REACTIVE MATERIALS AND ATTENUATION PROCESSES FOR PERMEABLE REACTIVE BARRIERS

Review Paper Environment

REACTIVE MATERIALS AND ATTENUATION PROCESSES FOR

PERMEABLE REACTIVE BARRIERS ANTHIMOS XENIDIS1, AGGELIKI MOIROU2,

IOANNIS PASPALIARIS3

ABSTRACT

Permeable reactive barrier (PRB) technology is a passive groundwater treatment method for the remediation of chlorinated hydrocarbons, metals and radionuclides. Various materials are used for the construction of the reactive zone, with the most common being granular zero valent iron (Fe0). Reduction, reductive precipitation, precipitation and sorption are the processes usually employed for the immobilization or destruction of the pollutants. Furthermore, reactive materials are required to be environmentally safe and to possess

stability, sufficient permeability and low cost. Permeable barriers in many US sites successfully intercept uranium plumes either in full or pilot scale installations. Zero valent iron and different forms of iron oxide are usually employed as reactive materials with encouraging results. In the present paper, currently available reactive materials are critically evaluated and the reaction mechanisms taking place are presented. Keywords: reactive materials, permeable barriers, groundwater, remediation, pollutants.

Introduction

Permeable Reactive Barrier (PRB) is one of the most promising passive treatment technologies, due to its effectiveness regarding various contaminants, and its low cost compared to other in situ technologies. A typical PRB configuration consists of a permeable treatment zone placed vertically to the flow path of groundwater, which

contains reactive material that immobilizes or decomposes the contaminants, as the ground water flows through it (Figure 1).

PRBs are installed as permanent, semi-permanent, or replaceable units. A wide variety of pollutants are degraded, precipitated, sorbed or exchanged in the reactive zone, including chlorinated solvents, heavy metals, radionuclides and other organic and inorganic species. Major advantages of the process include (Puls, et al. [1], USEPA [2]):

1 Dr. Mining Engineer and Metallurgist2 Mining Engineer and Metallurgist 3 Associate Professor

ΟΡΥΚΤΟΣ ΠΛΟΥΤΟΣ / MINERAL WEALTH 123/2002 35

FIGURE 1: Schematic view of a permeable reactive barrier installation (Powel, et al. [8]). ΣΧΗΜΑ 1: Απεικόνιση τρόπου λειτουργίας ενεργών υδροπερατών φραγµών.

• No need for expensive above-ground facilities for storage, treatment, transport, or disposal, other than monitoring wells.

• After the installation the above ground can be re-used.

• There are limited or no operational and maintenance costs.

• The in-situ contaminant remediation is more effective than the simple migration control achieved by the impermeable barriers.

• Contaminants are not brought to the surface; i.e. there is no potential cross media contamination.

Two installation schemes are more frequently used in field applications: the continuous and the funnel-and-gate PRB. The continuous PRB configuration is characterized by a single reactive zone installed across the contaminant plume, while the funnel-and-gate system consists of a permeable gate placed between two impermeable walls that direct the plume towards the reactive barrier (Figure 2).

The choice between the two configuration options depends on both the hydrogeological characteristics of the site and on the reactive materials cost. When a high-cost reactive material is used, the funnel-and-gate configuration is preferable since the reactive zone requires less material. However, if a cheap material (e.g. like granular iron) is used, it is more profitable to avoid the construction of the impermeable side-walls by employing a continuous barrier, provided that this configuration is in agreement with the hydrogeological study.

FIGURE 2: Funnel and gate (above) and continuous PRB systems (Powel, et al. [8]). ΣΧΗΜΑ 2: Τεχνικές εγκατάστασης ενεργών υδροπερατών φραγµών: funnel and gate (πάνω) και συνεχής (κάτω).

The design of a PRB requires the knowledge of site characteristics (geotechnical data, nature of the contaminants) and the investigation of critical parameters such as the required contaminant/reactant residence time and the stability of possible transformation products. Major issues that must be resolved before the installation of a PRB, are: the installation scheme, the selection of the suitable reactive material or mixture of materials, the reactive zone geometry (thickness, height, length) and the installation cost.

According to the USEPA [3] status report which summarizes data about the use of PRBs in the United States, Canada and Europe, there are currently 18 full scale installations, 15 of which are located in the United States, and 16 pilot scale installations 14 of which are located in the United States. Regarding the construction methods used, 15 installations are funnel-and-gate PRBs, 12 are continuous walls or continuous trenches, 2 are reaction vessels, 2 are

36 ΟΡΥΚΤΟΣ ΠΛΟΥΤΟΣ / MINERAL WEALTH 123/2002

constructed by hydraulic fracturing and 1 is a hanging barrier (not keyed into the aquitard) in metal frame. The widespread construction of funnel-and-gate PRBs, is due to the relative high cost that granular iron possessed during the early stages of the method’s implementation.

Although the technology of the PRBs is continuously progressing, there are still certain disadvantages such as (USEPA [2]): • The technology is restricted to shallow

plumes (up to 50 ft below ground surface). • Limited long-term field testing-data

concerning the longevity of the barrier’s reactivity are available.

• Detailed characterization of the plume and the surrounding environment is required.

