Potentialities of Agro-Based Wastes to Remove Cd, Hg, Pb, and As from Contaminated Waters

Apple residues, banana peel, eggshell, potato peel, and sweet potato peel were tested as biosorbents to remove top priority contaminants (As, Cd, Hg, and Pb) from contaminated waters. Sorption experiments were performed using 0.5 g of each biosorbent in 1 L of natural tap water spiked with a mixture of As, Cd, Hg, and Pb at realistic concentrations (50 μg L −1 ), under different pH values (4.5, 6.5, and 8.5) and water salinities (0, 10, and 30). Water was sampled periodically during the 48 h of the experiments and analyzed. The analysis by scanning electron microscopy showed differences among biosorbents, mainly in pore size and fibrous factures. Fourier-transform infrared spectroscopy identified cellulose, hemicellulose, pectin, and lignin in all biosorbents, except in eggshells, which are constituted mainly by carbonates. Apart from the eggshell, whose pH PZC is 10.1, all pH PZC of the biosorbents are between 3.99 and 5.90. Results showed that the levels of all contaminants in water were considerably reduced by the biosorbents in less than 3 h (more accentuated for apple and banana peel), apart from As, whose removal never surpassed 6%. Removal of Hg (up to 99%) was much higher than for Cd (76%) and Pb (86%). Overall, the pH had no notable effect on the removal, except for Cd, while ionic strength greatly influenced the removal of Cd by all the biosorbents, and the removal of Hg by eggshell. Findings point to the feasibility of reusing these agro-industrial wastes for water treatment, allowing water reuse for different purposes, which is perfectly aligned with the European agenda strategy for the Circular Economy. Graphical abstract