Physical-Chemical Permeable Reactive Barriers (Design Concept)

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Chromium Sources

Chromium (atomic number 24, atomic weight 51.996 g/mole) was discovered by a French chemist Louis Vauquelin in 1797. Vauquelin gave the element the Greek name ‘χρωμα’ (‘chroma’) which means colour due to the many different colours found in its compounds (Mohana and Pittman Jr, 2006). The gemstones ‘emerald’ and ‘ruby’ owe their colors to traces of chromium in the matrix. Chromium is the earth’s twenty-first most abundant element detected at a concentration of approximately 122 mg per kg of earths crust. Among the transitional metals, it is the sixth most abundant element. Notably, chromium does not occur in nature in pure elemental form, but is rather bonded in complex mineral forms. Chromium occurs in nature predominantly in the trivalent form (Cr(III)) mostly as chromite (FeOCr2O3) and crocoite (PbCrO4) in granitic rocks, serpentine rocks, and coal (Hintze, 1930; Merian, 1984). Small amounts of chromium in the hexavalent state (Cr(VI)) occur in silicate rich groundwater associated with Tertiary and Quaternary Alluvium filled basins. Continuous hydrolysis of silicates in the old alluvial sediments raises the pH of the water causing oxidation of Cr(III) to Cr(VI) (Robertson, 1975). Cr(VI) is also released into the atmosphere from forest fires, burning of coal, volcanic eruptions, automobile exhaust, and combustion of chromium containing materials (Merian, 1984; Xing and Okrent, 1993). The elemental form Cr(0) is also possible although it oxidizes quickly upon exposure to air.

Environmental and Health Effects

Hexavalent and trivalent chromium compounds differ in their health and environmental effects. Cr(VI) is toxic, carcinogenic and mutagenic to animals as well as humans and is associated with decreased plant growth and changes in plant morphology (Rosko et al., 1977, Silverberg, et al., 1977). The biotoxicity of Cr(VI) is largely due to its high reactivity, its ability to penetrate biological membranes as well as its high oxidizing capabilities (NAS, 1974). The natural intracellular Cr(VI) reduction pathway may involve an acceptance of electrons from organic electron donors such as NAD(P)H resulting in the formation of the transitory Cr(V) state (Horitsu et al., 1990). In humans and other mammals, acute exposure to Cr(VI) produces several health risks including allergic dermatitis, ulceration of the skin, irritation of the mucous membranes, nasal septum, renal tubular necrosis, and increase risk of respiratory tract infections. Super-active ionisation of water may result in the formation of the free radical (OH● ) which in turn results in excessive DNA damage (Flessel, 1979). Chronic exposure results in carcinogenesis and teratogenesis (abortions and premature still births) in mammals. Due to these and other observed toxic effects, the World Health Organisation (WHO) has set the maximum acceptable concentration of chromium in drinking water to 0.05 mg/L (50 µg/L) (Kiilunen, 1994; Lu and Yang, 1995; ACGIH, 2004).

Chemical Properties

Chromium can achieve nine oxidation states ranging from -2 to +6. Two of these, +3 and +6, are the stable forms found in the environment. The tetravalent [Cr(IV)] and pentavalent [Cr(V)] quickly reduces to Cr(III) and oxidizes to Cr(VI), respectively, in the presence of reducing or oxidising agents. Among all the oxidation states, Cr(III) is the most stable, it resides in the lowest energy trough among the oxidation states. The negative standard potential (Eo ) of the Cr(III)/Cr(II) metal ion couple signifies that Cr(II) is readily oxidized to Cr(III), and Cr(II) species are stable only in the absence of any oxidant (anaerobic conditions) (Kotas and Stasicka, 2000). In the aquatic environment, the redox potential of the medium affects the oxidation state of chromium where as the pH affects its complexation with anionic forms including the hydroxyl ion (OH- ) (Figure 2-2). This figure shows the predorminance of the insoluble form [Cr(OH)3(s)] in the pH range 5.5-10.5 under natural redox conditions (Eh ranging from -0.4 +0.6V). This correlates with the area where the majority of biological reactions occur. Figure 2-2 is adapted from Ball and Nordstrom (1998); Richard and Bourg (1991); Nieboer and Jusys (1988) and Rai et al. (1987, 1989).