In the present paper an attempt is made to present the most common pollutants and reactive materials studied so far. The materials are compared to each other regarding their efficiency, cost and availability in order to provide an overall assessment. The work is complemented with selected case studies concerning uranium groundwater contamination, as examples of PRBs’ implementation. Contaminants Table 1 summarizes the contaminants treated by

Contaminant

Chlorinated s

TCE, TCA, DFreon 113, CT

PCE, Cr6+

PCE, TCE, D

Metals and In

Cr6+, TCE

Ni, Fe, Sulfat

Cr6+

Pb, Cd, As, Z

Cr6+, PCE

Fuel Hydroca

BTEX

BTEX, TCE,

Nutrients

U, Tc, HNO3,

PO43-, NO3

-

Radionuclide

U

U, Tc, HNO3

Other organic

BHC, beta-BHethylbenzene,parathion

ΟΡΥΚΤΟΣ ΠΛΟΥΤΟΣ /

Table 1: Contaminants and treatment methods for field applications. Πίνακας 1: Ρύποι και µέθοδοι κατεργασίας σε εφαρµογές πεδίου

s Reactive materials No. of field applications

olvents

CE, PCE, DNAPL, VC,

Fe0, iron sponge 24

Surface modified zeolite (SMZ) 1

CE, VC Concrete, sand and iron 1

organics

Fe0 1

e Organic carbon 1

Sodium dithionite 1

n, Cu Limestone 1

Surface modified zeolite (SMZ) 1

rbons

O2 1

VC, DCE Fe0, O 1

NO3- Fe0 2

Fe/Ca oxides, high-Ca limestone, organic carbon

1

s

Fe0, Amorphous Ferric Oxide, PO4 1

Fe0 1

s

C, DDD, DDT, xylene, Activated carbon 1
lindane, methyl
MINERAL WEALTH 123/2002 37

installed PRBs (USEPA [3]). It is evident that the majority of cases involve organic substances usually degraded by zero-valent iron.

The most common organic contaminant is 1,1,2-trichloroethane (TCE), which is widely used for degreasing metals, followed by dichloroethene (three isomers 1,2-cis-, 1,2-trans- and 1,1-) (DCE). Also, tetrachloroethene (PCE or perchloroethene) and dichloromethane, which have been used for paint stripping and in the dry cleaning industry, are frequently found in contaminated groundwater. Most of the organic contaminants are very persistent in the ground water environment and as a consequence the USEPA and other US and European organisations have severely regulated their concentration in groundwater.

Frequently found inorganic contaminants include heavy metals like Cd2+, Co2+, Zn2+, Pb2+, Mn2+, Ni2+ and complex ions such as UO2

2+, as well as SO4

2-, NO3-, SeO4

2-, TcO4-, CrO4

2- and MoO4

2-. High concentrations of heavy metals or radionuclides are associated with industrial wastes, nuclear waste disposal sites and mine wastes.

Characterization of the contaminants is of great importance for the design and installation of a PRB. The organic and inorganic composition of groundwater is determined, to assess appropriate reactive materials and to predict possible precipitates that may affect the long-term efficiency of the barrier. Another important consideration is the variable distribution of the contaminants within the plume, while there may be more than one plume at a single site. For this reason, apart from the chemical and physical characteristics of the plume, it is also important to identify the three-dimensional distribution of the contaminant’s concentration. So far, plumes with 1000 m of width and 12 to 15 m of depth have been treated with PRBs (Gavaskar [4]). Reactive materials and attenuation processes

The selection of the reactive media is based on the following criteria (Gavaskar, et al. [5]): • Reactivity. The reactivity of the material is

quantitatively evaluated by the required residence time or the reaction rate constant. It is desirable to have high reaction rates and therefore only low residence times in order to

keep the barrier’s thickness within acceptable limits.

• Stability. The material is expected to remain active for long periods of time because its replacement is not easily achieved. Stability in changes of pH, temperature, pressure and antagonistic factors is also required.

• Availability and cost. The available amount of reactive material required for the construction of a reactive barrier is large enough. Therefore it is essential to have considerable quantities at low prices.

• Hydraulic performance. The hydraulic conductivity of the material depends on its particle size distribution and its value must be greater or equal to the value of the surrounding soil. However, an optimum particle size that would provide appropriate permeability and sufficient contact time must be determined.

• Environmental compatibility. It is important that the reactive media does not form any by-products when reacting with the contaminants and that it is not a source of contamination itself by solubilisation or other mobilization mechanisms.

• Safety. Handling of the material should not generate any risks for workers’ health.

The long-term efficiency of a PRB is a matter of great concern especially when the contaminants are chlorinated solvents or radionuclides, which persist for several decades. The concentration of inorganic constituents like Ca, Mg, and Na determines the formation and deposition of inorganic precipitates on the surface of the reactive medium thus inducing its performance. A change in the hydraulic conductivity of the barrier due to clogging of the iron surface can alter the flow of the plume, which either dips further down the aquifer or passes around the barrier. Geochemical models like MINTEQA2 (USEPA, [6]) and PHREEQC (USGS, [7]) may be used to predict the possible mineral phases formed under the specific conditions. Also, accelerated column experiments with percolation rates that greatly exceed the ground water flow provide indicative long-term results. Another important parameter for the longevity of the process is the dissolved oxygen value since its presence promotes the


formation of iron precipitates. For this reason, pretreatment zones where dissolved oxygen is consumed before entering the zero valent zone, are usually constructed (Gavaskar, [4]).

