Lewis Semprini

Influence of liberated silver from silver nanoparticles on nitrification inhibition of Nitrosomonas europaea

The ecotoxicity of silver nanoparticles (Ag-NPs) to wastewater biota, including ammonia oxidizing bacteria (AOB), is gaining increasing interest as the number of products containing Ag-NPs continues to rise exponentially and they are expected to accumulate in wastewater treatment plants. This research demonstrated that the addition order of Ag-NP and the media constituents had a profound influence on the stability of the Ag-NP suspension and the corresponding repeatability of results and sensitivity of Nitrosomonas europaea. N. europaea, a model AOB, was found to be extremely sensitive to ionic silver (Ag(+)) and two sizes of Ag-NPs (20 and 80 nm). Ag(+) exposures resulted in the highest level of toxicity with smaller Ag-NPs (20 nm) being more toxic than larger Ag-NPs (80 nm). The increased sensitivity of N. europaea to smaller Ag-NPs was caused by their higher rates of dissolved silver (dAg) release, via dissolution, due to a greater surface area to volume ratio. dAg was shown to be responsible for the vast majority of the observed Ag-NP toxicity, as determined by abiotic Ag-NP dissolution tests. For the sizes of Ag-NP studied (20 and 80 nm), there appears to be a negligible nanoparticle-specific toxicity. This was further supported by similarities in inhibition mechanisms between Ag(+) and Ag-NP, with both causing decreases in AMO activity and destabilization of the outer-membrane of N. europaea. Finally, equal concentrations of total silver were found to be tightly associated to both Ag(+) and Ag-NP-exposed cells despite Ag-NP concentrations being five times greater, by mass, than Ag(+) concentrations.

Anaerobic Transformation of Chlorinated Aliphatic Hydrocarbons in a Sand Aquifer Based on Spatial Chemical Distributions

We estimated the distribution of chlorinated aliphatic hydrocarbons (CAHs) from groundwater samples collected along three transects in a sand aquifer. Trichloroethylene (TCE) leaked and contaminated the aquifer probably more than a decade before we collected the measurements. The data show significant concentrations of TCE, cis-l,2-dichloroethylene (c-DCE), vinyl chloride (VC), and ethene. We attributed DCE, VC, and ethene to the reductive dehalogenation of TCE. The CAH concentrations varied significantly with depth and correlate with sulfate and methane concentrations. Anoxic aquifer conditions exist with methane present at relatively high concentrations at depth. High concentrations of TCE correspond with the absence of methane or low methane concentrations, whereas products of TCE dehalogenation are associated with higher methane concentrations and low sulfate concentrations. Indications are that the dechlorination of TCE and DCE to VC and ethene is associated with sulfate reduction and active methanogenesis. TCE dechlorination to DCE is likely occurring under the less reducing conditions of sulfate reduction, with further reductions to VC and ethene occurring under methanogenic conditions. We estimated that about 20% of TCE has dechlorinated to ethene. The analysis of the data enhanced our knowledge of natural in situ transformation and transport processes of CAHs.

Strategies for the aerobic co-metabolism of chlorinated solvents

Recent field and laboratory studies have evaluated the potential for aerobic co-metabolism of chlorinated solvents. Different co-metabolic substrates and different methods of application have been tried, including growing indigenous microbes in situ, and injecting into the soil subsurface strains grown in subsurface reactors for their co-metabolic potential. There is much potential for using co-metabolism for treating a broad range of chlorinated aliphatic hydrocarbons. Recirculation wells have potential for adding soluble co-metabolic substrates (i.e. phenol and toluene) into contaminated aquifers, while direct addition of gaseous substrates (i.e. methane and propane) into aquifers also holds promise. Aromatic substrates (phenol and toluene) are best used for treatment of chlorinated ethenes, whereas gaseous co-metabolic substrate (methane and propane) are better suited for the treatment of chlorinated methanes and ethanes. Many factors can enhance co-metabolic transformations, such as nutrients and available energy sources.

Laboratory, Field and Modeling Studies of Radon-222 as a Natural Tracer for Monitoring NAPL Contamination

