Benjamin Michen

Virus Removal in Ceramic Depth Filters Based on Diatomaceous Earth

Ceramic filter candles, based on the natural material diatomaceous earth, are widely used to purify water at the point-of-use. Although such depth filters are known to improve drinking water quality by removing human pathogenic protozoa and bacteria, their removal regarding viruses has rarely been investigated. These filters have relatively large pore diameters compared to the physical dimension of viruses. However, viruses may be retained by adsorption mechanisms due to intermolecular and surface forces. Here, we use three types of bacteriophages to investigate their removal during filtration and batch experiments conducted at different pH values and ionic strengths. Theoretical models based on DLVO-theory are applied in order to verify experimental results and assess surface forces involved in the adsorptive process. This was done by calculation of interaction energies between the filter surface and the viruses. For two small spherically shaped viruses (MS2 and PhiX174), these filters showed no significant removal. In the case of phage PhiX174, where attractive interactions were expected, due to electrostatic attraction of oppositely charged surfaces, only little adsorption was reported in the presence of divalent ions. Thus, we postulate the existence of an additional repulsive force between PhiX174 and the filter surface. It is hypothesized that such an additional energy barrier originates from either the phages specific knobs that protrude from the viral capsid, enabling steric interactions, or hydration forces between the two hydrophilic interfaces of virus and filter. However, a larger-sized, tailed bacteriophage of the family Siphoviridae was removed by log 2 to 3, which is explained by postulating hydrophobic interactions.

Characterization of Pores in Dense Nanopapers and Nanofibrillated Cellulose Membranes: A Critical Assessment of Established Methods.

Nanofibrillated cellulose (NFC) is a natural fibrous material that can be readily processed into membranes. NFC membranes for fluid separation work in aqueous medium, thus in their swollen state. The present study is devoted to a critical investigation of porosity, pore volume, specific surface area, and pore size distribution of dry and wet NFC nanopapers, also known as membranes, with various established techniques, such as electron microscopy, helium pycnometry, mercury intrusion, gas adsorption (N2 and Kr), and thermoporometry. Although these techniques can be successfully applied to inorganic materials (e.g., mesoporous silica), it is necessary to appraise them for organic and hydrophilic products such as NFC membranes. This is due to different phenomena occurring at the materials interfaces with the probing fluids. Mercury intrusion and gas adsorption are often used for the characterization of porosity-related properties; nevertheless, both techniques characterize materials in the dry state. In parallel, thermoporometry was employed to monitor the structure changes upon swelling, and a water permeance test was run to show the accessibility of the membranes to fluids. For the first time, the methods were systematically screened, and we highlighted the need of uniform sample treatments prior to the measurements (i.e., sample cutting and outgassing protocols) in order to harmonize results from the literature. The need for revising the applicability range of mercury intrusion and the inappropriateness of nitrogen adsorption were pointed out. We finally present a table for selecting the most appropriate method to determine a desired property and propose guidelines for results interpretation from which future users could profit.

Improved virus removal in ceramic depth filters modified with MgO.

Ceramic filters, working on the depth filtration principle, are known to improve drinking water quality by removing human pathogenic microorganisms from contaminated water. However, these microfilters show no sufficient barrier for viruses having diameters down to 20 nm. Recently, it was shown that the addition of positively charged materials, for example, iron oxyhydroxide, can improve virus removal by adsorption mechanisms. In this work, we modified a common ceramic filter based on diatomaceous earth by introducing a novel virus adsorbent material, magnesium oxyhydroxide, into the filter matrix. Such filters showed an improved removal of about 4-log in regard to bacteriophages MS2 and PhiX174. This is explained with the electrostatic enhanced adsorption approach that is the favorable adsorption of negatively charged viruses onto positively charged patches in an otherwise negatively charged filter matrix. Furthermore, we provide theoretical evidence applying calculations according to Derjaguin-Landau-Verwey-Overbeek theory to strengthen our experimental results. However, modified filters showed a significant variance in virus removal efficiency over the course of long-term filtration experiments with virus removal increasing with filter operation time (or filter aging). This is explained by transformational changes of MgO in the filter upon contact with water. It also demonstrates that filter history is of great concern when filters working on the adsorption principles are evaluated in regard to their retention performance as their surface characteristics may alter with use.

UV-protection of wood surfaces by controlled morphology fine-tuning of ZnO nanostructures

Abstract One of the most significant limitations for a wider utilisation of the renewable and CO2-storing resource wood is its low ultraviolet (UV) light stability. The protection of the wood surface without altering its aesthetic appeal requires an optically transparent but UV protective coating which should be strongly attached to the rough and inhomogeneous substrate. For this purpose, ZnO nanostructures were deposited onto the wood surface via a chemical bath deposition process. The morphology of crystalline ZnO was controlled by aluminium nitrate or ammonium citrate in the growth step resulting in nanorod arrays or platelet structures, respectively. Detailed structural, chemical and mechanical characterisations as well as accelerated weathering exposure revealed the effective performance of the platelet structure, which formed a dense and thin ZnO coating on spruce. The total colour change (ΔE in the CIE system) was calculated to be 20.5 for unmodified wood, while it was about three for the modified samples after 4 weeks accelerated weathering test. Moreover, the ZnO coating also suppressed crack initiation and propagation indicating a substantial increase in durability.