Some common reactive media along with the respective attenuation mechanisms are presented below.

Iron based materials

The most common reactive material in the current field applications is metallic granular iron (zero valent iron, ZVI). The extensive use of iron is attributed to its ability to degrade several organic substances like chlorinated hydrocarbons, alkanes, aromatics and to decompose or immobilise some inorganic compounds such as chromium, nickel, lead, uranium, sulfate, nitrate, phosphate and arsenic, among others. The process rate depends primary on the specific surface area of iron. The degradation mechanisms for certain pollutants (Powell, et al. [8]) are: • Degradation of chlorinated solvents by

oxidation of Fe(0) to Fe(+2). Fe + RCl + H+ → Fe2+ + RH + Cl- (1)

Iron concentrations within the reactive wall usually range from 0.5 to 14.8 mg/l, while Eh and pH values vary between –250 to –550 mV and 9 to 10.7 respectively (Puls R.W., et al. [9]). When the oxidation of Fe (0) is conducted under aerobic conditions, Fe (+2) is produced by the reaction: 2Fe + O2 + 2H2O → 2Fe2+ + 4OH- (2)

and oxidized further to Fe (+3) which can be precipitated as ferric hydroxide at elevated pH values (reaction 3), thus inducing the permeability of the barrier. Fe3+ + 3OH- → Fe(OH)3(s) (3)

Anaerobic oxidation of iron also results in the formation of ferrous oxides and hydroxides, as described by the reactions: Fe + 2H2O → Fe2+ + H2 + 2OH- (4) Fe2+ + 2OH- → Fe(OH)2(s) (5)

only in this case, the reaction rate is much lower and the products do not always inhibit the degradation of the organics. Also, in the presence of sufficient SO4

2-, sulphide formation is possible.

Among many field application studies, Vogan, et al. [10] highlight the effect of the inorganic constituents on the barriers' degradation performance for volatile organics (VOCs). The results from the monitoring wells indicate that mineral precipitates like CaCO3, FeCO3 and MgCO3 are formed by carbonate anions produced by the reaction: HCO3

- → CO32- + H+ (6)

due to pH increase. However, the existence of such species does not always affect the hydraulic conductivity or the reactivity of the iron wall. Muftikian, et al. [11] examined the dechlorination of TCE by elemental iron and found that intermediate reaction products are formed (1,1-dichloroethene and 1,2-dichloroethene), which delay the degradation process. For this reason they developed elemental iron that was surface modified with Pd according to the redox reaction: 2Fe + PdCl6

2- →2Fe2+ + Pd0 + 4Cl- (7) The improved material that was produced

displays considerably faster and complete degradation of TCE. Other bimetallic systems such as Fe/Cu, and Fe/Ni have also been shown to accelerate the degradation rates compared to the untreated metal. However, due to the high cost of palladium it is certain that these materials cannot be utilized in field applications. • Reduction of toxic Cr(+6) (as CrO4

2-) to Cr(+3) and subsequent precipitation.

CrO42- + Fe0 +8H+ → Fe3+ + Cr3+ + 4H2O (8)

(1-x)Fe3++(x)Cr3++2H2O→Fe(1-x)CrxOOH(s)+3H+ (9) or Fe+CrO4

2-+4H2O→(Fex, Cr1-x)(OH)3+5OH- (10) Iron-bearing oxyhydroxides and iron-bearing

aluminosilicate minerals may also conduct the reduction and precipitation of Cr (+6), but the reaction rate is greater when Fe0 is used. According to laboratory and field scale experiments during the removal of Cr (+6), pH increases from 6.5-8.5 to 9.5, while Eh drops from 100 mV to –300 mV, indicating the highly reducing conditions of the system. Chromium concentrations are reduced to less than 0.01 mg/l (Blowes, et al. [12], Puls, et al. [9], [1]). • Reductive precipitation of As(+3) and As(+5).


Arsenic naturally occurs in groundwater as AsO3

3- that is more mobile and highly toxic and as AsO4

3-, in respect to the inorganic redox reactions taking place. Both ions are hydrolyzed forming H3AsO3 and H3AsO4 respectively. Laboratory column experiments performed by McRae, et al. [13], proved that a mixture of 10 wt.% of zero valent iron, 40 wt % of agricultural limestone and 50 wt % of silica sand, successfully immobilises both As (+3) and As (+5), although Eh values obtained (134-480 mV) did not indicate reductive conditions. Regarding this observation, the authors assumed that the redox conditions prevailing at the surface of the iron grains are different from those indicated by the effluent quality. Initial concentration of As (1000 mg/l) was reduced to 18 mg/l while pH was maintained at constant values (7.6±0.2). • Reduction and precipitation of Se(+6) as

Se(0). HSeO4

- +3Fe(s)+7H+→3Fe2+ + Se(s)+ 4H2O (11) McRae, et al. [13] who studied the reduction of Se (+6) performing laboratory experiments, proposed the above reaction. Elemental iron achieved removal of 1000 mg Se (+6)/l from solution (Powell, et al. [8]). • Reductive precipitation of U (+6). Fe0 + UO2

2+(aq) → Fe2+ + UO2(s) (12)

An early study, which deals with the removal of UO2

2+, MoO42-, TcO4

- and CrO42- by Fe0 is the

one by Cantrell, et al. [14]. The removal rates provided by the laboratory experiments increased according to the sequence: CrO4

2- > TcO4- >

UO22+ >> MoO4

2-. The mechanism of UO22+

reduction was not clarified by the experiments. Three options were considered: i) reduction of UO2

2+ to the sparingly soluble

UO2xH2O amorphous phase, ii) UO2

2+ adsorption onto newly formed iron

oxide particles and iii) both reduction/precipitation and sorption onto

iron oxide occurred concurrently. Other cation-forming electroactive metals that are potentially treatable with reduction and precipitation by Fe0 are Cu and Hg.

