24 research outputs found
Hydrogen Gas Production from the Injection of Nanoscale Zero-Valent Iron and Sodium Borohydride Solutions: Potential Effects Near Injection Wells
The injection of nano-scale zero-valent iron (nZVI) is a remediation technique for the
treatment of organic and metal contamination in soil and groundwater. The hydrogen gas
(H2) produced during the reaction of nZVI and excess sodium borohydride (NaBH4) used
in nZVI synthesis with water can inhibit nZVI transport in the subsurface, potentially
limiting solution delivery to the target contaminant zone. Laboratory experiments were
completed in a thin flow cell using NaBH4 and nZVI solutions injected into watersaturated
medium sands, in which local gas saturations were quantified using a light
transmission technique to calculate H2 gas volumes. Hydraulic conductivity, under
water-saturated and quasi-saturated conditions, after gas exsolution and throughout gas
dissolution, was measured. The results showed that H2 gas volume produced as a result of
the reaction of nZVI with water was more than the H2 gas volume produced by the selfhydrolysis
of NaBH4 solution regardless of similar NaBH4 concentration used as excess
during nZVI synthesis. Pools of H2 gas were formed after injecting nZVI prepared with
excess 5 g/L NaBH4 or after injecting 5 g/L NaBH4 without nZVI. Gas accumulated
predominantly in a vertical layer of coarse sand, illustrative of a sand pack surrounding
an injection well. Lower hydraulic conductivity measurements were linked to higher gas
saturations and further reductions were evident as a result of gas pool accumulation at
the top of the flow cell. These results show that gas production during the application
of nZVI is an important process that must be considered during remediation design and
operation to ensure effective delivery to target zones
A flow cell simulating a subsurface rock fracture for investigations of groundwater-derived biofilms
Laboratory scale continuous-flow-through chambers (flow cells) facilitate the observation of microbes in a controlled, fully hydrated environment, although these systems often do not simulate the environmental conditions under which microorganisms are found. We developed a flow cell that mimics a subsurface groundwater-saturated rock fracture and isamenable to confocal laser scanning microscopy while allowing for the simple removal of the attached biomass. This flow cell was used to investigate the effect of toluene, a representative contaminant for non-aqueous phase liquids, on groundwater-derived biofilms. Reduced average biofilm biomass and thickness, and diminished diversity of amplifiable 16S rRNA sequences were observed for biofilms that developed in the presence of toluene, compared to the biofilms grown in the absence of toluene. The flow cell also allowed the detection of fluorescent protein-labelled cells
Insights and Modelling Tools for Designing and Improving Chlorinated Solvent Bioremediation Applications
The chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE) have been used extensively in industry and are now amongst the most common and hazardous groundwater contaminants. These solvents are typically present as dense, non-aqueous phase liquids (DNAPLs) and represent long-term source zones that produce persistent contamination plumes in aquifers. Under anaerobic conditions, chlorinated ethenes may be biodegraded via reductive dechlorination (the biologically mediated, step-wise removal of chlorine) to form ethene, a relatively innocuous end-product. The rate of reductive dechlorination can be enhanced by stimulating the activity of dechlorinating bacteria by injection of an electron donor (typically an organic substrate that generates hydrogen upon fermentation), nutrients and, in some cases, microbial communities known to dechlorinate effectively to ethene (i.e., bioaugmentation). Reductive dechlorination has been shown to be a viable technology for in situ treatment of dissolved chlorinated solvent plumes, and recent laboratory studies have suggested that this strategy may also be effective for chlorinated solvent DNAPL. Here, the source zone is targeted directly, with the aim of reducing its lifespan by enhancing dissolution from the DNAPL and sorbed phases and coupling this with effective and sustained dechlorination near the DNAPL-water interface and within the plume. This bulletin focuses on modelling of enhanced dechlorination processes in groundwater, including the modelling tools developed in the SABRE project (under which this report was written, http://www.claire.co.uk/index.php?option=com_content&task=view&id=53&Itemid=47&showall=1), insights gained from the models concerning factors controlling the rates and extent of enhanced source zone DNAPL bioremediation, and how the modelling tools can be used to assist future applications of this technology
Numerical modeling of the effects of roughness on flow and eddy formation in fractures
The effect of roughness on flow in fractures was investigated using lattice Boltzmann method (LBM). Simulations were conducted for both statistically generated hypothetical fractures and a natural dolomite fracture. The effect of increasing roughness on effective hydraulic aperture, Izbash and Forchheimer parameters with increasing Reynolds number (Re) ranging from 0.01 to 500 was examined. The growth of complex flow features, such as eddies arising near the fracture surface, was directly associated with changes in surface roughness. Rapid eddy growth above Re values of 1, followed by less rapid growth at higher Re values, suggested a three-zone nonlinear model for flow in rough fractures. This three-zone model, relating effective hydraulic conductivity to Re, was also found to be appropriate for the simulation of water flow in the natural dolomite fracture. Increasing fracture roughness led to greater eddy volumes and lower effective hydraulic conductivities for the same Re values
DNAPL accumulation in wells and DNAPL recovery from wells : model development and application to a laboratory study
