Contaminated land

In exceptional cases, the rocks, minerals and soils of the land surface contain compounds that generate natural chemical hazards. Uranium (U) and potassium (K), common elements in granitic rocks, are inherently unstable because of their radioactivity (see Section 2.8) and radioactive decay of isotopes of uranium to form radon (Rn) gas can be a health hazard (Box 4.13). Some chemicals, such as herbicides and pesticides, are present in soils because we put them there intentionally. Other chemicals arrive in soils because of unintentional or unavoidable releases, for example the byproducts of combustion in car engines. Exotic or synthetic chemicals (see Section 1.4) are ubiquitous in the environment today. In the USA alone, 20 X 109kg of ethylene and 1 X 109kg of benzene were produced in 1996. These compounds are the feedstock chemicals for synthetic processes that generate a huge array of synthetic organic compounds. Significant proportions of these compounds are released, usually by accident, into the atmosphere, hydrosphere and soils. There are a number of routes through which these compounds might reach soils, including aerial deposition, spillage, leaching and movement in groundwater. Thus, it is possible for some chemicals to reach areas remote from the site of compound production or use (see Section 7.4).

Where a substance is present in the environment at a concentration above natural background levels the term 'contaminant' is used. The term 'pollutant' is used when a contaminant can be shown to have a deleterious effect on the environment. Contaminants are broadly divided into two classes: (i) organic contaminants with chemical structures based on carbon, for example benzene; and (ii) inorganic contaminants, for example asbestos or lead, which may be in compound, molecular or elemental form. In the following section we concentrate on organic contaminants in soils; the inorganic contaminants mercury and arsenic are discussed in the context of water chemistry in Sections 5.6 and 5.7.2. This is an artificial division because organic and inorganic contaminants affect both soils and water. However, the chemical principles involved are similar in each case.

4.10.1 Organic contaminants in soils

Organic contaminants are compounds with a carbon skeleton, usually associated with atoms of hydrogen, oxygen, nitrogen, phosphorus and sulphur (see Section

Box 4.13 Radon gas: a natural environmental hazard

Radon gas (Rn) is a radioactive decay product of uranium (U), an element present in crustal oxides (e.g. uraninite — UO2), silicates (e.g. zircon—ZrSiO4) and phosphates (e.g. apatite — Ca5(PO4)3 (OH, F, Cl)). These minerals are common in granitic rocks, but are also to be found in other rocks, sediments and soils. Uranium decays to radium (Ra), which in turn decays to radon (Rn) (see Section 2.8). The isotope 222Rn exists for just a few days before it also decays, but, if surface rocks and soils are permeable, this gas has time to migrate into caves, mines and houses. Here, radon or its radioactive decay products may be inhaled by humans. The initial decay products, isotopes of polonium, 218Po and 216Po, are non-gaseous and stick to particles in the air. When inhaled they lodge in the lungs' bronchi, where they decay—ultimately to stable isotopes of lead (Pb) — by ejecting a radiation particles (see Section 2.8) in all directions, including into the cells lining the bronchi. This radiation causes cell mutation and ultimately lung cancer. Having said this, radon is estimated to cause only about one in 20 cases of lung cancer in Britain, smoking being a much more serious cause.

Radon gas is invisible, odourless and tasteless. It is therefore difficult to detect and its danger is worsened by containment in buildings. Radon is responsible for about half the annual radiation dose to people in England, compared with less than 1% from fallout, occupational exposures and discharges from nuclear power stations.

In England, about 100000 homes are above the government-adopted 'action level' of 200becquerels m-3. Various relatively low-cost steps can be taken to minimize home radon levels, including better underfloor sealing and/or ventilation. Building homes in low-radon areas remains an obvious long-term strategy, but such simple solutions are not always applicable, because of either geographic or economic constraints. For example, bauxite processing in Jamaica produces large amounts of waste red mud. This material binds together strongly when dry, and is readily available as a cheap building material. Unfortunately, the red mud also contains higher levels of 238U than most local soils. These cheap bricks are thus radioactive from the decay of 238U and a potential source of radon. Only careful consideration of the health risks in comparison with the economic benefits can decide whether red mud will be used as a building material.

