Polymer-assisted modification of metal-organic framework MIL-96 (Al): influence on particle size, crystal morphology and perfluorooctanoic acid (PFOA) removal

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. 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 48 where its benign effects on water quality are noted. 49 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, 50 HPAM must be anionic to initiate particle aggregation. In addition, PFOA adsorption performance by the modified material was benchmarked with a commercial AC to understand the interaction mechanisms better.


4
Of the treatments available for PFCs removal from water, 18 adsorption is extensively used given its simplicity, relatively lower cost and high selectivity. 19 Studied adsorbents range from activated carbon (AC), 20,21 minerals, 22,23 magnetic nanoparticles, 24 polymers, 25 resins, 26 functionalized cellulose 27 and metal-organic frameworks (MOFs). [28][29][30][31][32] However, except for MOFs, most adsorbents only exhibit between low and moderate PFOA uptake compared to the industrial standard of AC (>90% removal, 3M Company, USA). 33 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. 34 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. 35 Although various factors are involved in optimal bed design, lowering the pressure drop can be easily achieved by increasing the adsorbent particle size. 36 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 37 such as initiation solvents, 38,39 coordination modulators or capping agents, [40][41][42] surfactants, 43 and hard templates 44 or through synthetic techniques using pyrolysis, 45 ultrasound 46 and microreactor. 47 Some complexities regarding the respective methods are 5 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. 37 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 48 where its benign effects on water quality are noted. 49 Finally, a nontoxic 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, 50 HPAM must be anionic to initiate particle aggregation. In addition, PFOA adsorption performance by the modified material was benchmarked with a commercial AC to understand the interaction mechanisms better.  Desorption. Based on the result from the desorption solvent screening, the solvent composition was adjusted to get a mixed methanol/ water (3:1, v/v) stabilized with 10 mM AA buffer solution. To study the influence of NaCl on the desorption efficiency, four desorption solvents were prepared with varying NaCl concentrations namely: RS1 (no salt), RS2 (10 mM NaCl), RS3 (50 mM NaCl) and RS4 (500 mM NaCl). After 72 hours of adsorption, the adsorbent was recovered by filtration and placed respectively in 20 mL desorption solvent containing the above NaCl concentrations to be continuously stirred for another 72 hours at room temperature. Sample preparation for NMR analysis followed the steps mentioned previously.
Hydrothermal Synthesis of MOF. MIL-96 was synthesized according to the original method described previously 51 whereas the protocol for polymer addition was adapted from another study. 52

