Laboratory batch experiments were performed to: (i) select two individual and two mixtures of potential reactive materials for permeable barriers to treat groundwater contaminated with benzene and soluble lead (Pb2+); (ii) investigate the involved contaminant removal mechanisms; and (iii) determine the permeability and assess the environmental compatibility of the selected materials. Five individual reactive materials (zeolite, diatomaceous earth, brown coal, compost, and zero-valent iron as control) and four mixtures (compost:brown coal, compost:zeolite, compost:mulch, and mulch:diatomaceous earth) in different ratios were investigated. Benzene and Pb2+ were investigated separately using Pb2+/benzene spiked deionized water. Zeolite and brown coal were selected as individual materials for Pb and benzene based on their removal efficiencies. For the material mixtures, compost:brown coal (1:3) and compost:zeolite mixtures (1:3) were selected for Pb, whereas compost:zeolite (1:1) and compost:brown coal (1:5) were selected for benzene. The sequential extraction of Pb from these selected reactive materials showed that Pb was held mainly in the exchangeable fraction (52%–76%). Benzene was removed by biodegradation and sorption, with the latter contributing most to its removal (60%–99%). The selected materials were compatible with the environment considering the amounts of toxic metals leached from them, and their permeabilities were in the range of 4.2 × 10−5–2.14 × 10−3 m s−1.

INTRODUCTION

Soluble lead (Pb2+) and benzene are among the contaminants commonly found in groundwater. Benzene is widely used as a solvent for organic synthesis and equipment cleansing in many industries; however, it possesses carcinogenic and teratogenic properties. Lead is also well known for causing liver and kidney damage, anemia, infertility, and mental retardation (Singh et al. 2008; Weschayanwiwat et al. 2008). The removal of these contaminants from groundwater is therefore necessary to prevent these health problems.

Permeable reactive barriers (PRBs) that allow the utilization of a variety of removal mechanisms such as sorption, biodegradation, and precipitation have shown great promise in attenuating a variety of contaminants. The PRB technology involves the installation of a reactive medium/barrier across the flow-path of the contaminant plume to immobilize or degrade the contaminants as the plume migrates through it (Henderson & Demond 2007).

In the PRB system, the reactive media constitute a significant component; therefore, they require proper assessment before their selection. The selection of the most appropriate reactive material is usually influenced by a number of factors, such as reactivity, permeability, environmental compatibility, availability, and cost. It is desirable to use reactive materials with initial permeability reasonably higher than that of the aquifer matrix, and which can be maintained over time. In addition, the materials should be inexpensive to reduce the total cost of the PRB; be able to treat the contaminant(s) at a sufficient rate to give a reasonable wall thickness; and not release substances which have the potential to adversely impact the environment (Carey et al. 2002). Although a variety of treatment media are available, zero-valent iron (ZVI) has received considerable attention and has been frequently used in PRBs due to its effectiveness against a broad spectrum of contaminants, including heavy metals. However, some issues regarding its cost, accessibility, longevity (Henderson & Demond 2007), and inability to abiotically transform reduced organic contaminants such as BTEX have necessitated the search for alternative materials.

Mixtures of appropriate reactive materials are now being applied in PRBs to help eliminate or reduce the potential limitations of single/individual materials. Among other things, they can improve permeability, reduce cost of using the single/pure materials, provide multiple mechanisms for contaminant removal, and accelerate the removal rates (Obiri-Nyarko et al. 2014). The objective of this study was to screen individual and mixtures of reactive materials, with special emphasis on the latter, for PRBs to remediate groundwater contaminated with benzene and Pb2+.

METHODS

Reactive materials

The reactive materials investigated in this study included ZVI (as control), zeolite (clinoptilolite), brown coal (lignite), compost, mulch, and diatomaceous earth. These materials were considered because they are readily available, inexpensive and can be obtained in large quantities. Moreover, based on their properties it was presumed that mixtures of some of them may improve the permeability and/or provide different removal pathways to improve the removal efficiencies of the materials.

Compost, mulch and diatomaceous earth were included to additionally provide and/or stimulate micro-organisms for biodegradation/bioprecipitation of the contaminants. Municipal compost derived from food and plant waste was used in this study. It is naturally populated with a panoply of micro-organisms, and is often applied in biobarriers as a source of readily decomposable carbon (Ahmad et al. 2007; Simantiraki et al. 2013). Mulch usually consists of fractions of readily decomposable component (cellulose and hemicellulose) and slowly decomposable component (lignin, which is also porous in nature). Owing to the physical properties and chemical composition of the mulch, it has frequently been combined with materials such as compost in biobarriers to improve the permeability and serve as a long-term carbon source to maintain the long-term functionality of the barriers (Ahmad et al. 2007; AFCEE 2008). Diatomaceous earth (SiO2·nH2O) is a biogenic sedimentary material with unique physico–chemical properties (porous structure, high silica content, low density, low conductivity coefficient (Khraisheh et al. 2004). The diatomaceous earth used in this study was obtained from the brewery. Some physical and chemical properties of the studied reactive materials are presented in Table 1.

