Remediation of Thallium-contaminated Groundwater by Permeable Adsorptive Barrier

Procedia Environmental Sciences 25 ( 2015 ) 175 – 182

1878-0296 © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Scientific Committee of the IAHR Groundwater Symposium 2014

doi: 10.1016/j.proenv.2015.04.024

ScienceDirect

7th Groundwater Symposium of the

International Association for Hydro-Environment Engineering and Research (IAHR)

Remediation of thallium-contaminated groundwater by

permeable adsorptive barrier

D. Musmarra

, M. Di Natale

, I. Bortone

, A. Erto

and M. Ciarmiello

*

Abstract

The deterioration of groundwater quality is a widespread concern mainly originating by accidental discharges and soil/landfills leaching. Permeable Adsorptive Barriers (PAB) represents a challenging in situ remediation technology, which consist of a continuous trench penetrating the aquifer at a full depth. A PAB is filled with an adsorptive material; groundwater flow moves under natural gradient and the remediation naturally occurs.

This paper deals with the application of a PAB for the remediation of thallium Tl(I) contaminated groundwater in Falciano del Massico, Italy. The polluted site is a solid waste landfill, where many wastes were dumped over the past decades, particularly during the crisis of waste and landfill management in large areas of the region of Campania (Italy). Sawdust is chosen as reactive material for PAB, as it showed good Tl(I) removal capacity.

Based on the hydrogeological and geotechnical characterization of the polluted aquifer, a 3D numerical model is developed to describe pollutant transport and adsorption mechanisms onto the barrier. Numerical simulations are accurately performed over a long time span, by means of Computational Fluid Dynamic approach developed in COMSOL® _{Multiphysics. PAB configuration and design parameters are determined, in terms of location, shape and}

main dimensions, using a procedure previously developed. Results shows that the designed PAB is effective for the remediation of the contaminated aquifer, being Tl(I) concentration flowing out the barrier always lower than Italian regulatory limit. Furthermore PAB has been demonstrated to be an efficient long term method for groundwater protection.

© 2015 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the Scientific Committee of the IAHR Groundwater Symposium 2014. Keywords: Permeable adsorptive barriers (PABs); thallium; groundwater remediation.

* Corresponding author.

E-mail address: mena.ciarmiello@unina2.it

© 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Discharges and landfills leaching from solid waste landfill are often a significant source of contamination both by inorganic and organic compound, causing the deterioration of groundwater quality. The development of efficient technologies applicable to groundwater remediation is a fundamental objective for environmental sustainability, economic and social issues. However, the design and optimization of an effective groundwater remediation is rather complex, involving hydrogeological and geotechnical properties of the polluted aquifer, pollutant properties and costs evaluation. Among the

in-situ groundwater treatments, Permeable Adsorptive Barriers (PABs) are a suitable technology, assuring

both groundwater remediation and cost effectiveness. PABs consist in a trench penetrating the aquifer and filled of reactive material, whose hydraulic conductivity is higher than that of the surrounding soils, so that the contaminated plume moves under natural hydraulic gradient and it is passively treated without external energy inputs [1]. Furthermore, PABs allow to achieve high efficiency and relatively low operating and maintenance costs in the long term period and allow the productive use of the site almost immediately after installation [2]. Clogging phenomena due to salt precipitation are the main disadvantage of PRBs [3].

PABs have demonstrated to be an effective technology for the removal of both heavy metal and organic compound [4-11]. More in general, the wide application of the adsorption process is also due to the possibility of using different kinds of adsorbents, including natural materials, waste materials or by-products [12-18].

Tl(I) is a highly toxic, mobile element in the environment [19-20]; its removal from groundwater has been studied far less than other toxic element, due to the poor sensitivity of the classic analytical methods [21]. The design and optimization of an in-situ effective depuration technology must also take into account the hydrological and geotechnical properties of the entire polluted aquifer, the properties of the pollutants and the adsorption properties of the adsorbent.

