Design Of Permeable Reactive Barriers Research Articles

Machine learning vs. statistical model for prediction modeling and experimental validation: Application in groundwater permeable reactive barrier width design

Permeable reactive barrier (PRB) is an effective in-situ technology for groundwater remediation. The important factors in PRB design are the width and reactive material. In this study, the beaded coal mine drainage sludge (BCMDS) was employed as the filling material to adsorb arsenic pollutants in groundwater, aiming to design the width of PRB. The design methods involving traditional continue column experiments and empirical formulas, as well as machine learning (ML) predictions and statistical methods, which are compared with each other. Traditional methods are determined based on breakthrough curves under several conditions. ML method has advantages in predicting the width of mass transfer zone (WMTZ), which simultaneously consider the characteristics of material, pollutant, and environmental conditions, with data collected from articles. After data preprocessing and model optimizing, selected the XGBoost algorithm based on the high accuracy, which shows good prediction for WMTZ (R2 = 0.97, RMSE = 0.15). The experimentally derived WMTZ values were also used to validate the predictions, demonstrating the ML low error rate of 7.04 % and the feasibility. Subsequent statistical analysis of multiple linear regression (MLR) showed the error rate of 39.43 %, interpret superiority of ML due to the complexity of influencing factors and the insufficient precision of math regression. Compared to traditional width design methods, ML can improve design efficiency and save experimental time and manpower. Further expansion of the dataset and optimization of algorithms could enhance the accuracy of ML, overcoming existing limitations and gaining broader applications.

Utilizing machine learning for reactive material selection and width design in permeable reactive barrier (PRB)

Permeable reactive barrier (PRB) is an important groundwater treatment technology. However, selecting the optimal reactive material and estimating the width remain critical and challenging problems in PRB design. Machine learning (ML) has advantages in predicting evolution and tracing contaminants in temporal and spatial distribution. In this study, ML was developed to design PRB, and its feasibility was validated through experiments and a case study. ML algorithm showed a good prediction about the Freundlich equilibrium parameter (R2 0.94 for KF, R2 0.96 for n). After SHapley Additive exPlanation (SHAP) analysis, redefining the range of the significant impact factors (initial concentration and pH) can further improve the prediction accuracy (R2 0.99 for KF, R2 0.99 for n). To mitigate model bias and ensure comprehensiveness, evaluation index with expert opinions was used to determine the optimal material from candidate materials. Meanwhile, the ML algorithm was also applied to predict the width of the mass transport zone in the adsorption column. This procedure showed excellent accuracy with R2 and root‐mean‐square‐error (RMSE) of 0.98 and 1.2, respectively. Compared with the traditional width design methodology, ML can enhance design efficiency and save experiment time. The novel approach is based on traditional design principles, and the limitations and challenges are highlighted. After further expanding the data set and optimizing the algorithm, the accuracy of ML can make up for the existing limitations and obtain wider applications.

Analytical Model for the Design of Permeable Reactive Barriers Considering Solute Transport in a Dual-Domain System

A permeable reactive barrier (PRB) is an effective groundwater in situ remediation technology, and the design methods used for PRBs are significant in ensuring that they meet remediation goals. Steady-state analytical solutions are an effective tool to provide conservative and simple design methods. A steady-state analytical solution is proposed to describe organic contaminant transport through the PRB and aquifer in a PRB and cut-off wall system. The proposed analytical solution may serve as an effective tool to provide conservative and simple design methods. The shape factor (S) is introduced at the PRB-aquifer interface to investigate the effects of a PRB’s layout forms on its performance. The results show that the relative contaminant concentration at the point of compliance for a PRB with S=8 is 11 orders of magnitude larger than that without considering the shape factor. Effects of degradation, dispersion, and advection on PRB design are subjected to dimensionless analysis. Dimensionless analysis shows that degradation plays a key role in decreasing contaminant concentration in the PRB. In addition, increasing advection may promote contaminant transport from the source to the aquifer. Simplified solutions to estimate PRB thickness and source remediation time are derived for the practical design and performance evaluation of the PRB system.

