Álcool polivinil (PVA) para materiais de adsorção de corantes

Dye-laden effluent streams generated by textile finishing, leather tanning, and paper processing operations carry complex mixtures of anionic, cationic, and non-ionic chromophores that conventional activated carbon often fails to capture selectively at low concentrations across broad pH ranges. Polyvinyl alcohol (PVA) enters this gap as a synthetic, semi-crystalline polymer whose pendant hydroxyl groups can be physically or chemically crosslinked into hydrogel networks with tunable pore architecture, swelling capacity, and surface charge density. Industrial grades of PVA for dye adsorption are differentiated primarily by degree of hydrolysis (87–89 mol% partially hydrolyzed; 98–99 mol% fully hydrolyzed) and weight-average molecular weight (Mw) typically spanning 13 000–186 000 g mol⁻¹, which together govern crystallinity, solubility in cold water, and the density of reactive sites available for dye binding or subsequent functionalization. In contrast to chitosan, whose amine groups protonate and lose adsorptive capacity below pH 4.5, fully hydrolyzed PVA networks retain structural integrity from pH 2 to 12, making them viable for acid-heavy dye baths. Against commodity ion-exchange resins, PVA-based adsorbents offer a narrower operating temperature window—thermal degradation initiates near 230 °C—yet can be regenerated with dilute acid or alkaline eluents at ambient temperature without the osmotic shock cracking observed in styrenic beads.

How do the degree of hydrolysis and molecular weight define PVA grade suitability for anionic versus cationic dye adsorption?
Commercial PVA is supplied as a powder or granulate under designations such as Kuraray Poval™, Sekisui Selvol™, or Sinopec PVA, with specifications that map directly to dye-binding mechanisms. A fully hydrolysed grade (hydrolysis ≥ 98.5 %, residual acetyl content ≤ 0.5 wt%) yields a higher density of hydroxyl groups per chain segment, promoting hydrogen bonding with sulfonate or carboxylate groups on anionic dyes like Reactive Black 5 or Acid Orange 7. Partially hydrolysed grades (87–89 %) retain acetate moieties that reduce crystallinity, improve cold-water processability, and lower gelation temperature—properties that facilitate blending with filler particles such as bentonite or graphene oxide before crosslinking. The molecular weight controls solution viscosity and mesh size of the final hydrogel. A low-Mw grade (13 000–23 000) produces a tightly crosslinked network after reaction with glutaraldehyde (GA), resulting in a molecular weight cut-off low enough to exclude larger dye aggregates but also limiting intraparticle diffusion rates. High-Mw grades (146 000–186 000) require extended dissolution at 85–95 °C and yield hydrogels with equilibrium swelling ratios between 400 % and 900 % in deionised water, which correlates with faster uptake kinetics for small-molecule cationic dyes such as Methylene Blue (MB) and Crystal Violet. Batch adsorption isotherms fitted with the Langmuir model typically show monolayer capacities for MB on GA-crosslinked, fully hydrolysed PVA of 45–210 mg g⁻¹ depending on crosslinker-to-polymer ratio; these values are determined under stirring at 150 rpm in a thermostatic shaker bath set to 25 °C ± 0.5 °C, with residual dye quantified via UV-Vis spectrophotometry at λmax 664 nm following centrifugation at 6 000 rpm for 15 min.

The processing route introduces another layer of specification. When PVA is dissolved in water at 10 wt% and crosslinked via freeze-thaw cycling—typically 3–5 cycles of freezing at −20 °C for 12 h followed by thawing at 25 °C for 4 h—the resulting physical hydrogel exhibits a crystallite-mediated junction structure without chemical crosslinker residues. This is critical when the treated water must comply with drinking water standards that restrict glutaraldehyde leaching (WHO guideline value for GA in drinking water: 0.1 mg L⁻¹). Physical gels display tensile strengths in the range of 0.1–0.5 MPa and can be shaped into irregular chips suitable for packed columns. Their adsorption of methyl orange, an anionic azo dye, is markedly enhanced by blending 5–15 wt% chitosan into the PVA matrix prior to cryogelation, which introduces protonated primary amines that exert electrostatic attraction at pH 3–5. This composite approach leverages the mechanical resilience of PVA while offsetting its weak affinity for anionic species; a composition of 10 wt% PVA (Mw 89 000–98 000, 99 % hydrolysed) with 7.5 wt% chitosan (DD> 85 %) has been reported to achieve a qmax of 168 mg g⁻¹ for Congo Red in synthetic wastewater containing 500 mg L⁻¹ NaCl, tested according to the batch procedures outlined in ASTM D3860-98 (reapproved 2020) for adsorptive capacity determination.