Layer-by-layer deposition on a heterogeneous surface: Effect of sorption kinetics on the growth of polyelectrolyte multilayers

Surface functionalization by means of controlled deposition of charged polymers or nanoparticles using the layer-by-layer (LbL) approach has been used to modify mostly engineered materials with well-defined surface chemistry and morphology. In this regard, natural and inhomogeneous interfaces have gained very little attention. Furthermore, natural substrates are susceptible to alterations by factors commonly used to control the growth of multilayers, such as pH, temperature and ionic strength. Here, we study the impact of sorption kinetics of a bilayer system (Poly(diallyldimethylammonium chloride) (PDDA) and Poly(sodium 4-styrenesulfonate) (PSS)) on a natural heterogeneous wood surface at neutral pH, without salt addition, on the multilayer buildup. To overcome analytical limitations we introduce a complementary approach based on UV reflectance spectroscopy, atomic force microscopy (AFM) and zeta potential measurements. Compared to immersion times used for ideal substrates, we found that a high surface coverage requires relatively long immersion, approximately 30min, into polyelectrolyte solutions, while a sufficient removal of polyelectrolyte excess during the washing step, requires even longer, about 100min. Based on these findings, we show that film growth can be controlled kinetically. Long immersion times provide well-defined and regular multilayers. The obtained data points to specific requirements to be considered when LbL treatments are applied to rough, porous and heterogeneous surfaces, and thereby sets a basis for a successful transfer of various surface functionalization approaches already shown on ideal surfaces.

High-Resolution Adhesion Mapping of the Odd–Even Effect on a Layer-by-Layer Coated Biomaterial by Atomic-Force-Microscopy

The adhesion behavior of polyelectrolyte multilayers consisting of poly(diallyldimethylammonium chloride), PDDA, and poly(styrenesulfonate), PSS, toward a silicon AFM tip was studied during their build-up on wood, a chemically heterogeneous, micrometer rough biomaterial and compared with a nanometer rough substrate, namely quartz. The atomic force microscopy-based force mapping approach generated high-resolution topography-, and adhesion maps within the first bilayers, which point toward a homogeneous layer-by-layer build-up on the biomaterial surface, and therefore indicate an even charge distribution. By analyzing the force-distance curves in every pixel of the mapping, new insights into the specific interactions of the polyelectrolyte multilayers at the surface were achieved. The characteristic odd-even effect of polyelectrolyte multilayers cannot only be determined on quartz, but also on the biomaterial wood, however, only after an offset of two bilayers. This is potentially due to the specific roughness and charge of wood in comparison to commonly used quartz.

Application of response surface methodology to tailor the surface chemistry of electrospun chitosan-poly(ethylene oxide) fibers

Chitosan is a promising biocompatible polymer for regenerative engineering applications, but its processing remains challenging due to limited solubility and rigid crystalline structure. This work represents the development of electrospun chitosan/poly(ethylene oxide) blend nanofibrous membranes by means of a numerical analysis in order to identify and tailor the main influencing parameters with respect to accessible surface nitrogen functionalities which are of importance for the biological activity as well as for further functionalization. Depending on the solution composition, both gradient fibers and homogenous blended fiber structures could be obtained with surface nitrogen concentrations varying between 0 and 6.4%. Response surface methodology (RSM) revealed chitosan/poly(ethylene oxide) ratio and chitosan molecular weight as the main influencing factors with respect to accessible nitrogen surface atoms and respective concentrations. The model showed good adequacy hence providing a tool to tailor the surface properties of chitosan/poly(ethylene oxide) blends by addressing the amount of accessible chitosan.

Humic acid desorption from a positively charged nanocellulose surface

Trimethyl-ammonium functionalized cellulose nanofibers (CNF) display a positively charged surface over a wide pH and ionic strength range, and constitute therefore not only an interesting biobased sorbent material, but also a positively charged model surface to investigate sorption phenomena. In the current work, CNF were totally or partially covered by humic acid (HA) at pH ∼6, and the desorption process of HA from CNF was thereafter studied in batch and continuous filtration experiments at various pH and ionic strength. It is found that the desorbed amount of HA increases and the kinetic of desorption is faster via increasing the pH and ionic strength of the solution. Moreover, the desorption is affected by the extent of CNF coverage with HA and by the presence of free HA in solution. The present work gives insights into the interaction of HA with permanently positively charged surfaces and was successfully exploited for the removal of HA from CNF filters for regeneration allowing multiple filter utilization.

Investigating the time-dependent zeta potential of wood surfaces

This work reports on streaming potential measurements through natural capillaries in wood and investigates the cause of a time-dependent zeta potential measured during the equilibration of wood cell-walls with an electrolyte solution. For the biomaterial, this equilibration phase takes several hours, which is much longer than for many other materials that have been characterized by electrokinetic measurements. During this equilibration phase the zeta potential magnitude is decaying due to two parallel mechanisms: (i) the swelling of the cell-wall which causes a dimensional change reducing the charge density at the capillary interface; (ii) the transport of ions from the electrolyte solution into the permeable cell-wall which alters the electrical potential at the interface by internal charge compensation. The obtained results demonstrate the importance of equilibration kinetics for an accurate determination of the zeta potential, especially for materials that interact strongly with the measurement electrolyte. Moreover, the change in zeta potential with time can be correlated with the bulk swelling of wood if the effect of electrolyte ion diffusion is excluded. This study shows the potential of streaming potential measurements of wood, and possibly of other hygroscopic and nanoporous materials, to reveal kinetic information about their interaction with liquids, such as swelling and ion uptake.