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    • The effective process is chemically based and similar to

    2018-10-26

    The effective process is chemically based and similar to an acid–base neutralization reaction, rather than biologically dominated where the uptake and sequestering of heavy metals occurs directly in the algae. It is necessary to maintain physical separation of the algal colony from the effluent stream in order to prevent the algae from being affected by heavy metals toxicity. Consequently, the alkalinity produced by the Spirulina can continuously neutralize the acidic nature of AMD effluent. The system is reported to have consistently removed almost all heavy metals as recoverable hydroxide compounds from the treated AMD. Many studies have been carried out to identify species of both microalgae and macroalgae that can be applied to phycoremediation. A marine screen designed by Matsunaga et al. (1999) was used to analyze a chlorella strain that was capable of sustaining growth at 11.24 mg of Cd2+/l with a removal efficiency of 65% when exposed to an amount of 5.62 mg of Cd2+/l. A trial using a batch process was initiated by Travieso et al. (1999) using Chlorella and Scenedesmus strains at 20 mg Cr6+/l. It was found that the removal efficiencies were 48% and 31% respectively for both strains. Another strain known as Phaeodactylum tricornutum (CE50 = 22.3 mg/l) with high Cd2+ tolerance was characterized with respect to Mt III production pattern by Torres et al. (1997) and showed to have the potential to remove these two metals (Torres et al., 1998). Scenedesmus was also used in heavy metal removal studies and has demonstrated an effective removal efficiency for U6+ (Zhang et al., 1997), Cu2+ and Cd2+ as confirmed by Terry and Stone (2002) and Zn2+ (Aksu et al., 1998; Travieso et al., 1999; Cañizares-Villanueva et al., 2001). In a study carried out by Freitas et al. (2011) on the biosorption of heavy metals by algal species in AMD from coal mining in Brazil, it was found that algal biomass are able to accumulate heavy metals more specially Fe which constitutes 6.3% of the biomass. Their study showed the following order tranylcypromine capabilities: Fe > Al > Ca > Mg > Zn > Mn > Cu in all sampling sites. In addition their results also show that algal species such as Microspora, Eunotia, Euglena, Mougeotia, and Frustulia can survive in the AMD environment, with Microspora being found to be the most dominant. These acidophilic species were found to survive in water at pH values ranging from 2.9 to 4.1. Park et al. (2013) completed a study using a hybrid system comprising a pipe inserted microalgae reactor (PIMR) and an active treatment unit for the removal of Fe from AMD. The removal of Fe is very important because high levels of Fe can be detrimental to algae growth and lipids accumulation, thereby limiting the treatment ability of algae biomass when dealing with AMD. In their study, it was found that Nephroselmis sp. is effective in removing heavy metals and has a strong tendency to grow with pretreated AMD. Therefore, they recommended that PIMR is more effective for both metals removal and microalgae growth in AMD pretreated effluents. Compared to HRAP and SRB, this system is more protective of its algae biomass against environmental contaminations even though the area of operation between algae biomass is smaller. However, the treatment capacity and removal could be less compared to the HRAP system. Ben and Baghour (2013) reviewed at least fourteen algal species and their ability for uptake of heavy metal contaminants from AMD and other pollution sources. In their study individual species have showed to be “hyper-accumulators” or “hyper-absorbents” with a high selectivity for different metals. Also, Das et al. (2009) compiled some results achieved by few studies regarding the use of some microalgae species for AMD bioremediation. This table contains the removal efficiencies of heavy metals and sulphates which are the main contaminants in AMD. It also includes the operating conditions and the growth mode used to achieve the results. The table indicates the effectiveness and possibility of algae technology for AMD bioremediation. The main elements which are heavy metals such as Cu, Zn, Fe, Pb, Ni, Mn and sulphates can be removed by microalgae species under a known pH range.