Rapid dechlorination and full mineralization of para-chlorophenol (4-CP), a toxic contaminant, are unfulfilled goals in water treatment. Means to achieve both goals stem from the novel concept of coupling catalysis by palladium nanoparticles (PdNPs) with biodegradation in a biofilm. Here, we demonstrate that a synergistic version of the hydrogen (H 2 )-based membrane biofilm reactor (MBfR) enabled simultaneous removals of 4-CP and cocontaminating nitrate. In situ generation of PdNPs within the MBfR biofilm led to rapid 4-CP reductive dechlorination, with >90% selectivity to more bioavailable cyclohexanone. Then, the biofilm mineralized the cyclohexanone by utilizing it as a supplementary electron donor to accelerate nitrate reduction. Long-term operation of the Pd-MBfR enriched the microbial community in cyclohexanone degraders within Clostridium, Chryseobacterium, and Brachymonas. In addition, the PdNP played an important role in accelerating nitrite reduction; while NO 3 − reduction to NO 2 − was entirely accomplished by bacteria, NO 2 − reduction to N 2 was catalyzed by PdNPs and bacterial reductases. This study documents a promising option for efficient and complete remediation of halogenated organics and nitrate by the combined action of PdNP and bacterial catalysis.
Per-
and polyfluoroalkyl substances (PFASs) comprise a group of
widespread and recalcitrant contaminants that are attracting increasing
concern due to their persistence and adverse health effects. This
study evaluated removal of one of the most prevalent PFAS, perfluorooctanoic
acid (PFOA), in H2-based membrane catalyst-film reactors
(H2-MCfRs) coated with palladium nanoparticles (Pd0NPs). Batch tests documented that Pd0NPs catalyzed
hydrodefluorination of PFOA to partially fluorinated and nonfluorinated
octanoic acids; the first-order rate constant for PFOA removal was
0.030 h–1, and a maximum defluorination rate was
16 μM/h in our bench-scale MCfR. Continuous-flow tests achieved
stable long-term depletion of PFOA to below the EPA health advisory
level (70 ng/L) for up to 70 days without catalyst loss or deactivation.
Two distinct mechanisms for Pd0-based PFOA removal were
identified based on insights from experimental results and density
functional theory (DFT) calculations: (1) nonreactive chemisorption
of PFOA in a perpendicular orientation on empty metallic surface sites
and (2) reactive defluorination promoted by physiosorption of PFOA
in a parallel orientation above surface sites populated with activated
hydrogen atoms (Hads
*). Pd0-based catalytic reduction chemistry and continuous-flow treatment
may be broadly applicable to the ambient-temperature destruction of
other PFAS compounds.
Scalable
applications of precious-metal catalysts for water treatment
face obstacles in H2-transfer efficiency and catalyst stability
during continuous operation. Here, we introduce a H2-based
membrane catalyst-film reactor (H2-MCfR), which enables
in situ reduction and immobilization of a film of heterogeneous Pd0 catalysts that are stably anchored on the exterior of a nonporous
H2-transfer membrane under ambient conditions. In situ
immobilization had >95% yield of Pd0 in controllable
forms,
from isolated single atoms to moderately agglomerated nanoparticles
(averaging 3–4 nm). A series of batch tests documented rapid
Pd-catalyzed reduction of a wide spectrum of oxyanions (nonmetal and
metal) and organics (e.g., industrial raw materials, solvents, refrigerants,
and explosives) at room temperature, owing to accurately controlled
H2 supply on demand. Reduction kinetics and selectivity
were readily controlled through the Pd0 loading on the
membranes, H2 pressure, and pH. A 45-day continuous treatment
of trichloroethene (TCE)-contaminated water documented removal fluxes
up to 120 mg-TCE/m2/d with over 90% selectivity to ethane
and minimal (<1.5%) catalyst leaching or deactivation. The results
support that the H2-MCfR is a potentially sustainable and
reliable catalytic platform for reducing oxidized water contaminants:
simple synthesis of an active and versatile catalyst that has long-term
stability during continuous operation.
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