Factors That Affect Microalgal Bioremediation Capacity: Comparison
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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 PTEs from wastewater and may thrive in harsh environments. As a result, algal-based systems in bioremediation were studied and carefully analyzed.

  • bioremediation
  • microalgae
  • mine drainage waters
  • potentially toxic elements (PTEs)

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 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 [2][1]. 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 [26][2].
Non-living biomass biosorption benefits include heavy metal biosorption that is multiple times higher in non-living microalgae than in living microalgae [27][3]. Metal ions associated to the algal cell wall, for example, can be eliminated by washing the biomass with different desorption agents [28][4]. 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 [29][5].
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 [30][6].

2. Factors That Affect Microalgal Bioremediation Capacity

2.1. pH

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 [31][7]. 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 [32][8]. 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 [33][9].

2.2. Temperature

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 [34][10]. 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 [35][11], although other publications suggest that temperature has no influence on metal sorption [26][2], 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 [36][12]. 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 [37][13].

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 [26][2]. 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 [38][14].

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 [39][15]. 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 [27][3].
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 [40][16]. 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) [41][17]. Different bioremediation strategies, and their application benefits and limitations are summarized in Table 1.
Table 1. Bioremediation methods, benefits, and limitations [41,42,43].
Bioremediation methods, benefits, and limitations [17][18][19].
Techniques Examples Advantages Constraints Factors to Consider
In situ In situ bioremediationBiosparging

Bioventing

Bioaugmentation

Bioslurping

Permeable reactive barrier (PRB)		 Noninvasive and least expensive

Relatively inactive

Natural attenuation mechanisms

Treatment for soil and water		 Environmental restrictions

Treatment period lengthened

Difficulties with monitoring

Heavy metal and organic compound concentration inhibit the activity of some indigenous microorganisms

Acclimatization of microorganisms is frequently required for in situ bioremediation		 The depth of pollution, the kind of pollutant, the degree of pollution, the cost of remediation, and the geographical location of the contaminated site are all factors to consider
Ex situ Landfarming

Composting

Biopiles

Windrows

Biofilter		 Cutting costs

Low price

Can be completed on-site		 Heavy metal pollution and chlorinated hydrocarbons, such as trichloroethylene, are not covered Non-permeable soil needs further treatment

Before placing the pollutant in the bioreactor, it can be removed from the soil by soil washing or physical extraction		 See above
Bioreactors Slurry reactors

Aqueous reactors		 Kinetics of rapid deterioration

Environmental characteristics have been optimized

Improves mass transferInoculants and surfactants are used effectively		 Excavation is required for soil.

Capital at a somewhat high cost

Operating costs are rather expensive		 Process requires bioaugmentation

Amendment toxicity

Contaminant concentrations that are toxic
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 [4][20]. It should also be noted that after the heavy metal extraction, the algal biomass can be reused to increase the potential for biofuel production.