The aim of this paper is to investigate the effectiveness of a PAB for the remediation of a Tl(I)-contaminated groundwater. A Tl(I)-Tl(I)-contaminated aquifer in Falciano del Massico, a town in the north of the province of Caserta (Italy), in an area (known as Terra dei Fuochi) was considered as case study. This area was used as landfill in the late 70s and early 80s. Pollutant transport and capture by the barrier were modeled by a 3D COMSOL® _{Multiphysics code. The design of the barrier was performed by an} optimization procedure previously developed [22]. Activated sawdust [23] was considered as adsorbing material and the main barrier properties (i.e. location, orientation, dimensions) were evaluated, in order to keep Tl(I) concentration out-flowing the barrier below the Italian regulatory limit for groundwater, set to 2 µg L-1 _{(Italian Legislative Decree 152/06). Furthermore, Tl(I) concentration in the water flowing out the} barrier was calculated over a long period of time to check for the effectiveness of the technique.

Nomenclature

A polluted area total extend

a external specific surface area of adsorbent particles

C contaminant concentration

C* _{concentration in the liquid phase at thermodynamic equilibrium with the adsorbing solid} Csoil contaminant concentration on the soil

D tensor of the mechanical dispersion D*

d coefficient of molecular diffusion Dh hydrodynamic dispersion coefficient H aquifer bed height

h hydraulic head

J piezometric gradient

K Langmuir’s isotherm constant

k hydraulic conductivity

kb hydraulic conductivity of the barrier kc mass transfer coefficient

kd soil-water partition coefficient

L barrier length

nb porosity of the barrier ns porosity of the soil

R reaction term

t time u

_{unit flux vector}

w barrier width

Greek letters

αx longitudinal dispersivity αy transverse dispersivity αz vertical dispersivity ρb,s dry soil bulk density

ρb dry soil bulk density in the barrier

ω solid adsorbing material contaminant concentration ωmax Langmuir’s isotherm constant

2. Transport modelling

The transport of solute contaminants can be described by advection and dispersion mechanisms [24]. Therefore, for a three-dimensional system, the dissolved contaminant mass transport may be described as follows:

(

)

ρ

being C the pollutant concentration in groundwater, Csoil the pollutant concentration on the soil, ρb,s the dry soil bulk density, ns the soil porosity, u

the unit flux vector, Dh the hydrodynamic dispersion coefficient, R the generation/deplation rate of pollutant by chemical phenomena (in this case represented by the occurrence of adsorption phenomena) and t the time. The hydrodynamic dispersion coefficient, Dh, may be defined as the sum of mechanical dispersion tensor, D, and the molecular diffusion coefficient (a scalar), D* d, i.e.: * d h D D D = + (2)

Furthermore, the pollutant concentration on the soil, Csoil, may be written as follows

C

k

being kd the soil-water partition coefficient. In this work, kd is assumed to be 0.8 L kg-1 [25]. Furthermore, the unit flux vector, u
depends on the hydraulic conductivity of the aquifer, k, and the hydraulic head, h, and it can be calculated by the Darcy equation:

h

k

u

=

−

∇

Furthermore, in a PAB, the term R in Eq.1 describes adsorption phenomena and it can be expressed as follows:

( )

[

ω

]

where kc is the overall mass transfer coefficient for adsorption and a the specific surface area of adsorbent particles. The term C*(ω) describes the mass transfer driving force in the transport model

equation, according to Langmuir’s isotherm:

( )

( )

ω

ω

ω

ω

being ωmax and K the Langmuir’s constants.

The boundary conditions can be expressed as follows:

t z y X x C D t y x Z z z CC y y Y x z t t z y x C h∇ = = ∀ ∀ ∀ − ∀ ∀ ∀ = = = ∀ ∀ ∀ = = = ∀ ∀ ∀ = = , , 0 , , , 0 00 0 , , , , 0 0 (7)

assuming a reference frame coinciding with the boundary of the domain, where X, Y and Z are the size of the domain in x-, y- and z-direction, respectively. The model definition sketch is given in the Figure 1.

L w Groundwater Flow Contaminant Plume x y PAB

The modelling equations with their appropriate initial and boundary conditions were solved by a COMSOL® _{Multiphysics code. An activated sawdust was adopted as sorbent material; its characterization} for Tl(I) removal is reported in Memon et al. [23]. Numerical model parameters adopted in the performed simulations are reported in the Table 1.

