ReviewPhytoremediation strategies for soils contaminated with heavy metals: Modifications and future perspectives
Introduction
In the modern age of rapid industrialization, it is not possible to avoid the toxic chemicals and metals in the environment. Especially heavy metals pollution has become a serious threat to the environment and food security because of rapid growth in industries and agriculture and disturbance of natural ecosystem due to enormous increase in world population. Unlike organic pollutants, biodegradation of heavy metals is just out of question and hence are continuously accumulating in the environment (Sarwar et al., 2010). Accumulation of these heavy metals in agricultural soils and water resources poses a great threat to human health due to potential risk of their entry into food chain (Sarwar et al., 2010).
Heavy metals enter into the agro–ecosystem by natural as well as anthropogenic processes. Some soils inherit these metals from parent material from which they are being originated. Soils having a high background of these toxic metals are harmful to plants as well as animals, as the parent material naturally having high concentrations of these metals. For example, selenium (Se) toxicity problem in the Kesterson reservoirs in the West–central San Joaquin Valley was due to high Se concentration in parent material (Presser, 1994). Anthropogenic sources include application of phosphate fertilizers, sewage sludge and anthropogenic emissions from power stations, metal industries, urban traffic and cement industries (Wu et al., 2004). These processes contribute to higher concentrations of heavy metals to the agricultural soil–environment.
From soil, heavy metals are taken up by plants through the cortical tissues of roots due to their similarity with some essential micronutrients (like zinc) and adopt symplastic and/or apoplastic pathway to reach xylem transport system (Salt and Rauser, 1995). Heavy metals can cause plant growth reduction by decreasing photosynthetic rates and chlorophyll contents (Sarwar et al., 2015). Metals can cause water stress in some plants by decreasing stomatal conductance, transpiration rate and leaf relative water contents due to decrease in size and number of xylem vessels, chloroplasts and cell enlargement (Saifullah et al., 2014). These metals can accumulate in edible plant parts and thus enter into food chain. So, it is a matter of great importance to exclude these metals from the agro–ecosystem in order to maintain a safe food chain and healthy environment.
Heavy metals are categorized as essential and non–essential metals. Essential metals including; copper (Cu), zinc (Zn), manganese (Mn), nickel (Ni) and iron (Fe), have important regulatory roles in a number of biological processes such as in electron transferring proteins and as co–factors of numerous enzymes (Fageria et al., 2009, Chaffai and Koyama, 2011). While, non–essential metals are those having no known biological functions such as cadmium (Cd), mercury (Hg) and lead (Pb). Plants exposed to heavy metals stress respond by altering cellular mechanisms (Choppala et al., 2014) and gene expression (Hussain et al., 2004, Chaffai and Koyama, 2011). All heavy metals may cause the production of reactive oxygen species (ROS) beyond their toxic limits. However, non–essential metals inhibit various biological processes either by replacing essential metals or by altering the structure of biomolecules and important stress regulatory proteins (Sarwar et al., 2010). Heavy metals may cause severe toxicity in plants by; disturbing essential groups of enzymes, destructing the integrity of important biomolecules, modifying some macromolecules, replacing essential metal ions from structural formulae of biomolecules, and altering antioxidant defense mechanisms as a result of ROS production (Sarwar et al., 2010, Chaffai and Koyama, 2011, Choppala et al., 2014). Plants adopt different strategies to cope with metals toxicity which contribute to certain tolerance mechanisms such as metal sequestration, compartmentalization in certain cell organelles, exclusion and inactivation by exudation of organic ligands (Choppala et al., 2014).
A number of physical, chemical and biological techniques can be used to remediate metal contaminated soils. However, phytoremediation has been recognized as cost effective method for remediation of metal contaminated soils. This approach of decontamination of soils has great importance especially in case of metal contamination as contaminated soil has entirely different substrate then air or water. The reason behind this might be the longer persistence of heavy metals in soil than any other component of biosphere. Since plants are the primary recipients of heavy metals, so remediation of xenobiotic metals using plants (phytoremediation) seems an effective and attractive approach in the present scenario. In current review, we briefly discuss the sources and impacts of heavy metals in agro–ecosystem, factors affecting metals bioavailability, recent phytoremediation techniques with special reference to their advantages, disadvantages, mechanisms and future perspectives, and how these techniques are useful and economical for reclamation of heavy metal contaminated soils, and the scope of genetic engineering tools to develop effective transgenic hyperaccumulator plants producing large aboveground biomass for phytoremediation of deleterious metals.
