4. Ex-Situ Remedies

The chapter begins by introducing ex-situ remediation technologies, which are methods not conducted on-site to treat contaminated plumes. It delves into anoxic limestone drains (ALD), discussing their components, such as crushed limestone and plastic liners, and their mechanisms for removing trivalent and divalent metals through precipitation. The chapter then explores permeable reactive barriers (PRB), which use natural hydraulic gradients to drive contaminated plumes toward reactive materials. It discusses various PRB designs, including continuous walls and funnel-and-gate configurations, and the advantages and disadvantages of each. The chapter also covers the removal mechanisms of PRBs, which depend on influent conditions, reactive substrates, and hydraulic retention time. It highlights the use of zero-valent iron (ZVI) as a common reactive material and the challenges associated with its use, such as corrosion and clogging. Additionally, the chapter discusses biological PRBs, which utilize organic materials and reductants to support sulfate-reducing bacteria growth, and diffusion-active PRBs, which protect microbes from mine drainage toxicity. Finally, the chapter explores constructed wetlands, which are engineered systems that emulate natural wetlands to treat contaminated waters. It discusses the removal mechanisms of constructed wetlands, including biological reactions, phytoextraction, adsorption, and precipitation, and highlights their cost-effectiveness and environmental friendliness. Throughout the chapter, the author provides a detailed overview of the components, efficacies, and research gaps of each technology, making it a valuable resource for professionals in the field of environmental engineering and remediation.