Abstract:

Objective In China, over 20% of polyvinyl alcohol (PVA) is utilized as a sizing reagent in the textile industry, with virtually all of it being discharged into wastewater during the desizing process in fabric printing and dyeing. Traditional PVA wastewater treatment methods face significant challenges due to PVA's high degree of polymerization, electrical neutrality, and strong intramolecular/intermolecular hydrogen bonds, which collectively impede degradation and precipitation. These characteristics typically necessitate substantial chemical inputs and complex treatment processes. Therefore, developing an integrated technology that combines pre-oxidation and flocculation processes is particularly crucial for efficient PVA wastewater treatment.

Method A "pre-oxidation flocculation precipitation" technology based on Fe²⁺/CaO₂ was developed. The system releases H₂O₂ through CaO₂ hydrolysis, generates oxidative species under Fe²⁺ catalysis, enhances PVA flocculability, and achieves precipitation via Fe³⁺ and Ca²⁺ ions. Effects of reagent dosage, pH value, initial PVA concentration, and reaction time were investigated. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques were used to analyze precipitates for mechanism elucidation. The sludge was evaluated as an adsorbent and catalyst for Reactive Black 5 degradation.

Results The integrated treatment of PVA technology has been successfully achieved. The optimal conditions were determined to be 20 mmol/L Fe²⁺ and 2 mmol/L CaO₂, with a pH value of 3 for the pre-oxidation stage, pH value of 10 for the flocculation for sedimentation stage, and a pre-oxidation time of 60 minutes. When treating PVA within a concentration range of 0.5-2 g/L, the removal rates for PVA and chemical oxygen demand (COD) reached over 97.1% and 92.2%. The process also demonstrated effective treatment of actual desizing wastewater containing modified starch, achieving over 90% removal efficiency. XPS C1s spectra showed an increase in C—O bond proportion after treatment, with O—C═O bonds shifting to higher binding energy (289.3 eV) and the emergence of a new ketone (C═O) peak at 288.1 eV, indicating PVA chain breakage and oxidation to form oxygen-containing groups that improved flocculation. SEM revealed that precipitates consisted of irregularly stacked nanoparticles with compact structures, while EDS mapping showed Fe and Ca atomic percentages of 24.65% and 75.35%, respectively. TEM images displayed typical amorphous flocculent structures with localized lattice fringes, confirming the formation of CaFe₅O₅ ferrite complexes. Additionally, the integrated treatment technology produces sludge containing a significant amount of iron/calcium organic complexes and ferrites. This sludge can be repurposed for the treatment of difficult-to-degrade pollutants in wastewater. Specifically, when 0.10 g of sludge was added to 100 mL of wastewater containing 10 mg/L of Reactive Black 5, a removal rate exceeding 94.3% was achieved after 30 min.

Conclusion This study developed an Fe²⁺/CaO₂ system for treating PVA-containing desizing wastewater and investigated the resource utilization of generated sludge. The system functions by producing H₂O₂ through CaO₂ hydrolysis, generating oxidative species under Fe²⁺ catalysis, while Fe³⁺ and Ca²⁺ ions act as flocculants. Optimal conditions (20 mmol/L Fe²⁺, 2 mmol/L CaO₂, pH value 3 for pre-oxidation, pH value 10 for flocculation, 60 min reaction time) achieved removal efficiencies exceeding 97.1% for PVA and 92.2% for COD in 0.5-2 g/L PVA concentrations. The resulting sludge, rich in iron/calcium-organic complexes and ferrites (CaFe₅O₇), demonstrated exceptional value for treating recalcitrant pollutants, achieving 94.29% removal of Reactive Black 5 within 30 min.

Key words: desizing wastewater, polyvinyl alcohol, Fe²⁺/CaO₂ system, integrated wastewater treatment, pre-oxidation, flocculation precipitation, sludge utilization