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• Sludge, the massive by-product of the sewage system, became a major challenge for the wastewater treatment industry. Yet, conventional methods often face challenges like low efficiency, high energy consumption, and environmental pollution. Especially, the improper treatment and disposal of toxic sludge generated from different industrial processes or specific wastewater treatment operations exerted significant pressure and threat to hydrosphere, pedosphere, atmosphere and even biosphere. Deep dewatering technology can accomplish effective volume and mass reduction of the sludge, resulting from its water content being reduced to < 60.0 wt% [1]. Previously, incineration or landfill was considered as good treatment and disposal practice for the dewatered sludge cake. However, from the points of sustainable environment, carbon emission reduction and pollution reduction, resource recovery from the sludge was preferred as efficient and economical approach for future sludge treatment and disposal. Some researchers produced Fe-rich biochar via pyrolyzing sludge cake and Fenton reagent to activate H₂O₂ or yield H₂O₂ for sludge reduction and recycling. As an innovative and rapidly evolving approach, some researchers proposed that metal-organic frameworks (MOFs), as emerging functional materials, might provide new opportunities for sludge treatment. In this Editorial, the resource utilization and energy production from sludge with the aid of MOFs could demonstrate how MOF-mediated processes could significantly enhance the valorization of wastewater byproducts while addressing pressing environmental challenges (Scheme 1).

  Scheme 1

  

  Scheme 1.  Strategic implementations of MOFs in sustainable sludge management.

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  Lin and coworkers proposed a method for extracting Fe³⁺ and Ni²⁺ using H₂SO₄ (1.0 mol/L) from the iron/nickel-containing electroplating sludge, which reacted with fumaric acid and 2,6-naphthalenedicarboxylate tetrahydrate (NTD) to produce MIL-88A(Fe) and Ni(NTD), respectively [2]. It was well known that Fe³⁺ rather than Ni²⁺ preferred to form coordination interactions with fumaric acid under hydrothermal conditions. After the nearly complete consumption of Fe³⁺ in the leachate solution containing Fe³⁺ and Ni²⁺, NTD was introduced to coordinate the residue Ni²⁺ to produce Ni(NTD) under room temperature. This gradient recovery strategy toward the metal matrix could also be applied to other sludges containing iron and copper. Lin and coworkers also synthesized MIL-88A(Fe) and HKUST-1(Cu) using iron/copper containing sludge with the presence of fumaric acid and trimesic acid, respectively. Under low-concentration CO₂ conditions (10 vol%), the as-synthesized Ni(NTD) demonstrated record-breaking CO₂-to-CO photoconversion efficiency, while MIL-88A(Fe) preferentially facilitated H₂ generation. By strategically employing these Fe/Ni MOFs as modular catalytic components and optimizing their molar ratio, syngas composition with tunable CO/H₂ ratios spanning from 1:15 to 14:1 could be accomplished through photocatalytic CO₂ reduction. This remarkable tunability stems from the distinct adsorption affinities of Fe³⁺ and Ni²⁺ centers toward CO₂ and H₂O molecules, which are fundamentally governed by the differential 3d electron configurations of Fe³⁺(d⁵) and Ni²⁺(d⁸). The correlation between metal electronic structure and reactant adsorption preferences establishes a rational design principle for modular photocatalyst systems. Also, Chen and coworkers produced MIL-100(Fe) and MIL-88A adopting the leached Fe ions from the excess sludge (ES) in pesticide wastewater treatment plant (WWTP) [3]. The as-prepared MIL-100(Fe) was used as electrocatalyst to accomplish effective sulfamethoxazole degradation. This work further echoed the cost-effective approach for produce valued-added MOFs from the chemical excess sludge.

  Different from extracting metal ions from the electroplating sludge, Matovic and coworkers proposed an approach to produce MIL-53(Cr) using dried Cr-containing electroplating sludge (EPS) with the presence of HF [4]. The EPS was sampled from a TMP PPT plant in Trstenik, Serbia, which contained Cr (ca. 63%), Fe (ca. 12%), P (ca. 9%), Zn (ca. 6%), Ni (ca. 4%) and so on. The presence of co-existing metals did not exert negative impact to the production of MIL-53(Cr). The as-prepared rod-like MIL-53(Cr) with size of 5–10 µm displayed BET surface area of 770.48 m²/g, which could capture dibenzothiophene (DBT) (40.11 mg/g) from n-octane. The desulfurization performance overpassed nearly all the counterpart MOFs. This work provided the possibility that MOFs could be yielded directly from electroplating sludge rather than leaching metal ions from the sludge.

