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.