Adsorptive Removal of Pb(II) and As(V) from Aqueous Solution by PVDF/HMO Ultrafiltration Mixed Matrix Membrane

Lead and arsenic are among the designated chemicals of major concern by the World Health Organization, they affect millions of people worldwide with both acute and chronic effects on human health. In the current study, mixed matrix membranes (MMMs) were fabricated using polyvinylidene fluoride (PVDF) for the matrix and hydrous manganese dioxide nanoparticles to remove lead, Pb(II), and arsenic, As(V), form water. The objectives of the study were to fabricate membrane with HMO:PVDF ratios from 0 to 1 and to characterize their adsorption properties on the two metals and their filtration performances. The HMO nanoparticles were synthesized using manganese sulfate monohydrate and potassium permanganate. The membranes were fabricated using phase inversion, a dope solution was prepared with HMO:PVDF ratios between 0 and 1, NMP as a solvent at an NMP:PVDF ratio of 5.57 and PVP at a PVP:PVDF ratio of 0.1 to increase the permeance of the membranes. The dope solution was casted and then soaked in a water coagulation bath for phase inversion. The structure of the nanoparticles was observed using TEM, and the structure of the membranes was described using SEM. The adsorption tests assessed the adsorption capacity, the adsorption kinetic and the effect of the pH on the adsorption capacity for the two metals. The concentration in the samples was measured with ICP-OES. A FTIR analysis was performed as a proof of adsorption. The tests that were run to assess the filtration performance of the membranes were the following ones: pure water permeance test, hydrophilicity test, surface analysis using XPS, pore size with AFM and finally filtration capacity on lead and arsenic. The HMO particles that were produced had sizes in the nanometer range and were elongated, which provided them a large surface-to-volume ratio. The membranes had asymmetric structures and the membrane with large HMO ratios had the surface of their pores covered uniformly with HMO particles. The adsorption capacities of pure HMO particles were 36 [mg/g] for arsenic and 283 [mg/g] for lead. The adsorption capacity of the membranes with an HMO:PVDF ratio of 1 was 11 [mg/g] for arsenic and 146 [mg/g] for lead. The Freundlich isotherm was used to describe the adsorption capacity of the membranes for both lead and arsenic. The adsorption kinetic was described by a pseudo-second order kinetic model for arsenic and the results were not significant enough the assess the adsorption kinetic model for lead. The equilibrium was almost reached in 30 minutes for the adsorption of the two metals on pure HMO. The adsorption capacity was enhanced with increasing pH for lead adsorption and it was the opposite for arsenic adsorption. The sensitivity to the pH was higher for lead compared to arsenic and this sensitivity to the pH could be used to regenerate the membrane in case of lead adsorption. The average permeance of the membranes was around 1500 [L/m2h bar], the pore sizes were in the ultrafiltration range, between 41 [nm] and 70 [nm] and the membranes were in the hydrophilic range, with an average contact angle of 69 [°]. The HMO ratio in the membranes had a limited impact on these three parameters. The filtration capacity on arsenic was too low to remove arsenic at ppb concentrations and the results of this test on lead were not significant enough to be interpreted.

Adsorption Sites of Metal Atoms on MgO Thin Films and Rotational Quantum State Spectroscopy of Physisorbed H₂

Edgar Fernandes

The work presented in this thesis covers two different topics of surface science, both investigated with the scanning tunneling miscroscopy and spectroscopy at low temperatures (5~K). First, we are interested in the spectroscopic properties of physisorbed H2 (and D2) layers on metallic samples. The energy of the first rotational quantum state transition for H2 (D2) is characterized on a variety of metallic substrates. For homonuclear molecules, these rotational quantum states are directly dependent on the nuclear spin arrangement. Thus the rotational spectroscopy of molecular H2 (D2) allows for unprecedented spatial resolution of nuclear spin configurations. To isolate one physisorbed molecule from the surface, we grow Xe islands before exposing the sample to H2 (D2). Reducing the Xe islands size to support only one molecule would allow us to probe the rotational quantum state at the individual molecule level. We show that rotational spectroscopy of H2 (D2) on Xe islands is feasible. We find that this method is not viable to isolate a H2 (D2) molecule as small Xe islands become unstable in presence of them. The second topic deals with the adsorption of metallic atoms on thin MgO(100) films grown on a Ag(100) substrate. We use Ca-doping of the MgO layer to identify the Mg lattice positions. Subsequent adsorption of Ho allows the determination of their adsorption site, on top of the O and on the bridge site, halfway between two O (Mg) lattice positions. The abundance of the population over these sites differs with the MgO thickness. On the MgO monolayer, the Ho adatoms are distributed following the abundance of these sites, i.e., 1/3 and 2/3, respectively. On the MgO bilayer, exclusively the O site is populated. Atomic manipulation of the Ho adatoms with the STM tip permits the creation of a new species, adsorbing on the Mg site, which was correctly predicted by DFT. Adsorption sites for other metal atoms have further been measured. Dy and Er adsorb on the bridge and O site, with abundances depending upon the atomic species and the MgO thickness. Au adsorbs exclusively on bridge sites. Fe and Co populate uniquely the O site, independent of MgO thickness, while Tb adsorbs on all the three sites, i.e., O, bridge, and Mg. We finally investigated the Ho adsorption sites as a function of deposition and annealing temperature. Ho atoms on the O site become unstable at a temperature between 58 K and 77 K, while Ho atoms on the bridge site start to diffuse only at 125 K. This results in a hopping to the bridge site of all the Ho adatoms adsorbing on O between 58 K and 77 K. Additionally, we show that evaporation of Ho on the MgO bilayer held at 30 K gives rise to adsorption on almost uniquely bridge sites, in neat contrast to what is found for lower temperature (approximatively 10 K). We interpret this result as the action of entropy which changes the free energy of the two types of adsorption site as the temperature increases. Ideas for the source of entropy are given, but the amount of entropy needed to explain our observations is unrealistically large.