The Influence of pH on Subsurface Denitrification Stimulated with Emulsified Vegetable Oil

Treatment of nitrate rich groundwater using permeable reactive barriers (PRBs) established with injection of emulsified vegetable oil is receiving attention in areas where groundwater discharges contribute to eutrophication (e.g., Cape Cod, MA). To better understand the biogeochemical process kinetics when emulsified vegetable oil (EVO) is used to stimulate denitrification within the subsurface, microcosm experiments and process-based modeling were conducted for pH conditions ranging from 4 to 8. Biomass variability in soil and pH variations were found to affect denitrification, with limited nitrate reduction observed below pH 5.0. Different rates for denitrification associated with biomass variability suggest that a greater characterization of the indigenous biological community may improve PRB design and operation. Process-based modeling employed the activated sludge model No 3 (AMS3) framework that assumes denitrification as a two-step anoxic process dependent primarily on heterotrophic bacteria, soluble substrate, nitrate, and nitrite. Experimental data were used to calibrate the model under neutral to low pH, resulting in a robust set of equations that can be coupled with transport in future research to improve PRB effectiveness.

PCB containment using geosynthetics in Canada's Arctic

Two funnel-and-gate permeable reactive barriers (PRBs) with settling ponds comprised of a composite liner (geomembrane and geosynthetic clay liners (GCL)) were installed at Resolution Island (BAF-5), Nunavut to contain residual polychlorinated biphenyls (PCBs) moving with the sediment and annual snowmelt. The long-term performance of the geosynthetics used in the PRB funnels is studied for physical integrity and diffusive barrier properties after nine years of operation. Exhumed geomembrane specimens are compared with virgin material by index testing: diffusive resistance to benzene, toluene, ethylbenzene, and xylenes (BTEX) compounds, as well as puncture, burst, and tensile strength. Exhumed GCLs are evaluated concerning hydraulic conductivity, the mass of bentonite per unit area, and swell index. The migration of PCBs through the composite liner system by diffusion is modelled and the diffusive and sorptive properties of the geomembrane (Dg = 1.7 × 10−14 m2/s, Sgf = 160,000) and GCL (De = 3.1 × 10−10 m2/s, bentonite plus fibers layer Kd = 15 mL/g, cover geotextile Kd = 12,000 mL/g, and carrier geotextile Kd = 16,000 mL/g) were calculated. Modelling results estimate that the composite liner was successfully containing PCBs. This was confirmed by downgradient monitoring. The challenges of the location and terrain, PRB design, construction, and maintenance are discussed along with recommendations for designing PRBs in other remote and cold region environments.

A Review of the Hydraulic Performance of Permeable Reactive Barriers Based on Granular Zero Valent Iron

Permeable reactive barriers (PRBs) based on the use of zero valent iron (ZVI) represent an efficient technology for the remediation of contaminated groundwater, but the literature evidences “failures”, often linked to the difficulty of fully understanding the long-term performance of ZVI-based PRBs in terms of their hydraulic behavior. The aim of this paper is to provide an overview of the long-term hydraulic behavior of PRBs composed of ZVI mixed with other reactive or inert materials. The literature on the hydraulic performance of ZVI-based PRBs in full-scale applications, on long-term laboratory testing and on related mathematical modeling was thoroughly analyzed. The outcomes of this review include an in-depth analysis of factors influencing the long-term behavior of ZVI-based PRBs (i.e., reactive medium, contamination and the geotechnical, geochemical and hydrogeological characteristics of the aquifer) and a critical revision of the laboratory procedures aimed at investigating their hydraulic performance. The analysis clearly shows that admixing ZVI with nonexpansive granular materials is the most suitable choice for obtaining a long-term hydraulically efficient PRB. Finally, the paper summarizes a procedure for the correct hydraulic design of ZVI-based PRBs and outlines that research should aim at developing numerical models able to couple PRBs’ hydraulic and reactive behaviors.

Sensitivity analysis of factors influencing pollutant removal from shallow groundwater by the PRB method based on numerical simulation.

