Lab on a chip for a low-carbon future

Transitioning our society to a sustainable future, with low or net-zero carbon emissions to the atmosphere, will require a wide-spread transformation of energy and environmental technologies. In this perspective article, we describe how lab-on-a-chip (LoC) systems can help address this challenge by providing insight into the fundamental physical and geochemical processes underlying new technologies critical to this transition, and developing the new processes and materials required. We focus on six areas: (I) subsurface carbon sequestration, (II) subsurface hydrogen storage, (III) geothermal energy extraction, (IV) bioenergy, (V) recovering critical materials, and (VI) water filtration and remediation. We hope to engage the LoC community in the many opportunities within the transition ahead, and highlight the potential of LoC approaches to the broader community of researchers, industry experts, and policy makers working toward a low-carbon future.

Enhanced precipitation of magnesium carbonates using carbonic anhydrase

Carbonate precipitation, as part of the carbon dioxide (CO₂) mineralization process, is generally regarded as a high-temperature, high-pressure, and high-purity CO₂ process. Typical conditions consist of temperatures around 120 °C and a pressure of 100 bar of pure CO₂, making the process costly. A major challenge facing carbonate precipitation is performing the reaction at low temperatures and low partial pressures of CO₂ (p_CO2) such as 25 °C and CO₂ flue gas concentration. In this work, we investigated the effect of carbonic anhydrase (CA) to favor magnesium (Mg) carbonate precipitation at low temperatures and low p_CO2. CA is an enzyme that accelerates CO₂ hydration promoting its conversion into HCO₃^- and then CO₃^2-. This increases supersaturation with respect to Mg-carbonates. A geochemical model was implemented and used to identify supersaturated conditions with respect to Mg-carbonates. Tests were run at 25, 40, and 50 °C and at 1 bar of either pure CO₂ or 10 vol% CO₂ and 90 vol% N₂. The concentration of 10 vol% CO₂ was chosen to resemble CO₂ concentration in flue gas. In selected tests, the CA enzyme was added directly as bovine CA or through microalgae (Scenedesmus obliquus). Experiments were run for 48 hours; 24 hours to reach equilibrium, then another 24 hours until the supersaturated conditions were established. After 48 hours the experiments were interrupted and the solids were characterized. Results show that the addition of CA, either directly or through Scenedesmus obliquus, enhances Mg-carbonate precipitation. Regardless of the temperature, the precipitates were made entirely of nesquehonite (MgCO₃-3H₂O) when pure CO₂ was used. Otherwise, a solid solution containing brucite (Mg(OH)₂) and MgCO₃-3H₂O was formed. Overall, these findings suggest that CA can promote carbonate precipitation at low temperatures, pressures, and CO₂ purity. The enzyme is effective when added directly or supplied through microalgae, opening up the possibility for a CO₂ mineralization process to be implemented directly at a combustion plant as a CO₂ storage option without preliminary CO₂ capture.

Measuring and modeling the influence of salinity change on the transport behaviour of Escherichia coli through quartz sand

Pathogenic bacteria can be discharged in the environment through natural as well as anthropogenic activities. Once in the environment, they may contaminate soil and sediments and migrate towards water bodies. Transient chemical conditions may occur in soil/sediments and favor mobilization of bacteria, e.g., upon the reduction of salinity (or ionic strength). However, the magnitude of this phenomenon and its relationship with particle size is not well understood, yet. In this work, we investigated the transport of Escherichia coli under variable salinity conditions (between 1 and 20 part per thousand, ppt) and for different soil grain sizes (between 150 and 710 μm). A model developed in our group was applied in this work. It couples bacteria and salinity transport equations in order to account for transient water composition in the description of bacteria migration. The model was calibrated and validated with laboratory experiments. The tests were monitored continuously with UV-Vis spectroscopy, which allowed to record highly resolved concentration fronts. The results show that salinity increases the retardation of the bacteria. Upon salinity drop, a release of bacteria occurs forming a peak whose magnitude increases with salinity change. This effect becomes more important as the grain size decreases. Simulations suggest that the dominant retention mechanism is attachment for coarse sand and straining for fine sand. The retention can be reversed as the salinity is reduced causing a sudden bacteria mobilization. Such a behaviour may have important implications on microbial contamination of water bodies when soil/sediments undergo transient chemical conditions.

