Super-bridging fibrous materials for water treatment

The study addresses the challenge of improving sustainability, reducing costs, and enhancing efficiency in water treatment, where conventional aggregation/settling relies heavily on single-use metal coagulants and synthetic flocculants that increase sludge production and environmental burdens. The research hypothesizes that reengineered fiber-based materials—specifically Si-grafted cellulosic fibers formed into fibers and porous microspheres, and Fe-stabilized flakes—can act as super-bridging agents and ballast media to greatly increase floc size and density. This would accelerate solids separation (via settling or screening), reduce chemical demand (coagulants and flocculants), enable material reuse, and improve removal of conventional and emerging contaminants, ultimately decreasing plant footprint and environmental impact.

Background highlights include: widespread reliance on gravitational separation (settling) for solids removal in drinking water and wastewater treatment; substantial use and cost of coagulants/flocculants leading to increased sludge volumes and heavy-metal content; concerns and regulatory restrictions around synthetic flocculants (e.g., polyacrylamide) and the need to reduce their usage; and prior efforts to increase floc settling velocity, often requiring dense ballast media. The authors motivate the use of renewable, recycled, and waste fibers as reusable, versatile materials to reduce chemical consumption and sludge-related impacts while maintaining or improving treatment performance.

Materials development and characterization: (1) Si-grafted fibers and microspheres were synthesized by sol-gel grafting of SiO2 onto cellulosic fibers (pristine NISTRM8496 or recycled/deinked fibers). Fibers were washed, dried (with or without ethanol rinse), then reacted in ethanol/water with tetraethoxysilane (TEOS) and phosphotungstic acid catalyst for 24 h at 120 rpm at room temperature. Grafted materials were rinsed and dried. Dried pulp was manually ground and mixed in ethanol to form porous microspheres that were then stabilized via SiO2 grafting; synthesis conditions (shear, solvent ratios, TEOS amounts) controlled the proportion and size of microspheres. Microspheres were separated from fibers by short gravitational settling. (2) Metal-grafted flakes were formed by contacting cellulosic fibers (or dryer lint) with FeCl3 or Al2(SO4)3·14H2O (0.6–42 mM) at pH 7 (NaOH), hydrolyzing salts to Fe(OH)3/Al(OH)3 that precipitated onto fibers. After sieving, heating at 90 °C for 2 h, washing (including pH >9 to remove loosely bound metals/NOM), and drying, the metal (hydr)oxides functionally linked fibers into flakes. Flake size was tuned by fragmentation; mechanical reinforcement was achieved either by adding high-MW polyacrylamide during synthesis or by post-grafting a SiO2 shell with TEOS and phosphotungstic acid. Characterization included SEM-EDS, FTIR, XPS, and TGA; Si layer thickness was assessed by SEM; densities and surface areas were estimated from dimensions and material properties. Jar testing: Surface water and municipal wastewater were treated in 500 mL beakers: coagulation with alum (300 rpm, 2 min), flocculation (150 rpm, 4 min) with a mixed flocculant (50% starch, 50% polyacrylamide, total 0.30 mg/L). Fibers, Si-microspheres, or flakes were added at flocculation onset. Separation was evaluated by either settling (5–300 s) or in situ screening using nylon meshes (100, 500, 1000, 2000, 5000 µm). Turbidity was measured from supernatant collected 2 cm below the surface. Reuse protocol extracted materials from flocs (pH >9, shear), sieved (160 µm for fibers; 630 µm for microspheres/flakes), washed, and reinjected. Alternative flocculant: A starch-based polysaccharide was extracted from potato peels (pH 4.5 blending, 160 µm sieving, 100 kDa cutoff centrifugation, pH neutralization) to partially replace polyacrylamide. Contaminant assessments: Microplastics (140 µm PE) were quantified by filtration and stereomicroscopy; nanoplastics (200 nm PS) by fluorimetry; NOM (DOC/UV254) and phosphorus by standard methods/ICP-MS. Performance targets: residual turbidity <1 NTU; chemical demand reductions computed at this target.