2.7). These atoms may form an integral part of the molecule, alternatively they may be present in functional groups (see Section 2.7.1). Functional groups impart specific chemical properties on a molecule. Hydroxyl (-OH) and carboxyl (-COOH) functional groups increase polarity, making a molecule more soluble (Box 4.14), while -COOH also makes the molecule acidic due to dissociation of H+ (see Section 2.7.1). The various structural forms of organic molecules, for example saturated and unsaturated chains and rings (see Section 2.7), result in a diverse range of organic contaminants (Fig. 4.26).

Once in the soil environment organic contaminants may move in, or interact with, the soil atmosphere, soil water, mineral fractions and organic matter. Ultimately, however, the organic contaminants will either dissipate or persist (Fig. 4.27). Compounds persist if they are of low volatility, low solubility (Box 4.14) or have a molecular structure that resists degradation. Conversely, if compounds are highly volatile, highly soluble or are easily degraded, they will be destroyed or lost to other environments (e.g. the atmosphere or hydrosphere). The volatility of a compound is controlled by the vapour pressure (Box 4.14), while solubility is governed principally by polarity—a function of molecular structure, molecular weight and functional groups. Degradation results from both biological and abiological mechanisms, although given the profusion of microorganisms in soil (a gram of soil typically contains 106-109 culturable microorganisms) the potential for biodegradation is high. Abiological degradation occurs by hydrolysis, reduction, oxidation and photo-oxidation.

Organic contaminants interact mainly with either the mineral or organic components of soils. Two types of non-reactive interaction are possible: (i) adsorption (a surface phenomenon); and (ii) entrapment within the soil minerals or components (Fig. 4.28). The nature and extent of interaction is dependent on the properties of the molecule—its aqueous solubility, vapour pressure and hydro-phobicity (Box 4.14)—but also its concentration and the properties of the soil. Soil factors include the amount and type of soil organic matter, the clay content and mineralogy, pore size and pore structure, and the microorganisms present. The net result of both adsorption and entrapment is a decrease in both the biological and chemical availability of soil-associated contaminants with time. Thus, the proportion of compound in the 'available' fraction decreases with time (Fig. 4.29), while the proportion of compound in the 'non-available' fraction increases with time (Fig. 4.29). This process known as 'ageing', may cause a decrease in the rate, and the extent, of degradation an organic contaminant suffers in the soil environment. Organic contaminants attached to soil particles exist in four fractions distinguished by the ease with which they can be released or desorbed from

Fig. 4.26 Examples of organic contaminants.

Box 4.14 Physical and chemical properties that dictate the fate of organic contaminants

Polarity

Polarity is a measure of how skewed the electron distributions are within a molecule. We have seen previously that water (H2O) has an oxygen atom with a negative dipole (imbalance in charge) and two hydrogen atoms with positive dipoles (Box 4.1). These dipoles arise because oxygen is more electronegative (Box 4.2) and therefore draws more of the electron density in the oxygen-hydrogen bonds closer to itself. Conversely, hydrogen being more electropositive assumes a positive dipole in the molecule. Other molecules display polarity as shown below (Fig. 1).

Solubility

The solubility of a compound depends on its polarity. If water is the solvent, the compound will need to be of similar polarity to be dissolved in it, for example ethanol (CH3CH2OH). In ethanol the OH-group is polar and therefore the molecule has polar character (Fig. 1b). If the compound is nonpolar, for example ethane, it will not dissolve in polar water (Fig. 1c) because opposites do not mix. Ethanol will, however, dissolve in a non-polar solvent, for example hexane. Thus, the term 'solubility' should always be used with clarification of the solvent, for example 'aqueous solubility'. Solubility is expressed as the mass of a substance that will dissolve in a given volume of solvent, for example mgl-1.

Hydrophobicity

The hydrophobicity of a compound is a measure of its affinity for water. If a compound will not readily partition into the aqueous phase it is known as hydrophobic (fearing water). A converse term, often used when talking about organic compounds, is lipophilicity. If a compound is lipophilic it loves lipid (fat). These terms are used interchangeably when the environmental fate of organic contaminants is discussed. Hydrophobicity is measured as a partition coefficient between octanol/water (K0W). Octanol is chosen to represent lipid (fat) because it is an experimentally reproducible compound. K0W values are determined by allowing the compound of interest to equilibrate between the two phases: water and octanol. After equilibration the concentrations in each phase are determined and their ratio calculated (Fig. 2). Most organic compounds are very hydrophobic such that K0W values are often in the range 10000 to 1000000. In order to work with smaller numbers the log of the K0W value is usually used, such that the values are typically between 4 and 6.

