INTRODUCTION
Investigations about new technologies (Chen and Cutright, 2001) involving the removal of toxic metals from wastewater has direct attention to biosorption, based on metal binding capacities of various biological materials. Biosorption can be defined as the ability of biological material to accumulate heavy metals from wastewater through metabolically mediated or physico-chemical pathway of up takes (Stirk and Staden, 2002). It can be use to detoxify sites containing metals; pesticides, solvent, explosives, crude oil, hydrocarbons and land fill leachates. This is often referred to as bioremediation, botanical-bio remediation and green remediation (Bizily et al., 1999).
As a matter of fact that larger parts of agricultural soil are contaminated
with toxic metals including Pb, Cd, Hg and As (Strik et al., 2002). Therefore
approaches to develop techniques to reduced heavy metal from soil are important.
Many techniques have been developed to control the toxicity of heavy metals
in the environment. However, to reduce heavy metal in food chain, that transfer
less heavy metals to the shoot is required. The seaweed (Azmat et al.,
2006a) is an organic non pollutant renewable resource which helps in increasing
the water holding capacity of soil and conditioning the soil by absorbing (Same
et al., 2002) toxic metal on its surface. Biosorption of heavy metals
from soil by seaweeds offers a low cost method to reduced the toxic metals from
the sludge and some extracted metals may be recycled for value. Pb (Blaylock
et al., 1997) and Cr+6 may be inactivated in the soil by binding
the metal (Azmat et al., 2006b) with chelating compound found in seaweeds
(Strik and Staden, 2000).
Sludge originates from the process of treatment of wastewater. Due to physical-chemical processes involved in the treatment, the sludge (Joohyjun et al., 2003) tends to concentrate heavy metals and poorly biodegradable, trace organic compound as well as potentially pathogens (virus, bacteria etc.) present in wastewaters (Azmat et al., 2005). Sludge is however, rich in nutrients such as nitrogen and phosphorous and contains valuable organic matter that is useful when soils are depleted or subjected to erosion. Presence of metal in soil plays an integral role in the growth of plant. Some metals (e.g., Ca, Co, Cr, Fe, K, Mg, Na, Ni and Zn) are essential, serve as micronutrients and are used for redox process, to stabilize molecule through electrostatic interaction, while other metals (i.e., Ag, Al, Cd, Au, Pb and Hg) have no biological role and are toxic to physiological processes of plants.
Toxicity of non-essential metals occurs through the displacement of essential metals from their native binding sites or through ligand interaction. In addition, at high level both essential and non-essential metals can damage cell membrane; disrupt cellular functions and damage the structure of DNA.
Keeping these above views in sight, this research has been design to conditioning the solid sewage sludge (used for the cultivation of vegetables) from SITE area Karachi by green seaweeds, as a biosorbent to detoxify the metals found in sludge. The effect of seaweeds on sludge will test on plant growth and results will discus in relation with growth, health, nutritional quality, essential and non-essential ions uptake by plants.
MATERIALS AND METHODS
Solid sewage sludge from SITE farm of Karachi were collected from three different sampling stations in the month of April 2005 where vegetables like onion (Allium cepa), spinach (Spinacia oleracea) and turnip (Brassica rapa) were cultivated and referred as sludge A, sludge B and sludge C, respectively in the laboratory.
The green seaweed Codium iyengrii were harvested in the morning from
Bullijee coastal beach of Arabian Sea, Karachi, in the months of February and
March 2005 when it is abundant at the beach. The seaweed were dried at room
temperature and then finally grounded by rotary mill to dry seaweed powder.
The bean plants (Vigna radiata) were chosen to study the effect of sludge
on the physiological process of plants. Three pots of sludge A, B and C were
prepared and seeds of bean plants were soaked in water and surface sterilized
which later on introduced in to the pots of sludge. Similarly three pots of
sludge A, B and C in which 1 g of dry seaweeds were added and one pot of garden
soil were also prepared for comparative study. The pots were placed in natural
environment.
Morphology of root, shoot and leaves were compared with plants grown into garden soil.
Analysis of essential and non essential metals: The ash method was use to prepared the solution for detection of metals ions in the sludge and plants. The solutions were subjected to flame photometry and atomic absorption spectrophotometry for the analysis of essential, non-essential mineral and toxic metals ions.
Physiological process of bean plant cultivated into the sludge and treated sludge were observed by visible spectrophotometer as designated by Tandon (1993). These values were compared with control plant, grown in to the garden soil.
RESULTS AND DISCUSSION
Analysis of sludge A, B and C before addition of seaweeds showed that it contain essential and non essential macro and micronutrients ions with toxic metals (Table 1). Solid sewage shows high percentages of macronutrients like Na, K, Ca and Mg which is useful for plant growth.
