Electrostatic forces control nonspecific virus attachment to lettuce

Enteric viruses are key foodborne pathogens. The objective of this study was to compare the relative contributions of electrostatic and hydrophobic forces with the nonspecific attachment of virus to butterhead lettuce. The attachment of four viruses (echovirus 11, feline calicivirus [FCV], MS2, and phiX174) was studied. Three different conditions, namely (i) 1% Tween 80, (ii) 1 M NaCl, and (iii) 1% Tween 80 with 1 M NaCl, were investigated to determine the role of hydrophobic, electrostatic, and combined hydrophobic and electrostatic forces, respectively. Attachment above the pI of FCV and echovirus 11 was reduced or eliminated in the presence of NaCl, indicating an electrostatic interaction between the animal viruses and lettuce. The bacteriophage phiX174 was not significantly affected by any treatment, indicating a lack of electrostatic or hydrophobic interactions between the lettuce and phage phiX174. Overall, 1 M NaCl was the most effective treatment in desorbing viruses from the surface of lettuce at pH 7 and 8. The results imply that electrostatic forces play a major role in controlling virus adsorption to lettuce. The results indicate that 1 M NaCl solution would improve the recovery or elution of unenveloped viruses from lettuce.     

Virus retention and transport in chemically heterogeneous porous media under saturated and unsaturated flow conditions

Retention and transport of colloids and microorganisms are complex processes, especially in the vadose zone due to the more complicated water flow regime and additional interfacial reactions involved. In this study, we examined the retention and transport behavior of two bacteriophages, MS-2 and phiX174, in homogeneous and chemically heterogeneous media under variably saturated conditions. Column experiments with glass beads (treated to have either hydrophilic or hydrophobic surface properties) were conducted using a phosphate-buffered saline solution at different pore water ionic strengths ranging from 0.025 to 0.163 M. In columns packed with 100% hydrophilic glass beads, retention of the viruses increased with decreasing water content and increasing ionic strength, a result similar to those reported in the literature. However, greater retention of both MS-2 and phiX174 was observed in saturated columns than in unsaturated columns packed with a 1:1 mixture of hydrophilic and hydrophobic glass beads, especially at high ionic strengths. This result contradicts the common belief that viruses (and colloids in general) are subject to greater removal in unsaturated media. Our study suggests that while the mechanisms controlling colloid interfacial interactions (i.e., attachment on solid-water and air-water interfaces and film straining) on the pore scale are relevant, nonuniform wetting conditions due to heterogeneous grain surface hydrophobicity can strongly influence water flow and phase interconnection. Under these conditions, hydrodynamic effects on the mesopore scale will dominate pore-scale interfacial reactions in controlling the extent of colloid retention and movement in unsaturated media.     

Antimicrobial effect of silver particles on bacterial contamination of activated carbon fibers

Even though activated carbon fiber (ACF) filters have been widely used in air cleaning for the removal of hazardous gaseous pollutants, because of their extended surface area and high adsorption capacity, bacteria may breed on the ACF filters as a result of their good biocompatibility; ACF filters can themselves become a source of bioaerosols. In this study, silver particles were coated onto an ACF filter, using an electroless deposition method and their efficacy for bioaerosol removal was tested. First, various surface analyses, including scanning electron microscopy, inductively coupled plasma and X-ray diffraction were carried out to characterize the prepared ACF filters. Filtration and antimicrobial tests were then performed on the filters. The results showed that the silver-deposited ACF filters were effective for the removal of bioaerosols by inhibition of the survival of microorganisms, whereas pristine ACF filters were not. Two bacteria, Bacillus subtilis and Escherichia coli, were completely inhibited within 10 and 60 min, respectively. Electroless silver deposition did not influence the physical characteristics of ACF filters such as pressure drop and filtration efficiency. The gas adsorptive ability of the silver-deposited ACF filter, as represented by the micropore specific surface area, decreased by about 20% compared to the pristine filter because of the blockage of the ACF micropores by silver particles. Therefore, the amount of silver particles on the ACF filters needs to be optimized to avoid excessive reduction of their adsorptive characteristics and to show effective antimicrobial activity.     

