Cashew shells and millet stalks morphing into clean water droplets.

From Trash to Tap: How Food Waste Could Purify Your Drinking Water

Reese Marlowe in Climate & Environment August 2025 • 4 min read.

"Turning agricultural leftovers into activated carbon: A surprisingly sustainable solution for cleaner, fluoride-free water."

In a world grappling with increasing environmental concerns and health challenges, innovative solutions are often found in the most unexpected places. Researchers are exploring sustainable methods to purify drinking water, focusing on readily available resources like agricultural waste. The spotlight is on Senegal, where scientists are pioneering the use of activated carbon derived from cashew shells and millet stalks to tackle fluoride contamination in water sources.

Fluoride, while beneficial in small amounts for dental health, becomes a hazard at higher concentrations. The World Health Organization (WHO) sets a guideline of 1.5 mg/L as the safe upper limit. However, in many regions, particularly in developing countries, natural water sources far exceed this level, leading to significant health issues like dental and skeletal fluorosis. This is especially prevalent in areas of Senegal known as the groundnut basin.

Traditional methods of fluoride removal, such as membrane technologies, can be expensive and inaccessible for many communities. This has driven the search for more affordable and sustainable alternatives, leading researchers to explore the potential of activated carbon produced from agricultural waste. This approach not only addresses water purification but also repurposes waste materials, turning environmental liabilities into valuable assets.

The Science of Sustainable Filtration: Activated Carbon from Waste

Cashew shells and millet stalks morphing into clean water droplets.

The process begins with collecting agricultural residues—specifically cashew shells and millet stalks—common in Senegal. These materials undergo a combined pyrolysis and activation process using steam, avoiding the need for additional chemical compounds. The resulting activated carbonaceous materials, named CS-H2O (from cashew shells) and MS-H2O (from millet stalks), exhibit impressive carbon content, ranging from 71% to 86%.

Activated carbon's effectiveness lies in its porous structure, which provides a large surface area for adsorption. The materials created in this study boast significant surface areas, with CS-H2O reaching 942 m²/g and MS-H2O an even higher 1234 m²/g. This extensive surface area allows the activated carbon to efficiently trap fluoride ions from the water.

While both CS-H2O and MS-H2O show promise, a third activated carbon, FW/CFS-H2O, derived from food waste and coagulation-flocculation sludge, presents an interesting alternative. Key findings include:

Carbon and Calcium Content: FW/CFS-H2O has a carbon content of 32.6% and a calcium content of 39.3%.
Adsorption Capacity: It demonstrates a remarkable adsorption capacity of 28.48 mg/g with a high correlation coefficient (r² = 0.99) in synthetic water.
Efficiency: FW/CFS-H2O proves particularly effective in fluoride removal, making it a viable option for water treatment.

The adsorption process follows specific kinetic models, with the pseudo-first-order equation effectively describing the sorption kinetics. Isotherm studies using both synthetic and natural water sources help to understand how fluoride binds to the activated carbon. The Langmuir and Freundlich models are employed to analyze the experimental data, providing insights into the adsorption mechanisms and capacities of these materials.

A Promising Path to Clean Water

The research indicates that while CS-H2O and MS-H2O show good adsorbent properties, they are less efficient in fluoride removal compared to FW/CFS-H2O. The FW/CFS-H2O, with its high calcium content, demonstrates a strong capacity for fluoride adsorption, making it a potentially valuable resource for water treatment. This innovative approach offers a sustainable and cost-effective solution for communities affected by fluoride contamination, turning waste into a means of providing cleaner, safer drinking water.

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This article is based on research published under:

DOI-LINK: 10.5539/ijc.v8n1p8, Alternate LINK

Title: Sustainable Conversion Of Agriculture And Food Waste Into Activated Carbons Devoted To Fluoride Removal From Drinking Water In Senegal

Subject: Materials Chemistry

Journal: International Journal of Chemistry

Publisher: Canadian Center of Science and Education

Authors: Mohamad M. Diémé, Maxime Hervy, Saïdou N. Diop, Claire Gérente, Audrey Villot, Yves Andres, Courfia K. Diawara

Published: 2015-11-25

Everything You Need To Know

How are agricultural leftovers being used to purify drinking water in Senegal?

Scientists in Senegal are exploring the use of activated carbon derived from cashew shells and millet stalks to address fluoride contamination in water. The process involves converting these agricultural residues into activated carbonaceous materials like CS-H2O (from cashew shells) and MS-H2O (from millet stalks) through a process using steam. This provides a sustainable and cost-effective method for purifying water, repurposing waste materials, and turning environmental liabilities into valuable assets.

Why is it important to remove fluoride from drinking water, and what health problems can arise from excessive fluoride?

Fluoride becomes a hazard when its concentration exceeds the World Health Organization (WHO) guideline of 1.5 mg/L. Excessive fluoride intake can lead to significant health issues such as dental and skeletal fluorosis. The activated carbon materials, CS-H2O, MS-H2O, and FW/CFS-H2O, help to reduce fluoride levels in water sources, mitigating the risk of these health problems, particularly in regions like the groundnut basin in Senegal where natural water sources often have high fluoride concentrations.

How does activated carbon, like CS-H2O and MS-H2O, effectively remove fluoride from water?

Activated carbon materials, such as CS-H2O and MS-H2O, have a porous structure with a large surface area for adsorption. CS-H2O has a surface area of 942 m²/g, while MS-H2O boasts an even higher 1234 m²/g. This extensive surface area enables the activated carbon to efficiently trap fluoride ions from the water through a process described by kinetic models like the pseudo-first-order equation. Isotherm studies using models like the Langmuir and Freundlich models are employed to understand how fluoride binds to these materials.

What are the properties of FW/CFS-H2O, and how does it compare to CS-H2O and MS-H2O in terms of fluoride removal efficiency?

FW/CFS-H2O, derived from food waste and coagulation-flocculation sludge, has a carbon content of 32.6% and a calcium content of 39.3%. It demonstrates an adsorption capacity of 28.48 mg/g in synthetic water. While CS-H2O and MS-H2O show good adsorbent properties, FW/CFS-H2O proves more efficient in fluoride removal due to its high calcium content, making it a viable option for water treatment.

What are the broader implications of using agricultural waste for water purification on sustainable development and environmental conservation?

The utilization of agricultural waste like cashew shells and millet stalks to produce activated carbon contributes to a circular economy. By converting waste materials into valuable water purification tools, this approach reduces environmental pollution and offers an economically sustainable solution for communities facing fluoride contamination. This method could be particularly transformative in developing countries where resources for traditional water treatment methods are limited, aligning environmental protection with public health and economic development.