Mixed Matrix PVDF Membranes With in Situ Synthesized PAMAM

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Environmental Science & Technology

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Mixed Matrix PVDF Membranes With In Situ Synthesized PAMAM Dendrimer-like Particles:

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A New Class of Sorbents for Cu(II) Recovery from Aqueous Solutions by Ultrafiltration

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Madhusudhana Rao Kotte1, Alex T. Kuvarega2, Manki Cho1,

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Bhekie B. Mamba2 and Mamadou. S. Diallo1,3*

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Graduate School of Energy, Environment, Water and Sustainability (EEWS)

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Korea Advanced Institute of Science and Technology (KAIST)

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Daejeon, Republic of Korea

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Department of Applied Chemistry

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University of Johannesburg

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Johannesburg, Republic of South Africa

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Environmental Science and Engineering

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Division of Engineering and Applied Science

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California Institute of Technology

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Pasadena, CA, USA

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*Corresponding Author: Prof. Mamadou S. Diallo

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E-mail: [email protected] and [email protected]

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Phone: 011 1 626 578 0311 (USA)

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Fax: 011 626 585 0918 (USA)

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1 ACS Paragon Plus Environment


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Abstract

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Advances in industrial ecology, desalination and

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resource recovery have established that industrial

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wastewater, seawater and brines are important

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and largely untapped sources of critical metals

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and elements. A Grand Challenge in metal

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recovery from industrial wastewater is to design

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and synthesize high capacity, recyclable and

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robust chelating ligands with tunable metal ion

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selectivity that can be efficiently processed

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intolow-energy

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modules. In our efforts to develop high capacity chelating membranes for metal recovery from

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impaired water, we report a one-pot method for the preparation of a new family of mixed matrix

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polyvinylidene fluoride (PVDF) membranes with in situ synthesized poly(amidoamine) [PAMAM]

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particles. The key feature of our new membrane preparation method is the in situ synthesis of

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PAMAM dendrimer-like particles in the dope solutions prior to membrane casting using low-

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generation dendrimers (G0 and G1-NH2) with terminal primary amine groups as precursors and

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epichlorohydrin (ECH) as crosslinker. By using a combined thermally-induced phase separation

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(TIPS) and non-solvent phase separation (NIPS) casting process, we successfully prepared a new

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family of asymmetric PVDF ultrafiltration membranes with (i) neutral and hydrophilic surface layers

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of average pore diameters of 22-45 nm, (ii) high loadings (~48 wt%) of dendrimer-like PAMAM

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particles with average diameters of ~1.3-2.4 μm and (iii) matrices with sponge-like microstructures

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characteristics of membranes with strong mechanical integrity. Preliminary experiments show that

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these new mixed matrix PVDF membranes can serve as high capacity sorbents for Cu(II) recovery

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from aqueous solutions by ultrafiltration.

separation

materials

and

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Introduction Metals are key building blocks of the sustainable products, processes and industries of the 21st

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century.1-3 They are used to fabricate the critical components of numerous products and finished goods

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including airplanes, automobiles, smart phones, and biomedical devices. There is a growing awareness

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that the development and large-scale implementation of the clean and renewable energy technologies

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of the 21st century will also require sizeable amounts of technology metals.1-3 In addition to rare earth

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elements (REEs) and platinum group metals (PGMs), significant amounts of copper, silver, cobalt,

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nickel and gold will be needed to build (i) solar cells, (ii) wind turbines, (iii) electric vehicles, and (iv)

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energy-efficient lighting.3 Copper (Cu) is one of the most widely utilized technology metals.4 It has

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become the main metal used to fabricate electric wires and computer interconnects due to its excellent

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electrical conductivity. It is also widely employed as construction material because of its toughness,

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ductility and high corrosion resistance.4 Cu is a key component of high-efficiency thin film solar cells5

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and a leading metal for the preparation of catalysts for the electrochemical reduction of CO2 to liquid

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fuels and valuable products.6-7 Stresses in the global market of REEs have brought the availability and

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supply of technology metals such as Cu to the forefront in the United States and other industrialized

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countries.5 It is worth mentioning that China alone consumed 40% of the Cu produced by mining in

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2011 compared to 6% in 2000.3 This has led Lifton3 to posit that “while ample resources exist, it may

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not be possible to increase Cu production to meet the worldwide demand” if the current rate of

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increase of Cu consumption by China holds. Because there is a significant time lag between the

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commissioning of a new mine and the extraction/processing of a virgin ore, future shortages of Cu

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cannot be addressed by just opening new mines. Thus, there is a great need for new and more versatile

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strategies to rapidly augment the supply of critical metals such Cu as needed.

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Advances in industrial ecology, desalination and resource recovery have established that industrial

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wastewater, seawater and desalination plant brines are important and largely untapped sources of

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critical metals and elements.8-11 Industrial waste streams with significant amounts of dissolved Cu

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include (i) mining effluents12, (ii) spent electroplating liquors13 and (iii) chemical mechanical

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planarization (CMP) wastewater from semiconductor fabs.14 Allen11 has shown that there are no

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fundamental thermodynamic limitations (i.e. a large entropy barrier) in recovering metals from

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industrial liquid wastes. Combinations of physical-chemical processes (e.g. coagulation-precipitation,

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ion-exchange, sorption, etc) and membrane filtration (e.g. nanofiltration) have traditionally been used



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to treat industrial wastewaters contaminated by heavy metal ions.15-17 However, these processes are not

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very effective at recovering dissolved metal ions such as Cu(II) from liquid waste streams. Porous

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membranes with embedded or surface-grafted chelating polymers have emerged as promising sorbents

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for the recovery of dissolved Cu from aqueous solutions using microfiltration (MF) and ultrafiltration

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(UF).18-20 Bessbousse et al.18 showed that a 28-cm2 film (70 ± 5 m in thickness) of crosslinked

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polyvinyl alcohol (PVA) [molar mass Mw = 124-168 kDa] with in situ blended polyethylenimine (PEI)

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[Mw = 70 kDa] can bind 30 mg of Cu(II) per g of dry film following the completion of a dead-end

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filtration run (pressure = 3 bar) of an aqueous solution (400 cm3) with a Cu concentration of 100

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mg/L. Yoon and Kwak19 described the preparation of a new family of membrane absorbers by grafting

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hyperbranched poly(amidoamine) [HPAMAM] polymers to the surface of a poly(tetrafluoroethylene)

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[PTFE] MF membrane (FluoroporeTM FHUP 04700) from Millipore. The authors reported that the

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HPAMAM-grafted PTFE membranes can bind 1.42 g of Cu(II) per m2 of dry membrane following the

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completion of a dead-end filtration run (pressure = 0.25 bar) of an aqueous solution (50 mL) with a Cu

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concentration of 50 mg/L. Zhang et al.20 described the preparation of a new family of membrane

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absorbers by covalent grafting of PAMAM dendrons (G0-G5) with terminal NH2 groups to the outer

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surfaces of bromoethylated poly(2,6-dimethyl-1,4-phenylene oxide) [BPPO] hollow fiber membranes

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(HFMs). They reported that a G3-NH2 PAMAM-BPPO HFM absorber can bind ~37 mg of Cu(II) per

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g of dry membrane following immersion of a 50-mg sample of hollow fibers in an aqueous solution (8

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mL) with a Cu concentration of ~12 mg/L at room temperature for 72 h.

