Objective Deliming is a key operation connecting liming and bating in leather manufacture. However, ammonium salts, which are widely used as industrial deliming agents, markedly increase the concentration of ammonia nitrogen in wastewater and release ammonia gas, thereby causing environmental burdens. Composite amino acids prepared from keratinous wastes are regarded as a potential alternative to ammonium salts. However, after acid hydrolysis of keratin, large amounts of precipitates are formed during neutralization and usually removed as waste, which results in insufficient resource utilization. In this study, waste wool was used as the raw material, and its whole hydrolysate was directly utilized as a deliming agent (WKH). This strategy aims to reduce ammonia nitrogen pollution at the source while achieving resource utilization of waste biomass.

Methods The effects of sulfuric acid concentration and hydrolysis temperature on the degree of wool hydrolysis were investigated via single-factor experiments, with hydrolysis rate serving as the evaluation index. The hydrolysate was neutralized with sodium hydroxide to pH 1 to 14, and the neutralization endpoint was determined according to the precipitation characteristics. WKH powder, containing both the supernatant and precipitates formed in neutralization, was prepared under the optimal hydrolysis and neutralization conditions. Its total ash, crude protein, fat content, and amino acid composition were analyzed. Limed cattle hide was employed to compare the deliming performances of 3% ammonium sulfate and different dosages of WKH (4%-8%). During deliming, the pH variation, penetration time at different hide regions, and calcium removal rate were characterized. The delimed hides were processed into crust leather through subsequent procedures, and their softness and mechanical strength were determined. The concentrations of ammonia nitrogen, total organic carbon, and total nitrogen in the deliming float, as well as the release of NH₃ and H₂S, were measured to assess environmental impact.

Results The hydrolysis rate of wool increased significantly with elevated sulfuric acid concentration and hydrolysis temperature. Balancing hydrolysis efficiency and neutralization costs, the optimal hydrolysis conditions were identified as follows: sulfuric acid concentration of 3 mol/L, temperature of 110 ℃, hydrolysis duration of 6 h, and solid-to-liquid ratio of 1∶4. The hydrolysis rate reached approximately 85% under such conditions. During neutralization, the hydrolysate remained homogeneous at pH ≤ 1; precipitation occurred when the pH increased to 2; the amount of precipitate reached its maximum at pH 3-5; and the precipitate gradually redissolved when the pH was further adjusted to the alkaline range. This phenomenon indicated that the precipitate originated from reversible aggregation of amino acids and small peptides near their isoelectric points, which could dissolve in the alkaline environment of pH 12-13 at the initial stage of deliming. Therefore, removal of the precipitate was unnecessary. Accordingly, pH 3 was chosen as the neutralization endpoint. The WKH prepared under the optimal hydrolysis and neutralization conditions contained 59.6% total ash, 32.5% crude protein, and 7.9% fat. Seventeen amino acids were detected, with a total content of 30.8%. In terms of deliming efficiency, the float pH of the 7% WKH group reached 8.6 after deliming for 10 min, which was comparable to that of the 3% ammonium sulfate group. The pH remained stable at 8-9 throughout the deliming process, demonstrating that WKH exhibited a good buffering capacity. In the penetration test, WKH completely penetrated the back and belly regions within 30 min, while complete deliming of the butt and neck regions required 30-50 min. This observation indicated that WKH penetrated slightly more slowly than ammonium sulfate (which achieved complete deliming in all regions within 30 min) but was superior to most reported organic acid deliming agents. In terms of calcium elimination, the calcium removal rates of the 5%-8% WKH groups were all higher than those of the 3% ammonium sulfate group. This was attributed to the ability of carboxyl groups in amino acids to form soluble calcium salts with calcium ions that were fixed on the limed hide. The crust leather prepared by WKH deliming showed softness and mechanical strength comparable to those of the ammonium sulfate group, demonstrating that WKH had no adverse effect on the physical properties of the crust leather. Compared with ammonium sulfate deliming, 7% WKH deliming reduced the concentration of ammonia nitrogen in the wastewater from 4267.2 mg/L to 199.3 mg/L, corresponding to a removal efficiency of 95.3%. The carbon-to-nitrogen ratio increased from 0.2 to 2.0, which was beneficial for improving the biodegradability of wastewater. With respect to the release of odorous gases, the NH₃ release from WKH deliming was 4.5 mg/m³, representing an 80.3% reduction compared with ammonium sulfate deliming, which released 22.9 mg/m³. The H₂S release was only 37.9 μg/m³, much lower than that of the formic acid-treated group.

Conclusion In summary, a deliming agent was prepared via the full-component utilization of waste wool, which effectively avoids resource loss associated with the disposal of neutralization precipitates generated during conventional keratin hydrolysis. The deliming performance of WKH was comparable to that of ammonium sulfate, while it achieved a remarkable reduction in the sources of ammonia nitrogen and odorous gases. This work provides a feasible technical route for cleaner leather manufacturing and the resource utilization of waste.