Vapour pressure and volatility

Water vapour condenses to liquid water when cooled. At a constant pressure liquid water appears abruptly at a specific temperature and the pressure is known as the vapour pressure for this temperature. The vapour pressure of a compound is a measure of its volatility, i.e. its tendency to evaporate into the gas phase. A compound with high vapour pressure is described as volatile, while a compound with low vapour pressure is described as non-volatile. The vapour

Ethanol

H3C - CH3 Ethane

Fig. 1 Dipole distribution in (a) water (polar), (b) ethanol (polar) and (c) ethane (non-polar).

(continued)

After equilibration

After equilibration

Octanol (12 Xs)

X = Compound of interest

Octanol (12 Xs)

[water]

log Ko log 4

0.60

X = Compound of interest

Fig. 2 Schematic diagram illustrating experimental equilibration of compound X between water and octanol and determination of the partition coefficient (KOW).

pressure of water can be used as a 'yardstick' for comparison. At 25°C water has a vapour pressure of 3.17kPa. This means that liquid water is at equilibrium with water vapour at 25°C if the pressure is 3.17kPa. As temperature changes so too does the equilibrium vapour pressure. At 100°C the vapour pressure of water is 101.3kPa, equal to normal atmospheric pressure. At this temperature the vapour pressure of water molecules causes rapid formation of bubbles (boiling). Boiling occurs when the vapour pressure of the molecules in water equals (or exceeds) local atmospheric pressure and water vapour escapes to the atmosphere as steam.

Parent organic-Biodegradation-►

contaminant-Abiotic degradation

^ÎSSSSl. Volatilization

Parent organic-Biodegradation-►

contaminant-Abiotic degradation

Leaching

Groundwater

^ÎSSSSl. Volatilization

^Uptake in " 'plants

Metabolite

Water table

Fig. 4.27 Schematic diagram showing the fate of organic contaminants in soils.

Octahedral Sheet
Fig. 4.28 Interaction between organic contaminants and soil components. Of the inorganic components, clay minerals have the most potential to react with organic contaminants. T, tetrahedral sheet; O, octahedral sheet (see Fig. 4.12).
Fig. 4.29 Bioavailability of organic contaminants in soils as a function of time.

the particles: (i) a rapidly desorbable fraction; (ii) a slowly desorbable fraction; (iii) a very slowly desorbable fraction; and (iv) non-extractable (bound) residues. The non-extractable residue is the fraction of an organic compound (or its metabolites) that persists in the matrix following an extraction process that has not substantially changed either the compounds or the matrix. Bound residues usually represent an extreme end-member of ageing. The size of each of these fractions depends mainly on the length of contact time between the soil and contaminant.

Although the attraction between mineral surfaces and organic contaminants in soils can present a problem in cleaning up contaminated land, it can also be put to use by environmental chemists as a way of cleaning up contaminated water (Box 4.15).

4.10.2 Degradation of organic contaminants in soils

Degradation of organic contaminants in soils occurs typically by either chemical or microbiological pathways. The effectiveness of degradation is largely determined by the contaminant availability (Section 4.10.1), although the degree of persistence is influenced by the chemical structure of the contaminant. If the chemical structure of the contaminant is similar to that of a natural substance it is more likely to be degradable. In general, if the structure is complex the rate of degradation is slower and is more likely to be incomplete. Resistance to biodegradation is known as recalcitrance, which is caused by a number of factors:

1 Specific microbes or enzymes required for degradation may not be present in the soil.

2 Unusual or complex substitutions in a molecule (e.g. chlorine (Cl), bromine (Br) or fluorine (F)), or unusual bonds or bond sequences, may 'confuse' microorganisms that would otherwise recognize the molecule as a substrate (Box

3 A high degree of aromaticity (see Section 2.7), i.e. strongly bonded structures based on a number of fused benzene rings, results in molecules that are difficult to break down.

4 Large, complex and heavy molecules tend to be less water soluble, and therefore are physically unavailable to microorganisms that use intracellular degradation processes.