Permeable reactive barrier (PRB) is one of the most promising in situ treatment methods for shallow groundwater pollution. However, optimal design of PRB is very difficult due to a lack of comprehensive understanding of various complex influencing factors of PRB remediation. In this study, eight of the main factors of PRB, including hydraulic gradient I, permeability coefficient KPRB of PRB material, PRB length L, PRB width W, PRB distance from pollution source Dist., the ratio of the maximum adsorption capacity to Langmuir constant of PRB material Qmax/KL, the discharge rate of pollution source DR, and recharge concentration RC were investigated, to carry out the sensitivity analysis of PRB removal efficiency. The simulation experiments for Morris analysis were designed, and pollutant removal efficiency was numerically simulated by coupling MODFLOW and MT3DMS under two scenarios of high and low permeability and dispersivity. For a typical low permeability with low dispersity medium, the sensitivity ranking of factors from high to low is DR, RC, I, W, L, Dist., Qmax/KL, and KPRB, and for a typical high permeability with a high dispersity medium, the sensitivity ranking of factors from high to low is I, W, DR, Qmax/KL, L, RC, Dist., and KPRB. When considering multiple factors in PRB design, the greater the KPRB, L, W, Qmax/KL is, the higher the removal efficiency is; the greater the RC, I is, the lower the removal efficiency is. The rest factors remain ambiguous enhancement to removal efficiency.

Removal of Transition Metals from Contaminated Aquifers by PRB Technology: Performance Comparison among Reactive Materials.

The most common reactive material used for the construction of a permeable reactive barrier (PRB) is zero valent iron (ZVI), however, its processing can generate corrosive effects that reduce the efficiency of the barrier. The present study makes a major contribution to understanding new reactive materials as natural and synthetic, easy to obtain, economical and environmentally friendly as possible substitutes for the traditional ZHV to be used as filters in the removal of three transition metals (Zn, Cu, Cd). To assess the ability to remove these pollutants, a series of batch and column tests were carried out at laboratory scale with these materials. Through BACH tests, four of seven substances with a removal percentage higher than 99% were prioritized (cabuya, natural clinoptilolite zeolites, sodium mordenite and mordenite). From this group of substances, column tests were performed where it is evidenced that cabuya fiber presents the lowest absorption time (≈189 h) while natural zeolite mordenite shows the highest time (≈833 h). The latter being the best option for the PRB design. The experimental values were also reproduced by the RETRASO code; through this program, the trend between the observed and simulated values with respect to the best reactive substance was corroborated.

Hydro-biogeochemical processes and nitrogen removal potential of a tidally influenced permeable reactive barrier behind a perforated marine bulkhead

Denitrifying permeable reactive barriers (PRBs) installed in proximity to impaired waterbodies can help mitigate coastal eutrophication. We monitored the hydro-biogeochemical conditions and assessed nitrogen removal performance of a tidally influenced pilot PRB installed behind a perforated marine bulkhead. The pilot was installed in an area with appreciable submarine groundwater discharge (SGD) and elevated nitrogen concentration likely due to medium – high density residential septic input to the local watershed. The bulkhead PRB reduced groundwater nitrate concentrations from 285 to 429 μM to <1 μM. Data from the pilot PRB was used to inform a laboratory column experiment in which twelve columns filled with aged media retrieved from the pilot PRB received artificial groundwater and seawater mixtures (0, 1, and 11 ppt) at representative hydraulic loading rates (HLR) and influent nitrate concentrations. Nitrate removal was not affected by salinity nor by the presence of hydrogen sulfide that was produced in the 1 and 11 ppt treatments. Significantly less dissolved methane was observed in 1 and 11 ppt treatments compared to the 0 ppt treatment, indicating that the tidally driven supply of sulfate rich porewater can suppress the onset of methanogenesis. Extrapolating column experiment results, bulkhead PRBs have the potential to reduce nitrogen loading to adjacent water bodies by 1.6–3.2 kg N m−1 shoreline year−1 over a range of relevant groundwater nitrate concentrations and hydraulic settings. Integration of PRBs during bulkhead replacement is a promising strategy to remove legacy nitrate from groundwater to improve coastal water quality, and site-specific PRB design is critical to maximize nitrogen removal while minimizing the risk of pollution swapping.

