Some species of algae which can be naturally present in mine drainage waters, such as Spirogyra sp. and Chlorella sp., have a high capacity for absorbing potentially toxic elements (PTEs) from wastewater and may thrive in harsh environments. As a result, algal-based systems in bioremediation were studied and carefully analyzed.
1. Algae and Their Bioremediation Capacity
Because of its low cost and great efficacy in metal and sulphate removal, bioremediation with algal strains is a new and appealing biological way of acid mine drainage (AMD) therapy. Microalgae can operate as sorbents, and the metabolism of an algal biomass creates high alkalinity, which helps counteract the acidic character of the drainage stream and hence aids in metal precipitation. However, variations in the pH, oxygen content, and temperature of the acid stream have a significant impact on the treatment efficiency of this approach 
. However, in this fascinating world of microalgae, there are always several possibilities, and in this situation, non-living algae may also be employed, and there are noticeable differences in metal ion accumulation when compared to living algal cells 
Non-living biomass biosorption benefits include heavy metal biosorption that is multiple times higher in non-living microalgae than in living microalgae 
. Metal ions associated to the algal cell wall, for example, can be eliminated by washing the biomass with different desorption agents 
. Living microalgae, on the other hand, have little mechanical and chemical resistance to physical and chemical recycling treatments. A non-living algal biomass also eliminates the hazards of exposure to highly hazardous settings and does not need intense maintenance or the addition of further growth nutrients, being a cost reduction when considering a scale-up process 
To obtain the best removal effectiveness, the interaction between algal strains, dead or living cells, and contaminants should be adjusted, since several factors that will be discussed afterwards influence non-living and living algal heavy metal ion biosorption in different ways 
2. Factors That Affect Microalgal Bioremediation Capacity
pH is one of the most critical factors influencing metal adsorption by microalgal biomass. Metal intake pH dependency is intimately connected to metal chemistry in solution, as well as the acid-base characteristics of different functional groups on the microalgal cell surface. When related to acid mine drainage, pH is one of the most important variables, since the acid mining drainage is often extremely acidic (pH < 3.0), making metal ions available in solution, which can lead to competition between hydrogen ions and metal ions for the same adsorption site on the algae 
. However, pH > 7.0 is not good either, since it leads to metal ion precipitation into hydroxides, making them unavailable to be adsorbed, which can make adsorption rates drop. As a result, there is already an optimal pH value range for each metal’s sorption helping to promote adsorption, which is typically in the range of 4.0–6.0 
. The best pH range for microalgae, especially the Chlorella
sp., has been researched, and it has been shown that pH 6 appears to be most favorable for growth and lipid accumulation, which is relatively similar to the optimal pH value range for metal sorption 
The data available in the literature on the influence of temperature on the adsorption of PTEs by microalgae is not entirely consistent. As an example, increasing temperature lead to an increase in Ni2 adsorption by the dry biomass of C. vulgaris 
. Nevertheless, the same author claimed that increasing the temperature (from 20 to 50 °C) affected the biosorption capacity of cadmium(II) (from 85.3 to 51.2 mg/g) in a prior research paper 
, although other publications suggest that temperature has no influence on metal sorption 
, remaining unanimous and requiring further study.
2.3. Biomass Concentration
The quantity of metals taken from solution by microalgae is plainly impacted by the biomass concentration: it increases with the latter, which might be attributable to a greater number of metal-binding sites accessible. However, increasing the biomass level is only possible to a limited extent, because after a certain concentration, the values may lead to a decrease in metal binding 
. This may be explained by a possible overlap of biomass leading to a reduced surface area available for sorption, as well as a decrease in the average distance between the adsorption locations that are available 
2.4. PTE Interactions
Mine drainage often contains significant amounts of PTEs, resulting in a complex combination of heavy metals. Metal cation interactions may be studied using multiple metal solutions, which are more reflective of actual environmental issues than research on a single metal 
. The presence of other metals/co-ions in solution has a substantial effect (generally inhibits) on the sorption of PTEs into the microalgal biomass, owing to the competitive interactions between them and the adsorption binding sites on the cell surface, or precipitation 
2.5. Metals Speciation
PTE chemical speciation is a major component that impacts heavy metal toxicity. Their potential mobility, bioavailability, and environmental behavior are highly reliant on their precise chemical forms and current states, which are mostly influenced by pH 
. Metals in mine drainage can take many different chemical forms, including free ions, complexes with inorganic/organic ligands, and adsorbates on particulate phases. Nevertheless, free metal ions in solution are the most harmful to living creatures and bind the most to microalgae 
Because this procedure is so reliant on so many variables, algal-based bioremediation is frequently employed together with other treatment techniques, and is thus classified as a secondary or tertiary treatment.
As previously stated, the bioremediation of heavy metals and sulphates by algae is extremely variable, since it relies on the PTE interaction, biomass concentration, metal speciation, pH, temperature, and the season during which the removal process is carried out 
. The climate conditions can strongly influence the removal of contaminants, because algae are sensitive to parameters such as light, temperature, and water availability.
Depending on the degree of saturation and aeration of a region, several strategies are used. In situ procedures are those that are used on soil and groundwater on-site, with little disruption. Ex situ procedures are those that are used on soil and groundwater that have been removed from the site by excavation (soil) or pumping (water) 
. Different bioremediation strategies, and their application benefits and limitations are summarized in Table 1
Bioremediation methods, benefits, and limitations 
Algal bioremediation treatment can be carried out in situ, with the algae ideally growing in the contaminated effluent, followed by the collection of algal biomasses, drying to recover the content of adsorbed metal, and finally the conversion of heavy metals into recoverable oxides or other salts. Alternatively, an ex situ treatment would entail the algal biomass being grown in the laboratory and the adsorption capacity of the algae in the laboratory verified through the collection of samples of the effluent under study 
. It should also be noted that after the heavy metal extraction, the algal biomass can be reused to increase the potential for biofuel production.