Biodegradation of organic contaminants can be carried out by a single micro-bial species in pure cultures, but in nature the efforts of a mixture of microbes (a consortium) are usually required. The degradation process ranges from only minor structural changes to the parent molecule, known as primary degradation, to complete conversion to mineral constituents, for example CO2 or H2O, and termed mineralization:

Phenol

Catechol

Phenol

Catechol eqn. 4.20

Box 4.15 Use of clay catalysts in clean up of environmental contamination

The interaction between some organic contaminants and mineral surfaces has recently attracted attention as a way of cleaning up contaminants in natural waters. The large cation exchange capacity of smectite clay minerals (Section 4.5.2), in particular, has prompted research into their use as a catalyst, i.e. a substance that alters the rate of a chemical reaction without itself changing. Clay catalysts have potential applications as adsorbents to treat contaminated natural waters or soils.

The compound 2,3,7,8-tetrachlorodibenzo-p-dioxin is one of the most toxic priority pollutants on the US Environmental Protection Agency's list. Dioxin compounds act as nerve poisons and are extremely toxic. There is no lower limit at which dioxins are considered safe in natural environments. The destruction of dioxins by biological, chemical or thermal means is costly, not least because their low (but highly significant) concentrations are dispersed in large volumes of other (benign) material. Thus large volumes of material must be treated in dioxin destruction processes.

It is desirable to concentrate soluble contaminants like dioxin by adsorption on to a solid before destruction. The optimal solid adsorbent should be cheap, benign, recyclable and easy to handle and have a high affinity for — and be highly selective to - the contaminant. Finely ground activated carbon and charcoal have been used as adsorbents but they suffer from oxidation during thermal destruction of the contaminant, making them non-recyclable and expensive.

Smectite clay catalysts are potential alternative adsorbents, although some modifications of the natural mineral are necessary. Interlayer sites in smectite dehydrate at temperatures above 200°C, collapsing to an illitic structure. Since the ionexchange capacity of smectite centres on the interlayer site, collapse must be prevented if clay catalysts are to be used in thermal treatments of chemical organic toxins. The intercalation of thermally stable cations, which act as molecular props or pillars, is one method of keeping the interlayer sites open in the absence of a solvent like water (Fig. 1). Various pillaring agents can be used, for example the polynuclear hydroxyaluminium cation (Al13O4(OH)2+), which is stable above 500°C.

The pillar also increases the internal surface area of the interlayer site, making it more effective as an adsorbent. Moreover, by introducing cationic props of different sizes and spacings (spacing is determined by the radius of the hydrated cation and the charge), it is possible to vary the size of spaces between props. It is thus possible to manufacture highly specific molecular sieves, which could be used to trap large ions or molecules (e.g. organic contaminants) whilst letting smaller, benign molecules pass through.

Smectite clays do not have a strong affinity for soluble organic contaminants and this is improved using a surface-active agent (surfactant). A surfactant is a substance introduced into a liquid to affect (usually to increase) its spreading or wetting properties (i.e. those properties controlled by surface tension). Detergents and soaps are examples of surfactants. A soap molecule has two features essential for its cleansing action: a long, non-polar hydrocarbon chain and a

Octahedral Sheet
Fig. 1 Schematic diagram showing props or pillars in the interlayer position in smectite clay. T, tetrahedral sheet; O, octahedral sheet.

(continued)

polar carboxylate group, for example sodium octadecanoate (sodium stearate).

CH3(CH2 )16—COCTNa+ (non-polar hydrocarbon tail) (carboxylate head)

The polar carboxylate head dissolves in water, while the long, non-polar hydrocarbon-chain tail is hydrophobic (Box 4.14) but mixes well with greasy (lipophilic, Box 4.14) substances, effectively floating the grease into solution in a sheath of COO- groups.

Surfactant-coated interlayer sites in smectite have similar properties to detergents. A hydrophobic molecule or compound such as dioxin or other chlorinated phenol is attracted to the hydrophobic hydrocarbon tail projecting from the interlayer surface and immobilized.

The modified smectite thus has good affinity and selectivity for its target contaminant and is thermally stable, recyclable and economic to use (Fig. 2).

Pillared clay

Fig. 2 Scheme for recycling surfactant-modified pillared clay mineral adsorbents during thermal treatment of toxicant. From Michot and Pinnavaia (1991), with kind permission from the Clay Minerals Society.