Table 1

Physico–chemical properties of the reactive materials

MaterialspH
ρb (g/cm3)Moisture content (%)LOI (%)CEC (meq/100 g)
H2O1 M KCl
ZVI 8.5 n.d. 2.6 n.d. n.d. n.d. 
Compost 8.1 7.4 0.7 44.4 34.9 480.0 
Zeolite 7.1 5.5 0.8 5.2 5.2 435.5 
Brown coal 4.9 4.6 0.8 16.2 28 1,215.0 
Mulch 4.9 4.7 0.3 11.3 80.1 n.d. 
Diatomaceous Earth 5.2 5.0 1.2 62.5 33.9 n.d. 
MaterialspH
ρb (g/cm3)Moisture content (%)LOI (%)CEC (meq/100 g)
H2O1 M KCl
ZVI 8.5 n.d. 2.6 n.d. n.d. n.d. 
Compost 8.1 7.4 0.7 44.4 34.9 480.0 
Zeolite 7.1 5.5 0.8 5.2 5.2 435.5 
Brown coal 4.9 4.6 0.8 16.2 28 1,215.0 
Mulch 4.9 4.7 0.3 11.3 80.1 n.d. 
Diatomaceous Earth 5.2 5.0 1.2 62.5 33.9 n.d. 

LOI: loss on ignition; CEC: cation-exchange capacity; ρb: bulk density; n.d.: not determined.

Experimental design

In all the experiments benzene and Pb2+ were treated separately and the contaminated water used was prepared by spiking deionized water with these contaminants. The experiments to select the reactive materials were performed in three batch test series. The materials were selected based on their removal efficiency. In cases where the removal efficiencies were similar (not statistically different), other factors such as the anticipated contaminant removal mechanism(s) employed by the reactive materials were taken into consideration:

  • Batch test series 1 evaluated five individual materials, including ZVI (as control), zeolite, brown coal, compost, and diatomaceous earth. Mulch was not considered in this phase, due to its poor performance in a pretest with the individual materials.

  • Batch test series 2 evaluated the following mixtures:compost:zeolite, compost:brown coal, compost:mulch, and mulch:diatomaceous earth. The mixing ratio for the mixtures was 1:1 by weight.

  • Batch test series 3 evaluated the effects of different mixing ratios (by weight) for the mixtures selected in batch test series 2.

Subsequently experiments were performed to investigate the hydraulic properties (permeability), environmental compatibility, and the mechanisms by which Pb2+ and benzene were removed by the selected reactive materials.

Preparation of contaminated water and experimental procedure

Stock solutions were first prepared using analytical grades of Pb-nitrate (1,000 mg/L ± 4 mg/L; HNO3 = 2% w/w; Fluka) and benzene (Fluka, 99%). Thereafter, deionized water was spiked with part of the stock solution to achieve an initial concentration of approximately 2 mg/L for each contaminant. The reactive materials (individual and mixtures) were put into 250 mL amber bottles and filled with the contaminated water to zero headspace, resulting in a liquid to solid ratio of 20:1. The mixture of contaminated water (with Pb2+ or benzene) and the reactive materials were agitated for 48 hours on a 150 rpm orbital shaker at room temperature (22 ± 1 °C). At the end of the experiments, concentrations of Pb2+ and benzene in the liquid phase were determined using atomic absorption spectrophotometry (AAS 3, Corp Zeiss; detection limit of 0.001 mg/L) and gas chromatography (Shimadzu GC 17A, PAF/A/5/Sb; detection limit of 0.001 mg/L), respectively. The efficiency of the materials in removing the contaminants was calculated using the equation below: 
formula
1
where E is removal efficiency, ci is initial concentration of the contaminant in the water phase (mg/L), and cf is final concentration of the contaminant in the water phase (mg/L).

The pH and Eh of the solutions were also measured at the beginning and end of the tests using a multifunctional computer meter (Elmetron Cx-742).

Determination of removal mechanisms

The removal mechanisms considered for benzene were sorption and biodegradation. Two sets of experiments, A and B were performed. In experiment set A, 1% sodium azide (NaN3) was added to the samples to repress/exclude biodegradation. In experiment set B there was no addition of NaN3. This was to create a biologically active medium. In this experiment, benzene removal was considered to be due to both sorption and biodegradation. The amount of benzene biodegraded was then accounted for as the difference between the removal efficiency of the materials in experiments A and B, on the assumption that the amount of adsorbed benzene was large compared to the amount in the liquid phase at equilibrium.

The sequential extraction procedure (SEP) (Tessier et al. 1979) was employed to determine the distribution of Pb in the reactive materials. The SEP comprises the addition of reagents in an increasing reactivity order to extract the metal in different chemical binding forms. This protocol consists of five steps; however, it was truncated to three steps in this study, namely exchangeable fraction (i.e. removal via ion-exchange), carbonate fraction, and reducible fraction (i.e. removal via binding to Fe/Mn oxides). The remaining Pb was considered to be in the residual fraction, that is, Pb bound to organic matter (OM), sulfides, and in other chemical forms. A control experiment (no chemical addition) was performed to determine the background contribution.

