Heavy metal removal from industrial wastewater through Phytoremediation technique

In Bangladesh, industrial solid wastes and effluents are being discharged at random without treatments directly to soil, canals, and rivers. The solid wastes are also used in landfilling. It pollutes soils and natural water systems as well as groundwater endangering human health, aquatic lives, and crop production in Bangladesh. It contains heavy metals like Cu, Zn, Pb, Cr, Cd, As, Hg, Mn, and Fe.

Heavy metals are conventionally de?ned as elements with metallic properties and an atomicnumber>20.

Metal pollution has a harmful effect on biological systems and does not undergo biodegradation. Toxic heavy metals such as Pb, Co, Cd can be differentiated from other pollutants since they cannot be biodegraded but can be accumulated in living organisms, thus causing various diseases and disorders even in relatively lower concentrations. Heavy metals, with soil residence times of thousands of years, pose numerous health dangers to higher organisms. They are also known to have an effect on plant growth, ground cover, and have a negative impact on soil microflora. It is well known that heavy metals cannot be chemically degraded and need to be physically removed or be transformed into non-toxic compounds.

Some of them are toxic to plants and some others to both plants and animals. In areas where irrigation water is scarce, the use of industrial wastewater is an important source for supplementing water resources. The use of wastewater for irrigation may be an alternative method for recycling if used rationally and inappropriate concentrations (Kumawat et al., 2001). Although industrial wastewater contains many constituents which are phytotoxic at higher concentrations (Kaushik et al., 2005), many of these constituents can be helpful for plant growth at low concentrations (Swaminathan and Vaidheeswaran, 1991).

The effect of industrial effluent on different plant species has been investigated by several research (Dayama, 1987; Sujatha et al., 1992; Himabindu and Reddy, 2005). The use of industrial wastewater in forest tree species may contribute considerable to alleviate the heavy metal from industrial wastewater. Wastewater from different sources contains considerable amount of organic matter and plant nutrients (N, P, K, Ca, S, Cu, Mn & Zn) and has been reported to increase the crop yield (Pathak et al. 1998; Pathak et al.1999; Ramana et al. 200; Lubello et al.2004; Nagajyothi et al.2009; Nath et al. 2009). Sewage sludge consists of multi-element organic wastes that are also used commonly as manure (Otobbang et al. 1997).

Dash and Mishra (1999) has studied that, sewage enriched paper mill wastewater has a positive effect on the growth and pigment content of Westillopsis prolifica. Efforts have been made by different workers to determine the effect of different industrial wastewater on seed germination of various crops such as maze (Choudhury et at. 1987); rice (Behera& Mishra, 1982; Singh et al.1985); wheat (Agarwal et al.1995; Nagda et al. 2006); pine (Czabator, 1962); green gram (Subramani, 1999); mung bean (Nagda et al. 2006); pea, lentil & gram (Khan et al.2011); lettuce (Bazai & Achakzai, 2006); Vignaangularis, Vigna cylindrical and Sorghum cernum (Doke et.al.,2011). However, a little work has been done on the agro potentiality of industrial wastewater.

Phytoremediation is a bioremediation process that employs varieties of plants to eliminate, transfer, maintain, extract or degrade contaminants in the soil and groundwater.

 Phytoremediation has become an effective and affordable technological solution used to extract or remove inactive metals and metal pollutants from contaminated soil. Phytoremediation is the use of plants to clean up contamination from soils, sediments and water. This technology is environmentally friendly and potentially cost-effective. Plants with exceptional metal accumulating capacity are known as hyper accumulator plants. Phytoremediation takes the advantage of the unique and selective uptake capabilities of plant root systems, together with the translocation, bioaccumulation, and contaminant degradation abilities of the entire plant body.

Many species of plants have been successful in absorbing contaminants such as lead, cadmium, chromium, arsenic, and various radio nuclides from soils. One of the phytoremediation categories, phytoextraction can be used to remove heavy metals from soil using its ability to uptake metals which are essential for plant growth (Fe, Mn, Zn, Cu, Mg, Mo, and Ni). Some metals with unknown biological function (Cd, Cr, Pb, Co, Ag, Se, Hg) can also be accumulated.

There are different types of phytoremediation mechanisms that are used to eliminate or degrade contaminants from soil and water discussed as follows:

1. Rhizosphere biodegradation

2. Phytostabilisation

3. Phytoaccumulation (phytoextraction)

4. Rhizofiltration (Hydroponic systems for treating water streams)

5. Phytovolatilization

6. Phytodegradation:

* In this process, specific plant species are used for a particular contaminant on the basis of the degradation capability of plant species.

* In this process, plants actually metabolize and deteriorate contaminants within plant tissues.

7. Hydraulic control:

* In the hydraulic control process, trees are used as they have the potential to carry water from depth surface as compared to plants.

* They indirectly amend by controlling the groundwater movement.

* When roots of trees reach down towards the water table, they behave as natural pumps and forms a dense root mass that uptakes large quantities of water.

For example, A poplar tree pulls out 30 to 35 gallons of water per day from the ground, and cottonwood can uptake up to 340 gallons per day.

* Phytoremediation is applied for the elimination/treatment of metals, radio nuclides, pesticides, explosives, fuels, Volatile Organic Compounds (VOCs) and Semi-Volatile Organic Compounds (SVOCs).

* Researchers are also working to find out how phytoremediation plays a role to remediate per chlorate, a contaminant that has been shown to be consistent in surface and groundwater systems.

* It may be used to clean up contaminants present in soil and groundwater.

* For radioactive substances, chelating agents are sometimes used to make the contaminants accessible to plant uptake.

Heavy metals uptake, by plants using phytoremediation technology, seems to be a prosperous way to remediate the heavy metals-contaminated environment. It has some advantages compared with other commonly used conventional technologies. Several factors must be considered in order to accomplish a high performance of remediation results. The most important factor is a suitable plant species which can be used to uptake the contaminant. Even the phytoremediation technique seems to be one of the best alternatives, it also has some limitations. Prolong research needs to be conducted to minimize this limitation in order to apply this technique effectively.

The writer is a student Department of Environmental Science and Disaster Management, Noakhali Science and
Technology University                                  
Email: [email protected]