Box 4.16 Mechanisms of microbial degradation and transformation of organic contaminants

Mineralization

mineralization

Phenol mineralization

Phenol eqn. 1

Complete biodégradation of an organic contaminant into its inorganic components for example CO2 and H2O.

Polymerization

Cl Cl Cl Cl

Cl Cl Cl Cl

/ \ / \ polymerization // \ Cl-(' N)—OH Cl-(' N)—OH-Cl-< )— O -f > -OH

Cl Cl Cl Cl

Pentachlorophenol

Cl Cl Cl Cl

Para-pentachlorophenoxy-tetrachlorophenol eqn. 2

A metabolic transformation that involves the coupling or bonding of small molecules to form polymers. In equation 2, two pentachlorophenol molecules polymerize to form para-pentachlorophenoxy-tetrachlorophenol.

Detoxification

Cl Xl

Cl detoxification

Cl Xl

Pentachlorophenol

OCH3

OCH3

Pentachloroanisole eqn. 3

Detoxification involves a chemical change to a molecule. In equation 3, pentachlorophenol (PCP), a powerful biocide used in wood preservative, undergoes O-methylation. This transforms PCP to a far less toxic compound pentachloroanisole. Detoxification by one group of microorganisms often allows other organisms to continue biodegradation.

(continued)

Co-metabolism

// co-metabolism

Biphenyl

// co-metabolism

Biphenyl

Although many aromatic compounds are subject to microbial degradation, the presence of substituents (e.g. Cl, Br, CH3) on the molecule can result in an increase in their resistance to biodegradation. In these instances microbes that would normally recognize the molecule as a substrate fail to do so because of the presence of the novel substituents, and hence do not produce enzymes to destroy them. For example, biphenyl (unsubstituted) is readily degraded. However, the polychlorinated biphenyls (PCBs), for example 2,3,2',4'-tetrachlorinated biphenyl, are recalcitrant. Interestingly where biphenyls and PCBs are present together, both are degraded (eqn. 4). This is because the microbes recognize the biphenyl and produce enzymes to degrade it. The same enzymes degrade the PCBs and thus both contaminants are removed. This is co-metabolism, a process that accounts for the degradation of many xenobiotic (foreign to life) compounds.

Accumulation on or within microbes

fjf accumulation

Cl Cl

PCB (in organism)

PCB (in organism)

Certain microorganisms, particularly those with high lipid (fat) contents, can absorb water-insoluble chemicals, such as PCBs. Although this is not strictly biodegradation, it is a process that removes pollutants from the environment.

The main styles of biodegradation and transformation are described in Box 4.16. Even slight structural molecular transformations of the parent contaminant molecule brought about by biodegradation can alter significantly the original contaminant's toxicity, mobility (Fig. 4.30) and its affinity for soil surfaces (Fig. 4.28).

4.10.3 Remediation of contaminated land

In many cases, contaminated land can be treated in order to rehabilitate it for future use. The success of remediation techniques depends on the concentration, type and availability of the contaminants present, and on-site factors, such as soil texture, pH, availability of terminal electron acceptors (Section 4.6.5) and the age

Organic compounds in soil

Interaction with the soil

Degradation

Mineralization

Changes in mobility

Changes in toxicity

Enhanced mobility

Enhanced mobility

Reduced mobility

CH 1,3,5-Trinitrotoluene

N°2 Reduction to amino funtional groups enhances binding to 21 soil organic matter

NO CH

NO CH

Decreased toxicity

Decreased toxicity

Pentachlorophenol Toxicity

Increased toxicity Pentachlorophenol OH

Increased toxicity Pentachlorophenol OH

Cl Xl

Pentachlorophenoxy-tetrachlorophenol

Fig. 4.30 Changes in toxicity and mobility of organic contaminants caused by biodegradation and biotransformation. SOM, soil organic matter.

of the contamination. Social and economic considerations will also affect remediation options. Cost is always a key issue, and this might need to be balanced against the chances of successful remediation. It might be important to know how long the remediation will take or whether the remediation technique is sustainable (see below).

