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In this study, sediment, roots and leaves of Rhizophora mucronata collected at Poudre d'Or were analyzed for its heavy metal content (Fe, Ni,Co, Al,Cr, Mn, Zn, Pb, Cd). In this chapter, the trend obtained for Iron , Nickel, Cobalt ,Aluminum , Chromium , Manganese , Zinc , Lead and Cadmium between sediment, roots and leaves of Rhizophora mucronata were discussed. Finally, correlation between heavy metal analyzed in sediment with that of roots were discussed followed by correlation between heavy metal content in roots with that of leaves.
5.1 Heavy Metal accumulation in Mangrove Sediment at Poudre d'Or
Wetland sediments are generally considered a sink for metals and the anoxic zone may contain very high concentrations of metals in a reduced state ( Judith S. Weis and Peddrick Weis, 2004) In this study, the total average concentration of Iron in sediment (Table 4.1) was higher followed by Aluminum,Zinc, Manganese, Nickel,Cobalt, Lead, Chromium and finally Cadmium (Fe > A l> Zn > Mn > Ni > Co > Pb > Cr > Cd).This trend follows a distinctly similar pattern (Fe > Zn > Pb > Ni > Cu > Cr > Cd) in mangrove sediments from an area under intense development and industrialization in Hong Kong (Ong Che,1999).
The elevated concentration of heavy metal in mangrove ecosystem at Poudre d'Or is because of industrial and domestic wastes that are discharge in the ocean from industrial and residential area respectively. The presence of a hospital and motor way nearby is also noted which definitely contribute to pollution in the mangrove ecosystem. High amount of solid waste were also noted everywhere at our site of study.
Furthermore, all of the metal analyzed have been found to occur at the highest concentration in zone 2 (Substrate exposed at low tide only) and zone 1(Substrate submerge at low tide). Based on studies carried out by Sarva Mangala Praveena, Ahmad Zaharin Aris and Miroslav Radojevic (2010), they found that all heavy metals displayed relatively higher concentration at high tide compared to low tide. Therefore, it shows that tides control the water-flows that carry the sediments in the mangrove forest.
The elevated concentration of metals during high tide compared to low tide is due to redox conditions in sediment columns (Akpan et al., 2002). According to Marchand et al. (2004) estuarine sediments are usually in reduced condition and have pore water with high concentration of metals. Alloway (1995) explained that heavy metals in interstitial water are the mobile fraction. De Lacerda (2004) illustrated that mobile fraction of heavy metals tends to migrate in the sediment through interstitial water until it comes in contact with oxygen. Thus precipitation of hydrous metal oxides will occur. The precipitates of these heavy metals are no longer soluble and therefore incorporated into sediments at high tide, resulting in high concentrations of these metals at high tide (Grande et al., 2003). Thus, as expected Zone 1 and Zone 2 had the highest metal concentration while Zone 3 and 4 had the lowest metal concentration.
Also,a possible reason for the high level of heavy metal in sediment especially for Iron, Zinc and Manganese might be due to the high amount of litter production on the mangrove floor and the presence of crabs. Litterfall is an important factor in the cycling of heavy metals in mangrove ecosystem. Through litterfall, metals are transferred to sediments, incorporated into organic matter and eventually released by litter decomposition (C.A.R. Silva et al.,1998). Studies done by C.A.R. Silva et al, reported that decomposing leaves have got a high concentration of Iron and Zinc while Manganese concentration initially decreases in decaying leaves and afterward increases. The increase in manganese concentration is probably due to precipitation and adsorption of Mn4+, when reduced Mn2+ liberated from anoxic porewaters meet oxidizing conditons of the incoming tide and may be adsorbed to litter particles (Lacerda et al., 1988; Lacerda, 1993). Coming to the activity of crabs, Robertson (1986) showed that crabs have the ability to bury over 75% of total litterfall at low tide, speeding the “exportation” of deposited leaves. In our study area the activity of crabs, evaluated visually was very intense during low tide periods thus accounting for the high level of heavy metals in sediment.
Furthermore heavy metals such as Nickel ( χ² = 14.470, P< 0.05), Cobalt ( χ²= 14.830, P<0.05), Aluminum ( χ² = 20.544, P<0.05), Chromium ( χ² = 9.057, P<0.05) and Manganese ( χ² = 18.083, P<0.05) were found to be significantly difference in sediment concentration between the four zones whilst Iron ( χ² = 4.186, P>0.05), Zinc ( χ² = 5.937, P>0.05). and Lead ( χ² = 2.038, P>0.05) concentration were not. This statistically significant difference might be attributed to the effects of biological and physical phenomena, such as tidal inundation, salinity changes, wind and waves. These phenomena allow the processes of bioturbation, re-suspension and erosion that are known to affect the metal concentrations in surface sediments (Bellucci et al., 2002).
Base on our result obtained (Table 4.1 and 4.2), it can be deduced that heavy metals (Lead, Cadmium,Zinc, Nickel,Chromium) concentration in sediment was below the critical soil concentration (Table 2.1) thereby confirming no serious problem of pollution at our study site.
