A super-efficient polyquaternium gel that can remove over-10-times masses of lignins from wastewater for resourcefulness

Papermaking wastewater is a major industrial pollutant source, with lignin being a particularly difficult-to-degrade component. Conventional treatments (alkaline/acid precipitation, flocculation, biological degradation, adsorption) face limitations such as equilibrium constraints, high costs, waste residue generation, and limited removal efficiencies for lignin. Prior work by the authors showed polyquaternium gels could remove 3.89–8.28 times their own mass of lignin via synergistic adsorption and inner skeleton-space effects. This study hypothesizes that engineering an additional, effective outer skeleton-space (inter-chain segment region) by chain-segment modification will further improve lignin removal efficiency and enable resourceful reuse of treatment wastes.

Existing lignin treatment approaches include alkaline precipitation, acid precipitation, and flocculation based on precipitation mechanisms, which are limited by precipitation–dissolution equilibria, cost, and waste generation. Biological methods currently achieve only 60–70% lignin degradation, needing improvement. Adsorption is typically used as a polishing step after other treatments and struggles to directly remove lignin due to lignin’s polymeric structure and steric resistance to conventional adsorbents. The authors’ prior polyquaternium gel work showed enhanced adsorption via inner skeleton-space effects, suggesting potential for super-efficient lignin removal if outer skeleton-space interactions can be integrated.

Materials: CMDA (self-prepared), DMAC (distilled before use), APS (initiator), poly(epichlorohydrin-dimethylamine) (PED), lignin, and Reactive Scarlet 3BS dye were used. Synthesis of CSC crosslinker: PED (1.0 g), CMDA (1.248 g), NaOH (0.25 g), and deionized water (2 mL) were reacted at 60 °C for 3 h, neutralized to pH 7, and concentrated to obtain CSC solution. Synthesis of SGPQG/SGPQ: CSC and DMAC were copolymerized (55% total monomer mass fraction) at a mass ratio of 20/80 with APS (3% w/w of monomer) at 60 °C for 5 h, then dried at 100 °C for 8 h and crushed. Parallel crosslinking generated insoluble, highly crosslinked SGPQG gel (skeleton adsorbent) and micro-crosslinked, soluble SGPQ (flocculant). Optimization: An L(3)4 orthogonal design evaluated reaction temperature (A), monomer concentration (B), APS dosage (C), and time (D), using lignin removal percentage as index. Optimal conditions: 60 °C, 55% monomer concentration, 3% APS, 5 h. Structural composition optimization found the best CSC/DMAC mass ratio at 20/80. Component separation and quantification: SGPQG/SGPQ was eluted in deionized water at 50 °C with stirring (3 cycles, 48 h each) to separate components, yielding 60% (w/w) insoluble SGPQG and 40% (w/w) soluble SGPQ. Performance tests for lignin removal: 0.001–0.010 g of SGPQG/SGPQ was added to 50 mL of 800 mg·L⁻¹ lignin solution at 30 °C with continuous stirring for 48 h. Post-treatment absorbance (UV–Vis) provided R% via R% = (A0 − Ar)/A0 × 100%. The mass of lignin removed per unit sorbent mass, Q′ (mg·g⁻¹), was calculated from R% vs dosage relationships. Separate tests quantified SGPQG (adsorption) and SGPQ (flocculation) contributions. Isotherm and adsorption capacity (SGPQG): 0.001–0.010 g SGPQG in 50 mL of 800 mg·L⁻¹ lignin (30 °C, 48 h). Equilibrium concentration (Ce) and qe calculated by qe = (Co − Ce)V/m. Kinetics (composite): 0.005 g SGPQG/SGPQ in 50 mL of 800 mg·L⁻¹ lignin, stirred for 0.5–5 h. Residual Ct measured to fit zero-, first-, second-, and third-order reaction models; average removal rate AR computed. Kinetics (SGPQG as adsorbent): 0.008 g SGPQG in 50 mL of 800 mg·L⁻¹ lignin, 30 °C, 0.5–7 h. Data fitted to pseudo-first-order, pseudo-second-order, intraparticle diffusion, and particle diffusion models. Thermodynamics: 0.005 g SGPQG/SGPQ treating 50 mL of 800 mg·L⁻¹ lignin at 30–60 °C to equilibrium; thermodynamic parameters (Kc, ΔG, ΔH, ΔS) obtained. Microstructure/chemical analyses: Optical microscopy, FT-IR, SEM, XRD, EDS, and XPS compared SGPQG/SGPQ and components before/after lignin treatment. Resourceful reuse: SGPQG/SGPQ wastes after lignin treatment (S/S waste) were applied to 50 mL of 100 mg·L⁻¹ Reactive Scarlet 3BS dye at 30 °C for 48 h using 0.001–0.010 g; R% and adsorption capacity determined. Characterization (optical microscopy, SEM, FT-IR, XRD, EDS, XPS) assessed dye adsorption. Complex wastewater and real effluent tests: 50 mL model complex wastewater (800 mg·L⁻¹ lignin + 100 mg·L⁻¹ dye) treated with 0.001–0.010 g SGPQG/SGPQ for 48 h; time-course with 0.01 g for 1–48 h. Real papermaking wastewater (1.0 L) was treated with 0.14 g SGPQG/SGPQ; lignin removal quantified.

