Subsurface flow wetlands for the removal of arsenic and metals from contaminated water

The presence of arsenic (As) in aquatic environments is a worldwide concern due to its toxicity and chronic effects. In many cases, the choice of treatment technologies is limited due to the isolated location of the water source and the high cost of conventional treatment technologies. In addition, other pollutants are often found alongside As, such as iron (Fe) and boron (B). Constructed wetlands have shown capability to remove As and metals. However, few experimental studies have been undertaken to investigate As removal in wetland systems, leaving understanding of their removal mechanisms and performance wanting. This thesis has investigated the As, Fe and B removal capabilities of vertical and horizontal flow wetlands, using conventional and alternative media. Laboratory studies were performed with the objective of gaining further understanding of the processes that remove these pollutants. In vertical flow wetlands, alternative wetland media -limestone, zeolite and cocopeat- proved to be more effective than conventional gravel media. In horizontal flow wetlands, the zeolite media achieved high As and Fe removal rates. Wetlands made up of a sequential arrangement of limestone and cocopeat achieved similar As removal rates, higher B removal rates, but slightly lower Fe removal rates. Target pollutants were mainly retained in the wetland media instead of wetland plants, thus confirming the key role of wetland media in removing As, Fe and B. The main recommendation from this research is that subsurface flow wetlands should use alternative wetland media to enhance As, Fe and B removal from acidic water, and ideally they should be operated under continuous horizontal flow. Wetland media able to provide alkalinity and sorption sites, such as limestone and zeolite, are recommended, since they enhance As coprecipitation with Fe, and As and Fe sorption, respectively. Organic wetland media, such as cocopeat, have potential to enhance B removal by sorption. Continuous horizontal flow maximises contact time for sorption onto wetland media and also favours anaerobic processes such as precipitation of sulfides, which can also enhance As and Fe removal. This research has advanced understanding of the removal of arsenic, boron and iron in vertical and horizontal flow wetlands, offering recommendations to improve wetland design, with the ultimate aim to develop cost-effective technologies that can provide reliable water treatment for the protection of human health and aquatic ecosystems.