Review of Zero Valent Iron and Apatite as reactive materials for Permeable Reactive Barrier

Author: Luca Geranio

Tutor: Dr. Evert Elzinga

Term Paper SS 07/08, major in Biogeochemistry and Pollutant DynamicsDepartment of Environmental Sciences

ETH ZürichJune 2007

AbstractPermeable reactive barrier (PRB) is a technology developed recently in the last years. It has obtained promising results in the removal of several contaminants present in the groundwater. This Term paper focuses the attention on two reactive materials, Zero valent iron and Apatite, employed in the PRB system, giving an overview of the reactions and types of pollutants treated to date. The pollutants removal from groundwater by these two reactive materials is based on different processes, that is, reduction of organic or inorganic contaminants in the case of zero valent iron and immobilization of inorganic pollutants in the case of Apatite. Zero valent iron acts as reductant either reducing directly the contaminants in a less harmful form or also reducing pollutants which can subsequently precipitate. At the moment, Zero valent iron is the material most frequently used in the field installations and it is particularly effective in the chemical degradation of persistent chlorinated compounds into non-toxic and harmless by-products. In general perhalogenated hydrocarbons tend to be reduced faster than hydrocarbons less halogenated and also the dechlorination is more rapid at saturated carbons (e.g. CCl4) centers than at unsaturated carbons (e.g. TCE)[15]. Like anions, inorganic cations are reduced by Zero valent iron and precipitate as meagrely soluble solids. Zero valent iron seems to have effect in the remediation of heavy metals such as chromium, arsenic, technetium, selenium, copper, mercury and uranium.Apatite II is a particular form of apatite (natural waste of fish industry) that can immobilize and sequester a broad range of metals into new phosphate minerals and other low-solubility phases for a geologic period of time. The apatite II provides a low but sufficient concentration of PO43- in solution in order to exceed the solubility of the metal-apatite that rapidly precipitate, but only in the presence of an existing apatite structure which acts as nucleating site or seed crystal. Apatite acts also as a material for non-specific adsorption of most cationic metals from solution and is also an excellent buffer for neutralizing acidity through PO4

3-, OH-, and substituted CO3

2-, exerting control over chemical activities of other species leading to the precipitation of oxihydroxide- and carbonate-metal phases. Metals in solution are immobilized on the apatite mineral by surface sorption (main mechanism for most metals), precipitation (main mechanism for U, Pu, Pb, lanthanides) or co-precipitation (transition metals). The relative contribution of adsorption and precipitation to metal removal depends upon the environmental conditions, the mineral phases present, and the metal concentration in solution [16] [14].

Table of contents

1. INTRODUCTION..........................................................................................................................ii1.1 Permeable reactive barriers: the basic idea................................................................................ ii1.2 Site characterization ................................................................................................................. iv1.3 Choice of reactive media (RM) and laboratory tests.................................................................iv

2. GEOCHEMISTRY OF BARRIER MATERIALS..................................................................... v2.1 Zero Valent Iron......................................................................................................................... v

2.1.1 Reactions and types of pollutants treated ........................................................................... v2.1.2 Treatment of halogenated organic compounds with Zero Valent Iron.............................. vi2.1.3.1 Chromium ................................................................................................................... viii2.1.3.2 Technetium, Selenium, Arsenic ................................................................................... viii2.1.4 Zero valent iron and inorganic cations ............................................................................viii2.1.5 Precipitation problems........................................................................................................ix

2.2 Apatite ...................................................................................................................................... ix2.2.2 Apatite II..............................................................................................................................x2.2.2.1.1 Heterogeneous nucleation............................................................................................ xi2.2.2.1.2 pH buffering................................................................................................................ xii2.2.2.1.3 Surface chemi-adsorption............................................................................................xii2.2.2.1.4 Biological stimulation................................................................................................. xii2.2.2.2 Case study: use of apatite(II) in groundwater remediation contaminated with heavy metals by mine waste in Idaho,USA ......................................................................................... xii2.2.2.3 Benefits of use of Apatite II ......................................................................................... xiii

3. CONCLUSIONS AND REMAINING RESEARCH QUESTIONS....................................... xiv4. AKNOWLEDGEMENT ............................................................................................................. xv5. REFERENCES............................................................................................................................. xv

