This ability of algae to

This ability of algae to remove heavy metals varies with the different strains of algae and it is generally in the following green algae descending order Chlorophyta – Phaeophyta (red algae) Rhodophyta (Al-Shwafi and Rushdi, 2008). Furthermore, it was found that the lifeless algae beta lactamase inhibitor adsorbs more metals than living algae (Mehta and Gaur, 2005). Stigeoclonium sp. a fresh water algae can survive in mining water with high concentration of Zn of about 10 μM and it is also effective in its removal (Pawlik-Skowronska, 2001). In living algal cells, trace metals intracellular are accumulated by active biological transport (Ajjabi and Chouba, 2009; Kiran and Thanasekaran, 2011). The presence of various variety of algae coupled with their multilayer cell walls make them suitable as a cheap source of adsorbent for heavy metals (Bilal et al., 2013; Gupta et al., 2015; Wang and Chen, 2009). Algae are among the most commonly found photosynthetic eukaryotes (Anastopoulos and Kyzas, 2015) that have the ability to survive in both fresh and marine brackish waters. Spirogyra algal species was found to have a removal efficiency of 58–85% for Cu (II) at initial concentration of 20 mg/L after 30 min (Bishnoi and Pant, 2004). Caulerpa lentillifera dried green microalgae is also an effective bio adsorbent for multiple metals removal in aqueous solutions (Pavasant et al., 2006). Brown algae Turbinariaornate and green algae Ulothrix zonata are also very effective adsorbents in the removal of heavy metals (Nuhoglu et al., 2002; Vijayaraghavan and Prabu, 2006; Djati Utomo et al., 2016). Regarding the removal of sulphates, it is reported that microbial sulfate reduction generates alkalinity and the formation of metal sulphides allows the precipitation of metals from the solution. AMD environments are naturally carbon limited (Koschorreck, 2008), thus the addition of suitable carbon sources is required to promote the activity of SRB. For SRB to survive and for their better growth, nutrients are imperatively required. Furthermore, sulphates reduction and metal precipitation have to take place to allow an effective growth and survival. The expansion of technology using SRB for AMD remediation is constrained by costs and obtainability of a carbon source (Molwantwa et al., 2000). Previously, biotechnologies have been used to recycle wastewater with the help of microbial populations for the removal of pollutants. In the case of AMD, the most studied bioremediation for removal of heavy metals is based on sulphates reducing bacteria (Kiran et al., 2017). In this process heavy metals are removed through the production of metal sulphide precipitates. This technology was successful on a large scale for heavy metal removal at low pH values, stable sludge, very low operating costs and minimal energy consumption. Therefore, according to Sheoran et al. (2010), SRB could be a very effective alternative treatment method for AMD on sites with no power but are not suitable for sites with low temperatures such those under extreme winter conditions. However, it has the weaknesses of long residence times that can take weeks. Also, it requires a continuous supply of organic substrates and the use of large steel bioreactors (White et al., 1997; Sheoran et al., 2010). In addition, as it well known for any anaerobic process, the start-up is time consuming, requires an appropriate microbial population to work effectively and capital costs can be high. Hoffmann (1998) concluded that algae-based treatment can be used for the removal of inorganic substances from effluents. However, the greatest advantage of SRB is the simultaneous removal of both metals and sulphates, which does not occur with the traditional chemical processes (Kiran et al., 2017).
Previous studies The pollution of the aquatic environment by the discharge of AMD containing heavy metals has been a worldwide challenge and a concern for many decades (Nriagu and Pacyna, 1988). It was confirmed by many studies undertaken worldwide on the effects of AMD on the environment. Due to a number of negative effects produced by AMD on the environment, many countries have developed policies and regulations regarding the discharge of AMD. Through these policies and regulations, standards are suggested in order to prevent threat to human health and disastrous environmental impact, dropping concentrations in effluents, and developing cost effective technologies (Fan, 1996).