A super-efficient gel adsorbent with over 1000 times the adsorption capacity of activated carbon

Dyeing wastewater is a major water pollution source due to high chroma, toxicity, and poor biodegradability. Adsorption is widely used for removing low-concentration pollutants, with activated carbon as the prevalent adsorbent due to its porous structure and van der Waals interactions. However, activated carbon performs poorly for large-molecular-volume dyes because of pore size constraints (e.g., 1.85 mg g⁻¹ for Reactive Scarlet 3BS, M_r 1136.32 g mol⁻¹). Electrostatic (chemical) adsorption can form stronger ionic interactions than physical adsorption and overcome pore limitations. The authors previously showed polyquaternium gels can boost dye adsorption by 741-fold over activated carbon, motivating further enhancement. They hypothesized that transforming the conventional 3D distribution of cationic adsorption sites in gel adsorbents to a nearly two-dimensional flat distribution in water would enhance exposure of cationic sites, improve electrostatic interactions with anionic dyes, and surpass current capacity limits. They further posited that loading a flat-distributed polyquaternium gel onto a flat, hydrophobic carbon skeleton could consolidate this distribution and deliver super-efficient adsorption for dyeing wastewater purification.

Prior work establishes activated carbon as the standard adsorbent in water treatment but limited by pore size for large molecules. Electrostatic adsorption using cationic materials (e.g., polyquaternium gels, polycationic-modified substrates) can significantly increase dye uptake via ionic interactions, overcoming van der Waals limitations. The authors’ earlier studies reported strong electrostatic adsorption with polyquaternium gels, achieving up to 741-fold improvement over activated carbon for large dyes. Literature also documents various advanced adsorbents (biochar-based, MOFs, hydrogels, composites) targeting dye removal, yet few demonstrate ultra-high capacities under comparable conditions or mechanistic shifts away from 3D pore-controlled adsorption. No prior report described a solution-carbonized cyclodextrin (CDC) skeleton or a quasi-planar distribution strategy for cationic sites to enhance adsorption.

Design and materials: - Matrix: Reactive hetero-ring derived polyquaternium gel (RRQG) designed to distribute cationic adsorption sites in a nearly planar fashion in water. - Skeleton: Cyclodextrin carbide (CDC), a flat, hydrophobic carbonized structure obtained by solution carbonization of γ-cyclodextrin (γ-CD) in HCl at 100 °C. - Composite: RRQG loaded onto CDC (RRQG@CDC) to consolidate planar exposure of cationic sites and enhance electrostatic adsorption. Synthesis and optimization of RRQG: - Monomers: DHAC (N,N-diallyl-3-hydroxy-azacyclobutane ammonium chloride) and DMDAC (dimethyldiallylammonium chloride). - Orthogonal L(3) optimization (factors: monomer concentration (A), temperature (B), APS dosage (C), time (D)) using dye removal as index. Optimal conditions: 55% monomer concentration, 55 °C, APS 1% (w/w of monomer), 6 h reaction; optimal DHAC:DMDAC molar ratio 10:90 for maximum removal (R% 99.66%). RRQG exhibits flat distribution in water by optical microscopy (ring→transparent→flake forms). - Characterization: FT-IR (structural features, stability after adsorption), elemental analysis (C/H/N, confirming quaternary ammonium content), XRD (crystalline to amorphous transition upon dye doping), zeta potential (positive before adsorption; negative after dye adsorption). Preparation of CDC: - Dissolve γ-CD (e.g., 5.0 g) in HCl (1:9, v/v), evaporate/dry at 100 °C to form CDC via hydroxyl elimination. Non-soluble, particulate; increased carbon content; higher thermal stability vs CD; SEM shows fragmented porous-like morphology; FT-IR/XRD indicate hydroxyl elimination with skeleton retention and amorphization. Construction and optimization of RRQG@CDC: - Form γ-CD/RRQG complex by soaking DHAC-DMDAC prepolymer in 1% (w/w) γ-CD solution, evaporating/drying at 100 °C to in situ form RRQG in γ-CD. - Acidify γ-CD/RRQG complex in HCl (1:9, v/v) then evaporate/dry at 100 °C to carbonize γ-CD to CDC in situ and load RRQG, yielding RRQG@CDC. - L18(3^6) orthogonal optimization with factors: γ-CD mass fraction (A), HCl dilution (B), prepolymer soaking time (C) and temperature (D), acidification time (E) and temperature (F). Optimal: γ-CD 1% (mass fraction; γ-CD/RRQG mass ratio 1:2), HCl 1:9, soaking 3 h at 60 °C, acidification 2 h at 40 °C. Product achieved R% 96.99%. - Characterization: FT-IR (structural evolution showing OH elimination and changes in C–H/C–C), elemental analysis (C/H/N increase post-carbonization), SEM (surface smoothing/flattening), EDS (elemental changes pre/post adsorption), XRD (crystalline to amorphous after adsorption), zeta potential (+27.59 eV before, −14.44 eV after), XPS (N1s binding energy shifts and elemental content changes after adsorption). Adsorption experiments: - Isotherms, kinetics, thermodynamics for RRQG and RRQG@CDC using Reactive Scarlet 3BS (100 mg L⁻¹, 100 mL baseline tests). Dosage varied (0.001–0.01 g typical). Six parallel repeats for reproducibility. - Conditions variation: pH range 0–12; salinity with Na₂SO₄ and CaCl₂ at 0.5% (w/w) (total ion concentrations ~0.038 and 0.045 mol L⁻¹ initially); other dyes: Reactive Blue 19, Reactive Black KN-B, Methyl Blue. - Scale-up simulations: 1.5 L of 100 mg L⁻¹ dye treated using 0.09–0.15 g RRQG@CDC with proportional conditions. - Swelling permeability: RRQG@CDC SP% = 5680% (absorbs 56.8× its mass of water). Resource utilization tests of waste residues: - TG-DTG in N₂ and air to evaluate conversion to carbides and combustion fuel characteristics after dye adsorption. Determined ignition/burnout temperatures and combustion index; assessed feasibility as high-calorific fuel.

