Aggregation and disaggregation of iron oxide nanoparticles: Influence of particle concentration, pH and natural organic matter

The surface coating, aggregation behavior and aggregate structure of unpurified iron oxide nanoparticles (NPs) at variable pH and in the absence and presence of natural organic matter (NOM, Suwannee River humic acid, SRHA) have been previously studied in Baalousha et al. [Baalousha, M., Manciulea, A., Cumberland, S., Kendall, K., Lead, J.R., Aggregation and surface properties of iron oxide nanoparticles; influence of pH and natural organic matter. Environ Toxicol Chem 2008; 27: 1875-1882.]. Here the aggregation behavior of iron oxide NPs at variable concentrations of NPs and SRHA, and the disaggregation behavior of iron oxide NP aggregates in the absence and presence of SRHA are investigated. The increase of NP concentration enhances their aggregation, particularly at pH values close to the point of zero charge (PZC). High concentration of SRHA (100 mg l^− 1) shifts the NP (100 mg l^− 1) PZC charge and aggregation maximum towards lower pHs, while low concentration (10 mg l^− 1) shows low or no effect. The disaggregation behavior of iron oxide NP aggregates was investigated at pH 7 and at increasing concentrations of SRHA. High concentrations (50 and 100 mg l^− 1) of SRHA induced the disaggregation of iron oxide NP aggregates with time, which was not the case at lower concentrations (10 mg l^− 1) or in the absence of SRHA. The disaggregation was triggered by the enhanced surface charge induced by the sorption of SRHA molecules. The disaggregation rate increased with SRHA concentration and decreased with time. Two regimes of disaggregation were identified, a fast regime of “fragmentation” at the first 15 days of the experiment and a slow regime of “erosion” afterwards. The formation of small aggregates of about 170 nm and surface coating of several nanometers of SRHA on iron oxide NPs confirm the role of NOM in the disaggregation process and indicate that NPs might mimic the behavior of natural colloids.     

Evaluation of nanoscale zerovalent iron particles for trichloroethene degradation in clayey soils

The longevity and reactivity of nanoscale zerovalent iron (nZVI) and palladized bimetallic particles (BNP) were evaluated in batch and column experiments for remediation of a trichloroethene (TCE)-contaminated plume within a clayey soil from Oak Ridge Reservation (ORR). Comparative studies assessing the viability of BNP and nZVI confirmed that particle behavior is severely affected by clay sediments. Surface morphology and composition analyses using SEM and SEM-energy-dispersive spectroscopy spectrum revealed particle agglomeration through the formation of clay-iron aggregates of greater mass during the early phase of the experiment. Batch study results suggest that TCE degradation in ORR clayey soil follows a pseudo-first-order kinetic model exhibiting reaction rate constants (k) of 0.05-0.24 day^− 1 at varied iron-to-soil ratios. Despite high reactivity in water, BNP were less effective in the site-derived clay sediment with calculated TCE removal efficiencies of 98.7% and 19.59%, respectively.A column experiment was conducted to investigate particle longevity and indicator parameters of the TCE degradation process under flow conditions. It revealed that the TCE removal efficiency gradually declined over the course of the experiment from 86-93% to 51-52%, correlating to a progressive increase in oxidation-reduction potential (ORP) from − 485 to − 250 mV and pH drop from 8.2-8.6 to 7.4-7.5. The rate of nZVI deactivation reaction was found to be a first order with a k_d value of 0.0058 day^− 1. SEM images of residual nZVI revealed heavily agglomerated particles. However, despite widespread oxidation and agglomeration, particles managed to maintain some capacity for oxidation. A quantitative analysis of nZVI deactivation has the potential of predicting nZVI longevity in order to improve the design strategy of TCE remediation.     

