Table of Contents:
Soil Heavy Metal Pollution and its Bioremediation: An Overview
The development of civilization gave birth to industrialization and now it is causing pollution all over the world. Continuous discharging of the effluents to the soil and water without any protection has led to the accumulation of the heavy metals in the soil and water especially in the industrially developed and populated areas. The soil minerals and organic matter tightly bind these heavy metals, which helps them to sustain in the environment for a long time, and their effects could be long-lasting. Among them, some could be toxic at a very low concentration, e.g., arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, silver, zinc, etc. (Salem et al. 2000). These heavy metals are not only cytotoxic but also carcinogenic and mutagenic. Many of them are micronutrients and are essential for plant life but when the concentration of the heavy metal increases in the soil, it could be toxic for the plants and there could be a chance of them entering the food chains.
At some point, the environment needs rectification for human welfare. Some techniques are known to us including chemical precipitation, oxidation and reduction, filtration, ion-exchange, reverse osmosis, membrane technology, evaporation, and electrochemical treatments but they are not popular due to some reasons. First, these techniques are expensive and not easily applicable in the field condition. Another important reason is that these techniques become useless when the concentration of heavy metals goes below 100 mg/L (Ahluwalia and Goyal 2007). Salts of heavy metals are water-soluble and the physical separation of them is difficult (Hussein et al. 2004). In this condition, only the bioremediation process can show satisfactory outcomes. Bioremediation is a process where microorganisms are intentionally introduced to the environment to adsorb and breakdown the pollutants in order to clean the polluted sites and it is completely eco-friendly and economically feasible. The microorganisms mainly use bio-adsorption and bio-accumulation methods to remove the heavy metals from the contaminated site with high efficiency (Kapoor and Viraraghvan 1995). Though the exact mechanisms of these methods are still unknown, they are known to be the most sustainable techniques ever.
Bioaccumulation is the process where microorganisms spend energy and for the supply of the energy, oxygen is badly required which automatically increases the biological or chemical oxygen demand in the polluted soils. So, the bioaccumulation process could be considered as the active process. On the other hand, bio-adsorption process is a passive process that does not require any metabolic energy, only the structure of the cell wall is important. The microbial cell wall consists of polysaccharides, proteins and lipids and several functional groups like phosphate and amino groups, carboxyl and hydroxyl groups. Another important tiling is the maintenance of the healthy population of the microorganisms in the toxic environment. Some fungi and plant species are also capable of extracting heavy metals and their capabilities and potentials are going through different experiments. Nowadays, new approaches like biotechnology, designer plant approach, and rhizosphere modifications are extensively studied to improve the bioremediation technique.
Sources of heavy metals in the environment
Heavy metals are naturally present in the environment through the weathering of rocks, volcanic eruption, erosion of soils, etc. But the contamination depends on the intensity of anthropogenic activity. Different human activities like mining, disposal of by-products from different smelting, electroplating, fertilizers and pesticide industries as well as deposition of biosolids and atmospheric deposition intensify the contamination of the heavy metals in the soil environment (Wuana and Okieimen 2011, Sunnier et al. 2000) (Table 1). These ever-increasing human activities are responsible for heavy metal pollution which occurs quickly than the geochemical cycle (D’Amore et al. 2005). A mass balance equation was predicted by (Lombi and Gerzabek 1998):
M = heavy metals, p = parent materials, a = atmospheric deposition, f = fertilizer source, ag = agrochemical source, ow = organic waste, ip = inorganic pollutant, cr = crop removal, 1 = leaching, v = volatilization, op = other processes.
Status of heavy metal pollution
The metals whose atomic weight is more than iron (Fe 55.8 g mol'1) or density is more than 5 g cm"3 are called heavy metals. Some metals with atomic weight lower than Fe (e.g., Cr) and some metalloids (As, Se) are also considered as heavy metals. Among them, some play an important role as micronutrients in the plant body (Fe, Cu, Mn, Mo, Zn, and Ni) and others may have dangerous effects on the ecosystem (Costa et al. 2006). Heavy metals are electrostatically attracted to the binding sites of micronutrients in different cellular structures, which is responsible for the distortion of the cellular structures, mutagenesis and genetic disorder (Peipetuo et al. 2011. Raj endi an et al. 2003).
Not all the chemical forms of heavy metals are harmful to the environment. The various states of the chemical forms depend on the pH of the medium and the composition. Bioremediation techniques can be successfully used to biotransform heavy metals (Sutherland et al. 2000). The liquid phase has more adverse consequences than the solid phase as it can easily become a part of the nutrient cycle and can enter the plant body as well as the food chain causing bioaccumulation. After entering the food chain, the toxins pass from one trophic level to the next trophic level which increases the possibilities of biomagnification. Living tissues cannot metabolize the contaminants but these toxins can be stored in the tissues as they may be fat-soluble.
The term microorganism is mostly applied for bacteria, fungi, algae and yeast and sometimes also for other organisms. The relative proportion of the microorganisms in the soil depends upon types of soil, temperature and climate, pH of the soil, amount of water availability and some other reasons. But in general, bacteria and protozoa are at the top of the list regarding population followed by fungi. The proportion of algae and yeast varies.
Table 1. The sources of the heavy metals and their effect on human health.