Another study of U removal by zero valent iron and other reactive materials like peat, iron oxide and organic carbon is reported by Gu, et al. [15]. According to their results Fe0 was the most

effective material tested, demonstrating removal of 20,000 mg U/l. Reductive precipitation accounted for 96% of the uranium removed, while about 4% was attributed to adsorption on corrosion products of Fe0. The study also indicated that the adsorbed uranium could be mobilised by carbonate solutions, while the reduced species could also be re-oxidized under favourable conditions. However, the results of this evaluation were used to support the design of the permeable reactive barrier of Bear Creek at the US Department of Energy’s Y-12 Plant located in Oak Ridge, TN. Details regarding the experiments of the above work are presented in the USEPA [16]. • Reduction of NO3

- to NH4+.

The reaction has been studied by Rahman & Agrawal, [17] who observed that NO3

- is initially reduced to NO2

- and then finally to NH4+. The

kinetic evaluation proved that the reaction rate is very high. 4Fe0 + NO3

-+ 10H+→ 4Fe2+ + NH4+ + 3H2O (13)

Granular iron is the cheapest metallic medium available, and one of the cheapest reactive materials in general (typical price $350/ton.), which is one additional reason for its widespread use. In the applications of Table 2 where zero valent iron is the reactive material, the installation cost varies from $40,000 to $4,000,000 with an average value being $500,000, while the design cost ranges between $75,000 and $150,000. The funnel and gate installation, which is considered more effective, was generally used, except for certain cases where economic limitations imposed the use of continuous wall designs.

Organic materials

Many low cost materials are used as organic carbon sources including leaf, peat, sewage sludge, manure, sawdust, wood waste, and others. The organic materials and bacteria are usually mixed with limestone to maintain neutral pH, and with sand to control the permeability of the reactive wall. Reducing reactions are responsible for the immobilisation of the contaminants.


Table 2: Summarised data on permeable reactive barrier applications. Πίνακας 2: Συγκεντρωτικά στοιχεία εφαρµογής ενεργών υδροπερατών φραγµών

REACTIVE MATERIALS

TREATABLE CONTAMINANTS REMOVAL PROCESSESS FIELD APPLICATIONS COST DATA

Fe0

Iron sponge

Pd, Ni, Cu coated Fe

Fe0, O

Fe0, sand, concrete mixture

Zero valent iron pellets

Organic compounds:

1,1,1-trichloroethane, 1,1,2-trichloroethane, 1,1-dichloroethane, tetrachloromethane, trichloromethane, dichloromethane, tetrachloroethene, trichloroethene, cis-1,2-dichloroethene, trans-1,2-dichloroethene, 1,1-dichloroethene, vinyl chloride, 1,2-dichloropropane, Freon 113, benzene, toluene, ethylbenzene, hexachlorobutadiene, 1,2-dibromoethane, N-nitrosodimethylamine Inorganic materials:

Cr, Ni, Pb, U, Tc, Fe, Mn, Se, Cu, Co, Cd, Zn, SO4, NO3, PO4, As, Hg

• Reduction and co-precipitation:

Fe0 + RCl + H+ ⇒ Fe2++ RH + Cl-

CrO4

2-+Fe0 +8H+⇒ Fe3++Cr3++ 4H2O (1-x)Fe3++(x)Cr3++2H2O⇒ Fe(1-x)CrxOOH(s)+3H+

HSeO4

- + 3Fe0 + 7H+⇒ 3Fe2+ + Se0

(s) + 4H2O 3Fe0 + UO2

2+(aq) ⇒ Fe2+ + UO2(s)

4Fe0 + NO3

- + 10H+⇒ 4Fe2+ + NH4+

+ 3H2O with denitrifying bacteria Paracocus denitrificans

Full scale • Aircraft Maintenance Facility, OR • Caldwell Trucking, NJ • Federal Highway Administration (FHA) Facility, Lakewood, CO • Former Dry cleaning Site, Rheine, Westphalia, Germany • Former Manufacturing Site, Fairfield, NJ • Industrial Site, Belfast, Northern Ireland • Industrial Site, Coffeyville, KS • Industrial Site, NY • Industrial Site, SC • Intersil Semiconductor Site, Sunnyvale, CA • Kansas City Plant, Kansas City, MO • Lowry Air Force Base, CO • U.S. Coast Guard Support Center, Elizabeth City, NC • Y-12 Site, Oak Ridge National Laboratory, TN • Fry Canyon Site, UT Pilot Scale • Area 5, Dover Air Force Base (AFB), DE • Borden Aquifer, Ontario, Canada • Cape Canaveral Air Station, FL • Massachusetts Military Reservation CS-10 Plume, Falmouth,

