Ecological Tools for Remediation of Soil Pollutants
Soil is an extremely complex medium responsible for vital functions and ecosystem sendees linked with productivity and sustainability, environmental quality, biodiversity, and human wellbeing (Halvorson 2008, Cachada et al. 2018, Duraes et al. 2018). Various anthropogenic activities have resulted in the intentional or unintentional release of both organic and inorganic pollutants leading to decline in soil quality, which in turn reduces its capacity to perform ecosystem functions and services. Factors such as soil type, topograph)', geology, and the erosive processes influence the concentration, distribution and bioavailability of pollutants in the environment. Remediation of contaminated sites has been carried out by several strategies which involve biological, physicochemical, and thermal processes (Rubilar et al. 2011). Incineration, excavation, landfilling and storage are some of the expensive decontamination techniques and in most cases it is difficult to execute and involve generation of toxic byproducts (Bustamante et al. 2012). However, the selection of soil remediation approaches depends on soil type, soil composition physical properties, nature of contaminants, handling intensity, cost, etc. Bioremediation is a widely recognized technique of treating polluted soil because of its usability, eco-friendly and cost-effective nature. The rationale of using bioremediatiou method in the recent years has been justified with non-toxic inputs and byproducts throughout the processes of remediation. Hence, bioremediation has been modified or amalgamated with other methods to obtain better results in a lesser time frame. Cost-benefit analysis highlights the effectiveness of combined bioremediation technologies and their role in world environmental markets (Arora 2018).
Remediation of contaminated sites has been earned out by several biological methods which involve biological, physicochemical, and thermal processes (Rubilar et al. 2011). Methods such as incineration, excavation, landfilling and storage are expensive, sometimes difficult to execute, inefficient, and often exchange one problem for another (Bollag and Bollag 1995). On the other hand, biological processes offer several advantages over conventional technologies, because they are often more environmentally friendly, economic and versatile, and they can reduce the concentration and toxicity of a large number of contaminants (Bustamante et al. 2012).
Types of soil pollutants and their characteristics
There are several chemicals that may pollute soils, ranging from simple inorganic ions to complex organic molecules. The two major types of soil pollutants are organic pollutants (OPs) and the inorganic pollutants (IPs). Both IPs and OPs can have natural or anthropogenic origins and the majority of polluted soils in the world contain complex mixtures of each or from both.
Organic pollutants (OPs)
Several groups of compounds containing carbon in their structure (with or without functional groups) such as pesticides, hydrocarbons, PAHs, PCBs, polychlorinated dibenzo-p-dioxins (PCDDs) are counted in OPs. These OPs may occur naturally as a result of volcanic emission, forest flies, or related to fossil fuels. But, the greatest source is associated with anthropogenic production of a huge number of chemical compounds. These compounds vary with a wide range of properties like polarity, solubility, volatility, etc. even within the same group, resulting in different behaviors in the environment and toxicity to organisms (Duraes et al. 2018). Many organic chemicals are readily bio-transformed or degraded, while others are very resistant to both chemical and biochemical transformation and have long half-lives (e.g., polyhalogenated compounds). The most important factors of OPs with regard to its toxicity include persistence, solubility (water/organic solvents) and volatility (Walker et al. 2001). Persistent organic pollutants (POPs) are one of the most serious groups of organic contaminants and are considered as a global environmental issue due to their- potential for bioaccumulation, carcinogenicity and mutagenicity. Their- massive use or continued emission, persistence, and mobility throughout the environment make the situation more serious (Pacyna
2011). Sources of pollution fr om POPs include the incorrect use and/or disposal of agr ochemicals and industrial chemicals and improper burning. There are eight initial POPs pesticides such as aldriir, chlordaue. DDT, dieldrin, endrin, lreptachlor, hexaclrlorobeuzeue, nrirex, and toxaplrene and five new POP chemicals which may be categorized as pesticides, viz. alpha lrexachlorocyclohexane, beta lrexachlorocyclohexane, chlordecoue, lindane, and pentaclrlorobenzeue (Stojic et al. 2018). These chemical compounds have long half-lives in environment due to their resistance to biological, chemical, and plrotolytic degradation. Their- great affinity for lipids allows accumulation in food chants by storage in fatty tissues of organisms. The senri-volatile character of these pollutants also allows them to move long distances in the atmosphere causing a high spatial distribution away fr om the source (Jones and Voogt 1999).
