Prunus dulcis hull: a by-product of the agri-food industry as an adsorbent of Pb ion in water

Heavy metals are among the most hazardous aquatic pollutants due to their non-biodegradable nature and tendency to bioaccumulate along the food chain. In this class, lead (Pb²⁺) stands out for its well- documented neurotoxicity and persistence in biological systems. The development of efficient and cost-effective removal strategies is therefore a priority, and the use of agri-food industry by-products as adsorbents has proven to be a promising solution, offering both waste valorization and a low-cost alternative to conventional water treatment technologies [1,2]. In this context, this contribution proposes Prunus dulcis hull (PDH), a widely available and largely unexploited agricultural by- product, as a sustainable adsorbent for the removal of Pb ions from aqueous solutions. The PDH biomass was thoroughly washed, dried and ground into particles of size ranging from 0.1 to 0.2 mm. Then the adsorbent was characterized through ATR-FT-IR spectroscopy, ISE H+ potentiometric titrations, pHpzc, TGA, and SEM-EDX measurements. Kinetic and isotherm adsorption experiments in batch were carried out at different experimental conditions (pH, ionic medium, and temperature). Furthermore, the effect of some ligands with different binding groups (methylamine, acetate, and cysteine) was investigated. Finally, column experiments including breakthrough curves and adsorption-desorption experiments were performed to study the efficiency in continuous flow and the recyclability of the adsorbent. The adsorption equilibrium was reached within 700/750 min. The adsorption kinetics was well described by the pseudo first-order model. The adsorption isotherms were well described by Langmuir equation. The highest affinity was found at pH = 5.0, I → 0 mol L and T = 284.15 K whilst, the maximum adsorption capacity was found in 2+ -1 -3 -1 solutions containing cysteine 5 10 mol L (q = 101 mg g ). The adsorption process was m -1 spontaneous and exothermic. The magnitude of ΔG and ΔH suggested a physisorption mechanism. 00 Column experiments showed a comparable adsorption behaviour of PDH under equilibrium and non- equilibrium conditions and a good reuse capacity. Thanks to the easy preparation procedure, PDH is a cost-effective adsorbent (€ 1.62 kg ) and has no impact on the environment.