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1. INTRODUCTION

1.1 Permeable reactive barriers: the basic idea

Permeable reactive barrier (PRB) is a technology developed beginning from the 90s as a remediation for polluted groundwater. It is an implementation consisting of a permeable zone which cleans up a contaminated plume through immobilization or transformation of the pollutants in a less harmful form. In the subsurface the flow of the water is intercepted by a perpendicular “wall” of reactive materials that can degrade, precipitate, sorb or exchange contaminants which can so reach the innocuous or legal concentration downgradient the barrier [1][2][8]. The principle of applying PRB's in remediating contaminated ground water is illustrated in Figure 1. This treatment technology is so called “in situ passive method” because typically exploits the natural gradient of the flow allowing the water but not the contaminants to pass across the barrier [2][8][23]. It is a new valid alternative to traditional Pump and Treat systems due to its low cost and mantenance once installed and its potential long duration and effectiveness.A benefit of its application is the large number of pollutants that can be treated often bringing their concentration below the detection limit. Moreover the site on the surface above the treated aquifer is available for other activities and can be economically re-used once the installation is concluded and during the remediation process. There is the possibility to treat waste plumes that are heterogeneous in composition and concentration and a reduction of costs (~50%) and energy compared to Pump and treat (P&T) methodology could be achieve. In addition PRB has lower impact to the rate of groundwater flow compared to P&T. Finally it could be a strong potential application in urban areas. [2][8]One of the mains drawbacks for the utilization of this technology is that only the part of plume moving downstream from the typically immobile source across the reactive materials can be treated.Another drawback is that the barrier is permanent and fixed and can not be shifted if there is a deviation in the movement of plume. PRB technology needs careful study and characterization of the site prior the installation in order to avoid this drawback. All the possible subsurface changes must be taken into account and the configuration and the size of the barrier must be able to respond to them.Other requirements are the extensive knowledge of hydrology in the aquifer and the precise localization and description of the polluted plume. Furthermore PRB is applicable only for shallow plumes ( not deeper than 50 feet down the ground surface). In addition to these hassles a long process of remediation is needed if the aquifer has a low hydraulic conductivity. Finally a possible lessening of permeability of the PRB due to corrosion, clogging and fouling of the reactive media could happen after a certain time. Unfortunately few field data concerning the longevity of the reactivity of the PRB are available. [2][8][23]The PRB is emplaced digging a trench in the ground and subsequently filled with adapted reactive media in accordance with the hydrogeology characteristics of the site and the pollutant to treat.

Figure 1. Principle of groundwater remediation by using PRB [8]

Basically there are two main types of configuration for the installation of the PRB that are used for the field application: the continuous PRB and the Funnel-and-Gate Design.

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In the continuous scheme the plume encounters a wall of reactive media for all its width and height. The reactive zone such as in Funnel and Gate configuration has permeability equal or bigger than that of the aquifer and so the groundwater changes very little or does not change its flux velocity and natural gradient. It also does not deviate around the reactive media. It is good norm to place the inferior part of the barrier into impermeable strata in order to prevent the contaminant underflow phenomena.In the Funnel and Gate design the groundwater is directed by means of a impermeable funnel (commonly sheet pilings or slurry walls) towards the permeable zone or “gate” constituted to the media. The cross sectional area of the gate is usually rectangular and much smaller than that of the plume in the aquifer and so the flow velocity within the reactive site will be higher [23].

1.2 Site characterization

A complete understanding of the site destined to PRB implementation is compulsory. You have to characterize the stratigraphy, its variation in fracturing and permeability, and the local position and extent of plume. Naturally the aquifer location, the groundwater flow direction, flow velocity and contaminant concentrations are also fundamental in the appraisal and characterization of the site. The reactive zone of PRB must be large enough that the entire plume will pass through, moving under the natural ground-water gradient, also considering the recharge and seasonal variation. It is also necessary to place PRB in order to avoid that the contaminant plume migrates partially beyond site boundaries. Microbial activity can have beneficial or negative effects. Microbes can degrade contaminants but can also biofoul and diminish the permeability of the barrier [23].

1.3 Choice of reactive media (RM) and laboratory tests

The chemical or physical processes, involved in the treatment with PRB, comprise reduction, sorption, precipitation and biochemical degradation (aerobic or anaerobic) of contaminants.Sorption includes adsorption, absorption and ion exchange to reactive media. In general, adsorption is the adhesion of vapour or dissolved matter to the surface of a solid by physical and/or chemical forces, absorption is the incorporation of one substance into or through another of a different state (e.g., liquids in solids, gases in liquids) [31] [32] [8]. Remediation based on sorption phenomena usually use media like activated carbon, zeolite, peat for organic compounds and heavy metal removal.In the precipitation the contaminants pass from the soluble forms into insoluble solid states which are detained. In the degradation the pollutants are transformed in less harmful compounds by chemical or biological reactions. The type or types of processes ,mentioned above, that actually occur in PRBs and remove pollutants from solution, depend on both the pollutant and the barrier material itself [8][23].Once obtained the information from the site characterization it has to be chosen a suitable reactant.This information is extracted based on laboratory experiments.The reactive materials may be mixed with sand to facilitate the passage of the water through the barrier and its amount vary proportionally to the mass flux of contaminants requiring remediation [2][8]. Among all the factors that influence the choice of an appropriate media, the chemical composition of the contaminant is the main and most important. You have to consider if the reactive medium reacting with the detrimental compounds in the water promotes the formation of toxic by-products. The medium has to be known in depth and the ideal one, beyond to be not a source of pollutant itself, has to be inexpensive and durable (i.e. reactive over a long time scale). It has to have a size of the particles that consents the passage of the flow without constrains and it has not to be much heterogeneous, avoiding clogging [2][8][23]. In addition the environmentally compatibility of reaction products and by-products has to be considered (e.g. Fe2+, Fe3+, oxides, carbonates) . In order to evaluate the suitable of the reactive materials laboratory tests concerning the rate and mechanism (including the formation of by-products) of pollutant removal are performed. These laboratory tests coupled with site characterization information are the base for the designing and the implementation of the PRB. Sometimes when there are a huge well-known data concerning the removal rate of the contaminant, the laboratory tests can be eliminated. This is not applicable when there is a mixture of pollutants.The tests should be carried out with the ground water coming from the plume.Batch studies: it is the test more appropriated for the selection of the reactive materials for the barrier. The rate in the remediation of pollutants and the longevity of different materials can be evaluated under