A review on design, material selection, mechanism, and modelling of permeable reactive barrier for community-scale groundwater treatment

Over the last thirty years, several techniques of groundwater (GW) remediation based on the principles of physical (air sparging), biological (bioventing), and chemical (e.g., ion exchange) processes have proven to be effective; however, only a handful of them could successfully be implemented at a community or regional scale due to issues like longevity, a requirement of significant investment and operation cost, skilled labours, and others. Therefore, considering the scope of Permeable Reactive Barriers (PRBs) to be implemented on a regional scale and its capability to be a significant replacement for several existing GW treatment methods, this review was prepared with the following objectives: (i) to compare the PRB method with the conventional methods of groundwater treatment along with the possibility and problems associated with the PRB installation in pilot-scale; (ii) to enlist all the probable sets of adsorbents (reactive materials) that can be used for different types of organic and inorganic contaminants; (iii) to understand the key mechanisms of degradation/removal of contaminants involved in PRB design; and (iv) to put forward the future research perspectives of this domain. Review augments that PRBs certainly has a low maintenance cost and a longer life span of ̃30 years that requires very ordinary skills. PRBs promise to be effective in developing countries like India, Bangladesh, and Sri Lanka for the removal of geogenic contaminants like arsenic and fluoride given the appropriate aquifer depth and hydrogeological settings like hydraulic gradient and transmissivity. Furthermore, reactive fillers required in PRBs are readily available, have longer expected life, and operate with no surrounding disturbances. With the advent of several green nanomaterials based adsorbents, PRB’s performance can achieve another height, but it needs the experiences from several pilot and larger scale projects. Indeed PRBs are the need of the hour, but a more programming-based investigation would be expected for its superior comprehension.

Multi-objective optimization of permeable reactive barrier design for Cr(VI) removal from groundwater.

The present study aims to develop a practical approach for the optimal permeable reactive barrier (PRB) design towards Cr(VI) removal from groundwater. Batch and column experiments were performed to investigate the characteristics of the four proposed reactive materials; nanoscale zero-valent iron (Fe0), bimetallic nanoscale zero-valent iron (Fe0/Cu), activated carbon (AC) and sand/zeolite mixture (S/Z). Kinetic analysis and dynamic modeling of the experimental data were implemented to determine the controlling conditions of the reactive performance of the PRB's materials. The sensitivity index of the design parameters was examined as an indicator of their effect on the reactive responses. Moreover, the Response Surface Methodology (RSM) was considered for optimizing the design variables of the PRB based on the practical factorial analysis. Results revealed that Fe0 and Fe0/Cu showed high performance in Cr(VI) removal, with a slight superiority to Fe0, with final removal efficiency values of 89.7 and 84.1%, respectively. Kinetic analysis depicted that pseudo second order was the best fitting model for Cr(VI) removal in the four materials' cases. ANOVA statistical analysis revealed that quadratic polynomial model was the best model, corresponding to the highest correlation efficiency and adequate precision, to describe the relationships in the four PRB's cases between the selected dependent variables; resident time (tR), reactive material mass per sectional area of contaminant plume (M/A) and reactive material cost (CostPRB) towards the independent parameters; barrier thickness (b) and permeability (Kr). Additionally, sensitivity analysis has been conducted which depicted the high sensitivity, in the four PRB's cases, of average pore water velocity within the barrier (vr) vr and Kr with the highest and the second-highest sensitivity index (SI) values towards tR, respectively. The RSM-optimization revealed that Fe0 is the most feasible reactive material, comparing to the other considered materials, with respect to the optimal conditions regarding the long residency (tR=22 days) and low cost (b=0.521m), with around 95.2% desirability of its optimal solution. Overall, the current study represents a significant contribution and a vital step towards an accurate PRB's design based on previously determined optimal conditions.

