## 4 Conclusions  
This study investigated the remediation of Zn\({}^{2+}\), Ni\({}^{2+}\), and Cr(VI) contamination using the crude sword bean urease-based enzyme-induced carbonate precipitation (EICP) method. Both liquid batch tests and geochemical simulations were conducted, complemented by microscopic analysis results, and the following conclusions were drawn:  
(1) The Zn\({}^{2+}\), Ni\({}^{2+}\), and Cr(VI) can be effectively immobilized by the crude sword bean urease-induced EICP method. The highest immobilization percentage was observed for Zn\({}^{2+}\), reaching up to 99%. Ni\({}^{2+}\) and Cr(VI) could be immobilized at 62.4% and 24.4%, respectively.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type** & **Advantages** & **Disadvantages** & **Ref** \\ \hline Activated carbon & Higher surface area and active sites & Expensive & [71; 72] \\ Agricultural waste & Low cost & Low adsorption capacity & [73; 74] \\ Clay minerals & Abundant and cheap & Modification may result in additional & [75] \\ Zeolite-based adsorbents & Eco-friendly and regenerative & Complex pre-treatment steps & [76] \\ Chitosan-based adsorbents & Regenerative and low cost & Sensitivity to pH, low thermal stability, & [77] \\ Nanomaterials & High adsorption capacity & Solubility, difficulty in manipulation, & \\ EICP in this study & Low cost, excellent immobilization capability in multiple heavy metals & Regeneration & / \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparisons of different adsorbents in literature and the EICP method in this study.  
Figure 15: SEM-EDS results of precipitates in liquid batch tests.  
(2) The addition of Ca\({}^{2+}\) in the EICP method facilitates the immobilization of heavy metal ions. As long as urease activity is not severely inhibited, the immobilization percentages of heavy metals increase with the concentration of the added Ca\({}^{2+}\). This indicates that calcium carbonate adsorption plays a significant role in the bioremediation of heavy metal ions.  
(3) Organic molecules in sword bean urease positively influence the immobilization of heavy metals. The differences between simulation and liquid batch results suggest that the organic molecules in sword bean urease may adsorb heavy metal ions and promote the precipitation of ZnCO\({}_{3}\) and NiCO\({}_{3}\).  
(4) The liquid batch test results, simulation results, and microscopic analysis results indicate that the mechanism of EICP for remediating heavy metal contaminated solutions can be summarized as biomineralization to form heavy metal carbonate precipitates and metal hydroxide precipitates; adsorption by calcium carbonate; and adsorption, complexation, or nucleation promotion by organic molecules with heavy metal ions.  
Y.B.: Conceptualization, methodology, experiment conduction, writing--original draft. Y.C.: Methodology, investigation, writing--review, and editing. L.Z.: Conceptualization, methodology, writing--review, and editing. H.K.: Conceptualization, methodology, writing--review, and editing. Y.G.: Conceptualization, methodology, writing--review, and editing. Q.W.: Conceptualization, experiment conduction, writing--review and editing. G.Q.: Conceptualization, methodology, writing--review, and editing. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Key Research and Development Program of Zhejiang (No. 2022C03095), the National Natural Science Foundation of China (Grants 51988101, 52208373, and 52208357), the National Key Research and Development Program of China (No. 2023 YFC3007104), and the Fundamental Research Funds for the Central Universities (No. 226202400057).