Effectiveness of fish scales in the adsorption of heavy metals: a systematic review

Industrial activity has increased the presence of heavy metals in surface waters and effluents, posing an environmental and health threat due to their persistence, toxicity, and bioaccumulation potential. In this context, fish scales are a promising alternative as an adsorbent due to their composition, which is rich in proteins, collagen, calcium salts, hydroxyapatite, and chitosan—components that promote the retention of metal ions. This systematic review evaluated the effectiveness of fish scales in adsorbing heavy metals, with a particular focus on Pb, Cd, Cr, Zn, and Cu. To this end, studies published between 2015 and 2026 were analyzed, and 44 articles were selected based on inclusion criteria, following the PRISMA methodology. The results showed that the most commonly used species were black tilapia (Oreochromis niloticus), Mozambique tilapia (Oreochromis mossambicus), and Indian carp (Catla catla), which highlights their experimental relevance and availability as raw materials. Likewise, removal efficiency was influenced by variables such as pH, adsorbent dose, and contact time, reaching efficiency values between 80.37% and 100% under optimized conditions. It was also found that chemically modifying the scales—through acid treatments and the incorporation of hydroxyapatite or a combination with chitosan to improve porosity— increases the number of active sites and optimizes interaction with the metals. In conclusion, the evidence supports the use of fish scales as a low-cost, technically viable alternative— consistent with the circular economy—for the remediation of water contaminated with heavy metals.