Permeability and environmental compatibility tests

The hydraulic properties (permeabilities) of the selected reactive materials were determined according to the method of Head & Keeton (2008). A leaching test according to the toxicity characteristic leaching procedure (TCLP)-method 1311 (USEPA 1992) was performed to assess the environmental compatibility of the materials. The extractant for the leaching test was prepared from a mixture of glacial acetic acid, sodium hydroxide, and deionized water.

RESULTS AND DISCUSSION

The results of batch test series 1 for the individual reactive materials are presented in Table 2. A decrease in Eh was observed for all the materials in both Pb and benzene experiments. In the Pb experiments, a rise in pH was observed for all the materials whereas the converse was observed in the case of benzene. The rise in pH should promote the removal of heavy metals; however, this may also (inevitably) cause some problems, the main ones being reduction of permeability and removal efficiency. This is however dependent on the type of material and the magnitude of the pH increase.

Table 2

Results of batch test series 1

MaterialsPb
Benzene
Removal efficiency (%)pHEh (mV)Removal efficiency (%)pHEh (mV)
Initial solution – 3.2 (0.0) +303.0 (7.8) – 8.8 (0.2) +211.0 (14.9) 
Compost 91.9 (0.2) 7.1 (0.3) +28.6 (34.4) 73.2 (0.5) 7.5 (0.3) −72.8 (9.1) 
Brown coal 99.9 (0.0) 5.8 (0.1) +204.0 (8.3) 93.2 (0.0) 5.1 (0.3) +16.2 (15.5) 
Diatomaceous earth 76.2 (0.1) 5.5 (0.1) −179.9 (7.2) 66.8 (0.3) 5.7 (0.2) −207.7 (10.5) 
Zeolite 98.7 (0.0) 6.2 (0.1) +226.1 (6.0) 99.9 (0.0) 6.5 (0.0) +190.3 (4.0) 
ZVI 99.9 (0.0) 6.9 (0.1) +153.7 (17.9) 55.7 (0.1) 6.4 (0.1) +159.4 (9.7) 
MaterialsPb
Benzene
Removal efficiency (%)pHEh (mV)Removal efficiency (%)pHEh (mV)
Initial solution – 3.2 (0.0) +303.0 (7.8) – 8.8 (0.2) +211.0 (14.9) 
Compost 91.9 (0.2) 7.1 (0.3) +28.6 (34.4) 73.2 (0.5) 7.5 (0.3) −72.8 (9.1) 
Brown coal 99.9 (0.0) 5.8 (0.1) +204.0 (8.3) 93.2 (0.0) 5.1 (0.3) +16.2 (15.5) 
Diatomaceous earth 76.2 (0.1) 5.5 (0.1) −179.9 (7.2) 66.8 (0.3) 5.7 (0.2) −207.7 (10.5) 
Zeolite 98.7 (0.0) 6.2 (0.1) +226.1 (6.0) 99.9 (0.0) 6.5 (0.0) +190.3 (4.0) 
ZVI 99.9 (0.0) 6.9 (0.1) +153.7 (17.9) 55.7 (0.1) 6.4 (0.1) +159.4 (9.7) 

Removal efficiency, pH, and Eh represent averages of quadruplicate and triplicate measurements (± standard deviation) for Pb and benzene, respectively.

The ZVI acts primarily as a reductant in most systems due to its high standard reduction potential of −440 mV. Accordingly, it has been frequently reported in the literature that there is a reduction of Eh with a concomitant rise of pH following the addition of ZVI to water or vice-versa. In the subsurface (or in ZVI-H2O systems), the reduction of Eh has been attributed to the oxidation of the ZVI (and the dissolved reaction products, e.g. Fe2+) by dissolved oxygen, water, electro-active contaminants or other redox-active species (O'Carroll et al. 2013). Considering the overwhelming amount of water as the solvent (and especially under anoxic conditions), the reduction of water by the ZVI can be expected to be the principal redox process leading to the reduction of the Eh in this study.

The rise of pH in the Pb experiment may be due to a number of anoxic ZVI dissolution reactions such as those occurring in Equations (2), (3), and (5) in Table 3. The ZVI either consumed H+ directly, leading to the reduction of H+ (Equation (2)) or was corroded by H2O, leading to the production of OH as shown in Equation (3). Ferrous iron (Fe2+), a product of ZVI dissolution, may also reduce water to increase the pH (Equation (5)). These reactions are expected to raise the pH to/above 8 (O'Carroll et al. 2013), which is higher than we observed in our study. This may be due to the low initial solution pH (Wilkin & McNeil 2003) or the occurrence of other reactions (e.g. Equations (4) and (6)). The decrease of pH in the experiments with benzene may be explained with Equations (3)–(6). The ZVI initially may have been corroded by H2O to produce Fe2+ and OH (Equation (3)). An initial increase in alkalinity is expected as a sequel of the OH produced. The produced Fe2+ either reacts with OH (decrease in pH) (Equation (4)) or undergoes further oxidization to yield ferric iron (Fe3+) (Equation (5)), which can also undergo further reaction to yield H+ (Equation (6)).

