Passive Phyto-Remediation of Heavy Metals by Biosorption of Sargassum cinereum

Introduction

Globally, heavy metals contamination in marine coastal and estuarine ecosystem due to chemical disposal and effluents from various industries, anthropogenic activities and metal mining is one of the major problems. The load of high accumulation of toxic heavy metals to the environment, gradually increase the inclusion of toxic heavy metals to the food chain of marine organism indirectly affecting the health of human beings as they consume marine fishes and arthropods as food items. The bioaccumulation and persistence of heavy metals in environment, consequently biomagnifications of heavy metals in food chain causes the irreparable damage at very low concentration in the environment [1]. It has been reported that some of the heavy metals are mutagenic, carcinogenic and have dangerous health effects [2]. Trying to solve this problem, some conventional expensive techniques are applied to bio-remediate the toxic heavy metals but phytoremediation is regarded as the feasible and economically affordable approach [3]. It had been reported that seaweeds are bio-indicator of heavy metals pollution, as specific seaweed preferably grow in dense population in the area of specific metal pollution in the environmental. As they have contribution in the nutrients cycle of the aquatic ecosystem, so they easily indicate the water quality changes [4]. Bio-sorption is an inactive physiochemical process of naturally occurring biomass to allow passively concentrates and bind contaminant onto its structure. Some algae, fungi and microbial biomass had this inactive property due to their cell structure. Another opposite process of removal of contaminants from living environment by active living cells are known as bioaccumulation. Bio-sorption has several benefits such as high efficiency, less expensive, less production of chemical or biological sludge; no additional nutrients required and there is some possibility of metal recovery. Biosorption mechanisms are extra or intracellular accumulation or precipitation including cell surface precipitation and absorption. For filtration of contaminants from polluted environment and also filtering the harmful pollutants from industrial effluent is much beneficial than the use of ion exchange resin which is expensive. As bio- sorption capacity depends on surface area and cell structure of bio absorber, so, which seaweeds have large surface area have the power of high bio-sorption capacity. Some seaweed such as Sargassum, Macrocystis, Turbinaria and Cystoseria has large surface area and quite develop and complex thallus structure, so these seaweeds may be useful as bio-sorbent. Among these seaweeds, Sargassum cinereum were used for our study because this brown seaweed is easily and abundantly available as attach and floating condition during growth season, Sargassum cinereum, grow so densely which make huge dense forest like condition of brown seaweed along the coastal areas of Gulf of Mannar, Rameshwaram. Also, this species of brown seaweed is easily cultivatable. So, in this study, we used Sargassum cinereum to test its bio-sorbent capacity for metals such as iron, lead and mercury. Similarly, the combination of two metals biosorption capacity of this brown seaweed also done, because sometimes different industrial effluents may release couple of metals as compound form. The combined bio- sorption capacity of Sargassum cinereum will explain the metal-metal replacement mechanism during bio-sorption capacity. If environment is polluted with multiple heavy metals, then only a single seaweed will be enough and useful for biosorption to detoxify the environment.

Results

We selected five species of Sargassum and analysed their heavy metals content and it showed that Sargassum cinereum contained highest amount of Fe, Pb, Cr, As and Hg rather than Sargassum cinctum, Sargassum cristaefolium, Sargassum polycystum, and Sargassum wightii. The brown seaweed Sargassum cinereum contain 3520.35ppb Fe, 5.582ppb Pb, 65 ppb Cr and similarly 0.12ppb As and 0.35ppb Hg which clearly indicated its high metal content capacity. Among five different Sargassum Sp. Sargassum polycystum contained lowest amount of all heavy metals (Table 1).

The ICP results showed that Sargassum cinereum bio-sorption capacity increased with increase of metal concentration in solution. For this study, the control was 10gm fresh Sargassum cinereum which was kept in 250ml normal seawater. The iron concentration of control was 3520.35ppb (Table 1) but Sargassum cinereum which was treated with various concentration of iron solution, showed high concentration of iron deposition. After 48 hours of treatment with 0.25mM iron solution Sargassum cinereum absorbed 6679.70ppb iron and treated with high concentration of iron solution such as 4mM which absorbed maximum iron such as 21781.3089ppb. The results of this study reported that S. cinereum had an excellent capacity of iron bio-sorption (Figure 3). So, this brown seaweed may be use as phytoremediator for removal of iron from highly polluted water bodies or from any industrial effluents after more extensive experiment in field condition. The anatomical and morphological structures of different parts of the thallus treated with different iron concentration were listed below (Figure 2). The parts of treated seaweed showed dark appearance due to accumulation of high iron. The vesicles on leaf like part was very dark due to high accumulation of iron which explained that this seaweed may be accumulated high iron due to its favourable morphological structures (Figure 2).