EPFL2017

Molecular Level Interfacial Studies of Nanoemulsions: Surface Structure, Specific Ion Effects and Stability

Evangelia Zdrali

This thesis is a study on the molecular structure of the interface of nanometer-sized oil droplets dispersed in water, a system known as oil-in-water nanoemulsion. The motivation for this research is the large significance of the interfacial nanostructures both for in industry and biology. Here we apply the nonlinear optical techniques of sum frequency scattering and second harmonic scattering to study the stabilization mechanism of nanoemulsions, as well as specific ion effects at the nanoscale. We begin with the study of the interfacial structure of oil nanodroplets stabilized with a positively charged, a negatively charged and a neutral surfactant, along with the stability of each system. We show that the surface density of charged surfactants on nanodroplets is an order of magnitude lower than on planar interfaces, due to repulsive interactions between like charges on opposing sides on the droplets surface through the oil phase, allowed by the small droplet size. Moreover, we find no experimental correlation between stability and surfactant surface density. Instead droplet stability is found to depend on the relative cooperativity between charge-charge, charge-dipole and hydrogen bonding interactions. Then, we study the effect of the inversion of the oil and the water phases on the droplet stability for nanometer-sized and micrometer-sized droplets. We employ an oil-soluble neutral surfactant, a water-soluble anionic surfactant, and the combination of the two. We find that, while microdroplets and water-in-oil nanodroplets follow the widely accepted empirical rules, and are stabilized only with a surfactant soluble in the continuous phase, nanometer-sized oil-in-water emulsions can be stabilized with an oil-soluble surfactant. Moreover, the structure of the surfactant is different when it approaches the interface from the dispersed or the continuous phase. Next, we study the interaction of four anions (SCN-, NO3-, Cl- and SO4-2) with different molecular structure with the nanointerface of droplets stabilized with a surfactant with a positively charged headgroup (trimethylammonium) for different anionic concentrations. Our results reveal a unique adsorption pattern for each anion that changes with concentration, possibly involving reorientation of the anions and adsorption to different patches of the interface. Interestingly, not only the weakly-hydrated SCN- and NO3 , but also the well-hydrated SO¬4-2 approaches the nanointerface, inducing strong interfacial water ordering. Last, we further study the interaction of SCN- with the positively charged nanointerface employing ab-initio molecular dynamics simulations. We find ion-pairing of SCN- with the interfacial trimethylammonium groups at concentrations as low as 5 millimolar. Moreover, a variety of ion species emerge at different ionic strengths, with differently oriented SCN- groups adsorbed on hydrophilic and/or hydrophobic parts of the surface. This diverse and heterogeneous chemical environment is surprisingly different from the behaviour at extended liquid planar interfaces, where ion pairing is typically detected at molar concentrations.

EPFL2019

Contact angle and adsorption energies of nanoparticles at the air-liquid interface determined by neutron reflectivity and molecular dynamics

Mauro Moglianetti, Evgeniy Ponomarev, Javier Reguera Gomez, Francesco Stellacci

Understanding how nanomaterials interact with interfaces is essential to control their self-assembly as well as their optical, electronic, and catalytic properties. We present here an experimental approach based on neutron reflectivity (NR) that allows the in situ measurement of the contact angles of nanoparticles adsorbed at fluid interfaces. Because our method provides a route to quantify the adsorption and interfacial energies of the nanoparticles in situ, it circumvents problems associated with existing indirect methods, which rely on the transport of the monolayers to substrates for further analysis. We illustrate the method by measuring the contact angle of hydrophilic and hydrophobic gold nanoparticles, coated with perdeuterated octanethiol (d-OT) and with a mixture of d-OT and mercaptohexanol (MHol), respectively. The contact angles were also calculated via atomistic molecular dynamics (MD) computations, showing excellent agreement with the experimental data. Our method opens the route to quantify the adsorption of complex nanoparticle structures adsorbed at fluid interfaces featuring different chemical compositions.