Measuring and modeling nanoparticle transport by foam in porous media

In this paper, an experimental study of nanoparticle transport by foam is presented. Bubbles made of N₂-gas were stabilized with either a cationic surfactant (Cetyl Trimethyl Ammonium Bromide, CTAB), silica nanoparticles, or a combination of them. The concentrations of the surface active materials were selected upon foamability and stability tests. Column-flood tests were run until steady-state changing nanoparticle concentration, foam quality (f_g), and flow rate. A synergistic behaviour of surfactant and nanoparticles help the formation of a strong foam. The measurements were used to validate a mechanistic model, presented in our earlier work (Li and Prigiobbe, 2020), which couples foam and nanoparticles transport with agglomeration and extended-DLVO theory. The model agrees well with the measurements and results show that an high-quality (ca. 90% gas fraction) can be used to carry nanoparticles and the efficient increases with flow velocity. This opens the opportunity for the application of foam as a carrier of nanoparticles in subsurface applications such as the remediation of contaminated sites and makes the model a valuable tool to design and predict such operations.

Transport of produced water through reactive porous media

During hydraulic fracturing (or fracking) large volumes of wastewater (flow-back and produced water) are generated, which are naturally rich in heavy metals and radionuclides, such as radium. Spills may occur during operations and contaminate the groundwater. Therefore, there is an urgent need to identify a practice that can mitigate the negative impact of accidental leaks on water resources. Here, we present an experimental and modeling work on the transport of alkaline earth elements in produced water, which are congeners of radium, namely, barium (Ba²⁺), strontium (Sr²⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) in addition to sodium (Na⁺). Column-flood tests were conducted using produced water from a shale-gas site and reactive porous media made of ubiquitous minerals such as sand, hydrous ferric oxide, activated alumina, and manganese oxide. In all cases, no retardation of the ions was observed at the salinity conditions of the produced water, but strong retardation in the pH front was measured, indicating that adsorption indeed occurred. When using manganese oxide and upon dilution of produced water, the concentration fronts of all major divalent cations were retarded. However, a fast wave of solute, traveling at the average flow velocity, emerged. This phenomenon confirmed that significant adsorption occurred under those conditions. But, pH-dependent adsorption and hydrodynamic dispersion favored fast solute transport. Overall, these results suggest that manganese oxide could be used as a reactive material in the lining of temporary storage tanks and in the well cases in order to retard the migration of the major toxic elements in produced water. However, mixing must be controlled to avoid the emergence of an instability at the concentration fronts favoring the formation of fast waves.

Relationship between infiltration, sewer rehabilitation, and groundwater flooding in coastal urban areas

The aging of sewer networks is a serious issue in urban areas because of the reduced functionality of the system that can have negative impact on the urban environment. Aging pipes are not water-tight anymore and they can leak untreated sewage or allow infiltration of groundwater. In the latter case, more frequent combined sewer overflows (CSOs) may occur. Generally, prompt intervention to repair damaged conduits is envisaged. However, in low-lying coastal regions, sewer systems may provide an unplanned drainage that controls the groundwater table from flooding the urban ground. Here, a study is presented to investigate the influence of the repair of damaged sewer on the water table of an urban shallow aquifer. Sewer and groundwater models were built to describe the effect of sewer replacement. Based on a real dataset, simulations were run for a city located along an estuary. Results show that the presence of infiltration into the sewer system increases the frequency of CSOs, which trigger the discharge of untreated sewage after a minor precipitation or even in dry weather conditions. As the sewer is repaired, CSO spills diminish occurring only upon significant precipitation. However, the water table rises and eventually, during the high tide, the groundwater floods the low-lying part of the city. Overall, this work highlights the susceptibility of shallow aquifers in coastal urban areas and suggests that they should be regarded in flooding predictions.

