Avner Adin

Silver nanoparticle-E. coli colloidal interaction in water and effect on E. coli survival.

Silver nanoparticles exhibit antibacterial properties via bacterial inactivation and growth inhibition. The mechanism is not yet completely understood. This work was aimed at elucidating the effect of silver nanoparticles on inactivation of Escherichia coli, by studying particle-particle interactions in aqueous suspensions. Stable, molecularly capped, positively or negatively charged silver nanoparticles were mixed at 1 to 60microgmL(-1) with suspended E. coli cells to examine their effect on inactivation of the bacteria. Gold nanoparticles with the same surfactant were used as a control, being of similar size but made up of a presumably inert metal. Log reduction of 5log(10) and complete inactivation were obtained with the silver nanoparticles while the gold nanoparticles did not show any inactivation ability. The effect of molecularly capped nanoparticles on E. coli survival was dependent on particle number. Log reduction of E. coli was associated with the ratio between the number of nanoparticles and the initial bacterial cell count. Electrostatic attraction or repulsion mechanisms in silver nanoparticle-E. coli cell interactions did not contribute to the inactivation process.

Trihalomethane formation in chlorinated drinking water : a kinetic model

Abstract Aquatic humic and fulvic acids are the main trihalomethane (THM) precursors formed during surface water chlorination. The plot of THM concentration against humic substance levels is a “convex”-shape, suggesting that a multi-step reaction occurs. In the first step, organochlorine intermediates are produced, and, in the second step, these are converted to THM. A mechanistic model, based on the reaction of chlorine with model compounds, is used to derive kinetic equations for THM formation as a function of the precursor and chlorine concentrations under controlled pH and temperature conditions. This paper describes the application of this model and the correlation between calculated THM levels and experimentally measured values.

Characterization of alginate-like exopolysaccharides isolated from aerobic granular sludge in pilot-plant.

To understand functional gel-forming exopolysaccharides in aerobic granular sludge, alginate-like exopolysaccharides were specifically extracted from aerobic granular sludge cultivated in a pilot plant treating municipal sewage. The exopolysaccharides were identified by the FAO/WHO alginate identification tests, characterized by biochemical assays, gelation with Ca(2+), blocks fractionation, spectroscopic analysis as UV-visible, FT-IR and MALDI-TOF MS, and electrophoresis. The yield of extractable alginate-like exopolysaccharides was reached 160+/-4mg/g (VSS ratio). They resembled seaweed alginate in UV-visible and MALDI-TOF MS spectra, and distinguished from it in the reactions with acid ferric sulfate, phenol-sulfuric acid and Coomassie brilliant blue G250. Characterized by their high percentage of poly guluronic acid blocks (69.07+/-8.95%), the isolated exopolysaccharides were capable to form rigid, non-deformable gels in CaCl(2). They were one of the dominant exopolysaccharides in aerobic granular sludge. We suggest that polymers play a significant role in providing aerobic granular sludge a highly hydrophobic, compact, strong and elastic structure.

Electrocoagulation versus chemical coagulation: Coagulation/flocculation mechanisms and resulting floc characteristics

Electrocoagulation (EC) and chemical coagulation (CC) are employed in water treatment for particle removal. Although both are used for similar purposes, they differ in their dosing method - in EC the coagulant is added by electrolytic oxidation of an appropriate anode material, while in CC dissolution of a chemical coagulant is used. These different methods in fact induce different chemical environments, which should impact coagulation/flocculation mechanisms and subsequent floc formation. Hence, the process implications when choosing which to apply should be significant. This study elucidates differences in coagulation/flocculation mechanisms in EC versus CC and their subsequent effect on floc growth kinetics and structural evolution. A buffered kaolin suspension served as a representative solution that underwent EC and CC by applying aluminum via additive dosing regime in batch mode. In EC an aluminum anode generated the active species while in CC, commercial alum was used. Aluminum equivalent doses were applied, at initial pH values of 5, 6.5 and 8, while samples were taken over pre-determined time intervals, and analyzed for pH, particle size distribution, ζ potential, and structural properties. EC generated fragile flocs, compared to CC, over a wider pH range, at a substantially higher growth rate, that were prone to restructuring and compaction. The results suggest that the flocculation mechanism governing EC in sweep floc conditions is of Diffusion Limited Cluster Aggregation (DCLA) nature, versus a Reaction Limited Cluster Aggregation (RLCA) type in CC. The implications of these differences are discussed.