Chemical Reactive Barriers

Several types of treatment walls have been studied to attenuate the movement of metals in groundwater at contaminated sites. Trench materials that have been investigated include zeolite, hydroxyapatite, elemental iron, and limestone (Vidic and Pohland, 1996). Elemental iron has been tested for chromium (VI) reduction and other inorganic contaminants (Powell et al., 1995) and limestone for lead precipitation and adsorption (Evanko and Dzombak, 1997) Permeable reactive barriers are an emerging alternative to traditional pump-and-treat systems for groundwater remediation. Such barriers are typically constructed from highly impermeable emplacements of materials such as grouts, slurries, or sheet pilings to form a subsurface “wall.” Permeable reactive barriers are created by intercepting a plume of contaminated groundwater with a permeable reactive material.

Biological Permeable Reactive Barriers (BPRB)

These are PRBs specifically designed to utilise microorganisms in the treatment processes. A typical design comprises of a double-layer with an aeration zone followed by the bioremediation zone. One such system was evaluated against the removal of methyl-tert-butyl-ether (MTBE) contaminated groundwater (Figure 2-4) (Liu et al, 2006). The aeration in this case was achieved chemically by the oxidation of calcium peroxide (CaO2) to release oxygen into the medium. Other growth nutrients were added to encourage the growth of MTBE degrading organisms in the second layer. Notably, inorganic salts such as potassium dihydrogen phosphate (KH2PO4) and ammonium sulphate ((NH4)2SO4) can act as buffers against pH changes caused by the oxidation of CaO2 into carbonates (CO3 2-). Thus, nutrients added in the second layer must include the phosphate buffer for the proper functioning of the barrier.

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Diversity of Cr(VI) Reducing Microorganisms

Microbial Cr(VI) reduction was first reported in the late 1970s when Romanenko and Koren’Kov (1977) observed Cr(VI) reduction capability in Pseudomonas sp. grown under anaerobic conditions. Since then, several researchers have isolated new microorganisms that catalyse Cr(VI) reduction under varying conditions (Ackerley et al., 2004; Chirwa and Wang, 1997a; Ohtake et al. 1990; Ganguli and Tripathi, 2002; Suzuki et al., 1992; Ramírez-Ramírez et al., 2004; Baldi et al., 1990). Lately, genetic sequences of 16S rDNA have been used to supplement the conventional methods of species identification and characterisation (Blackall et al.,1998; Molokwane et al., 2008; Molokwane and Chirwa, 2009).
This allows identification of a wide range of organisms which are unculturable using the conventional solid agar culturing methods. It also helps uncover species that have not been identified before. The cumulative list of known Cr(VI) reducing bacteria and their growth conditions is shown in Table 2-1. Table 2-1 illustrates a number of known chromium reducing bacteria. Most of the bacterial species were isolated from chromium (VI) contaminated environments (i.e. sediments, wastewater treatment plants, soil etc). Although earlier isolates grew mostly on aliphatic carbon sources, later studies have shown diversity in the preferred carbon sources and electron donors. For example, consortium cultures were shown to grow in the absence of organic carbon sources – utilising only bicarbonate (HCO3 – ) as the carbon source (Molokwane and Chirwa, 2009)