Mechanistic Study of Lead Adsorption on Activated Carbon

Activated carbon (AC) is a carbonaceous material broadly applied in filters to remove lead (Pb(II)) from drinking water through adsorption. However, the chemical interactions between Pb(II) and the reactive sites on AC or other carbonaceous materials are not well understood, yet. The understanding of the mechanism of Pb(II) adsorption onto AC would allow to optimally design AC-based materials even in the presence of a complex liquid phase. Here, the interaction between Pb(II) and functional groups on AC was investigated at the molecular scale to help identifying the chemical reactions at the solid-liquid interface. Spectroscopic analyses and chemical quantum calculations were performed and indicated the formation of monodentate mononuclear Pb(II)-phenol and bidentate mononuclear Pb(II)-carboxyl complexes on AC. Competitive adsorption behavior was observed between Pb(II) and calcium (Ca(II)) because of their similar adsorption configurations on AC. In contrast, anions, including sulfate and phosphate, were observed to enhance Pb(II) adsorption on AC by forming ternary complexes. On the basis of these observations, a new surface complexation model of Pb(II) adsorption onto AC was formulated and validated with batch tests. Overall, this work presents a new set of chemical reactions at the solid-liquid interface between Pb(II) and AC under various conditions of interest for the application of AC or other carbonaceous materials in water treatment.

Effect of ionic strength on barium transport in porous media

Hydraulic fracturing (or fracking) is a well stimulation technique used to extract resources from a low permeability formation. Currently, the most common application of fracking is for the extraction of oil and gas from shale. During the operation, a large volume of brine, rich in hazardous chemicals, is produced. Spills of brine from wells or pits might negatively impact underground water resources and, in particular, one of the major concerns is the migration of radionuclides, such as radium (Ra²⁺), into the shallow subsurface. However, the transport behaviour of Ra²⁺ through a reactive porous medium under conditions typical of a brine, i.e., high salinity, is not well understood, yet. Here, a study on the transport behaviour of barium (Ba²⁺, congener of radium) through a porous medium containing a common mineral such as goethite (FeO(OH)) is presented. Batch and column flood tests were carried out at conditions resembling the produced brine, i.e., large values of ionic strength (I), namely, 1 to 3mol/kg. The measurements were described with the triple layer surface complexation model coupled with the Pitzer activity coefficient method and a reactive transport model, in the case of the transport tests. The experimental results show that the adsorption of Ba²⁺ onto FeO(OH) increases with pH but decreases with I and it becomes negligible at the brine conditions. Moreover, even if isotherms show adsorption at large I, at the same conditions during transport, Ba²⁺ travels without retardation through the FeO(OH) porous medium. The triple layer model agrees very well with all batch data but it does not describe well the transport tests in all cases. In particular, the model cannot match the pH measurements at large I values. This suggests that the chemical reactions at the solid-liquid interface do not capture the mechanism of Ba²⁺ adsorption onto FeO(OH) at large salinity. Finally, this study suggests that barium, and potentially its congeners, namely, radium, calcium, magnesium, and strontium, may travel at the average flow velocity through a soil where the dominant reactive mineral is goethite.

Measuring and modeling the magnetic settling of superparamagnetic nanoparticle dispersions

In this paper, we present settling experiments and mathematical modeling to study the magnetic separation of superparamagnetic iron-oxide nanoparticles (SPIONs) from a brine. The experiments were performed using SPIONs suspensions of concentration between 3 and 202g/L dispersed in water and separated from the liquid under the effect of a permanent magnet. A 1D model was developed in the framework of the sedimentation theory with a conservation law for SPIONs and a mass flux function based on the Newton's law for motion in a magnetic field. The model describes both the hindering effect of suspension concentration (n) during settling due to particle collisions and the increase in settling rate due to the attraction of the SPIONs towards the magnet. The flux function was derived from the settling experiments and the numerical model validated against the analytical solution and the experimental data. Suspensions of SPIONs were of 2.8cm initial height, placed on a magnet, and monitored continuously with a digital camera. Applying a magnetic field of 0.5T of polarization, the SPION's velocity was of approximately 3·10(-5)m/s close to the magnet and decreases of two orders of magnitude across the domain. The process was characterized initially by a classical sedimentation behavior, i.e., an upper interface between the clear water and the suspension slowly moving towards the magnet and a lower interface between the sediment layer and the suspension moving away from the magnet. Subsequently, a rapid separation of nanoparticle occured suggesting a non-classical settling phenomenon induced by magnetic forces which favor particle aggregation and therefore faster settling. The rate of settling decreased with n and an optimal condition for fast separation was found for an initial n of 120g/L. The model agrees well with the measurements in the early stage of the settling, but it fails to describe the upper interface movement during the later stage, probably because of particle aggregation induced by magnetization which is not accounted for in the model.