5.2 Heavy metal accumulation in roots and leaves of Rhizophora mucronata
Mangrove plant absorb heavy metal from the sediment through their roots system. Heavy metals available for plant absorption are those that are geochemically mobile in the sediment. The concentration of heavy metals in roots is expected to be proportional to the level of those metal in sediment. (SD Mremi et al., 2003) .
Based on the trend obtained for heavy metal concentration in roots between the 4 zones, it was found that all of them with the exception of Zinc and Cadmium were found in higher concentration in zone 1 and 2 compare to the other zones. One reason for this might be because the concentration of heavy metal in sediment was higher in these zones.
Moreover, Of all the heavy metal examined, it was found that the total mean concentration of Zinc (Table 4.4) was higher in the roots of Rhizophora mucronata followed by Iron, Cadmium, Chromium, Manganese, Cobalt, Lead, Nickel and finally Aluminum in all four zones. (Zn > Fe > Cd > Cr > Mn > Co > Pb > Ni > Al). Based on the general order of transfer coefficient of heavy metals from soil to plant roots proposed by Kloke et al, it was found that Zinc had the highest transfer coefficient and are more readily taken up (Alloway.,1995) thereby accounting for its high concentration. In addition, Zinc along with Iron are considered as essential elements which mangrove plants requires for their growth and metabolism (Tam and Wong 1997).
Furthermore, significantly higher heavy metals concentration was recorded in roots of Rhizophora mucronata in comparison to their leaves especially for Zinc and Iron (Tam and Wong.,1997). This shows the high capacity of Rhizophora mucronata to take up and accumulate heavy metals from the sediment in their roots (Mohd Fazlin Nazli and Nor Rasidah Hashim., 2010) with little transportation to the leaves of the plant (Tam and Wong., 1997). It is to be noted that factors such as species of the plant, the particular metal and a number of environmental conditions affects the degree of upward translocation of metals (Judith S. Weis and Peddrick Weis.,2004). In fact the roots of mangrove have been known to be a good absorptive sponge for heavy metals in soil and water. Metal absorbed or absorbed by roots are often bound with the cell wall material or other macromolecules to prevent them
from being translocated to the leaves (Tam and Wong., 1997). That is why all of the heavy metals analyzed were found to be in higher concentration in Rhizophora mucronata roots supporting the fact that the mangrove roots act as a barrier for heavy metals translocation to the leaves.
Metal mobility within Rhizophora mucronata was in descending order of Fe > Mn > Cr > Co > Cd > Zn > Ni > Pb > Al thus showing that excess Iron, Manganese and Chromium were more likely to be translocated from the root to the aerial part of the plants rather than Nickel and Lead. High Iron concentrations in leaves may be attributed to the fact that being an essential micro-nutrients, it is highly required by the mangrove plants for their proper functioning. It is known that iron serve as a catalyst for the synthesis of chlorophyll and also function as as an activator for biochemical processes such as respiration, photosynthesis (Manjula V. Nathan, University of Missouri, 2009 ). The presence of non-essential metals such as Nickel and lead in leaves in low concentration maybe because the defensive strategy of the roots which act as a barrier has exhausted at high concentration of heavy metals in the surrounding thereby promoting the translocation of Nickel and lead to the leaves. Chaney and Giordano classified Manganese as the element which is readily translocated to the plant top,Cobalt and Nickel as intermediate and lead which is translocated to the least extent (Alloway.,1995) thereby accounting for the high Manganese concentration and low Lead concentration in leaves.
Furthermore, studied done by Chua and Hashim (2008) have found that plants are capable of deriving significant amount of certain heavy metals through foliar absorption in addition to their ability of accumulating heavy metals through their roots. Since the mangrove forest at Poudre d'Or is located near industries and the latter are the source of air pollution containing a huge amount of heavy metals, therefore, presence of heavy metal such as Lead and Cadmium in leaves can be attributed to foliar absorption.
Also high level of heavy metal in leaves can be based on the maturity of leaves of Rhizophora mucronata. Studies carried out by Zheng et al (1997) have shown that as leaves of Rhizophora stylosa develop from young to old, Zinc and Nickel can be translocated and reused before defoliation, while amounts of other heavy metal elements such as Lead, Cadmium, Chromium and Manganese increased in older leaves. Thus, high level of Manganese, Zinc, Lead and Cadmium in the leaves collected at Poudre d'Or in zone 1 in comparison to other zones might be because the leaves analysed were older.
In all Four Zones, the concentration of Aluminum in leaves were 0 mg/g confirming the statement that the roots actually serve as a barrier for heavy metal thus preventing them from accumulating in aerial part of the mangrove tree. Furthermore, the result shows (Table 4.5 and 4.6) that the leaves can be consider as a good bio-indicator for Iron and Manganese.
Concentration of Iron, Chromium, Nickel, Cadmium and Cobalt in roots and leaves (Table 4.3 to 4.6) of Rhizophora mucronata lies within or above the critical level range of heavy metal in plant thereby causing phototoxicity.