  • Synopsis
  • Declaration
  • Dedications
  • Acknowledgements
  • List of Figures
  • List of Tables
  • List of Nomenclature
    • 1.1 Background
    • 1.2 Unique Methods
    • 1.3 Objectives
    • 1.4 Main Findings
    • 2.1 Chromium Sources
    • 2.2 Chromium Uses and Pollution
    • 2.3 Environmental and Health Effects
    • 2.4 Chemical Properties
    • 2.5 Pollution Remediation Strategies
    • 2.5.1 Physical-Chemical Treatment Methods
    • 2.5.2 Chemical Reactive Barriers
    • 2.5.3 Physical-Chemical Permeable Reactive Barriers (Design Concept)
    • 2.5.4 Biological Permeable Reactive Barriers
    • 2.6 Physical-Chemical Treatment Methods
    • 2.6.1 Microbial Resistance to Cr(VI) Toxicity
    • 2.6.2 Diversity of Chromium Reducing Microorganisms
    • 2.6.3 Cr(VI) Reduction Pathways
    • 2.7 Current and Future Biotechnology Solutions
    • 2.7.1 Suspended Culture Systems
    • 2.7.2 Attached Growth Systems
    • 2.7.3 In situ Inoculation
    • 2.7.4 Bioaugmentation
    • 2.8 Chapter Summary
    • 3.1 Source of Cr(VI) Reducing Organisms
    • 3.2 Mineral Media
    • 3.3 Culture Isolation
    • 3.4 Gram Staining
    • 3.5 Microbial Culture Characterisation
    • 3.5.1 Aerobic Culture
    • 3.5.2 Anaerobic Culture
    • 3.6 Cr(VI) Reduction Experiments
    • 3.6.1 Aerobic Batch Experiments
    • 3.6.2 Anaerobic Batch Culture Experiments
    • 3.6.3 Microcosm Reactor Studies
    • 3.6.4 Mesocosm Reactor Studies
    • 3.7 Analytical Methods
    • 3.7.1 Elemental Analysis
    • 3.7.2 Cr(VI) and Total Chromium
    • 3.7.3 Viable Biomass
    • 3.7.4 Total Biomass (Suspended cells)
    • 3.7.5 Cr(VI) Reduction Activity
    • 4.1 Modelling Methodology
    • 4.2 Mixed Culture Performance
    • 4.2.1 Biotic versus Abiotic
    • 4.2.2 Cr(VI) Reduction under Aerobic Conditions
    • 4.2.3 Cr(VI) Reduction under Anaerobic Conditions
    • 4.2.4 Decisions from Observed Trends
    • 4.3 Enzymatic Cr(VI) Reduction Capacity of Cells
    • 4.4 Cr(VI) Reduction Capacity of Cells
    • 4.5 Parameter Determination
    • 4.5.1 Aerobic Batch Kinetics
    • 4.5.2 Anaerobic Batch Kinetics
    • 4.6 Sensitivity Analysis
    • 4.7 Chapter Summary
    • 5.1 Microcosm Study Conceptual Basis
    • 5.2 Performance of Vadose System Microcosm
    • 5.2.1 Cr(VI) Removal Efficiency
    • 5.2.2 Cr(VI) Speciation in the Vadose Microcosm Reactors
    • 5.2.3 Microbial Culture Dynamics in Vadose Systems
    • 5.3 Performance of the Main Aquifer Microcosm
    • 5.3.1 Evaluation of the Abiotic Process in the Microcosms
    • 5.3.2 Cr(VI) Reduction by Inoculated Natural Soil without Carbon Source
    • 5.3.3 Cr(VI) Reduction by Inoculated Natural Soil with Added Carbon Source
    • 5.3.4 Cumulative Cr(VI) Reduction in the Microcosm Systems
    • 5.3.5 Performance Summary
    • 5.4 Microbial Culture Dynamics in Aquifer Media Microcosm Reactor
    • 5.4.1 Analysis under Anaerobic Conditions
    • 5.4.2 Characteristics of Microorganisms in the Microcosm after 45 days
    • 5.5 Simulation of Cr(VI) Reduction in Microcosm Systems
    • 5.5.1 Model Description – Advection/Reduction Model
    • 5.5.2 Simulation of Control Conditions
    • 5.5.3 Evaluation of the Effect of Carbon Source using the Model
    • 5.6 Summary of Parameters
    • 5.7 Chapter Summary
    • 6.1 Background
    • 6.2 Simulation Reactive Barrier: Mesocosm Reactor
    • 6.3 Barrier Performance evaluation (Quantitative)
    • 6.4 Performance Evaluation (Qualitative)
    • 6.5 Spatial Variation at Discrete Time
    • 6.6 Chapter Summary


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