MA • Moffett Federal Airfield, Mountain View, CA • Savannah River Site TNX Area, Aiken, SC • SGL Printed Circuits, Wayne, NJ • Somersworth Sanitary Landfill, NH • U.S. Naval Air Station, Alameda, CA • X-625 Groundwater Treatment Facility, Portsmouth Gaseous

Diffusion Plant, Piketon,OH

Capital cost ($) 600,000 1,120,000 1,000,000 123,000 875,000 375,000 400,000 797,000 400,000 1,000,000 1,500,000 530,000 500,000 500,000 1,000,000 170,000 800,000

- 809,000 160,000 465,000 120,000 48,000 400,000 400,000 400,000 4,000,000

Organic carbon containing materials: leaf, peat, sewage sludge, manure, sawdust, wood waste, composted leaf mulch, pine mulch, pine bark

NO3, SO4, Cd, Pb, Co, Cu, Ni, Zn

• Reductive precipitation with bacteria 5CH2O(s) + 4NO3

-⇒ 2N2 + 5HCO3

- + 2H2O + H+

2CH2O(s) + SO42- ⇒ H2S(aq) + 2HCO3

Me2+ + H2S(aq) ⇒ MeS(s) + 2H+

• Sorption/Ion exchange

Full Scale • Nickel Rim Mine Site, Sudbury, Ontario, Canada Pilot scale • Public School, Langton, Ontario, Canada

30,000 5,000

Table 2 (continued) Πίνακας 2 (συνέχεια)

REACTIVE MATERIALS

TREATABLE CONTAMINANTS REMOVAL PROCESSESS FIELD APPLICATIONS COST DATA

Limestone, hydrated lime

Cr, U, As, Mo, PO4, Se • Precipitation Full Scale • Tonolli Superfund Site, Nesquehoning, PA Pilot scale • Public School, Langton, Ontario, Canada

5,000

Phosphates U, As, Mo• Precipitation Full scale

• Fry Canyon Site, UT 170,000

Ferrous sulfate Cr, U, As, Mo • Precipitation

Natural zeolites: clinoptilolite, mordenite Surfactant modified zeolite (SMZ)

Sr, Ba, Cr, PCE

• Ion exchange/sorption Full-scale • LEAP Permeable Barrier Demonstration Facility,

Portland, OR

100,000

Iron oxide, Basic oxygen furnace oxide (BOF), amorphous ferric oxide (AFO)

U, As, PO4, Sr

• Oxidation/sorption Full scale • Fry Canyon Site, UT • Y-12 Site, Oak Ridge National Laboratory, TN

170,000 1,000,000

Activated alumina As, PO4, Sr • Sorption

Organic polymers: Cyclophane I, II

Halogenated hydrocarbons (e.g. chloroform) aromatic compounds (e.g. benzene)

• Surface sequestration

Sodium dithionite Cr • Reduction Pilot scale

• 100D Area, Hanford Site, WA 480,000

Activated carbon

Alpha-hexachlorobenzenes (BHC), beta-BHC, DDD, DDT, xylene, ethylbenzene, lindane, methyl parathion

• Sorption Full scale • Marzone Inc./Chevron Chemical Company, Tifton, GA

750,000

Microorganisms: G. matallireducens, A. putrefaciens

Tertiary butyl ether (MTBE), U, Ag, Cd, Co, Cu, Fe, Ni, Pb, Zn

• Reduction/Reductive precipitation

½UO2(CO3)22- + H+ ⇒ ½UO2(s) +

HCO3-

Full Scale • Nickel Rim Mine Site, Sudbury, Ontario, Canada

30,000

• Reduction of NO3-.

5CH2O(s)+4NO3-→2N2+5HCO3

-+2H2O+H+ (14) The denitrification reaction by organic carbon

is rapid while usually it is catalysed by bacteria of the Pseudomonas group. Application of various low-cost organic materials like sawdust and wood wastes has been studied for the remediation of domestic and institutional septic systems (Powell, et al. [8]). Small-scale field experiments using organic carbon materials were initiated for the remediation of groundwater containing nitrate, as described by Robertson & Cherry, [18]. The authors report results derived from 4 operational years which indicate that the barrier is very stable since it constantly retains 50-100% NO3-N. • Reductive precipitation of heavy metals (Cd,

Pb, Co, Cu, Ni, Zn). 2CH2O(s) + SO4

2- → H2S(aq) + 2HCO3- (15)

Me2+ + H2S(aq) → MeS(s) + 2H+ (16)

where, Me2+ denotes a divalent metal. The reactions take place anaerobically in high

pH values. The bacterial reduction and metal precipitation of metals solubilized from tailings-derived acid drainage has been investigated by Blowes, et al. [19]. The test cells designed for the study contained, apart from the organic carbon sources (composted leaf mulch, pine mulch and pine bark), agricultural limestone for pH adjustment and coarse sand and gravel in order to maintain the permeability of the mixture. In addition, sulphate-reducing bacteria (SRB), working as catalysts for the reduction of SO4

2- were inserted into the system. Long-term data concerning the reactivity of the materials are not available from this study except for 6 months measurements of Fe2+ and SO4

2- that were quite satisfactory.

The reactivity of the organic materials depends on the lability or availability of the contained carbon and according to the study of Waybrant, et al. [20], the combination of more than one organic source is more successful than the use of solely one material. This is due to the fact that a mixture of organic materials contains compounds with varying complexity, some of them decomposing fast and others over a long time period, thus achieving long term reactivity of the barrier. It is noteworthy that among other organic

sources, sewage sludge achieved very rapid degradation rates causing, however, increasing nickel solubilization. As a result it was considered as an unacceptable reactive material.