Results and Discussion
SEM. For MIL-96, HPAM is proven effective in changing the MOF particle morphology as observed in Figure 1-3. Generally, starting from the normally reported small hexagonal bipyramidal shaped (HB) crystals, 53 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 PXRD measurements, the apparent HS morphology was revealed to be a hybrid form of two elongated hexagonal pyramids seamlessly connected at the long axis. 54 It was reported that the hydrothermal synthesis conditions for MIL-96 must be strictly controlled given the possibility of forming two larger pore Al trimesates, MIL-100 and MIL-110, in the same reaction system despite all having distinct crystal structures. MIL-100 is the kinetically stable product which forms at short reaction times and low pH condition whereas the thermodynamically favored MIL-96 can be obtained after longer reaction times. MIL-110, on the other hand, crystallizes in a much more acidic or basic medium. 55 Their corresponding crystal morphologies have also been solved with MIL-96, MIL-100 and MIL-110 presenting a flat hexagonal, an octahedral and a hexagonal needle-like habits respectively. 56 Therefore, the influence of HPAM on the crystal morphology agrees well with the stability order of these phases; MIL-96 > MIL-110 > MIL-100.
The addition of HPAM seems to play multiple roles in the crystal elongation phenomenon, firstly as a competing modulator. Since the pKa value for HPAM (4.5) 57 is similar to that of TMA (pKa1 = 3.51, pKa2 = 3.89, pKa3 = 4.70), there will be competition with the linker to form coordination bonds with available Al. As a result, nucleation rate slows down and permits slow growth of large MOF crystals. 58 Secondly, the amine (-NH2) groups in both HPAM and PAM increase the basicity of the reacting solution, providing ideal conditions for the formation of MIL-110 ascribed by the existence of HR crystals. HPAM is a copolymer composed of PAM and Na-PAA. To identify the dominant functional groups responsible to the formation of HR, MIL-96 was synthesized using equivalent concentrations of PAM and Na-PAA separately.
Before the reaction, the solutions were very acidic upon addition of PAM (pH = 2), Na-PAA (pH = 2.4) and HPAM (pH =2.2). Such acidic environment increases the likelihood of -NH2 groups protonating to form primary ammonium ions (-NH3 + ) in preference to the ionization of the -COOH groups on the TMA due to higher pKa values. It is therefore expected to see the -NH2 groups contained in HPAM and PAM to govern the crystal elongation mechanism, transforming the HB shape into the HR habit more so than using Na-PAA.
Following the addition of 20 mL Na-PAA, it is unclear what drives the formation of new hexagonal lumps (HL) with some truncated and larger in dimension among the HR. What might have happened is, from the instant of mixing Na-PAA in water, the dissociated Na + cations may end up competing with the Al 3+ species to form bonds with the linker, causing deceleration in the crystal growth and altering the original template. Another factor that may have undermined Na-PAA's influence is due to its low molecular weight (~2100 g/mol). By substituting higher molecular weight PAA for the synthesis, its influence toward crystal elongation may be          Moreover, with a higher quantity of added polymers, the subtle width broadening of the strongest peak corresponding to the (102) crystal face implies a slight decrease in particle size.
In addition, the relative Bragg peaks of (200) and (201) also become broader and less intense compared to the original MIL-96, supporting the minor reduction in particle size and crystallinity.
FTIR. FTIR spectroscopy was used to understand PFOA adsorption mechanism onto MIL-96-RHPAM2 and the linker interaction with added polymer ( Figure S5). Detailed analysis of the IR peaks is displayed in Table 1. The collected spectra of spent MIL-96-RHPAM2 in Figure   8 confirms that PFOA was adsorbed through electrostatic interaction with the MOF. A previous study also inferred the increased intensities of vs to vas (COO -) after PFOA adsorption in relation to an inner-sphere complexation with the metal centers. In this case, the carboxylate head groups of PFOA may have formed covalent bond to the surface of MIL-96-RHPAM2. 61  BET. The use of PAM, Na-PAA and HPAM for this particle size modification led to a significant reduction in the apparent surface area (SABET), see Figure 9. The pore size distribution plot in Figure S6 clearly proves complete pore blocking when HPAM was used, asserting its  To gain a detailed insight in the PFOA adsorption kinetics for MIL-96-RHPAM2, vapor phase adsorption experiments were conducted with a hydrocarbon analogue to PFOA; n-octane (C8H18) (Figure S11 -S12). The fundamental differences in liquid-phase adsorption and vapor adsorption are acknowledged but, in both cases, the kinetic rate is nonetheless similar (denoted by the proximity of k2 = 2 × 10 -4 g/mg.min). Vapor phase adsorption time of 1000 minutes is still insufficient to realize an equilibrium adsorption state. The BET result shows that although increasing the HPAM content increases the particle size ( Figure 10 and Table 2), it has compromised the material's porosity. In turn, there is an increase in the diffusion path length inside the adsorbent that hindered effective diffusion and prolonging the time required to reach equilibrium. 68 It may appear that the approach outlined here has created another problem but, there is a future opportunity to retain the beneficial effect provided by HPAM by tuning MIL-96 adsorption capacity with temperature, taking into account the high thermal stability of HPAM 69 and the flexibility of changing, increasing, its pore size. 70 20 Figure 10. Hydrated particle size distribution (diameter in µm) of polymer-modified MIL-96 in pure water by laser light diffraction. The particle size ranges shown reflect the true size as particles were pre-sonicated prior to the laser light diffraction measurement.  