Ziziphus Seed-Based Adsorbents: A Review Study on Preparation, Adsorption Mechanisms, and Environmental Applications

Ziziphus seeds, an abundant and low-cost lignocellulosic residue generated across the value chains of Z. mauritiana, Z. spina-christi, and Z. jujuba in arid and semi-arid regions, have emerged as a promising but under-synthesized feedstock for adsorption-based water treatment. This review provides a critical, mechanism-focused assessment of Ziziphus seed-derived adsorbents-raw biomass, biochars, activated carbons, and magnetic or nanocomposite hybrids-covering preparation routes, physicochemical characterization, adsorption mechanisms, and environmental performance for dye and heavy metal remediation. Comparative analysis of the literature (2020-2025) shows that adsorption capacity is dictated primarily by the intensity and nature of thermochemical/chemical modification rather than by botanical origin alone: chemical activation with H₃PO₄, H₂SO₄, or ZnCl₂ and alkali or magnetic functionalization yield specific surface areas ranging from approximately 100 to 1900 m² g^-1 and substantially enhance dye and metal uptake relative to unmodified biomass. Mechanistic evidence indicates that dye removal is governed predominantly by electrostatic attraction, hydrogen bonding, π-π interactions, and pore filling, whereas heavy metal sequestration (particularly Cr(VI), Cd(II), and Pb(II)) proceeds via surface complexation, ion exchange, and, in some systems, redox transformation, with markedly pollutant-specific pH optima. Kinetic and equilibrium data are best described by pseudo-second-order and Freundlich models for heterogeneous carbons and metal systems, while pseudo-first-order and Langmuir behavior are more prevalent for rapidly adsorbing dyes and structurally hybridized materials; thermodynamic parameters confirm spontaneous, but mechanistically variable (endothermic or exothermic), adsorption. Despite promising bench-scale capacities, including magnetic biochars achieving >98% Cr(VI) removal, the field remains constrained by an overreliance on idealized single-solute batch systems and negligible data on the removal of nutrients, pharmaceuticals, and per- and polyfluoroalkyl substances. Future research should prioritize application-specific functionalization, continuous-flow and multicomponent testing, long-term regeneration studies, and integrated sustainability assessment to advance Ziziphus-derived adsorbents toward field-scale deployment.