Guar gum solutions for improved delivery of iron particles in porous media (Part 2): Iron transport tests and modeling in radial geometry

In the present work column transport tests were performed in order to study the mobility of guar-gum suspensions of microscale zero-valent iron particles (MZVI) in porous media. The results were analyzed with the purpose of implementing a radial model for the design of full scale interventions. The transport tests were performed using several concentrations of shear thinning guar gum solutions as stabilizer (1.5, 3 and 4 g/l) and applying different flow rates (Darcy velocity in the range 1 · 10− 4 to 2 · 10− 3 m/s), representative of different distances from the injection point in the radial domain. Empirical relationships, expressing the dependence of the deposition and release parameters on the flow velocity, were derived by inverse fitting of the column transport tests using a modified version of E-MNM1D (Tosco and Sethi, 2010) and the user interface MNMs (www.polito.it/groundwater/software). They were used to develop a comprehensive transport model of MZVI suspensions in radial coordinates, called E-MNM1R, which takes into account the non Newtonian (shear thinning) rheological properties of the dispersant fluid and the porous medium clogging associated with filtration and sedimentation in the porous medium of both MZVI and guar gum residual undissolved particles. The radial model was run in forward mode to simulate the injection of MZVI dispersed in guar gum in conditions similar to those applied in the column transport tests. In a second stage, we demonstrated how the model can be used as a valid tool for the design and the optimization of a full scale intervention. The simulation results indicated that several concurrent aspects are to be taken into account for the design of a successful delivery of MZVI/guar gum slurries via permeation injection, and a compromise is necessary between maximizing the radius of influence of the injection and minimizing the injection pressure, to guarantee a sufficiently homogeneous distribution of the particles around the injection point and to prevent preferential flow paths

Guar gum solutions for improved delivery of iron particles in porous media (Part 1): Porous medium rheology and guar gum-induced clogging

The present work is the first part of a comprehensive study on the use of guar gum to improve delivery of microscale zero-valent iron particles in contaminated aquifers. Guar gum solutions exhibit peculiar shear thinning properties, with high viscosity in static conditions and lower viscosity in dynamic conditions: this is beneficial both for the storage of MZVI dispersions, and also for the injection in porous media. In the present paper, the processes associated with guar gum injection in porous media are studied performing single-step and multi-step filtration tests in sand-packed columns. The experimental results of single-step tests performed by injecting guar gum solutions prepared at several concentrations and applying different dissolution procedures evidenced that the presence of residual undissolved polymeric particles in the guar gum solution may have a relevant negative impact on the permeability of the porous medium, resulting in evident clogging. The most effective preparation procedure which minimizes the presence of residual particles is dissolution in warm water (60 °C) followed by centrifugation (procedure T60C). The multi-step tests (i.e. injection of guar gum at constant concentration with a step increase of flow velocity), performed at three polymer concentrations (1.5, 3 and 4 g/l) provided information on the rheological properties of guar gum solutions when flowing through a porous medium at variable discharge rates, which mimic the injection in radial geometry. An experimental protocol was defined for the rheological characterization of the fluids in porous media, and empirical relationships were derived for the quantification of rheological properties and clogging with variable injection rate. These relationships will be implemented in the second companion paper (Part II) in a radial transport model for the simulation of large-scale injection of MZVI-guar gum slurries

Pilot Injection of Microscale Zerovalent Iron for Aquifer Remediation

Background/Objectives. Concentrated suspensions of microscale and nanoscale zerovalent iron particles (MZVI and NZVI) have been studied in recent years for the remediation of contaminated aquifers. In the framework of the research project AQUAREHAB (FP7 - G. A. Nr. 226565), a pilot injection test of guar gum stabilized microsized zerovalent iron has been designed and performed under low pressure in a CAHs contaminated site in Belgium and the resulting radius of influence was determined. Approach/Activities. A shear thinning guar gum solution (2 g/l) was selected as an environmentally friendly stabilizer of the iron particles. The relevant properties of the iron slurry (iron particles size and concentration, polymeric stabilizer type and concentration, slurry viscosity) were designed in the laboratory based on several tests (namely iron reactivity tests towards contaminants, sedimentation tests and rheological measurements). Since the injection regime of iron slurries depends on subsurface geotechnical parameters, aquifer hydraulic conductivity, and fluid properties, a specific injection well and monitoring strategy have been developed in order to achieve high discharge rates and radii of influence, and a more homogeneous distribution of the iron particles through low pressure injection. The injection well has been designed and sealed in order to sustain average to high discharge rates, preventing the daylighting of the product. Moreover the well has been hydraulically tested by means of innovative water and guar gum step rate tests in order to determine the most suitable injection rate for the iron slurry. The injection of 50 kg of microsized iron particles (BASF, Germany), dispersed in 5 m3 of a 2 g/l guar gum suspension, was performed at a discharge rate of 1.5 m3/h. The monitoring of the process has been conducted measuring injection rate and pressure as well as iron concentration by means of a magnetic susceptometer. After the injection, the iron distribution in the subsurface was determined through liners extraction and the iron concentration measured both via non-invasive magnetic susceptibility measurements and chemical analysis. Results/Lessons Learned. Even if the field test was specifically designed to inject in a permeation regime, or on the threshold between permeation and fracturing, the results of monitoring injection pressure and iron distribution proved that particles migration in the porous medium occurred via preferential flow. Nevertheless significant radius of influence was achieved during the pilot test

