii, in situ activation produced smaller, fewer, and more scattered calcite crystals under the same conditions. Calcite crystal nucleation and growth were influenced by cementation solution concentration. 6) The urease-producing bacteria in tailings sand exhibited good tolerance to heavy metals and had a long survival time, continuously converting exchangeable heavy metals into stable forms. After in situ activation, the cation exchange capacity (CEC) of the tailings sand increased, effectively enhancing soil fertility for mine site revegetation after heavy metal fixation.Conclusions1) In situ activation of urease-producing microbes offers stronger heavy metal immobilization and deeper, more uniform solidification compared to traditional MICP methods. 2) After 15 rounds of treatment, tailings sand solidified using native urease-producing bacteria achieved mechanical properties comparable to those treated with Bacillus pasteurii after 10 rounds, while reducing costs by two-thirds. 3) In situ activation altered the microbial community in the tailings sand, making urease-producing bacteria dominant. These bacteria induced calcium carbonate precipitation, filling sand particle pores, and converting exchangeable heavy metals into stable forms such as metal carbonates, metal-calcium carbonate co-precipitates, or adsorbed forms on iron and aluminum oxides. This improved both mechanical properties and heavy metal immobilization. 4) By increasing the cation exchange capacity, this method enhanced soil fertility, facilitating mine site revegetation. It is an economically and environmentally viable approach for engineering applications.