Optimization of permeable reactive barrier dimensions and location in groundwater remediation contaminated by landfill pollution

One of the well-known methods is the use of permeable reactive barrier (PRB) to treat groundwater. In order to simulate the PRB design due to the complexity of the equations governing hydraulic flow and pollutant transfer and the integration of these two sets of equations, there is need to use simulated flow and pollutant transfer models. MODFLOW as quantitative model for groundwater flow modeling and MT3DMS qualitative model for groundwater contamination modeling as well as GMS software as graphical interface were used. In the optimization section, due to the extent of decision space in the objective function and the need for high reliability of the selective optimization algorithm, metaheuristic algorithm (GA) is used. The model is implemented under the non-barrier condition and then comparing it with the optimum PRB condition indicates that cost of treatment and total costs are reduced despite consideration costs of manufacturing and active materials presented in PRB and then effect of changes in landfill pollution concentration level was studied in different scenarios. Also, the simulation model was used to investigate the effect of changes of PRB’s active material, length, width and height parameters on pollutant concentration and treatment cost. Of the total PRB costs, 37 % is related to manufacturing costs and 63 % to active materials used in PRB. A comparison of the total costs reveals 45 % benefit in the using condition and the optimum barrier width is 3 m, its length is 35 m and its height is 25 m, located 21 m from the well.

Biogeochemical Behavior of Metals Along Two Permeable Reactive Barriers in a Mining‐Affected Wetland

AbstractThe biogeochemistry of two alkaline permeable reactive barriers (PRBs) installed for remediation in a mining‐affected wetland was investigated in order to assess the importance of colloidal particles on metal removal processes in such systems. At the time of investigation, both PRBs were effective in removing U, Cu, and Zn (&gt;95%) from groundwater but were slightly less efficient for Ni and Co (&lt;90%). Previously installed groundwater wells allowed an in‐depth analysis of groundwater passing through the first PRB. Here, in an alkaline environment (pH 6.0–9.7), 11–14% of Ni, 36–37% of Co, 77–81% of Cu, 14–17% of U, and 10–19% of Fe were associated with organic matter and inorganic colloids, while upgradient in the more acidic environments (pH &lt;6.0), ionic species and complexes (e.g., Co2+, Ni2+, Cu2+, and UO2H3SiO4+) dominated. Copper and U preferentially bound to larger colloidal fractions (&gt;1 kDa), which might have promoted their sequestration. Uranium removal was likely further enhanced by U (VI) reduction in the alkaline and oxygen‐depleted conditions of the PRBs. The less efficient removal of Ni and Co, being target metals for remediation, was explained by a combination of their high solubility, unfavorable redox and pH conditions created by the alkaline PRBs, and their limited association with colloidal particles. These considerations are critical in the design of future PRBs for the remediation of similar systems.

The removal efficiency and long-term hydraulic behaviour of zero valent iron/lapillus mixtures for the simultaneous removal of Cu2+, Ni2+ and Zn2+

Granular mixtures composed of zero valent iron (ZVI) and lapillus at two different weight ratios (i.e. 30:70 and 50:50) were tested through column experiments for the simultaneous removal of Cu2+, Ni2+ and Zn2+ present in aqueous solutions at high concentrations. The results were used to evaluate the feasibility of the above-mentioned granular mixtures as reactive media in permeable reactive barriers (PRB) for the remediation of groundwater polluted by metals. Test results showed that the two granular reactive media efficiently removed the three heavy metals under study according to the following removal sequence Cu > Zn > Ni. The granular mixture with the higher iron content showed a proportionally higher removal rate but also a higher reduction of hydraulic conductivity over time. Different removal mechanisms occurred for the three contaminants in question. Considering that for Ni and Zn the main removal mechanism was probably adsorption, we used different mathematical models, in order to predict the breakthrough curves for the adsorption mechanisms. The Adams-Bohart model showed the best fit with the experimental data and it was thus used to predict the zinc removal front within the barrier thickness. Finally, we showed that the mathematical approach may be used for the design of PRBs for the reactive media and contaminants used in this research.

