Hydrated electrons and other reductive species - properties, formation and applications in advanced reduction processes for degradation of emerging organic pollutants – a review
Bakhtiar Ali Samejo, Manoj P. Rayaroth, Chongqing Wang, Xun Sun, Grzegorz Boczkaj
Abstract
The hydrated electron (e−aq) has received significant attention in advanced oxidation processes (AOPs) and currently in advanced reduction processes (ARPs). This review offers an up-to-date information about the structure, reactivity, and stability of e−aq and environmental applications. Various ARPs, particularly generating e−aq such as UV/reductants (sulfite, iodide, dithionite and ferrous ion), and innovative technologies (e-beam, plasma, sonochemistry, photocatalysis) are discussed. Optimal conditions needed to produce enough e−aq, benefits and drawbacks of each system are given. Moreover, the degradation effectiveness of chlorinated and fluorinated chemicals, nitro-derivatives, among others are discussed. It has been found that effectiveness of ARPs, varied with the utilized processes. Per- and polyfluoroalkyl substances (PFAS) were reduced effectively using UV/Indole and UV/sulfite processes. UV/iodide needed more time to degrade them. Research suggested that dissolved organic matter was a crucial e−aq scavenger. In addition, e−aq-based photochemical methods effectively reduced inorganic compounds i.e., BrO32−, NO3−, arsenic in a form of As(V) and As(III). Types of scavengers as well as methods for identification of reductive species contributing to degradation mechanism were discussed. Decrease of the matrix effect provide additional processes which will allow to preconcentrate target pollutants and eliminate most of matrix components – such approaches based on effective separation techniques like adsorption or membrane processes are currently under high interest. Finally, impact of process variables on ARPs effectiveness, such as pH, dissolved oxygen, temperature, source chemical concentration, and UV intensity are discussed. Still debatable is effect of dissolved oxygen, high scavenging effect was reported for several systems, including UV/iodide – anaerobic conditions obtained by nitrogen or argon purging where needed. Other studies (UV/sulfite) showed was minor, probably by oxygen removal using excess of reductant. Preference of basic pH, high selectivity makes the ARPs a promising alternative to AOPs.