Introduction heavy metal pollution

Table 1: Sources of heavy metals

Metal	Industry
Chromium (Cr)	Mining, industrial coolants, chromium salts manufacturing, leather tanning
Lead (Pb)	lead acid batteries, paints, E-waste, Smelting operations, coal- based thermal power plants, ceramics, bangle industry
Mercury (Hg)	Chlor-alkali plants, thermal power plants, fluorescent lamps, hospital waste (damaged thermometers, barometers, sphygmomanometers), electrical appliances etc.
Arsenic (As)	Geogenic/natural processes, smelting operations, thermal power plants, fuel burning
Copper (Cu)	Mining, electroplating, smelting operations
Vanadium (Va)	Spent catalyst, sulphuric acid plant
Nickel (Ni)	Smelting operations, thermal power plants, battery industry
Cadmium (Cd)	Zinc smelting, waste batteries, e-waste, paint sludge, incinerations & fuel combustion
Molybdenum (Mo)	Spent catalyst
Zinc (Zn)	Smelting, electroplating

(Source: INSA, 2011)

Remedial Measures
Strategies for remedy of heavy metal pollution involve reducing the bioavailability, mobility and toxicity of heavy metals. This can be achieved by three complementary functions viz., technological, management and regulatory. Technological methods involve development of the treatment system whereby pollution load in the waste or effluent is brought within the safe limits before discharge in the environment. Technology for remediation should be cost effective and environmentally sustainable. Management function is important to ensure that the right technologies are being adopted and also monitor the end results. Regulations ensure the safety and health of workers as well as the public in general by regulating the toxic metal levels in effluent release in the environment. Most conventional techniques such as thermal processes, physical separation, electrochemical methods, washing, stabilization/solidification and burial are too expensive, require high energy and may generate secondary pollutants that affect biological functioning of an ecosystem.

Therefore, alternate techniques such as bioremediation, particularly plants microbe interaction is gaining much attention for heavy metal pollution and eco-restoration of contaminated environment. Bioremediation technologies are more acceptable and offer many advantages over conventional treatment methods, for example, cost effectiveness, high efficiency, minimizing the disposable sludge volume and it also offers the flexibility for desorption techniques for biomass regeneration and/or recovery of metals (Eapen & D’Souza, 2005). Plants based technology capable of extracting and accumulating significant level of heavy metal. Phytoremediation approaches with its subset (e.g. phytoextraction, phytovolatilisation and phytostabilisation) for heavy metal pollution abatement has been well documented in recent years. At present, more than 400 plant species of 45 families are known accumulate heavy metals (Pal & Rai, 2010; Reeves & Baker, 2000; Guerinot & Salt, 2001). Several plant species e.g. Alyssum bertolonii, Brassica juncea, Eichhornia crassipes, and Iberis intermedia have been found to sequester various metals in their tissues (Pinto et al. 1987; Robinson et al. 1997; Brooks et al. 1998; Anderson et al. 1999; Boominathan et al. 2004). Success of the process of absorption and transformation of heavy metals into plant system strictly depends on their solubility and complexity (Rungwa, et al. 2013). However, Plants have constitutive and adaptive mechanisms for extracting, accumulating and tolerating high concentrations of their rhizospheric contaminants. Plants have developed range of potential mechanism to tolerate and avoid toxic effect of high metal concentration, which are as follows:

1. 
   Immobilization of heavy metals in cell walls, preventing their contact with protoplasm. Plant cell wall acts as a cation exchanger and can hold variable quantities of metal (Rauser, 1999).
   
2. 
   Compartmentalization and formation of complexes with inorganic and organic acid, phenol derivatives and glycosides in the vacuole (Singh et al. 2010).
   
3. 
   Chelation in the cytoplasm by peptide ligands such as metallothioneins (MTs) and phytochelatins (PCs). MTs are cysteine-rich polypeptides. PCs are trace metal binding peptides play key role in metal tolerance. PCs protect plant enzymes from trace metal poisoning (Pal & Rai, 2010; Singh et al. 2010).
   

Microorganisms associated with plants root system also play significant role in plants mediated heavy metal remediation technologies. Such microbial community can be classified into two major group viz., michorrhizal fungi and plant growth promoting rhizobacteria (PGPR). These microbes in rhizosphere provide a critical link between plant and soil, which is described in Figure 1.


Figure 1. Rhizosphere microorganisms as a critical link between plants and soil

(Source: Richardson et al. 2009; Hrynkiewicz & Christel, 2012)
Michorrhizal fungi form major component of rhizosphere and show mutualistic association with most plants (Azcón – Aguillar & Barea, 1992; Marques et al. 2009). Michorrhizal fungi such as, arbuscular mycorrhizal fungi (AMF) can benefits plant (Marques, et al. 2009) in following ways:

1. 
   Improve nutrient absorption through extensive extra radical hyphal networks, which explore the soil, absorb nutrients, and translocate them to the roots.
   
2. 
   Modify root system resulting in a more extensive length and increased branching and therefore enhanced nutrient absorption capacity of roots.
   
3. 
   Changes the chemical composition of root exudates and influences soil pH thus quantitatively affecting the microbial populations in the rhizosphere.
   
4. 
   Improve soil structure.
   
5. 
   Regulate hormones.
   
6. 
   Tolerance and protection against biotic and abiotic stress such as soil-borne plant pathogens, insect herbivores, drought and high levels of heavy metals.
   

On the basis of relationship with plants, plant growth-promoting rhizobacteria (PGPR) communities can be divided into two groups (a) symbiotic bacteria and (b) free-living rhizobacteria (Khan, 2005). These organisms are able to enhance plant growth through various mechanisms (Marque et al. 2009), such as:

1. 
   Allowing plants to develop longer roots during early stages of growth by reducing ethylene production.
   
2. 
   Nitrogen fixation.
   
3. 
   Specific enzymatic activity.
   
4. 
   Supply bioavailable phosphorous and other trace elements for plant uptake.
   
5. 
   Production of phytohormones such as auxins, cytokinins, and gibberellins.
   
6. 
   Produce antibiotic that protect plants from diseases.
   
7. 
   Increase plant tolerance against flooding, salt stress, and water deprivation.
   
8. 
   Produce siderophores (low molecular mass compounds, 400-1000 K dalton). Play key role in solubilizing unavailable forms of heavy metal bearing minerals by complexation reaction (Rajkumar, et al. 2012).
   

Different microorganisms apply different mechanisms for growth and metal tolerance in plants, so it can be beneficial to design the process of phytoremediation in combination with appropriate microbial consortium, which may include AMF and PGPR.