Differential vs. absolute UV absorbance approaches in studying NOM reactivity in DBPs formation: Comparison and applicability

Differential absorbance at wavelengths near 272 nm (−ΔA₂₇₂) has been used to track the halogenation of NOM, but its performance for different drinking water sources before and after water treatment processes has not been thoroughly ascertained. In this study, the behavior of −ΔA₂₇₂ during the halogenation process was determined to be strongly correlated with DBPs' concentrations regardless of the NOM properties. However, chlorination of different NOM samples resulted in different patterns when DBP concentrations were plotted vs. −ΔA₂₇₂. In order to quantify the reactivity of NOM in DBPs formation an alternative index, denoted as −ΔA₂₇₂(t = 2 h), that is the differential absorbance at 272 nm obtained at 2 h of reaction time and pH 7.0, was proposed. This parameter was strongly correlated with DBPs' concentrations regardless of the major chlorination conditions (chlorine dose, water temperature) and NOM properties (raw, treated and fractionated samples). Its performance was found better than that of other widely used surrogate parameters (i.e. DOC, SUVA₂₅₄, A₂₅₄, A₂₇₂) and it presents several options for field applications.     

Investigating dissolved air flotation performance with cyanobacterial cells and filaments

Dissolved air flotation (DAF) performance with two different naturally occurring cyanobacterial morphologies was investigated with respect to the biomass removal efficiency, the toxin release to water and the coagulant demand by different water background natural organic matter (NOM). Coagulation (C)/Flocculation (F)/DAF bench-scale experiments (2 min coagulation at 380 s⁻¹ with polyaluminium chloride (0.5-4 mg/L Al₂O₃, the dose depending on the water NOM content); 8 min flocculation at 70 s⁻¹; 8 min DAF with 5 bar relative pressure and 8% pressurised recycle) were performed with single cells of Microcystis aeruginosa and Planktothrix rubescens filaments spiked in synthetic waters with different NOM contents (hydrophobic vs. hydrophilic NOM; moderate (2-3 mgC/L) vs. moderate-high concentration (ca. 6 mgC/L)). For both morphologies, the results show no apparent cyanobacterial damage (since the water quality did not degrade in dissolved microcystins and the removal of intracellular microcystins matched the removal of chlorophyll a) and high biomass removal efficiencies (93-99% for cells and 92-98% for filaments) provided optimal coagulant dose for chlorophyll a removal was ensured. Charge neutralisation by the polyaluminium chloride was the main coagulation mechanism of the M. aeruginosa cells and most likely also of the P. rubescens filaments. The specific coagulant demand was severely affected by NOM hydrophobicity, hydrophobic NOM (with a specific UV_254nm absorbance, SUVA, above 4 L/(m mgC)) requiring ca. the triple of hydrophilic NOM (SUVA below 3 L/(m mgC)), i.e. 0.7 vs. 0.2-0.3 mg Al₂O₃/mg DOC.     

Secondary effects of anion exchange on chloride, sulfate, and lead release: systems approach to corrosion control

Water treatment processes can cause secondary changes in water chemistry that alter finished water quality including chloride, sulfate, natural organic matter (NOM), and metal release. Hence, the goal of this research was to provide an improved understanding of the chloride-to-sulfate mass ratio (CSMR) with regards to chloride and sulfate variations at full-scale water treatment plants and corrosion potential under simulated premise plumbing conditions. Laboratory corrosion studies were conducted using Pb-Sn solder/Cu tubing galvanic cells exposed to model waters with low (approx. 5 mg/L Cl⁻ and 10 mg/L SO₄^2-) and high (approx. 50 mg/L Cl⁻ and 100 mg/L SO₄^2-) concentrations of chloride and sulfate at a constant CSMR of ∼0.5. The role of NOM during corrosion was also evaluated by changing the type of organic material. In addition, full-scale sampling was conducted to quantify the raw water variability of chloride, sulfate, and NOM concentrations and the changes to these parameters from magnetic ion exchange treatment. Test conditions with higher concentrations of chloride and sulfate released significantly more lead than the lower chloride and sulfate test waters. In addition, the source of NOM was a key factor in the amount of lead released with the model organic compounds yielding significantly less lead release than aquatic NOM.     