Removal and inactivation of waterborne viruses using zerovalent iron

A daunting challenge facing the water industry and regulators is how to simultaneously control microbial pathogens, residual disinfectant, and disinfection byproducts in drinking water, and to do so at an acceptable cost. Of the different pathogens, viruses are especially problematic due to their small size, high mobility, and resistance to chlorination and filtration. In the past decade, zerovalent iron has been used to treat a wide variety of organic and inorganic contaminants from groundwater. However, iron has not been tested against biological agents. This study examined the effectiveness of commercial zerovalent iron to remove two viruses, phiX174 and MS-2, from water. Removal of these viruses by iron granules in batch reactors was first-order, and the rate was likely controlled by external mass transfer. Most of the viruses removed from solution were either inactivated or irreversibly adsorbed to iron. In a flow-through column containing zerovalent iron (with 20 min of iron contact time), the removal efficiency for both viruses was 4-log in an initial pulse test, and over 5-log in the second pulse test after passage of 320 pore volumes of artificial groundwater. We assume that the improved efficiency was due to continuous formation of new iron (oxyhydr)oxides which served as virus adsorption sites. To our knowledge, this is the first demonstration of biological agent removal from water by zerovalent iron. Results of this study suggest zerovalent iron may be potentially useful for disinfecting drinking water and wastewater, thereby reducing our dependence on chlorine and reducing the formation of disinfection byproducts.     

Effect of humic acid on pyrene removal from water by polycation-clay mineral composites and activated carbon

Pyrene removal by polycation-montmorillonite (MMT) composites and granulated activated carbon (GAC) in the presence of humic acid (HA) was examined. Pyrene, HA, and sorbent interactions were characterized by FTIR, fluorescence and zeta measurements, adsorption, and column filtration experiments. Pyrene binding coefficients to the macromolecules were in the order of PVPcoS (poly-4-vinylpiridine-co-styrene) > HA > PDADMAC (poly diallyl-dimethyl-ammonium-chloride), correlating to pyrene-macromolecules compatibility. Electrostatic interactions explained the high adsorption of HA to both composites (～100%), whereas HA adsorption by GAC was low. Pyrene removal by the composites, unlike GAC, was enhanced in the presence of HA; removal by PDADMAC-MMT increased from ～50 (k(d) = 2.2 × 10(3) kg/L) to ～70% (k(d) = 2.4 × 10(3) kg/L) in the presence of HA. This improvement was attributed to the adsorption of pyrene-HA complexes. PVPcoS-MMT was most efficient in removing pyrene (k(d) = 1.1 × 10(4) kg/L, >95% removal) which was explained in terms of specific π donor-π acceptor interactions. Pyrene uptake by column filters of GAC reached ～50% and decreased to ～30% in the presence of HA. Pyrene removal by the PVPcoS-MMT filter was significantly higher (100-85% removal), exhibiting only a small decrease in the presence of HA. The utilization of HA as an enhancing agent in pollutant removal is novel and of major importance in water treatment.     

Linear adsorption of nonionic organic compounds from water onto hydrophilic minerals: silica and alumina

To characterize the linear adsorption phenomena in aqueous nonionic organic solute-mineral systems, the adsorption isotherms of some low-molecular-weight nonpolar nonionic solutes (1,2,3-trichlorobenzene, lindane, phenanthrene, and pyrene) and polar nonionic solutes (1,3-dinitrobenzene and 2,4-dinitrotoluene) from single- and binary-solute solutions on hydrophilic silica and alumina were established. Toward this objective, the influences of temperature, ionic strength, and pH on adsorption were also determined. It is found that linear adsorption exhibits low exothermic heats and practically no adsorptive competition. The solute-solid configuration and the adsorptive force consistent with these effects were hypothesized. For nonpolar solutes, the adsorption occurs presumably by London (dispersion) forces onto a water film above the mineral surface. For polar solutes, the adsorption is also assisted by polar-group interactions. The reduced adsorptive forces of solutes with hydrophilic minerals due to physical separation by the water film and the low fractions of the water-film surface covered by solutes offer a theoretical basis for linear solute adsorption, low exothermic heats, and no adsorptive competition. The postulated adsorptive forces are supported by observations that ionic strength or pH poses no effect on the adsorption of nonpolar solutes while it exhibits a significant effect on the uptake of polar solutes.     