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controlled condition. Typically, different samples are prepared with in each one a mixture of the reactive material to test and an aqueous solution containing dissolved contaminants. It is possible to test the reactivity of different materials simultaneously [23]. The mixtures react for a given period of time and the concentrations of the contaminants at the beginning and at the end of the contact time are measured [13].Column studies: the conditions of this test, like flow velocity, are more similar to those of the field. Based on this studies, you can obtain the residence time of the contaminant in the reactive zone that can be used , with the flow rate, to determine the thickness of the media [23]. Column tests are less cheap than batch tests but approximate better the reality and can be useful for the estimation of long term performance. In this method the reactive materials are packed in granular form into a columns fed with contaminated water [13].In synthesis laboratory tests serve to assess the effectiveness and rate of pollutants removal of potential barrier materials. In addition lab experiments evaluate the reaction products that are formed in the remediation process and their eventually toxicity.

2. GEOCHEMISTRY OF BARRIER MATERIALS

In the following sections, two often used barrier materials will be discussed in terms of the (geo)chemical processes that remove pollutants from solution. These materials are zero valent iron and apatite.The choice to focus the attention on these two medium is due to the fact that they engage in different types of geochemical processes leading to contaminant removal. Zero valent iron is the most used reactive medium in PRB remediation, whereas Apatite is a very promising and cheap media which can remove several metals from groundwater. Zero valent iron has the ability to degrade diverse organic and inorganic toxic compounds. Zero valent iron acts as reductant either reducing directly the contaminants in a less harmful form or also reducing pollutants which can subsequently precipitate. Apatite is an effective reactive material in PRB for the removal of toxic metal forms from solution [16][14]. The apatite II provides a low but sufficient concentration of PO43- in solution (about 100 ppb PO4

3-or less resulting in no phosphate loading or eutrophication, particularly important in ecosystem restoration and maintenance) in order to exceed the solubility of the metal-apatite. The resulting and rapid precipitation of phases such as Pb-pyromorphite or U-autunite happens only in the presence of an existing apatite structure which acts as nucleating site or seed crystal. Apatite also acts as an excellent material for specific adsorption of most cationic metals from solution. Apatite is an excellent buffer for neutralizing acidity through PO4

3-, OH-, and substituted CO3

2-, exerting control over chemical activities of other species leading to the precipitation of oxihydroxide- and carbonate-metal phases. Environmental conditions, mineral phases present and metal concentration in solution determine the relative contribution of adsorption and precipitation to metal removal. The evaluation of which mechanism dominate at any particular site is possible with simple feasibility studies on the contaminated groundwater and soil under site conditions. [16][14]

2.1 Zero Valent Iron

2.1.1 Reactions and types of pollutants treated Zero valent iron (ZVI) has a demonstrated effectiveness against a broad variety of contaminants, especially towards the chlorinated aliphatic hydrocarbons (CAHs) [2][8][23]. Zero valent Iron is the most common reactive medium used in PRB remediation. ZVI is instable under natural conditions and it has to be fabricated at high temperature. In the normal atmospheric oxygen condition and at low temperatures, the oxidation of ZVI is negligible by means of the formation of oxide films that act as inhibitors and do not enable the surface exposure. The standard potential of Fe0/Fe2+ couple is -0.440V and this negative potential permits ZVI to act as an electron donor and reduce several redox-labile compounds (oxidized species) [12]. The favourable thermodynamical electrochemical corrosion of ZVI, which happens in the aqueous system, is necessary for the remediation of contaminants using this material [30]. There is a passage of electrons from the iron surface to the oxidized organic pollutant, which becomes reduced and completely or less harmful. At the same time there is a production of soluble cations of the metal. As long as electron acceptors are present, corrosion processes and electron transfer can continue[2][8][23].

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Fe0 Fe2+ + 2e- 2H+ + 2e- H2(g)

Fe0 + 2H+ Fe2+ + H2(g) Net reaction

The surface area of Iron per unit volume of pore water, so called specific surface area, plays a determining role in the rate of degradation of the contaminants. It influences directly the number of active surface sites exposed to the groundwater plume that are very important for the initialization, mediation and degradation reactions, independently from the nature of the pollutant. However an elevated specific surface can lead to a lessening of the permeability of the barrier which should be much greater than that of the surrounding aquifer layer. Other intrinsic factors, for ZVI, like the grain size and shape, the manufacturing process, the content in alloying element (P, Ni, S, C, Cr), have an additional role. The chemistry of the groundwater and the consequent influence in the corrosion process is important too: for example many dissolved species like chloride, carbonate, sulphate enhance the corrosion processes and can lead to the formation of unstable minerals[23]. The mineral precipitation is a factor of great importance in the appraisal of the performances of a reactive permeable barrier in the long term [3]. If water contains a high amount of carbonates, there is an increase in calcite (CaCO3) and siderite (FeCO3) precipitates. It can moreover be observed the precipitation of iron oxides and hydroxides, among which the ferric hydroxide ( Fe(OH)3 ), the ferrous hydroxide (Fe(OH)2 ), the magnetite (Fe3O4) and maghemite ( Fe2O3 ); these are responsible of the ” effect coating”[15][23][12]. In field applications of PRBs iron is always used in mixture with sand to avoid a complete clogging[11].