- SGPQG/SGPQ achieved a lignin removal per unit mass Q′ = 10,157.71 mg·g⁻¹ (removing 10.16 times its own mass of lignin), outperforming similar materials by 1.23–50.55×. - Lignin removal percentages exceeded 90.2% at dosages ≥0.005 g (in 50 mL of 800 mg·L⁻¹ lignin at 30 °C, 48 h). - Component roles: mass composition was 60% insoluble SGPQG (skeleton adsorbent) and 40% soluble SGPQ (flocculant). The SGPQG skeleton exhibited high adsorption capacity for lignin (qe ≈ 7507.66 mg·g⁻¹), 1.89× higher than a prior non-chain-segmented gel (PPG). - Kinetics and thermodynamics: Lignin removal increased with temperature; ΔH = 55.69 kJ·mol⁻¹ and ΔS = 189.28 J·mol⁻¹·K⁻¹; ΔG ≤ 0 indicates spontaneity with a critical temperature ≈21.0 °C (spontaneous above room temperature). Model fitting supported effective interaction kinetics. - Microstructural and spectroscopic evidence (FT-IR, XRD, SEM, EDS, XPS) confirmed extensive lignin uptake both within the gel skeleton (electrostatic binding/ion exchange, self-condensation) and outside in inter-chain segment regions (flocculation/aggregation), validating an integrated skeleton-space mechanism. - Average lignin removal rate was 1.85–3.34× higher than pre-products (PPG, SHPCG). - Resourceful reuse: SGPQG/SGPQ waste after lignin treatment (S/S waste) adsorbed Reactive Scarlet 3BS dye with capacity 799.04 mg·g⁻¹ and removed up to 99.27% at 0.006 g dosage (50 mL, 100 mg·L⁻¹), 443.9× higher capacity than activated carbon. - Complex wastewater (lignin + dye): Q′ for lignin = 7561.50 mg·g⁻¹; Q′ for dye = 750.14 mg·g⁻¹; lignin and dye removal proceeded nearly synchronously. Premature dye uptake can compete for skeleton space, emphasizing optimal treatment order (lignin first, dyes second via waste reuse). - Real papermaking wastewater: 0.14 g SGPQG/SGPQ treated 1.0 L, achieving 91.9% lignin removal, demonstrating practical applicability.

The study demonstrates that integrating outer inter-chain segment regions (via chain-segment grafting) with the inner gel skeleton creates a coordinated skeleton-space mechanism that overcomes steric and adsorption limitations typical for lignin. The insoluble SGPQG skeleton provides high-capacity adsorption and promotes lignin self-condensation inside the network, while the soluble SGPQ facilitates flocculation and macroparticle aggregation in the outer inter-chain regions. This synergy yields super-efficient lignin removal (over 10× mass uptake per sorbent mass) and faster removal rates than previous polyquaternium gels. Thermodynamic analysis indicates an endothermic, spontaneous process above room temperature, supporting practical deployment. Spectroscopic and microscopic evidence corroborates ion exchange/electrostatic binding and extensive lignin accumulation both inside and outside the skeleton. The approach also enables circular use: post-lignin-treatment wastes effectively adsorb dyes at capacities far exceeding activated carbon, thereby valorizing waste streams and enhancing overall sustainability. Successful treatment of real papermaking wastewater underscores translational relevance.

The authors developed an SGPQG/SGPQ composite polyquaternium gel that achieves super-efficient lignin removal from water (Q′ ≈ 10,157.71 mg·g⁻¹) through an integrated skeleton-space effect combining inner-skeleton adsorption/self-condensation with outer-region flocculation/aggregation. The material shows improved performance over prior gels, robust thermodynamics, and mechanistic validation by multi-technique characterization. Wastes from lignin treatment can be directly reused to adsorb dyes with very high capacity, establishing a high-efficiency resourcefulness pathway. The material effectively treats complex model wastewater and real papermaking effluent (91.9% lignin removal), indicating strong application potential. Specific future research directions are not detailed in the paper.

The paper does not explicitly discuss limitations or constraints; no dedicated limitations section is provided.