References

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  2. Monteiro, C.M.; Castro, P.M.L.; Malcata, F.X. Metal Uptake by Microalgae: Underlying Mechanisms and Practical Applications. Biotechnol. Prog. 2012, 28, 299–311.
  3. Mehta, S.K.; Gaur, J.P. Use of Algae for Removing Heavy Metal Ions from Wastewater: Progress and Prospects. Crit. Rev. Biotechnol. 2005, 25, 113–152.
  4. Chen, C.Y.; Chang, H.W.; Kao, P.C.; Pan, J.L.; Chang, J.S. Biosorption of Cadmium by CO 2-Fixing Microalga Scenedesmus Obliquus CNW-N. Bioresour. Technol. 2012, 105, 74–80.
  5. Arunakumara, K.K.I.U.; Zhang, X. Heavy Metal Bioaccumulation and Toxicity with Special Reference to Microalgae. J. Ocean Univ. China 2007, 7, 60–64.
  6. Kong, Q.X.; Li, L.; Martinez, B.; Chen, P.; Ruan, R. Culture of Microalgae Chlamydomonas Reinhardtii in Wastewater for Biomass Feedstock Production. Appl. Biochem. Biotechnol. 2010, 160, 9–18.
  7. Lee, M.G.; Lim, J.H.; Kam, S.K. Biosorption Characteristics in the Mixed Heavy Metal Solution by Biosorbents of Marine Brown Algae. Korean J. Chem. Eng. 2002, 19, 277–284.
  8. Ferreira, L.S.; Rodrigues, M.S.; de Carvalho, J.C.M.; Lodi, A.; Finocchio, E.; Perego, P.; Converti, A. Adsorption of Ni2+, Zn2+ and Pb2+ onto Dry Biomass of Arthrospira (Spirulina) Platensis and Chlorella Vulgaris. I. Single Metal Systems. Chem. Eng. J. 2011, 173, 326–333.
  9. Qiu, R.; Gao, S.; Lopez, P.A.; Ogden, K.L. Effects of PH on Cell Growth, Lipid Production and CO2 Addition of Microalgae Chlorella Sorokiniana. Algal Res. 2017, 28, 192–199.
  10. Aksu, Z. Determination of the Equilibrium, Kinetic and Thermodynamic Parameters of the Batch Biosorption of Nickel(II) Ions onto Chlorella Vulgaris. Process Biochem. 2002, 38, 89–99.
  11. Aksu, Z. Equilibrium and Kinetic Modelling of Cadmium(II) Biosorption by C. Vulgaris in a Batch System: Effect of Temperature. Process Biochem. 2001, 21, 285–294.
  12. Esposito, A.; Pagnanelli, F.; Lodi, A.; Solisio, C.; Vegliò, F. Biosorption of Heavy Metals by Sphaerotilus Natans: An Equilibrium Study at Different PH and Biomass Concentrations. Hydrometallurgy 2001, 60, 129–141.
  13. Ahuja, P.; Gupta, R.; Saxena, R.K. Zn2+ Biosorption by Oscillatoria Anguistissima. Process Biochem. 1999, 34, 77–85.
  14. Mehta, S.K.; Singh, A.; Gaur, J.P. Kinetics of Adsorption and Uptake of Cu2+ by Chlorella Vulgaris: Influence of PH, Temperature, Culture Age, and Cations. J. Environ. Sci. Heal.—Part A Toxic/Hazard. Subst. Environ. Eng. 2002, 37, 399–414.
  15. Suresh Kumar, K.; Dahms, H.U.; Won, E.J.; Lee, J.S.; Shin, K.H. Microalgae—A Promising Tool for Heavy Metal Remediation. Ecotoxicol. Environ. Saf. 2015, 113, 329–352.
  16. Elbaz-Poulichet, F.; Dupuy, C.; Cruzado, A.; Velasquez, Z.; Achterberg, E.P.; Braungardt, C.B. Influence of Sorption Processes by Iron Oxides and Algae Fixation on Arsenic and Phosphate Cycle in an Acidic Estuary (Tinto River, Spain). Water Res. 2000, 34, 3222–3230.
  17. Mary Kensa, V. Bioremediation—An Overview. J. Ind. Pollut. Control. 2011, 27, 161–168.
  18. Sharma, I. Bioremediation Techniques for Polluted Environment: Concept, Advantages, Limitations, and Prospects. In Intech; IntechOpen: London, UK, 2021; p. 16.
  19. Hussain, A.; Rehman, F.; Rafeeq, H.; Waqas, M.; Asghar, A.; Afsheen, N.; Rahdar, A.; Bilal, M.; Iqbal, H.M.N. In-Situ, Ex-Situ, and Nano-Remediation Strategies to Treat Polluted Soil, Water, and Air—A Review. Chemosphere 2022, 289, 133252.
  20. Bwapwa, J.K.; Jaiyeola, A.T.; Chetty, R. Bioremediation of Acid Mine Drainage Using Algae Strains: A Review. South Afr. J. Chem. Eng. 2017, 24, 62–70.
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