Table 1. Contaminant treatable by reactive material in PRBs[23]

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2.1.2 Treatment of halogenated organic compounds with Zero Valent Iron

ZVI is one of the reactive medium more used for the treatment of numerous organic compounds and some inorganic compounds present as contaminants in the groundwater (see Table 1). In the last years several researches have been focused above all to the degradation of chlorinated compounds like TCE and PCE, by means of reactions occurring on the Fe0 surface. In general perhalogenated hydrocarbons tend to be reduced faster than hydrocarbons less halogenated and also the dechlorination is more rapid at saturated carbons (e.g. CCl4) centers than at unsaturated carbons(e.g. TCE)[15].The degradation process of the chloroether derivates can involve three steps[15]:1. the contaminant adsorption on the reactive sites of the barrier;2. reaction on the ZVI surface reaction;3. final products desorption;Nowadays the reaction between Iron and Chlorinated solvents is considered as an abiotic reductive dehalogenation [23] which can take place based on three degradation mechanisms, according to the conditions of the groundwater ,such as the amount of oxygen. This reaction occurs at the surfaces of Fe(0) that, through their corrosion and donation of electrons, permits the chlorinated hydrocarbon’s reduction and dehalogenation to hydrocarbon (non-toxic product). The resulting chloride is then dispersed in the water phase[23].The three mechanisms are[15]:

A)Fe0 Fe2++2e- Anodic reaction

RCl + 2e- + H+ RH + Cl- Cathodic reaction

Fe0 + RCl + H+ Fe2+ + RH + Cl- Net reaction

The reduction happens for transferring electrons from surface metal to the chlorurated molecule adsorbed on it.B) The ferrous ions (Fe2+) yielding through the ZVI corrosion by water are further oxidated to ferric ions(Fe3+) and than the chlorinated compounds are reduced. The net reaction is:

2Fe2++RCl+H+ 2Fe3++ RH + Cl-

2Fe0+O2+2H2O 2Fe2++ 4OH-

4Fe2++4H++O2 4Fe3++ 2H2O

C) H2 produced through the iron corrosion by the water shifts its electron to the chlorurated substance.

H2+RCl RH +H++ Cl-

It is most likely that the dehalogenation proceeds via sequence A).

When the compounds to treat, like the chlorinated aliphatic compounds, and the oxygen have similar oxiding potential, they can compete for being the favoured electrons acceptor. If the oxygen is sufficiently present, Fe2+ is further oxidized to Fe3+and can precipitate as hydroxide or (oxy)hydroxides at the elevated pH in the reactive zone of the ZVI[12].Aerobic system[2][12]:

2Fe0+O2+2H2O 2Fe2++4OH-

4Fe2++4H++O2 4Fe3++2H2OFe3++3OH- Fe(OH)3(s)

Under anaerobic conditions the corrosion reaction proceeds more slowly, yields an increase in the pH (due to the consumption of H+ like in the aerobic condition) and leads to the formation of ferrous (oxy)hydroxides instead of ferric (oxy)hydroxides[2][12].

Fe0+2H2O Fe2++ H2+ 4OH-

Fe2++2OH- Fe(OH)2(s)

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2.1.3 Zero valent iron and inorganic anions or oxyanions

Elements which can be in the oxyanion form under environmental conditions include selenium, arsenic, chromium, technetium, antimony, nitrate, phosphate, sulphate. The anionic species are very soluble in water and they are not attracted to the common negative surface of the minerals in aquifer [28]. This fact implies that these oxyanions can be potentially persistent in high concentration in ground water. These elements have in common that the reduced forms (which can be generated by the ZVI) are the immobile ones. The ZVI is then used in the barrier to reduce the oxidized species in the groundwater to the reduced forms which then precipitate out of solution and are thus removed from the groundwater[23].

2.1.3.1 Chromium Cr can appear commonly in environment as Cr(VI) or Cr(III). Cr(III) is relatively a micronutrient, non–toxic is adsorbed by some minerals and forms scarcely soluble hydroxide precipitates . Hexavalent Cr, instead is carcinogenic, more soluble and then more persistent and mobile[23].Several researches have been done on Cr interactions with Fe(0), and actually there are PRBs that have been specifically designed to treat Cr-contaminated groundwater with Fe(0).In the ground water Cr(VI) usually appears in the CrO4

2- form and it is not adsorbed by aquifer materials. In the ZVI system, the reduction of Cr(VI) to Cr(III) coupled with the oxidation of Fe(0) to Fe(II)and Fe(III) and the subsequent precipitation of Fe(III)-Cr(III) oxyhydroxides or hydroxides seems to be the main mechanism. Laboratory tests have investigated some materials containing reduced iron to see which was the most effective has been seen being in the Cr(VI) removal through reduction and precipitation: the most suitable medium was found to be Fe(0). Also column tests and field applications have shown a rate of Cr(VI) reduction and precipitation by Fe(0) suitable for ground water remediation system. Sequestration of Cr(VI) provokes a steep increase in pH from initial neutral condition(6,5<pH<8,5) to more basic conditions(pH>9,5) and a sharp decrease in the Eh (from Eh>100mV to Eh<-300mV)[23].The overall reactions are: CrO4

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

Cr(III) and Fe(III) hydroxide phase in the same spot on the surface of reacted iron. This fact demonstrates coprecipitation and the probability of the (Crx,Fe1-x)(OH)3 solid solution phase formation.The dominant product ,as said above, is a mixed Fe-Cr oxyhydroxide phase with the mineral structure of goethite (FeOOH). In the precipitate Cr and Fe are only in the Cr(III) and Fe(III) oxidation state[23].