Impact of natural organic matter and divalent cations on the stability of aqueous nanoparticles

The stability of nanoparticles in aquatic environment plays an important role in determining their environmental implication and potential risk to human health. This research studied the impact of natural organic matter (NOM) and divalent cations (Ca²⁺) on the stability of engineered metal oxide nanoparticles (e.g. ZnO, NiO, TiO₂, Fe₂O₃ and SiO₂). When nanoparticles were present in neutral water, a relatively weak electrolyte concentration (0.01 M KCl) could result in their aggregation; however, with the addition of 1 mg/L NOM, the negative surface charge of nanoparticles increased significantly and therefore their propensity to aggregate is reduced. 4 mg/L NOM stabilized most nanoparticles by producing −30 mV or higher zeta potentials. On the other hand, the negative charge that NOM imparted to nanoparticles could be neutralized by divalent cations (calcium ions). 0.04 M-0.06 M Ca²⁺ induced the aggregation of NOM-coated nanoparticles. It should be noted that among all the studied nanoparticles, SiO₂ exhibited the unique stability due to its low NOM adsorption capacity and small Hamaker constant. SiO₂ remained stable no matter whether the solution contained NOM or Ca²⁺.     

Aerobic treatment of N-nitrosodimethylamine in a propane-fed membrane bioreactor

N-Nitrosodimethylamine (NDMA) is a suspected human carcinogen that has recently been detected in wastewater, groundwater and drinking water. Treatment of this compound to low part-per-trillion (ng/L) concentrations is required to mitigate cancer risk. Current treatment generally entails UV irradiation, which while effective, is also expensive. The objective of this research was to explore potential bioremediation strategies as alternatives for treating NDMA to ng/L concentrations. Batch studies revealed that the propanotroph Rhodococcus ruber ENV425 was capable of metabolizing NDMA from 8 μg/L to <2 ng/L after growth on propane, and that the strain produced metabolites that do not pose a significant risk at the concentrations generated (Fournier et al., 2009). A laboratory-scale membrane bioreactor (MBR) was subsequently constructed to evaluate the potential for long-term ex situ treatment of NDMA. The MBR was seeded with ENV425 and received propane as the primary growth substrate and oxygen as an electron acceptor. At an average influent NDMA concentration of 7.4 μg/L and a 28.5 h hydraulic residence time, the reactor effluent concentration was 3.0 ± 2.3 ng/L (>99.95% removal) over more than 70 days of operation. The addition of trichloroethene (TCE) to the reactor resulted in a significant increase in effluent NDMA concentrations, most likely due to cell toxicity from TCE-epoxide produced during its cometabolic oxidation by ENV425. The data suggest that an MBR system can be a viable treatment option for NDMA in groundwater provided that high concentrations of TCE are not present.     

Synergistic effect of nickel ions on the coupled dechlorination of trichloroethylene and 2,4-dichlorophenol by Fe/TiO₂ nanocomposites in the presence of UV light under anoxic conditions

The coupled removal of priority pollutants by nanocomposite materials has recently been receiving much attention. In this study, trichloroethylene (TCE) and 2,4-dichlorophenol (DCP) in aqueous solutions were simultaneously removed by Fe/TiO₂ nanocomposites under anoxic conditions in the presence of nickel ions and UV light at 365 nm. Both TCE and DCP were effectively dechlorinated by Fe/TiO₂ nanocomposites, and the pseudo-first-order rate constants (k_obs) for TCE and DCP dechlorination were (1.39 ± 0.05)×10⁻² and (1.08 ± 0.05)×10⁻² h⁻¹, respectively, which were higher than that by nanoscale zerovalent iron alone. In addition, the k_obs for DCP dechlorination was enhanced by a factor of 77 when Fe/TiO₂ was illuminated with UV light for 2 h. Hydrodechlorination was found to be the major reaction pathway for TCE dechlorination, while DCP could undergo reductive dechlorination or react with hydroxyl radicals to produce 1,4-benzoquinone and phenol. TCE was a stronger electron acceptor than DCP, which could inhibit the dechlorination efficiency and rate of DCP during simultaneous removal processes. The addition of nickel ions significantly enhanced the simultaneous photodechlorination efficiency of TCE and DCP under the illumination of UV light. The k_obs values for DCP and TCE photodechlorination by Fe/TiO₂ in the presence of 20-100 μM Ni(II) were 30.4-136 and 13.2-192 times greater, respectively, when compared with those in the dark. Electron spin resonance analysis showed that the photo-generated electron-hole pairs could be effectively separated through Ni ions cycling, leading to the improvement of electron transfer efficiency of TCE and DCP by Fe/TiO₂.     

