Papermaking wastewater poses a significant environmental challenge due to the presence of lignin, a recalcitrant pollutant. Current lignin removal methods, such as alkaline/acid precipitation, flocculation, and biological methods, have limitations in efficiency and cost-effectiveness. Adsorption methods show promise but are often used as a secondary treatment. This study aims to develop a novel material that overcomes these limitations. Previous research by the authors showed that polyquaternium gel adsorbent materials could remove several times their weight in lignin due to adsorption and self-condensation within the gel's structure. Building upon this, the current work hypothesizes that modifying the gel structure to introduce a controlled space outside the gel skeleton can further enhance lignin removal efficiency. This hypothesis led to the design and synthesis of a novel chain segment polyquaternium gel composite material (SGPQG/SGPQ), which comprises a highly cross-linked, insoluble skeleton component (SGPQG) and a micro-crosslinked, soluble flocculant component (SGPQ). The integrated structure is designed to create an enhanced 'skeleton-space' effect for improved lignin removal and waste resourcefulness.

The uncontrolled discharge of papermaking wastewater, a major industrial pollutant, seriously impacts societal sustainability. Lignin, a complex phenolic polymer, is particularly challenging to remove from this wastewater. Existing methods, including alkaline precipitation, acid precipitation, flocculation, and biological treatment, suffer from limitations in removal efficiency and often generate large amounts of waste. While adsorption offers a potential solution for deep wastewater treatment, its direct application for lignin removal is hindered by lignin's polymeric structure and spatial resistance. The challenge lies not only in efficient lignin removal but also in the resourceful reuse of the resulting waste materials. This study draws upon the authors' prior work demonstrating the potential of polyquaternium gel materials for lignin removal, attributing their success to a synergistic adsorption and self-condensation mechanism within the gel's internal structure.

The SGPQG/SGPQ composite material was synthesized via cross-linking copolymerization of a chain segment crosslinker (CSC) and dimethyldiallylammonium chloride (DMAC) monomers. Orthogonal experiments (L(3)4) were conducted to optimize synthesis conditions (reaction temperature, monomer concentration, initiator dosage, and reaction time), maximizing lignin removal. The optimal mass ratio of CSC to DMAC was determined to be 20:80. The resulting material was characterized and its lignin removal performance assessed. The lignin removal efficiency and the differential contributions of SGPQG and SGPQ were evaluated using various dosages and concentrations. Isothermal adsorption curves for SGPQG were determined. Kinetic and thermodynamic studies were performed to understand the interaction mechanisms at different temperatures and reaction times, fitting the data to zero-order, first-order, second-order, and third-order reaction kinetics equations, and pseudo-first-order, pseudo-second-order, intraparticle diffusion, and particle diffusion adsorption kinetics equations. Microstructural changes before and after lignin treatment were analyzed using optical microscopy, FT-IR, SEM, XRD, and XPS. The spent SGPQG/SGPQ material (S/S waste) was tested for its capacity to adsorb Reactive Scarlet 3BS dye from a simulated dyeing wastewater, and its adsorption capacity was compared to activated carbon. Finally, the SGPQG/SGPQ material was applied to a complex wastewater containing both lignin and dye, and a real papermaking wastewater sample, to evaluate its real-world applicability. The methods included UV-Vis spectrophotometry for measuring lignin and dye concentrations, and various characterization techniques to analyze the material's structure and composition.

The optimized SGPQG/SGPQ material exhibited a remarkable lignin removal capacity of 10,157.71 mg·g⁻¹, significantly surpassing existing materials. This high efficiency is attributed to a newly identified integrated skeleton-space effect mechanism involving both adsorption and self-condensation of lignin within the gel's internal structure and flocculation and aggregation outside the gel structure. The SGPQG component contributed significantly to adsorption (7507.66 mg·g⁻¹), further enhanced by chain segment modification. The SGPQ component played a crucial role in flocculation. Kinetic studies indicated that the lignin removal followed a pseudo-second-order kinetic model. Thermodynamic analysis confirmed the spontaneous nature of the lignin removal process (ΔG ≤ 0) at temperatures above 21.0 °C. Microstructural analysis using optical microscopy, FT-IR, SEM, XRD, and XPS confirmed the interaction of lignin with both the internal and external spaces of the SGPQG/SGPQ material. The spent SGPQG/SGPQ (S/S waste) exhibited a superior dye adsorption capacity (799.04 mg·g⁻¹), which is 443.9 times greater than activated carbon. Application of SGPQG/SGPQ to a complex wastewater solution showed simultaneous removal of lignin and dye, though the efficiency was slightly lower compared to treating lignin alone. The material also effectively treated real papermaking wastewater, achieving a lignin removal rate of 91.9%.

The results demonstrate the superior performance of the SGPQG/SGPQ material for lignin removal from papermaking wastewater compared to previously reported methods. The integrated skeleton-space effect mechanism, a key innovation of this study, offers a more efficient and sustainable approach to lignin removal. The high adsorption capacity and successful removal of both lignin and dyes from complex wastewater highlight the material's potential for practical applications. The remarkable dye adsorption capacity of the S/S waste not only addresses the problem of waste disposal but also offers a valuable resource recovery pathway. The slightly reduced efficiency observed when treating the complex wastewater compared to pure lignin solutions could be attributed to the dye competing for adsorption sites. The study's findings have significant implications for the environmental remediation of papermaking wastewater and waste resource utilization.

This study successfully developed a super-efficient polyquaternium gel composite material (SGPQG/SGPQ) for removing lignin from papermaking wastewater. The exceptional performance is attributed to a novel integrated skeleton-space effect mechanism. The resourceful reuse of the spent material for dye adsorption further enhances its sustainability. Future research could focus on optimizing the material's synthesis and exploring its potential for treating other types of industrial wastewater containing recalcitrant pollutants.

The study primarily focused on a specific type of lignin and dye. Further investigations are needed to assess the material's performance with other types of lignin and dyes encountered in diverse industrial wastewaters. While the material demonstrated excellent performance in laboratory settings, further large-scale testing is required to validate its effectiveness and economic feasibility for industrial applications. The long-term stability of the material under industrial operating conditions also requires further investigation.