Table 3

Possible reactions in ZVI-H2O system under anoxic conditions

Reaction/equationNumberReference
 
formula
 
(2) Wilkin & McNeil (2003)  
 
formula
 
(3) O'Carroll et al. (2013)  
 
formula
 
(4) Fu et al. (2014)  
 
formula
 
(5) Suponik & Blanco (2014)  
 
formula
 
(6) Suponik & Blanco (2014)  
Reaction/equationNumberReference
 
formula
 
(2) Wilkin & McNeil (2003)  
 
formula
 
(3) O'Carroll et al. (2013)  
 
formula
 
(4) Fu et al. (2014)  
 
formula
 
(5) Suponik & Blanco (2014)  
 
formula
 
(6) Suponik & Blanco (2014)  
The removal of metals by the ZVI corrosion process has been shown to occur via a number of mechanisms including adsorption, reduction, (co)precipitation, and oxidation/reoxidation. The specific removal mechanism(s), however, depends on the standard redox potential of metal and the aqueous phase pH or accompanying physico–chemical changes (O'Carroll et al. 2013). Considering the slightly greater standard reduction potential (−130 mV) of Pb, the reduction of Pb2+ by ZVI (as depicted in Reaction (7)) could be expected in this study. The adsorption of Pb (possibly in both its zero-valent and bivalent forms) could also be expected, as this has also been reported (O'Carroll et al. 2013). In the case of benzene, the removal may have occurred through its adsorption onto the ZVI. 
formula
7
Compost neutralized the pH whereas the Eh was decreased in both Pb and benzene experiments. The increase of pH in the case of Pb was possibly due to consumption of protons by surface functional groups while a release of protons from the surface functional groups or the production of organic acids from OM mineralization may explain the decrease of pH in the case of benzene (Brady 2001). The reduction of Eh in experiments with both benzene and Pb may be due to microbial metabolism, based on the results of batch test series 4. An increase in microbial activity is usually accompanied by an increase in oxygen uptake, which consequently leads to a reduction in the oxidation–reduction potential (Herbel et al. 2007). Similarly, the decrease in Eh in experiments with benzene and Pb with diatomaceous earth may be attributed to microbial metabolism, whereas the resulting low pH may be due to the nature of the reactive material as shown in Table 1.

In the case of zeolite, the redox potential was slightly reduced at the end of the experiments. The rise of pH may be attributed to the consumption of protons through ion-exchange reactions (Park et al. 2002) or an interaction of protons with the Lewis basic sites of the zeolite. The decrease of pH in the case of benzene may be due to the interaction of the OH with the Brønsted acidic sites of the zeolite (Rivera et al. 2000).

The Eh measurements indicate the prevalence of oxidative conditions in the experiments with Pb, benzene, and brown coal. Structurally, brown coal consists of aromatic rings as well as functional groups such as carboxyls, phenolics and hydroxyls and carbonyls, which are able to buffer pH through protonation or deprotonation. The rise of pH in the case of Pb may be due to protonation or consumption of protons through ion-exchange with cations such as Na+ and K+ (Kwiatkowska et al. 2008), whereas the decrease of pH in the case of benzene may be due to deprotonation of the surface functional groups. Brown coal and zeolite were selected for the separate contaminants for further evaluation, considering their removal efficiencies.

The results of batch test series 2 are presented in Table 4. In the case of benzene, the mulch:diatomaceous earth and compost-mulch mixtures performed poorly (their removal efficiencies were below 40%). In the case of Pb, the pH was raised by all the materials while it was decreased in the case of benzene. The Eh decreased in both experiments with Pb and benzene.

Table 4

Results of batch test series 2

MaterialsPb
Benzene
Removal efficiency (%)pHEh (mV)Removal efficiency (%)pHEh (mV)
Initial solution – 3.2 (0.1) +303.0 (1.4) – 8.8 (0.2) +211.0 (14.9) 
Compost:brown coal (1:1) 93.6 (0.0) 5.6 (0.2) +207.3 (2.1) 86.8 (0.0) 5.9 (0.0) +81.5 (4.6) 
Compost:mulch (1:1) 92.1 (0.2) 5.3 (0.1) −190.7 (3.4) 36.5 (1.1) 5.2 (0.0) −174.7 (4.5) 
Mulch:diatomaceous earth (1:1) 91.5 (0.0) 5.0 (0.1) −68.3 (137.3) 29.1 (0.1) 4.6 (0.1) −67.5 (12.9) 
Compost:zeolite (1:1) 93.9 (0.1) 6.9 (0.1) −25.0 (52.9) 99.9 (0.0) 6.7 (0.2) −32.0 (44.8) 
MaterialsPb
Benzene
Removal efficiency (%)pHEh (mV)Removal efficiency (%)pHEh (mV)
Initial solution – 3.2 (0.1) +303.0 (1.4) – 8.8 (0.2) +211.0 (14.9) 
Compost:brown coal (1:1) 93.6 (0.0) 5.6 (0.2) +207.3 (2.1) 86.8 (0.0) 5.9 (0.0) +81.5 (4.6) 
Compost:mulch (1:1) 92.1 (0.2) 5.3 (0.1) −190.7 (3.4) 36.5 (1.1) 5.2 (0.0) −174.7 (4.5) 
Mulch:diatomaceous earth (1:1) 91.5 (0.0) 5.0 (0.1) −68.3 (137.3) 29.1 (0.1) 4.6 (0.1) −67.5 (12.9) 
Compost:zeolite (1:1) 93.9 (0.1) 6.9 (0.1) −25.0 (52.9) 99.9 (0.0) 6.7 (0.2) −32.0 (44.8) 

Removal efficiency, pH, and Eh represent averages of quadruplicate and triplicate measurements (± standard deviation) for Pb and benzene, respectively.