Section snippets
Heavy metals in agro–ecosystem
From plant health point of view, heavy metals are well known to be divided into two categories. Some metals are required by plants as essential micronutrients for proper plant growth such as Zn, Cu, Fe, Mn and Ni at very low concentration (Fageria et al., 2009). But excessive levels of these metals are toxic to plants and can cause growth inhibition, soil quality deterioration, yield reduction and poor quality of food with a potential health risk to human and animals (Seth et al., 2007, Seth,
Harmful effects of heavy metals
Heavy metals once enter into agro–ecosystem become very hazardous for life especially human life. Certain heavy metals show duality in plant tolerance like Fe, Cu, Zn and Mn are beneficial at low concentrations by improving plant growth and yield or/and biofortification and hence beneficial for the whole food chain linkages (Verkleij et al., 2009, Imran et al., 2016b). But higher concentrations of these metals and other nonessential elements (mentioned above) not only exert damaging effects on
Factors affecting metal bioavailability
From plant uptake point of view, the bioavailable concentration of a metal is of great concern. The term bio availability can be defined as “a part of total concentration of a metal that is available to plants, microbes etc.” and this bioavailable concentration of a metal is important regarding its uptake and accumulation in plant rather than total metal concentration in soil. A number of factors control bioavailability of a metal in the soil including soil organic matter, soil pH, competitive
Phytoremediation of heavy metals: traditional concept
The term phytoremediation refers to the use of green plants and associated microorganisms to minimize the toxic effects of potential contaminant in the environment (Greipsson, 2011). The word “phytoremediation” is derived from Greek word phyto (mean plant) and Latin word Remedium (to correct or remove an evil). This technique can be used for remediation of heavy metals and metalloids from soil and found economically feasible and efficient approach as compared to engineering techniques like
Mechanism of heavy metals phytoremediation
Generally, plant uptake metals from soil solution which act as bioavailable pool of heavy metals and plant nutrients as well. Factors like soil pH, organic matter, root exudates, microbial biomass, and competitive cations affect the availability of heavy metals in soil (Sarwar et al., 2010). A specific heavy metal once taken up by plant roots may either accumulate in root tissues (phytoimmobilization) or translocate to the aerial parts of plant through xylem vessels via symplastic and/or
Phytoremediation of heavy metals: new concept
The use of green plants to remediate a contaminated soil seems an attractive approach to tackle the heavy metal problems and having vast research background. Still the traditional phytoremediation techniques lack large scale applications because of a number of limitations. Naturally accruing hyperaccumulator plant species are either slow growing producing low above ground plant biomass or not well adapted to variety of environmental conditions (Saifullah et al., 2009). Traditional
Molecular mechanisms of heavy metals tolerance in higher plants
To understand molecular mechanisms of heavy metal tolerance in plants, it is necessary to have a brief background about toxic effects of these metals. Many heavy metals (e.g. Cd, Cu. Fe etc.) are transition metals and have capacity to oxidize as well as to reduce different biomolecules (e.g. GSH) and thus can disturb the harmony of redox status of plant cell (Chaffai and Koyama, 2011). These effects on redox status of cell may be further enhanced due to coupling reaction of these metals with
Economic evaluation of phytoremediation
Unsustainable economic activities use environmental resources–land, air, water etc.–as input in production process and give rise to negative externalities in the form of pollution and degradation of environmental resources (Champ et al., 2003). Empirical evidence shows that there is inverse relationship between environmental quality and economic growth (suggesting an inverse U relationship) and developing countries tend to use environmentally non–friendly cheaper technologies that promote
Conclusions and future perspectives
Heavy metal contamination of agricultural soils is a major environmental and health concern of today due to potential risk of food chain contamination and other associated health risks. In this situation, phytoremediation techniques may prove as important tool to tackle the problem as other physical and chemical approaches to decontaminate the polluted soils seems economically unfeasible and time consuming with less effective results. Hyperaccumulator plants can be effectively used to extract
Conflict of interest and assurance
All the authors have no any conflict of interest.
We assured that no human being or animal was used for experimental purpose in the study.
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