Electrochemical multiwalled carbon nanotube filter for viral and bacterial removal and inactivation

Nanotechnology has potential to offer solutions to problems facing the developing world. Here, we demonstrate the efficacy of an anodic multiwalled carbon nanotube (MWNT) microfilter toward the removal and inactivation of viruses (MS2) and bacteria (E. coli). In the absence of electrolysis, the MWNT filter is effective for complete removal of bacteria by sieving and multilog removal of viruses by depth-filtration. Concomitant electrolysis during filtration results in significantly increased inactivation of influent bacteria and viruses. At applied potentials of 2 and 3 V, the electrochemical MWNT filter reduced the number of bacteria and viruses in the effluent to below the limit of detection. Application of 2 and 3 V for 30 s postfiltration inactivated >75% of the sieved bacteria and >99.6% of the adsorbed viruses. Electrolyte concentration and composition had no correlation to electrochemical inactivation consistent with a direct oxidation mechanism at the MWNT filter surface. Potential dependent dye oxidation and E. coli morphological changes also support a direct oxidation mechanism. Advantages of the electrochemical MWNT filter for pathogen removal and inactivation and potential for point-of-use drinking water treatment are discussed.     

Electrochemical carbon-nanotube filter performance toward virus removal and inactivation in the presence of natural organic matter

The performance of an electrochemical multiwalled carbon nanotube (EC-MWNT) filter toward virus removal and inactivation in the presence of natural organic matter was systematically evaluated over a wide range of solution chemistries. Viral removal and inactivation were markedly enhanced by applying DC voltage in the presence of alginate and Suwannee River natural organic matter (SRNOM). Application of 2 or 3 V resulted in complete (5.8 to 7.4 log) removal and significant inactivation of MS2 viral particles in the presence of 5 mg L(-1) of SRNOM or 1 mg L(-1) of alginate. The EC-MWNT filter consistently maintained high performance over a wide range of solution pH and ionic strengths. The underlying mechanisms of enhanced viral removal and inactivation were further elucidated through EC-MWNT filtration experiments using carboxyl latex nanoparticles. We conclude that enhanced virus removal is attributed to the increased viral particle transport due to the applied external electric field and the attractive electrostatic interactions between the viral particles and the anodic MWNTs. The adsorbed viral particles on the MWNT surface are then inactivated through direct surface oxidation. Minimal fouling of the EC-MWNT filter was observed, even after 4-h filter runs with solutions containing 10 mg L(-1) of natural organic matter and 1 mM CaCl(2). Our results suggest that the EC-MWNT filter has a potential for use as a high performance point-of-use device for the removal of viruses from natural and contaminated waters with minimal power requirements.     

Natural polysaccharides and their interactions with dye molecules: applications in effluent treatment

Dyeing effluent is one of the largest contributors to textile effluent and such colored wastewater has a seriously destructive impact on the environment. Adsorption can be a very effective treatment for decolorization of textile dyeing effluent, but current techniques employ adsorption chemistry that is not particularly environmentally friendly, such as the use of alum. In this study, natural polysaccharides were used as adsorbents for removal of dye molecules from effluent. The results showed that naturally cationic polysaccharides such as chitin and chitosan gave excellent levels of color removal, and this was attributed to a combination of electrostatic attraction, van der Waals forces, and hydrogen bonding. Nonionic galactomannans (locust bean gum, guar gum, cassia gum) were also highly effective in removing dye from effluent, whereas other nonionic polysaccharides, such as starch, were not effective. This was attributed to the structure of the polysaccharides and the relative degree of inter- and intramolecular interactions between separate polymer chains. The pendant galactose residues of galactomannans prevented strong interaction, allowing greater hydrogen bonding with dye; comparatively, starch has extensive chain interactions, and as such had limited potential for hydrogen bonding with the dye molecules at the temperature of application. In addition, hydrophobic interactions between the hydrophobic parts of the dye and the alpha-face of the pendant galactose residues may have contributed to the superior performance. Repulsion between anionic polysaccharides and the dye anions prevented any hydrogen bonding and as such pectin, carrageenans, and alginic acid were not effective in dye removal from effluent. The use of galactomannans derived from plants in this system presents a sustainable method of effluent treatment. The raw materials are derived from renewable plant sources and are available in tonnage quantities, the adsorption system itself is highly effective and does not involve any additional chemical input or treatment other than the use of the adsorbent, and the adsorption agents themselves are nontoxic and biodegradable.     