A comparison of the results of batch test 2 with batch test 1 suggests that some of the material mixtures had a positive effect while others had a negative effect on the removal of the contaminants. For instance, the addition of mulch to compost and diatomaceous earth gave better results for the removal of Pb than when the latter two were used alone. In the case of benzene, however lower removal was observed for the same material mixtures in comparison with the individual materials. This effect may be attributed to the mechanisms of removing the contaminants. For instance, mulch together with compost or diatomaceous earth were intended to remove benzene via sorption and biodegradation, but the contribution of the latter mechanism in the case of mulch was possibly negligible due to its ligniferous nature and the fact that the duration of the experiment was too short to obtain significant biodegradation. In the case of compost:brown coal and compost:zeolite mixtures, the obtained removal efficiencies may be attributed to the material dose. The removal efficiency recorded for the brown coal:compost mixture in the case of Pb was lower compared to when brown coal alone was used but higher when compost was used alone. Brown coal alone was demonstrated to be more effective in Pb removal than compost. Thus by reducing the amount of brown coal in the compost:brown coal mixture, the preceding statements are in general logical and could be expected. A similar explanation is given in the case of benzene for the same mixture (compost:brown coal) and also for the compost:zeolite mixture. Generally, the removal efficiencies obtained for the individual reactive materials were higher than for the mixtures. This may be due to some of the reasons mentioned as well as the total amount of the mixtures. The combination resulted in a reduction of the total amount of the mixtures of reactive materials in comparison to the total amount of individual materials used. This may have reduced the number of reactive sites available for contaminant removal. Based on the results, compost:brown coal and compost:zeolite were selected to investigate the effects of different mixing ratios on the removal of benzene and Pb in batch test series 3. The other reactive materials were not evaluated further.

The results of batch test series 3 are presented in Table 5. The pH was raised by all the materials investigated in the Pb experiments. However, the increase was higher for the compost:zeolite mixtures than the compost:brown coal mixtures. Moreover, between the same material mixtures, there was not much noticeable difference in the pH. The reasons for the observed results may be similar to those cited in batch test series 2 for the same material mixtures. A similar observation was made in terms of their effect on Eh; only in this case the Eh was reduced for all the materials tested. In the benzene experiments, the pH was lowered but with no noticeable difference for the different ratios for the different mixtures. The Eh, however, decreased in the compost:brown coal mixtures for both 1:3 and 1:5 ratios (due to microbial metabolism) but remained almost unchanged in the case of compost:zeolite mixtures (1:3 and 1:5). The reason for the inconsistencies between the Eh measured in this experiment with those of batch test series 2 is unclear.

Table 5

Results of batch test series 3

MaterialsPb
Benzene
Removal efficiency (%)pHEh (mV)Removal efficiency (%)pHEh (mV)
Initial solution – 3.2 (0.1) +317.6 (22.1) – 8.8 (0.4) +200.4 (4.9) 
Compost:brown coal (1:3) 99.8 (0.0) 5.8 (0.1) +231.4 (5.4) 86.7 (0.2) 5.9 (0.1) −133.9 (30.0) 
Compost:brown coal (1:5) 99.3 (0.0) 5.9 (0.1) +227.5 (4.2) 93.4 (0.1) 5.8 (0.1) −82.6 (10.1) 
Compost:zeolite (1:3) 97.9 (0.0) 6.8 (0.1) +272.4 (13.0) 99.9 (0.0) 7.2 (0.1) +205.9 (3.0) 
Compost:zeolite (1:5) 95.6 (0.1) 6.6 (0.1) +275.9 (13.3) 99.9 (0.0) 7.4 (0.0) +203.8 (1.9) 
MaterialsPb
Benzene
Removal efficiency (%)pHEh (mV)Removal efficiency (%)pHEh (mV)
Initial solution – 3.2 (0.1) +317.6 (22.1) – 8.8 (0.4) +200.4 (4.9) 
Compost:brown coal (1:3) 99.8 (0.0) 5.8 (0.1) +231.4 (5.4) 86.7 (0.2) 5.9 (0.1) −133.9 (30.0) 
Compost:brown coal (1:5) 99.3 (0.0) 5.9 (0.1) +227.5 (4.2) 93.4 (0.1) 5.8 (0.1) −82.6 (10.1) 
Compost:zeolite (1:3) 97.9 (0.0) 6.8 (0.1) +272.4 (13.0) 99.9 (0.0) 7.2 (0.1) +205.9 (3.0) 
Compost:zeolite (1:5) 95.6 (0.1) 6.6 (0.1) +275.9 (13.3) 99.9 (0.0) 7.4 (0.0) +203.8 (1.9) 

Removal efficiency, pH, and Eh represent averages of quadruplicate and triplicate measurements (± standard deviation) for Pb and benzene, respectively.

