Per and polyfluoroalkyl substances (PFAS), legacy chemicals used in firefighting and the manufacturing of many industrial and consumer goods, are widely found in groundwater resources, along with other regulated compounds, such as chlorinated solvents. Due to their strong C− F bonds, these molecules are extremely recalcitrant, requiring advanced treatment methods for effective remediation, with hydrated electrons shown to be able to defluorinated these compounds. A combined photo/ electrochemical method has been demonstrated to dramatically increase defluorination rates, where PFAS molecules sorbed onto appropriately functionalized cathodes charged to low cell potentials (−0.58 V vs Ag/ AgCl) undergo a transient electron transfer event from the electrode, which "primes" the molecule by reducing the C−F bond strength and enables the bond's dissociation upon the absorption of a hydrated electron. In this work, we explore the impact of headgroup and chain length on the performance of this two-electron process and extend this technique to chlorinated solvents. We use isotopically labeled PFAS molecules to take advantage of the kinetic isotope effect and demonstrate that indeed PFAS defluorination is likely driven by a twoelectron process. We also present density functional theory calculations to illustrate that the externally applied potential resulted in an increased rate of electron transfer, which ultimately increased the measured defluorination rate.
The
growth of mineral crystals on surfaces is a challenge across
multiple industrial processes. Membrane-based desalination processes,
in particular, are plagued by crystal growth (known as scaling), which
restricts the flow of water through the membrane, can cause membrane
wetting in membrane distillation, and can lead to the physical destruction
of the membrane material. Scaling occurs when supersaturated conditions
develop along the membrane surface due to the passage of water through
the membrane, a process known as concentration polarization. To reduce
scaling, concentration polarization is minimized by encouraging turbulent
conditions and by reducing the amount of water recovered from the
saline feed. In addition, antiscaling chemicals can be used to reduce
the availability of cations. Here, we report on an energy-efficient
electrophoretic mixing method capable of nearly eliminating CaSO4 and silicate scaling on electrically conducting membrane
distillation (ECMD) membranes. The ECMD membrane material is composed
of a percolating layer of carbon nanotubes deposited on porous polypropylene
support and cross-linked by poly(vinyl alcohol). The application of
low alternating potentials (2 Vpp,1Hz) had a dramatic impact
on scale formation, with the impact highly dependent on the frequency
of the applied signal, and in the case of silicate, on the pH of the
solution.
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