Waste-derived Fe-Co zeolitic imidazolate framework as an additive for enhanced methane production during anaerobic digestion of food waste

The growing demand for renewable energy and sustainable waste management has increased interest in anaerobic digestion (AD) of food waste; however, limitations in substrate conversion and methane-production kinetics can restrict energy recovery. This study investigated waste-derived iron–cobalt zeolitic imidazolate framework (Fe-Co ZIF) as an additive for enhancing methane production during anaerobic co-digestion of food waste. Fe-Co ZIF was synthesized using iron recovered from waste iron filings and characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy and Brunauer–Emmett–Teller (BET) analysis. Batch digestion was conducted under mesophilic conditions using Fe-Co ZIF dosages of 20, 40, and 60 mg g -1 volatile solids (C20, C40, and C60), with an untreated control (C). Methane-production kinetics were evaluated using the Double-Gompertz model, while treatment effects were assessed using one-way ANOVA, effect size ( η 2 ), coefficient of variation and Tukey HSD multiple comparisons. Fe-Co ZIF produced a significant, non-linear dosage response. Cumulative methane production increased from 324.93 ± 13.46 mL g -1 VS in the control to 376.10 ± 10.59 and 384.80 ± 31.87 mL g -1 VS at C20 and C40, representing increases of 15.7% and 18.4%, respectively, whereas C60 produced only 329.00 ± 14.70 mL g -1 VS. The overall dosage effect on cumulative methane production was significant (F (3,8) = 7.624, p = 0.0099, η 2 = 0.741), with C20 and C40 significantly exceeding the control but remaining statistically comparable to each other. C20 exhibited the highest first-phase methane potential ( P 1 = 117.42 ± 4.85 mL g -1 VS) and maximum first- and second-phase production rates ( R 1 = 70.04 ± 6.15 and R 2 = 24.25 ± 2.14 mL g -1 VS Day -1 ), indicating superior kinetic acceleration and response consistency. Conversely, C40 produced the highest second-phase methane potential ( P 2 = 266.07 ± 6.08 mL g -1 VS) and total methane potential ( P total = 380.31 ± 15.31 mL g -1 VS), demonstrating greater ultimate methane-production capacity. Increasing the dosage to C60 provided no statistically supported methane-production advantage over the control while significantly prolonging the kinetic lag phases. Overall, the results identify 20–40 mg g -1 VS as the favorable Fe-Co ZIF dosage region, with C20 preferable for rapid and consistent methane generation and C40 advantageous for maximizing methane-production capacity. These findings demonstrate the potential of waste-derived Fe-Co ZIF for integrating iron-waste valorization with enhanced renewable methane recovery from food waste.