The influence of dissolved and surface-bound humic acid on the toxicity of TiO₂ nanoparticles to Chlorella sp

NOM is likely to coat TiO₂ nanoparticles (nano-TiO₂) discharged into the aquatic environment and influence the nanotoxicity to aquatic organisms, which however has not been well investigated. This study explored the influence of nanoparticle surface-bound humic acid (HA, as a model NOM) as well as dissolved HA on the toxicity of nano-TiO₂ to Chlorella sp., with a specific focus on adhesion of the nanoparticles to the algae. Results showed that nano-TiO₂ and the dissolved HA could inhibit the algal growth with an IC₅₀ of 4.9 and 8.4 mg L⁻¹, respectively, while both dissolved and nanoparticle surface-bound HA could significantly alleviate the algal toxicity of nano-TiO₂. IC₅₀ of nano-TiO₂ increased to 18 mg L⁻¹ in the presence of 5 mg L⁻¹ of the dissolved HA and to 48 mg L⁻¹ as the result of surface-saturation by HA. Co-precipitation experiment and transmission electron microscopy observation revealed that both dissolved and nanoparticle surface-bound HA prevented the adhesion of nano-TiO₂ to the algal cells due to the increased electrosteric repulsion. The generation of intracellular reactive oxygen species (ROS) was significantly limited by the dissolved and nanoparticle surface-bound HA. The prevention of adhesion and inhibition of ROS generation could account for the HA-mitigated nanotoxicity.     

Methanogenic community development in anaerobic granular bioreactors treating trichloroethylene (TCE)-contaminated wastewater at 37 °C and 15 °C

Four expanded granular sludge bed (EGSB) bioreactors were seeded with a mesophilically-grown granular sludge and operated in duplicate for mesophilic (37 °C; R1 & R2) and low- (15°; R3 & R4) temperature treatment of a synthetic volatile fatty acid (VFA) based wastewater (3 kg COD m⁻³ d⁻¹) with one of each pair (R1 & R3) supplemented with increasing concentrations of trichloroethylene (TCE; 10, 20, 40, 60 mg l⁻¹) and one acting as a control. Bioreactor performance was evaluated by % COD removal efficiency and % biogas methane (CH₄) content. Quantitative Polymerase Chain Reaction (qPCR) was used to investigate the methanogenic community composition and dynamics in the bioreactors during the trial, while specific methanogenic activity (SMA) and toxicity assays were utilized to investigate the activity and TCE/dichloroethylene (DCE) toxicity thresholds of key trophic groups, respectively. At both 37 °C and 15 °C, TCE levels of 60 mg l⁻¹ resulted in the decline of % COD removal efficiencies to 29% (Day 235) and 37% (Day 238), respectively, and in % biogas CH₄ to 54% (Day 235) and 5% (Day 238), respectively. Despite the inhibitory effect of TCE on the anaerobic digestion process, the main drivers influencing methanogenic community development, as determined by qPCR and Non-metric multidimensional scaling analysis, were (i) wastewater composition and (ii) operating temperature. At the apical TCE concentration both SMA and qPCR of methanogenic archaea suggested that acetoclastic methanogens were somewhat inhibited by the presence of TCE and/or its degradation derivatives, while competition by dechlorinating organisms may have limited the availability of H₂ for hydrogenotrophic methanogenesis. In addition, there appeared to be an inverse correlation between SMA levels and TCE tolerance, a finding that was supported by the analysis of the inhibitory effect of TCE on two additional biomass sources. The results indicate that low-temperature anaerobic digestion is a feasible approach for the treatment of TCE-containing wastewater.     

