Abstract

The contamination of containing heavy metal has become an environmental issue globally that has attracted public attention associated with the protection of agricultural products. Soil is containing heavy metals which attract some useful heavy metals of biological toxicity, including mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), and arsenic (As), etc. In recent years, human activities gradually increases which causes the contamination of heavy metals in soil. Pollution of Heavy metal causes a bad impact to the health and human beings due to potential accumulation risk through the food chain are well-being of organisms. Remediation using chemical, physical, and biological methods has been adopted to solve the problem. Therefore, in this paper an attempt has been made to discuss the effects of contamination of heavy metals in soil, sources of heavy metals in soil and remedies available to overcome the contamination of heavy metals in soil.

1. Introduction

Heavy metals are those metals whose densities are greater than equal to 5g/cm³ such as mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), and arsenic (As), etc. Anthropogenic activities such as industry emissions, metal processing etc. are the primary sources of heavy metal pollution. When the heavy metals are present in excessive amount, soil quality decreases. The contamination of heavy metals in soil is a serious threat to the environment, human health and ecosystem. For example, exposure to arsenic (As) would results in cardiovascular and other serious symptoms such as dieseases of respiratory systems etc. which may eventually lead to cancer. In addition, lead (Pb) is not an important metal for humans however its excessive amount causes significant impact on skeletal, endocrine, immune and circulatory systems. Harmful effects of some heavy metals are presented in Table 1.

In past, soil contamination was not considered important as water and air pollution. However, in recent years soil contamination in developed countries are becoming serious such that it attracts public globally for the agricultural products security.

According to recent study conducted by there 5 million sites globally contaminated by soil pollution having excessive amount of heavy metals as comparable to regulatory levels. In the study of Cd ranks first in soil that exceed 7%, according to acceptable levels. CPCB identified highly polluted industrial areas based on Comprehensive Environmental Pollution Index (CEPI) rating. In sixteen states, forty three polluted zones were reported whose CEPI rating was more than 70 (Table 2) ². Globally 80 million of ha in china exceeds heavy contamination in soil (Hu et al., 2015). In Europe necessary reductions on the annual emissions of Cd, Pb and Hg to avoid harmful effects have been introduced in the United Nations–Economic Commission Europe Protocol on heavy metals in 1998.

In order to understand the current situation, this study discuss harmful effects, sources of heavy metals and remedies for soil contamination by heavy metals.

2. Sources of Heavy Metals

There are various sources which are responsible for excessive amount of heavy metals in soil which include atmospheric deposition, sewage irrigation, industrial solid waste, use of pesticide and fertilizers etc. Table 3 shows various sources associated with heavy metals.

The parent material is the heavy metal present in the soil through which they are generated. 95 pr cent of the earth crust is made of ingenious rocks and 5 per cent is made of sedimentary rocks. Basically, basaltic ingenious rocks are rich in heavy metals such as Cu, Cd, Ni, and cobalt (Co), whereas shales contain large amounts of Pb, Cu, Zn, manganese (Mn), and Cd. Rocks contains heavy metal can directly mixed in soil environment by the natural process such as, terrestrial, biogenic, meteoric and volcanic process erosion; leaching; and surface winds. The nature’s disturbance slowly occurs heavy metals geochemical cycle by anthropogenic process as a result one or more heavy metals accumulation inside the soil. Recent changes occur in agricultural sector, urbanization and industrialization have contributing essentially to cut down the metal contamination in the soil. Mining and smelting are the anthropogenic activities ³, combustion of fossil fuel refining, disposal of municipal wastes, application of pesticides,sewage irrigation, and fertilizer application are contributed to gain heavy metals concentrations in the agricultural soil environment. Last few decades, worldwide annual release of heavy metals crossed 22,000 t (metric ton) for Cd, 939,000 t for Cu, 783,000 t for Pb, and 1, 35,000 t for Zn.

3. Remediation of Heavy Metal Polluted Soils

So many strategies and methods are used to solving of the contamination problem the soil. Generally, technologies remediation can be divided in two strategies, situ remediation and ex situ remediation. The treatment of Situ remediation is the pollutants in the original place, contaminated soil itself without moving. Ex situ remediation are having excavation and polluted soil removal for the treatment. As compared with ex situ remediation, in situ remediation is giving so many potential technical, environmental and economic advantages. As well as, soil remediation appropriate selection method is depend on the factors of site, containment concentration, types of pollutants to be removed and the last use of the contaminated medium. Remediation cab be bring by the chemical, physical and biological methods.

In the physical remediation process of stopping or reversal of soil damage includes soil replacement, isolation, containment method and thermal treatment by physical treatment. Replacement of soil by including techniques such as landfilling, capping encapsulation. Removal of soil can dilute effectively the concentration of pollutant and soil environmental capacity increases and also contaminated soil who are serious in nature is suitable for the small place because of high cost and heavy workload. Isolation can be cause in pollutants and it is containing installing barrier walls, to stop further dispersion from the site. Impermeable material is made by physical barriers, such as, steel, cement, bentonite, and for capping grout are used, horizontal and vertical containment. The soil isolation method or containment is not a direct remediation process, but a measurement was used to reduce obviously heavy metals migration into the ground water (Jankaite and Vasarevicius, 2005). The treatment of thermal is organised on the basis of containments volatility to heating the subsurface to remediate the soil. The methods of heating including conductive heating mainly, steam-based heating and electrical resistive heating and radio frequency heating (Song et al., 2017). So the technology shown that the removing contaminants effectively with high vapour pressure such as Hg; as well as, it is affecting greatly properties of soil ⁴. Soil has shown that exhibit drastic changes physiochemical composition and mineralogical composition factors fewer than 600°C treatments, which was removed intentionally Hg from the soil. Huang et al. suggest that the heavy metals removal by chemical extraction and heavy washing can be done before thermal treatment for removal of Hg, because of thermal decontamination can leads metals repartitioning. For example, heavy metals in iron or Mn oxides transformed into acid- extractable, organic-matter-bound, and residual forms after thermal treatment at 550°C, whereas Cr, Cu, and Ni became less mobilized and difficult to extract, which may greatly affect the effectiveness of subsequent decontamination approaches.