Breakthrough Curve Analysis and Fixed-Bed Column Specifications
Continuous-flow adsorption trials on PVA hydrogel beads (diameter 2–3 mm after swelling, sphericity> 0.92) packed in borosilicate glass columns of internal diameter 1.5 cm and bed height 10 cm provide engineering data for scale-up. Typical operating conditions impose an inlet dye concentration of 50 mg L⁻¹ at a volumetric flow rate that yields an empty bed contact time (EBCT) of 2–5 min. The breakthrough point, defined at Ct/C0 = 0.05 as per the column test method adapted from ASTM D6586-03, is reached after 80–120 bed volumes for Methylene Blue on chemically crosslinked PVA beads; this declines to 30–50 bed volumes when the same beads treat Reactive Red 120, reflecting weaker hydrogen bonding compared with electrostatic interactions available in cation-exchange designs. The Thomas rate constant kTh obtained from fitting the breakthrough profile falls in the range 0.5–1.2 mL mg⁻¹ min⁻¹, and the maximum solid-phase concentration q0 aligns with batch isotherm data within ± 8 %, confirming mass-transfer limited kinetics with negligible axial dispersion effects under the applied superficial velocity of 0.4–0.8 cm min⁻¹.


Representative specification ranges for PVA grades applied in dye adsorption hydrogel formation.
Grade Parameter
Partially Hydrolysed Low-MW
Partially Hydrolysed High-MW
Fully Hydrolysed Medium-MW
Hydrolysis (mol%)
87.0–89.0
87.0–89.0
98.5–99.2
Viscosity of 4 % aq. soln. at 20 °C (mPa·s)
4.0–6.0
40.0–50.0
20.0–30.0
Ash content (wt%)
≤ 0.5
≤ 0.5
≤ 0.5
pH of 4 % soln.
5.0–7.0
5.0–7.0
5.5–7.5
Equilibrium swelling of 10 % GA-crosslinked gel (%)
250–350
600–900
400–650
Typical MB qmax (Langmuir, mg g⁻¹)
48–72
120–180
85–145


When chitosan-based adsorbents lose amine activity at acidic pH, PVA crosslinked networks maintain structural integrity
The primary operational advantage of PVA over natural polysaccharide adsorbents manifests under strongly acidic dye baths. Chitosan, despite its high amine density (degree of deacetylation commonly 75–95 %), undergoes progressive protonation below its pKa (~6.3), and at pH 3 the material dissolves or disintegrates unless heavily crosslinked with agents such as epichlorohydrin, which introduces toxicity concerns. PVA hydrogels, by contrast, do not rely on ionizable surface groups for structural cohesion; the network is maintained by hydrogen bonds and crystalline junctions that persist down to pH 1. Even chemical crosslinks formed with glutaraldehyde are stable under acidic regeneration conditions using 0.1 M HCl. In side-by-side column runs treating a simulated textile effluent containing Acid Blue 113 at pH 2.8, fully hydrolysed PVA beads retained 93 % of their initial dynamic binding capacity after five adsorption-desorption cycles, whereas chitosan beads crosslinked with 2.5 % glutaraldehyde lost 40 % of their initial bed height due to compaction and suffered a capacity drop exceeding 50 %. The PVA cycle was executed with regeneration solution of 0.1 M NaOH at 1 BV h⁻¹ for 2 h, followed by rinsing with deionised water until neutral pH. The absence of amine-based by-products during incineration of spent PVA adsorbent (ash residue <1 wt%) further simplifies disposal under waste management codes aligned with EU Directive 2008/98/EC.