Despite ban on several organic compounds, there are many others still being manufactured and widely in use (e.g., PBDEs, fluorinated compounds, synthetic musk fragrances, organoplrosphate esters). Undoubtedly, the number of chemicals introduced by many different applications like pesticides, pharmaceuticals, personal care products, detergents, flame-retardants, di-electric fluids and combustion by-products has been drastically increasing in the environment (Caclrada et al. 2018). PAHs are important soil pollutants due to the potential risks to human health as some of them are probable human carcinogens (having natural or anthropogenic origin). Due to their- lrydroplrobicity, these pollutants are readily adsorbed by soil particles, and have difficulty being degraded (Wang et al. 2000), whereas the polar nature of phenolic compounds makes it easier to transport in soils and can exist as dissolved in soil solution, sorbed to soil particles, or polymerized in humic compounds. For this reason, they are easily degraded under aerobic conditions (Min et al. 2000). Tire organic based pollutants have the potential to disrupt hormonal systems and modify the natural growth of humans and annuals and to significantly alter the diversity of organisms in the soil system. However, the impact of many of the OPs in soil quality is not w-ell recognized and hence, monitoring and identifying then fate in the soil environment is much more difficult. For example, perfluorinated compounds, used since 1960 as surfactants or fire retardants, are widely distributed but little is known about then- toxicity and behavior in the environment (Thonraidi et al. 2016).
Inorganic pollutants (IPs)
Inorganic pollutants (IPs) are released into the environment due to anthropogenic activities like mining, smelting, electroplating, usage in agriculture of pesticides, phospliatic fertilizers and biosolids, sludge dumping, and emission from industry (Fangueiro et al. 2018). Natural causes such as weathering of minerals, erosion, and volcanic activity release inorganic pollutants to the environment. Environmental risks associated with inorganic pollutants vary widely due to several complex interactions at both intracellular and extracellular levels (Saha et al. 2017). Over the last decades, heavy metals have been recognized as the group of metal(loid)s widely associated with contamination or toxicity processes in the environment. Heavy metals include elements with an atomic density greater than 4 g cm'3. Therefore, concerning the high toxicological relevance of this group of metals and metalloids, they are also referred as Potential Toxic Elements (PTEs) and include elements such as arsenic (As), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb), and zinc (Zn) (Patinlia et al. 2018).
PTEs are the best examples of noxious inorganic pollutants due to then non-degradability and persistence in environment for longer periods allowing their transfer from the contamination sources (e.g., mining or industrial areas) to other locations where direct exposure to living organisms may be more favored. Soil is a major reservoir for PTEs and can occur in various forms in association with the solid fractions (Duraes et al. 2018). Ionic, molecular, chelated and colloidal forms of PTEs in soil confirm their high mobility. On the basis of availability, PTEs forms are exchangeable ions in mineral or organic particles > complexed or chelated to organic colloids > sorbed to inorganic constituents > incorporated in supergenic phases as (oxy) hydroxides, clay minerals, or insoluble salts, and > fixed in ciystal lattice of the minerals (Romic 2012). The higher content of available PTEs in soil environment can compromise the ecosystem sendees and the growth of plants. Free ionic forms of PTEs present in the soil solution may be toxic to the soil microflora as it leads to reduction in biota activity, population size, biodiversity and inactivation of some extracellular enzymes responsible for the cycling of many nutrients (Patinha et al. 2018). As a result, the cycling of organic matter will be disturbed due to its limited biodegradation leading to reduction in soil nutrients and plant productivity. Plants are also sensitive to higher PTEs content as it limits their metabolic activities (Reeve and Baker 2000). Higher amounts of PTEs absorbed by plants from soils may enter in human and other animals by direct (inhalation or ingestion of soil particles) or indirect (plant consumption) routes, causing remarkable physiological or metabolic disorders (Patinlia et al. 2018).
Both PTEs and OPs are present in majority of polluted soils in the world as a complex mixture of one or both types of pollutants. However, these two types of pollutants highly differ in their behavior in soils due to the non-degradable nature of PTEs over OPs, which can be decomposed by living organisms (Domene 2016). Thus, PTEs may experience increased or decreased availability over tune, depending on their form when deposited in soil and changes in physicochemical conditions during then accumulation in soil (Wuana et al. 2014). On the other hand, the decreased availability of OPs over time is closely related to their susceptibility to degradation for producing simple units or functional groups of lowest toxicity (Wuana et al. 2014).