Remediation of chemical is a method in which reactions and reagents of chemical, principles are used for the removal of contaminants (Song et al., 2017). Including the main remediation stabilization/solidification, vetrification, soil washing, soil flushing and electro kinetics Solidification/stabilization technology is always applying for the mixing of contaminated soils with materials or reagents to decrease the heavy metal Contaminants mobility. Physical encapsulation is solidification of the contaminants in a solid matrix formed by cement, bitumen, asphalt, and thermoplastic binders, at the time of stabilization including chemical reactions to decrease contaminant mobility. Additions of Bonemeal (finely ground, poorly crystalline apatite, Ca10(PO4)6OH2) has a capability to stop pollutant metals in soil and also decrease metal bioavailability by the generation of matal phosphates (Hodson et al., 2000). A cost effective variety and eco friendly resources of waste have been found not only for stopping metals in contaminated soils, also to improve quality of soil, such as lime-based agents (Lim et al., 2013), calcined oyster shells (Yong et al., 2010), eggshells (Soares et al., 2015), waste mussel shells (Otero et al., 2015), and calcined cockle shell (Islam et al., 2017).

Washing and flushing of soil are essential methods of remediation by water or proper water solution for the removal of contaminants from the soil (U.S EPA, 2006). For getting the removal of optimal heavy metal, by washing agents such as water (Dermont et al., 2008), saponin (Maity et al., 2013), organic acid (Kim et al., 2013), chelating agents (Jiang et al., 2011), surfactants (Sun et al., 2011), and low-molecular-weight organic acids (Almaroai et al., 2012) are shown to be effective on stimulating desorption of soil contaminants. EDTA has been found are the most efficient chelating agent for the heavy metals removal from contaminated soils (Lestan et al., 2008). The EDTA advantages includes low biodegradability, high efficiency of metal removal, proper recycling techniques availability and decreasing effect on soil microorganism and activity of enzyme (Qiao et al., 2017).

Bioremediation is having technique of sate of the art which is used for the in a certain degree heavy metal contaminated soil ecosystem restoration. This technique is used for biological mechanism inherent in plants and microorganism to destroy or to remove or stopping hazardous contaminants from the polluted environment ¹. It’s a cost effective and eco-friendly technique for removal of heavy metal with physical methods and conventional chemical, which are very expensive often and ineffective for down concentrations of metal, for forming the essential amount of toxic sludge. Blaylock et al. ⁵ observed that 50 to 60 per cent saving was done when the treatment by bioremediation of one acre of pb-contaminated soil is compared for the use of conventional methods such as landfill and excavation. Heavy metals bioremediation can be done by the use of plants, microorganism or the both. The commonly used microorganism for heavy contaminated soil removal are fungi and bacteria, however, algae and yeast are also applied frequently ⁶. Microorganism studies example are used treatment of bioremediation for heavy metals including, Sporosarcina ginsengisoli, Pseudomonas putida ⁷, and Bacillus subtilis. Microorganism bioremediation will be succeed if it is consortium of bacterial strains is used in place of single strain culture. studied that the essential effect of mixture of bacteria mixtures (Viridibacillus arenosi B-21, Sporosarcina soli B-22, Enterobacter cloacae KJ-46, and E. cloacae KJ-47) on the bioremediation of a mixture of Cd, Cu, and Pb from contaminated soils. They studied that demonstration of bacterial mixture higher resistance and heavy metals remediation efficiency are by comparing with single strain cultures. In remediation of heavy metals mechanism is used from contaminates soils which including microorganism by the precipitation process, bio absorption by sequestration by intracellular metal binding proteins, and conversion of metals to get the innocuous forms by the enzymes.

4. Conclusion

Frequent growth in urbanization and industrialization accompanied, heavy metals soil pollution is a major concern, by its heavy impact on animal and human health. For example, industrial waste water used for irrigation develop large area of land is polluted and it also leads to the contamination of grain in million tons per year. However, there is necessity of effective remediation methods. The chemical and physical traditional techniques for clean-up and restoration of soils contamination by heavy metals have serious effects on cost, and transformation of soil properties and a related micro flora, or capable creation of problems of secondary pollutions. By the comparison, phytoremediation is major option for problem solving. It is a cost effective and eco-friendly, with no requirements of expensive equipment’s and contaminated sites special care. As well as, phytoremediation is also having some weak points. For example, long time has been taken for the contaminants removal from the compared sites with chemical and physical methods, and application scope related to confine is required background site knowledge and characteristics of contamination remediation objectives, remediation efficiency, cost-effectiveness, remediation time, and public acceptability. Research is in progress to visible native plant species to polluted soil remediate with toxic metals and to assessing the various parameters availability on phytoremediation. As well as, biotechnical approaches application are making progress in plants for getting heavy metals phytoremediation. However, needs of some more effects to expose for the application success of this technique on commercial scale for the cleave of soil which is contaminated.

References