Yoram Oren

Removal of chromium from aqueous solutions by treatment with porous carbon electrodes: Electrochemical principles

The possibility of removing hexavalent chromium from waste water by electrochemical treatment using a graphite felt electrode and synthetic electrolytes is investigated. It is suggested that the process proceeds in two steps: electrochemical reduction of the hexavalent chromium to chromic ion followed by the formation of an insoluble chromic hydroxide in an electrochemically generated high pH environment. The chromic hydroxide adheres to the electrode surface as a charged colloidal particle. The electrochemical dissolution of the hydroxide layer by potential inversion is also discussed as a possible regeneration procedure.It is concluded that these steps can be included within a single separating column provided that the feed pH and the column voltage are carefully controlled.

The role of alginate in Pseudomonas aeruginosa EPS adherence, viscoelastic properties and cell attachment

Among various functions, extracellular polymeric substances (EPS) provide microbial biofilms with mechanical stability and affect initial cell attachment, the first stage in the biofilm formation process. The role of alginate, an abundant polysaccharide in Pseudomonas aeruginosa biofilms, in the viscoelastic properties and adhesion kinetics of EPS was analyzed using a quartz crystal microbalance with dissipation (QCM-D) monitoring technology. EPS was extracted from two P. aeruginosa biofilms, a wild type strain, PAO1, and a mucoid strain, PAOmucA22 that over-expresses alginate production. The higher alginate content in the EPS originating from the mucoid biofilms was clearly shown to increase both the rate and the extent of attachment of the EPS, as well as the layers thickness. Also, the presence of calcium and elevated ionic strength increased the thickness of the EPS layer. Dynamic light scattering (DLS) showed that the presence of calcium and elevated ionic strength induced intermolecular attractive interactions in the mucoid EPS molecules. For the wild type EPS, in the presence of calcium, an elevated shift in the distribution of the diffusion coefficients was observed with DLS due to a more compacted conformation of the EPS molecules. Moreover, the alginate over-expression effect on EPS adherence was compared to the effect of alginate over-expression on P. aeruginosa cell attachment. In a parallel plate flow cell, under similar hydraulic and aquatic conditions as those applied for the EPS adsorption tests in the QCM-D flow cell, reduced adherence of the mucoid strain was clearly observed compared to the wild type isogenic bacteria. The results suggest that alginate contributes to steric hindrance and shielding of cell surface features and adhesins that are known to promote cell attachment.

The influence of antiscalants on biofouling of RO membranes in seawater desalination.

Antiscalants are surface active polyelectrolyte compounds commonly used in reverse osmosis (RO) desalination processes to avoid membrane scaling. In spite of the significant roles of antiscalants in preventing membrane scaling, they are prone to enhance biofilm growth on RO membranes by either altering membrane surface properties or by serving as nutritional source for microorganisms. In this study, the contribution of antiscalants to membrane biofouling in seawater desalination was investigated. The effects of two commonly used antiscalants, polyphosphonate- and polyacrylate-based, were tested. The effects of RO membrane (DOW-Filmtec SW30 HRLE-400) exposure to antiscalants on its physico-chemical properties were studied, including the consequent effects on initial deposition and growth of the sessile microorganisms on the RO membrane surface. The effects of antiscalants on membrane physico-chemical properties were investigated by filtration of seawater supplemented with the antiscalants through flat-sheet RO membrane and changes in surface zeta potential and hydrophobicity were delineated. Adsorption of antiscalants to polyamide surfaces simulating RO membranes polyamide layer and their effects on the consequent bacterial adhesion was tested using a quartz crystal microbalance with dissipation monitoring technology (QCM-D) and direct fluorescent microscopy. A significant increase in biofilm formation rate on RO membranes surface was observed in the presence of both types of antiscalants. Polyacrylate-based antiscalant was shown to enhance initial cell attachment as observed with the QCM-D and a parallel plate flow cell, due to rendering the polyamide surface more hydrophobic. Polyphosphonate-based antiscalants also increased biofilm formation rate, most likely by serving as an additional source of phosphorous to the seawater microbial population. A thicker biofilm layer was formed on the RO membrane when the polyacrylate-based antiscalant was used. Following these results, a wise selection of antiscalants for scaling control should take into account their contribution to membrane biofouling propensity.

Effect of surface-exposed chemical groups on calcium-phosphate mineralization in water-treatment systems.