McGregor, et al. [21] tested a reactive wall made of a leaf-compost material for the remediation of groundwater polluted with Cd, Cu, Ni, Pb and Zn, originating from a mine site in Western Canada. The permeable barrier achieved pH increase from 5.88 to 6.50, accompanied by an increase in the alkalinity from 89 mg/l to 202 mg/l which is due to dissolution of dolomite [CaMg(CO3)2] and calcite [CaCO3]. The Eh value was decreased by the metals’ immobilization although the dissolution of Ca and Mg affected its value. Finally, measurements conducted for 21 months after the installation showed that Cu, Zn, Cd, Ni and Pb concentrations were reduced by 99.8%, 96.6%, 99.4%, 95.4% and 50% respectively upon exiting the reactive wall.

According to the data of table 2 [2], two field installations with organic reactive materials are reported, both of them located in Canada. The organic full scale barrier which contained municipal compost, leaf compost, and wood chips reduced the SO4

2- and Fe concentrations of the groundwater by 50-95% and 91-100%. The capital cost for the full scale barrier including design, construction, materials and the reactive mixture is $30,000, while the corresponding figure for the pilot scale barrier is $5,000. Moreover, a typical value regarding the cost of peat is $10-30/t while other organic sources are practically considered as waste materials. In general the organic materials combine low cost and increased availability but their effectiveness is largely confined to inorganic pollutants.

Alkaline materials-complexing agents • Precipitation of Cr (III), U, As, Mo, Se and

PO43-.

3Ca(OH)2+(UO2)3(OH)5++OH-→3CaUO4(s)+6H2O (17)

Morrison & Spangler [22], [23] studied the removal of U, As and Mo using various natural materials like hydrated lime, ferrous sulphate, peat, phosphate and others that incorporate either precipitation or adsorption reactions. Lime barriers cause pH increase to 12-12.5 in order to facilitate the formation of metal hydroxides. The


results were successful for both cationic and anionic species indicating that the use of such materials in permeable barriers can be effective. Similar observations were made by Baker et al. [24] regarding the reactivity of limestone and calcium oxide for PO4

3- precipitation after its adsorption on iron oxide. The system demonstrated fast removal rates in batch experiments and considerable stability in subsequent column tests. Lime is considered as an inexpensive material with a typical price of $50/t. • Hydroxyapatite (Ca10(PO4)6(OH)2) is another precipitating agent, which also exhibits sorption properties. The in-situ immobilisation of Pb in soil and in aqueous solutions has been demonstrated by Ma, et al. [25] who suggested that Pb is precipitated as hydroxypyromorphite by the reaction: 10Pb2++Ca10(PO4)6(OH)2→Pb10(PO4)6(OH)2+10Ca2+ (18)

The process is rapid but the solution pH is a very important parameter since it must be maintained low enough (5-6) to dissolve apatite and supply P that will react with Pb, but not low enough to inhibit the formation of hydroxypyromorphite. In addition, various other metals like Al, Cu, Cd, Zn and Fe are potential competitors of the phenomenon since they also form insoluble precipitates (Ma, et al. [26]). Important disadvantages regarding the use of hydroxyapatite are its low availability in natural form and its high cost when being synthesized. A source of natural apatite is bone, and for this reason bone char has been tested as a potential phosphate-rich material with low cost, at an abandoned uranium site in Fry Canyon, UT (see Table 2) (Feltcorn, [27]). • Studies concerning the use of sodium dithionite (NaS2O4) as a reductant or precipitant for the degradation of halogenated hydrocarbons and the immobilisation of chromate have been reported (Vidic & Pohland, [28]). The additive has the reducing ability to transform naturally occurring ferric oxides to the ferrous form that is capable of immobilising various pollutants. Being in liquid form, it is injected into the subsurface and after the reaction is completed it is flushed along with the reaction by-products, creating a permeable reaction zone in advance of a contaminant plume. Laboratory experiments

have shown that the method is effective for easily reducible compounds such as UO2

2+. Sodium dithionite is an environmentally safe and inexpensive material, which is readily available, while having the capacity to reduce a variety of organic compounds. A possible disadvantage of its use in reactive walls is its physical state (liquid) that requires special handling.

Zeolites

The well-known crystalline aluminosilicate minerals with the cage like structure have been used for the retention of most heavy metals and many radionuclides from aqueous solutions. • Sorption of 90Sr by clinoptilolite and

mordenite. Batch and column experiments were carried out

by Fuhrmann, et al. [29] in order to evaluate two zeolitic minerals (mordenite, clinoptilolite), as reactive materials for the construction of a full-scale in-situ barrier. All the experiments were conducted using groundwater from the specific site in order to test both the sorptive capacity and the stability of the materials with time. The results proved that clinoptilolite retained Sr, Ca and Ba more effectively than mordenite while releasing to the solution Na, Mg and K due to the exchange with Ca and Sr. Also, 4*10-4 of the zeolites total Si content was released in the solution due to its partial dissolution after 50 days of operation.