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Although the previous investigations have established that a crosslinked PVA membrane with

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embedded PEI macromolecules, a HPAMAM surface-grafted PTFE membrane and PAMAM

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dendronized HFMs can extract Cu(II) from aqueous solutions, their viability as building blocks for

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low-pressure UF/MF modules for Cu recovery from industrial wastewater remains to be established.

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The crosslinked PVA-PEI membranes have a very low water flux, i.e. ~3.5 L/m2/hr (LMH) at 3 bar.18

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In contrast, the HPAMAM surface-grafted PET membrane shows the high water flux (635 ± 9 LMH)

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of typical MF membranes.19 However, the procedures used to activate the surface of the PTFE

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membranes prior to HPAMAM grafting are lengthy and tedious, i.e. 2 days of hydrazine-assisted UV

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amination in a specially designed tubular quartz reaction chamber followed by 4-hr immersion in a 1.0

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mM solution of 2,4,6-trichlotriazine in chloroform.19 Similarly, the preparation of a generation 1 (G1)

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PAMAM-BPPO HFM requires 21 days to complete including a 7-day amination step followed by


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esterification and Michael addition of amine with methyl acrylate (14 days).20 In our efforts to develop

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high capacity chelating membranes, we report a one-pot method for the preparation of a new family of

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mixed matrix polyvinylidene fluoride (PVDF) membranes with in situ synthesized PAMAM particles.

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Preliminary experiments show that these PVDF-PAMAM membranes can serve as high capacity

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sorbents for Cu(II) recovery from aqueous solutions by low-pressure UF.

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Experimental Methods and Procedures

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PVDF (Kynar 761) was provided by Arkema (King of Prussia, PA, USA). G0-NH2 and G1-NH2

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PAMAM dendrimers were purchased as methanol solutions (~34 wt%) from Dendritech Inc, USA.

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Epichlorohydrin (ECH) was purchased from Sigma-Aldrich. Triethyl phosphate (TEP), ethanol and

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nitric acid (60 wt% HNO3) were purchased from Daejung Chemicals (South Korea). Hydrochloric acid

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(12 M HCl) was purchased from Junsei (South Korea). Sodium hydroxide (NaOH pellets) and

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copper(II) nitrate trihydrate (ACS purus grade) were purchased from Sigma-Aldrich. A standard

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solution of copper (Cu) [10 mg/L in 5wt% HNO3] (Multi-element calibration standard-2A) was

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purchased from Agilent Technologies. All chemicals were used as received. All aqueous solutions

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were prepared using Milli-Q deionized water (DIW) with a resistivity of 18.2MΩcm and total organic

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content < 5 ppb.

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Membrane Preparation and Characterization

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The membrane preparation and characterization procedures were adapted from Kotte et al. 21-23 The

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cross section and surface morphology of each membrane and the average thickness and pore diameter

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of the membrane surface layers were characterized using field emission scanning electron microscopy

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(FESEM) and N2 adsorption permporometry, respectively. The sizes of the embedded PAMAM

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particles of each membrane were characterized using FESEM and dynamic light scattering (DLS).

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Membrane surface composition, hydrophilicity and charge were characterized by attenuated total

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reflectance (ATR) Fourier transform infrared (FT-IR) spectroscopy, x-ray photoelectron spectroscopy

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(XPS), contact angle and zeta potential measurements, respectively. The supporting information (SI)

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provides a detailed description of all membrane preparation and characterization procedures.

Chemicals and Materials

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Copper Filtration and Binding Studies

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The Cu(II) filtration and binding experiments were conducted on a custom-made cross-flow UF system

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with an active filtration area of 24 cm2. The filtration cell (l7.62 cm in length; 2.54 cm in width and 0.3

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cm in depth), pump head, reservoir and tubing were built using Teflon and polyvinyl chloride (SI

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Figure S1) to eliminate metal ion sorption onto the system components. The flow rate was maintained

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at ~ 1.7 L / min with a crossflow velocity of ~37.2 cm/s. Each filtration experiment consisted of four

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steps. The pH of the feed water was adjusted with a solution of 0.1 N HCl or 0.1 N NaOH as needed.

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Each membrane was first compacted by running DIW for 1 hour at a pressure of 3 bar. The pressure

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was then reduced to 2 bar and aliquots of permeate were collected every 5 minutes for 1 hour to

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estimate membrane water flux. Following this, a constant-pH solution was pumped through each

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membrane and aliquots of permeate were collected every 5 minutes for 30 minutes. After the

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completion of the constant-pH water flux measurements, a 2 L of a solution of Cu(II) [10 mg/L] at

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constant pH (3, 7 and 9) was pumped trough each membrane at 2 bar. In this case, permeate samples

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were collected every 5 minutes for 3 hours. Following the flux measurements, the permeate samples

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were poured back into the UF system feed tank (SI Figure S1) to keep the volume of the feed (2 L)

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constant; i.e. within 2%. The SI provides a detailed description of the experimental and data analysis

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procedures that were utilized in the Cu(II) binding experiments. Following the completion of the metal

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binding experiments, a sample of Cu(II) loaded PVDF membrane absorber with in situ synthesized

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PAMAM particles was characterized by FESEM, XPS and FT Raman spectroscopy.