Calcium-phosphate-scale formation on reverse osmosis (RO) membranes is a major limiting factor for cost-effective desalination of wastewater. We determined the effects of various organic chemical groups found on membrane surfaces on calcium-phosphate scaling. Langmuir films exposing different functional groups were equilibrated with a solution simulating the ionic profile of secondary effluent (SSE). Surface pressure-area (Langmuir) isotherms combined with ICP elemental analyses of the interfacial precipitate suggested acceleration of calcium-phosphate mineralization by the surface functional groups in the order: PO(4) > COOH ∼ NH(2) > COOH:NH(2) (1:1) > OH > ethylene glycol. Immersion of gold-coated silicon wafers self-assembled with different alkanethiols in SSE solution showed formation of a hydroxyapatite precipitate by X-ray diffraction and ATR-IR analysis. Data showed diverse influences of functional groups on mineralization, implying low calcium-phosphate scaling for uncharged surfaces or surfaces coated with both positively and negatively charged groups. This information is valuable for understanding scaling processes, and for designing of novel low-scaling membranes for water desalination.

Improved compact accelerated precipitation softening (CAPS)

CAPS is a softening process in which adjusting water pH to the range 8-10.5 reduces calcium and carbonate alkalinity by accelerated CaCO 3 nucleation and growth in 2 regions: (a) in a pre-prepared slurry made of calcite small particles and, (b) within a CaCO 3 layer (cake) formed on the top of the filter through which water is pumped out. Whilst the largest degree of precipitation occurs within the slurry, the cake process is a polishing step in which calcium concentration is reduced further. Within the dense cake structure, the removal of the smaller calcium concentrations is possible within short contact times due to enhanced mass transfer rates made possible by large solution velocities within narrow pores and much larger surface to volume ratio. CAPS was first suggested for water softening and later tested for the possibility of simultaneous silica removal. CAPS was also studied as a pretreatment for RO (reverse osmosis) with water taken from fish ponds. The capability of reducing SDI, organics and hardness to levels satisfactory for prolonged RO treatment was demonstrated. In a previous paper, water was mixed with CaCO 3 particles, the slurry was circulated through a microfiltration module and the clear and softened permeate was then RO treated with a recovery rate above 80%. CAPS may be used as a stand-alone water treatment process or in conjunction with pressure and electrical driven membrane processes (UF, NF, RO, ED) as an effective pretreatment routine for increasing recovery and decreasing fouling rates. In this work, a new concept for CAPS, which comprises in-tank mixing and filtration is presented. This makes the CAPS device more attractive due to compactness and the process more attractive technically and in terms of cost. The advantages of in-tank filtration were appreciated in the past and it has been a subject for intensive investigation. Laboratory CAPS units were run continuously (up to 250 h) and for shorter time periods in order to investigate tap water softening. The effect of the initial CaCO 3 slurry concentration; residence time or pumping rate; pH; backwash frequency, duration and mode (dry or wet) and slurry mixing rate was investigated and analyzed in terms of Saturation Index (SI) reduction, separated effects of the slurry and the cake on the softening action and filter cake load.

Laser-Induced Graphene Layers and Electrodes Prevents Microbial Fouling and Exerts Antimicrobial Action

Prevention of fouling on surfaces is a major challenge that broadly impacts society. Water treatment technologies, hospital infrastructure, and seawater pipes exemplify surfaces that are susceptible to biofouling. Here we show that laser-induced graphene (LIG) printed on a polyimide film by irradiation with a CO2 infrared laser under ambient conditions is extremely biofilm resistant while as an electrode is strongly antibacterial. We investigated the antibacterial activity of the LIG surface using LIG powder in suspension or deposited on surfaces, and its activity depended on the particle size and oxygen content. Remarkably, the antimicrobial effects of the surface were greatly amplified when voltages in the range of 1.1-2.5 were applied in an electrode configuration in bacterial solutions. The bactericidal mechanism was directly observed using microscopy and fast photography, which showed a rapid bacterial movement toward the LIG surface and subsequent bacterial killing. In addition, electrochemical generation of H2O2 was observed; however, the bacterial killing mechanism depended strongly on the physical and electrical contact of the bacterial cells to the surfaces. The anti-biofilm activity of the LIG surfaces and electrodes could lead to efficient protection of surfaces that are susceptible to biofouling in environmental applications by incorporating LIG onto the surfaces.

