# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 7 Conclusions and Future Outlook  
The potential of various adsorbents for the removal of heavy metals is demonstrated in this review. Adsorption is an economical method which has certain advantages over conventional methods such as being inexpensive, readily available, producing no sludge, being regenerated, and having technological feasibility, engineering applicability and affinity for the removal of heavy metals. Among the natural adsorbents like lignin, natural bentonite, tailored bentonite, the lignin was found to be the best adsorbent for lead ions (1865 mg/g), and chitosan was found to be a better adsorbent for cadmium ions with a removal capacity of 420 mg/g. It was also found that synthetic polymer poly(2-hydroxy ethyl methacrylate-methacrylolmidophenylalanine) showed good adsorption capacity for toxic metal ions like mercury (Hg = 669.4 mg/g), lead (Pb = 584.4 mg/g), cadmium (Cd = 268.4 mg/g), in comparison to poly(ethylene glycol dimethylate-n-vinyl imidazole, poly(hydroxyethyl methacrylate-L-glutamic acid), acrylonitrile onto polypropylene fiber having a diethylene tri-amine group. Also, the isotherm and kinetics study reveals that the rate of sorption methods depends on the physical or chemical properties of the sorbent and is also influenced by various parameters like pH, time, amount of adsorbent, etc. The pseudo-second-order kinetic model best fit the majority of the processes, which follow the Langmuir or Freundlich isotherms. Furthermore, regeneration tests revealed that acidic eluents typically yield the highest desorption percentages. Usually, the rate of metal ions' removal increased with time until the equilibrium was obtained for metal ions in the solid and liquid phases. From the current literature review, it is found that the sorption method is the best substitute for the traditional technology in extracting toxic metals. Additionally, more active functional groups on the adsorbent are required to remove a wide range of contaminants, maintain high mobility for an extended period of time, and meet the requirements for practical wastewater treatment. To change the adsorbent surface and boost toxin removal efficiency against a wide range of contaminants, certain dynamic compounds with high proclivity have been used. This review article offers a wide perspective on the use of different adsorbents but that are only applied at a laboratory scale. Further research is important to validate the finding on real industrial wastewaters for single and multi-pollutants.  
Conceptualization, V.S. and A.G.; methodology, V.S., A.G. and V.K.; resources, V.S., A.G., V.K., K.S., P.M. and S.C.; data curation, V.S., A.G., V.K., K.S., P.M., S.C., B.K. and P.K.M.; writing--original draft preparation, A.G., V.S., P.M., S.C., B.K. and P.K.M.; writing--review and editing, V.S., A.G., V.K., K.S., P.M., S.C., B.K., H.M., A.M., A.E. and P.K.M.; supervision, V.S.; project administration, V.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors [PERSON] and [PERSON] are thankful to the support provided by the Polish National Agency for Academic Exchange (PPN/ULM/2019/1/00289).  
The authors declare no conflict of interest.