3. Case study

In this work, a polluted site in Falciano del Massico, in the north are of the province of Caserta (Italy) known as Terra dei fuochi, was considered as a case study to evaluate PAB effectiveness in groundwater remediation. This site was used as landfill during the late 70’s and early 80’s. In early 90’s, the landfilled wastes were removed, while the site characterization was carried out in 2000s, showing the presence of a large number of pollutants into both groundwater and soil, mainly inorganics, and in particular Tl(I).

3.1. Site characterization

The groundwater is located about 10 cm below the ground surface. The aquifer consists of two stratigraphic units, both 5.00 m depth, whose hydraulic conductivity is 1×10-5 _{m s}-1 _{and 1×10}-4 _{m s}-1 respectively. The groundwater flow lines are west/east oriented, under a piezometric gradient, J, of 0.004 m m-1_{. Tl(I) contamination on the soil was detected in the upper 1.00 m depth soil layer below the ground} surface and in the soil layer situated 4.00 m and 5.00 m below the ground surface. In detail, Tl(I)contamination on the soil is variable and a maximum value of about 7 times higher than the Italian regulatory limit in dry soil (set to 1 mg kg-1_{, Italian Legislative Decree 152/06) was found. The main} hydrogeological and geological site properties are reported in the Table 2, together with Tl(I) pollutant concentration and the main properties of adsorbing material chosen for the barrier.

Table 1. Model grid parameters. Numerical model parameters

Horizontal space step, Δx 2.50 PAB horizontal space step, Δxb Δx/10

Transversal space step, Δy 1.00 PAB transversal space step, Δyb Δy/10

Vertical space step, Δz 1.00 PAB vertical space step, Δzb Δz

Table 2. Aquifer characteristics and adsorbing material properties.

Aquifer characteristics Longitudinal dispersivity, αx 7.50 m Polluted area total extent, A 7000 m2 _{Transverse dispersivity, α}

y αx/10

Aquifer bed height, H 10 m Vertical dispersivity, αz αx/100

Piezometric gradient, J 0.004 m m-1 _{Adsorbing material properties}

Porosity, ns 0.35 Reactive media Activated sawdust

Dry bulk soil density, ρb,s 1300 kg m-3 Dry bulk density, ρb 520 kg m-3

Hydraulic conductivity in the soil layer 1, k1 1×10-5 m s-1 Porosity, nb 0.40 Hydraulic conductivity in the soil layer 2, k2 1×10-4 m s-1 Hydraulic conductivity, kb 7×10-5 m s-1

4. Results

Several numerical simulations were carried out in order to determine the optimal PAB position and dimensions. In this work, the optimum PAB configuration was obtained for a continuous full depth trench penetrating the aquifer perpendicular to groundwater flow, with constant thickness, w, and length, L equal to 3.5 m and 100 m, respectively. Longitudinal and cross sections of the contaminant plume at different time steps, t, (for t=1, 50, 100 and 200 years) are reported in Figures 2-3, respectively. In details, the Figure 2 refers to longitudinal sections taken 1 m below the ground surface, while the Figure 3 shows the cross sections obtained for y = 30 m. It is possible observing that in both the directions the designed PAB is able to intercept the contaminant flow for the whole plume. In Figure 4, Tl(I) inlet and outlet concentrations as a function of the run time are reported as breakthrough curves, showing that during a run time of about 200 years the out-flowing Tl(I) concentration is always lower than the regulatory limit.

Fig. 2. Longitudinal Tl(I) iso-concentration during the simulation time. Snapshots taken for z = 9 m.

Fig. 4. Tl(I) inlet and outlet concentration during the simulation time. 5. Conclusions

This paper deal with the design of a Permeable Adsorptive Barrier (PAB) for the remediation of a contaminated aquifer. A 3D numerical model was developed to describe pollutant transport within groundwater and pollutant adsorption onto the barrier. The numerical model was applied to a case study represented by a Tl(I)-contaminated aquifer in Falciano del Massico, in the North of the province of Caserta (Italy). The optimal barrier properties (location, orientation and dimensions) were defined by an iterative numerical procedure. The results show that Tl(I) concentration outflowing the barrier is always lower than the regulatory limit throughout the computational domain. Finally, the PAB designed allows an effective and long-term thallium retention and aquifer protection.

Acknowledgements

The authors are grateful to “Settore Ambiente, Ecologia e Gestione Rifiuti” of the Province of Caserta – Italy for granting permission to use site characterization data.

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