Occupational exposure and food-contact regulations form another differentiator. PVA is listed as an indirect food additive under FDA 21 CFR 177.1670, and its aqueous solutions are not classified as hazardous according to Regulation (EC) No 1272/2008. This contrasts with adsorbents based on acrylamide or acrylic acid monomers, where residual monomer limits for drinking-water contact are enforced at sub-ppm levels (e.g., acrylamide ≤ 0.1 µg L⁻¹ under EU Directive 98/83/EC), necessitating extensive post-synthesis washing that increases production cost and water usage. PVA’s synthetic route via vinyl acetate polymerization and subsequent saponification leaves no toxic monomers; the residual methanol and methyl acetate volatilize during drying at 105 °C, and final product specifications typically cap volatile organics at ≤ 0.2 wt% measured by headspace GC per ASTM D4526-20.

A frequently overlooked differentiator is the compatibility of PVA with electrospinning into nanofibrous mats. Solutions of fully hydrolysed PVA (10–12 wt% in deionised water, conductivity adjusted with 0.1 vol% Triton X-100) electrospun at a feed rate of 0.5–1.0 mL h⁻¹, a tip-to-collector distance of 15 cm, and an applied voltage of 18–22 kV yield continuous fibres with average diameter 150–300 nm. These mats, after crosslinking by thermal treatment at 140 °C for 1 h in the presence of maleic acid as a crosslinking catalyst, exhibit specific surface areas of 25–45 m² g⁻¹ as determined by BET nitrogen adsorption (ASTM D6556-21). While lower than the 800–1200 m² g⁻¹ typical of activated carbon, the nanofibrous geometry provides rapid interstitial flow and reduces pressure drop in depth filtration mode to <0.5 kPa at a face velocity of 5 cm min⁻¹, enabling high-throughput polishing of colloidal dye aggregates that would plug microporous carbon. Regeneration is performed by backwashing with 60 °C alkaline solution (pH 11, NaOH adjusted), recovering over 95 % of initial flux within 20 min.



Standards and test methods applicable to PVA-based dye adsorbents for industrial water discharge certification.
Application Criterion
Standard/Method
Measured Parameter
Batch adsorptive capacity
ASTM D3860-98 (2020)
qe (mg g⁻¹) at equilibrium
Column breakthrough service life
ASTM D6586-03 (2021)
Bed volumes to Ct/C0 = 0.05
Swell ratio and gel fraction
Gravimetric immersion in DI water 25 °C/24 h
Swelling (%) and insoluble fraction (%)
Residual colour in treated effluent
ISO 7887:2011 Method B
Absorbance at Hg line 436 nm; ADMI colour value
Total organic carbon (leaching)
ISO 8245:1999
TOC ≤ 5 mg L⁻¹ after 72 h soak
Biodegradability of spent adsorbent
OECD 301B (modified Sturm test)
CO2 evolution ≥ 60 % of ThO₂ after 28 days


Thermal regeneration—an alternative to chemical elution—imposes operational boundaries. When PVA hydrogel beads are heated above their glass transition temperature (~85 °C for fully hydrolysed grades) in an inert atmosphere, the network collapses, desorbs bound dye, and re-swells upon cooling if the temperature is kept below the onset of decomposition (230 °C). Published data for this specific configuration is limited, but pilot-scale trials using a rotating drum oven at 190 °C under nitrogen flow 2 L min⁻¹ have demonstrated recovery of 78–85 % of initial MB capacity across three cycles, beyond which embrittlement of the bead surface reduced sphericity below 0.8 and generated fines that increased column backpressure by 35 %. Users evaluating thermal regeneration must therefore balance energy cost against chemical consumption and effluent neutralization requirements, a trade-off not encountered with mineral adsorbents like bentonite that do not undergo reversible swelling.