“To break free from the constraints of costly commercial materials, I chased circular‑material innovation, uplifting discarded banana‑peel waste into accessible, MOF‑inspired porous filter media to deliver a green cure for mixed heavy‑metal and dye water pollution.”

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During a study trip to Fenghuang Ancient Town in Hunan, I saw wastewater from local traditional dyeing activities flowing into nearby rivers. The colored wastewater made me start thinking about how difficult some dyes can be to remove once they enter the water system.

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Back in Shanghai, I became interested in another side of the same problem. Household wastewater can contain synthetic dyes released from fabrics, along with small amounts of metal ions such as Cu²⁺ and Pb²⁺. This made me wonder whether my original filter could do more than simply reduce clogging.

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Could I upgrade the design so that it could remove both organic dyes and heavy-metal ions at the same time?

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✨ Research Spark: Learning from MOF Materials

 

In July 2025, I completed a two-week laboratory assistant internship at Shanghai Normal University. During the internship, I synthesized Fe-, Co-, Ni-, and Cu-based MOFs and learned about their possible applications in environmental remediation.

What interested me most about MOFs was their highly porous structure. Their large surface area, adjustable pore sizes, and different surface functional groups allow them to interact with different pollutants. I learned that they can be used to remove heavy-metal ions and organic dyes through adsorption, and in some cases photocatalytic degradation.

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I then tried putting lab-synthesized MOF powder into my bionic-alveolar filter prototype. The material worked well as an idea, but I quickly ran into a practical problem: MOFs are relatively complicated to synthesize, and the raw materials can be expensive. That made them difficult to imagine in a low-cost water-treatment device for everyday use.

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So I started looking for a cheaper material, especially one that could come from household waste.

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Through further research and comparison of multiple materials including orange peel, apple peel, pomelo peel, coffee grounds, and tea residues, I became interested in banana peels. After dehydration and simple citric-acid esterification, banana-peel-based materials can develop more porous structures and improved adsorption properties. This gave me a new direction for the project: using waste materials to help treat wastewater.

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This became the starting point of my “treating waste with waste” water-purification project.

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❓Research Questions

 

My investigation focused on five core research questions:

 

• How does citric acid modification affect the functional group structure and pore characteristics of banana peel?

• What are the optimal citric acid modification concentration and TiO₂ loading amount to achieve the highest removal efficiency for methylene blue?

• How does the composite material achieve the synergistic effect of adsorption enrichment and photocatalytic degradation, thereby improving overall purification efficiency?

• How can a continuous-flow reaction device be designed to achieve dynamic and continuous operation of the material?

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🧪Experimental Materials & Procedures

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​Table 1: Experimental Materials & Equipment

 

 

 

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Figure 1: Experimental Workflow

 

Step 1 and 2 Peel pretreatment and modification：

Fresh banana peels were cut, oven‑dried and pulverized, then modified separately with citric acid and sodium hydroxide. Citric acid grafts abundant “adsorption hooks” onto fiber surfaces; NaOH etches open internal pores to create extra adsorption sites.

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​Figure 2: Schematic illustration of the banana peel modification procedure

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Step 3 Performance test with pure CA powder：

Methylene‑blue solution simulated dye wastewater. I used spectrophotometry to measure adsorption‑removal efficiency of pure CA powder and identify the optimal citric‑acid modification concentration.

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Figure 3: Methylene blue adsorption experiment

 

Step 4 TiO2 Loading:

TiO₂ was loaded onto modified banana peels via the impregnation method to construct CA‑TiO₂ heterojunctions and extend the material’s light response range.

 

Step 5 Degradation Potential Test:

Methylene blue simulated wastewater was used to assess the adsorption‑photocatalysis synergy of pure CA and CA‑TiO₂ samples under UV irradiation, with pure CA set as the control group. Removal efficiency was measured spectrophotometrically.​​​

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Figure 4: Photocatalytic adsorption-degradation experiment for methylene blue

 

Step 6 Material Characterization:

SEM was adopted to observe microscopic pores, FTIR to analyze functional groups, BET to characterize pore structures, and UV‑Vis spectroscopy to investigate optical absorption features, confirming the success of material modification and composite preparation.

 

Step 7 Real-Situation test with Archimedean Screw Reactor:

I built a horizontal continuous‑flow Archimedean‑screw prototype integrating UV lighting. Cotton filler physically retained composite powder inside the chamber and extended pollutant‑material contact time.

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Figure 5: Archimedean screw reactor prototype building process

 

📈Key Experimental Findings

1. **Citric‑acid‑modified banana‑peel powder**: Sample CA1.5 (1.5 mol/L citric‑acid treatment) delivered best performance with **97.74 % methylene‑blue removal**. Esterification generates high‑density carboxyl groups and hierarchical pore networks for effective dye capture.

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2. **CA‑TiO₂ composite**: The 5:1.5 CA‑to‑TiO₂ mass‑ratio sample achieved **99.56 % methylene‑blue removal**, nearly complete dye mineralization. Chemical Ti‑O‑C bonds (not simple physical mixing) form between TiO₂ nanoparticles and banana‑peel carbon matrix, significantly improving photocatalysis and breaking dyes down into harmless CO₂ and H₂O.

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3. Control experiments confirm far higher pollutant‑removal performance under UV illumination versus dark conditions, proving effective adsorption‑photocatalysis synergy. The composite also shows promising heavy‑metal‑ion‑adsorption capacity.

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4. Over 40‑hour continuous operation of the Archimedean‑screw reactor reached **46.68 % methylene‑blue removal**, validating continuous‑flow feasibility. Performance gaps between lab batch tests and the physical prototype stem from two factors: lower filter‑material loading constrained by reactor geometry, plus reduced‑power UV lamps adopted for human‑safety concerns, which lowers photocatalytic activity. These observations point out clear directions for future hardware optimization.​

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Figure 6: Photocatalytic adsorption-degradation performance of methylene blue

 

 

Final conclusion: This work successfully demonstrates a "waste-to-resource" strategy, proving that a citric acid-modified banana peel-TiO₂ composite offers strong synergistic effects for dye removal. The integrated spiral reactor provides a viable and scalable framework for continuous wastewater treatment, with significant potential for improvement through enhanced reactor design to fully exploit the material's capabilities.

 

 

🎯 Project Takeaways

This research grew out of real‑world pollution I had witnessed. I aimed to build practical hardware instead of only conducting textbook‑style lab work. Banana peels are abundant and require only simple modification. This technology could be developed for low‑cost household filter cartridges or small‑scale sewage‑treatment solutions for underserved regions. To me, “treating waste with waste” is more than an environmental slogan — it is a practical solution I want to realize.

 

The performance gap between small‑batch lab tests and full‑prototype results taught me a vital lesson: outstanding laboratory metrics do not guarantee real‑world engineering outcomes. Environmental engineering requires constant trade‑offs among material performance, operational safety and manufacturing cost.

 

My research journey stretches from my original 10th‑grade bionic filter design to developing banana‑peel‑derived filter media in 11th grade. Starting from high‑performance but costly MOF materials, I pivoted toward low‑cost waste‑based alternatives. Repeated modification failures and iterative hardware tuning were driven entirely by my curiosity and real‑world environmental problems. This experience strengthens my ambition to pursue interdisciplinary studies bridging materials chemistry and environmental engineering. Moving forward, I hope to keep exploring waste‑resource‑conversion technologies and create more practical engineering solutions addressing genuine ecological challenges.

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