Hydraulic and pollutant removal performance of fine media stormwater filtration systems

Stormwater runoff from urban areas has multiple negative hydrologic and ecological impacts for receiving waters. Fine media stormwater filtration systems have the potential to mitigate these effects, through flow attenuation and pollutant removal. This work provides an overall assessment of the hydraulic and pollutant removal behavior of sand- and soil-based stormwater filters at the laboratory scale. The influence of time, cumulative inflow sediment, cumulative water volume, wetting and drying, and compaction on hydraulic capacity was investigated. The results suggested that the primary cause of hydraulic failure was formation of a clogging layer at the filter surface. Loads of sediment and heavy metals were effectively retained; however,the soil-based filters leached nitrogen and phosphorus for the duration of the experimental period. Media pollutant profiles revealed significant accumulation of all pollutants in the top 20% of the filter profile, suggesting that elevated discharges of nutrients was due to leaching of native material, rather than failure to remove incoming pollutants. It is recommended that the top 2-5 cm of the filter surface be scraped off every two years to prevent hydraulic failure; this will also avoid excessive accumulation of heavy metals, which may otherwise have been of concern.     

Sorption and desorption of arsenic to ferrihydrite in a sand filter

Elevated arsenic concentrations in drinking water occur in many places around the world. Arsenic is deleterious to humans, and consequently, As water treatment techniques are sought. To optimize arsenic removal, sorption and desorption processes were studied at a drinking water treatment plant with aeration and sand filtration of ferrous iron rich groundwater at Elmevej Water Works, Fensmark, Denmark. Filter sand and pore water were sampled along depth profiles in the filters. The sand was coated with a 100-300 microm thick layer of porous Si-Ca-As-contaning iron oxide (As/Fe = 0.17) with locally some manganese oxide. The iron oxide was identified as a Si-stabilized abiotically formed two-line ferrihydrite with a magnetic hyperfine field of 45.8 T at 5 K. The raw water has an As concentration of 25 microg/L, predominantly as As(II). As the water passes through the filters, As(III) is oxidized to As(V) and the total concentrations drop asymptotically to a approximately 15 microg/L equilibrium concentration. Mn is released to the pore water, indicating the existence of reactive manganese oxides within the oxide coating, which probably play a role for the rapid As(III) oxidation. The As removal in the sand filters appears controlled by sorption equilibrium onto the ferrihydrite. By addition of ferrous chloride (3.65 mg of Fe(II)/L) to the water stream between two serially connected filters, a 3 microg/L As concentration is created in the water that infiltrates into the second sand filter. However, as water flow is reestablished through the second filter, As desorbs from the ferrihydrite and increases until the 15 microg/L equilibrium concentration. Sequential chemical extractions and geometrical estimates of the fraction of surface-associated As suggest that up to 40% of the total As can be remobilized in response to changes in the water chemistry in the sand filter.     

Effect of process variables and natural organic matter on removal of microcystin-LR by PAC-UF

The release of cyanobacterial toxins, such as microcystin-LR, in drinking water supplies is of increasing concern. In this study, we investigated the use of ultrafiltration (UF) combined with adsorption on powdered activated carbon (PAC) for the removal of microcystin-LR from drinking water. Process variables examined included PAC type, PAC dosage, membrane characteristics (material and pore size), and the presence of natural organic matter (NOM). Due to greater mesopore volume, wood-based activated carbon was up to 4-times more effective at removing microcystin-LR than coconut-based carbon, depending on contact time. Cellulose acetate (CA) membranes with a molecular weight cutoff (MWCO) of 20,000 Da did not reject or adsorb microcystin-LR. Membranes composed of polyethersulfone (PES) of similar pore size, on the other hand, adsorbed microcystin-LR presumably through hydrophobic interactions. A PES membrane with a MWCO of 5000 Da sorbed microcystin-LR, and also rejected 8.4% of the toxin through a size exclusion mechanism. When PAC was coupled to UF using PES membranes, greater removal of microcystin-LR occurred compared to when CA membranes were used due to sorption of the toxin to the PES membrane surface. The presence of Suwannee River fulvic acid (SRFA) reduced microcystin-LR removal by PAC-UF, primarily due to competition between SRFA and microcystin-LR for sites on the PAC surface.     

