Bioavailability of Organic Xenobiotics in the Environment

The paper provides a review of contemporary information on the concentrations, burdens and fate of polycycylic aromatic hydrocarbons (PAHs) — representing a group of persistent organic pollutants (POPs) — in the atmosphere, sea and inland waters, sediments, soils, wastes, vegetation and food products. The main anthropogenic sources of PAH emission are discussed and the partitioning of PAHs in the natural environment is evaluated. Special attention is paid to terrestrial environment and the data on soil contamination with PAHs are presented. The aspect of biodegradation and bioavalibility of these compounds in soils is discussed widely.

For 30 years, light oil products and rocket fuels were stored at this former military fuel depot. Light and dense non-aqueous phase liquids have polluted natural environment beneath the territory of Valciunai Oil Product base. Pollutants occur in groundwater both as a dissolved aqueous plume and as pure product. In the polluted areas hazardous compounds migrate both laterally and vertically (downward). The main ecological problems are: firstly, the polluted area situated in the watershed of two rivers and shallow groundwater flowing from the base’s territory drains into the one of these. Secondly, it is within the third protection zone of the groundwater reservoir supplying water to Vilnius; chemical pollution in this zone is prohibited. The investigations of polluted underground in this site are carrying out more than 3 years. A monitoring network for regular observation of the evolution of the extent of underground pollution has been created. Oil products have accumulated on the shallow groundwater table below the central part of the base. Free phase oil covers an area of 3,000 – 3,500 m², with a maximum thickness of c. 2 m. Local accumulation of oil product has been controlled by the lithology and structure of aquifer sediments. The size of the polluted area is thus increasing. In order to hinder the migration of oil products, remediation of the subsurface has started. In the initial five months of a pump-treat-recirculate system, more than 150 m³ of liquid oil products have been extracted.

Microorganisms can metabolize many aliphatic and aromatic contaminants, either to obtain carbon and/or energy for growth, or as co-substrates, thus converting them into carbon dioxide, water, chloride and biomass. To be able to exploit these biotransformations for the remediation of contaminated environments, a number of prerequisites have to be fulfilled. The chemical has first to reach the microbial cell, then it must be transported into the cell. Toxicity should be absent or limited at the in situ concentration the cell is exposed to. This chapter reviews a number of metabolic reactions that convert the contaminant into common intermediary metabolites.

The natural biodegradation of chemicals in the environment is controlled by both the compounds’ characteristics (e.g., hydrophobicity), the environment’s characteristics (e.g., soil organic matter content) and the activity of the microbial population. Simplified biodegradability tests under laboratory conditions and in the absence of soil often show higher rates (given that sufficient and appropriate microorganisms are present). This could be explained by differences in compound bioavailability. Thus, standardized laboratory tests are not by themselves good predictors of environmental half-lives. In order to rapidly construct an environmental database, over 800 scientific articles were examined providing a subset of 75 publications with 26 different soils and 55 different compounds (given a total of 150 data). Sensitivity studies based on derived correlations show that not all environmental characteristics are relevant to predicting environmental half-lives. In addition, most chemicals studied are reputed to be difficult to degrade, which lessens the influence of environmental characteristics relative to molecular ones. In general, however, derived correlations show that under optimum or near optimum environmental conditions, biodegradation rates are controlled by molecular characteristics whereas under more extreme conditions biodegradation rates are controlled by environmental conditions.

The capability of microorganisms to degrade a great variety of xenobiotic compounds in laboratory cultures is in contrast to their frequent failure to remediate soils and aquifers. The most probable explanation is the low bioavailability of the chemicals in the natural environment. An overview is given of recent studies, which attempt to identify the processes by which chemical are withheld from active soil microorganisms. To unravel the complexity of natural soils, individual features of subsurface systems were mimicked using defined materials. The results of these experiments gave evidence for the unavailability of solid, liquid, sorbed, and micelle-solubilized chemicals, the substrate deficiency of bacteria associated with non-sorbing surfaces, and the enhanced transfer of volatile chemicals in the presence of a gas phase. The studies furthermore indicated the dependence of active populations on the mass transfer capacity of their habitat.