Even though all these heavy metals were found in high concentration (sediment > roots > leaves) in the mangrove weland and their tissues at Poudre d'Or, it was observed that Rhizophora mucronata was capable of surviving and developing under a considerable pollution load and neither the leaves nor the roots showed any sign of injury thereby suggesting that it can be efficiently used as a phytoremediator for these heavy metals.
5.3.1 Relationship between heavy metal in sediment and heavy metal in roots of Rhizophora mucronata.
Spearman's rho analysis (Table 4.1 and 4.2) showed that heavy metals in sediment such as Iron in zone 1 (Substrate submerge at low tide) and zone 2 ( Substrate expose at low tide only), Nickel in zone 2 and zone 3 (Substrate expose by tide heights at intermediate tide between high and low tide only), Cobalt in zone 1,zone 3 and zone 4 (Substrate expose at high tide), Aluminum in zone 2 and 4, Chromium in zone 4, Manganese in Zone 1 and 2, Zinc in zone 2 and Lead in zone 2 were negatively correlated with Rhizophora mucronata roots content.
From the correlation obtained, it was found that heavy metals such as iron in zone 4,Nickel in zone 1 and 4, Cobalt in zone 2, Aluminum in zone 1 and 2. Chromium in Zone 1, 2 and 3, Manganese in Zone 3 and 4, Zinc and lead in zone 1,3 and 4 in sediment were positively correlated with heavy metal content in roots of Rhizophora mucronata. This implies that these heavy metals were absorbed from the sediment into the roots and thereby contributed to the concentration of metal in the mangrove roots. However, only for Manganese in zone 3 (P < 0.01) that there was enough evidence to prove that indeed it contributed to the concentration of heavy metal in roots. Also in all 4 zones, no correlation were obtained for Cadmium between sediment and roots.
5.3.2 Relationship between heavy metal in roots and heavy metal in leaves of Rhizophora mucronata
From the correlation obtained ( Table 4.3 and 4.4), heavy metals such as Iron in zone 1 and 2, Nickel in zone 1, Cobalt in zone 2,3 and 4, Chromium in zone 2, Manganese in zone 1, Zinc in zone 1 and 2 and Cadmium in zone 3 in roots were found to be positively correlated with heavy metals in leaves of Rhizophora mucronata. This implies that these heavy metal were translocated from the roots to the leaves thereby contributing to the concentration of these metal in the leaves of the mangrove plants. However, all of the correlation were insignificant which mean that there was not enough evidence at 5% level two-tailed test to prove the above statement.
Spearman's rho analysis showed that heavy metal in roots were negatively correlated with heavy metals in leaves of Rhizophora mucronata ( Iron in zone 3 and 4, Nickel and Manganese in zone 2,3 and 4, Cobalt in zone 1, Chromium in zone 1,3 and 4, Zinc in zone 4, Lead in zone 1,2 and 4 and Cadmium in zone 1,2 and 4). This suggest that maybe the concentration of these metal in the leaves were not due to translocation from the roots but rather due to foliar absorption from the atmosphere. Also, for Zinc and lead in Zone 3 and 4 respectively, no correlation was obtained while for Aluminum no correlation was obtained in all four zones.
Conclusion, Limitation and Recommendation
Heavy metals such as Iron, Nickel, Cobalt, Aluminum, Chromium, Manganese, Zinc, Lead and Cadmium were analyzed in sediment, roots and leaves of Rhizophora mucronata collected at Poudre d'Or. Heavy metal concentration was found to be high in sediment especially in zone 2(substrate expose at low tide) but was below the critical soil concentration. This indicate no serious problem of pollution at Poudre d'Or. However, the result confirm that wetland forest does act as a sink for heavy metals. It was also found that heavy metal concentration with the exception of Iron, Zinc and Lead were statistically difference when all four zones were pooled together. Moreover, this study has shown that Rhizophora mucronata possess the capacity to take up heavy metals via its roots and translocate them, with a preference for essential micro-nutrient such as Iron into their leaves thereby suggesting that it can be used as a phytoremediation species. Presence of non-essential element such as Nickel and lead in leaves is also related to foliar absorption, exhaustion of root to act as a barrier and maturity of leaves. From the correlations, it can be deduced that only heavy metal in sediment contribute to heavy metal concentration in roots whereby heavy metal from sediment such as Manganese from zone 3 showed a significant relationship ( rs = 0.900, p<0.01) with root content in mangrove forest.
- Breakdown of microwave digestor
- Heavy metal analysis using atomic absorption spectrophotometry was delayed as acetylene gas was over.
- Performing analysis using Graphite was impossible due to fluctuation of the apparatus.
- Further studies need to be carried out at different mangrove sites across the island in order to be able to compare level of pollution.
- Increase sample size in order to have a better statistical result.
- Monitoring of physical parameters such as pH, salinity, temperature and dissolved oxygen need to be done at approximately the same time during the day.
- Further studies is recommended to ascertain the true potential of Rhizophora mucronata as a phytoremediation species