Variability of virus attachment patterns to butterhead lettuce

Enteric viruses account for most foodborne illness in the United States. The objective of this study was to determine whether the isoelectric point (pI) of viruses such as feline calicivirus (FCV), echovirus 11, and bacteriophages phiX174 and MS2 had any effect on their attachment to butterhead lettuce. The adsorption of virus particles to the lettuce was variable. Bacteriophage MS2 was the only virus that fit the current Derjaguin-Landau-Verway-Overbeek model of virus attachment. Echovirus 11 had the highest affinity to lettuce surface. Echovirus 11 appeared to exhibit reversible attachment above its pI, whereas below its pI strong adsorption was observed. Adsorption of FCV was at its maximum above its pI. Bacteriophage phiX174 exhibited the most complex adsorption pattern, with attachment occurring only at the pH extremes (pH 3.0 and 8.0). These results suggest the current model for virus adsorption to sediment does not adequately explain the attachment of virus to lettuce. Importantly, the results indirectly suggest that current sample processing methods to recover viruses from lettuce may differentially select for the recovery of only certain virus types.     

Bacterial community structure in the drinking water microbiome is governed by filtration processes

The bacterial community structure of a drinking water microbiome was characterized over three seasons using 16S rRNA gene based pyrosequencing of samples obtained from source water (a mix of a groundwater and a surface water), different points in a drinking water plant operated to treat this source water, and in the associated drinking water distribution system. Even though the source water was shown to seed the drinking water microbiome, treatment process operations limit the source water's influence on the distribution system bacterial community. Rather, in this plant, filtration by dual media rapid sand filters played a primary role in shaping the distribution system bacterial community over seasonal time scales as the filters harbored a stable bacterial community that seeded the water treatment processes past filtration. Bacterial taxa that colonized the filter and sloughed off in the filter effluent were able to persist in the distribution system despite disinfection of finished water by chloramination and filter backwashing with chloraminated backwash water. Thus, filter colonization presents a possible ecological survival strategy for bacterial communities in drinking water systems, which presents an opportunity to control the drinking water microbiome by manipulating the filter microbial community. Grouping bacterial taxa based on their association with the filter helped to elucidate relationships between the abundance of bacterial groups and water quality parameters and showed that pH was the strongest regulator of the bacterial community in the sampled drinking water system.     

Biogenic Fe(III) minerals lower the efficiency of iron-mineral-based commercial filter systems for arsenic removal

Millions of people worldwide are affected by As (arsenic) contaminated groundwater. Fe(III) (oxy)hydroxides sorb As efficiently and are therefore used in water purification filters. Commercial filters containing abiogenic Fe(III) (oxy)hydroxides (GEH) showed varying As removal, and it was unclear whether Fe(II)-oxidizing bacteria influenced filter efficiency. We found up to 10(7) Fe(II)-oxidizing bacteria/g dry-weight in GEH-filters and determined the performance of filter material in the presence and absence of Fe(II)-oxidizing bacteria. GEH-material sorbed 1.7 mmol As(V)/g Fe and was ~8 times more efficient than biogenic Fe(III) minerals that sorbed only 208.3 μmol As(V)/g Fe. This was also ~5 times more efficient than a 10:1-mixture of GEH-material and biogenic Fe(III) minerals that bound 322.6 μmol As(V)/g Fe. Coprecipitation of As(V) with biogenic Fe(III) minerals removed 343.0 μmol As(V)/g Fe, while As removal by coprecipitation with biogenic minerals in the presence of GEH-material was slightly less efficient as GEH-material only and yielded 1.5 mmol As(V)/g Fe. The present study thus suggests that the formation of biogenic Fe(III) minerals lowers rather than increases As removal efficiency of the filters probably due to the repulsion of the negatively charged arsenate by the negatively charged biogenic minerals. For this reason we recommend excluding microorganisms from filters (e.g., by activated carbon filters) to maintain their high As removal capacity.     

Impact of surface heterogeneity on mercury uptake by carbonaceous sorbents under UHV and atmospheric pressure

