Fate of pollutants in soil-Management of soil pollution-Bio and Phyto-remediation of polluted soil 

Soil contamination occurs through either point source or diffuse pollution; the main difference between the two types of contamination lies in how the contaminants are transferred to the soil. Point sources, such as manufacturers, landfills, incinerators, use soil as a support and are linked to the activities that necessarily transfer pollutants into the soil. Diffuse sources are associated with natural phenomena (long range transport, atmospheric deposition, sedimentation by surface water), with agricultural practices, with recycling and inadequate waste treatments.

The most dangerous contaminants in soil are, in general, persistent organic pollutants (POPs) and inorganic pollutants, above all heavy metals. Persistent organic pollutants have an anthropic origin and are characterized by high lipoaffinity, semivolatility and resistance to degradation. In the case of heavy metals, that cannot be degraded or destroyed, the presence in the soil could be due to natural processes, for example the formation of soil, and to anthropogenic activities. Some are important essential elements (Cu, Fe, Mn, Zn, Co), if present in optimal concentration ranges, while others (Hg, Pb, Cd) are potentially toxic elements

The nature and behavior of inorganic contaminants

Heavy metals are one of the numerous classes of substances that can reach critical levels in terms of human health, food safety, soil fertility and ecological risks. Heavy metals are common contaminants in the soil and bioaccumulate, thus their concentration in the organism increases over time compared to the level measured in the environment. This is because the absorption rate is higher than the excretion rate in the organism.

The distribution of heavy metals between the solid phase and the soil solution is considered to be the key factor when assessing the environmental consequences of the accumulation of metals in the soil

A physical and chemical analysis along with an analysis of the soil profile is essential for assessing the soil as a barrier against inorganic contaminants, particularly heavy metals. The retention of heavy metals in the solid phase of the soil is dependent primarily on the pH, and is linked to clay minerals, humic substances, iron oxides and hydroxides, and manganese found in the soil, which all control the attenuation effect even on anionic forms

The retention and release process of heavy metals includes precipitation and decomposition, ionic exchange, and adsorption and desorption. The precipitation/release reactions may involve discrete solid phases or solid phases, which are absorbed onto the soil surface. The ion-exchange reactions derive from an exchange between an ionic species in the soil solution and an ionic species retained in sites with permanent charge on the soil surface. The absorption and desorption processes can affect all ionic or molecular species and generally concern absorbent sites with a pH-dependent charge. These surfaces are iron, aluminum and manganese oxides and hydroxides, clay minerals and humic substances.

pH

pH is the most important parameter governing concentrations of metals in soil solutions that regulate precipitation– dissolution phenomena. Metal solubility tends to decrease at a higher pH. In alkaline conditions the precipitation of solid phases diminishes the concentration of metal ions in solutions and the reverse happens with a lower pH. pH values also regulate specific adsorption and complexation processes. The sorption of metals is often directly proportional to soil pH due to the competition of H+ (and Al3+) ions for adsorption sites, however this competition may be reduced by specifi c adsorption. Metal hydrolysis at higher pH values also promotes the adsorption of the resulting metal hydroxo complexes, which beyond a threshold pH level (which is specific for each metal) drastically reduce the concentration of metal ions in the soil solution. At low pH levels, on the other hand, sorption processes are reduced due to the acid catalysed dissolution of oxides and their sorption sites, whereas the complexation by organic matter tends to decrease with increasing acidity.

Clay content

Ion exchange and specific adsorption are the mechanisms by which clay minerals adsorb metal ions. This is done through the adsorption of hydroxyl ions followed by the attachment of the metal ion to the clay by linking to the adsorbed hydroxyl ions or directly to sites created by proton removal. Highly selective sorption occurs at the mineral edges. However notable differences exist among clay minerals in their ability to retain heavy metals which are more strongly adsorbed by kaolinite than montmorillonite. This is probably due to a higher amount of weakly acidic edge sites on kaolinite surfaces. In expandable clays (vermiculite and smectite) the sorption processes essentially involve the inter-layer spaces, and are greater than in non-expandable clays such as kaolinite. The importance of clay minerals, and of soil texture in determining the distribution of heavy metals between the solid and the liquid phases of soil has direct consequences on the metal bioavailability of plants. For the same total concentration it is well known that heavy metals are more soluble and plant available in sandy soil than in clay soil.