Remediation options are broadly physical, chemical or biological in approach. Physical remediation includes dig-and-dump, incineration or containment of contaminants on site. Generally, these approaches provide a guaranteed 'quickfix', but at a cost. Some of these options are expensive (incineration), while others simply pass the problem on without addressing the root problem of contamination (dig and dump and containment). Soil washing with, for example, surfactants (Box 4.15) and/or solvents is a chemical remediation option, as is the addition of chemically active reagents to promote contaminant degradation and/or immobilization. Overall, physical and chemical remediation options are non-sustainable because typically they alter a soil's structure, chemistry or biology.

Biological options are described by the term bioremediation, 'the elimination, attenuation or transformation of polluting or contaminating substances by the use of biological processes, to minimize the risk to human health and the environment'. In contrast to physical and chemical methods, biological options generally maintain soil integrity with respect to its chemical and biological elements. However, bioremediation often takes months or years to complete, and success cannot be guaranteed. Although bioremediation is driven by biological vectors, much chemistry (biochemistry) is involved. Biodegradation occurs under both aerobic and anaerobic conditions, although aerobic degradation is generally faster and more extensive for most contaminants (Section 4.6.5). Thus, aerobic conditions are generally promoted during bioremediation strategies, for example air-venting.

The same physical and chemical properties that dictate a contaminant's fate in soils (Box 4.14) also dictate the amenability of contaminants to bioremediation. In general, lighter molecules that are non-halogenated (e.g. without Cl) and with high polarity are more readily biodegraded. This is because lighter and simpler molecules are inherently more biodegradable and because polar molecules are more soluble, and hence available for degradation. The bioavailability of a contaminant may also be influenced by its length of contact with the soil; longer contact usually lowering bioavailability. Toxicity is also an issue because some compounds are highly toxic to microbes (e.g. pentachlorophenol (PCP), the fungicide used in wood preservers).

Bioremediation is either done in situ, with contamination treated where it occurs, or ex situ, where contaminated soil is removed by excavation prior to treatment. In some cases treatment can be ex situ on-site, i.e. where soil is excavated but treated on site in heaps. In situ and ex situ approaches have their advantages and disadvantages, as outlined in Table 4.11.

In situ bioremediation by biostimulation— Exxon Valdez oil spillage

Biostimulation is the promotion of favourable conditions to facilitate the degradation of contaminants by in situ microorganisms. Stimulation can be achieved

Table 4.11 Consideration of typical factors relating to in situ or ex situ treatment of contaminated land.

In situ

Ex situ

Less expensive Creates less dust

Causes less release of contaminants Treats larger volumes of soil

Against Slower

Difficult to manage Not suited to high clay soils or compacted sites

Against

More expensive

Creates dust during excavation

May disperse contaminants

Limited in scale — batches treated individually

For Faster

Easier to manage — ensure results Suited to a variety of sites including high clay and compacted sites by addition of nutrients, addition of air/oxygen (a process called 'bioventing'), addition of other terminal electron acceptors (e.g. hydrogen peroxide) or addition of co-metabolic substrates.

On 24 March 1989 the Exxon Valdez oil tanker ran aground on Bligh Reef, Prince William Sound, Alaska spilling 37000 tonnes of oil. Despite efforts to contain the spill, tidal currents and winds caused a significant proportion of the oil to be washed ashore. Approximately 15% (~2000km) of shoreline in Prince William Sound and the Gulf of Alaska became oiled to some degree. Bio-remediation was one of a number of techniques applied in the clean-up operation. Bioremediation was favoured because the majority of molecules in crude oil are biodegradable and because shorelines often support large populations of oil-degrading microorganisms.

Biostimulation of the shoreline microbes was effected through addition of fertilizers. Two products were applied: Inipol EAP22, a urea-based product designed to stick to oil, and Customblen a slow-release granular fertilizer containing ammonium nitrate, calcium phosphate and ammonium phosphate. These products were selected to minimize nutrient losses with the tides and thereby optimize nutrient input to the oiled areas. By late summer 1989 approximately 120 km of shoreline had been treated in this way.

Hydrocarbon degradation by this method starts with attack of the methyl group (-CH3) at the extremity of the hydrocarbon chain (terminal -CH3), a process called methyl-oxidation, resulting in formation of a carboxylic acid group (-COOH; Fig. 4.31). The reaction then proceeds by b-oxidation (explained in Fig. 4.31), a process that cleaves C2 units from the hydrocarbon chain as ethanoic (acetic) acid (CH3COOH). Ethanoic acid is then utilized in the tricarboxylic acid (TCA) cycle through which the microbes derive energy. This mechanism is limited to straight-chain molecules; branch chains have to be removed by other degradation pathways before b-oxidation can proceed.