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Results and Discussion

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A Grand Challenge in metal recovery from industrial wastewater is to design and synthesize high

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capacity, recyclable and robust chelating ligands with tunable metal ion selectivity that can be

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efficiently processed into low-energy separation materials and systems (e.g. ultrafiltration membrane

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absorbers and modules). In aqueous solutions and industrial wastewater, dissolved Cu is predominantly

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found as cationic species.14,24 Chelating agents are the most effective ligands for recovering cationic

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species from aqueous solutions. Metal ion complexation is an acid-base reaction that depends on

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several parameters including (i) ion size and acidity, (ii) ligand basicity and molecular architecture and

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(iii) solution physical-chemical conditions.24 The invention of dendrimers may be considered as

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significant milestone in ligand architecture and coordination chemistry.25 PAMAM dendrimers were

Membrane Preparation, Morphology and Bulk Properties


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the first class of dendrimers to be commercialized.25 They possess functional nitrogen and amide

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groups arranged in regular “branched upon branched” patterns, which are displayed in geometrically

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progressive numbers as a function of generation level. This high density of N and O donors make

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PAMAM dendrimers particularly attractive as high capacity and selective chelating agents for

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transition metal ions such as Cu(II).26-30 Diallo et al.31 have developed a dendrimer enhanced filtration

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(DEF) process that can recover Cu(II) from aqueous solutions using UF. Although higher generation

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PAMAM dendrimers (e.g. G4-NH2and-G5-NH2) have shown excellent potential as high capacity,

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selective and recyclable macroligands for Cu(II) recovery from aqueous solutions using DEF31 and

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dendronized PAMAM hollow fiber membranes26, they are expensive to produce due to the large

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number of synthetic and purification steps required to prepare such macromolecules. In our efforts to

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exploit the exceptional metal ion chelating capability of PAMAM dendrimers for Cu(II) recovery from

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aqueous solutions, we report the preparation of a new family of mixed matrix PVDF membrane

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absorbers with in situ synthesized particles using low-generation PAMAM dendrimers (G0-NH2 and

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G1-NH2) as precursors.

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A standard procedure for the preparation of mixed matrix membranes (MMMs) involves the dispersion

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of preformed micro/nanoparticles in a suitable polymer solution followed by membrane casting.

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However, the synthesis of PAMAM micro/nanoparticles will require the use of surfactant-stabilized

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inverse suspensions systems32-33 followed by tedious and lengthy purifications to produce the clean

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particles required for the preparation of high quality dope solutions for the synthesis of MMMs. Figure

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1A illustrates the one-pot method that was employed to prepare our new mixed matrix PVDF

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membranes including (i) dope preparation, (ii) in situ PAMAM particle synthesis and (iii) phase

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inversion casting. Figure 1B depicts the crosslinking reaction between a G1-NH2 PAMAM

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macromolecule and an ECH molecule in the membrane casting solution. Kotte et al.22 provide a

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detailed discussion of the reactions of ECH with the primary/secondary amine groups of branched

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polyethyleneimine (PEI). The selection of PVDF, ECH and G0/G1-NH2 PAMAM dendrimers as

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building blocks for our new membrane absorbers was motivated by several considerations. Firstly,

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PVDF is widely used as base polymer in the fabrication of commercial UF/MF membrane due to its

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high tensile strength, and thermal and chemical resistance.33 PVDF membranes can be prepared by

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phase inversion casting usingthermally-induced phase separation (TIPS) and/or non-solvent phase

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separation (NIPS).32 This provides many degrees of freedom for optimizing the microstructures of our

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new mixed matrix PVDF membrane absorbers by selecting the appropriate synthesis conditions. 7 ACS Paragon Plus Environment


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Secondly, we have learned to exploit the high reactivity of ECH toward functional macromolecules

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and oligomers containing primary and secondary amino groups to prepare a broad range of separation

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membranes and media.21-23, 31-32 Thirdly, the low-generation PAMAM dendrimers (G0 and G1-NH2)

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have (i) well defined compositions, (ii) low molecular weights (Mn of 517 and 1430 Da) and (iii) high

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density of functional N and O donors (~19meq/g) for metal ion complexation (SI Table S1). Moreover,

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the G0 and G1-NH2 PAMAM are much less expensive to produce and have the required primary

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amine groups (NH2) for in situ crosslinking with ECH21-23 in the dope solutions prior to membrane

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casting (Figure 1B). Two mixed matrix PVDF membranes with in situ synthesized PAMAM particles

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(MDP-G0 and MDP-G1) and a control (neat) PVDF membrane were prepared in this study. SI Table

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S2 lists the composition of the casting solution for each membrane. Table 1 lists the estimated

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compositions (on a dry basis) of the neat PVDF membrane and mixed matrix PVDF membranes that

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were prepared in this study. The recipes used to prepare our new mixed matrix membranes (MMMs)

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were adapted from our previous studies21-23 to achieve a high loading (~48 wt%) of in situ synthesized

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PAMAM particles with a degree of crosslinking (i.e. particle ECH wt%) of 40%.

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A TIPS and NIPS casting process was employed to prepare the mixed matrix PVDF-PAMAM

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membranes. For the characterization experiments, the membranes were prepared without a

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polyethylene terephthalate (PET) microporous support. Figure 2 shows FESEM cross-section

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micrographs of the neat PVDF membrane (Panels A and B), MDP-G0 membrane (Panels C and D) and

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MDP-G1 membrane (Panels E and F). Table 2 lists selected physicochemical properties of these

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membranes. Figure 2 and Table 2 indicate that the neat PVDF membrane exhibits an asymmetric

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structure with a dense skin (~15.0 m) and a matrix with a sponge like microstructure consisting of

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PVDF spherulites. Similarly, the MDP-G0 and MDP-G1 membranes are asymmetric with dense skins

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(~8-12 m) and matrices with sponge-like microstructures containing mixtures of PAMAM particles

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and PVDF spherulites (Figure 2). We attribute the sponge-like microstructures of the MMMs primarily

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to the crystallization-induced gelation process that occured during the TIPS process32 when the hot

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membrane casting solutions (80 ˚C) were cooled down to ambient temperature prior to immersion into

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the DIW bath. SI Figure S2 shows magnified FESEM micrographs (1000 X) of the top cross-sections

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of the MDP-G0 and MDP-G1 membranes. These micrographs confirm that the PAMAM particles are

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present in both the matrices and top surfaces of the MMMs. SI Figure S3 indicates that the top

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surfaces of theMMMs appear to be more porous than that of the neat PVDF membrane as illustrated by

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the corresponding SEM micrographs. The N2 permporometry measurements (SI Figures S4-S6 and 8 ACS Paragon Plus Environment

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Table 2) indicate that the skin layer of the MDP-G0 membrane has larger pore diameters (~27-45 nm)

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than those of the MDP-G1 membrane (~23-28 nm) and neat PVDF membrane (~13-17 nm). Figure 2

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and SI Figure S2 also show that the PAMAM particles are uniformly distributed throughout the cross-

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section of MMMs. We subsequently utilized the SEM analysis Image J software35 to extract estimates

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of the size ranges of the embedded PAMAM particles of the MMMs (SI Table S3). Table 2 indicates

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that the average diameters of PAMAM particles of the MDP-G0 and MDP-G1 membranes are,

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respectively, equal to ~1.5 and 2.3 m. To validate the FESEM particle size estimates, we carried out

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DLS measurements of dispersions of PAMAM particles obtained by dissolving the MMMs in TEP.