Results showed that sludge A contain high concentration of toxic metal, in
this sludge onion was cultivated where as sludge b showed less concentration
of toxic metals, spinach were grown in this area which is a leafy vegetable
and leafy vegetable accumulate maximum (Azmat et al., 2005) concentration
of toxic metals in their broad leaf size while C shows moderate concentration
of non-essential metals, turnip were cultivated in this place. Plants grown
in to this sludge showed that it accumulate (Hammaini, 2003) maximum amount
of essential and non-essential metals as compared to plant grown in garden soil
(Table 2).
| Table 1: |
Analysis of macro nutrient, micronutrient and toxic metal (mg kg-1)
found in sewage sludge |
|
| Table 2: |
Accumulation trend of essential and non essential metals (mg kg-1)
from sewage sludge into bean plant |
|
Nutritive values like chlorophyll a and b, proteins and carbohydrate were found
to be less as compared with control pot which contain garden soil while amino
acids were absent both in roots and shoots (Table 3). Table
3 shows the growth rate of seedling in sewage sludge. Comparison of average
growth rate of bean plant with the seedling grown into garden soil, indicated
that length of root and shoot of bean plant were less as compared with the plant
grown in garden soil.
Addition of seaweed powder to the sludge reduces the concentration of essential and non-essential metals available to plant (Table 4) while concentration of micronutrients were found to be increased and remarkable differences were recorded on the seed germination (El-Sheekh et al., 2000) and leaves size (Table 5). Seaweed also reduced the metal accumulation in the roots of plants results in the healthy growth of plant consequently change in the morphology of plants were observed with larger and broader size of leaves. It also helps in increasing the length of the root and makes it powerful due to which healthy growth of bean plants were observed (Table 5). This also represent the improvement in the nutritive value of edible plant.
It was also observed that toxic metals like Pb, Hg, Cd and As were not reported in shoot and root of plant (Table 6). This indicated that seaweeds correct the soil condition by providing the ligands interaction with heavy metals (Kamnev and lelie, 2000).
Mechanism of complexation: The biosorption of metals (Nortan, 2003) take place through both adsorption and formation of coordination bonds between metals and amino and carboxyl groups of cell wall polysaccharides of seaweeds. The metal removal from sewage sludge may also take place by complex formation on the cell surface after the interaction between the metal and the active groups of proteins and amino acids found in green algae. Complexation was found to be only mechanism responsible for calcium, magnesium, cadmium, zinc, copper and mercury accumulation by marine algae.
Investigation showed that application of dry seaweed powder to the sludge provides
multiple levels of potential benefits. These potential benefits have been identified
during seaweed spray including nutritional level, physiological process, morphology,
mineral and metal ion (Schiewer and Wong, 2000) uptake by plants (Table
6). The physico-chemical interaction occurs between the toxic metal and
the surface polysaccharides of the biomass (algae), ion-exchange, complexation
and adsorption takes place and the phenomena is not metabolism dependent.
| Table 3: |
Average growth rate and nutritive values in bean plant before treatment |
 |
| Table 4: |
Analysis of macro, micronutrient and toxic metal (mg kg-1)
in sewage sludge after addition of seaweed |
|
| Table 5: |
Average growth rate and nutritive values in bean plant after treatment |
 |
| Table 6: |
Accumulation trend of essential and non essential metals (mg kg-1)
from sewage sludge into bean plant after treatment |
 |
The surface of the seaweeds is constituted of polysaccharides and proteins
that provide a wide range of ligands for heavy metal ions. These processes are
rapid and reversible. Seaweed contains all known trace element and these elements
can be made available to plant by chelating i.e., by combining the mineral ion
with organic molecules, starches, sugars and carbohydrates in seaweed and seaweed
products possess such chelating properties (Kapoor and Viraroghvan, 1998). As
a result, these constituents are in natural combination with the iron, cobalt,
copper, manganese, zinc and other trace elements found naturally in seaweed.
That is why these trace elements in seaweed product do not settle out in alkaline
soils, but remain available to plants, which need them. Table
5 shows that when seaweeds mixed with the sludge, biosorption of toxic metals
takes place, which stimulate the growth rate and physiological processes.
CONCLUSION
Today’s industrial world has contaminated our soil, sediments and aquatic resources with hazardous material. Metal water interaction is often result of industrial activities, such as mining, refining and electroplating. Hg, Pb, As, Cd and Cr are often prevalent at highly contaminated sites. Therefore it is our responsibility to check and develop the low cost techniques to remove the toxic metals by methylation, complexation or changes in valance state from the environments for humanity. This research suggests that marine green algae may be use for removal of heavy metals from contaminated soil as a very low cost method.
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