- Super-efficient adsorption: RRQG@CDC achieved maximum adsorption capacity (Qmax) of 2312.54 mg g⁻¹ for Reactive Scarlet 3BS under salt-free conditions (at 0.004 g dosage), about 1250× that of commonly used activated carbon under equivalent conditions. RRQG (matrix alone) reached Qmax 1881.59 mg g⁻¹ at 0.005 g dosage, 1017.08× activated carbon. - Salt effects: In 0.5% (w/w) salt solutions, RRQG@CDC maintained or improved capacity: Qmax 2635.35 mg g⁻¹ (Na₂SO₄) and 2960.84 mg g⁻¹ (CaCl₂), with near-complete removal (98.57–100%) at dosages ≥0.005 g. Presence of salts restored conventional dosage–capacity trends (diffusion-driven). - pH adaptability: High removal under acidic to neutral conditions (pH ≤7: 99.14–99.71%); lower removal (78.18–83.67%) under alkaline conditions. - Broad dye applicability: For Reactive Blue 19, Reactive Black KN-B, and Methyl Blue, best removals were 98.04–98.84%; capacities reached 3288.5, 1630.17, and 1280.43 mg g⁻¹, respectively. - Kinetics: • RRQG: Intra-particle diffusion model best fit (R² = 0.99), y = 554.24x + 346.35; k = 554.24 mg g⁻¹ h⁻¹/². • RRQG@CDC: Multiple models fit well (R² > 0.99). Second-order model qe = 2433.09 mg g⁻¹ (close to isotherm qe = 2312.54 mg g⁻¹). Intra-particle diffusion: y = 629.89x + 805.95, indicating faster internal diffusion vs RRQG. - Thermodynamics: • RRQG: Exothermic adsorption with ΔH = −128.13 kJ mol⁻¹, ΔS = −399.95 J mol⁻¹ K⁻¹; spontaneous (ΔG < 0) when T < 48.13 °C. • RRQG@CDC: Endothermic adsorption with ΔH = 171.09 kJ mol⁻¹, ΔS = 581.11 J mol⁻¹ K⁻¹; reported AG ≤ 0 when T < 21.42 °C; dye removal increased with temperature. - Zeta potential shifts: RRQG from +23.17 eV to −14.90 eV after adsorption; RRQG@CDC from +27.59 eV to −14.44 eV, evidencing strong electrostatic capture of anionic dyes. - Structural changes: XRD showed crystalline peaks pre-adsorption and amorphous pattern post-adsorption for both RRQG and RRQG@CDC, indicating dye doping and structural disruption. FT-IR indicated weakening of C–N⁺ features after adsorption. - Mechanism: Established an enhanced quasi-planar electrostatic adsorption model. CDC forms a flat hydrophobic carrier that associates with RRQG’s hydrophobic skeleton, exposing cationic sites quasi-planarly and strengthening electrostatic interactions with anionic dyes. - Scale-up feasibility: Treating 1.5 L of 100 mg L⁻¹ Reactive Scarlet 3BS achieved 98.57% removal with 0.09 g RRQG@CDC and 99.15% with 0.15 g, matching small-scale performance. - Resource utilization: Post-adsorption residues are suitable as high-calorific fuels. In air, ignition at 341.95 °C, burnout at 601.9 °C, final residue 1.85%, and a high comprehensive combustion characteristic index (6.169 × 10¹). Conversion to stable carbides in N₂ was limited (residual mass 7.48% at 900 °C), indicating instability as carbonized materials. - Cost: A preliminary calculation suggests RRQG@CDC cost is lower than activated carbon (per Supplementary Information).