Organic matter content

The organic matter content of soils is often small compared to clay. However, the organic fraction has a great influence on metal mobility and bioavailability due to the tendency of metals to bind with humic compounds in both the solid and solution phases in soil.

The formation of soluble complexes with organic matter, in particular the fulvic fraction, is responsible for increasing the metal content of soil solutions. However higher molecular weight humic acids can greatly reduce heavy metal bioavailability due to the strength of the linkages. Both complexation and adsorption mechanisms are involved in the linking of metals by organic matter thus including inner sphere reactions and ion exchange. Negatively-charged functional groups (phenol, carboxyl, amino groups etc.) are essential in metals retained by organic matter. The increase in these functional groups during humification produces an increase in the stability of metal organic complexes, which also show a greater stability at higher pH values.

Cation exchange capacity

The density of negative charges on the surfaces of soil colloids defines the CEC of soil. This capacity is governed by the type of clay and amount of organic colloids present in the soil. Montmorillonitic type clays have a higher net electrical charge than kaolinitic type clays; consequently, they have a higher cation exchange capacity. Soils containing a high percentage of organic matter also tend to have high cation exchange capacities. The surface negative charges may be pH dependent or permanent, and to maintain electroneutrality they are reversibly balanced by equal amounts of cations from the soil solution. Weak electrostatic bonds link cations to soil surfaces, and heavy metals can easily substitute alkaline cations on these surfaces by exchange reactions. Moreover, specifi c adsorption promotes the retention of heavy metals, also by partially covalent bonds, although major alkaline cations are present in soil solutions at much greater concentrations.

Redox potential

Reduction-oxidation reactions in soils are controlled by the aqueous free electron activity pE often expressed as Eh redox potential. High levels of Eh are encountered in dry, well aerated soils, while soils with a high content of organic matter or subject to waterlogging tend to have low Eh values. Low Eh values generally promote the solubility of heavy metals. This can be ascribed to the dissolution of Fe–Mn oxyhydroxides under reducing conditions resulting in the release of adsorbed metals. However under anaerobic conditions, the solubility of metals could decrease when sulphides are formed from sulphates. Differences in individual metal behaviour and soil characteristics result in confl icting reports regarding the effects of redox conditions on metal solubility. Iron and manganese oxides Hydrous Fe and Mn oxides, are particularly effective in infl uencing metal solubility in relatively oxidising conditions. They are important in reducing metal concentrations in soil solution by both specifi c adsorption reactions and precipitation. Although Mn oxides are typically less abundant in soils than Fe oxides, they are particularly involved in sorption reactions with heavy metals. Mn oxides also adsorb heavy metals more strongly, thus reducing their mobility. This action is particularly important in contaminated soils. Specific adsorption of metals by hydrous oxides follows the preferential order: Pb > Cu >> Zn > Cd.

Other factors

There are a number of other factors which may affect the solubility of metals in soils. Temperature, which infl uences the decomposition of organic matter, can modify the mobilisation of organo-metal complexes and consequently plant uptake. An increase in the ionic strength of soil solutions reduces the sorption of heavy metals by soil surfaces due to the increased competition from alkaline metals. Similar effects also derive from the simultaneous presence in soil solutions of many heavy metals which compete for the same sorption sites. This results in an increase in mobility in contaminated soils due to the saturation of adsorption sites. The living phase of soil is also of great importance in determining metal solubility, which is dependent to some extent both on microbial and root activity. In the rhizosphere, plants can increase metal mobility by increasing their solubility. This happens following the release in the exudates both of protons which increase the acidity, and organic substances which act as complexing agents. Microbial biomass may promote the removal of heavy metals from soil solutions by precipitation as sulphides and by sorption processes on new available surfaces characterized by organic functional groups.

