Valorization of Sodium Lignosulfonate for Enhanced Humification and Copper Immobilization During Municipal Sludge Composting

Valorization of industrial by-products like sodium lignosulfonate (SL) offers a sustainable strategy for environmental remediation. This study provides a systematic evaluation of SL's dual-purpose application to enhance humification and immobilize copper (Cu) during municipal sludge composting. The effects of SL addition at 0% (CK), 3% (T3), and 9% (T9) dry weight over a 60-day composting period were investigated. The 9% SL treatment (T9) significantly accelerated compost maturity, achieving a germination index (GI) of 96.4% compared to 85.9% in the control. EEM-PARAFAC analysis revealed that SL addition promoted the transformation of protein-like and quinone-like intermediates into stable humic acid (HA)-like substances. Consequently, the final HA content and degree of polymerization (DP) in T9 were substantially higher than in the control. Critically, this enhanced humification directly facilitated Cu immobilization. The DTPA-extractable (bioavailable) Cu in T9 was reduced to 96.40 mg/kg, a 66% reduction from its initial value and significantly lower than the 150.73 mg/kg in the final control compost. Pearson analysis confirmed a strong negative correlation between HA content and bioavailable Cu. The findings demonstrate that SL is a highly effective amendment for producing safer, higher-quality compost, offering a novel strategy for the synergistic valorization of industrial and municipal waste streams.