Compost combined with brown coal in a ratio of 1:5 was more effective at benzene removal than when combined in a ratio of 1:3. Increasing brown coal content in the mixture gave results similar to when brown coal alone was used. In the case of the compost:zeolite the removal efficiencies were the same for the different mixing ratios. A comparison of the removal efficiencies was made for the mixtures of reactive materials in order to select the appropriate mixing ratio (1:1, 1:3, and 1:5). In the case of Pb, the removal efficiencies obtained for the different mixing ratios for the different reactive material mixtures, i.e. compost:brown coal and compost:zeolite, were statistically different (α = 0.05). Therefore, compost:brown coal (1:3) and compost:zeolite (1:3) were selected. In the case of benzene, the removal efficiencies obtained for the compost:zeolite mixtures were all not statistically different whereas in the case of the compost:brown coal, the ratio (1:5) was found to be statistically different from the other ratios (i.e. 1:1 and 1:3 were not statistically different). Therefore, the compost:brown coal (1:5) was chosen. In the case of the compost:zeolite, the mixing ratio with the possibility of enhancing biodegradation (i.e. 1:1 based on the amount of compost) was selected.

The results of experiments to determine the mechanisms of Pb and benzene removal are presented in Figures 1(a) and 1(b), respectively. As shown in Figure 1(a), Pb was held mainly in the exchangeable fraction. The amount of Pb removed in this fraction was not significantly increased when compost was added to zeolite, but decreased significantly when compost was added to brown coal. This is reasonable because as shown in Table 1, brown coal has the highest cation-exchange capacity (CEC) among the three materials whereas compost and zeolite have similar CECs. With the exception of zeolite, the amount of Pb held in the carbonate fraction was low for all the reactive materials. This may be due to the amount of carbonates in the materials and/or the pH. A low proportion of the Pb was held within the reducible fraction (with Fe and Mn oxides). This may be due to the low quantity of these oxides in the materials or the low pH. At high pH, the surfaces of oxides are deprotonated thus favoring adsorption of cationic metals. Conversely, at low pH positive charges develop on the oxides, which disfavor the adsorption of cations. This is, however, dependent on the iso-electric point or point of zero charge (PZC), which can be defined as the pH at which the net total charge is zero. For Fe-oxides the PZC is high (between pH 7 and pH 9) (Mohamed & Antia 1998). This implies that the charge on these oxides under acidic and natural pH conditions is positive, thus favoring the adsorption of anions. On the contrary, the PZC found for most Mn-oxides are very low (Tan et al. 2008). A high proportion of Pb was found in the residual fraction, which comprised the association of Pb with OM, sulfides and other chemical forms not determined in the previous steps. As can be seen in Figure 1(a), the amount of Pb bound in this fraction increased when compost (increase of OM) was added to zeolite and brown coal. According to Mohamed & Antia (1998), OM have low PZC, therefore, they have a high capacity to adsorb cations under most natural conditions.

Figure 1

Percentage contribution of different processes/chemical fractions in the removal of (a) Pb and (b) benzene.

Figure 1

Percentage contribution of different processes/chemical fractions in the removal of (a) Pb and (b) benzene.

Ion-exchange mechanisms are reversible since the binding forces are relatively weak (Mohamed & Antia 1998). Thus, remobilization of Pb can occur and the presence of other cations may affect its uptake. The carbonate form is also loosely bound and susceptible to changes in environmental conditions such as pH and salinity. Metals in the reducible fraction can be released at pH lower than the uptake pH, and if the redox conditions change from oxic to anoxic. Metals bound to OM are also liberated under oxidizing conditions; however, they are generally strongly bound and would be released slowly (Tessier et al. 1979). As can be seen from the results, the addition of compost to the materials may provide long-term retention of Pb as it reduces the amount in the exchangeable phase and increases the amount strongly bound. It is important to note, however that the results of the sequential extraction analysis are only an indication of chemical speciation rather than a precise quantitative analysis as there are methodological limitations due to the restricted selectivity of extractants and operational conditions (Tessier et al. 1979).

Benzene was removed mainly by sorption as shown in Figure 1(b). The sorption of benzene was possibly onto the OM (represented by loss on ignition (LOI)) in the brown coal and compost-based materials as removal of petroleum hydrocarbons such as benzene by sorption is dependent, among other factors, on the LOI of the adsorbent. Natural zeolites generally have low LOI, thus they have low affinity for organic compounds such as benzene (Seifi et al. 2011). Considering the low LOI of the zeolite (Table 1), the removal of benzene by zeolite may be attributed to the diffusion of benzene molecules into zeolite intraparticle pores (Jousse & Auerbach 1997) or the weak interaction of the π-electrons of benzene with the zeolite terminal silanol (Si–OH) groups (Rungsirisakun et al. 2006). Biodegradation was mostly observed in samples with compost and tended to increase as the amount of compost increased. The low contribution of biodegradation may, however, be attributed to the recalcitrance of benzene to biodegradation due to the stability of its π-electron cloud (Weelink et al. 2010), the short duration of the experiment, the lack of oxygen (Vogt et al. 2011) or the effect of sorption on biodegradation (Kim et al. 2003).