Two-objective Optimization for Optimal Design of the Multilayered Permeable Reactive Barriers

The design, construction and management of engineering projects tend to be large scale, indivisible, and long-term facilities, with investments taking place in waves. This investment process due to constant further development has become a critical issue in the world management of civil engineering with regard to sectors of the national and private economy. These kinds of investments cannot be limited to solving just the mechanical problem. They should be designed with consideration of their life cycle, all costs of the project and construction stage, as well as what will be obtained during the lifetime of the investment. Due to the increasing environmental contamination, especially soil and ground-water, the need of restoration of these degraded lands for construction investment has emerged. Nowadays, the remediation technologies are more often focused on using passive engineering constructions installed in the ground, for example permeable reactive barriers (PRB) which allow for land use during decontamination processes. As with any investment, information about the costs of remediation technologies is as significant in determining their final commercial success as are efficiencies in the data. Technology investors need to have consistent cost information to conclude whether the technology will be economical or not. In the case of PRB technologies, the installation is a major investment cost, where one of the biggest drivers are material costs. The most important parameters for PRB cost and design are dimensions (thickness, length and height should be enough to treat the entire width of contaminants and to prevent their migration). The most important challenge is to determine the optimal thickness of a PRB, which provides the residence time appropriate for reducing the concentration of contaminants. In an engineering investment which involves designing, systems and decision making, optimization is crucial in creating best design subjects irrespective of constraints. In this paper two-objective optimization methods for multi-layered PRB design are considered. The proposed methods are characterized by following special features: elimination of a timeconsuming simulation model; application of a universal, simple Excel spreadsheet-based optimisation model that calculates minimum cost of PRB using solver; and the usage of real input variability based on literature and laboratory tests. In view of minimizing cost of reactive materials and maximizing the resident time of contamination, the required thicknesses of PRB layers: activated carbon, zeolite and zero valent iron were calculated.

A design solution of PRB with multispecies transport based on a multi-domain system

Based on a multi-domain system involving an up-gradient permeable reactive barrier (PRB) and a down-gradient aquifer, analytical solutions were developed to solve the multispecies solute transport. Multi-domains configuration of the transformation approach used in the previous studies is validated by comparing the calculation results from these solutions and numerical results computed by using the numerical reactive transport code. The solutions are calculated and compared with the numerical model results. When the compliance plane is far from the PRB exit face, the aquifer reaction capability was found important for the required thickness calculation of the PRB. A new PRB design equation for multi-species solute transport is developed. For the remediation multiple species sharing a single parental species, the proposed analytical solution provides meaningful designing information for the PRB remediation settlement.

Phosphate Removal Through Nano-Zero-Valent Iron Permeable Reactive Barrier; Column Experiment and Reactive Solute Transport Modeling

This study investigates the efficacy of nano-zero-valent iron (nZVI) permeable reactive barriers (PRBs) as an in situ phosphate removal method. Batch equilibrium experiments were conducted to determine maximum adsorption capacity of nano-iron particles for phosphate to be 54.34 mg-P/g-Fe. Short-term experiment was performed for a period of a month through three sandy soil columns with different configurations of nZVI reactive layers. Initial concentration of 25 (PO4–P) mg/L was introduced to the columns, while effluent samples were collected for analysis. Numerical model was developed to simulate the 1-D advective–dispersive reactive transport of phosphate through the porous media in the three columns. Sensitivity analysis of model parameters was performed, expressed by the change in the effluent concentration with respect to variation in the value of model critical parameters. Optimization of transport process in columns was based on minimizing the sum of squared error values between measured and predicted effluent concentration. Phosphate breakthrough curves implied that Column 2 (C2) with two reactive layers showed a slight better performance in phosphate removal than Column 1 (C1) with maximum efficiency of 98.9% after only 17 h from the beginning of the experiment, whereas phosphate concentration in Column 3 (C3) reached the full saturation by the 9th day. The model verification with experimental data showed a reasonable agreement with a correlation coefficient (R2) ranging from 0.97 to 0.99. The results in this study confirmed that such presented model can be used for the promotion of the preliminary design of PRBs.