Transforming the cationic adsorption sites from a 3D gel network into a nearly planar distribution substantially improves exposure and accessibility of electrostatic sites to bulky anionic dyes. Loading RRQG onto a flat, hydrophobic CDC skeleton consolidates this quasi-planar arrangement and further enhances electrostatic interactions. The resulting RRQG@CDC displays dramatically higher capacities than activated carbon and previously reported gel adsorbents, validating the central hypothesis. Kinetic analyses show fast internal diffusion and strong agreement with second-order and diffusion models, consistent with chemisorption and efficient mass transfer in the quasi-planar architecture. Thermodynamic behavior differs between RRQG (exothermic) and RRQG@CDC (endothermic with a reported critical temperature), suggesting that the composite’s structural consolidation and activation may alter energetic requirements. Performance is robust across acidic to neutral pH and in saline matrices, and extends to multiple dye classes, indicating broad applicability. Structural and surface analyses (FT-IR, XRD, SEM-EDS, XPS, zeta potential) consistently support a dominant electrostatic adsorption mechanism with dye-induced structural changes. Simulated scale-up maintains high removals, implying practical feasibility. Moreover, enabling resource utilization of dye-laden residues as fuels addresses secondary pollution concerns and supports a circular approach to wastewater treatment.

This work introduces a super-efficient adsorbent system (RRQG@CDC) by loading a flat-distributed polyquaternium gel onto a solution-carbonized cyclodextrin skeleton, achieving up to 2312.54–2960.84 mg g⁻¹ capacity for dyes and approximately 1250-fold improvement over activated carbon. The study establishes an enhanced quasi-planar electrostatic adsorption mechanism that departs from traditional 3D pore-dominated adsorption, explains the high capacities and kinetics, and demonstrates robustness across pH, salinity, and dye types. Simulated scale-up confirms engineering feasibility, and thermogravimetric combustion analyses show that dye-laden residues can be repurposed as high-calorific fuels, mitigating secondary pollution. Future work could include long-term cycling and regeneration studies, testing with real industrial effluents containing mixed contaminants, pilot-scale demonstrations, comprehensive cost and life-cycle assessments, and exploration of the quasi-planar strategy with other cationic matrices and carbon skeletons.

- Scale-up tests were simulated at bench scale (up to 1.5 L); no pilot-plant or continuous-flow trials were reported. - Thermodynamic constraints: RRQG shows an upper temperature limit for spontaneity (ΔG < 0 when T < 48.13 °C). For RRQG@CDC, a critical temperature for spontaneity was reported (AG ≤ 0 when T < 21.42 °C) alongside endothermic behavior; this may constrain operating windows. - Stability of carbonized residues: Conversion of waste residues to stable carbides in N₂ was limited (only 7.48% mass remaining at 900 °C), indicating instability as carbonized materials. - Regeneration and reusability: Adsorbent regeneration, longevity, and performance over multiple cycles were not assessed. - Real wastewater complexity: Tests focused on model dye solutions; performance with real textile wastewater containing diverse co-contaminants was not evaluated.