Chemical and morphological heterogeneities of carbon sorbents play important roles in gas-phase adsorption. However, the specific chemical complexes and topological structures of carbon that favor or impede elemental mercury uptake are not well understood and are the subject of this study. Temperature programmed desorption (TPD) with a model carbonaceous material (highly oriented pyrolytic graphite, HOPG) under ultrahigh vacuum (UHV) conditions and fixed bed adsorption by activated carbon (BPL) at atmospheric pressure were combined to investigate the effects of chemical and morphological heterogeneities on mercury adsorption by carbonaceous surfaces. TPD results show that mercury adsorption at 100 K onto HOPG surfaces with and without chemical functional groups and topological heterogeneity created by plasma oxidation occurs through physisorption. The removal of chemical functionalities from the HOPG surface enhances mercury physisorption. Plasma-oxidation of HOPG provides additional surface area for mercury adsorption. However, the pits created by plasma oxidation are more than 10 nm in diameter and do not simulate microporosity that predominates in activated carbons. Mercury adsorption by activated carbon at atmospheric pressure occurs through two distinct mechanisms. Physisorption governs mercury adsorption at lower temperatures (i.e., below 348 K), while chemisorption predominates at high adsorption temperatures (i.e., above 348 K). Presence of water on activated carbon surface enhances mercury uptake by both physisorption and chemisorption. Oxygen containing functional groups reduce mercury uptake by physisorption by blocking access to the micropores. No significant impact of oxygen functionalities was observed in the chemisorption regime. The key findings of this study open the possibility to apply scientific information obtained from the studies with simple surfaces such as HOPG under ideal conditions (UHV) to industrial sorbents under realistic process conditions.     

Gas/solid partitioning of semivolatile organic compounds (SOCs) to air filters. 3. An analysis of gas adsorption artifacts in measurements of atmospheric SOCs and organic carbon (OC). WHen using teflon membrane filters and quartz fiber filters

Adsorption of gaseous semivolatile organic compounds (SOCs) onto the filter(s) of a filter/sorbent sampler is a potential source of measurement error when determining specific SOCs as well as organic carbon (OC) levels in the atmosphere. This work examines partitioning to both Teflon membrane filters (TMFs) and quartz fiber filters (QFFs) for purposes of predicting the magnitude of the compound-dependent gas adsorption artifact as a function of various sampling parameters. The examination is based on values of Kp,face (m3 cm(-2)), the gas/filter partition coefficient expressed as [ng sorbed per cm2 of filter face]/[ng per m3 in the gas phase]. Values of Kp,face were calculated based on literature values of the gas/solid partition coefficient Kp,s [ng sorbed per m2 of filter]/[ng per m3 in gas phase] for the adsorption of various polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzodioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) to TMFs, and for the adsorption of PAHs to QFFs. At relative humidity (RH) values below approximately 50%, the Kp,face values for PAHs are lower on TMFs than on ambient-backup QFFs. The gas adsorption artifact will therefore be lower for PAHs with TMFs than with QFFs under these conditions. In the past, corrections for the gas/filter adsorption artifact have been made by using a backup filter, and subtracting the mass amount of each compound found on the backup filter from the total (particle phase + sorbed on filter) amount found on the front filter. This procedure assumes that the ng cm(-2) amounts of each SOC sorbed on the front and backup filters are equal. That assumption will only be valid after both filters have reached equilibrium with each of the gaseous SOCs in the incoming sample air. The front filter will reach equilibrium first. The minimum air sample volume Vmin,f+b required to reach gas/filter sorption equilibrium with a pair of filters is 2Kp,face Afilter where Afilter (cm2) is the per-filter face area. Kp,face values, and therefore Vmin,f+b values, depend on the compound, relative humidity (RH), temperature, and filter type. Compound-dependent Vmin,f+b values are presented for PAHs and PCDD/Fs on both TMFs and QFFs. Compound-dependent equations which give the magnitude of the filter adsorption artifact are presented for a range of different sampling arrangements and circumstances. The equations are not intended for use in actually correcting field data because of uncertainties in actual field values of relevant parameters such as the compound-dependent Kp,face and gas/particle Kp values, and because of the fact that the equations assume ideal step-function chromatographic movement of gas-phase compounds through the adsorbing filter. Rather, the main utility of the equations is as guidance tools in designing field sampling efforts that utilize filter/sorbent samplers and in evaluating prior work. The results indicate that some backup-filter-based corrections described in the literature were carried out using sample volumes that were too small to allow proper correction for the gas adsorption artifactfor some specific SOCs of interest. Similar conclusions are reached regarding artifacts associated with the measurement of gaseous and particulate OC.     

