New sustainable utilization approach of livestock manure: Conversion to dual-reaction-center Fenton-like catalyst for water purification

Rural pollution, significantly impacted by the accumulation of livestock manure and crop straw, poses a major global challenge. Existing disposal methods, such as biogasification, composting, and fermentation, are often energy-intensive, release harmful substances, and generate greenhouse gas emissions, hindering the achievement of UN Sustainable Development Goals (SDGs). The untreated fecal waste contributes significantly to sanitation-related deaths worldwide. Livestock manure, however, contains valuable organic and metallic components. Chicken manure (CM), for example, possesses elements like Ca, Cu, Fe, Si, C, and O. While metal-organic frameworks (MOFs) and metal-organic complex polymers (MOCPs), often used in various fields, typically require expensive metal species and energy-intensive synthesis, this research explores using waste materials, specifically livestock manure, as a sustainable source for catalyst creation. Previous work demonstrated the benefit of metal cation-π structures in creating dual-reaction-centers (DRCs) for efficient Fenton-like reactions, but these methods still involved resource-intensive synthesis. This study aims to overcome this limitation by directly converting unprocessed CM into a highly efficient Fenton-like catalyst, simultaneously addressing waste disposal and reducing energy consumption in water treatment.

The introduction extensively reviews existing literature on rural pollution, the environmental impacts of livestock manure disposal, and the synthesis and applications of MOFs and MOCPs. It highlights the limitations of current waste management strategies and the potential of utilizing waste biomass as raw materials for catalyst synthesis. The authors cite numerous studies to support their claims, emphasizing the urgency to develop sustainable and eco-friendly solutions. The review also establishes a foundation for the proposed method by referencing previous research on the development of dual-reaction-center (DRC) catalysts and their superior performance in Fenton-like reactions.

The study employs an innovative two-stage in situ calcination-annealing process to convert chicken manure (CM) into a copper-doped CM nanoparticle (CCM-Nps) catalyst. Initially, CM undergoes purification to create a precursor. This precursor is then calcined to remove volatile substances. Subsequently, a trace amount of copper nitrate solution is added, followed by another calcination step to recrystallize the CM with Cu species. This process results in the formation of a DRC catalyst through ordered bonding of intrinsic metal-organic species. Various characterization techniques are used to analyze the structural properties of the resulting CCM-Nps, including field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and Fourier-transform infrared spectroscopy (FTIR). The catalytic performance is evaluated through several experiments measuring the removal efficiency of various emerging pollutants (bisphenol A (BPA), 2-chlorophenol (2-CP), diphenhydramine (DP), and ciprofloxacin (CIP)) in the CCM-Nps/H2O2 system under varying conditions of catalyst concentration, H2O2 concentration, and pH. The stability of the catalyst is assessed in a continuous reactor over a prolonged period. The study further investigates the influence of various anions on catalytic activity and uses electron paramagnetic resonance (EPR) spectroscopy to identify the reactive oxygen species (ROS) involved. Finally, liquid chromatography-mass spectrometry (LC-MS) is used to analyze the decomposition products of ciprofloxacin (CIP) to gain insights into the degradation pathways.

The results demonstrate that the in situ two-stage calcination-annealing process successfully converts chicken manure into CCM-Nps, a highly efficient Fenton-like catalyst with a unique nanosheet structure decorated with Cu and O species. Characterization reveals the formation of graphene-like structures and Cu-O-C bond bridges, confirming the ordered bonding of intrinsic metal-organic species. The catalyst exhibits exceptionally high removal efficiency for various emerging pollutants, achieving complete degradation within 1 hour, with some pollutants (e.g., DP) showing >90% degradation in just 1 minute. The CCM-Nps/H2O2 system displays remarkable performance even at low H2O2 concentrations (2 mM) and over a broad pH range (3.86-9.63). Interestingly, the presence of various anions (Cl-, SO42-, NO3-, and PO43-) does not inhibit catalytic activity and may even enhance it. Long-term stability tests indicate that the catalyst maintains approximately 60% BPA degradation efficiency after 900 hours of continuous operation. EPR studies confirm the generation of hydroxyl radicals (•OH), superoxide radicals (•O2-), and singlet oxygen (¹O2), with •OH and ¹O2 identified as the main active species responsible for pollutant degradation. The mechanism involves electron transfer from pollutants to the catalyst's electron-rich centers, activating dissolved oxygen and H2O2, leading to pollutant degradation via both surface cleavage and ROS attack, thus minimizing H2O2 consumption. LC-MS analysis of CIP decomposition products confirms both hydroxylation and surface cleavage pathways. The comparison with existing Fenton catalysts highlights the superior efficiency and resource-saving aspects of the CCM-Nps catalyst, requiring significantly lower catalyst dosage and H2O2 concentration.

The findings demonstrate the feasibility of converting a significant waste product, livestock manure, into a valuable resource for water purification. The study's success in creating a highly efficient, stable, and pH-tolerant Fenton-like catalyst from waste materials significantly advances the field of sustainable resource utilization and environmental remediation. The low H2O2 consumption and the catalyst's ability to utilize energy from pollutants are particularly noteworthy advancements. The comprehensive characterization and mechanistic studies offer valuable insights into the catalyst's structure-activity relationship, paving the way for further optimization and the development of similar catalysts from other waste biomass sources. The observed synergistic effects of different anions on catalytic activity warrant further investigation, potentially leading to tailored catalyst designs for specific wastewater applications.

This research successfully demonstrates a sustainable strategy for converting livestock manure into a highly effective Fenton-like catalyst for wastewater purification. The catalyst exhibits excellent performance, long-term stability, and adaptability to various conditions. This approach offers a significant contribution to waste management and environmental sustainability, promoting a circular economy model. Future research could explore expanding the application to other waste biomass sources, optimizing the synthesis process for larger-scale production, and investigating the long-term environmental impact of the proposed technology.

While the study demonstrates impressive results, some limitations should be noted. The study primarily focused on chicken manure; further research is needed to determine the applicability of this method to other types of livestock manure. The long-term environmental impact of the CCM-Nps catalyst, particularly concerning potential leaching of copper ions, requires further investigation. The current study focuses on specific pollutants; testing the catalyst's efficacy against a broader range of pollutants is necessary for complete evaluation. Although the catalyst exhibited excellent stability in the continuous reactor, the long-term performance under diverse real-world conditions needs further assessment.