Biopolymer-enzyme-induced carbonate precipitation (EICP) for the green solidification/stabilization of graphite tailings: Mechanical, leaching, and microstructural characterization.

The rapidly increasing global demand for graphite has increased the generation of graphite tailings. However, graphite tailings are rich in harmful substances, such as heavy metals, which pose serious threats to the environment and human health. Traditional recycling methods of graphite tailings face challenges, including high costs, substantial CO emissions, and limited effectiveness in heavy-metal stabilization, which inhibit their environmental sustainability in large-scale applications. In this study, a novel ecofriendly strategy was proposed for the green solidification and stabilization (S/S) of graphite tailings. Chitosan (CTS), a biopolymer, was introduced during the enzyme-induced carbonate precipitation (EICP) of graphite tailings. The potential of CTS-EICP in binding the loose particle structure of graphite tailings and inhibiting the release of heavy-metal pollutants was discussed from dual dimensions of mechanical strength and environmental effects. Results revealed that the unconfined compressive strength (UCS), split tensile strength (STS), and calcium carbonate generation rate of CTS-EICP-treated graphite tailings significantly improved compared with those of EICP-treated tailings. The effects were optimal when the CTS content was 0.15 %, reaching 897.8 kPa, 258.1 kPa, and 8.03 %, respectively. After CTS-EICP treatment, the pH of the tailing leachate stabilized at 7.90-8.26 and the fixation rate of heavy-metal ions was 92.61 %-100 %. CTS promoted the formation of carbonate crystals via urease stabilization mechanism, which were embedded in the three-dimensional network formed via the crosslinking of CTS molecules and yielded a multilayer composite barrier structure of "tailings-CTS-carbonate-CTS-tailings." CTS-EICP provided a new perspective for employing biopolymer-EICP synergistic remediation interactions in multidimensional green S/S of heavy metal-containing tailings.