Elsevier

Chemosphere

Volume 262, January 2021, 128072
Chemosphere

Polymer-assisted modification of metal-organic framework MIL-96 (Al): influence of HPAM concentration on particle size, crystal morphology and removal of harmful environmental pollutant PFOA

https://doi.org/10.1016/j.chemosphere.2020.128072Get rights and content

Highlights

  • Addition of HPAM during MIL-96 synthesis facilitated particle size enlargement.

  • HPAM also functions as a crystal shape modifying agent.

  • HPAM-grafted MIL-96 showed improvement in PFOA uptake quantity.

Abstract

A new synthesis method was developed to prepare an aluminum-based metal organic framework (MIL-96) with a larger particle size and different crystal habits. A low cost and water-soluble polymer, hydrolyzed polyacrylamide (HPAM), was added in varying quantities into the synthesis reaction to achieve >200% particle size enlargement with controlled crystal morphology. The modified adsorbent, MIL-96-RHPAM2, was systematically characterized by SEM, XRD, FTIR, BET and TGA-MS. Using activated carbon (AC) as a reference adsorbent, the effectiveness of MIL-96-RHPAM2 for perfluorooctanoic acid (PFOA) removal from water was examined. The study confirms stable morphology of hydrated MIL-96-RHPAM2 particles as well as a superior PFOA adsorption capacity (340 mg/g) despite its lower surface area, relative to standard MIL-96. MIL-96-RHPAM2 suffers from slow adsorption kinetics as the modification significantly blocks pore access. The strong adsorption of PFOA by MIL-96-RHPAM2 was associated with the formation of electrostatic bonds between the anionic carboxylate of PFOA and the amine functionality present in the HPAM backbone. Thus, the strongly held PFOA molecules in the pores of MIL-96-RHPAM2 were not easily desorbed even after eluted with a high ionic strength solvent (500 mM NaCl). Nevertheless, this simple HPAM addition strategy can still chart promising pathways to impart judicious control over adsorbent particle size and crystal shapes while the introduction of amine functionality onto the surface chemistry is simultaneously useful for enhanced PFOA removal from contaminated aqueous systems.

Graphical abstract

Schematic mechanism for size and morphology-tunable synthesis of MIL-96 with HPAM.

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Introduction

Adequate, clean and safe water supply is indispensable to healthy wellbeing of humans and living organisms (Tiedeken et al., 2017). Unfortunately, our surface and ground waters continue being polluted by industrial waste streams containing toxic and persistent chemicals including perfluorinated compounds (PFCs). PFCs have recently received much attention because they are ubiquitous contaminants in water, wildlife, and humans. The most commonly found PFCs in surface waters are perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) (Rivera-Utrilla et al., 2013). Surprisingly, thus far, there is no mutually agreed international drinking water standard for PFOA and PFOS, but the United States Environmental Protection Agency have recommended a strict combined concentration limit of 70 ng/L (Cordner et al., 2019). Recent analysis of global water samples even revealed concentrations exceeding the provisional threshold (Andersson et al., 2019; Feng et al., 2020; Li et al., 2018; Lorenzo et al., 2019; Scher et al., 2018; Valsecchi et al., 2017).

Although most fluorochemical manufacturers have ceased their production of PFOA and PFOS in the early 2000s, this measure does not address the legacy pollution caused by these compounds (Wang et al., 2017). The ubiquitous presence and long-range mobility of PFCs is attributed to the high chemical stability of their C–F bonds (bond dissociation energy, D°298 K = 485 kJ/mol) (Richardson, 2008), rendering them suitable as ingredients to make fluoropolymers (McNamara et al., 2018), fire-fighting foams (Cordner et al., 2019), and stain-repellent products (Richardson, 2008). Typical contamination hot spots then include the vicinity of industrial areas, military training sites, airports and wastewater treatment plants (Hu et al., 2016). Moreover, exposures to PFCs adversely affect body immunity (Sunderland et al., 2019), neurological functions (Piekarski et al., 2020), reproductive systems (Rashtian et al., 2019), as well as giving strong correlations to coronary heart disease (Huang et al., 2018), and cancer (Mancini et al., 2020) among many others. As a result, there is an urgent need to investigate efficient materials for PFCs remediation from contaminated water.

Of the treatments available for PFCs removal from water (Ateia et al., 2019), adsorption is extensively used given its simplicity, relatively lower cost and high selectivity (Du et al., 2014). Studied adsorbents range from activated carbon (AC) (Du et al., 2015; Xu et al., 2020), minerals (Shih and Wang, 2013; Wang and Shih, 2011), magnetic nanoparticles (Badruddoza et al., 2017), polymers (Xiao et al., 2017), resins (Yang et al., 2018), functionalized cellulose (Ateia et al., 2018), graphene oxide-silica hybrid (Ali et al., 2020), and metal-organic frameworks (MOFs) (Chen et al., 2016; Jun et al., 2019; Liu et al., 2015b; Sini et al., 2018, 2019). However, except for MOFs, most adsorbents only exhibit between low and moderate PFOA uptake compared to the industrial AC standard (>90% removal, 3 M Company, USA) (Vecitis et al., 2009).

