Mercury is one of the toxic heavy metal because it is toxic to both aquatic and human life. Main source from which mercury released into the environment is industry. It is released by industrial activities including; mining, coal combustion, smelting, waste incineration etc. it may release in both aqueous and gaseous form. Methylmercury (MeHg) is one of the compound of the mercury and it is more toxic and hazardous. Mercury and its compounds are toxic because of their penetration into blood-brain barrier; it can interfere with uptake of metal ions; it can disrupt protein and metal thiolate bond. There are several diseases which are caused by mercury and its compounds such as antibiotic resistance, mental retardation, it can also disturb the reproductive system and many other diseases are caused by mercury.
There are many ways to use microalgae for the bioremediation of mercury, but most useful are given here:
Microalgae can biotransform/convert mercuric ion (more toxic) into the mercury(less toxic) and to metacinnabar (-HgS) into some extent. After this reduction or conversion which is catalyzed by mercuric reductase, the volatile mercury is now removed. This is removed by both biological and non-biological volatilization. Considering the conditions of both, non-biological volatilization is such type of volatilization which happens under the presence of light, while biological volatilization is such types of volatilization which was light independent but require metal concentration and cell density. Volatilization happens more rapidly, usually within 20min to few hours and it was observed in some microalgae species such as Selenastrum minutum, Chlorella fusca var. fusca, Galdiera sulphuraria.
Using genetic engineering, mercuric reductase was expresses from B.Megaterium in chlorella sp.DT. It was observed that this transgenic strain removes more level of mercury and it was estimated that it removes up to 2 fold greater than their control. It was also observed that oxidative stress was reduced.
In this polyelectrolyte self assembly together with biomemtic mineralization which used to pretreat C.vulgaris. These microalgae cells wrapped in polyelectrolyte coating, and the mineral layer was doped with sulfur atom. This mineral layer was transparent amorphous calcium phosphate layer. This layer provides protection to microalgae cell from heavy metal poisoning, so the microalgae cells grows in high concentration of heavy metals. The concentration would be in range of 100 g/L. It was also observed that PH plays an important role in the removal of mercury. At PH 5.5 raw C.vulgaris removes the mercury up to 62.85% while, the efficiency of mineralized C.vulgaris to absorb mercury is up to 94.7% at PH 7.
Nowadays, membrane filtration and activated sludge system become famous because of their efficiency. So, Microalgae dynamic membrane formed using C. vulgaris powder which was then used in dynamic membrane bioreactor to remove mercury. Advantage of this membrane bioreactor includes: increased removal of mercury; reduction in fouling; reduction in expensive cost of membrane recovery. It was observed that it show greater metal removal capacity than their control.
However, other techniques that can be used for mercury removal can be thiol-chelation and bio-Methylation.
There are mainly two algal based biotechnologies that are used. These are used to remove pollutants such as metals and organic nutrients.
This is consists of biomass of green algae (chlorella), cyanobacteria (Anabaena) or sometimes both. It was used for treating underground water so that removals of heavy metals and chlorinated compounds make possible.
There is greater removal of metals is possible because of high PH. It was observed by Toumi et al. (2000). He also observed that this high PH was due to their photosynthesis.
There are currently many technologies that are using microalgae alone or with other microorganism. Some are described here:
This uses algae to remove heavy metals from aqueous solution and further consist of many processes. Two main are given here:
a) The algae-silica preparation (AlgaSORB-scy):
This is made from the natural dead cells of the blue-green cyanobacterial algal and used to remove arsenic heavy metal.
b) The use of inactivated micro algal biomass:
This is used as commercial adsorbent in the form of biotraps to remove heavy metals from industrial effluents.
Bio-fix is made of wide variety of biomass such as algae, moss, yeast, bacteria and aquatic flora.
Metals recovered from bio-fix could offer more income than other process such as lime precipitation. Bio-fix shows more affinity for removal towards Al, Cd, Zn and Mn.