Waste-derived natural polymers offer a route to low-cost sorbents for toxic metal ions, but reported performance is difficult to compare because precursor composition, polymer processing, modification, and experimental conditions vary widely. This critical review examines silk fibroin, banana peel, orange peel, and pectin as structurally distinct platforms and evaluates how macromolecular chemistry, esterification, ionic crosslinking, morphology, and regeneration influence metal uptake. Particular attention is given to the distinction between untreated biomass, isolated biopolymers, and chemically engineered materials; to mechanistic evidence from spectroscopy, elemental analysis, ion exchange, and controlled structural modification; and to the limitations of interpreting Langmuir or pseudo-second order fitting as direct proof of adsorption mechanism. The literature supports strong roles for pectic carboxyl groups and Ca²⁺-mediated exchange in several cation-binding systems, while silk fibroin provides a reusable protein-based matrix with different donor-site chemistry. Across the four platforms, high q_max values are not transferable material constants and do not by themselves establish selectivity, practical treatment performance, or analytical sensitivity. Major gaps include precursor standardization, matched comparison of raw and modified sorbents, multicomponent and real-water testing, nonlinear model evaluation, continuous-flow studies, regeneration stability, metal recovery, and end-of-life management. Diffuse-reflectance sensing is considered as a downstream value-added function once a reproducible biosorbent has been established. An evidence-based roadmap is proposed for converting polymer-rich wastes into controlled, regenerable, and application-oriented biosorbents.

Fibrous polyacrylonitrile (PAN) is a versatile platform for solid-phase analytical chemistry because of its chemically transformable nitrile groups, mechanical robust-ness, and favourable fibre morphology. This review critically examines PAN-based fi-brous sorbents for sorption–spectroscopic analysis (SSA) of toxic metal ions, focusing on polyamine-modified PAN/polyethylenepolyamine/1,2-dichloroethane (PPD) and PAN/polyethylenepolyamine/acrylonitrile (PPA) matrices, iminodiacetate-containing fibrous ion-exchange (FIBAN) fibres, triethanolamine-modified PAN (PAN-T), and electrospun amidoximated PAN nanofibres (AOPAN). Relationships among polymer composition, fibre morphology, surface functionalisation, reagent immobilisation, ion transport, and optical response are evaluated with respect to sensitivity, selectivity, stability, reuse, and practical applicability. Particular attention is given to electrostat-ic/physical versus covalent immobilisation, matrix interference, and the distinction between adsorption capacity and direct solid-phase optical performance. Representa-tive systems include PPD–Arsenazo III for Pb(II), with a minimum detectable concen-tration of 0.24 μg L⁻¹, and AMADA immobilised on a polyeth-ylene-polyamine-modified PAN matrix for Mn(II), with a reported determination limit of 0.02 μg mL⁻¹. The review also considers sustainability, automation, nanostructured architectures, additive manufacturing, and portable optical detection. Current evi-dence indicates that no single PAN architecture is universally superior; future progress requires improved sorbent standardisation, systematic interference and leaching stud-ies, reproducible optical calibration, and validation under realistic sample conditions. Fibre diameter and porosity are also discussed as coupled determinants of diffusion, scattering, and reproducibility.