pH-dependent transport of metal cations in porous media

We study the effect of pH-dependent adsorption and hydrodynamic dispersion on cation transport through a reactive porous medium with a hydrophilic surface. We investigate how competitive adsorption between a proton and a metal (which in some situations of practical interest may also be a radionuclide) can facilitate the migration of a certain fraction of the latter. We performed laboratory experiments using a chromatographic column filled with silica beads coated with iron oxide and flooded initially with an acidic solution (pH ≈ 3) and then with an alkaline solution (pH > 7) containing either sodium, potassium, lithium, calcium, magnesium, or barium. The composition of each injected solution was chosen to represent one of two possible theoretical predictions, either a retarded shock and a fast pulse, that is, traveling at the interstitial fluid velocity, or only a retarded shock. Highly resolved breakthrough curves measured with inline ion chromatography allowed us to observe in all cases agreement with theoretical predictions, including numerous observations of a fast pulse. The fast pulse is the result of the interaction between pH-dependent adsorption and hydrodynamic dispersion and has previously been observed in systems with strontium. Here, we show the fast pulse arises also in the case of other cations allowing a generalization of the physical mechanism underlying this phenomenon and consideration of it as a new fast transport behavior. A one-dimensional reactive transport model for an incompressible fluid was developed combining surface complexation with mass conservation equations for a solute and the acidity (difference between the total proton and hydroxide concentration). In all cases, the model agrees with the measurements capturing the underlying physics of the overall transport behavior. Our results suggest that the interplay between pH-dependent adsorption and hydrodynamic dispersion can give rise to the rapid migration of metals through reactive porous media with potential effects on, for example, the performance of subsurface engineering infrastructures for pollutant containment, the mobilization of metal contaminants by brine acidified upon contact with CO2 during geologic carbon storage, and the chromatographic separation processes in industrial applications.

Quantification of sewer leakage by a continuous tracer method

Water authorities interested in the evaluation of the structural state of a sewer must quantify leakage to plan strategic intervention. However, the quantification of the exfiltration and the localisation of structural damage are challenging tasks that usually require expensive and time-consuming inspections. Herein, we report one of the first applications of the QUEST-C method to quantify the exfiltration in a continuously operating sewer by dosing two chemical tracers, sodium bromide (NaBr) and lithium chloride (LiCl). The method was applied at the catchment scale in a 14-year-old sewer in Rome, Italy. Preliminary laboratory tests, field measurements, and numerical simulations showed that reliable results require the QUEST-C method to be applied to sewers without lateral inflows, during periods of quasi-steady flow, and that the travel time of the NaBr tracer is minimised. Three sewer reaches were tested and the estimated exfiltration, as a fraction of the dry weather flow (DWF), increased from 0.128 in the agricultural area to 0.208 in the urban area. Although our estimates are at the lower end of the range given in the literature (0.01-0.56 DWF), the exfiltration was not negligible, and interventions should focus on the sewers in urban areas. This illustrates the capability of the QUEST-C method to guide strategic intervention at low cost and without an interruption of sewer operation. However, careful interpretation of the results is recommended for sewers with many lateral inflows, where leakage may be overestimated.

Quantification of sewer system infiltration using delta(18)O hydrograph separation

The infiltration of parasitical water into two sewer systems in Rome (Italy) was quantified during a dry weather period. Infiltration was estimated using the hydrograph separation method with two water components and delta(18)O as a conservative tracer. The two water components were groundwater, the possible source of parasitical water within the sewer, and drinking water discharged into the sewer system. This method was applied at an urban catchment scale in order to test the effective water-tightness of two different sewer networks. The sampling strategy was based on an uncertainty analysis and the errors have been propagated using Monte Carlo random sampling. Our field applications showed that the method can be applied easily and quickly, but the error in the estimated infiltration rate can be up to 20%. The estimated infiltration into the recent sewer in Torraccia is 14% and can be considered negligible given the precision of the method, while the old sewer in Infernetto has an estimated infiltration of 50%.