**5. Conclusions**

Based on the results obtained from the aqueous batch experiments, OSP and HAP had the optimum removal efficiencies to Cr, Zn, Cu, and Ni with CB taking the third place. As for Hg, FeS and CB showed the best immobilizing ability. Kaolinite presented the weakest removal performance towards all of these five metals, and lack of the presence of kaolinite did not show significant influence on the hydraulic stability of mixed caps.

Based on the lab-scale microcosm experiments, the mixing ratio of 10% kaolinite + 2.5% FeS + 2.5% CB + 35% HAP + 50% OSP (i.e., Column 5) had the most prominent effect to immobilize the five metals present in the test sediment. Although Column 4 (the lowest cost one) with 80% OSP showed unsuccessful results for reducing Ni and MeHg, it performed well in inhibiting the release of the other metals. When considering cost effectiveness and environmental impact, using caps mainly composed of OSP can yet be a good choice. But when applied to real sites, decisions should be made after comprehensive evaluations based on actual conditions.

This study helps to construct guidelines of using mixed materials mostly prepared from recycled materials to remediate multi-contaminated sediments and provide some references for active capping application and development. Results from this study would also be helpful in reducing the human health and ecological risks by reducing the potential toxic metal release from sediment to overlying water.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4441/12/7/1886/s1, Figure S1: title, Table S1: Oxidation reduction potential (ORP) during operation, Table S2: pH values of microcosm during operation, Table S3: Dissolved oxygen (DO) (mg L−1) of the overlying water in the microcosm, Table S4: Electrical conductivity (EC) of the overlying water in the microcosm, Table S5: THg concentration of the overlying water in the microcosm, Table S6: MeHg concentration of the overlying water in the microcosm, Table S7: Ni concentration of the overlying water in the microcosm, Table S8: Cr concentration of the overlying water in the microcosm, Table S9: Cu concentration of the overlying water in the microcosm, Table S10: Zn concentration of the overlying water in the microcosm.

**Author Contributions:** Conceptualization, M.-Y.O., Y.T., T.-C.C., and H.-C.H.; methodology, M.-Y.O., Y.T., B.-L.C., and C.C.; formal analysis, M.-Y.O., Y.T., B.-L.C., and Y.-H.C.; investigation, M.-Y.O.; data curation, M.-Y.O. and Y.-H.C.; writing—original draft preparation, M.-Y.O.; writing—review and editing, T.-C.C. and H.-C.H.; visualization, M.-Y.O.; funding acquisition, H.-C.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was financially supported by the Taiwan Environmental Protection Administration (No. 08BT547001).

**Acknowledgments:** We are very grateful to all members of the project team and the Groundwater Pollution Remediation Funds of Taiwan Environmental Protection Administration for their funding support.

**Conflicts of Interest:** The authors declare no conflict of interest.