Accordingly mordenite released Na in the solution as exchangeable cation while 5*10-4 of its Si content was dissolved in 21 days of operation. Also, the desorption tests performed with NaCl and HCl solutions proved that both zeolites are easily regenerated for re-use since 97% and 100% of Sr retained respectively was solubilised during the reverse experiments. • Sorption of CrO4

- and PCE. A new surfactant-modified zeolite (SMZ) was

tested in a pilot-scale permeable reactive barrier for its ability to clean up a simulated CrO4

2- and PCE containing plume. The modification of the zeolite results in the formation of a positively charged surface bilayer where anions can be exchanged. In addition, the organic carbon content is increased by the modification, enabling the zeolite to retain organic compounds. The nature of the process as a whole does not affect the initial cation exchange capacity of the


material. In this manner, the modified zeolite is capable of retaining three kinds of contaminants simultaneously. The results provided by the pilot test proved that both contaminants were completely removed from the simulated plume. The cost of the zeolite plus the surfactant and the processing is about $425/t (Bowman, [30]). • Sorption of U(IV).

Adsorption of U(IV) on synthetic zeolite X under various experimental conditions i.e. uranium concentration, pH, contact time, temperature, has been studied by Akyil, et al. [31], which concluded that adsorption is favoured at low metal concentration. Also Misaelides, et al. [32] report that the retention of U(VI) by natural HEU-zeolites (clinoptilolite, heulandite) is mostly due to adsorption and surface precipitation rather than ion exchange in the zeolitic channels.

The price of a natural zeolitic tuff is about $75/t (sized and packaged to customers specifications), while synthetic zeolites like A, X, and Y usually cost $300-600/t. However, the exchange capacity of synthetic zeolites is 3 to 6 times higher than the capacity of natural zeolites, thus the selection of the appropriate zeolite sample depends on the demands of the specific case. When approaching a real situation, it would be more reasonable to test extensively the effectiveness of a natural zeolitic sample with no pre-treatment.

Metal oxides

The use of various forms of iron oxide (amorphous ferric oxyhydroxide (AFO), basic oxygen furnace (BOF) oxide) and alumina for groundwater remediation is well documented. The contaminants are removed by adsorption and/or oxidation mechanisms. • Oxidation of As (III) to As (V).

BOF-oxide is an inexpensive and abundant material produced as a waste by-product of steel manufacturing. It is a fine-grained material and it contains mainly oxidized basic iron oxide, considerable amounts of Ca, Mg and traces of Mn. The use of BOF-oxide in ground water remediation is both economically and environmentally beneficial. McRae, et al. [13] studied the use of BOF-oxide for the remediation of As which is achieved by oxidation of As (III) to As (V) and subsequent adsorption onto Fe and Mn oxides contained in BOF-oxide. Also, BOF-

oxide has been used for the removal of PO4 by adsorption mechanisms (Baker, et al. [24]). • Sorption of As3+, PO4

3- and 90Sr. Activated alumina is chemically produced as a

porous dehydrated aluminum oxide. Its retention capacity for inorganic cations and anions (As3+, PO4

3-) is due to surface adsorption, and it has proven to be greater than the BOF-oxide capacity (McRae, et al. [13], Xu, et al. [33]). Fuhrmann, et al. [29] compared the reactivity of activated alumina with natural zeolites and concluded that for the specific 90Sr contaminated groundwater sample, alumina sorbs 99% of the contained 90Sr but it shows limited stability with time.

Microorganisms

• Biodegradation of methyl tertiary-butyl ether (MTBE).

Thomson et al. [34] report the design considerations of a permeable barrier, which intercepts a plume contaminated by gasoline components and specifically methyl tertiary-butyl ether (MTBE). The barrier is based on air stripping and biodegradation of the organic substance and according to monitoring results it achieves 100% removal of the pollutant. The total construction cost of the barrier, including the installation of a monitoring well, was less than $5,000, while operating cost for one year were approximately $180. • Biological reduction of U.

Uranium removal by reductive precipitation has been studied by many researchers in regard to the potential microorganisms such as iron- and sulfate-reducing bacteria that can be used under various conditions (Abdelouas, et al. [35], Abdelouas, et al. [36], Uhrie, et al. [37]). Studies have shown that the reduction of U is an enzymatically catalyzed reaction, reagardless the experimental conditions (type of bacteria, composition of growth medium, uranium concentration, incubation temperature). Abdelouas, et al. [38] found out that uranium concentrations between 0.25 and 235 mg/l in groundwater contaminated with large amounts of nitrate, sulfate, and carbonate, can be efficiently reduced according to the reaction:

½UO2(CO3)22- + H+ → ½UO2(s) + HCO3

- (19)

The method has not been tested in the field yet.


Polymers

• Binding of organic molecules Brookhaven National Laboratory has developed

synthetic polymers with hydrophobic cavities similar to molecules, which have the capacity to sequestrate organic pollutants from groundwater. The structure of the organic polymers can be easily modified, according to the contaminants characteristics (size, shape and polarity) and therefore they can be utilized in variable applications. Cyclophane I and II are two polymer molecules designed for binding planer aromatic molecules like benzenes and tetrahedral halogenated hydrocarbons such as chloroform in order to be used in permeable reactive barriers (Callot, [39]). • Binding of inorganic ions

Dynaphore Inc. has developed a cellulose sponge by the trade name Forager @ Sponge which is a polymer possessing selective affinity for both cationic (heavy metals) and anionic (CrO4

