Hybrid thin film nano-composite membrane reactors for simultaneous separation and degradation of pesticides

Highlights

• Catalytic thin film composite membranes were synthesized.

• Catalytic behaviours provided by silver nano-particles incorporated into nano-sized metal organic framework crystals.N

• Nanofiltration separation performance and ability to degrade model pesticide demonstrated.

Abstract

Membrane reactors typically combine chemically reactive pathways with separation opportunities to increase conversion and chemical processes efficiency in liquid effluents treatment. The treatment of industrial bio-products, waste mixed solvents and agro-chemicals with such reactors are however challenging due to the natural affinity of such reactive materials for organic and biological matter leading to surface adsorption and fouling tendencies. Here, hybrid thin film composite catalytic membranes offering superior flow permeation characteristics, extremely high retention of low molecular weight organics and partial salt rejection capabilities were for the first time synthesized. Catalytic silver-metal nano-materials were for the first time homogeneously templated and encapsulated across metal organic frameworks nano-particles and incorporated across the top surface of poly(amide) thin films during interfacial polymerization. These novel materials offer high catalytic/anti-microbial behaviours due to the nano-structure of the metal nano-particles reduced within the metal organic framework template, forming unique hierarchical sub-100 nm hybrid nano-structures. These ultra-thin but yet dense membranes were able to simultaneously degrade chemicals and filter contaminants, opening new pathways for the design of the next generation thin film nano-composite membranes. Catalytic properties and homogeneity were evaluated for the Fenton-like heterogeneous catalytic degradation of 2,4-Dichlorophenoxyacetic acid, a waste pesticide contained in agricultural wastewater.

Introduction

Membrane reactors typically combine chemically reactive pathways with separation opportunities to increase the efficiency of both conversion and chemical processes in the treatment of liquid effluents [1]. Among the different classes of recently developed membrane reactor systems, catalytic membrane reactors (CMR), have shown promises in chemical synthesis, contaminants degradation, pathogens destruction or effluents purification by controlling chemical reactions at the surface of membranes or at the interface between different phases across pores within membrane materials [2]. Such reactors may operate in partial vapour or liquid phase and were demonstrated to exhibit high yields during hydrogenation [3], [4] or dehydration [5], [6] reactions. The treatment of industrial bio-products, waste mixed solvents and agro-chemicals, such as pesticides, are particularly challenging due to the natural affinity of such reactive materials for organic and biological matter leading to strong surface adsorption and fouling tendencies [1], [2], [7].

2,4-Dichlorophenoxyacetic acid (2,4-D) is a common systemic herbicide used in the control of broadleaf weeds, which although shown to exhibit mild toxicity to mammals and birds at low concentrations, may be extremely toxic if concentrated to levels reached in intensive farming area [8]. 2,4-D offers a lifespan in wastewater of up to 15 days, well beyond the duration of a recycling water treatment processes in developed countries [9]. Upon hydrolysis in water, 2,4-D also generates ester forms which were demonstrated to impact kidney efficiency during laboratory experimentations [8]. 2,4-D was also shown to strongly bind to inorganic matter, such as silicate or carbonate materials within soils, therefore raising concerns on its long term viability. The small molecular weight of this pesticide, at 221 g mol⁻¹, makes it highly permeable to current separation systems and the design of more efficient degradation and capture processes are therefore required to alleviate risks associated with the sustainable use of agro-chemicals [9].

The design of catalytic TFC membranes implies the controlled incorporation of nano-scale metal or metal oxide moieties across the 100-nm thick active layer of the membrane. Such layers are typically dense, exhibiting free volume distribution on the order of 4–6 Å selectively allowing for water transport and rejecting most low molecular weight contaminants and salts [10]. Current CMRs typically include nano or meso porous substrates, coated with photo-catalytic or chemically active materials exhibiting high specific surface area to enhance kinetics of reactions [11], [12]. Although, these reactors offer versatile platforms which can be tuned through specific decoration or anchoring of catalytic metal or metal oxide materials [7], [13], surface grafting of catalyst precursors [14], the long term stability of both the catalytic moeties and the substrates strongly limit their industrial applications. Furthermore, the selectivity of degradation by-products generated during the chemical reactions are typically extremely low due to the large size of the pores across the separation barrier [7]. Therefore, new routes to control the microstructure and nano-texturation of CMRs, as well as selectivity and ability to treat low molecular weight chemicals efficiently must be designed to allow for advanced and cost-effective remediation of complex industrial liquid waste effluents [2].

Recent design of nano-composite TFC membranes have included the incorporation of nanotubes, graphene, nano-wires, or NPs as a loading to either bring new functionalities or promoting water permeation by enhancing interfacial pathways between the dense polymer matrix and the fillers [10], [15]. One of the most relevant strategies consist in physically or covalently binding the NPs within the PA material through incorporation into one of the two phases used during interfacial polymerization [15]. However, the incorporation of metal or metal oxide NPs or nano-wires, prerequisite to generate CMR materials, was previously shown to be challenging due to agglomeration of NPs during membrane preparation and to the lack of interactions between the PA and the metal [16]. Furthermore, CMRs require extremely high specific surface area catalytic materials for membranes to efficiently behave during operation [17]. New routes to homogenously incorporate fine metal NPs across TFC membranes were therefore required.

In order to tackle these issues, hybrid thin film composite (TFC) catalytic membranes were for the first time synthesized and characterized in this work. Catalytic silver metal nano-materials were templated across sub nano-porous scaffolds, called Metal Organic Frameworks (MOFs) and incorporated across the top surface of poly(amide) (PA) TFC membranes by controlled solubilization and diffusion at the PA surface during interfacial polymerization. These novel materials offer high catalytic behaviours due to the nano-structure of the metal nano-particles reduced within the metal organic framework template, forming unique hierarchical sub-100 nm hybrid nano-structures. The catalytic properties of the materials were found to be highly stable and these ultra-thin but yet dense membranes were able to simultaneously degrade chemicals, filter low molecular weight molecules and partially reject salts – opening the door for the first time to the design of nano-scale texturation of TFC membranes for advanced fouling control and chemical reactions. The impact of the hybrid structure metal loading, the size of the nano-structures and the homogeneiety of the thin films were evaluated for the Fenton-like heterogeneous catalytic degradation of a common waste pesticide materials, 2,4-Dichlorophenoxyacetic acid (2,4-D), contained in agricultural wastewater.