In comparison to the reported kinetic data for PFOA removal by several other adsorbents, MIL-96-RHPAM2 can adsorb PFOA at loading levels on a par with other adsorbents, but the extremely slow uptake would be a significant drawback for its practical use. Relative to other adsorption studies displayed in Table 3, the PFOA concentration used in this work is the highest. regeneration and economical reuse of the spent adsorbents is critically important. Although thermal regeneration is preferably used by carbon manufacturers, adsorbed PFCs are not easily removed from the carbon surfaces by decomposition as C-F is the strongest single bond in organic chemistry after B-F, Si-F and H-F. While the next best alternative is to use higher regeneration temperatures to break the C-F bond, this often results in high carbon losses. 18 To develop an efficient PFOA regeneration method from spent MOF, the nature of adsorption processes must first be understood. An earlier report on PFOA adsorption by MOF had confirmed the electrostatic interaction between the cationic MOF species (Al 3+ ) with the anionic carboxylate groups (COO -) of PFOA as the main binding mechanism, supplemented by H-bonding and hydrophobic interaction to a lesser extent. 30 So, with electrostatic interaction being the most prevalent, it was decided to trial saline eluents to evaluate their destabilization effect on the electrostatic PFOA adsorption complex.
The positively charged Al 3+ center of MIL-96 is surmised to have a strong interaction with the anionic carboxylate groups (COO -) of PFOA. Contrary to expectations, increasing the eluent ionic strength has minimal effect on PFOA desorption by MIL-96-RHPAM2 22,23 except for a slight drop at the highest tested salt concentration (500 mM NaCl). Such salt concentration may be sufficiently high enough to reduce the electrostatic forces between the Al 3+ center and the negatively charged PFOA molecules resulting from the competitive PFOA adsorption with the background ions (Na + and Cl -). 61 However, due to the well-established acid-base and electrostatic bonds between the Al 3+ , carboxylate (COO -), hydroxyl (OH), NH2 and NH3 + functional groups on the MOF, the driving force to establish new bonds with the incoming Na + and Clions is not as favorable, hence, not accommodating the desorption of adsorbed PFOA molecules into the eluent phase.
Apart from the Al 3+ center being a major MOF binding site, it is probable that the NH2, which will also exist in a protonated form as a NH3 + , contained in HPAM also attaches to the anionic carboxylate of PFOA via strong electrostatic adsorption forces. 29,71 In addition, the van der Waals interaction between the carbon chain of PFOA (8 carbons on the PFOA) and the main skeleton (CH2-CH)n of HPAM may also contribute to the adsorption process. Altogether, these explanations for the incomplete desorption of PFOA from aminated adsorbents such as MIL-96-RHPAM2 are in line with the obtained result from batch desorption studies. Figure 12 illustrates the nuanced NaCl influence in the eluent to the PFOA desorption performance of MIL-96-RHPAM2. Across the tested salinity window, the highest PFOA fraction desorbed only reached 77% but not full recovery, providing future research opportunities to clarify the best solvent/ salt systems capable of complete PFOA desorption. Descriptions on the possible PFOA adsorption mechanisms onto MIL-96-RHPAM2 can be found in Figure S14 (Scheme S1) and Figure S15 (Scheme S2) for the pristine form.
Although PFOA desorption efficiency from AC is inferior to MIL-96-RHPAM2 at lower ionic strengths, AC's desorption efficiency is improved at the highest ionic strength tested here.
Similar to MOF, PFOA adsorption onto AC shares similar adsorption mechanisms, 22,72 confirming the importance of surface chemistry on AC. There is a clear correlation with a marked increase in PFCs sorption with more availability of basic groups in the adsorbents.
Thus, to enhance the adsorption process, an abundance of hydroxyl and carboxyl oxygencontaining functional groups on the AC surface will then interact with the PFOA anions via acid-base/electrostatic interactions. 73 Along the same lines, the oxygen-containing basic and 25 acidic surface groups on AC may form H-bond interactions with the amphoteric COOH functional groups within PFOA. An increase in the eluent ionic strength can potentially weaken these PFOA to AC H-bonding interactions by the Na + and the Clions shielding the electrostatic interactions whilst also directly competing for the H-bonding sites. While these results show that NaCl was not an effective PFOA desorption facilitating agent, only catalytic degradation has so far offered superior performance. 19,74

Conclusions
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. Incorporating HPAM into the MIL-96 structure resulted in the introduction of amine (-NH2) functionality, which can form a primary ammonium species (-NH3 + ) that in turn led to improved PFOA adsorption capacity relative to original pristine form, while also promoting higher PFOA affinity compared to AC.
Higher concentrations of NaCl in the eluent solvents did not yield enhanced PFOA desorption from both spent adsorbents, MIL-96-RHPAM2 (maximum PFOA desorbed was 77%) and AC (74%), primarily due to the strong PFOA adsorption to both adsorbents.

Associated Content
Supporting Information. SEM images of HPAM-modified MIL-101 (Cr), IR spectra analysis of HPAM-treated linker, data pertaining to sample's pore size distribution, TGA-MS, batch adsorption, n-octane vapor sorption study, particle size measurements, elemental analysis, and illustration of PFOA adsorption mechanisms by MIL-96-RHPAM2 are included.