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These measurements confirm that the sizes of the PAMAM particles in the TEP dispersions (~1.4 to

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2.3 m) are comparable to the FESM particle size estimates (Table 2 and SI Figures S7 and S8).

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Membrane Surface Composition and Physicochemical Properties The FT-IR spectra corroborate the presence of PAMAM particles at the surfaces of the MDP-G0 and

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MDP-G1 membranes. Figure 3A shows that the mid IR spectra of both membranes exhibit four new

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peaks including (i) -OH and -NH stretching (3313 cm-1) from the amide, secondary amino or hydroxyl

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groups of the ECH crosslinked PAMAM particles; (ii) -C=O stretching (1640 cm-1) from the amide

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groups of the PAMAM particles; (iii) -NH bending (1540 cm-1) from the amide/amine groups of

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PAMAM particles and (iv) -C-N stretching (1270 cm-1) from the amine groups of PAMAM

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particles.21-23,36 The near IR spectra (Figure 3B) provide additional supporting evidence for the

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presence of PAMAM particles with -OH groups at the surfaces of these membranes including: (i) first

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overtone of -OH stretching vibrations (6914 cm-1), (ii) first overtones of –CH and –CH2 stretching

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vibrations (5783 cm-1) and (iii) combination of –NH2 stretching and bending vibrations (5115 cm-1)21-

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23

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groups at the surfaces of the mixed matrix PVDF membranes. SI Figure S9 shows that the atomic

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concentrations of oxygen (O1s) of the MDP-G0 and MDP-G1 membranes are, respectively, equal to

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2.71 and 3.81 wt%. It is worth mentioning that the concentrations of nitrogen (N1s) are significantly

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lower for both MMMs; i.e. 0.22 wt% for MDP-G0 and 0.47 wt% for MDP-G1. The zeta potential (ZP)

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measurements (Table 2) indicate that the PVDF-PAMAM membrane absorbers have neutral surface

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charges with ZP values respectively equal to 0.93 mV and 0.46 mV for the MDP-G0 and MDP-G1

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membranes compared to -5.9 mV for the neat PVDF. These results suggest that the ECH crosslinked

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PAMAM particles expose their OH groups at the surface of the mixed matrix PVDF membranes. We

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attribute this observation primarily to more favorable interactions between the PAMAM particles and

. The XPS experiments also corroborate the presence of PAMAM particles with high density of OH



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the non-solvent (DIW) during the coagulation phase of the membrane casting process (Figure 1A).

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This causes the ECH crosslinked PAMAM particles to migrate at the surface of the MMMs and expose

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their OH groups as they become incorporated in the membrane surface layers (SI Figure S2).

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Copper Filtration and Binding Studies The overall results of the characterization experiments indicate that the MDP-G0 and MDP-G1

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membranes are asymmetric with (i) neutral and hydrophilic surface layers of average pore diameters of

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23-45 nm and (ii) high loadings of in situ synthesized PAMAM particles (~48 wt%) containing ~9.0

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meq of N and O donors per g of dry membrane (Table 1). These membranes also exhibit the sponge-

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like microstructures (Figure 2) that are typically found in UF membranes with strong mechanical

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integrity.37 We carried out filtration experiments to evaluate the utilization of MDP-G0 and MDP-G1

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as membrane absorbers for Cu(II) recovery from aqueous solutions by low-pressure UF at 2 bar. Three

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key objectives of these experiments were to: 1) assess the effects of solution pH on Cu(II) uptake by

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the PVDF-PAMAM membrane absorbers, 2) evaluate the scaling potential of these membranes and 3)

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gain insight into the mechanisms of Cu(II) coordination with the N and O donors of their embedded

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PAMAM particles. SI Figure S10 shows the flux of DIW through the MMMs as a function of time at 2

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bar. The average water flux of the MDP-G0 and MDP-G1 membranes are, respectively, equal to ~ 427

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± 13 and 107 ± 4 LMH (SI Figure S10). In contrast, the neat PVDF membrane has a very low water

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flux; i.e. less than 3.0 LMH (data not shown). We attribute the higher flux of the MDP-G0 membrane

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to its lower contact angle (i.e. higher hydrophilicity) and the larger pore diameters of its surface layer;

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i.e. ~27-45 nm compared to ~22-28 nm for those of the MDP-G1 membrane. Figures 4-5, SI Figure

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S10 and SI Table S4-S5 summarize the results of the Cu(II) filtration and binding experiments at 2 bar

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and pH 3.0, 7.0 and 9.0. It is worth mentioning that the absence of Cu(OH)2 precipitates in the feed

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solution is consistent with the differences between the measured fluxes of the Cu(II) solutions for the

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MDP-G0 and MDD-G1 membranes. If there were Cu(II) precipitation in the feed solutions, the

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measured permeate fluxes at 2 bar would drop significantly (close to zero) due to the formation and

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buildup of micron-size Cu(OH)2 scales at the surface of the membranes. However, Figure 4 indicates

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that the permeate flux of aqueous solutions of Cu(II) through the MDP-G1 membrane initially

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decreases and then stabilize around a value of 60, 60 and 42 LMH at pH 3.0, 7.0 and 9.0 respectively.

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A similar trend (with a less pronounced initial flux decline at pH 9) is also observed for the MDP-G0

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membrane. In this case, the steady state permeate fluxes of the Cu(II) solutions are, respectively, equal

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to 395, 275 and 213 LMH at pH 9.0, 7.0 and 3.0 (Figure 4 and SI Table S4). These results are 10 ACS Paragon Plus Environment

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consistent with a flux decline mechanism caused by pore blockage due to Cu(II) binding to the

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embedded PAMAM particles of the MDP-G0 and MDP-G1 membranes. Figure 5 and SI Table S5

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show that Cu(II) sorption onto the MDP-G0 membrane reaches saturation during the course of the

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filtration run (3 hours) at pH 3.0, 7.0 and 9.0. In all cases, the MDP-G0 membrane binds less than 25%

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of the amount of Cu(II) in the feed solution with a binding capacity of ~46-52 mg of Cu(II) per mL of

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dry membrane (SI Table S5). In contrast, the MDP-G1 membrane can bind ~51 ± 3.6 mg of Cu(II) per

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mL of dry membrane at pH 9 without reaching saturation (i.e. with a mean percentage of bound copper

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of~99%) [SI Table S5]. At pH 3.0 and 7.0, the MDP-G1 membrane can bind 54-57 mg of Cu(II) per

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mL of dry membrane with mean percentages of bound copper of 75 and 82%, respectively.