Strength and Numerical Analysis in the Design of Permeable Reactive Barriers

Permeable reactive barriers are one of the most important in situ technologies in groundwater remediation. Most of the installed PRBs have tended to use singular reactive media, but there is an increasing number of applications using combined or sequenced media to treat mixtures of contaminants within a groundwater plume. The concept of a multi-layered permeable reactive barrier (MPRB) to prevent and protect groundwater along traffic routes, especially in ecologically and naturally valuable areas, was developed following several field and laboratory investigations conducted in the Department of Geotechnical Engineering of the Warsaw University of Life Sciences. In accordance with the guidelines of the Interstate Technology & Regulatory Council for the selection of reactive materials, numerous laboratory and field investigations should be performed to determine the environmental conditions, type and concentrations of the contaminants, and the physical-chemical and permeability properties of the reactive materials. However, the deformation and strength properties of the reactive materials should be also considered in the design and evaluation of the safety conditions. In this paper, strength and deformation properties of silica spongolite, zeolite, and activated carbon were investigated using direct shear and oedometer tests. The laboratory test results were used in numerical calculations with the application of the finite element method. The aim of this study was to define the impact of the installation stages of a multi-layered permeable reactive barrier on the stability of a road embankment. Numerical analysis may prevent, reduce or eliminate the risk in the case of a breakdown during the construction or/and exploitation of a PRB.

Optimization Model for the Design of Multi-layered Permeable Reactive Barriers

Permeable reactive barriers (PRBs) are employed as in situ groundwater remediation technology. The installation of PRBs is usually a major investment, where one of the biggest cost drivers are material costs. PRBs are barriers against contaminants moving under the natural gradient, however not against groundwater contaminants. The most common construction of a PRB is a single barrier, but in the case of contaminant mixtures a multi-layered construction, i.e. a combination of different reactive materials and removal processes, is required. The most important parameters for PRB design are dimensions. The barrier must be long enough to treat the entire width of the plume (dimension perpendicular to groundwater flow) and should extend to and be keyed into an impermeable layer. The problem is to determine the optimal thickness of a PRB, which should provide a residence time appropriate for reducing the concentration of contaminants to the desired effluent concentration. In PRBs, design is accomplished using numerical methods or simulators, which are useful to predict the scenarios and evaluate the resulting groundwater flow systems to specific site conditions. On the other hand, numerical methods are complicated and may have significant errors if the discretization is too coarse or is incorrectly aligned. This paper deals with a simple, conceptual model of a one-approach optimization method for multi-layered PRB design. Based on literature and laboratory test results (residence time, density and hydraulic coefficient), a selection of layers of reactive materials was determined. Considering the lowest cost of the reactive materials, the required thicknesses of activated carbon, zeolite and zero valent iron were calculated using two different algorithms. The simple model may be used for preliminary barrier design and cost calculations. Using the optimization model in a preliminary design stage, it is possible to reject the PRB concept and avoid losing time for the complicated analysis.

Fe0/pumice mixtures: from laboratory tests to permeable reactive barrier design

Permeable reactive barriers containing zero valent iron (ZVI-PRBs) represent an in situ technology for groundwater remediation. One of the main design issues of ZVI-PRBs is the determination of the expected lifespan. Therefore, the objective of the present work is to determine, by interpreting column experiments data through a first-order kinetic model, the thickness and duration of a hypothetical PRB constituted respectively by ZVI and by a ZVI/pumice mixture to be built for remediating groundwater contaminated by nickel at two different initial concentrations (40 and 8 mg/L). The kinetic constant values derived from this model are intended solely as an engineering tool for the design of ZVI systems and not as an interpretation of the actual reactions involving ZVI, water, and its constituents. Column test results show a linear decrease over time of the kinetic constant values. The use of the mixture implies an increase of the barrier thickness (from 2·5 to 15·5 times the ZVI thickness) but allows a reduction in the amount of ZVI needed to reach remediation objectives (from 84·5 to 87% considering 4 years of PRB operation) and an increase in the lifespan of the barrier (about 7·2 times for the lowest values of nickel concentration used).