Wang, Zhongli
(2023)
Remediation and recovery of Selenium (Se) from contaminated water environment.
PhD thesis, University of Nottingham.
Abstract
The current shortage of resources, such as water and raw materials, is a global and critical challenge. In addition, human industrial activities are worsening the shortage of available water resources through pollution and harming the sustainability of the ecological environment. These pollutants not only harm water resources, but also enter the food chain, affecting human health. Selenium (Se) is an environmental toxic contaminant that seriously threatens human health and water ecosystems. Meanwhile, it is also a scarce element that cannot be mined as a primary product but plays a crucial role in the industry.
The primary aim of this project was to achieve in-situ Se remediation and recovery via an applicable approach. Reduction is a widely recognised approach to achieve Se remediation, as the most common form of Se released to the water environment are Se oxyanions (SeO42- and SeO32-), which are more toxic than Se with a lower oxidation state such as Se(0). Using organic or gaseous electron donors can achieve satisfactory Se remediation results for ex-situ biological water treatment, and the produced Se(0) can be recycled as a valuable product for various industries such as electronics, medicine, and agriculture. However, the produced Se(0) would stay in the water system for in-situ remediation and pose an environmental risk due to its nanoparticle form and tendency to be oxidised into Se(VI). Another widely studied reducing agent for Se(VI) remediation is zero-valent iron (ZVI), which has been shown to effectively reduce Se oxyanions to less toxic Se(0). In addition, the low cost of ZVI and its potential for adsorption and coprecipitation of Se due to ZVI corrosion during Se oxyanions reduction make ZVI a promising candidate for Se remediation and its recovery. Under natural water environmental conditions, microorganisms are ubiquitous and can affect the migration and transformation of Se in water. Understanding this process can provide an important scientific basis for the remediation and recovery of Se. Therefore, it is crucial to explore the behaviour and transformation of Se mediated by natural microorganisms. However, most ZVI studies have been carried out in water matrices, the universal microorganisms’ role is neglected, and the recovery of reduced Se has not been fully investigated.
The work presented in this thesis aims to investigate the use of ZVI for in-situ Se remediation and recovery in a water-sediment system constructed using natural sediment. Adding ZVI enhanced the removal of Se(VI) in the water phase mediated by microorganisms in the fresh sediment, with a removal efficiency of 92.7 ± 1.1% within 7 days when 10 mg L-1 Se(VI) was present. Furthermore, soluble Se(VI) was removed from the water and precipitated to the sediment phase (74.8 ± 0.1%), and this process was further enhanced by the addition of ZVI (83.3 ± 0.3%). Furthermore, adding ZVI decreased the formation of Se(0), which is biohazardous, in the sediment phase. The study also investigated the recovery proportion of Se, which was found to be 34.2 ± 0.1% and 92.5 ± 0.2% through wet and dry magnetic separation with 1 g L-1 ZVI added, respectively. The changes in the microbial community of the sediment in the water-sediment system were also studied, and it was found that the addition of ZVI and Se led to variations in the microbial community, i.e., species abundance of Actinobacteriota decreased significantly. However, magnetic separation was potentially shown to mitigate these changes in the long term.
This work provides a novel technique to achieve in-situ Se remediation and recovery by combining ZVI reduction and magnetic separation. The findings have major implications for developing effective strategies to manage Se environmental contamination. Further research should be carried out to study the potential of using ZVI in combination with magnetic separation for different pollutants’ remediation and recovery.
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