2.1.3.2 Technetium, Selenium, Arsenic

Tc(VII) is rapidly reduced to Tc(II), Se(VI) is transformed in Se(0) due to interactions with Fe(0).A reaction proposed by McRae et al.(1997) for the Se(VI) reduction is the following :

HSeO4-+3Fe0+7H+ 3Fe2++Se0

(s)+4H2O

The two principal oxidation states for Arsenic ,As(V) and As(III), appear respectively as H3AsO4 and H3AsO3

dissolved species with their dissociation products. Both As(V) and As(III) are mobile in natural ground water system and the first one, for the human health, is more hazardous than the second one. Possible removal mechanisms for the As have been proposed among which the reduction of Arsenic to As(0) or the incorporation into a secondary arsenic sulphide. Although batch experiments indicate a rapid and effective elimination of As(V) and As(III) by ZVI, in the laboratory further research is needed to test the feasibility of this method for field application[23].

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2.1.4 Zero valent iron and inorganic cations

Like anions, inorganic cations are reduced and precipitate as meagrely soluble solids. Industrial, mine and nuclear sites are the principal sources of Hg, Cu, Tc and complex such as UO2

2 , responsible of their high concentration. Laboratory experiments have shown for mercury, technetium, uranium and copper treated with ZVI a reduction and a subsequent coprecipitation within secondary precipitates.For U(VI) the reaction proposed is the following[23]:

Fe0+UO2+2(aq) Fe2++UO2(s)

UO2(s) (uraninite) can be amorphous or crystalline precipitate and its solubility is in the range of 10-8 mol/l in a pH range between 4 and 14. Its solubility can enhance under oxidising conditions and below pH 4. UO(VI) is reduced to U(IV) spontaneously by elemental iron[23].

2.1.5 Precipitation problems

Geochemical changes such as pH increases and oxygen elimination, occur in water passing a barrier of Fe (0). These changes can lead to precipitation of secondary minerals onto the reactive surface which can influence the reactivity and permeability of the ZVI system over the time[22]. These redox conditions permits precipitation of secondary minerals from ions typically present in ground water as well as some ground water contaminants [29]. The typically secondary minerals formed in PRBs are magnetite (Fe3O4), hematite(α-Fe2O3), goethite (α-Fe3+O(OH)), lepidocrocite ( FeOOH) , calcite (CaCOɤ 3), aragonite (CaCO3), siderite (FeCO3), green rust ([Fe(1–x)

2+ Fex3+ (OH)2]x+ [x/n An–·m H2O]x–, where x is the ratio Fe3+/Fetot), ferrous

hydroxide Fe(OH)2, ferrous sulfide (FeS2), and marcasite (FeS2). In general calcium carbonates and siderite are found close to the entrance of a PRB, while magnetite, ferrous hydroxide, green rust and iron oxyhydroxides generate throughout a PRB. Accumulation of secondary minerals is responsible of loss of pore space and reactive surface area of the medium in PRBs, which can alter flow paths, residence times, and effectiveness of a PRB treatment. Due to site-specific geochemical and hydrogeological conditions, and the relatively long period over which mineral deposition occurs, it is complicated developing a general assessment of mineral precipitation in PRBs using field and/or laboratory data [29].The effects that an implementation of a ZVI barrier produces intra-wall and down gradient are a loss of dissolved oxygen, decreasing Eh, reduction in carbonate alkalinity increasing in pH value up to 9 or 10 and in Fe2+ that can also precipitate as oxyhydroxide colloids. Loss of cementation between the grains and precipitate formation can generate mobile colloidal particles that can transport toxic substances. Deeper explorations have to be carried out to know which type of geochemical characteristic of the plume can influence the liberation and mobilization of immobilized metals and the transportation of the colloidal materials. Iron hydroxides, with the proceed of the corrosion, can form a more and more thick passivating layer on the surface of iron grains. This phenomenon gradually occludes the Iron surface and reduces its reactivity [23]. Since Ferrous (oxy) hydroxide is thermodynamically unstable, it might be further oxidized to magnetite or goethite that is non-passivating and seems to allow sufficient contaminant degradation instalments over the years [23]. Under conditions of pH neutral, it can be formed a mixed valence (Fe2+/Fe3+)compound, said “green rust”, as intermediate product of magnetite (Fe3O4) formation from iron hydroxide. The “green rust” is stable only at low grades of oxide reduction and its oxidation leads usually to the formation of maghemite (Fe2O3) or lepidocrocite ( FeOOH), with a prevalence in the formation of theɤ first compound compared to the second[7]. The maghemite, differently from the magnetite, is responsible for iron passivation. If ”effect coating” prevents the Ionian ferrous passage in solution, the surface of the iron is polarized by work of Fe2+ ion and diminishes therefore the tendency of the iron to corrode itself. It has also been seen that, if the concentration of organic chlorinated compounds increases, the half life of the polluting agents increases too. That depends on the increasing of the passivated iron surface and on the emphasized saturation of the reaction site. The passivation phenomenon is influenced moreover from the composition of the iron [10]. The mineral precipitation can involve, at the same time of reactivity reduction, also various negative impacts on the hydraulic conditions of the barrier [3]. The filling of the pores can raise the heterogeneity inside the barrier, leading therefore to the formation of preferential paths [9], increasing of fluid speed and meaningful lessening of the time of residence inside of the PRB. Today there are studies describing the performances in the long term of the PRB containing zerovalent iron.