The recently developed natural radon tracer method has potential as a rapid, low‐cost, nondestructive, and noninvasive method for quantifying NAPL contamination. In the subsurface, radon‐222 (radon) is produced by the decay of naturally occurring radium‐226 contained in the mineral fraction of aquifer solids. In groundwater radon occurs as a dissolved gas, with a half‐life of 3.83 days. In the absence of NAPL, the radon concentration in groundwater quickly reaches a maximum value that is determined by the mineral composition of the aquifer solids, which controls the rate of radon emanation. In the presence of NAPL, however, the radon concentration in the groundwater is substantially reduced due to the preferential partitioning of radon into the organic NAPL phase. A simple equilibrium model and supporting laboratory studies show the reduction in radon concentration can be quantitatively correlated with residual NAPL saturation. Thus, by measuring the spatial distribution in radon it may be possible to identify locations where residual NAPL is present and to quantify the NAPL saturation. When the basic processes of partitioning, radon emanation from the aquifer solids, and first‐order decay are incorporated into an advective/dispersive transport model, good agreement is obtained with the results of laboratory and field experiments. Model sensitivity analyses shows many factors can contribute to the radon concentration response, including the length of the NAPL zone, NAPL saturation, groundwater velocity, porosity, and radon emanation. Thus, care must be taken when applying the radon method to locate and quantify NAPL contamination in the subsurface.

Influence of Bovine Serum Albumin and Alginate on Silver Nanoparticle Dissolution and Toxicity to Nitrosomonas europaea

Bovine serum albumin (BSA), a model protein, reduced the toxicity of 20 nm citrate silver nanoparticles (AgNP) toward Nitrosomonas europaea, a model ammonia oxidizing bacteria, through a dual-mode protection mechanism. BSA reduced AgNP toxicity by chelating the silver ions (Ag(+)) released from the AgNPs. BSA further reduced AgNP toxicity by binding to the AgNP surface thus preventing NH3-dependent dissolution from occurring. Due to BSAs affinity toward Ag(+) chemisorbed on the AgNP surface, increased concentrations of BSA lead to increased AgNP dissolution rates. This, however, did not increase AgNP toxicity as the dissolved Ag(+) were adsorbed onto the BSA molecules. Alginate, a model extracellular polysaccharide (EPS), lacks strong Ag(+) ligands and was unable to protect N. europaea from Ag(+) toxicity. However, at high concentrations, alginate reduced AgNP toxicity by binding to the AgNP surface and reducing AgNP dissolution rates. Unlike BSA, alginate only weakly interacted with the AgNP surface and was unable to completely prevent NH3-dependent AgNP dissolution from occurring. Based on these results, AgNP toxicity in high protein environments (e.g., wastewater) is expected to be muted while the EPS layers of wastewater biofilms may provide additional protection from AgNPs, but not from Ag(+) that have already been released.

Aerobic Cometabolism of Chloroform and 1,1,1-Trichloroethane by Butane-Grown Microorganisms

Abstract Aerobic cometabolism of chloroform (CF) and 1,1,1-trichloroethane (1,1,1-TCA) was observed by subsurface microorganisms grown on butane. Studies performed in batch incubated microcosms were screened for CF transformation potential using the following cometabolic substrates: ammonia, methane, propane, butane, propene, octane, isoprene, and phenol. CF transformation was observed in microcosms fed ammonia, methane, propane, and butane. The butane microcosms achieved the most effective transformation. The transformation of CF and 1,1,1-TCA was strongly correlated with butane utilization and oxygen consumption. CF transformation ceased in the absence of butane or when oxygen was depleted to low concentrations in the microcosms. No transformation of carbon tetrachloride was observed. With successive additions of CF and butane to the microcosms, complete transformation of CF was achieved at solution concentrations as high as 1 mg/L. High CF concentrations appeared to inhibit butane utilization. Maximum ...

Expression of merA, amoA and hao in continuously cultured Nitrosomonas europaea cells exposed to zinc chloride additions

The effects of ZnCl2 additions on a mercuric reductase, merA, ammonia monooxygenase, amoA, and hydroxylamine (NH2OH) oxidoreductase, hao, gene expression were examined in continuously cultured Nitrosomonas europaea cells. The reactor was operated for 85 days with a 6.9 d hydraulic retention time and with four successive additions of ZnCl2 achieving maximum concentrations from 3 to 90 µM Zn2+. Continuously cultured N. europaea cells were more resistant to Zn2+ inhibition than previously examined batch cultured cells due to the presence of Mg2+ in the growth media, suggesting that Zn2+ enters the cell through Mg2+ import channels. The maximum merA up‐regulation was 45‐fold and expression increased with increases in Zn2+ concentration and decreased as Zn2+ concentrations decreased. Although Zn2+ irreversibly inactivated ammonia oxidation in N. europaea, the addition of either 600 µM CuSO4 or 2250 µM MgSO4 protected N. europaea from ZnCl2 inhibition, indicating a competition between Zn2+ and Cu2+/Mg2+ for uptake and/or AMO active sites. Since ZnCl2 inhibition is irreversible and amoA was up‐regulated at 30 and 90 µM additions, it is hypothesized that de novo synthesis of the AMO enzyme is needed to overcome inhibition. The up‐regulation of merA during exposure to non‐inhibitory Zn2+ levels indicates that merA is an excellent early warning signal for Zn2+ inhibition. Biotechnol. Bioeng. 2009;102: 546–553.