Effects of UV/H2O2 advanced oxidation on chemical characteristics and chlorine reactivity of surface water natural organic matter

The advanced oxidation process utilizing ultraviolet and hydrogen peroxide (UV/H₂O₂) is currently applied in commercial drinking water applications for the removal of various organic pollutants. Natural organic matter (NOM) present in the source water can also be oxidized and undergo changes at the fluence and H₂O₂ concentrations applied in commercial drinking water UV/H₂O₂ applications (fluences less than 2000 mJ cm⁻², initial H₂O₂ concentrations less than 15 mg L⁻¹). In this study, the impact of UV/H₂O₂ on NOM’s aromaticity, hydrophobicity, and potential to form trihalomethanes (THMs) and haloacetic acids (HAAs) was investigated for raw surface water and the same water with the very hydrophobic acid (VHA) fraction of NOM removed. During UV/H₂O₂ treatments, NOM in the raw surface water was partially oxidized to less aromatic and hydrophobic characteristics, but was not mineralized, confirming findings from past research. Below fluences of 1500 mJ cm⁻² UV/H₂O₂ treatment of the raw water did not lead to reduction in the formation potential of THMs. The formation potential of HAAs was reduced at a fluence of 500 mJ cm⁻² with only small additional reductions as fluence further increased. For the water from which the VHA fraction was removed, UV/H₂O₂ treatment led to mineralization of NOM suggesting that, when coupled with a pre-treatment capable of removing the VHA fraction, UV/H₂O₂ could achieve further reductions in NOM. These subsequent reductions in NOM led to continuous reductions in the formation potentials of THMs and HAAs as fluence increased.     

Low-temperature (7 °C) anaerobic treatment of a trichloroethylene-contaminated wastewater: microbial community development

The feasibility of low-temperature (7 °C) anaerobic digestion for the treatment of a trichloroethylene (TCE) contaminated wastewater was investigated. Two expanded granular sludge bed (EGSB) bioreactors (R1 and R2) were employed for the mineralisation of a synthetic volatile fatty acid based wastewater at an initial organic loading rate (OLR) of 3 kg COD m⁻³ d⁻¹, and an operating temperature of 15 °C. Successive reductions in OLR to 0.75 kg COD m⁻³ d⁻¹, and operational temperature to 7 °C, resulted in stable bioreactor operation by day 417, with COD removal efficiency and biogas CH₄ content ≥74%, for both bioreactors. Subsequently, the influent to R1 was supplemented with increasing concentrations (10, 20, 30 mg l⁻¹) of TCE, while R2 acted as a control. At an influent TCE concentration of 30 mg l⁻¹, although phase average TCE removal rates of 79% were recorded, a sustained decrease in R1 performance was observed, with COD removal of 6%, and % biogas CH₄ of 3% recorded on days 595 and 607, respectively. Specific methanogenic activity (SMA) assays identified a general shift from acetate- to hydrogen-mediated methanogenesis in both R1 and R2 biomass, while toxicity assays confirmed an increased sensitivity of the acetoclastic community in R1 to TCE and dichloroethylene (DCE), which contributed to acetate accumulation. Quantitative Polymerase Chain Reaction (qPCR) analysis of the methanogenic community confirmed the dominance of hydrogenotrophic methanogens in both R1 and R2, representing 71-89% of the total methanogenic population, however acetoclastic Methanosaeta were the dominant organisms, based on 16S rRNA gene clone library analysis of reactor biomass. The greatest change in the bacterial community, as demonstrated by UPGMA analysis of DGGE banding profiles, was observed in R1 biomass between days 417 and 609, although 88% similarity was retained between these sampling points.     