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2.2 Apatite

2.2.1 Structure

Apatite is a microporous mineral described with general formula Ca5(PO4)3(OH,F,Cl), or more accurately by the unit cell contents [Ca4][Ca6][(PO4)6][F]2. [Ca4][Ca6][(PO4)6][F]2 is composed by CaO6 columns linked together with PO4 groups in order to form “a hexagonal network like a honeycomb with channels extending right through the structure in c direction “ [25]. This one-dimensional tunnel structure is very stable and strong, and is chiefly due to the arrangement of calcium and phosphate. The structure can be considered a tunnel structure with walls composed of corner-connected CaO6 and PO4 polyhedra as relatively invariant units. Compared to fluorapatite, in the hydroxyl apatites the presence of OH-, that is a little larger than F-, is responsible for the resulting expanded structure. The microporus structure with tunnels filled to Calcium and anions (OH,F) leads to an adjustment that best satisfies bond-length requirements [25]. Little changes in the ionic radii of the tunnel atoms cause the expansion or contraction of the channel. The channels can accommodate different contents, accepting large cations of different valence through the introduction of framework counter ions. Apatites are reversible ion exchangers for some anions and cations, are stable in the subsurface and do not induce microbial blooms[25].

2.2.2 Apatite II

2.2.2.1 Reactions of apatite and metals

Apatite II has the general formula [Ca10-xNa(PO4)6-x(CO3)x(OH)2 where x<1] and is generated as a waste product from fish industry. It is an excellent buffer for neutralize the acidity through PO4

3- and is one of the best available non-specific metal sorbent. The use or the production of apatite II has not environmental impacts and there is no phosphate loading to the environment due to its low solubility, Ksp>10-20 [16] . Metals in solution are immobilized on the apatite mineral by surface sorption (main mechanism for most metals), precipitation (main mechanism for U, Pu, Pb, lanthanides) or co-precipitation (transition metals).It has particular characteristics that render it fit for optimal performance in the field, namely: its substituted CO3

2- and sodium that destabilize the structure and make the material more soluble, no substituted F, low trace metal concentration, poor crystallinity (>90% amorphous) and high microporosity [6]. These properties influence the kinetics and solubility. F as minor constituent in the structure raises lattice stability and decreases solubility and dissolution rate. Instead carbonate has the opposite effect[16]. Differences in the performance among various apatite phases result from differences in those properties which influence the kinetics and solubility, e.g., crystallinity and minor element chemistry. A higher degree of crystallinity decreases solubility and dissolution rate, making the apatite less reactive and less effective, whereas lower crystallinity increases solubility. Thus, the amorphous form of the solid is the most reactive. The presence of F as a minor constituent in the apatite structure increases lattice stability, decreasing solubility and dissolution rate. The presence of carbonate as a minor constituent in the apatite structure decreases lattice stability, increasing solubility and dissolution rate[16].Phosphate-Induced Metal Stabilization (PIMS) is a remediation technology that stabilizes a broad range of radionuclides and metals like Pb, U, Cd, Zn, Cu, and Al which are bound into new phosphate minerals and low solubility phases. The metal–phosphate phase is very stable over geological time because of its low solubility products ( Ksp), as shown in table 2 [6][14]. These metal-phosphate products are thermodynamically stable over a broad range of environmental conditions, and precipitate rapidly, thus ensuring a stable and long term immobilization of metal pollutants. They have great durability, low solubility and are stable over a wide range of conditions [14].

x

Table2-Solubility products of metal phosphates [27]

Metals and apatite react very quickly ([17], [20], [26], [4], [5], [21])on molecular scale, but at a macroscopic dimension, limiting parameters like grain size, flow rate, barrier design, influence the efficiency with which dissolved metals come into contact with the surface of the reactive media. Apatite II has treated successfully contaminated soils, groundwater and wastewater for Pb, U, Cd, Zn, Al stabilizing between 5% and 50 % of its weight in metals depending upon the metal and the environmental conditions[16] . Pb,Cd, and Zn are sorbed by apatite with different mechanisms. Pb is primarily removed with this mechanism: the dissolution of the apatite that provides PO4

3- and the following precipitation of hydroxyl fluoropyromorphite on Apatite surface. Minor otavite (CdCO3) precipitation was observed in the interaction of the apatite with aqueous Cd. However other sorption mechanisms, such as surface complexation, ion exchange, and the formation of amorphous solids, are primarily responsible for the removal of Zn and Cd [5]. There are four general factors that aid in the effective sequestration of metals by apatite II, depending upon the type of metal and its concentration and the groundwater chemistry: 1) heterogeneous nucleation; 2) buffer acidity to pH 6.5 to 7; 3) surface chemi-adsorption; 4) biological stimulation; [21]