Comparison of oil degradation between treated plots and adjacent control plots indicated that after 109 days the treated plots had experienced about 90% consumption of hydrocarbons, whereas no significant changes had occurred

The Chemistry of Continental Solids 133

Methyl-oxidation |

B-oxidation |

R(CH2)n-CH2-CH2-CH3

R(CH2)n-CH=CH2-COOH

R(CH2)n-CH2-CH2-CH2OH

R(CH2)n-CH-CH2-COOH

R(CH2)n-CH2-CH2-CHO

R(CH2)n-CH-CH2-COOH

R(CH2)n-CH2-CH2-COOH

Tricarboxylic acid (TCA) cycle

Fig. 4.31 Biodegradation pathways of hydrocarbon. Methyl-oxidation, by attack of the methyl group (—CH3) at the extremity of the hydrocarbon chain, results in formation of a carboxylic acid group (-COOH). b-oxidation indicates that oxidation occurs at the second carbon atom (counted from the end that bears the -COOH group, the a-carbon atom being immediately adjacent to the -COOH group). b-oxidation continues removing C2 units, and in effect unzips the hydrocarbon chain until it no longer exists.

Fig. 4.31 Biodegradation pathways of hydrocarbon. Methyl-oxidation, by attack of the methyl group (—CH3) at the extremity of the hydrocarbon chain, results in formation of a carboxylic acid group (-COOH). b-oxidation indicates that oxidation occurs at the second carbon atom (counted from the end that bears the -COOH group, the a-carbon atom being immediately adjacent to the -COOH group). b-oxidation continues removing C2 units, and in effect unzips the hydrocarbon chain until it no longer exists.

on the control plots. Hydrocarbon consumption rate in the control plot was 0.052% d-1, but this increased to 0.45% d-1 when the plot was fertilized, a rate enhancement of 8.6 times.

Ex situ on-site bioremediation by composting—Finnish sawmills

Gardeners know that compost is added to soil to provide a source of nutrients and to aid soil aeration by creating a more open soil structure. Gardeners might not know, however, that most compost is also a rich source of microorganisms; the compost effectively inoculates the soil with microbes. Under composting conditions heat is generated though the processes of degradation (see eqn. 5.20). This heat changes the microbial community and the rate at which it degrades organic matter, including any bioavailable organic contaminants. Conditions within a soil/compost mixture can thus be optimized in terms of aeration, nutrients and temperature, to achieve the most efficient degradation.

As part of its operations between 1955 and 1977, a Finnish sawmill had been impregnating timber with a preservative to inhibit microbial degradation. This product, called Ky-5, contained a mixture of chlorophenols, namely, 2,4,6-trichlorophenol (7-15%), 2,3,4,6-tetrachlorophenol (~80%) and pentachlorophenol (6-10%). Ky-5 also contained traces of polychlorinated phenoxyphenols and dibenzo-p-dioxins as impurities. Over the years this product had contaminated the soils around the sawmill. A cost-effective bioremediation strategy was needed that could be used at this site but also throughout Finland where 800 other sites of this type existed.

The bioremediation strategy used compost and composting materials mixed with the excavated soil in heaps known as biopiles. Two-parts contaminated soil were mixed with one-part inoculant that contained straw-compost, bark chips, lime (to adjust the pH) and nutrients (supplied by a commercial fertilizer). The biopiles contained 7500 kg of material (volume of 13 m3) and were built on a layer of bark chips to provide insulation. The entire biopile was covered with a plastic sheet and moisture content was adjusted by watering.

Chlorophenol degradation proceeds via dechlorination (removal of Cl-groups) with hydroxylation (addition of OH-groups) at the dechlorinated sites. The microbes are effectively manipulating the molecule to make it susceptible to degradation by cleavage of the benzene ring.

Cleavage of benzene ring (see Fig. 4.33)

pentachlorophenol eqn. 4.21

In the case of pentachlorophenol (eqn. 4.21) the Cl-group opposite the OH-group on the benzene ring is replaced by an OH-group first. The next dechlori-nation/hydroxylation reaction yields a molecule with a total of three OH-groups, two of which are adjacent to each other. At this stage ring cleavage occurs between these adjacent OH-groups (see benzene degradation in Fig. 4.33) and the resultant straight-chain hydrocarbon degrades to derive smaller chlorinated and unchlorinated products.