Organic pollutants

Among the many organic compounds present in soil, the most dangerous are the “persistent organic pollutants” that derive, in general, from anthropic activity, are extremely persistent in the environment and are transported for long distances. In specific environmental conditions they bioaccumulate and biomagnify, reaching considerable concentrations that represent a threat for human health and ecosystems. Of the twelve groups of persistent organic pollutants, the following three are acknowledged internationally: polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). PCBs are high hydrophobic extremely stable compounds and have very good dielectric and thermostability properties; these characteristics led to the diffusion of PCB for industrial and civil use. After accidental ingestion or due to their presence in food compounds, PCBs are absorbed through the gastrointestinal tract, and then accumulate in body fats as a consequence of their hydrophobicity. The International Agency for Research on Cancer (IARC) has classifi ed PCBs as potential carcinogenic agents for humans: experimental tests suggest, in fact, that these compounds may increase the risk of skin, liver and brain cancer. In order to protect human health and the preservation of the environment, the European Community banned the commercial use of PCBs in 1990. However, these persistent compounds are still present both in natural soils, owing to long-distance transport, and in soils that have been contaminated by specific industrial activities. PCDDs and PCDFs, which are generally known as “dioxins”, are the undesired by-products of chemical and combustion processes and are also produced from natural events, such as accidental fi res and volcanic eruptions. The dioxins are a group of 210 chlorine-containing chemicals, 17 of which have a toxicological interest owing to their carcinogenic potential and their effects on reproductive, endocrine and immune systems. Owing to their high persistence in the environment, they remain in soil, which become pollutant reservoirs. In humans, the main route of exposure to dioxins is through food, which represents 90% of the total exposure

EDCs

Over the last few years there has been an increasing interest in identifying the long-term damage to reproduction and development; xenobiotics with potential endocrine activities or endocrine disrupter chemicals (EDCs) have been identifi ed as the main possible risk factors

Endocrine disrupters are a heterogeneous group of persistent organic and inorganic pollutants including dioxins, PCBs, pesticides, and industrial compounds. They are characterized by their potential to affect the correct functions of the endocrine system, especially the homeostasis of sexual and thyroid hormones. These molecules may enter the soil environment by agricultural practices or industrial waste disposal. The risks derived from EDCs are determined by the distribution of these compounds among the soil phases. Depending on the chemical properties of the molecules, EDCs can be either strongly retained by solid soil phases, or leached to deeper layers. Their mobility is largely determined by adsorption – desorption processes on solid soil phases. A probable role of endocrine disrupters is attributable to polybrominated byphenyls (PBDEs), a class of manufactured chemicals structurally similar to PCBs, which were used in the past as fl ame retardants. Even though most PBDEs were banned within the European Union in 2006, studies have revealed that PBDE levels have increased both in the environment and in human tissues and body fluids

Pesticides are a class of compounds used to kill harmful organisms, especially in agriculture. However many are also toxic for other organisms, including humans. The presence and bioavailability of pesticides in soil can adversely impact soil quality with related consequences on water and air quality. Soil characteristics regulate the processes that affect the behavior of pesticides such as adsorption, degradation, volatilization adsorption by crops. Pesticide adsorption to soil depends on both the chemical properties of the pesticide and properties of the soil, in particular organic matter. Organochlorinated pesticides have been used for many decades and one of their main features is their high persistence in soil and transfer into the food chain, with the consequence of well known toxic effects in biota

Behavior of organic contaminants

Organic molecules in soil are a carbon source for microorganisms. Therefore, the conditions that influence the breakdown of organics by microflora should be considered. Microflora are not always able to attack organic molecules and digest them completely, but often only partially break them down. This results in compounds that are even more toxic than the initial ones. The intrinsic toxicity and health risks following the ingestion of organic compounds are well known, both natural compounds and those deriving from productive processes. On the other hand, there is less information about the potential contamination, caused by organic compounds present in the soil, on the food chain (plants-animals-humans). Organic compounds should be evaluated in terms of their chemical properties and their relative absorption potential by plants, but also in terms of the influence that the soil has on them. In fact, these compounds can be volatized, absorbed and therefore immobilized, or transported along the soil profile even to underground water. The most important chemical properties of organic molecules are those dealing with their absorption in the food chain: the distribution coefficient (octanol/water (Kow), the Henry constant, solubility, half-life, and the bioconcentration factor (BCF).

