**6. Conclusions**

The presence of heavy elements from the effluents of factories and industries has a severely destructive effect on the structure of living organisms and their performance. Because these elements cause various severe kidney, nervous, genetic, and cancer diseases in humans, therefore their absorption from polluted wastewater is very important. Absorption of heavy elements with the help of algae is a suitable, productive, and useful method in this field due to its availability, very low cost, and significant effect on the removal of pollutants.

Biological treatment of wastewater containing various heavy metals with biological absorption by microalgae is a simple method that has received much attention in recent years due to its high efficiency, leaving minimal secondary waste and using cheap materials. The research shows that these bioabsorbents have a very good performance in removing heavy metals from large amounts of wastewater with low concentrations of heavy metals. Microalgae can be grown in large quantities in different climates and in fresh and salty waters. The performance of microalgae in absorbing heavy metals is mainly due to the presence of various functional groups on the surface of microalgae and the tendency to replace the alkaline earth metal ions present in them with heavy metal ions. The efficiency of biological absorption of metals by microalgae depends on the characteristics of microalgae (particle size, growth conditions, biomass pretreatment, etc.) and process operating conditions (solution pH, adsorbent dose, contact time, temperature, work method, adsorbent bed height, the wastewater flow rate through the absorption column, heavy metal concentration, etc.) that should be optimized.

The biochemical sorption process can be easily modeled using several well-known equilibrium and kinetic models, which provide useful information about the mechanism of heavy metal sorption onto microalgae. Preliminary absorption experiments should be performed in batch systems to obtain preliminary information. Then the design and sizing of industrial-scale absorption system equipment and the necessary economic estimates are examined by conducting tests in continuous and dynamic systems. In general, the studies conducted in this research show that microalgae have a very high potential to be used as a cost-effective and efficient bio-absorbent in the removal of all types of heavy metals from industrial wastewater.