The extent of degradation in the biopiles was proportional to the starting chlorophenol concentration. Overall, however, chlorophenol loss was between 80 and 90%. Where highly contaminated soil was treated (original chlorophenol concentration 850mgkg-1) the rate of loss was between 2 and 5mg (kgdrywt)-1 d-1. This is a fast rate of chlorophenol loss and the remediation process was complete within 3 months.

OH OH OH

Cl J^ Xl pentachlorophenol

Cleavage of benzene ring (see Fig. 4.33)

Cl" ^f "Cl C^^ "CM ) Cl' ^f "Cl

Cl OH OH

Ex situ on-site bioremediation using a bioreactor—

polycyclic aromatic hydrocarbons

A bioreactor is a silo containing a slurry of contaminated soil mixed with water. The slurry is aerated and enriched with nutrients and microorganisms, as necessary, to control and optimize decomposition. Bioreactors are usually small-volume closed systems that allow collection and treatment of volatile components. Their small volume usually limits their use to batches of soil treated individually, such that on large sites bioremediation will be slow and expensive. Bioreactors are particulary useful for the treatment of contaminant hot spots at a site, deployed alongside other bioremediation techniques.

In the 1980s the US Environmental Protection Agency (EPA) embarked on a programme known as Superfund, to clean up abandoned hazardous waste sites. The Superfund site at Burlington Northern (Minnesota) has a historic burden of creosote contamination. Creosote is principally a mixture of polycyclic aromatic hydrocarbons (PAHs). PAHs are a group of compounds based on fused benzene rings resulting in the formation of chains and clusters. The US EPA has listed 16 PAHs as 'priority pollutants' (Fig. 4.32) on account of their toxicity, typically as carcinogens and/or mutagens. PAHs exhibit a wide range of physical and chemical properties, governed principally by the number of fused benzene rings. Naphthalene, for example, the smallest PAH, comprises only two benzene rings (Fig.

4.32), has a molecular weight of 128, an aqueous solubility of 32 mgl-1 and a log Kow value of 3.37 (Box 4.14). By contrast, benzo[a]pyrene is composed of five benzene rings (Fig. 4.32), has a molecular weight of 252, an aqueous solubility of 0.0006 mgl-1 and a log Kow value of 6.04 (Box 4.14). The heavier PAHs with more than four rings are a problem for bioremediation, being relatively insoluble and strongly bonded molecules that are difficult to degrade.

Biodegradation of PAHs is analogous to the biodegradation of benzene (Fig.

4.33). Initial ring oxidation yields 1,2-dihydroxybenzene (commonly known as catechol). The benzene ring is then broken (cleaved). Ring-cleavage occurs either between the -OH groups or adjacent to one of them, known as ortho- and meta-cleavage, respectively (Fig. 4.33). Ring cleavage occurs at these positions because -OH groups are involved as reaction sites. Furthermore, because the process is enzymatically mediated the presence of adjacent -OH groups enables recognition of the molecule by the enzymes responsible for the degradation. Ring cleavage yields straight-chain products, namely cis,cis-muconic acid and 2-hydroxymuconic semialdehyde (Fig. 4.33). These products are further degraded to yield simple molecules, such as pyruvate, citrate and acetaldehyde, used in the tricarboxylic acid (TCA) cycle through which the microbes derive energy.

In the case of naphthalene (two-ringed PAH) initial ring oxidation yields 1,2-dihydroxynaphthalene (Fig. 4.34). Ring cleavage then occurs, followed by removal of side-chains to yield salicylic aldehyde. Salicylic aldehyde is then converted to catechol via salicylic acid (Fig. 4.34). Catechol is then degraded as illustrated for benzene in Fig. 4.33. For heavier PAHs the initial phases of degradation yield a catechol analogue to the PAH containing one less benzene ring than the original PAH. By way of illustration phenanthrene is converted to 1,2-

Benzene Converted Catechol
Fig. 4.32 Sixteen PAHs listed as priority pollutants by the US Environmental Protection Agency.

If 1 Ortho cleavage f ^ COOH

Benzene Catechol 2

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