Para-Chlorophenol (4-CP) Removal by a Palladium-Coated Biofilm: Coupling Catalytic Dechlorination and Microbial Mineralization via Denitrification

Abstract

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.

H2-Based Membrane Catalyst-Film Reactor (H2-MCfR) Loaded with Palladium for Removing Oxidized Contaminants in Water

“…Furthermore, these results in the mild condition document the fastest saturation of aromatic rings, even at least one order of magnitude faster than those at high temperature or with addition of activating chemicals like selected Lewis acids . While 4-AP and 4-aminocyclohexanone are unregulated in water, the conversion of 4-AP to 4-aminocyclohexanone potentially mitigates ecological risk, as cyclohexane derivatives are generally more bioavailable than their aromatic counterparts …”

“…66 While 4-AP and 4-aminocyclohexanone are unregulated in water, the conversion of 4-AP to 4aminocyclohexanone potentially mitigates ecological risk, as cyclohexane derivatives are generally more bioavailable than their aromatic counterparts. 67 Interestingly, further hydrogenation of 4-AP only took place for high Pd loadings (e.g., 0.44 g/m 2 ) but not for low Pd loadings (e.g., 0.02 g/m 2 ) (data not shown). This suggests that catalyst dosage and/or nanoparticle size affected the reaction rate and product selectivity.…”

“…On the one hand, the success of Pd 0 catalysis in this study paves the way for applying Pd and other precious metals for removing a broad spectrum of oxidized contaminants. − On the other hand, the practical implementation of the MCfR can be built on the foundation of the commercially available H 2 -MBfR, the MCfR’s biological counterpart. , In fact, the two approaches can be combined beneficially in some situations. Our previous studies , reported that the membrane-hosted biofilm in the H 2 -MBfR was capable of in situ synthesizing and stabilizing Pd 0 NPs; the “palladized” biofilm thus functioned as a synergistic platform for coupled catalytic and enzymatic processes that reinforce the removal efficiency and selectivity. In the future, the effects of other components in real-world contaminated water, particularly sulfur species, , on deactivating and/or poisoning Pd 0 catalysts need to be further evaluated and addressed.…”

Biotechnological synthesis of Pd-based nanoparticle catalysts

“… 238 In a following study the same system was effectively applied to the degradation of the toxic contaminant 4-chlorophenol (4-CP). 239 Pd(0) catalysed the reductive dechlorination of 4-CP, with over 90% selectivity to cyclohexanone. Bacteria within the biofilm then utilised cyclohexanone as an electron donor to accelerate nitrate reduction.…”

Simultaneous Construction of Silica Nanotubes Loaded with Pd Nanoparticles for Catalytic Hydrodechlorination of Chlorophenols

“…The common catalysts used for HDC are supported noble metals, − for example, Pd, ,− Ru, and Rh, ,− while transition metals, such as Ni and Co, are effective for this reaction but require relatively critical reaction conditions. To achieve better performance, bimetallic alloy catalysts, for instance, PdAg, PtNi, NiFe, and PdNi, have also been studied, and their performances are usually dependent on the compositions of bimetallic alloys. , Among various catalysts, Pd has been recognized as efficient catalysts for HDC of various chlorophenols, which can be performed at atmospheric H 2 pressure and low temperatures.…”