In-situ identification of iron electrocoagulation speciation and application for natural organic matter (NOM) removal

In this work, iron speciation in electrocoagulation (EC) was studied to determine the impact of operating parameters on natural organic matter (NOM) removal from natural water. Two electrochemical EC parameters, current density (i) and charge loading rate (CLR), were investigated. Variation of these parameters led to a near unity current efficiency (φ = 0.957 ± 0.03), at any combination of i in a range of 1–25 mA/cm² and CLR in a range of 12–300 C/L/min. Higher i and CLR led to a higher bulk pH and limited the amount of dissolved oxygen (DO) reduced at the cathode surface due to mass transfer limitations. A low i (1 mA/cm²) and intermediate CLR (60 C/L/min) resulted in low bulk DO (<2.5 mg/L), where green rust (GR) was identified by in-situ Raman spectroscopy as the primary crystalline electrochemical product. Longer electrolysis times at higher i led to magnetite (Fe₃O₄) formation. Both higher (300 C/L/min) and lower (12 C/L/min) CLR values led to increased DO and/or increased pH, with lepidocrocite (γ-FeOOH) as the only crystalline species observed. The NOM removal of the three identified species was compared, with conditions leading to GR formation showing the greatest dissolved organic carbon removal, and highest removal of the low apparent molecular weight (<550 Da) chromophoric NOM fraction, determined by high performance size exclusion chromatography.     

Stabilization of highly concentrated suspensions of iron nanoparticles using shear-thinning gels of xanthan gum

Nanoscale zerovalent iron (NZVI) particles have recently become subject of great interest in the field of groundwater remediation for their ability to treat a wide variety of organic and inorganic contaminants. However, the field application of this technology is strongly hindered by the lack of stability of NZVI water suspensions.This study demonstrates that highly concentrated NZVI slurries (15 g/L) can be stabilized for more than 10 days adding 6 g/L of xanthan gum biopolymer. Stability against aggregation and sedimentation was achieved in the range of ionic strength 6 × 10⁻³–12 mM and is mainly due to the formation of a viscous gel characterized by shear-thinning behaviour.     

Influence of nanoscale zero-valent iron on geochemical properties of groundwater and vinyl chloride degradation: A field case study

A 200 m² pilot-scale field test successfully demonstrated the use of nanoscale zero-valent iron (NZVI) for effective remediation of groundwater contaminated with chlorinated organic compounds in Taiwan within six months. Both commercially available and on-site synthesized NZVI were used. A well-defined monitoring program allowing to collect three-dimensional spatial data from 13 nested multi-level monitoring wells was conducted to monitor geochemical parameters in groundwater. The degradation efficiency of vinyl chloride (VC) determined at most of monitoring wells was 50-99%. It was found that the injection of NZVI caused a significant change in total iron, total solid (TS) and suspended solid (SS) concentrations in groundwater. Total iron concentration showed a moderate and weak correlation with SS and TS, respectively, suggesting that SS may be used to indicate the NZVI distribution in groundwater. A decrease in oxidation-reduction potential (ORP) values from about −100 to −400 mV after NZVI injection was observed. This revealed that NZVI is an effective means of achieving highly reducing conditions in the subsurface environment. Both VC degradation efficiency and ORP showed a correlative tendency as an increase in VC degradation efficiency corresponded to a decrease of ORP. This is in agreement with the previous studies suggesting that ORP can serve as an indicator for the NZVI reactivity.     

