Investigation Of Se Removal Mechanisms From The Water Column Using Chlorella Vulgaris

As a naturally occurring trace element,selenium(Se)acts as an essential micronutrient for humans and animals,although there is a narrow margin between beneficial and toxic effects.For instance,high Se concentrations in drainage water have been blamed for gastrointestinal illness and hair loss among residents in Enshi County,Hubei Province,and deformation,reproductive failure or juvenile mortality in waterfowl and fish in southern California,USA.Of all the technologies compared for treatment of Se contaminated waters,bioremediation approaches may have the lowest construction and operation costs for Se removal.our previous studies have shown Chlorella sp.,in particular Chlorella vulgaris,were able to remove up to 96% of Se from the water column,mainly through volatilization(61%).In order to optimize field conditions and developing an algal water treatment system for the Se removal by C.vulgaris,we further looked into the factors that might be influential.Therefore selenium(Se)removal efficiency by C.vulgaris was evaluated under various conditions defined by the exposure Se concentration,algal density,pH and temperature and two approaches to harvest the Se containing algae from the system were investigated.The results show Se uptake,accumulation and volatilization rates increased with Se concentration until 6000 ?g Se/L and then declined,indicating maximum tolerance of Se toxicity was reached;however,a maximum Se removal efficiency plateau of approximately 90% was observed between 1000-3000 ?gSe/L.Maximum volatilization rates were reached on day 2 for all the scenarios,followed by a decline on day 3,indicating it might take at least one day for C.vulgaris to reach full capacity to volatilize selenite.Over a range of algal densities(0.08 to 4.03 g/L DW),C.vulgaris was most effective at 0.75 g/L DW,since the total amount of Se absorption,volatilization and accumulation reached maximum at an algal density of 0.75 g/L DW when treated with 1580 ?g Se/L,and algal volatilization was dominant at densities below 1.37 g/L DW.Moreover,Se removal efficiency reached minimum at the lowest algal density of 0.08 g/L,with the minimum value of Fv/Fm,indicating the negative effects of selenite toxicity.Se was volatilized two times faster at 25°C than at 20°C in the first 24 h,indicating facilitated algal metabolism of Se by higher temperatures.Se volatilization rates reached maximum at pH of 6.5 by C.vulgaris while the Se removal efficiency reached maximum at pH of 8.0.To avoid potential ecotoxicity of Se as a side of an algal treatment system,the Se laden alga was further dewatered and burned to ashes or filtered out from the water column by Anodonta woodiana.After burning,biomass Se was reduced by 99%,with organic-led Se converted into inorganic Se,significantly lowering Se bioavailability,which might reduce the risks of Se access to organisms.The algal filtration rate of the bivalves dropped by 27% in the presence of Se containing algae in the experimental group,as compared to the control group that was fed with Se free algae,suggesting that selenite pretreated algae appeared to have a negative impact on the filtering capacity of A.woodiana for algae.A.woodiana removed 54% of Se in algal suspensions in 24 h through ventilation and filtration,leading to significant bioaccumulation of Se in soft tissues(4.85 ?g Se/g DW),which requires additional research to avoid potential Se biomagnification.