Accumulation of Metals by Mangrove Species and Potential for Bioremediation

Abstract

The Indian Sundarbans region at the apex of the Bay of Bengal in Eastern India is impacted by industrial and urban development, including the discharge of large volumes of wastewater. We analyzed the concentrations of zinc (Zn), copper (Cu) and lead (Pb) in the root, stem and leaves of four dominant mangrove species, Avicennia officinalis, Avicennia marina, Avicennia alba and Excoecaria agallocha in samples collected from 12 different monitoring stations in the Indian Sundarbans. At all of the stations, the magnitude of accumulation of the metals in the vegetative parts of the mangroves was in the order, root > stem > leaf. The maximum concentrations of zinc in the roots (116.1 ± 4.2 μg/g dry weight) and the stems (100.4 ± 6.2 μg/g dry weight) was observed in A. officinalis collected at the Kakdwip site, whereas in leaf tissues, the maximum mean zinc concentration (66.8 ± 3.5 μg/g dry weight) was observed in A. marina collected at the Chemaguri site. The concentrations of copper were lower, with the maximum mean concentration (51.1 ± 3.0 μg/g dry weight) in the roots of A. officinalis at Kakdwip. In all vegetative parts, the concentrations of lead were lowest relative to the other metals, with the maximum mean concentration (30.4 ± 2.0 μg/g dry weight) observed in the roots of A. officinalis at Kakdwip. The concentrations of metals were generally lower in the vegetative parts of E. agallocha relative to the other mangrove species. Simultaneous analysis of dissolved metals in the surface waters at the monitoring stations revealed that zinc was present in high concentration, followed by copper and lead. Among the 12 selected stations, the highest concentrations of dissolved zinc, copper and lead were observed at Kakdwip at concentrations of 668.5 ± 5.4, 176.3 ± 4.6 and 48.4 ± 3.1 μg/L, respectively. This site is located in the western part of the Indian Sundarbans. The site at Bagmara in the eastern part of the study area had the lowest concentrations of dissolved heavy metals, as well as the lowest concentrations of the metals in sediments. High metal concentrations in sediments observed at the Frasergunj site (117.3 ± 4.3 μg/g dry weight for zinc, 37.7 ± 2.4 μg/g dry weight for copper and 10.8 ± 1.0 μg/g dry weight for lead) may be due to the presence of a fish landing station at this site. There were significant correlations between the levels of the metals in ambient media (i.e. water, sediments) and the concentrations of the metals in the vegetative parts of the mangroves; indicating that the levels of metals in mangroves reflect environmental exposures. The results indicate that mangrove species can accumulate these metals from the marine environment to ppm concentrations. Bioaccumulation factors for A. officinalis vegetation relative to sediment appeared to be >1 for lead, which is discussed in the context of using this species for bioremediation; a new dimension in the spectrum of ecosystem services offered by the mangroves.