Fruit-processing residues represent a largely underutilized class of agricultural waste with considerable potential for low-cost water treatment applications. This study investigated the capacity of dried watermelon peel (Citrullus lanatus) — designated WMP — to adsorb Pb(II), Zn(II), Cu(II), Cr(VI), and Ni(II) ions from synthetic aqueous solutions through systematic batch experimentation. The effects of contact time (30–180 min), adsorbent dose (1–5 g), initial metal concentration (10–90 mg/L), solution pH (2–10), and temperature (30 70°C) were each evaluated. Optimal removal of cationic metals occurred at pH 6, while Cr(VI) showed maximum uptake at pH 4 rather than the strongly acidic pH 2 commonly reported for lignocellulosic adsorbents, peaking at 67% removal — a divergence attributed to the pectin-rich surface chemistry of watermelon peel. Nickel achieved the highest overall removal of 97.0% at 180 minutes, and lead maintained an exceptional 90% efficiency across the entire tested concentration range of 10–50 mg/L. Equilibrium data for Pb²⁺, Zn²⁺, Cu²⁺, and Ni²⁺ conformed best to the Langmuir isotherm (R² = 0.972–0.991), while Cr(VI) data fitted the Freundlich model more closely (R² = 0.939), indicating a mechanistic divergence in chromium uptake relative to the four cationic metals. The Pseudo-Second-Order kinetic model provided an excellent description of adsorption rates for all five metals (R² = 0.998 0.999), implicating chemisorption as the dominant rate-controlling mechanism. Thermodynamic analysis yielded negative ΔG° values (−0.400 to −0.610 kJ/mol), negative ΔH°, and positive ΔS° across all systems, confirming spontaneous, mildly exothermic, and entropy-driven adsorption. Scanning electron micrographs revealed a porous, spongy pre adsorption surface that became densely packed after metal loading, while FTIR spectroscopy identified carboxylate, amine, sulfate, and ester functional groups as binding sites, with pronounced post-adsorption spectral shifts in the carboxylate region confirming the involvement of pectin-derived carboxyl groups in metal complexation. The findings collectively establish WMP as a high-performing, mechanistically disti

References: 

Adeyemo, A. A., Adeoye, I. O., & Bello, O. S. (2022). Thermodynamics of heavy metal biosorption onto agricultural waste-derived materials: a review. Environmental Monitoring and Assessment, 194(6), 415. https://doi.org/10.1007/s10661-022-10028-3

Akhtar, K., Mannan, H. A., & Bhanger, M. I. (2024). Equilibrium and mechanistic insights into chromium(VI) biosorption by agricultural waste biomass. Journal of Hazardous Materials Advances, 13, 100410. https://doi.org/10.1016/j.hazadv.2024.100410

Anastopoulos, I., Robalds, A., Tran, H. N., Mitrogiannis, D., Giannakoudakis, D. A., Hosseini Bandegharaei, A., & Dotto, G. L. (2022). Removal of heavy metals by agricultural waste-derived biosorbents. Environmental Chemistry Letters, 17(1), 629–650. https://doi.org/10.1007/s10311-018 0818-7

Balali-Mood, M., Naseri, K., Tahergorabi, Z., Khazdair, M. R., & Sadeghi, M. (2021). Toxic mechanisms of five heavy metals: mercury, lead, chromium, cadmium, and arsenic. Frontiers in Pharmacology, 12, 643972. https://doi.org/10.3389/fphar.2021.643972

Bashir, A., Malik, L. A., Ahad, S., Manzoor, T., Bhat, M. A., Dar, G. N., & Pandith, A. H. (2023). Removal of heavy metal ions from aqueous solution by adsorption onto agricultural by-products: a review. Environmental Chemistry Letters, 21(2), 1059–1081. https://doi.org/10.1007/s10311-022 01540-y

Bilal, M., Ihsanullah, I., Shah, M. U. H., Almomani, F., & Adam, M. R. (2022). A review on environmental contamination of heavy metals and their toxic effects: source, fate and remediation approaches.Journal of Cleaner Production, 379, 134836. https://doi.org/10.1016/j.jclepro.2022.134836 Foo, K. Y., & Hameed, B. H. (2010). Insights into the modeling of adsorption isotherm systems. Chemical Engineering Journal, 156(1), 2–10. https://doi.org/10.1016/j.cej.2009.09.013

Genchi, G., Lauria, G., Catalano, A., Sinicropi, M. S., & Carocci, A. (2023). Nickel: human health effects, mechanisms, and toxicology. Toxics, 11(8), 690. https://doi.org/10.3390/toxics11080690 Ho, Y. S., & McKay, G. (1999). Pseudo-second order model for sorption processes. Process Biochemistry, 34(5), 451–465. https://doi.org/10.1016/S0032-9592(98)00112-5

Lagergren, S. (1898). About the theory of so-called adsorption of soluble substances. Kungliga Svenska Vetenskapsakademiens Handlingar, 24(4), 1–39.