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The biosorption of lead was increased with increase of metal concentration in the solution. So, the maximum absorption was estimated for seaweed treated with 4mM lead solution, the biosorption was 21490.22 ppb and the minimum iron absorption was 9437.70 ppb estimated at 0.25mM concentration (Figure 4). For this study, the control was 10gm fresh Sargassum cinereum which was kept in 250ml normal seawater. The concentration of Pb of control seaweed was 5.582 ppb (Figure 4).

The seaweed treated with both iron and lead, explained different pattern of bio-absorption in comparison to the single metal treatment. The solution concentration from

0.25mM to 1mM, the metal absorption was gradually increased, but after 1mM concentration, the absorption capacity decreased gradually. It may be due to both metals reaction and formation of any unknown compounds which was not absorbed by Sargassum cinereum. The combined effect of metals showed that Sargassum was absorbed more iron than lead. But up to 0.2mM to 1mM concentration both metal absorption was increased and above that concentration absorption decreased gradually (Figure 5). For this study, the control was 10gm fresh Sargassum cinereum which was kept in 250ml normal seawater. The concentration of iron and Pb of control seaweed were 3520.35ppb and 5.582ppb (Figure 5).

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The seaweed treated with different concentration of only mercury had completely different effect and had no absorption up to 2mM concentration of mercury treatment. But only at 4mM concentration absorption of mercury was detected and it was very less (3.2ppb). It may be due to high concentration treatment; basically, Sargassum cinereum had no absorption capacity of mercury because no absorption was detected up to 2mM. But the biosorption was detected at 4mM mercury treatment may be due to high stress of mercury (Figure 6). It was also observed from combined metals effect of mercury and iron. So, from this study, it indicated that Sargassum cinereum absorbed iron in all concentration from 0.25mM to 4mM. But no absorption was found for mercury treatment. So, from single metal effect and combined metals effect, it can be concluded that Sargassum cinereum had no absorption capacity of mercury or may be if only mercury will very high in the environment, then it will be absorbed, even above 4mM concentration (Figure 7).

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Discussions

Different species of marine algae accumulated heavy metals in their different body parts which study conducted along the Turkish Coast of the black Sea during 1998-2000 to study the heavy metal monitoring of seaweeds [5]. There are many previous literatures available on bio-accumulation and biosorption of metals and heavy metals reportedly. The plenty of work of elemental composition of different seaweeds have been reported periodically. But there is little gap of study of heavy metals biosorption capacity of seaweeds especially brown seaweed. As we saw huge mass of brown seaweed Sargassum Sp. is available along the coastal area, so we can utilize that mass of seaweed for bio-sorption of heavy metals and phycoremediate the polluted environment. Similarly, the brown seaweed, Sargassum cinereum naturally carrying high amount of Fe, Pb, Cr, As and Hg which have been observed from the result of heavy metal content of five different species of Sargassum. So, my focus was to test its bio-sorption capacity of these five heavy metals. Some South Korean brown seaweeds (Hizikia fusiformis, Laminaria japonica, and  Undaria pinnatifida) reported for its high capacity of pH dependent biosorption of ions of heavy metals (Pb2+, Cd2+, Mn2+, Cu2+, and Cr2O72-). For positively charged ions like Pb2+, Cd2+, Mn2+, Cu2+, towards acidic pH 2 to 4, biosorption was maximum and towards basic pH 5 to 8, solution minimum biosorption was for negatively charged ions (Cr2O7 2-) [6]. The brown seaweed Sargassum tenerrimum after five sorption-desorption cycles, uptake of 63.43mg/g cadmium [7]. It has been reported that brown algae,  Cystoseria indica also have temperature and pH dependent biosorption capacity of cadmium and nickel [8]. The brown seaweed Turbinaria conoides uptake of 220.1mg/g lead from column successfully [9]. In this present study, Sargassum cinereum showed high capacity of biosorption of iron 6679.70ppb to 21718.30ppb iron. It clearly indicates that this brown seaweed has iron biosorption capacity. This brown seaweed also has high biosorption capacity of lead and maximum absorption is 21490.22ppb after 4mM lead treatment. Similarly, the combined effect of iron and lead, also revealed that Sargassum cinereum also have biosorption capacity of iron and lead in combination. In nutshell, brown seaweed will be helpful to remove pollutant from polluted environment and water system after extensive experiment with Sargassum cinereum in field condition.

Acknowledgements

The authors are sincerely thanking to Dean and Director C.A.S in Marine Biology, Faculty of Marine Sciences, Annamalai University. The authors also thankful to DST, Govt. of India for funding support. All authors sincerely thank to all higher authorities of Annamalai University.

Funding

Authors are thankful to Department of Science and Technology, New Delhi, Government of India.

Conflict of Interest

Authors have no conflicts of interest to be disclosed.