The bioavailability of pollutants is significantly influenced by its interactions with sedimentary organic matter. Natural organic matter (OM) is heterogeneous and exists as a multiphase macromolecular organic matrix formed from remnants of plant biobiopolymers degraded to varying degrees, dissolved and solid humic materials from plant degradation, refractory cross-linked organic matter derived from geologic processes, and deposited atmospheric combustion particles. Recent models have qualitatively described pollutant interactions with OM in soils but they often fail to adequately predict biological responses to aged contaminants. Our ability to predict pollutant behavior and bioavailability is limited by the lack of established techniques capable of probing the relevant molecular interactions of OM and our ignorance concerning the chemical composition of OM. Current models do not include chemical processes that transform plant materials to OM, nor do they include analytical methods for characterizing OM at the molecular level of detail. Thus, there is a gap in our knowledge about OM structure, maturation, and their subsequent effects on pollutant bioavailability and aging mechanisms.

Differing experimental conditions were studied in an attempt to evaluate the effect that these conditions have on the values obtained for PCB concentrations in soil and straw material. The methodological aspects of these biodegradation experiments included different methods for the extraction of PCBs from soils in studies on biodegradation by white rot fungi. Extraction experiments were performed on soils with and without inactivated white rot fungi. To obtain information on the fate of a specific PCB during the extraction, ¹⁴C tetrachlorobiphenyl was used to check the extraction selectivity with commercial mixtures of PCBs. The methods that were compared included the Soxhlet and Ultrasonic extractions with and without Triton X-100 detergent. During the biodegradation experiments, PCBs were sorbed, evaporated and then extracted after a 10 day incubation period. To quantify possible sources of PCB loss, different methods of soil preparation and contamination with PCBs were compared. Furthermore, four different sorbents were tested to evaluate evaporation of PCBs from soils. Radiolabelled PCB analysis were performed using a liquid scintillation analyzer. However, the analyses of commercial mixtures containing PCBs were carried out by GC/ECD.

A theoretical method was developed to evaluate the intrinsic bioremediation potential of an aquifer contaminated with an organic xenobiotic. The hazard associated with the presence of the contaminant was also evaluated. Since contaminant bioavailability in an aquifer results from different kinetics, the method is based on bioavailability rates. In this study, pollutant bioavailability is described from the standpoint of environmental receptors. The fraction of bioavailable contaminant may have two different fates: biodegradation leading to a positive bioavailability rate, or migration through groundwater flow leading to a negative bioavailability rate. These rates allow the calculation of the intrinsic bioremediation potential and the hazard index which can then be applied to assess the feasibility of various bioremediation techniques.

To study the bioavailability of pollutants in a direct way, whole-cell living biosensors can be used. These are genetically constructed microorganisms, which upon sensing (bioavailable) pollutant concentrations express an easy detectable signal and may or may not degrade the pollutant as well. Biosensors are constructed by combining a sensor element (the regulatory protein) with a reporter gene fused to an inducible promoter. The most suitable reporter genes for the usage in biosensors are those coding for bioluminescent or fluorescent proteins like the luciferase and the Green Fluorescent Protein. Biosensors which are used to determine the bioavailability of pollutants in the environment should be sensitive, respond in a quantitative manner and be selective. Bioreportes should not be considered as an alternative for traditional chemical analyses but regarded as a valuable extension to these well-established techniques. By using both techniques, a better control in bioremediation processes may be obtained.

Many synthetic organic chemicals are introduced into soils where their fate is determined by a number of physical, chemical and biological factors. In this report, an overview is proposed of the important role that abiotic reactions play in the transformation of xenobiotics in soils, as well as an assessment of the consequences of these reactions on the bioavailability of the xenobiotics.