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Characterization of a Cu(II) saturated PVDF-PAMAM membrane absorber by SEM, XPS and FT-

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Raman spectroscopy

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Interestingly, Figure 5 and SI Table S5 indicate that the mean percentage of bound copper for the

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MDP-G1 membrane (~99%) at pH 9.0 is significantly larger than that of the MDP-G0 membrane

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(~20%) even though both sorbents have equal concentrations of N and O donors (~9.0 meq/g) [Table

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1]. At the present time, we do not have a definite explanation for this observation. We attribute this to a

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dendritic effect in metal ion chelation30. This dendritic effect is a characteristic signature of higher

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generation PAMAM dendrimers as soft colloids with covalently bonded polymeric chains. This

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endows these macromolecules with the ability to serve as high capacity and selective macroligands that

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can form both coordination and inclusion complexes with metal ions such Cu(II)30.We hypothesize that

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the in situ synthesized PAMAM particles of the MDP-G1 membrane behave as high generation

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dendrimer-like particles (DLPs) that can bind Cu(II) through several mechanisms including (i)

326

coordination with their N and O donors and (ii) non specific binding to water molecules and/or

327

counterions trapped inside the DPLs.27-30,38-40 To gain insight into the mechanisms of Cu(II) binding to

328

our new PVDF-PAMAM membrane absorbers, a sample of Cu(II) saturated MDP-G0 membrane was

329

characterized by FESEM , FT Raman spectroscopy and XPS. The FESEM micrographs (Figure 6)

330

show that no solid copper precipitates were formed at the surface and inside the matrix of the Cu(II)

331

laden MDP-G0 membrane. In contrast, Zhang et al.20 reported the formation of Cu2(OH)3Cl crystals

332

(SI Figure S12) following the incubation of a G3-NH2 dendronized PAMAM HFM with an aqueous

333

solution of Cu(II) [~12 mg/L] at room temperature for 72 h. The FT Raman spectra (Figure 7A )

334

exhibit the typical PVDF bands including (i) CH2 bending (1421 cm−1) and (ii) CF stretching (796 11 ACS Paragon Plus Environment


Page 12 of 26

335

cm−1).41 We attribute the intense Raman peak (1050 cm-1) of Figure 7A to C-N stretching resulting

336

from the coordination of Cu(II) with the amino groups of the embedded PAMAM particles of the

337

MDP-G0 membrane. The XPS spectra (Figures 7B, 7C and 7D) provide further supporting evidence

338

for the coordination of Cu(II) with the N and O donors of the membrane embedded PAMAM particles.

339

The shift of the C1s peak toward higher binding energy (283.8 eV) [Figure 7C] 42, the splitting and

340

increase in the intensity of the O1s1/2 peak42 (Figure 7D) and the appearance of a N1s peak around 400

341

eV (SI Figure S11) are all consistent with Cu(II) coordination with the membrane N and O donors.

342

Figure 8 provides a cartoon of our postulated copper coordination sites within the embedded PAMAM

343

particles of our new mixed matrix PVDF membrane absorbers. Based on the results of our previous

344

work and published literature on the mechanisms of Cu(II) binding to PAMAM dendrimers in aqueous

345

solutions27-30,38-40, we hypothesize that three classes of complexes (Figure 8) could be formed

346

depending on metal ion loading and solution pH including (i) complexes of Cu(II) with four nitrogen

347

donors (Complexes A1, A2, A3 and A4), (ii) complexes of Cu(II) with two nitrogen donors and two

348

oxygen donors (Complexes B1 and B2) and (iii) complex of Cu(II) with six water molecules.

349

However, more in-depth investigations will be required to validate our postulated mechanisms of

350

Cu(II) binding to the embedded PAMAM particles of our new mixed matrix PVDF membrane

351

absorbers (Figure 7).

352 353 354

Environmental Implications As stated in the Introduction, porous membranes with embedded or surface-grafted chelating polymers

355

have emerged as promising sorbents for the recovery of dissolved Cu from industrial liquid waste

356

stream using low-pressure membrane filtration. The one-pot methodology described in this manuscript

357

provides a simple, fast and potentially scalable route for the preparation of high capacity PVDF-

358

PAMAM membrane absorbers for the recovery of Cu(II) from aqueous solutions. It is worth

359

mentioning that the amount of Cu(II) bound by our MDP-G0 membrane absorber (19-21 g/m2 of dry

360

membrane) is significantly higher that those of a crosslinked PVA membrane with embedded PEI

361

networks18 (~11 g/m2 of dry membrane) and HPAMAM surface-grafted PTFE membrane19 (1.42 g/m2

362

of dry membrane). Moreover, the MDP-G0 membrane has a high water flux (~ 427 ± 13 LMH at 2 bar

363

and pH 7) with a neutral surface layer and a matrix with a sponge-like microstructure characteristic of

364

UF membranes with strong mechanical integrity. Additional investigations are being conducted in the

365

authors’ laboratory to optimize the performance (e.g., sorption capacity and regeneration efficiency) of


Page 13 of 26


366

our PVDF membrane absorbers with in situ synthesized PAMAM particles for the recovery of Cu(II)

367

from relevant industrial liquid waste streams.

368 369 370

Supporting Information The Supporting Information (SI) includes a detailed description of the methods and procedures used to

371

prepare, characterize and evaluate the membranes along with supporting tables and figures. This

372

material is available free of charge via the Internet at http://pubs.acs.org.

373 374

Acknowledgments

375

This research was carried out at the Korea Advanced Institute of Science and Technology (KAIST)

376

and at the California Institute of Technology (Caltech). The membrane casting, characterization and

377

filtration experiments were carried out at KAIST. The basic chemistry and methodology used to

378

synthesize the PAMAM particles were developed at Caltech. Funding for KAIST was provided by the

379

National Research Foundation of Korea (NRF) [MEST grant No. 2012M1A2A2026588] and the

380

EEWS Initiative (NT080607C0209721). Funding for Caltech was provided by the National Science

381

Foundation (NSF) [CBET EAGER Award 0948485]. ATK thanks the University of Johannesburg for

382

financial support.

