Critical Review of UV-Advanced Reduction Processes for the Treatment of Chemical Contaminants in Water

UV-advanced reduction
processes (UV-ARP) are an advanced water
treatment technology characterized by the reductive transformation
of chemical contaminants. Contaminant abatement in UV-ARP is most
often accomplished through reaction with hydrated electrons (eaq–) produced
from UV photolysis of chemical sensitizers (e.g., sulfite). In this
Review, we evaluate the photochemical kinetics, substrate scope, and
optimization of UV-ARP. We find that quantities typically reported
in photochemical studies of natural and engineered systems are under-reported
in the UV-ARP literature, especially the formation rates, scavenging
capacities, and concentrations of key reactive species like eaq–. The absence
of these quantities has made it difficult to fully evaluate the impact
of operating conditions and the role of water matrix components on
the efficiencies of UV-ARP. The UV-ARP substrate scope is weighted
heavily toward contaminant classes that are resistant to degradation
by advanced oxidation processes, like oxyanions and per- and polyfluoroalkyl
substances. Some studies have sought to optimize the UV-ARP treatment
of these contaminants; however, a thorough evaluation of the impact
of water matrix components like dissolved organic matter on these
optimization strategies is needed. Overall, the data compilation,
analysis, and research recommendations provided in this Review will
assist the UV-ARP research community in future efforts toward optimizing
UV-ARP systems, modeling the eaq–-based chemical transformation kinetics,
and developing new UV-ARP systems.