Kinetic assessment and modeling of an ozonation step for full-scale municipal wastewater treatment: Micropollutant oxidation, by-product formation and disinfection

The kinetics of oxidation and disinfection processes during ozonation in a full-scale reactor treating secondary wastewater effluent were investigated for seven ozone doses ranging from 0.21 to 1.24 g O₃ g⁻¹ dissolved organic carbon (DOC). Substances reacting fast with ozone, such as diclofenac or carbamazepine (kP,O₃ > 10⁴ M⁻¹ s⁻¹), were eliminated within the gas bubble column, except for the lowest ozone dose of 0.21 g O₃ g⁻¹ DOC. For this low dose, this could be attributed to short-circuiting within the reactor. Substances with lower ozone reactivity (kP,O₃ < 10⁴ M⁻¹ s⁻¹) were only fully eliminated for higher ozone doses.The predictions of micropollutant oxidation based on coupling reactor hydraulics with ozone chemistry and reaction kinetics were up to a factor of 2.5 higher than full-scale measurements. Monte Carlo simulations showed that the observed differences were higher than model uncertainties. The overestimation of micropollutant oxidation was attributed to a protection of micropollutants from ozone attack by the interaction with aquatic colloids. Laboratory-scale batch experiments using wastewater from the same full-scale treatment plant could predict the oxidation of slowly-reacting micropollutants on the full-scale level within a factor of 1.5. The Rct value, the experimentally determined ratio of the concentrations of hydroxyl radicals and ozone, was identified as a major contribution to this difference.An increase in the formation of bromate, a potential human carcinogen, was observed with increasing ozone doses. The final concentration for the highest ozone dose of 1.24 g O₃ g⁻¹ DOC was 7.5 μg L⁻¹, which is below the drinking water standard of 10 μg L⁻¹. N-Nitrosodimethylamine (NDMA) formation of up to 15 ng L⁻¹ was observed in the first compartment of the reactor, followed by a slight elimination during sand filtration. Assimilable organic carbon (AOC) increased up to 740 μg AOC L⁻¹, with no clear trend when correlated to the ozone dose, and decreased by up to 50% during post-sand filtration. The disinfection capacity of the ozone reactor was assessed to be 1-4.5 log units in terms of total cell counts (TCC) and 0.5 to 2.5 log units for Escherichia coli (E. coli). Regrowth of up to 2.5 log units during sand filtration was observed for TCC while no regrowth occurred for E. coli. E. coli inactivation could not be accurately predicted by the model approach, most likely due to shielding of E. coli by flocs.     

Impact of nano zero valent iron (NZVI) on methanogenic activity and population dynamics in anaerobic digestion

Nano zero valent iron (NZVI), although being increasingly used for environmental remediation, has potential negative impact on methanogenesis in anaerobic digestion. In this study, NZVI (average size = 55 ± 11 nm) showed inhibition of methanogenesis due to its disruption of cell integrity. The inhibition was coincident with the fast hydrogen production and accumulation due to NZVI dissolution under anaerobic conditions. At the concentrations of 1 mM and above, NZVI reduced methane production by more than 20%. At the concentration of 30 mM, NZVI led to a significant increase in soluble COD (an indication of cell disruption) and volatile fatty acids in the mixed liquor along with an accumulation of H₂, resulting in a reduction of methane production by 69% (±4% [standard deviation]). By adding a specific methanogenesis inhibitor-sodium 2-bromoethanesulfonate (BES) to the anaerobic sludge containing 30 mM NZVI, the amount of H₂ produced was only 79% (±1%) of that with heat-killed sludge, indicating the occurrence of bacterially controlled hydrogen utilization processes. Quantitative PCR data was in accordance with the result of methanogenesis inhibition, as the level of methanogenic population (dominated by Methanosaeta) in the presence of 30 mM NZVI decreased significantly compared to that of the control. On the contrary, ZVI powder (average size <212 μm) at the same concentration (30 mM) increased methane production presumably due to hydrogenotrophic methanogenesis of hydrogen gas that was slowly released from the NZVI powder. While it is a known fact that NZVI disrupts cell membranes, which inhibited methanogenesis described herein, the results suggest that the rapid hydrogen production due to NZVI dissolution also contribute to methanogenesis inhibition and lead to bacterially controlled hydrogenotrophic processes.     

