Reutilization of nickel-loaded humic acid coated magnetite spent adsorbent for enhanced biomethane production from tofu wastewater

This study demonstrates a circular strategy by converting spent nickel-loaded humic acid-coated magnetite (Ni-HA@Fe 3 O 4 ) adsorbent into a value-added supplement for anaerobic digestion (AD) of Tofu wastewater (TW). The spent Ni-HA@Fe 3 O 4 composites were generated through nickel removal from synthetic nickel wastewater using synthesized HA@Fe 3 O 4 , yielding materials (R-0 to R-8) with nickel content declining from 68.48 to 49.68 mg·g⁻¹ over eight regeneration cycles. In batch AD of tofu wastewater (TW), the spent adsorbent (R-0) substantially enhanced biomethane production. Kinetic analysis using the modified Gompertz model showed a 2.54-fold increase in biomethane production potential ( G m ) relative to the control, and outperformed nickel-only and HA@Fe 3 O 4 -only groups by factors of 1.41 and 1.49, respectively. Correspondingly, the maximum daily biomethane production rate R m was elevated by 2.73-, 1.41-, and 1.44-fold, while the lag phase ( λ ) was reduced by 2.73-, 1.59-, and 1.84-fold compared to the same groups. Although repeated regeneration gradually diminished efficacy, all experimental groups (R-0 to R-8) still improved the biomethane production potential compared to the control group. These results confirm that spent Ni-HA@Fe 3 O 4 can be effectively utilized as an additive for AD, offering a sustainable route that integrates wastewater treatment, solid waste management, and enhanced bioenergy recovery.