The behavior of an organic contaminant in the soil depends on the interactions that are established with the solid, liquid and gas phases of the soil, and with the living phase. These relations give rise to the major phenomena that rule the fate of the organic contaminants concerning adsorption, biotic and abiotic decay, leaching and volatilization.

Adsorption and desorption

The adsorption processes of organic compounds on the active surfaces of the soil are particularly important because they delay mobilization and leaching of organic contaminants. The distribution of the contaminants between the liquid and solid phase of the soil can be synthetically described by the distribution coeffi cient Kd, which in turn can be expressed as a function of organic carbon (Koc) and of Kow. The compounds that have high levels of Kow and low solubility will be mostly retained by the soil surfaces and be less available to environmental processes.

Biodegradation

Biodegradation is the most important mechanism for the removal of organic compounds in the soil. Degradation by the microbial fl ora can increase the solubility and therefore the availability of recalcitrant compounds for microorganisms in the soil. The chemical characteristics of each specific compound affect the time required for biodegradation. Various parameters have been identified that could be correlated with the degradation period. For example, the half-life of polycyclic aromatic hydrocarbons, PCBs and dioxins are all related to the Kow. This coefficient is also related to the leaching process and to the persistence of contaminants in the soil. In fact, compounds characterized by a log Kow > 4.0 rarely mobilize. Therefore the same compounds mentioned above, as well as several organochlorinated pesticides are very persistent and have a very low leaching potential. Monocyclic aromatic hydrocarbons, some chlorobenzenes, short chain aliphatic compounds and phenols, on the other hand, degrade rapidly and are more easily leached from the soil. Photolysis, hydrolysis and oxidation (abiotic degradation) also contribute to the disappearance of some organic compounds. These reactions mostly affect compounds with simple molecular structures, such as phenols and some polycyclic aromatic hydrocarbons (PAHs) with less than four benzene rings. Volatilization also affects volatile substances, which are generally characterized by a reduced molecular complexity.

Remediation

There are several principal strategies for remediation:

·         Excavate soil and take it to a disposal site away from ready pathways for human or sensitive ecosystem contact. This technique also applies to dredging of bay muds containing toxins.

·         Aeration of soils at the contaminated site (with attendant risk of creating air pollution)

·         Thermal remediation by introduction of heat to raise subsurface temperatures sufficiently high to volatize chemical contaminants out of the soil for vapor extraction. Technologies include ISTD, electrical resistance heating (ERH), and ET-DSP.

·         Bioremediation, involving microbial digestion of certain organic chemicals. Techniques used in bioremediation include landfarming, biostimulation and bioaugmentating soil biota with commercially available microflora.

·         Extraction of groundwater or soil vapor with an active electromechanical system, with subsequent stripping of the contaminants from the extract.

·         Containment of the soil contaminants (such as by capping or paving over in place).

·         Phytoremediation, or using plants (such as willow) to extract heavy metals.

·         Mycoremediation, or using fungus to metabolize contaminants and accumulate heavy metals.

·         Remediation of oil contaminated sediments with self-collapsing air microbubbles.

·         Surfactant leaching

By country

Various national standards for concentrations of particular contaminants include the United States EPA Region 9 Preliminary Remediation Goals (U.S. PRGs), the U.S. EPA Region 3 Risk Based Concentrations (U.S. EPA RBCs) and National Environment Protection Council of Australia Guideline on Investigation Levels in Soil and Groundwater.

People's Republic of China

The immense and sustained growth of the People's Republic of China since the 1970s has exacted a price from the land in increased soil pollution. The State Environmental Protection Administration believes it to be a threat to the environment, to food safety and to sustainable agriculture. According to a scientific sampling, 150 million mu (100,000 square kilometres) of China’s cultivated land have been polluted, with contaminated water being used to irrigate a further 32.5 million mu (21,670 square kilometres) and another 2 million mu (1,300 square kilometres) covered or destroyed by solid waste. In total, the area accounts for one-tenth of China’s cultivatable land, and is mostly in economically developed areas. An estimated 12 million tonnes of grain are contaminated by heavy metals every year, causing direct losses of 20 billion yuan($2.57 billion USD).

