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 CaCl₂) 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 CaCl₂ 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 Iron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