The results of the permeability tests presented in Table 6 show that brown coal had a permeability of 4.2 × 10−5 m s−1 which is two orders of magnitude lower than that of zeolite (2.14 × 10−3 m s−1). The permeability slightly increased when compost was added to brown coal whereas it decreased for the compost:zeolite mixtures. In general, however, the permeability values obtained for all the materials in this study are similar to those reported in the literature (Moraci & Calabrò 2010), implying that these materials can be applied in PRBs, however, at sites with aquifer permeabilities that are reasonably lower.

Table 6

Permeabilities (Ki) of the selected materials

MaterialsRatioKi (m s−1)
Brown coal – 4.20 × 10−5 
Zeolite – 2.14 × 10−3 
Compost:brown coal 1:3 7.17 × 10−5 
Compost:zeolite 1:3 1.20 × 10−3 
Compost:brown coal 1:5 5.77 × 10−5 
Compost:zeolite 1:1 1.42 × 10−3 
MaterialsRatioKi (m s−1)
Brown coal – 4.20 × 10−5 
Zeolite – 2.14 × 10−3 
Compost:brown coal 1:3 7.17 × 10−5 
Compost:zeolite 1:3 1.20 × 10−3 
Compost:brown coal 1:5 5.77 × 10−5 
Compost:zeolite 1:1 1.42 × 10−3 

The results of the leaching tests are presented in Table 7. The toxic substances considered were mainly heavy metals. Among the materials, compost released the highest amounts of the metals considered. Generally, however, negligible amounts of heavy metals were leached from the studied materials when compared with the permissible limits according to the Polish Standards: the Decree of Minister of Environmental Protection on the conditions required when discharging wastewater to water and soil, and on substances particularly hazardous for aquatic environments (PMH 2007). This suggests that the selected materials are environmentally safe for use in a PRB, according to the leaching procedure used.

Table 7

Results of leaching test in comparison with the Polish standard values (PMH 2007)

Heavy metalPolish standard (μg/L)Amounts of heavy metals leached from the materials (μg/L)
CompostZeoliteBrown coal
Pb 500 10.0 <0.4 <0.4 
Cu 500 56.0 8.3 9.6 
Zn 2,000 140.0 23.0 19.0 
Ni 500 6.0 13.5 <0.4 
Heavy metalPolish standard (μg/L)Amounts of heavy metals leached from the materials (μg/L)
CompostZeoliteBrown coal
Pb 500 10.0 <0.4 <0.4 
Cu 500 56.0 8.3 9.6 
Zn 2,000 140.0 23.0 19.0 
Ni 500 6.0 13.5 <0.4 

CONCLUSION

The studied reactive materials (individual and mixtures) demonstrated different abilities in removing benzene and Pb2+ from the respective contaminated solutions. Among the individual materials, zeolite and brown coal were selected for both contaminants. The mixtures of compost:zeolite (1:3) and compost:brown coal (1:3) were selected for Pb2+, whereas compost:zeolite (1:1) and compost:brown coal (1:5) were selected for benzene. The mixtures of materials had either antagonistic or synergistic effect on Pb2+ and benzene removal, depending on the materials in the mixture. The Pb removal efficiencies of the selected reactive materials were either similar to or slightly lower than that of the control material, ZVI (99.9%). In the case of benzene, the removal efficiencies of the selected materials were higher than that of ZVI (55.7%). Generally, the selected materials changed the pH from acidic/alkaline to near neutral values and the redox conditions from highly oxidizing to moderately oxidizing or reducing conditions. Benzene removal occurred mainly via sorption (60–99%) onto the materials. Lead was weakly bound to the materials as 52–76% associated with exchangeable fraction of the materials. The permeabilities of the selected materials were in the range similar to those reported in the literature. The amounts of toxic substances (mainly heavy metals) leached from these materials, based on the TCLP-method 1311 were also below the regulatory limits according to the Polish Standards. Further studies are needed to test the selected materials under dynamic conditions with natural groundwater, and to investigate their longevity and the effects of groundwater constituents on their performance.

ACKNOWLEDGEMENTS

The research leading to these results has received funding from the European Community's Seventh Framework Program (FP7/2007-2013 under grant agreement no. 265063). The manuscript was improved by the insightful comments of anonymous reviewers from Water Science and Technology.