Clay minerals, metal oxides, and humic substances are a complex mixture of soil components that abiotically promotes a number of reactions relevant to the environmental impact of organic xenobiotics. Adsorption is the primary stage determining the transformation of organic substances, followed by chemical reactions of the activated forms on the surface. Hydrolytic, oxidation and polymerization reactions catalyzed by clay minerals and Mn and Fe oxides dominate the abiotic transformation of xenobiotics. The efficiency of the catalytic surface processes depends on the structure and properties of the clay mineral or metal oxide and on the nature of the xenobiotic as well as on the reaction conditions. Soil organic matter appears to be involved to a lesser extent in the direct transformation reactions of xenobiotics, but it has a major role as accumulation phase of many polar and non-polar pollutants. The bioavailability and toxicity of the bound residues depend upon the possible release of bound pollutants from humic substances. Direct and indirect photolytic transformation of pollutants on soil surfaces is also discussed. The transformed products can be sorbed to the surfaces of minerals and organic matter through a variety of chemical processes. Sorption to soil will affect their availability for microbial degradation. However, soil surfaces directly and indirectly affect microbial activity. These interactions complicate the knowledge of the rate and the extent of bioavailability. An understanding of abiotic transformations under conditions that enable both biotic and abiotic transformations to occur is essential to achieve the remediation of soils contaminated with xenobiotics.

Adsorption is probably the most important form of interaction between pesticides and the soil, since it governs the amount of active substance available. Understanding of the extent and the mechanisms of adsorption of pesticides is an indispensable premise to any type of evaluation of the efficacy of their active principles and their possible impact on the environment. Laboratory studies of the adsorption and desorption of pesticides enable forecasts to be made of their behaviour in the field when the features of the soils on which they are to be applied are known. Tests can be carried out on whole soils or purified fractions: in the first case, the contribution of each adsorbent phase can only be evaluated by determining the correlations between the extent of adsorption and the chemical and physical properties of the soil. Pesticides can be adsorbed on a soil through variously strong physicochemical bonds. The type and degree of adsorption and also the extent to which it is reversible will depend on the properties of both the soil and the pesticide.

The bioavailability of organic xenobiotics in the environment is currently the object of considerable attention from scientists, environmental activists, and policy makers. Yet, in the literature that this interest has stimulated in recent years, the concept of bioavailability itself is seldom defined precisely, with the result that different definitions are used by different people. In the present chapter, we attempt to provide a set of definitions of biovailability, and to relate these definitions to the concepts of exposure and dose traditionally used in (eco)toxicology. Assumptions used to assess the bioavailability of xenobiotics in soils and sediments are critically reviewed, as well as the experimental evidence concerning changes in bioavailability over time (aging). Based on a number of recent publications, it is argued that the key determinant of the bioavailability of organic xenobiotics in subsurface environments is not the (supposedly fixed) rate of their release by the soil matrix but instead the ability of microbial cells and higher organisms to act as sinks for these compounds. This viewpoint is supported by experiments carried out with near-perfect sinks (resin beads), which have shown limited aging of organic xenobiotics and heavy metals in soils. The “sink theory” of the bioavailability of organic xenobiotics, introduced in this chapter, has a number of very practical consequences, in particular in terms of environmental policy decisions.

Soil organic matter is one of the five main constituents of the soils with minerals, solutions, gases and organisms. It contributes to the original, specific and major properties of soils. It is strongly involved in the control of the availability and cycling of elements and water and in the behaviour and fate of inorganic and organic pollutants. The knowledge of the fundamental chemical, physico-chemical and biological properties of soil organic matter is of great interest to understand soil functioning processes and to define soil quality.

All anthropogenic organic chemicals form non-extractable residues to some extent after entering soils. This well known phenomenon has been studied intensively in the field of soil agrochemistry. Similar processes have been observed during the bioremediation of oil-contaminated soils. Thus, the residue formation of toxic and carcinogenic polycyclic aromatic hydrocarbons is of particular concern. Beside mineralisation by microbial activity, the formation of non-extractable residues is a major sink for anthropogenic pollutants in soils. Microbial activity often stimulates the formation of bound residues. On a molecular scale, bound residue formation is suggested to be a covalent binding of xenobiotic substances and their metabolites to macromolecular natural organic matter in soils and sediments. When this binding occurs, the xenobiotic substance loses its chemical identity. A strong reduction of the bioavailability of xenobiotic carbon is one major consequence of bound residue formation in soils. In this overview we will discuss the present state and new trends in the field of bound residue research regarding the application of isotopically labeled tracer substances. Carbon budgets are emphasized to study the fate of isotopically labeled PAH in soils, including bound residue formation.