International audienceDense nonaqueous phase liquid (DNAPL) accumulation and recovery from wells cannot be accurately modeled through typical pressure or flux boundary conditions due to gravity segregation of water and DNAPL in the wellbore, the effects of wellbore storage, and variations of wellbore inflow and outflow rates with depth, particularly in heterogeneous formations. A discrete wellbore formulation is presented for numerical modeling of DNAPL accumulation in observation wells and DNAPL removal from recovery wells. The formulation includes fluid segregation, changing water and DNAPL levels in the well and the corresponding changes in fluid storage in the wellbore. The method was added to a three-dimensional finite difference model (CompSim) for three phase (water, gas, DNAPL) flow. The model predictions are compared to three-dimensional pilot scale experiments of DNAPL (benzyl alcohol) infiltration, redistribution, recovery, and water flushing. Model predictions match experimental results well, indicating the appropriateness of the model formulation. Characterization of mixing in the extraction well is important for predicting removal of highly soluble organic compounds like benzyl alcohol. A sensitivity analysis shows that the incorporation of hysteresis is critical for accurate prediction. Among the multiphase flow and transport parameters required for modeling, results are most sensitive to soil intrinsic permeability
Colloid Transport in Dolomite Rock Fractures: Effects of Fracture Characteristics, Specific Discharge, and Ionic Strength
The effects of fracture characteristics, specific discharge,
and
ionic strength on microsphere transport in variable-aperture dolomite
rock fractures were studied in a laboratory-scale system. Fractures
with different aperture distributions and mineral compositions were
artificially created in two dolomite rock blocks. Transport tests
were conducted with bromide and carboxylate-modified latex microspheres
(20, 200, and 500 nm diameter). Under overall unfavorable attachment
conditions, there was significant retention of the 20 nm microsphere
and minimal retention of the 500 nm microsphere for all conditions
examined. Aperture variability produced significant spatial variation
in colloid transport. Flushing with low ionic strength solution (1
mM) following microsphere transport at 12 mM ionic strength solution
produced a spike in effluent microsphere concentrations, consistent
with retention of colloids in secondary energy minima. Surface roughness
and charge heterogeneity effects may have also contributed to the
effect of microsphere size on retention. Matrix diffusion influenced
bromide transport but was not a dominant factor in transport for any
microsphere size. Calibrated one-dimensional, two-site kinetic model
parameters for colloid transport in fractured dolomite were sensitive
to the physical and chemical properties of both the fractured dolomite
and the colloids, indicating the need for mechanistic modeling for
accurate prediction
Forces of Interactions between Bare and Polymer-Coated Iron and Silica: Effect of pH, Ionic Strength, and Humic Acids
The interactions between a silica substrate and iron
particles
were investigated using atomic force microscopy-based force spectroscopy
(AFM). The micrometer- and nanosized iron particles employed were
either bare or coated with carboxymethyl cellulose (CMC), a polymer
utilized to stabilize iron particle suspensions. The effect of water
chemistry on the forces of interaction was probed by varying ionic
strength (with 100 mM NaCl and 100 mM CaCl<sub>2</sub>) or pH (4,
5.5, and 8) or by introducing 10 mg/L of humic acids (HA). When particles
were uncoated, the forces upon approach between silica and iron were
attractive at pH 4 and 5.5 and in 100 mM CaCl<sub>2</sub> at pH 8,
but they were negligible in 100 mM NaCl buffered to pH 8 and repulsive
in water buffered to pH 8 and in HA solutions. HA produced electrosteric
repulsion between iron particles and silica, likely due to its sorption
to iron particles. HA sorption to silica was excluded on the basis
of experiments conducted with a quartz-crystal microbalance with dissipation
monitoring. Repulsion with CMC-coated iron was attributed to electrosteric
forces, which were damped at high ionic strength. An extended DLVO
model and a modified version of Ohshima’s theory were successfully
utilized to model AFM data
Effect of Water Chemistry and Aging on Ironî—¸Mica Interaction Forces: Implications for Iron Particle Transport
The transport of particles through groundwater systems
is governed
by a complex interplay of mechanical and chemical forces that are
ultimately responsible for binding to geological substrates. To understand
these forces in the context of zero valent iron particles used in
the remediation of groundwater, atomic force microscopy (AFM)-based
force spectroscopy was employed to characterize the interactions between
AFM tips modified with either carbonyl iron particles (CIP) or electrodeposited
Fe as a function of counterion valency, temperature, particle morphology,
and age. The measured interaction forces were always attractive for
both fresh and aged CIP and electrodeposited iron, except in 100 mM
NaCl, as a consequence of electrostatic attraction between the negatively
charged mica and positively charged iron. In 100 mM NaCl, repulsive
hydration forces appeared to dominate. Good agreement was found between
the experimental data and predictions based on the extended DLVO (XDLVO)
theory. The effect of aging on iron particle composition and morphology
was assessed by X-ray photoelectron spectroscopy (XPS), scanning electron
microscopy (SEM), and energy dispersive spectroscopy (EDS) revealing
that the aged particles comprising a zero valent iron core passivated
by a mixture of iron oxides and hydroxides. Force spectroscopy showed
that aging caused variations in the adhesive force due to the changes
in particle morphology and contact area