European Union

According to the received data from Member states, in the European Union the number of estimated potential contaminated sites is more than 2.5 million and the identified contaminated sites around 342 thousand. Municipal and industrial wastes contribute most to soil contamination (38%), followed by the industrial/commercial sector (34%). Mineral oil and heavy metals are the main contaminants contributing around 60% to soil contamination. In terms of budget, the management of contaminated sites is estimated to cost around 6 billion Euros (€) annually.

United Kingdom

Generic guidance commonly used in the United Kingdom are the Soil Guideline Values published by the Department for Environment, Food and Rural Affairs (DEFRA) and the Environment Agency. These are screening values that demonstrate the minimal acceptable level of a substance. Above this there can be no assurances in terms of significant risk of harm to human health. These have been derived using the Contaminated Land Exposure Assessment Model (CLEA UK). Certain input parameters such as Health Criteria Values, age and land use are fed into CLEA UK to obtain a probabilistic output.

Guidance by the Inter Departmental Committee for the Redevelopment of Contaminated Land (ICRCL)^[16] has been formally withdrawn by DEFRA, for use as a prescriptive document to determine the potential need for remediation or further assessment.

The CLEA model published by DEFRA and the Environment Agency (EA) in March 2002 sets a framework for the appropriate assessment of risks to human health from contaminated land, as required by Part IIA of the Environmental Protection Act 1990. As part of this framework, generic Soil Guideline Values (SGVs) have currently been derived for ten contaminants to be used as "intervention values". These values should not be considered as remedial targets but values above which further detailed assessment should be considered;

Three sets of CLEA SGVs have been produced for three different land uses, namely

·         residential (with and without plant uptake)

·         allotments

·         commercial/industrial

It is intended that the SGVs replace the former ICRCL values. The CLEA SGVs relate to assessing chronic (long term) risks to human health and do not apply to the protection of ground workers during construction, or other potential receptors such as groundwater, buildings, plants or other ecosystems. The CLEA SGVs are not directly applicable to a site completely covered in hardstanding, as there is no direct exposure route to contaminated soils.

To date, the first ten of fifty-five contaminant SGVs have been published, for the following: arsenic, cadmium, chromium, lead, inorganic mercury, nickel, selenium ethyl benzene, phenol and toluene. Draft SGVs for benzene, naphthalene and xylene have been produced but their publication is on hold. Toxicological data (Tox) has been published for each of these contaminants as well as for benzo[a]pyrene, benzene, dioxins, furans and dioxin-like PCBs, naphthalene, vinyl chloride, 1,1,2,2 tetrachloroethane and 1,1,1,2 tetrachloroethane, 1,1,1 trichloroethane, tetrachloroethene, carbon tetrachloride, 1,2-dichloroethane, trichloroethene and xylene. The SGVs for ethyl benzene, phenol and toluene are dependent on the soil organic matter (SOM) content (which can be calculated from the total organic carbon (TOC) content). As an initial screen the SGVs for 1% SOM are considered to be appropriate.

Canada

India

In March 2009, the issue of Uranium poisoning in Punjab attracted press coverage. It was alleged to be caused by fly ash ponds of thermal power stations, which reportedly lead to severe birth defects in children in the Faridkot and Bhatindadistricts of Punjab. The news reports claimed the uranium levels were more than 60 times the maximum safe limit.^[17][18] In 2012, the Government of India confirmed^[19] that the ground water in Malwa belt of Punjab has uranium metal that is 50% above the trace limits set by the United Nations' World Health Organization(WHO). Scientific studies, based on over 1000 samples from various sampling points, could not trace the source to fly ash and any sources from thermal power plants or industry as originally alleged. The study also revealed that the uranium concentration in ground water of Malwa district is not 60 times the WHO limits, but only 50% above the WHO limit in 3 locations. This highest concentration found in samples was less than those found naturally in ground waters currently used for human purposes elsewhere, such as Finland. Research is underway to identify natural or other sources for the uranium.