383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403

References 1. Diallo, M. S.; Fromer, N.; Jhon, M. Nanotechnology for sustainable development: Retrospective and outlook. J. Nanop. Res. 2013, 15: 2044. 2. Fromer, N.; Diallo, M. S. Nanotechnology and clean energy: Sustainable utilization and supply of critical materials. J. Nanop. Res. 2013, 15: 2011. 3. Fromer, N.; Eggert, R. G.; Lifton, J. Critical materials for sustainable energy applications. Resnick Institute Report, California Institute of Technology 2011. Available online at http://resnick.caltech.edu/docs/R_Critical.pdf (Accessed March 6, 2014). 4. Kunding, K. J. A. Copper and Copper Alloys. In Handbook of Materials Selection. 1st Edition. 2002, Kutz, M. Ed.; Wiley; ISBN: 978-0-471-35924-1. 5. Dhere, N. G. Toward GW/year of CIGS production within the next decade. Sol. Energy Mater. Sol. Cells 2007, 91, 1376–1382. 6. Constentin, C.; Robert, M.; Savéant, J. M. Catalysis of the electrochemical reduction of carbon dioxide. Chem. Soc. Rev. 2013, 42, 2423-2436. 7. Spinner, N. S.; Vega, J. A.; Mustain, W. E. Recent progress in the electrochemical conversion and utilization of CO2. Catal. Sci. Technol. 2012, 2, 19-28. 8. Bardi, U. Extracting minerals from seawater: An energy analysis. Sustain. 2010, 2, 980-992. 9. Nakazawa, N.; Tamada, M.; Ooi, K.; Akagawa, S. Experimental studies on rare metal collection from seawater. Proceedings of the Ninth ISOPE Ocean Mining Symposium, Maui, Hawai (USA), June 19-24, 2011, ISBN 978-1-880653-95-1, 184-189. 13 ACS Paragon Plus Environment


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10. Gilbert, O.; Valderrama, C.; Peterkóva, M.; Cortina, J. L. Evaluation of selective sorbents for the extraction of valuable metal ions (Cs, Rb, Li, U) from Reverse Osmosis Rejected Brine. Solvent Extr. Ion Exc. 2010, 28, 543-562. 11. Allen, D. T.; Shonnard, D. R. Sustainable Engineering: Concepts, Design and Case Studies. ISBN-13: 978-0-13-275654-9, 2011. 12. Lazaridis, N. K.; Peleka, E. N.; Karapantsios, Th. D.; Matis, K. A. Copper removal from effluents by various separation techniques. Hydrometallurgy 2004, 74, 149-156. 13. Peng, C.; Liu, Y.; Bi, J.; Xu, H.; Ahmed, A. S. Recovery of copper and water from copperelectroplating wastewater by the combination process of electrolysis and electrodialysis. J. Hazard. Mater. 2011, 189, 814-820. 14. Maketon, W.; Ogden, K. L. Synergistic effects of citric acid and polyethyleneimine to remove copper from aqueous solutions. Chemosphere. 2009, 75, 2006-2011. 15. Johnson, P. D.; Girinathannair, P.; Ohlinger, K. N.; Ritchie, S.; Teuber, L.; Kirby, J. Enhanced removal of heavy metals in primary treatment using coagulation and flocculation. Water Environ. Res. 2008, 80, 472-479. 16. Al-Rashdi, B.; Somerfield, C.; Hilal, N. Heavy metals removal using adsorption and nanofiltration techniques. Sep. Purif. Rev. 2011, 40, 209-259. 17. Dabrowski, A.; Hubicki, Z.; Podkoscienly, P.; Robens, E. Selective removal of the heavy metal ions from waters and industrial wastewaters by ion-exchange method. Chemosphere. 2004, 56, 91-106. 18. Bessbousse, H.; Rhlalou, T.; Verchère, J-F.; Lebrun, L. Removal of heavy metal ions from aqueous solutions by filtration with a novel complexing membrane containing poly(ethyleneimine) in a poly(vinyl alcohol) matrix. J. Membr. Sci. 2008, 307, 249-259. 19. Yoo, H.; Kwak, S-Y. Surface functionalization of PTFE membranes with hyperbranched poly(amidoamine) for the removal of Cu2+ ions from aqueous solution. J. Membr. Sci. 2013, 448, 125-134. 20. Zhang, Q.; Wang, N.; Zhao, L.; Xu, T.; Cheng, Y. Polyamidoamine dendronized hollow fiber Membranes in the Recovery of Heavy Metal Ions. ACS Appl.Mat. Interfaces 2013, 5, 19071912. 21. Kotte, M. R.; Cho, M.; Diallo, M. S. A facile route to the preparation of mixed matrix polyvinylidene fluoride membranes with in-situ generated polyethyleneimine particles. J. Memb. Sci. 2014, 450, 93-102. 22. Kotte, M. R.; Hwang, T.; Han, J-I.; Diallo, M. S. A one-pot method for the preparation of mixed matrix polyvinylidene fluoride membranes with in situ synthesized and PEGylated polyethyleneimine particles. J. Membr. Sci. 2015, 474, 277–287. 23. Hwang, T.; Kotte, M. R.; Han, J-I.; Oh, Y-K.; Diallo, M. S. Microalgae recovery by ultrafiltration using novel fouling-resistant PVDF membranes with in situ PEGylated polyethyleneimine particles. Water Res. 2015, 73, 181-192. 24. Martell, A. E.; Hancock, R. D. Metal Complexes in Aqueous Solutions. Plenum Press, New York, 1996. 25. Tomalia, D. A.; Henderson, S. A.; Diallo, M. S. Dendrimers – an enabling synthetic science to controlled organic nanostructures. In Handbook of Nanoscience, Engineering and Technology. 2nd Edition. 2007; Goddard, W. A. III.; Brenner, D. W.; Lyshevski, S. E.; Iafrate, G. J. Eds.; CRC Press: Boca Raton.