A preliminary investigation on the influence of some heavy metals on the rate of PAHs dissipation from soil as well as on biological activity of the soil contaminated with these compounds were performed as a pot experiment. The sandy soil contaminated with mixture of four PAHs (fluorene, anthracene, pirene and chrysene) at the level of Σ4PAHs=10 mg/kg was used for the study. It was found that the amendment of the soil with salts of Zn, Pb and Cd at the levels corresponding to those find in the highly contaminated areas (Zn=1000mg/kg, Pb=500mg/kg and Cd=3mg/kg) decreased the indexes of soil microbiological activity (the intensity of respiration, alkaline and acidic phosphatase activity and the total number of soil bacteria) and increased the persistence of PAH compounds.

The concept of non-extractable organic residues is well accepted in the EU-legislation for pesticides. Making pollutants less bioavailable by increasing physical sorption represents a pragmatic approach to contractors and regulators. For organic pollutants with acceptable concentration in the soil solution of the order of 1 mg/1, a sorptive loading of the order of 10 000 mg pollutant per kg activated carbon respectively organic matter appears a workable assumption. In case of toxic substances such as pesticides which have a 1000 times lower acceptable level, a sorptive loading of up to 10 mg organic pollutant per kg sorbent can be used. Non-bioavailable pollutants can be considered as representing no direct harm to the environment. In practice, the application of up to 100 – 200 kg dry weight quality compost per ton dry weight soil or alternatively the supplementation of other sorbents such as powdered activated carbon (up to 100 kg per ton soil) offers possibilities to cost-effective remediation of organic pollutants.

Accumulation of organic or mineral micropollutants in soils and waters may alter the functioning of ecosystems and contaminate the food chain. The remediation of these contaminated environments by currently available physico-chemical methods is either costly or impossible. Phyto-remediation, i.e. remediation based on the use of plants, would be more economic and more environmentally friendly, leaving soil material without major alterations in biological properties. Living plant roots transform the soil environment through many processes including uptake of water and elements and release of organic compounds, i.e. exudates, in the surrounding soil. Presence of exudates stimulates the soil microflora and induces changes in the soil structure as well as in the mobility of mineral ions. Hence, plants significantly alter the fate of pollutants in soils, and are suitable candidates for management of contaminated soils, i.e. phytoremediation. Phytoremediation utilizes the numerous capabilities that plants have to change their close environment. Covering of contaminated soils by adapted plants, i.e. phytostabilisation, helps the stabilization of the soil surface, and reduces the risk of transport to water streams of pollutants adsorbedon the fine solid phase. Also, growing tolerant plants reduces the water movement into the soil profile, thus limiting the leaching of soluble pollutants. Plants have also the ability to extract and accumulate non-essential trace elements in their tissues making it possible to removed metals from polluted environments. Hyperaccumulators of metals are a specialized class of plants able to accumulate metals to very high concentrations (up to 1 % by dry weight) in their above-ground tissues. They proved to be efficient for removing significant amounts of metals from soils polluted by sewage sludge or industrial activities, with little changes in other soil properties, i.e. phytoextraction. Plants are not only a sink for pollutants, they exert changes in the compounds present in their rhizosphere. The release of exudates modifies the chemistry and physics of the soil and may subsequently alter the mobility of metals Enhanced microbial activity is also observed in the rhizosphere, which makes plants useful in the management of environments contaminated with organic pollutants. In soils and waters, pesticides and hydrocarbons are degraded at a rate that depends on molecule type, soil properties, and the state of the microflora. In presence of plants, the process of degradation of organic pollutants, e.g. pesticides and hydrocarbons, is accelerated. Extraction by plants of organic compounds at high rates has not been demonstrated uniquevoquely yet. Phyto-remediation can be suitable for many polluted sites, and research is underway to make this approach a routine technique for soil and water remediation.

Hydrophobic organic chemicals (HOCs) often exhibit limited bioavailability to microorganisms and can persist in the subsurface for long periods of time. The use of surfactants has been proposed to enhance the effectiveness of both in-situ bioremediation and ex-situ slurry-reactor bioremediation by increasing HOC bioavailability. However, the fate of HOCs in response to surfactant addition at both the laboratory and field scale is difficult to predict; in some cases, surfactants sometimes even inhibit HOC biodegradation. The objective of this review is to identify factors that influence the effectiveness of surfactant-enhanced bioremediation (SEB) of soils contaminated with HOCs.