2.2.2.1.1 Heterogeneous nucleation

A small, but sufficient, amount of phosphate is provided to solution in order to overtake the solubility limits of the metal-apatites. The (Ksp) of the resulting metal-apatites is very low for the metals Pb, U, Cd, Zn, Cu and Al and in particular for Pb that is equal to Ksp=10-167[21].Pb-pyromorphite can precipitate only through heterogeneous nucleation, namely, a seed crystal with the apatite crystal structure is necessary for the precipitation , unless Pb concentrations exceed about 10 ppm ( not frequent in environmental conditions) [18][19].The Apatite II amorphous structure is relatively reactive and supplies a slight excess of phosphate ions to solution. The random nanocrystal inclusions of crystalline apatite, provide the seeds for nucleating the precipitates. Pb in the system precipitates only as microscope Pb-pyromorphite minerals which over the time will grow and join together eventually forming larger mineral bunches [22]. The presence of apatite II- supplied phosphate in this process ,that can take many years, keeps the concentration of Pb in solution extremely low, <15 ppb, and so no leaching of Pb above drinking water limits happens[16].Two steps occur for the Pb immobilization, the dissolution reaction (of apatite) and subsequently the precipitation (of pyromorphite on Apatite II surfaces) reaction[16]:

a) Ca10-xNax(PO4)6-x(CO3)x(OH)2 + 14H+ (10-x)Ca2+ + xNa+ + (6-x)[H2(PO4)] + xH2CO3 + 2H2 O

Apatite

b) 10Pb2+ + 6H2(PO4)- + 2OH- Pb10(PO4)6(OH)2 + 12H+ Pyromorphite

Similar reactions immobilize U, Mn, Ce and Pu and other metals for which precipitation is the primary process of removal from solution. Because of the low solubility of the new metal phosphate phases, the concentrations of these metals (involved in the precipitation) in solution are usually below legal or detection

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limits[16]. The degree of protonation, namely the number of hydrogen ions attached to the PO43-, in the

intermediate reactions, depends upon the solution pH. The example above is for the range of acid soils or acid mine drainage, pH<6. Not always the reaction a) leads to the reaction b) . However the apatite supplies constantly phosphate to the solution and induces reaction b) whenever Pb2+is in contact with apatite. The dissolution reaction a) conduces to a strong pH buffering.In order to be effective the solubility of the original apatite must be adequately low to persist in the environment for many years, preventing phosphate loading and it must be sufficiently high to be reactive. The rate of dissolution of apatite in batch test diminishes strongly over the time because the system approaches the equilibrium. In the open system like PRB this fact does not occur because the system rarely reach the equilibrium due to the rapid removal of the dissolved species from the system through flushing [16].

2.2.2.1.2 pH buffering

Apatite II neutralizes acidity through PO43-,OH-,and substituted CO3

2-groups and buffers to neutral pH (6.5 to 7)that alone precipitates many metal phases such as Al and Fe [6].

2.2.2.1.3 Surface chemi-adsorption

Because of presence of phosphate and hydroxyl surface functional groups that induce metal sorption, apatite II is an effective metal (above all transition metals) sorbent. Through this process apatite can adsorb up to 5% of its weight. [6][21].

2.2.2.1.4 Biological stimulation

P and readily bioavailable organic carbons at low but optimal concentration are continuously supplied by apatite II to solution for stimulating the activity of microbial communities that can remediate contaminants. For example in presence of high sulphate concentration, Zn and Cd are reduced to sulphides [16]. E.g.

a) SO42-+ 2CH2O H2S + 2HCO3

- CH2O represents the organic carbon from the Apatite II. It acts at the same time as electron donor and carbon sources for the sulfate microbial reducers. b) Zn2++HS- ZnS + H+ Sphalerite ( and other metal sulphides) precipitates quickly on Apatite II surface.

Biological stimulation process also diminishes the bioavailability of ingested metal-apatite[24]. Under most environmental conditions, adsorption or precipitation into non apatite phase are the principal mechanisms of subtraction of Zn, Cd, and other transition metals from the solution. Hence, the performance for the removal of these metals, depends on the apatite adsorption characteristics and features like the surface properties, the grain size, amount and type of organics, solution chemistry, degree of crystallinity and competing ions[16].

2.2.2.2 Case study: use of apatite(II) in groundwater remediation contaminated with heavy metals by mine waste in Idaho, USA [6][26].

Apatite II is being used as reactive media in a subsurface PRB installed at the Success Mine and Mill site in northern Idaho. Success Mine and Mill operated from 1886 to 1956 and. 100 tons of apatite II at cost of $ 350/ton was put in subsurface. The shallow ground water to treat contains elevated levels of Zn, Cd, P, sulphate and nitrate. Apatite II was chosen based upon the laboratory studies of 9 reactive media. This field application is an example where all the four processes, that the Apatite II can provoke, work together. The remediation of the groundwater occurs before it enters the East Fork of Ninemile Creek, a tributary to the Coeur d’Alene River.For three years, since it was emplaced in January 2001, less than half of the Apatite was consumed and the PRB operated successfully reducing the levels of Pb and Cd below detection (2 μg/l), Zn from over 100 ppm to near background (about 100 μg/L)and sulfate and nitrate from over 200 ppm to below detection (50 μg/l).Apatite has served as an ideal substrate and nutrient source for microbial communities in the reactive zone. Analyzing the differences in pH, sulfate, nitrate, ammonia, phosphate, and bicarbonate concentrations between the inflow and outflow of the barrier, it has been seen that Zn is being incorporated into sulphide