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26. Ottaviani, M. F.; Montali, F.; Turro, N. J.; Tomalia, D. A. Characterization of starburst dendrimers by the EPR technique, 1. Copper-complexes in water solution. J. Am. Chem. Soc. 1994, 116, 661-671. 27. Ottaviani M. F.; Montalti, F.; Turro, N. J.; Tomalia, D. A. Characterization of starburst dendrimers by the EPR technique. Copper(II) ions binding full-generation dendrimers. J. Phys. Chem. B 1997, 101, 158-166. 28. Diallo, M. S.; Balogh, L.; Shafagati, A.; Johnson, J. H. Jr.; Goddard, W. A. III; Tomalia, D. Poly(amidoamine) dendrimers : a new class of high capacity chelating agents for Cu(II) ions. Environ. Sci. Technol. 1999, 33, 820-824. 29. Zhou, L.; Russell D. H.; Zhao, M. Q.; Crooks R. M. Characterization of poly(amidoamine) dendrimers and their complexes with Cu2+ by matrix-assisted laser desorption ionization mass spectrometry. Macromolecules 2001, 34, 3567-3573. 30. Diallo, M. S.; Christie, S.; Swaminathan, P.; Balogh, L.; Shi, X.; Um, W.; Papelis, C.; Goddard, W. A. III.; Johnson, J. H. Jr. Dendritic chelating agents 1. Cu(II) binding to ethylene diamine core poly(amidoamine) dendrimers in aqueous solutions. Langmuir 2004, 20, 26402651. 31. Diallo, M. S.; Christie, S.; Swaminathan, P.; Johnson, J. H. Jr.; Goddard, W. A. III. Dendrimer enhanced ultrafiltration. 1. Recovery of Cu(II) from aqueous solutions using Gx-NH2 PAMAM dendrimers with ethylene diamine core. Environ. Sci. and Technol. 2005, 39, 1366-1377. 32. Chen, D. P.; Yu, C. J.; Chang, C-Y.; Wan, Y.; Frechet, J. M. J.; Goddard, W. A. III.; Diallo, M. S. Branched polymeric media: perchlorate-selective resins from hyperbranched polyethyleneimine. Environ. Sci. Technol. 2012, 46, 10718-10726. 33. Mishra, H.; Yu, C.; Chen, D. P.; Dalleska, N. F.; Hoffmann, M. R.; Goddard, W. A. III.; Diallo, M. S. Branched polymeric media: boron-chelating resins from hyperbranched polyethyleneimine. Environ. Sci. Technol. 2012, 46, 8998–9004 34. Liu, F.; Hashim, N. A.; Liu, Y.; Moghareh Abed, M. R.; Li, K. Progress in the production and modification of PVDF membranes. J. Membr. Sci. 2011, 375, 1-27. 35. Abramoff, M. D.; Magalhaes, P. J.; Ram, R. J. Image processing with Image J software. Biophotonics Int. 2004, 11, 36-42. 36. Popescu, M. C.; Filip, D.; Vasile, C.; Cruz, C.; Rueff, J. M.; Marcos, M.; Serrano, J. L.; Singurel, Gh. Characterization by Fourier transform infrared spectroscopy (FT-IR) and 2D IR correlation spectroscopy of PAMAM dendrimer. J. Phys. Chem. B 2006, 110, 14198–14211. 37. Strathmann, H. Introduction to Membrane Science and Technology; Wiley-VCH Verlag: Weinheim (Germany), 2011. 38. Tran, M. L.; Gahan, L. R.; Gentle, I. R. Structural studies of Copper(II)-amine terminated dendrimer complexes by EXAFS. J. Phys. Chem. B 2004, 108, 20130-20136. 39. Krot, K. A.; de Namor, A. F. D.; Aguilar-Cornejo, A.; Nolan, K. B. Speciation, stability constants and structures of complexes of copper(II), nickel(II), silver(I) and mercury(II) with PAMAM dendrimer and related tetramide ligands. Inorg. Chim. Acta 2005, 358, 3497-3505. 40. Camarada, M. B.; Zuniga, M.; Alzate-Morales, J.; Santos, L. S. Computational study of metal ions with poly(amidoamine) PAMAM G0 dendrimers. Chem. Phys. Lett. 2014, 616-617, 171177. 41. Boccaccio, T.; Bottino, A.; Capannelli, G.; Piaggio, P. Characterization of PVDF membranes by vibrational spectroscopy, J. Membr. Sci. 2002, 210, 315 – 329.



492 493 494 495 496

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42. Song, L.; Zhang, Z.; Song, S.; Gao, Z. Preparation and characterization of the modified polyvinylidene fluoride (PVDF) hollow fibre microfiltration membrane. J. Mater. Sci. Technol. 2007, 23 (1), 55 – 60.


Page 17 of 26

497 498 499 500 501 502 503 504 505 506 507 508 509 510 511


Table 1. Estimated compositions of the mixed matrix PVDF membranes with in situ synthesized PAMAM particles and neat PVDF membranes that were prepared in this study. a

512 513 514 515 516 517 518 519 520 521 522 523

WPVDF

b

Mixed Matrix

e

f

g

h

Membrane

(Wt%)

(Wt%)

(Wt%)

(meq/g)

(meq/g)

(meq/g)

(meq/g)

(meq/g)

MDP-G0

52.29

47.71

39.62

0

2.65

1.33

2.65

9.29

MDP-G1

52.29

47.71

39.62

0

1.91

1.43

2.86

9.06

PVDF (Neat)

100

0

0

0

0

0

0

0

WXLP

c

WECH

d

CPamine

CSamine

CTamine

CAmide

CLigand

a

WPVDF: estimated mass fraction of PVDF in the dry membrane WXLP: the mass fraction of crosslinked PAMAM particles in the dry membrane was estimated based on the following assumptions: i) All ECH crosslinker molecules were reacted with the segregated PAMAM molecules (Figure 1B). ii) Each ECH molecule produces one molecule of hydrogen chloride (HCl) following the crosslinking reaction (Figure 1B). iii) All unreacted PAMAM molecules were washed away in the coagulation bath and subsequent membrane washes with methanol and DIW. c WECH: the mass fraction of ECH was taken as a surrogate for the degree of crosslinking of the in situ synthesized PAMAM particles based on our previous work on the synthesis of perchlorate-selective PEI resin beads32. b

524 525

d

526

g

527

gram of dry membrane, respectively.

CPamine, eCSamine and fCTamine are the estimated concentrations of primary, secondary and tertiary amine groups in milli equivalents (meq) per gram of dry membrane respectively. CAmide and hCLigand are the estimated concentration of amide and ligands (i.e. N and O donors) meq per

528 529 530 17 ACS Paragon Plus Environment


531 532

Table 2. Selected physicochemical properties of the mixed matrix PVDF membranes with in situ synthesized PAMAM particles and neat PVDF membrane that were prepared in this study. Property

MDP-G0

MDP-G1

PVDF (Neat)

Thickness of membrane surface layers (m) Thickness of dry membrane (m) b Contact angle (Degree) c Zeta potential at pH 7.0 (mV) d Average pore diameter of membrane surface layer (nm) Adsorption Desorption e Crosslinked PAMAM particle diameter by FESEM (nm) Minimum Maximum Average f Crosslinked PAMAM particle diameter by DLS (nm) Minimum Maximum Average