MOFs refer to a novel class of porous materials made from organic bridging linkers and metal atoms. Distinguishing MOF features include their high surface areas, ready structure tunability and reusability via simple washing procedures which means that these materials are seen as a promising future substitute for AC (Xue et al., 2016). These features are befitting to resolve long-standing problems in PFOA removal by other conventional wastewater treatments especially given their dilute levels existence, low molecular weight, and high transportability (Chen et al., 2020; Gagliano et al., 2020). Further to that, in commercial water purification processes, adsorbents are generally deployed into a packed bed for continuous filtration. Fluids flowing through some packed beds may at times result in high pressure drops. This directly increases the energy requirement to pump the fluid through the bed and decreases the operational lifetime of the pumps, impacting on the economics of the process (Gebald et al., 2019). Although various factors are involved in optimal bed design, lowering the pressure drop can be easily achieved by increasing the adsorbent particle size (Mandić et al., 2017). Larger particles also facilitate easier recovery in between the washing cycles.

Size and crystal morphology of MOFs can be precisely controlled via the presence of additives (Seoane et al., 2016) such as initiation solvents (Liu et al., 2015a; Long et al., 2011), coordination modulators or capping agents (Guo et al., 2018; Han et al., 2015; Wang et al., 2013), surfactants (Falcaro et al., 2011), and hard templates (Yan et al., 2015) or through synthetic techniques using pyrolysis (Lee and Kwak, 2017), ultrasound (Vaitsis et al., 2019), and microreactor (Watanabe et al., 2017). Some complexities regarding the respective methods are usage of harmful organic solvents, use of acidic or basic modulators that increase risk of safety hazards and difficulty in isolating the hard templates after preparation. Whilst these three MOF synthetic techniques can only be carried out with specialized instrument, a common limitation of these approaches is the formation of predominantly nanoparticle products. The work reported here uses a surfactant-assisted method which serves as a multifaceted approach; functioning as a nanoreactor, a capping agent as well as a molecular template (Seoane et al., 2016).

The current study highlights a new one-pot surfactant-based synthesis method to control particle size and morphology of MOF crystals by utilizing a synthetic polymer, hydrolyzed polyacrylamide (HPAM). HPAM is a low cost and water-soluble polymer widely used for enhanced oil recovery in petroleum drilling processes, as well as being used as a flocculant for wastewater treatment (Rellegadla et al., 2017) where its benign effects on water quality are noted (Edomwonyi-Otu and Adelakun, 2018). Finally, a non-toxic and hydrothermally stable aluminum-based MOF, MIL-96 (Al) was chosen to be tested for liquid phase adsorption of PFOA. Given the positive surface charge for MIL-96 (Azhar et al., 2018), HPAM must be anionic to initiate particle aggregation. Such polymer-MOF composite strategy may also improve separation selectivity (Chang et al., 2020). In this case, the PFOA adsorption performance by the modified material was benchmarked with a commercial AC to understand the interaction mechanisms better.

Section snippets

Chemicals

Polyacrylamide (PAM) (FLOPAM FA 920 SH, non-ionic, molecular weight (Mw) = 10–12 MDa) and partially hydrolyzed polyacrylamide (HPAM) (FLOPAAM FP 3630 S, anionic, 25–35 mol% hydrolyzed, Mw = 18 MDa) were supplied by SNF Floerger (France). Polyacrylic acid sodium salt (PAA, average Mw∼2100), 2,2,2-trifluoroethanol (TFE, 99.5%), perfluorooctanoic acid (PFOA, 95%), ammonium acetate (AA, 99.0%), trimesic acid (TMA, 95%), sodium chloride (NaCl, 99%) were purchased from Sigma Aldrich. Deuterium oxide

SEM

For MIL-96, HPAM is proven effective in changing the MOF particle morphology as observed in Fig. 1a – 1d. Generally, starting from the normally reported small hexagonal bipyramidal shaped (HB) crystals (Benzaqui et al., 2017), higher HPAM concentrations facilitate incremental increases in the aspect ratio (length: diameter), transitioning from hexagonal spindles (HS) to elongated hexagonal rods (HR), particularly when 20 mL HPAM was used (MIL-96-RHPAM2). Upon closer inspection from out-of-plane

Conclusions

In summary, the addition of an anionic polymer (HPAM) to a cationic aluminum-based MOF (MIL-96) during the hydrothermal synthesis stage allowed the formation of larger MOF crystals with well-defined crystal habits. Taking advantage of HPAM being a low cost and an environmentally friendly additive, a HPAM-modified MIL-96 (MIL-96-RHPAM2) showed promising particle size growth from 3.2 μm to 10.4 μm as well as the ability to control the crystal morphology using this simple modification procedure.

CRediT author statement

Luqman Hakim Mohd Azmi: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Visualization. Daryl R. Williams: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision. Bradley Ladewig: Conceptualization, Methodology, Resources, Data curation (Zenodo), Writing - review & editing, Visualization (Graphical Abstract and Adsorption Mechanism Schemes), Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Luqman Hakim Mohd Azmi gratefully acknowledges the financial support provided by the sovereign wealth fund of Malaysian Government, Yayasan Khazanah for his PhD studies. The authors also thank Prof. Paul D. Lickiss, Dr. Pavani Cherukupally, Elwin Hunter-Sellars, Dr. Shanxue Jiang and Tingwu Liu for useful discussion on material design and characterization.

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