Table 4.

Different adsorbents involved in HMs removal with their advantages and disadvantages.

Adsorbents based methods	Overview	Disadvantages	References
Chitosan-based adsorbents	Chitosan has an affinity toward pollutants because of the hydroxyl and amino groups	Low porosity, surface area, and high crystallinity	(Jain et al., 2021)
Carbon-based adsorbents	Graphene-activated carbon and carbon nanotubes have a high surface area and are used to remove HMs. Phenyl, carboxy, and lactone groups can enhance the HMs uptake	Surface modification demands high pressure, heat, intensive oxidation/reduction reactions which makes the process expensive	(Gu et al., 2019)
Mineral adsorbents	Clay, zeolite, and silica are considered good adsorbents because of their low operating costs. Extraordinary cation exchange capacity and selectivity along with surface hydrophilicity and surface electronegativity are seen in clay.	After a few cycles, the removal efficiency is decreased which is the only disadvantage.	(Qasem et al., 2021)
Magnetic adsorbents	They have a specific material matrix that hosts iron particles such as Fe3O4. Easily synthesized, low cost, extraordinary surface charge, and reusability are the main advantages of magnetic adsorbents.	pH, irradiation time, the concentration of the adsorbent, temperature, and the initial dosage of pollutants affect the HMs removal	(Dang et al., 2017)
Adsorbents based on metal-organic framework (MOFs)	MOFs are synthesized using reticular synthesis. The metal ions are bonded strongly to organic linkers.	Organic ligands which form MOFs are toxic and expensive	(Chen et al., 2020)