• Floc size: Conventional treatment produced flocs of 520 ± 50 µm. Si-fibers yielded 4950 ± 480 µm flocs (~10× larger), and Si-microspheres yielded 6630 ± 540 µm flocs (~13× larger). Across conditions, fiber-based flocs were 3,930–5,590 µm (fibers) and 5,770–7,520 µm (microspheres).
• Settling performance to 1 NTU: Conventional treatment required >180 s; pristine fibers required ~24 s (~8× faster); Si-fibers ~14 s (~13× faster); Si-microspheres ~7 s (~26× faster). Correspondingly, much smaller settling tanks could be used without compromising turbidity removal.
• Chemical demand reductions: Si-fibers reduced coagulant demand by ~20% and flocculant demand by >60%; Si-microspheres reduced coagulant demand by ~40% (based on achieving 1 NTU). Partial replacement of synthetic polyacrylamide with extracted starch was demonstrated.
• Screening instead of settling: Conventional flocs required 100 µm mesh; ballasted flocculation required ~500 µm. Fiber-based materials enabled screening with much larger meshes: 2000 µm for fibers and 5000 µm for Si-microspheres, facilitating low-clogging, easily cleaned, low-footprint screens. Materials were recovered from screens and reused (recovery >95%).
• Reusability: Si-fibers and Si-microspheres were extracted, washed, and reused at least 20 cycles without loss of turbidity removal performance.
• Material properties: Si grafting of fibers reached 4–26 wt% Si, increasing relative density from ~1.40 to ~1.54; Si layers ~50–600 nm thick. Si-microsphere density was 1.61 ± 0.23. Estimated cellulose fiber surface area ~280 m²/g versus ~0.017 m²/g for silica sand (~16,000× higher).
• Contaminant removal: Fiber-based screening improved removal of microplastics and nanoplastics compared to conventional treatment. Fe-grafted flakes (1–9% atomic Fe by XPS; 6–32% iron (hydr)oxides by TGA) provided adsorption capacities up to ~1.7 mg NOM/g flakes and ~0.6 mg P/g flakes, lowering residual NOM to 5.2–4.2 mg C/L and phosphorus to 0.80–0.17 mg P/L over 0.3–7.5 g/L flake doses, with no detectable Fe release.
• Versatility and sustainability: Flakes could be synthesized from recycled fibers or dryer lint; flakes and microspheres can be reinforced (polyacrylamide or SiO2) and potentially filled with media to tailor hydrodynamics. Large flocs enable compact screen-based clarification, reducing plant footprint and potentially capital and operating costs.

The findings validate the hypothesis that reengineered fiber-based materials can act as super-bridging agents and ballast media to dramatically increase floc size and density. This translates into much faster clarification, allowing either smaller settling tanks or complete replacement of settling with coarse screening. The ability to use larger mesh sizes reduces clogging and simplifies maintenance, enhancing process robustness. Significant reductions in coagulant and flocculant requirements reduce sludge production, energy and landfill burdens, and heavy-metal content in sludge, improving environmental sustainability and potentially enabling beneficial reuse. The materials’ high surface area and porosity, combined with tunable density, underlie their superior performance compared to conventional ballast media. Fe-grafted flakes add adsorption functionality for NOM and phosphorus while also bridging and ballasting, addressing both particulate and soluble contaminants in one step. Reusability over multiple cycles with high recovery indicates practical viability and cost-effectiveness. Collectively, these results point to a pathway for designing more compact, lower-chemical, and lower-footprint water treatment systems while improving removal of emerging contaminants such as micro- and nanoplastics.

This work introduces reusable Si-grafted fibers and porous Si-microspheres as super-bridging and ballasting materials, and Fe-grafted cellulose flakes as three-in-one bridging/ballasting/adsorbing media. These fiber-based materials produced flocs up to ~13× larger than conventional treatment, achieved target turbidity in seconds rather than minutes, reduced coagulant demand by up to ~40% and flocculant demand by >60%, and enabled replacement of settling tanks with coarse screening. Fe-grafted flakes provided additional adsorption of NOM and phosphorus while maintaining effective solids removal. The materials were recoverable and reusable with high efficiency, compatible with recycled/waste fibers, and amenable to mechanical or chemical reinforcement. Future research should optimize material synthesis for scale-up (e.g., control of size, density, and metal coverage), evaluate long-term durability and life-cycle impacts, extend functionalization to other metal (hydr)oxides or polymers, investigate integration into full-scale screen-based clarification systems, and assess performance across diverse water matrices and contaminant classes.