2-, AsO43-, Au(CN)2

-, SeO42-, HgCl3

-, Ag(S2O3)3-, SiO3

2-, UO42-) species. Laboratory

column experiments showed that the removal efficiency of the material is about 90% at a flow rate of 2-3 bed volumes/min with low hydrostatic pressure, due to its highly porous structure. The study also proved that the sponge is capable of scavenging metals in concentration levels of ppm and ppb. According to the results the affinity sequence for metals was:

Cd2+>Cu2+ >Hg2+ >Pb2+ >Au3+> Zn2+ > Fe3+>Ni2+

>Co2+ >>Al3+>Ca2+ >Mg2+ >>Na+

This technology was accepted into the SITE Demonstration program in June 1991 and it was demonstrated in April 1994 at the National Lead Industry site in Pedricktown, New Jersey. Also, the sponge was tested in a field-scale installation

at a photoprocessing operation at a cost of $1,100 per month (Ott, [40]). Conclusions

Reactive materials of different nature, employing various fixation mechanisms, are currently available for the construction of permeable reactive barriers, according to the demands of the specific site. Research on new materials and optimization of the existing ones is continuously progressing, while laboratory studies are being carried out to provide a deeper understanding regarding the immobilisation processes. Based on monitoring results from field applications in full and pilot scale, the following conclusions can be drawn regarding the practical implementation of the materials presented: • Barriers containing zero valent iron in varying forms (iron foam, iron pellets, iron/sand mixtures) have proven their efficiency against both organic and inorganic pollutants, thus providing a safe solution for most cases. Moreover, the materials' cost is reasonable enough to allow the construction of both continuous barriers and funnel designs in respect to the site characteristics. • Other promising categories of reactive medium are the organic materials (peat, leaf compost, etc.) and the alkaline materials (lime, sodium dithionite, hydroxides), which are considerably effective, especially for inorganic pollutants and their cost is minimum. Acknowledgements The authors would like to acknowledge the financial support of the European Commission under the Research & Technological Development Project, Contract No. EVK1-CT-1999-00035.


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ΠΕΡΙΛΗΨΗ

ΕΝΕΡΓΑ ΥΛΙΚΑ ΚΑΙ ∆ΙΕΡΓΑΣΙΕΣ ΚΑΤΑΚΡΑΤΗΣΗΣ ΡΥΠΩΝ ΣΤΑ ΣΥΣΤΗΜΑΤΑ ΕΝΕΡΓΩΝ Υ∆ΡΟΠΕΡΑΤΩΝ ΦΡΑΓΜΩΝ

Άνθιµος Ξενίδης1, Αγγελική Μοίρου2, Ιωάννης Πασπαλιάρης3

H τεχνολογία των Ενεργών Υδροπερατών Φραγµών είναι µια παθητική µέθοδος κατεργασίας υπόγειων νερών τα οποία έχουν ρυπανθεί από οργανικές ενώσεις, µέταλλα ή ραδιενεργά στοιχεία. Μέχρι σήµερα, έχουν χρησιµοποιηθεί διάφορα είδη ενεργών υλικών για την κατασκευή υδροπερατών φραγµών, ενώ παράλληλα µελετάται µεγάλος αριθµός νέων υλικών για µελλοντική χρήση. Ένα από τα πιο διαδεδοµένα ενεργά υλικά είναι ο µεταλλικός σίδηρος (Fe0), ο οποίος χρησιµοποιείται σε διάφορες µορφές (κόκκων, ρινισµάτων, σφαιριδίων, αφρού, κ.ά.). Η αποµάκρυνση ή

καταστροφή των ρύπων επιτυγχάνεται συνήθως µε διεργασίες όπως η αναγωγή, η αναγωγική καταβύθιση, η προσρόφηση, η καταβύθιση και η ιοντοεναλλαγή. Τα χαρακτηριστικά που θα πρέπει να διαθέτουν τα ενεργά υλικά είναι αποδοτικότητα, κατάλληλη διαπερατότητα, σταθερότητα, χαµηλό κόστος, καθώς επίσης να είναι ασφαλή για το περιβάλλον. Στην παρούσα εργασία πραγµατοποιείται ανασκόπηση και αξιολόγηση όλων των διαθέσιµων ενεργών υλικών και των αντίστοιχων φυσικοχηµικών διεργασιών που λαµβάνουν χώρα στους ενεργούς υδροπερατούς φραγµούς. Επίσης, δίνονται οικονοµικά και τεχνικά στοιχεία για ορισµένες εφαρµογές της συγκεκριµένης τεχνολογίας σε πιλοτική κλίµακα στις Ηνωµένες Πολιτείες.

1 ∆ρ. Μηχανικός Μεταλλείων - Μεταλλουργός 2 Μηχανικός Μεταλλείων - Μεταλλουργός 3 Αναπληρωτής Καθηγητής

Manuscript received from: - the authors on 19.7.01 - the Review Committee on 11.12.01

Manuscript received from: - the authors on 19.7.01 - the Review Committee on 11.12.01

48

Παραλαβή εργασίας - αρχική από τους συγγραφείς 19.7.01 - τελική από την Κριτική Επιτροπή 11.12.01

ΟΡΥΚΤΟΣ ΠΛΟΥΤΟΣ / MINERAL WEALTH 123/2002

REACTIVE MATERIALS AND ATTENUATION PROCESSES FOR PERMEABLE REACTIVE BARRIERS

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