Filter and electrostatic samplers for semivolatile aerosols: physical artifacts

Adsorptive and evaporative artifacts often bias measurements of semivolatile aerosols. Adsorption occurs when the sampling method disrupts the gas-particle partitioning equilibrium. Evaporation occurs because concentrations of semivolatiles are rarely constant over time. Filtration is subject to both adsorptive and evaporative artifacts. By comparison, electrostatic precipitation reduces these artifacts by minimizing the surface area of collected particles without substantially disrupting the gas-particle equilibrium. The extent of these artifacts was determined for filter samplers and electrostatic precipitator samplers for semivolatile alkane aerosols in the laboratory. Adsorption of gas-phase semivolatiles was lower in electrostatic precipitators by factors of 5-100 compared to the filter method. Particle evaporation from the electrostatic sampler was 2.3 times lower than that from TFE-coated glass-fiber filters. Use of a backup filter to correct for compound-specific adsorption artifacts can introduce positive or negative errors to the measured particle-phase concentration due to competition among the adsorbates for available adsorption sites. Adsorption of evaporated particles from the front filter onto the backup filter increased the measured evaporative artifact by a factor of 1.5-2.     

Arsenic removal from groundwater by household sand filters: comparative field study, model calculations, and health benefits

Arsenic removal efficiencies of 43 household sand filters were studied in rural areas of the Red River Delta in Vietnam. Simultaneously, raw groundwater from the same households and additional 31 tubewells was sampled to investigate arsenic coprecipitation with hydrous ferric iron from solution, i.e., without contact to sand surfaces. From the groundwaters containing 10-382 microg/L As, < 0.1-48 mg/L Fe, < 0.01-3.7 mg/L P, and 0.05-3.3 mg/L Mn, similar average removal rates of 80% and 76% were found for the sand filter and coprecipitation experiments, respectively. The filtering process requires only a few minutes. Removal efficiencies of Fe, phosphate, and Mn were > 99%, 90%, and 71%, respectively. The concentration of dissolved iron in groundwater was the decisive factor for the removal of arsenic. Residual arsenic levels below 50 microg/L were achieved by 90% of the studied sand filters, and 40% were even below 10 microg/L. Fe/As ratios of > or = 50 or > or = 250 were required to ensure arsenic removal to levels below 50 or 10 microg/L, respectively. Phosphate concentrations > 2.5 mg P/L slightly hampered the sand filter and coprecipitation efficiencies. Interestingly, the overall arsenic elimination was higher than predicted from model calculations based on sorption constants determined from coprecipitation experiments with artificial groundwater. This observation is assumed to result from As(lll) oxidation involving Mn, microorganisms, and possibly dissolved organic matter present in the natural groundwaters. Clear evidence of lowered arsenic burden for people consuming sand-filtered water is demonstrated from hair analyses. The investigated sand filters proved to operate fast and robust for a broad range of groundwater composition and are thus also a viable option for mitigation in other arsenic affected regions. An estimation conducted for Bangladesh indicates that a median residual level of 25 microg/L arsenic could be reached in 84% of the polluted groundwater. The easily observable removal of iron from the pumped water makes the effect of a sand filter immediately recognizable even to people who are not aware of the arsenic problem.     

Sustainable colloidal-silver-impregnated ceramic filter for point-of-use water treatment

Cylindrical colloidal-silver-impregnated ceramic filters for household (point-of-use) water treatment were manufactured and tested for performance in the laboratory with respect to flow rate and bacteria transport. Filters were manufactured by combining clay-rich soil with water, grog (previously fired clay), and flour, pressing them into cylinders, and firing them at 900 degrees C for 8 h. The pore-size distribution of the resulting ceramic filters was quantified by mercury porosimetry. Colloidal silver was applied to filters in different quantities and ways (dipping and painting). Filters were also tested without any colloidal-silver application. Hydraulic conductivity of the filters was quantified using changing-head permeability tests. [3H]H2O water was used as a conservative tracer to quantify advection velocities and the coefficient of hydrodynamic dispersion. Escherichia coli (E. coli) was used to quantify bacterial transport through the filters. Hydraulic conductivity and pore-size distribution varied with filter composition; hydraulic conductivities were on the order of 10(-5) cm/s and more than 50% of the pores for each filter had diameters ranging from 0.02 to 15 microm. The filters removed between 97.8% and 100% of the applied bacteria; colloidal-silver treatments improved filter performance, presumably by deactivation of bacteria. The quantity of colloidal silver applied per filter was more important to bacteria removal than the method of application. Silver concentrations in effluent filter water were initially greater than 0.1 mg/L, but dropped below this value after 200 min of continuous operation. These results indicate that colloidal-silver-impregnated ceramic filters, which can be made using primarily local materials and labor, show promise as an effective and sustainable point-of-use water treatment technology for the world's poorest communities.