Gaikwad and Gupta associate acid mine drainage

Gaikwad and Gupta (2008) associate angiopoietin mine drainage, also known as abandoned mine drainage, to the reaction of water and oxygen specifically with coal mining. Water passing through the rocks from mining operations that have been deposited on the surface and the underground voids left behind by mining activities causes the formation of acidic drainage effluents. This occurs by the reaction involving oxygen, water and pyrite sulfidic and non-sulfidic minerals to produce acid sulfate rich wastewater know as AMD. Kuyucak (2002) terms the AMD phenomenon as “acid rock drainage” (ARD) and notes that the same chemical reactions can take place in roads and bridge construction, and in the construction of tunnels through rock formations. With the formation of net acidity, effluent water becomes increasingly laden with Fe, Mn, Al, Zn, Cu, Ni, Pb, As and Cd as the principle heavy metal contaminants. Ben and Baghour (2013) found that Cd is typically the most mobile and therefore has the highest bioavailability. Furthermore, the presence of microorganisms promoting geochemical processes together with mineral oxidation reduces the amount of dissolved oxygen in water (Bhattacharya et al., 2006). Generally, the most common uses of algae as phycoremediators reported in the literature are their character of being alkalinity boosters rather than as direct absorbers. The removal of heavy metal contaminants by algae species assists in monitoring the levels of heavy metals in an environment. However, in terms of efficiency and ease of maintenance, the turnover of algae due to toxicity of the accumulated heavy metals makes continuous processes much more attractive. The performance of such systems over time is also more easily monitored. According to Costello (2003) many examples of these passive systems are operational and are still undergoing research to improve their functional characteristics. The use of various algae strains to remove heavy metals from AMD has only more recently become the subject of closer scrutiny. Gok and Yel (2009) carried out studies of the kinetic characteristics of heavy metals uptake by microalgae. These characteristics would be of fundamental importance in designing effective and long-lasting phycoremediation systems. Perales-Vela et al. (2006) completed a study on heavy metals detoxification mechanisms in microalgae examining the biomolecular way by which the algae isolate and sequester those environmental materials to which they are exposed. They particularly examined the formation of metallothionein peptides in the algae.
Absorbent and adsorbent properties of algae biomass The removal of heavy metals and sulphates by algae is very flexible. It depends on the metal type, the taxon and age of material (Novis and Harding, 2007). Optimum heavy metals and contaminants removal vary with season type (Elbaz-Poulichet et al., 2000; Brake et al., 2004). Seasons of the year can highly influence the contaminants removal because of parameters such as light and temperatures. Algae are very sensitive when it comes to parameters related to seasons such temperature and light intensity. The absorbency and adsorbency mechanisms are commonly used by algae species to remove nutrients, heavy metals (depending on the specie type) and other minerals from wastewater. While these elements are removed from wastewater algae growth takes place because species need also nutrient elements to grow. Some are taken by the surface and other taken into the inner cells as presented in Fig. 1. According to Mehta and Gaur (2005), algae in aqueous solution is very effective for the removal of low concentration of metal ions and also bio accumulate these metals within their cells more specially in the cell vacuoles and/or in the intercellular spaces (Afkar et al., 2010; Chen et al., 2012; Kumar and Gaur, 2011). Cladophora glomerata and Oedogonium rivulare are amongst species used to continuously remove Co, Ni, Pb, Cd, Mn, Fe, Cr and Cu from wastewater (Vymazal, 1984).