Ion exchange membrane bioreactor for treating groundwater contaminated with high perchlorate concentrations

Perchlorate contamination of groundwater is a worldwide concern. The most cost efficient treatment for high concentrations is biological treatment. In order to improve and increase the acceptance of this treatment, there is a need to reduce the contact between micro organisms in the treatment unit and the final effluent. An ion exchange membrane bioreactor (IEMB), in which treated water is separated from the bioreactor, was suggested for this purpose. In this study, the IEMBs performance was studied at a concentration as high as 250mgL(-1) that were never studied before. In the bioreactor, glycerol was used as a low cost and nontoxic carbon and energy source for the reduction of perchlorate to chloride. We found that high perchlorate concentrations in the feed rendered the anion exchange membrane significantly less permeable to perchlorate. However, the presence of bacteria in the bio-compartment significantly increased the flux through the membrane by more than 25% in comparison to pure Donnan dialysis. In addition, the results suggested minimal secondary contamination (<3mgCL(-1)) of the treated water with the optimum feed of carbon substrate. Our results show that IEMB can efficiently treat groundwater contaminated with perchlorate as high as 250mgL(-1).

Removal of cadmium and associated contaminants from aqueous wastes by fibrous carbon electrodes

Abstract The electrochemical treatment of aqueous wastes containing 1–20 ppm cadmium, nickel and cyanide was investigated. Graphite felt electrodes were used in flow-by and flow-through type electrochemical cells. The conditions for the simultaneous removal of cadmium and nickel and cadmium and cyanide down to the ppb concentration level were determined. Cell performances were discussed in terms of normalized space velocity. Generally, it was found that the flow-by cell yield is much larger than that of the flow-through cell. However, as the flow velocity decreases, the yields of the two cell types become similar.

The effect of electric fields on bacterial attachment to conductive surfaces

Bacterial adhesion has been studied in various conditions. The bactericidal effect of strong electric voltages has been mainly tested within the context of preventing device-related infections in hospital environments. Little evidence is found in the literature on the mechanism of bacterial deposition as a function of an applied electric field. In this study, we assumed that an electric field, which was applied perpendicularly to the flow of a bacterial suspension, might impact the electrostatic energy barrier between the negatively charged bacterial cell and the positively charged electrode substrate. Experiments were carried out with a QCM-D electro-chemical module which allowed monitoring the adhered mass as well as the dissipation factor, while a bacterial suspension was passed through the module in a diffusion dominated flow regime and while an electric field was applied perpendicularly to the flow of the suspension. A Pseudomonas fluorescens strain was used in order to detect and estimate any bacterial adhesion on the electrode surface. While the results confirmed that a decrease of the recorded resonance frequency was in direct proportion to the amount of adhered bacteria (estimated by a visual control of the bacterial signal on the electrode sensor at the end of each experiment), the change in the resonance frequency as a function of the applied electric potential was rather counter-intuitive: with negative potential the rate of bacterial adhesion was faster and steadier than with a positive electric potential, when the bacterial rate of adhesion tended to level off. Moreover, analyses of the measured dissipation to the measured frequency shifts (ΔD/Δf) during the deposition experiment, which indicate the fluidic character of the adhered layer of bacteria, revealed that with negative electric potential the bacterial cells were rigidly connected, while with positive electric potentials the cells were rather loosely bound to the electrode. The measured data suggested that the applied electric potential might have an impact on the conformation of the surface appendages on the bacterial cell surface of P. fluorescens, causing a state of steric repulsion with an extended conformation induced by positive electric potentials, and a state of entangled conformations induced by negative potentials, which allowed the cells to overcome the electrostatic energy barrier.

Saline Groundwater from Coastal Aquifers As a Source for Desalination

Reverse osmosis (RO) seawater desalination is currently a widespread means of closing the gap between supply and demand for potable water in arid regions. Currently, one of the main setbacks of RO operation is fouling, which hinders membrane performance and induces pressure loss, thereby reducing system efficiency. An alternative water source is saline groundwater with salinity close to seawater, pumped from beach wells in coastal aquifers which penetrate beneath the freshwater-seawater interface. In this research, we studied the potential use of saline groundwater of the coastal aquifer as feedwater for desalination in comparison to seawater using fieldwork and laboratory approaches. The chemistry, microbiology and physical properties of saline groundwater were characterized and compared with seawater. Additionally, reverse osmosis desalination experiments in a cross-flow system were performed, evaluating the permeate flux, salt rejection and fouling propensities of the different water types. Our results indicated that saline groundwater was significantly favored over seawater as a feed source in terms of chemical composition, microorganism content, silt density, and fouling potential, and exhibited better desalination performance with less flux decline. Saline groundwater may be a better water source for desalination by RO due to lower fouling potential, and reduced pretreatment costs.