REFERENCES

REFERENCES
AFCEE
2008
Air Force Center for Environmental Excellence, Technical Protocol for Enhanced Anaerobic Bioremediation Using Permeable Mulch Biowalls and Bioreactors
.
Parsons Infrastructure & Technology Group, Inc.
,
Denver, CO
.
Ahmad
F.
McGuire
T. M.
Lee
R. S.
Becvar
E.
2007
Considerations for the design of organic mulch permeable reactive barriers
.
Remediation Journal
18
(
1
),
59
72
.
Brady
N. C.
2001
The Nature and Properties of Soil
, 10th edn,
Macmillan Publishing Co.
,
New York
,
USA
.
Carey
M. A.
Fretwell
B. A.
Mosley
N. G.
Smith
J. W. N.
2002
Guidance on the Use of Permeable Reactive Barriers for Remediating Contaminated Groundwater
.
National Groundwater and Contaminated Land Centre Report NC/01/51
,
UK Environment Agency
,
Bristol
, p.
140
.
Head
K. H.
Keeton
G. P.
2008
Manual of Soil Laboratory Testing, Volume 2: Permeability, Shear Strength & Compressibility Tests
.
Whittles Publishing
,
UK
.
Henderson
A. D.
Demond
A. H.
2007
Long-term performance of zero-valent iron permeable reactive barriers: a critical review
.
Environmental Engineering Science
24
(
4
),
401
423
.
Herbel
M. J.
Suarez
D. L.
Goldberg
S.
Gao
S.
2007
Evaluation of chemical amendments for pH and redox stabilization in aqueous suspensions of three California soils
.
Soil Science Society of America Journal
71
(
3
),
927
939
.
Khraisheh
M. A. M.
Al-Degs
Y. S.
McMinn
W. A. M.
2004
Remediation of wastewater containing heavy metals using raw and modified diatomite
.
Chemical Engineering Journal
99
,
177
184
.
Kim
S. B.
Hwang
I.
Kim
D. J.
Lee
S.
Jury
W. A.
2003
Effect of sorption on benzene biodegradation in sandy soil
.
Environmental Toxicology and Chemistry
22
(
10
),
2306
2311
.
Mohamed
A. M. O.
Antia
H. E.
1998
Geo-environmental Engineering
.
Elsevier, Amsterdam, The Netherlands
.
O'Carroll
D.
Sleep
B.
Krol
M.
Boparai
H.
Kocur
C.
2013
Nanoscale zero valent iron and bimetallic particles for contaminated site remediation
.
Advances in Water Resources
51
,
104
122
.
PMH
2007
Polish Ministry of Health Regulation: DZ.U.2007.61.417 of the Polish Ministry of Health 29th of March 2007 on the quality of water for human consumption. (In Polish.)
Rivera
A.
Rodriõguez-Fuentes
G.
Altshuler
E.
2000
Time evolution of a natural clinoptilolite in aqueous medium: conductivity and pH experiments
.
Microporous and Mesoporous Materials
40
(
1
),
173
179
.
Rungsirisakun
R.
Nanok
T.
Probst
M.
Limtrakul
J.
2006
Adsorption and diffusion of benzene in the nanoporous catalysts FAU, ZSM-5 and MCM-22: a molecular dynamics study
.
Journal of Molecular Graphics and Modelling
24
(
5
),
373
382
.
Seifi
L.
Torabian
A.
Kazemian
H.
Bidhendi
G. N.
Azimi
A. A.
Nazmara
S.
AliMohammadi
M.
2011
Adsorption of BTEX on surfactant modified granulated natural zeolite nanoparticles: parameters optimizing by applying Taguchi experimental design method
.
CLEAN–Soil, Air, Water
39
(
10
),
939
948
.
Simantiraki
F.
Kollias
C. G.
Maratos
D.
Hahladakis
J.
Gidarakos
E.
2013
Qualitative determination and application of sewage sludge and municipal solid waste compost for BTEX removal from groundwater
.
Journal of Environmental Chemical Engineering
1
,
9
17
.
Singh
C. K.
Sahu
J. N.
Mahalik
K. K.
Mohanty
C. R.
Mohan
B. R.
Meikap
B. C.
2008
Studies on the removal of Pb (II) from wastewater by activated carbon developed from Tamarind wood activated with sulphuric acid
.
Journal of Hazardous Materials
153
(
1
),
221
228
.
Suponik
T.
Blanco
M.
2014
Removal of heavy metals from groundwater affected by acid mine drainage
.
Physicochemical Problems of Mineral Processing
50
(
1
),
359
372
.
Tessier
A.
Campbell
P. G. E.
Bisson
M.
1979
Sequential extraction procedure for the speciation of particulate trace metals
.
Analytical Chemistry
51
(
7
),
844
851
.
USEPA
1992
Method 1311–Toxicity Characteristic Leaching Procedure (TCLP
). In:
Test Methods for Evaluating Solid Waste, Physical and Chemical Methods, SW-846
.
USEPA
,
Washington, DC
.
Vogt
C.
Kleinsteuber
S.
Richnow
H. H.
2011
Anaerobic benzene degradation by bacteria
.
Microbial Biotechnology
4
(
6
),
710
724
.
Weelink
S. A. B.
van Eekert
M. H. A.
Stams
A. J. M.
2010
Degradation of BTEX by anaerobic bacteria: physiology and application
.
Reviews in Environmental Science and Bio/Technology
9
(
4
),
359
385
.