Alginate nanohydrogels as a biocompatible platform for the controlled release of a hydrophilic herbicide

The large-scale application of volatile and highly water-soluble pesticides to guarantee crop production can often have negative impacts on the environment. The main loss pathways are vapor drift, direct volatilization, or leaching of the active substances. Consequently, the pesticide can either accumulate and/or undergo physicochemical transformations in the soil. In this scenario, we synthesized alginate nanoparticles using an inverse miniemulsion template in sunflower oil and successfully used them to encapsulate a hydrophilic herbicide, i.e., dicamba. The formulation and process conditions were adjusted to obtain a unimodal size distribution of nanohydrogels of about 20 nm. The loading of the nanoparticles with dicamba did not affect the nanohydrogel size nor the particle stability. The release of dicamba from the nanohydrogels was also tested: the alginate nanoparticles promoted the sustained and prolonged release of dicamba over ten days, demonstrating the potential of our preparation method to be employed for field application. The encapsulation of hydrophilic compounds inside our alginate nanoparticles could enable a more efficient use of pesticides, minimizing losses and thus environmental spreading. The use of biocompatible materials (alginate, sunflower oil) also guarantees the absence of toxic additives in the formulation

Exploring the potential of graphene oxide nanosheets for porous media decontamination from cationic dyes

Graphene oxide (GO) nanosheets, often embedded in nano-composites, have been studied as promising materials for waste water purification, in particular to adsorb heavy metals and cationic organic contaminants. However, a broader range of potential applications of GO is still unexplored. This work investigated the potential applicability of GO for enhanced in-situ soil washing of secondary sources of groundwater contamination (i.e. the controlled recirculation of a washing GO suspension via injection/extraction wells). The laboratory study aimed at quantifying the capability of GO to effectively remove adsorbed methylene blue (MB) from contaminated sand. The tests were conducted in simplified conditions (synthetic groundwater at NaCl concentration of 20 mM, silica sand) to better highlight the key mechanisms under study. The results indicated a maximum sorption capacity of 1.6 mgMB/mgGO in moderately alkaline conditions. Even though the adsorption of MB onto GO slightly reduced the GO mobility in the porous medium, a breakthrough higher than 95% was obtained for MB/GO mass ratios up to 0.5. This suggests that a very high recovery of the injected particles should be also expected in the field

Estensione dell'analisi di rischio sanitario ambientale a contaminazioni di nanomateriali in sistemi acquiferi

Il largo impiego dei nanomateriali in numerosi processi industriali ha sollevato particolare attenzione attorno alle questioni sanitarie connesse ad una loro potenziale diffusione in ambiente. Tuttavia, a causa della natura polidispersa dei nanomateriali, il rischio sanitario ambientale associato ad una contaminazione da nanoparticelle (NP) non può essere quantificato tramite la procedura ASTM comunemente applicata per le altre sostanze chimiche. NP di dimensione diverse sono infatti caratterizzate da differente tossicità e mobilità nei comparti ambientali. In questo studio viene proposto un approccio per l’adattamento della procedura ASTM al caso di acquiferi contaminati da NP. Le soluzioni analitiche usate per l’analisi di rischio di Livello 2 di sostanze disciolte sono qui adattate ed estese per tenere conto dei meccanismi di trasporto propri delle NP. Vengono infine presentati due casi applicativi della procedura proposta, uno teorico ed uno sperimentale-modellistico, facendo particolare riferimento al ruolo dell’eterogeneità granulometrica delle NP nella quantificazione del rischio sanitario ambientale

Field assessment of guar gum stabilized microscale zerovalent iron particles for in-situ remediation of 1,1,1-trichloroethane

A pilot injection test with guar gum stabilized microscale zerovalent iron (mZVI) particles was performed at test site V (Belgium) where different chlorinated aliphatic hydrocarbons (CAHs) were present as pollutants in the subsurface. One hundred kilograms of 56 μm-diameter mZVI (~ 70 g/L) was suspended in 1.5 m3 of guar gum (~ 7 g/L) solution and injected into the test area. In order to deliver the guar gum stabilized mZVI slurry, one direct push bottom-up injection (Geoprobe) was performed with injections at 5 depths between 10.5 and 8.5 m bgs. The direct push technique was preferred above others (e.g. injection at low flow rate via screened wells) because of the limited hydraulic conductivity of the aquifer, and to the large size of the mZVI particles. A final heterogeneous distribution of the mZVI in the porous medium was observed explicable by preferential flow paths created during the high pressure injection. The maximum observed delivery distance was 2.5 m. A significant decrease in 1,1,1-TCA concentrations was observed in close vicinity of spots where the highest concentration of mZVI was observed. Carbon stable isotope analysis (CSIA) yielded information on the success of the abiotic degradation of 1,1,1-TCA and indicated a heterogeneous spatio-temporal pattern of degradation. Finally, the obtained results show that mZVI slurries stabilized by guar gum can be prepared at pilot scale and directly injected into low permeable aquifers, indicating a significant removal of 1,1,1-TCA