Lakshmipathy, R., & Sarada, N. C. (2015). Watermelon rind as a biosorbent: a review. Reviews in Environmental Science and Bio/Technology, 14(3), 413–426. https://doi.org/10.1007/s11157-015 9363-6

metal ions Macena, M., Pereira, H., Cruz-Lopes, L., Grosche, L., & Esteves, B. (2023). Competitive adsorption of by lignocellulosic materials: a review. Separations, 12(3), 70. https://doi.org/10.3390/separations12030070

Mathew, B. B., Yusuf, A. K., & Adeniyi, A. A. (2023). Pectin-rich agricultural waste biomass as a biosorbent for heavy metal remediation: prospects and limitations. Sustainable Chemistry and Pharmacy, 33, 101079. https://doi.org/10.1016/j.scp.2023.101079

Mitra, S., Chakraborty, A. J., Tareq, A. M., Emran, T. B., Nainu, F., Khusro, A., Idris, A. M., Khandaker, M. U., Osman, H., & Alhumaydhi, F. A. (2022). Impact of heavy metals on the environment and human health: novel therapeutic insights. Journal of King Saud University – Science, 34(3), 101865. https://doi.org/10.1016/j.jksus.2022.101865

Murphy, O. P., Vashishtha, M., Palanisamy, P., & Kumar, K. V. (2023). A review on the adsorption isotherms and design calculations for the optimization of adsorbent systems. ACS Omega, 8(19), 17407–17430. https://doi.org/10.1021/acsomega.3c00762

Okam, P. C., Eze, S. I., & Igwe, J. C. (2022). Surface characterization and heavy metal sorption behaviour of fruit-peel derived biosorbents. Heliyon, 8(11), e11486. https://doi.org/10.1016/j.heliyon.2022.e11486

Onyancha, R. B., Ukhurebor, K. E., Aigbe, U. O., Kusuma, H. S., & Darmokoesoemo, H. (2024). Adsorptive removal of heavy metals from contaminated wastewater using agricultural waste biomass: kinetic and equilibrium considerations. Environmental Quality Management, 33(2), 89–104. https://doi.org/10.1002/tqem.22042

Raji, Z., Karim, A., Karam, A., & Khalloufi, S. (2023). Adsorption of heavy metals: mechanisms, kinetics, and applications of various adsorbents in wastewater remediation. Waste, 1(3), 775–805. https://doi.org/10.3390/waste1030045

Sen, T. K. (2023). Agricultural solid wastes based adsorbent materials in the remediation of heavy metal ions from water and wastewater by adsorption: a review. Molecules, 28(14), 5575. https://doi.org/10.3390/molecules28145575

Shehu, Z., & Ibrahim, A. A. (2023). Pectin-bearing fruit waste as biosorbents for aqueous heavy metal removal: kinetics, equilibrium, and mechanistic considerations. Biomass Conversion and Biorefinery, 13(17), 12455–12470. https://doi.org/10.1007/s13399-021-02143-7

Tran, H. N., You, S. J., & Chao, H. P. (2021). Kinetic and thermodynamic analysis of heavy metal biosorption onto agricultural residue. Adsorption Science & Technology, 2021, 5594375. https://doi.org/10.1155/2021/5594375

Vunain, E., Mishra, A. K., & Mamba, B. B. (2023). Dendrimers, mesoporous silicas and chitosan based biosorbents for heavy metal removal: a review. Environmental Science and Pollution Research, 30(14), 11423–11458. https://doi.org/10.1007/s11356-022-19312-y

Wang, J., Wang, S., Vieira, B. J. C., Manjarrez Buenrostro, K. M., & Waite, C. A. (2022). Adsorption mechanisms and models for heavy metal removal using agricultural waste adsorbents: a review. Frontiers in Environmental Science, 10, 977822. https://doi.org/10.3389/fenvs.2022.977822

WHO. (2022). Guidelines for drinking-water quality (4th ed., incorporating the 2017 and 2022 addenda). World Health Organization.