Temporal variability of combined sewer overflow contaminants: Evaluation of wastewater micropollutants as tracers of fecal contamination

A monitoring program was initiated for two sewage outfalls (OA and OB) with different land uses (mainly residential versus institutional) over the course of a year. Eleven CSO events resulting from fall and summer precipitations and a mixture of snowmelt and precipitation in late winter and early spring were monitored. Median concentrations measured in CSOs were 1.5 × 10⁶ Escherichia coli/100 mL, 136.0 mg/L of Total Suspended Solids (TSS), 4599.0 ng/L of caffeine (CAF), 158.9 ng/L of carbamazepine (CBZ), in outfall OA and 5.1 × 10⁴ E. coli/100 mL, 167.0 mg TSS/L, 300.8 ng CAF/L, 4.1 ng CBZ/L, in outfall OB. Concentration dynamics in CSOs were mostly related to the dilution by stormwater and the time of day of the onset of overflows. Snowmelt was identified as a critical period with regards to the protection of drinking water sources given the high contaminant concentrations and long duration of events in addition to a lack of restrictions on overflows during this period. Correlations among measured parameters reflected the origins and transport pathways of the contaminants, with E. coli being correlated with CBZ. TSS were not correlated with E. coli because E. coli was found to be mostly associated with raw sewage whereas TSS were additionally from the resuspension of in-sewer deposits and surface runoff. In receiving waters, E. coli remained the best indicator of fecal contamination in strongly diluted water samples as compared to WWMPs because WWMPs can be diluted to below their detection limits.     

Modeling monochloramine loss in the presence of natural organic matter

A comprehensive model describing monochloramine loss in the presence of natural organic matter (NOM) is presented. The model incorporates simultaneous monochloramine autodecomposition and reaction pathways resulting in NOM oxidation. These competing pathways were resolved numerically using an iterative process evaluating hypothesized reactions describing NOM oxidation by monochloramine under various experimental conditions. The reaction of monochloramine with NOM was described as biphasic using four NOM specific reaction parameters. NOM pathway 1 involves a direct reaction of monochloramine with NOM (k_doc1=1.05×10⁴-3.45×10⁴ M⁻¹ h⁻¹). NOM pathway 2 is slower in terms of monochloramine loss and attributable to free chorine (HOCl) derived from monochloramine hydrolysis (k_doc2=5.72×10⁵-6.98×10⁵ M⁻¹ h⁻¹), which accounted for the majority of monochloramine loss. Also, the free chlorine reactive site fraction in the NOM structure was found to correlate to specific ultraviolet absorbance at 280 nm (SUVA₂₈₀). Modeling monochloramine loss allowed for insight into disinfectant reaction pathways involving NOM oxidation. This knowledge is of value in assessing monochloramine stability in distribution systems and reaction pathways leading to disinfection by-product (DBP) formation.     

Alternating electric fields combined with activated carbon for disinfection of Gram negative and Gram positive bacteria in fluidized bed electrode system

Strong electric fields for disinfection of wastewaters have been employed already for several decades. An innovative approach combining low strength (7 V/cm) alternating electric fields with a granular activated carbon fluidized bed electrode (FBE) for disinfection was presented recently. For disinfection performance of FBE several pure microbial cultures were tested: Bacillus subtilis, Bacillus subtilis subsp. subtilis, Enterococcus faecalis as representatives from Gram positive bacteria and Erwinia carotovora, Pseudomonas luteola, Pseudomonas fluorescens and Escherichia coli YMc10 as representatives from Gram negative bacteria. The alternating electric field amplitude and shape were kept constant. Only the effect of alternating electric field frequency on disinfection performance was investigated. From the bacteria tested, the Gram negative strains were more susceptible and the Gram positive microorganisms were more resistant to FBE disinfection. The collected data indicate that the efficiency of disinfection is frequency and strain dependent. During 6 h of disinfection, the decrease above 2 Log units was achieved with P. luteola and E. coli at 10 kHz and at dual frequency shift keying (FSK) modulated signal with frequencies of 10 kHz and 140 kHz. FBE technology appears to offer a new way for selective bacterial disinfection, however further optimizations are needed on treatment duration, and energy input, to improve effectiveness.