  Besides the metal resources in electroplating sludge, the organic matters like proteins and polysaccharides in waste activated sludge from municipal wastewater treatment plant might be transformed into renewable energy like CH₄ during anaerobic digestion. However, the corresponding bioconversion efficiency was limited by poor hydrolysis of organic matters and syntrophic methanogenesis of anaerobes. Dai and co-workers introduced MOF-808 into the anaerobic digestion process to overcome the above-mentioned limitations [5]. With the presence of MOF-808 (150 mg MOF-808 per gram of volatile solid), there was an increase of approximately 26.7% in the production of methane and 15.6% in the proportion of methane in biogas, respectively. MOF-808 significantly enhanced both the degree and rate of sludge organic matter hydrolysis during anaerobic digestion through dual mechanisms: promoting biological enzymatic hydrolysis and exhibiting intrinsic catalytic hydrolysis activity. Specifically, the introduction of MOF-808 led to remarkable increases in key enzymatic activities, with protease and α-glucosidase activities enhanced by up to 68.9% and 91.2%, respectively, during the anaerobic digestion process. Furthermore, MOF-808′s inherent catalytic properties contributed to a substantial 60% increase in the maximum proportion and an impressive 583.7% acceleration in the initial rate of low molecular weight organic matter generation through its catalytic hydrolysis activity. MOF-808 could boost the bioconversion efficiency of hydrolysis-acidification products, particularly volatile fatty acids, to methane during anaerobic digestion. MOF-808 demonstrated remarkable catalytic activity, as evidenced by an 867.4% surge in acetate consumption rate between day 4 and day 8 of the anaerobic digestion process. Mechanistic studies revealed that MOF-808 facilitated proton transfer dynamics and modulated methanogenic pathways, with a particularly pronounced effect on CO₂ reduction. This enhancement was attributed to the selective enrichment of mixotrophic methanogens, as indicated by a 35.5% increase in the relative abundance of Methanosarcina species, which are known for their versatile metabolic capabilities in methane production.

  Besides adsorption and catalysis toward pollutants removal in different sludges, MOFs offer immense potential for revolutionizing sludge treatment in the fields of resource and energy recovery. In future, more efforts should be made to transform sludge treatment into a more efficient, sustainable, and resource-recovering process, with the purpose of environmental protection and circular economy goals. The metal resources in the sludge could be selected as metal centers of MOFs. However, the present work suffered from large input of extra resources (like strong acid), extra energy (like high temperature), small scale (reaction volume < 100 mL) and batch operation. More focus should be put to develop continuous MOF production technology using sludges containing metals as raw materials, in which continuous microwave technology and equipment should be considered. It was confirmed that MOF-808 significantly improved both the degree and rate of hydrolysis-acidification of sludge substrates while optimizing the functional microbial community structures involved in hydrolysis, acidification, and methanation processes. However, MOF-808 was expensive due to high synthesis costs and small scalability. Some counterpart MOFs or even some cheaper and environmentally friendly MOFs should be considered. Also, some attention should be paid to the effect, transformation and fate of selected MOFs in the environment.

  In conclusion, sludge represents a valuable reservoir of resources and energy that can be harnessed through innovative approaches like MOF-based technologies. By converting sludge into high-value MOFs and renewable energy, we can align sludge management with the principles of a circular economy, sustainable development, and global carbon neutrality goals. Continued research and technological advancements will be essential to fully realize the potential of MOFs in transforming sludge treatment into a more efficient, sustainable, and resource-recovering process, in which the life cycle assessment should be fully considered.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Chong-Chen Wang: Writing – review & editing, Writing – original draft, Resources, Investigation, Funding acquisition, Conceptualization. Xiaohang Xu: Writing – original draft.

  Acknowledgments

  This work was supported by National Natural Science Foundation of China (Nos. 52370025, 22176012), BUCEA Post Graduate Innovation Project (No. PG2024086).

  
  
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  Scheme 1  Strategic implementations of MOFs in sustainable sludge management.

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