11.78 150.0 56.0 0.93

7.58 151.5 59.0 0.46

15.0 103.5 87.0 -5.9

44.6 26.7

28.03 22.48

16.87 12.79

335 2890 1501

816 3341 2284

g

816 1309 1373

1801 3179 2442

g

a

533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558

Page 18 of 26

a)

NA NA g NA g

NA NA g NA g

Estimated average thickness of each membrane surface layer from the FESEM micrographs using the Image J software35. b) Measured contact angle after 120 seconds. c) Measured zeta potential at pH 7.0. d) Average pore diameter of the top layer (i.e. skin) of each membrane. The pore diameters were estimated from the N2 adsorption permporometry experiments using the Barrett-Joyner-Halenda (BJH). methodology (SI Figure S4-S6). e) Estimated size range of crosslinked PAMAM embedded particles using FESEM micrographs analyzed by Image J software (SI Table S3). f) Estimated size range of crosslinked PAMAM embedded particles from DLS particle size analysis. g NA: Not Applicable


Page 19 of 26

559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604


Figure 1. Procedures and reaction schemes used to prepare mixed matrix PVDF membranes with in situ synthesized PAMAM particles. Panel A highlights and visualize the preparation procedures: (i) preparation of membrane casting solution by dissolution of PVDF in TEP, (ii) addition of PAMAM and ECH to the membrane casting solution to initiate the in situ crosslinking reactions between PAMAM and ECH, (iii) membrane preparation by phase inversion casting (Figure 1A). Panel B illustrates the reaction schemes: (i) reaction of ECH epoxy groups with the primary amino groups of the segregated PAMAM macromolecules in the dope solution (Figure 1B) via ring opening nucleophilic substitution followed by the nucleophilic displacement of the ECH chloro groups via reaction with the remaining primary/secondary amino groups of the PAMAM macromolecules. 19 ACS Paragon Plus Environment


Page 20 of 26

605 606 607 608 609 610

A 1 1 1

B

C

D

E

F

C

C

611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631

Figure 2. FESEM micrographs showing the overall cross-sections of the neat PVDF membrane and mixed matrix PVDF membranes with in situ synthesized PAMAM particles. Panels A and B: neat PVDF membrane; Panels C and D: mixed matrix MDP-G0 membrane; Panels E and F: mixed matrix MDP-G1 membrane. The estimated composition of each membrane is listed in Table 1.

632 20 ACS Paragon Plus Environment

Page 21 of 26

633


A. Mid IR Spectra

634 635 636 637 638 639 640 641 642 643 644

B. Near IR Spectra

645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663

Figure 3. Mid and near FTIR spectra of the neat PVDF membrane and mixed matrix PVDF membranes with in situ synthesized PAMAM particles. Panel A highlights the main absorption bands of the mid IR region: (a) 3313 cm-1: -OH and –NH stretch from amide, secondary amino or hydroxyl groups of PAMAM particles; (b) 1640 cm-1: -C=O stretch from amide groups of PAMAM particles; (c) 1540 cm-1: -NH bending from the amide/amine groups of PAMAM particles and (d) 1270 cm-1: -C-N stretch from the amine groups of PAMAM particles. Panel B highlights the main absorption bands of the near IR region: (e) 6850 cm-1: -OH overtone from the ECH crosslinked PAMAM particles. (f) 5788 cm-1: overtones of –CH/-CH2 stretching from the ECH crosslinked PAMAM particles and (g) 5116 cm-1: combination of asymmetric OH stretching/bending from the ECH crosslinked PAMAM particles. The estimated composition of each membrane is listed in Table 1. 21 ACS Paragon Plus Environment


664

Page 22 of 26

A. MDP-G0 PVDF-PAMAM membrane

665 666 667 668 669 670 671 672 673 674 675 676

B. MDP-G1 PVDF-PAMAM membrane

677 678 679 680 681 682 683 684 685 686 687 688 689 690 691

Figure 4. Permeate flux of aqueous solutions of Cu(II) through the mixed matrix MDP-G0 and MDP-G1 membranes as a function of solution pH and filtration time at 2 bar. The composition of each membrane is listed in Table 1. A 2 L solution of Cu(II) [10 mg/L] at constant pH (3, 7 and 9) was pumped trough each membrane at 2 bar.


Page 23 of 26


692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717

Figure 5. Extent of binding [mg of Cu(II) per mL of membrane] and mean % Cu(II) bound in deonized water by the mixed matrix PVDF membranes with in situ synthesized PAMAM particles as a function of filtration time and solution pH. The composition of each membrane is listed in Table 1. A 2 L solution of Cu(II) [10 mg/L] at constant pH (3, 7 and 9) was pumped trough each membrane at 2 bar.

718



Page 24 of 26

719 720 721 722 723 724 725 726 727 728

Figure 6. FESEM micrographs showing the surface and cross-sections of the bare and Cu(II) saturated PAMAM-PVDF MDP-G0 membrane using different electron detectors. A 2 L solution of Cu(II) [10 mg/L] at pH 9 was pumped trough the membrane at 2 bar. Panel A shows a micrograph of the surface of the bare MDP-G0 membrane using FESEM with a through-the-lens detector (TLD); Panels B and C show micrographs of the Cu(II) MDP-G0 membranes using FESEM with TLD and a concentric backscatter (CBS) detector, respectively; Panel D shows a micrograph of the cross section of the bare MDP-G0 membrane using FESEM with a Everhart-Thornley detector (ETD) and Panels E and F show the micrographs of the Cu(II) MDP-G0 membranes using FESEM with TLD and CBS detector.

729 730


Page 25 of 26


731 732 733 734 735

Figure 7. Characterization of a Cu(II) laden MDP-G0 membrane by FT-Raman spectroscopy and XPS. The membrane composition is listed in Table 1. A 2 L solution of Cu(II) [10 mg/L] at pH 9 was pumped trough the membrane at 2 bar. Panel A shows the FT-Raman spectra; Panel B shows the overall XPS scans while Panels C and D highlight the C1s and O1s scans, respectively.

736 737 738 739 740 741



Page 26 of 26

742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773

Figure 8. Postulated mechanisms of Cu(II) complexation with the N and O donors of a mixed matrix PVDF membrane with in situ synthesized crosslinked PAMAM particles. Table 1 lists the estimated composition of each membrane. The postulated mechanisms of Cu(II) binding to the membrane PAMAM particles were derived based on the results of FT-Raman and XPS characterization a Cu(II) saturated PVDF-PAMAM membrane absorber (Figure 7) and published literature data27-30,38-40. These hypothetical mechanisms have not been validated by independent experiments and/or atomistic simulations.

774 775


Mixed Matrix PVDF Membranes With in Situ Synthesized PAMAM

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