Self-enhancement of phosphorus recovery from anaerobic fermented sludge with MOF-808 synthesized with an improved green method

Abstract

Although sludge anaerobic fermented is rich in phosphate, its complicated composition might be a barrier for adsorption approach due to the competing effects of ions and organics. In this study, an interesting phenomenon was found with MOF-808-(25) which exhibited a higher adsorption capacity in anaerobic fermented sludge than in phosphate-water solution. A green synthesis of MOF-808-(25) was first developed by optimizing the microwave synthesis method to reduce the cost of the adsorbent. Under the optimized conditions, i.e., Zr4+/BTC3− ratio of 4:1, 25 min, and microwave power of 350 W, the space–time yield of MOF-808-(25) reached 236.03 kg·m−3d−1, approximately 100 times higher than those achieved through conventional hydrothermal methods. In phosphate-water solution, the maximum adsorption capacity of MOF-808-(25) for phosphate was 66.61 mg P/g (pH = 7, 25 °C). Molecular dynamics simulation revealed that the adsorption of phosphate by MOF-808 was primarily dominated by coordination effects, while, electrostatic interactions, hydrogen bonding, and steric hindrance were also involved. MOF-808-(25) exhibited good adsorptive selectivity over coexisting anions and organic matters in anaerobic fermentation broth. Experimental results also suggest that a higher temperature or a lower pH value favored the adsorption capacity of MOF-808-(25). With this feature, the phosphate adsorption capacity of MOF-808-(25) was surprisingly ∼ 5 mg P/g higher than that obtained in phosphate-water solution due to the acidogenesis of anaerobic fermentation. Additionally, MOF-808-(25) improved the hydrolysis of proteins through non-biological catalysis. These findings depict a promising vision for phosphorus recovery by MOF-808-(25).