Iron-oxidation processes in an electroflocculation (electrocoagulation) cell

The processes of iron oxidation in an electroflocculation cell were investigated for a pH range of 5-9 and electric currents of 0.05-0.4A (equivalent current densities of 8.6-69 A/m(2)). At all pH values and electric currents investigated, it was demonstrated and proven that for all practical purposes, the form of iron that dissolves from the anode is Fe(2+) (ferrous). The difference between the amount of theoretical dissolution as calculated by Faradays law and the amount of observed dissolved iron ions may indicate two phenomena in electrochemical cells. The first is possible dissolution of the anode even without the operation of an electric current; this led to higher theoretical dissolution rates at lower pH. The second is the participation of some of the electrons of the electric current in reactions other than anode dissolution which led to lower theoretical dissolution rates at higher pH. Those other reactions did not lead to an increase in the local oxidation saturation level near the anode and did not affect iron-oxidation rates in the electroflocculation processes. The oxidation rates of the dissolved Fe(2+) (ferrous) to Fe(3+) (ferric) ions in electroflocculation processes were strongly dependent on the pH and were similar to the known oxidation rates of iron in non-electrochemical cells.

Fluorescent dye labeled bacteriophages--a new tracer for the investigation of viral transport in porous media: 1. Introduction and characterization.

A new method for the study of pathogen transport in porous media is presented. The method is based on conjugation of fluorescent dyes to target bacteriophages and application of the modified bacteriophages for tracer studies. We demonstrate that the relevant transport determining properties of Rhodamine and several fluorescein-labeled phages are practically identical to those of the native bacteriophages. The advantages of the proposed method relative to direct enumeration of bacteriophages by plaque forming unit method, turbidity, fluorescent microspheres, and other alternative tracers are discussed. Notable advantages include simple quantitation by optical methods, unbiased signals even when virus aggregates are formed, and the ability to decouple inactivation kinetics from transport phenomena. Additionally, the signal reflects the removal and transport of the studied microorganism and not a surrogate.

Control of biofilm formation in water using molecularly capped silver nanoparticles

Control of biofouling and its negative effects on process performance of water systems is a serious operational challenge in all of the water sectors. Molecularly capped silver nanoparticles (Ag-MCNPs) were used as a pretreatment strategy for controlling biofilm development in aqueous suspensions using the model organism Pseudomonas aeruginosa. Biofilm control was tested in a two-step procedure: planktonic P. aeruginosa was exposed to the Ag-MCNPs and then the adherent biofilm formed by the surviving cells was monitored by applying a model biofilm-formation assay. Under specific conditions, Ag-MCNPs retarded biofilm formation, even when high percentage of planktonic P. aeruginosa cells survived the treatment. For example, Ag-MCNPs (10 microg mL(-1)) retarded biofilm formation (>60%), when 50 percent of the planktonic P. aeruginosa cells survived the treatment. Moreover, stable low value of relative biomass has been formed in the presence of fixed Ag-MCNPs concentrations at various biofilm incubation times. Our results showed that Ag-MCNPs pretreated cells were able to produce EPS although they succeeded to form relatively low adherent biofilm. These pretreated cells appear well preserved and undamaged under TEM HPH/freeze micrographs, yet the intra cellular material seems to be pushed towards the peripheral parts of the cell, possibly indicating a survival strategy to the presence of Ag-MCNPs. The lower value of relative biomass formed in the presence of Ag-MCNPs could be associated with molecular mechanisms related to biofilm formation or continuous release of silver ions in the sample. However, further research is required to examine these factors.