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phases by precipitation that it has resulted being a mixture of sphalerite and recrystallized apatite. Within the first few feet of the PRB Pb and Cd vanish completely from solution, Zn concentration instead diminishes continuously along the barrier, finally dropping to about 0.1 mg/L or below once exited from it.In order to understand which mechanisms are operating and how much time the barrier can last, analysis of material captured from barrier and geochemical modelling using MINTEQ and PHREEQ were performed. MINTEQ indicates that pyromorphite and sphalerite (ZnS) are the steadiest phases within the barrier for Pb and Zn. These results are based to the dates displayed in the table 3. The other metals such as Mn,Cu, Al and U are also being reduced from ppm/ subppm level to the pbb level or below detection. This indicates that biological reduction, pH buffering, precipitation and surface adsorption all act in the PRB. Cd can undergo adsorption onto the Apatite II or precipitate as a sulphide similar to Zn.

Table 3- Changes in some groundwater constituents entering/exiting the Apatite II PRB in August 2002 [14]

2.2.2.3 Benefits of use of Apatite II [6]

The use of Apatite II is a new remediation technology that has several advantages.First at all Apatite II can sequester most heavy metals and radionucleotides for geologically long time periods. Many metals and radionuclide contaminants precipitate as mineral phases in which the metals are not bioavailable. Apatite II is one of the best non-specific surfaces sorbent for species that do not precipitate as a separate phase and is the most cost-effective reactive media for most metals and radionuclides, e.g., immobilizes up to 20% of its weight in Pb, U, Pu and perhaps other metals.Apatite II can be mixed into contaminated soil or waste, emplaced in PRB or used as a disposal liner and can be combined with most other reactive media e.g., grout, bentonite, zero valent iron, and other remediation technologies, e.g., bioremediation, etc.

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3. CONCLUSIONS AND REMAINING RESEARCH QUESTIONS

The feasibility of PRB’s installations depends on the chemical, geological and environmental settings of the PRB’s capture zone. The reactivity of the barriers can be impaired by the remediation processes themselves (e.g. oxidation), by the reaction products like for example precipitates and by exhaustion of sorption capacity[11]. The types of materials used in barriers are those changing redox potential (e.g. zero valent iron – degradation of chlorinated hydrocarbons), those causing precipitation, materials with high sorption capacity, those releasing nutrients/oxygen to enhance biological degradation [12];

Zero Valent Iron is the reactive medium in Permeable Reactive Barrier system most used because of its vast capacity to remediate a large number of contaminants through its corrosion and donation of electrons for the reduction of the pollutants in a harmless state. Hence, are available numerous terms of comparison from which can be extrapolated the guide lines for the planning and the emplacement of new installations[15]. ZVI is very effective towards the chlorinated aliphatic hydrocarbons acting as reductant and degrading them through a dehalogenation. For the contaminants in the form of inorganic cations and anions, their reduction and their consequent precipitation are the processes involved when is utilized the ZVI as reactant[23]. To date the mechanisms of remediation of ZVI towards several compounds are fairly noted and they have been studied in depth. Not completely clear and requiring further studies are the long term effects of some precipitates like maghemite (Fe2O3), ferric hydroxide ( Fe(OH)3 ), ferrous hydroxide ( Fe(OH)2 , a mixed valence (Fe2+/Fe3+)compound, said “green rust”, on the maintenance of the PRB’s functionality.

Apatite II is a particular configuration of apatite that appears very suitable for the remediation of groundwater contaminated by metals, including uranium and plutonium [14].In the case study considered in this paper, apatite II has demonstrated high efficiency in the abatement of the heavy metals Pb, Cd and Zn. A not negligible benefit in the utilization of apatite II as medium in PRB is the fact that it has not environmental impacts and there is no phosphate loading to the environment due to its low solubility, Ksp<10-20 [16]. In addition, as a byproduct of fish industry, is cheap and abundant. Another consideration is that apatite II can immobilize the pollutants for a very long period [16].

Since PRB is a relative new technology, further studies should be made for the improvement of the effectiveness of contaminants remediation with the actual used reactive materials.In general, other researches, regarding other possible reactive materials that could be applied in PRB’s systems, could be carried out in order to find the optimal media for the remediation of certain contaminants. Many pollutants are not treatable by Zero Valent Iron-PRB. Studies on new combinations of reactive materials could also be carried out. Moreover should be fulfilled further investigations on the long term performance of PRBs for a better understanding of the remediation processes [12] and for monitoring the potential formation of toxic byproducts and secondary precipitates.Laboratory and field experiments should be made in order to identify methods to avoid an excessive loss of reactivity and “refresh” the reactivity of the Fe(0) surface over time.Loss of cementation between the iron grains and precipitates can generate mobile colloidal particles that can transport toxic substances. Deeper explorations have to be accomplished to know which type of geochemical characteristic of the plume could influence the liberation and mobilization of immobilized metals and the transportation of the colloidal materials. Additional research is needed on these possible disruptive effects to establish whether certain types of intercepted contaminants can be transported colloidally [23].Other possible researches could be undertaken for testing the effectiveness of the use of Apatite II as a nutrient source for microrganisms in bioremediation of organic contaminants.

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4. AKNOWLEDGEMENT Particular thanks to my tutor who has supported me with patience in developing this term paper, giving me precious hints and councils. Further thanks to my reviewers.

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