Continuous-flow column study of reductive dehalogenation of PCE upon bioaugmentation with the Evanite enrichment culture

A continuous-flow anaerobic column experiment was conducted to evaluate the reductive dechlorination of tetrachloroethene (PCE) in Hanford aquifer material after bioaugmentation with the Evanite (EV) culture. An influent PCE concentration of 0.09 mM was transformed to vinyl chloride (VC) and ethene (ETH) within a hydraulic residence time of 1.3 days. The experimental breakthrough curves were described by the one-dimensional two-site-nonequilibrium transport model. PCE dechlorination was observed after bioaugmentation and after the lactate concentration was increased from 0.35 to 0.67 mM. At the onset of reductive dehalogenation, cis-dichloroethene (c-DCE) concentrations in the column effluent exceeded the influent PCE concentration indicating enhanced PCE desorption and transformation. When the lactate concentration was increased to 1.34 mM, c-DCE reduction to vinyl chloride (VC) and ethene (ETH) occurred. Spatial rates of PCE and VC transformation were determined in batch-incubated microcosms constructed with aquifer samples obtained from the column. PCE transformation rates were highest in the first 5 cm from the column inlet and decreased towards the column effluent. Dehalococcoides cell numbers dropped from approximately 73.5% of the total Bacterial population in the original inocula, to about 0.5% to 4% throughout the column. The results were consistent with estimates of electron donor utilization, with 4% going towards dehalogenation reactions.

Influence of ammonia on silver nanoparticle dissolution and toxicity to Nitrosomonas europaea

Nitrosomonas europaea, a model ammonia oxidizing bacterium, was sensitive to both ionic silver (Ag(+)) and 20 nm citrate capped silver nanoparticles (AgNPs). AgNP toxicity has been previously shown to be primarily due to the dissolution of Ag(+). The rate of AgNP dissolution dramatically increased in test medium containing ammonium sulfate ((NH4)2SO4) and HEPES buffer compared to test medium containing either deionized water or HEPES buffer alone. The AgNP dissolution rates accelerated with increases in ammonia (NH3) concentrations either through increases in pH or through higher (NH4)2SO4 concentrations. Ammonia likely participated in the oxidation of the AgNP to form [Formula: see text] in solution leading to the observed increase in AgNP dissolution rates. AgNP toxicity was enhanced as NH3 concentrations increased. However, Ag(+) toxicity was constant at all NH3 concentrations tested. Therefore, it can be concluded that the increased AgNP toxicity was due to increased Ag(+) release and not due to a synergistic effect between NH3 and Ag(+). The results of this study may provide insights in the fate and toxicity of AgNPs in high NH3 environments including wastewater treatment plants, eutrophic waterways and alkaline environments.

Bioaugmentation of butane-utilizing microorganisms to promote cometabolism of 1,1,1-trichloroethane in groundwater microcosms

The transformation of 1,1,1-trichloroethane (1,1,1-TCA) in ioaugmented and non-augmented microcosms was evaluated. The microcosms contained roundwater and aquifer materials from a test site at Moffett Field, Sunnyvale, CA. The initial inoculum for bioaugmentation was a butane-utilizing enrichment from the subsurface of the Hanford DOE site. The non-augmented microcosm required 80 days of incubation before butane-utilization was observed while the augmented microcosms required 3 days. Initially the augmented microcosms were effective in transforming 1,1,1-TCA, but their transformation ability decreased after prolonged incubation. The non-augmented microcosms initially showed limited 1,1,1-TCA transformation but improved with time. After 440 days, both the non-augmented and augmented microcosms had similar transformation yields (0.04 mg 1,1,1-TCA/mg butane) and had similar microbial composition (DNA fingerprints). Subsequent microcosms, when bioaugmented with a Hanford enrichment that was repeatedly grown in 100% mineral media, did not effectively grow or transform 1,1,1-TCA under groundwater nutrient conditions. Microcosm tests to study the effect of mineral media on transformation ability were performed with the Hanford enrichment. Microcosms with 50% mineral media in groundwater most effectively utilized butane and transformed 1,1,1-TCA, while microcosms with groundwater only and microcosms with 5% mineral media in groundwater lost their 1,1,1-TCA transformation ability. DNA fingerprinting indicated shifts in the microbial composition with the different mineral media combinations. Successful bioaugmentation was achieved by enriching butane-utilizers from Moffett Field microcosms that were effective in groundwater with no mineral media added. The results suggest that successful in-situ bioaugmentation might be achieved through the addition of enriched cultures that perform well under subsurface nutrient conditions.