A comparative study of the physicochemical properties of natural bentonite clays of Pogodayevo (Republic of Kazakhstan, mod. 1) and Dash-Salakhli (Republic of Azerbaijan, mod. 2) deposits and modification of the bentonite clay with polyhydroxocations of iron (III) and aluminium (III). The amount of bentonite in the concentration of iron (aluminum) was 5 mmol Me³⁺/g. It was established that the modification of natural bentonites using polyhydroxocations of iron (III) (mod. 1_Fe_5-c, mod. 2_Fe_5-c) and aluminum (III) (mod. 1_Al_5-c, mod. 2_Al_5-c) by the method of “co-precipitation” leads to a change in their chemical composition, structural and sorption properties. The results showed that hydroxy-aluminum cations ([Al₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺) and poly-hydroxyl-Fe or polyoxo-Fe were intercalated into clay layers, which led to an increase in the values of d₀₀₁ and specific surface areas compared to those of the original bentonite, from 37 to 120 m²/g for the Pogodaevo bentonite and from 51 to 172 m²/g respectively, for bentonite from the Dash-Salakhli deposit. It is shown that modified sorbents based on natural bentonite are finely porous objects with a predominance of pores of 1.5–8.0 nm in size. As a result, there is a significant increase in the specific surface area of sorbents. Modification of bentonite with polyhydroxocations of iron (III) and aluminum (III) by the “co-precipitation” method also leads to an increase in the sorption capacity of the sorbents obtained with respect to nickel (II) cations. Modified bentonites were used for the adsorption of Ni (II) ions from the model solution. Ni (II) was absorbed in a neutral pH solution. The study of equilibrium adsorption showed that the data are in good agreement with the Langmuir isotherm model. The maximum adsorption capacity of the Ni (II) obtained from the Langmuir equation was 25.0 mg/g (mod. 1_Al_5-c), 18.2 mg/g (mod. 2_Al_5-c) for Al-bentonite and 16.7 mg/g (mod. 1_Fe_5-c), 10.1 (mod. 2_Fe_5-c) for Fe-bentonite. The kinetics of adsorption is considered. The high content of Al-OH anion exchange centers in them determines the higher sorption activity of Al-modified bentonites.

A comparative study of the physicochemical properties of natural bentonite clays of Pogodayevо (Republic of Kazakhstan, mod.1) and Dash-Salakhli (Republic of Azerbaijan, mod.2) deposits and modification of the bentonite clay with polyhydroxocations of iron (III) and aluminium (III). It-wasestablished that the modification of natural bentonitesusing polyhydroxocations of iron (III) and aluminium(III) by the method of "co-precipitation" leads to a change in their chemical composition, structural and sorption properties. The results showed that hydroxy-aluminum cations ([Al3O4(OH)24(H2O)12]7+) and poly-hydroxyl-Fe or polyoxo-Fe were intercalated into clay layers, which led to an increase in the values of d001 and specific surface areas compared to those of the original bentonite, from 37 to 120 for the Pogodaevo bentonite and from 51 to 172 respectively for bentonite from the Dash-Salakhli deposit. It is shown that modified sorbents based on natural bentonite are finely porous objects with a predominance of pores of 1.5−6.0 nm in size. As a result, there is a significant increase in the specific surface area of sorbents. Modification of bentonite with polyhydroxocations of iron (III) and aluminium (III) by the "co-precipitation" method also leads to an increase in the sorption capacity of the sorbents obtained with respect to nickel (II) cations. Modified bentonites were used for the adsorption of Ni (II) ions from the model solution. Ni (II) was absorbed in a neutral pH solution. The study of equilibrium adsorption showed that the data are in good agreement with the Langmuir isotherm model. The maximum adsorption capacity of the Ni (II) monolayer obtained from the Langmuir equation was 25.0 mg/g (mod. 1_Al_5-c), 18.2 mg/g (mod. 2_Al_5-c) for Al-bentonite and 16.7 mg/g (mod. 1_Fe_5-c), 10.1 (mod. 2_Fe_5-c) for Fe-bentonite. The kinetics of adsorption is considered. The high content of Al-OH anion exchange centresin them determines the higher sorption activity of Al-modified bentonites.

The physicochemical and adsorption properties of granular sorbents based on natural bentonite and modified sorbents based on it have been studied. It was found that modification of natural bentonite with iron (III) polyhydroxocations (mod. 1_Fe_5 GA) and aluminum (III) (mod. 1_Al_5 GA) by the “co-precipitation” method leads to a change in their chemical composition, structure, and sorption properties. It is shown that modified sorbents based on natural bentonite are finely porous (nanostructured) objects with a predominance of pores measuring 1.5–8.0 nm, with a specific surface area of 55–65 m²/g. Modification of bentonite with iron (III) and aluminum compounds by the “co-precipitation” method also leads to an increase in the sorption capacity of the obtained sorbents with respect to bichromate and arsenate anions and nickel cations by 5-10 times compared with natural bentonite. The obtained sorption isotherms were classified as Langmuir type isotherms. Kinetic analysis showed that at the initial stage the sorption process is controlled by an external diffusion factor, i.e. refers to the diffusion of sorbent from solution into a liquid film on the surface of the sorbent. Then the sorption process begins to proceed in a mixed diffusion mode, when it limits both the external diffusion factor and the internal diffusion factor (the diffusion of the sorbent to the active centers through the system of pores and capillaries). To determine the contribution of the chemical stage to the rate of adsorption of bichromate and arsenate anions and nickel(II) cations with the studied granular sorbents, kinetic curves were processed using the equations of chemical kinetics (pseudo-second-order model). As a result, it was found that the adsorption of the studied anions by modified sorbents based on natural bentonite is best described by a pseudo-second-order kinetic model. It is shown that the use of natural bentonite for the development of technology for the production of granular sorbents based on it has an undeniable advantage, firstly, in terms of its chemical and structural properties, it is easily and effectively modified, and secondly, having astringent properties, granules are easily made on its basis, which turn into ceramics during high-temperature firing. The result is a granular sorbent with high physical and mechanical properties. Since bentonite is an environmentally friendly product, the technology of recycling spent sorbents is also greatly simplified.

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.