The effect of chemical oxidants on effluent constituents for drip irrigation

Abstract One of the main problems concerning drip irrigation by effluents is the clogging of drippers and emitters, which, due to their small diameters, are extremely vulnerable to clogging by effluent constituents. Earlier observations revealed that this phenomenon can be reduced if effluents are treated with oxidants. In order to realize the role of oxidants in these processes, and to obtain meaningful and reproducible results, synthetic effluents rather than authentic effluents were used. However, the synthetic effluents were composed so as to provide the best possible resemblance to natural effluents, and the effects of Cl2 and ClO2 on their constituents were studied. The demand of these effluents for Cl2 was 5–8 mg l−1 and for ClO2 3–4 mg l−1. Two mg l−1 of either oxidant caused a very fast bacteria inactivation that reached four orders of magnitude after 1 min. However, with respect to algae, concentrations up to 20 mg l−1 of either oxidant did not affect the number of algae cells, although they caused a remarkable decrease in algal viability as expressed by its chlorophyll content and replication ability. Both oxidants have demonstrated a notable aggregation effect on the effluents. The conclusions of the results described above were examined in a pilot system. Continued chlorination by 5–10 mg l−1 Cl2 applied directly to the drippers was not very effective. The reason for this was the presence of clogging agents, “immune” to low Cl2 concentrations, produced as early as in the reservoir, and carried down to the drippers by the effluent stream. Batch treatment combined with settling was much more efficient, and reduced the clogging significantly, because in this case the Cl2 reacted not only as a disinfectant, but also as a coagulant due to the oxidation of humic constituents.

Size and structure evolution of kaolin-Al(OH)3 flocs in the electroflocculation process: a study using static light scattering.

Electroflocculation (EF) is gaining recognition as an alternative process to conventional coagulation/flocculation. The electrical current applied in EF that generates the active coagulant species creates a unique chemical/physical environment in which competing redox reactions occur, primarily water electrolysis. This causes a transient rise in pH, due to cathodic formation of hydroxyl ions, which, in turn, causes a continuous shift in coagulation/flocculation mechanisms throughout the process. This highly impacts the formation of a sweep floc regime that relies on precipitation of metal hydroxide and its growth into floc. The size and structural evolution of kaolin-Al(OH)(3) flocs was examined using static light scattering techniques, in aim of elucidating kinetic aspects of the process. An EF cell was operated in batch mode and comprised of two concentric electrodes - a stainless steel cathode (inner electrode) and an aluminum anode (outer electrode). The cell was run at constant current between 0.042A and 0.22A, and analyses performed at pre-determined time intervals. The results demonstrate that EF is able to generate a range of flocs, exhibiting different growth rates and structural characteristics, depending on the conditions of operation. Growth patterns were sigmoidal and a linear correlation between growth rate and current applied was observed. The dependency of growth rate on current can be related to initial pH and aluminum dosing, with a stronger dependency apparent for initial optimal sweep floc regime. All flocs exhibited a fragile nature and undergo compaction and structural fluctuations during growth. This is the first time size and structural evolution of flocs formed in the EF process is reported.

Fouling mitigation by iron-based electroflocculation in microfiltration: Mechanisms and energy minimization

High-energy demand presents a major obstacle in the application of advanced water-purification systems. In this work, energy minimization and fouling mitigation by iron-based electroflocculation in dead-end microfiltration were investigated. Highly pure water contaminated with Silica-CMP (chemical mechanical polishing) particles were pretreated by electroflocculation at short operation times and a constant electrical current intensity of 0.4 A, followed by different slow-mixing times and filtration without any sedimentation step. By using a new method for filtration-energy appraisal, we found that an over 90% reduction in filtration energy could be achieved. The improvement was observed at all pH values examined (pH 6-8); pH values below 7 were problematic because the permeate turned yellow as a result of residual iron. The appearance of residual iron was explained by the dependence of Fe(2+) to Fe(3+) reaction rates on pH. Scanning electron micrographs of the fouled membrane surface showed the important role played by the sweep-coagulation mechanism in mitigating fouling. When internal fouling was the dominant mechanism, the amorphous iron-hydroxide solids formed a layer that filtered out the primary particles, protecting the membrane pores from plugging. Iron-hydroxide particles also reduced the hydraulic resistance of the cake when the external fouling mechanism dominated. Significant energy reduction was observed, even without the slow-mixing step, as a result of the local flocculation conditions near the membrane surface. Additional energy savings were obtained due to the significantly higher initial-flux restoration rates (>90%) resulting from electroflocculation pretreatment.