Amazonian Sacha Inchi shells become cheap carbon filters for mercury removal

Deep in the Colombian Amazon, a humble crop is being grown as an alternative to illicit trafficking, and its discarded shells are now at the center of a remarkable scientific breakthrough. Sacha Inchi, a plant prized for its oil-rich edible seeds used in food, cosmetics, and healthcare industries, produces brown external shells that have long been dumped in landfills with no productive use. Now, researchers have transformed these agricultural waste products into highly efficient carbonaceous materials capable of removing mercury from contaminated water, offering a sustainable solution to one of the world’s most persistent pollutants.

Mercury contamination is a global crisis with particularly devastating consequences in the Amazon region. Human activities such as mineral extraction and coal combustion release mercury into soil, air, and water, where it disperses freely. In water bodies, the problem becomes especially acute. Concentrations of mercury have been detected in domestic wastewater at levels reaching 2.8 × 10³ nanograms per liter, and due to absent or inefficient treatment plants, this mercury flows into rivers, where reported concentrations range between 2.41 and 2.02 nanograms per liter. Once in the aquatic environment, bacteria transform mercury under reducing conditions into methylated forms, which are exceptionally hazardous because they accumulate and magnify in food chains, posing serious threats to both human health and wildlife.

The urgency of eliminating mercury from water bodies has driven scientists to explore multiple removal techniques, including coagulation, ultrafiltration, precipitation, and adsorption. Among these, adsorption stands out as particularly promising due to its ease of operation, high adsorption rate, and selective removal capability. Carbonaceous materials—porous substances enriched in carbon—can be prepared from agro-industrial waste, providing added value to organic residues while contributing to the circular economy. These materials are generally obtained by pyrolysis of biomass under oxygen-free conditions and feature high surface areas with a variety of functional groups such as carboxylic, hydroxyl, amino, ether, and thiol groups, along with adequate pore distribution across micro-, meso-, and macropores.

While advanced porous materials like metal–organic frameworks and hydrogen-bonded organic frameworks have attracted considerable attention for mercury removal due to their tunable pore structures and abundant functional adsorption sites, they often involve complex synthesis routes, expensive precursors, and limited stability under real aqueous conditions. Carbonaceous materials derived from agro-industrial biomass represent an attractive alternative, offering porous architectures and oxygenated surface functionalities comparable to those of advanced framework materials while providing lower cost, simpler preparation, higher chemical stability, and sustainable valorization of agricultural residues. Previous studies have demonstrated the potential of such materials, with pistachio green shells achieving efficiencies of up to 98 percent in mercury removal, and corn cob materials modified with potassium hydroxide reaching removals of up to 95 percent.

Colombia, with an annual production of approximately 72 million tons of agro-industrial waste, presents abundant raw material opportunities. The research team collected external Sacha Inchi shells from an agroindustry located in Caquetá, Colombia, and subjected them to rigorous preparation protocols. The raw material was ground, sieved, and washed to remove impurities, then dried at 105°C. Subsequently, the shells were pyrolyzed at different temperatures—300, 400, and 500°C—using a horizontal furnace with three zones of controlled heating purged with nitrogen gas. The temperature was increased at 5°C per minute and maintained for two hours to complete the process, yielding materials designated SE-300, SE-400, and SE-500 according to their preparation temperatures.

Beyond simple thermal treatment, the researchers explored chemical modifications using potassium hydroxide and phosphoric acid, as well as a hydrothermal method requiring fewer chemical agents and lower temperatures. The potassium hydroxide-modified material, SE-KOH-500, achieved a remarkable specific surface area of 668.95 square meters per gram, as this base reacts with the precursor carbon to generate gases such as carbon dioxide and carbon monoxide that create pores within the material. The phosphoric acid-modified material, SE-H₃PO₄-500, stood out for its higher fixed carbon content of approximately 85 percent, as the acid dehydrated the material and reduced oxygen content to form aromatic structures. The hydrothermal material, SE-Hydro, was prepared in a stainless steel reactor at just 180°C using only water.

When tested for mercury removal, all materials demonstrated adsorption capability, but their efficiency varied dramatically. The raw material achieved approximately 43 percent removal, while the phosphoric acid-modified material reached about 86 percent, the unmodified SE-500 achieved roughly 96 percent, and the potassium hydroxide-modified SE-KOH-500 topped the list at approximately 98 percent. Surprisingly, when evaluating materials prepared only through thermal treatment at different temperatures, SE-300 proved to be the most effective adsorbent among the thermally treated materials, maintaining mercury adsorption efficiencies above 95 percent. This unexpected result was attributed to the fact that at 300°C, the collapse of the material’s porosity is avoided, allowing greater development of surface area and maintaining an adequate pore diameter to adsorb the mercury ion, which has an ionic radius of 0.11 nanometers.

The characterization of SE-300 revealed fascinating properties. Despite being prepared at a lower temperature than its counterparts, SE-300 exhibited a specific surface area of 37.69 square meters per gram with a pore diameter of 2.67 nanometers and a total pore volume of 0.031 cubic centimeters per gram. Scanning electron microscopy showed strand-like carbonaceous surfaces without fragmentation. Fourier transform infrared analysis confirmed significant C=C bands at 1600 wavenumbers, which favor contaminant removal, along with oxygenated functional groups including alcohols at 3330 wavenumbers and ethers corroborated through a broad band between 1050 and 1250 wavenumbers corresponding to the C-O bond. The material presented a point of zero charge of 8.1, preserving its basic character while increasing oxygen content.

The adsorption process followed monolayer behavior according to the Langmuir isotherm model, with a maximum adsorption capacity of 120.6 milligrams of mercury per gram of SE-300—remarkably higher than values reported for other carbonaceous materials such as rice husk-activated carbon at 55.87 milligrams per gram and mango seed-activated carbon at 56.1 milligrams per gram. Kinetic analysis revealed that the process occurs in two stages, fitting the pseudo-second-order model. Thermodynamic parameters indicated the process is spontaneous, with negative Gibbs free energy values, and endothermic, with positive enthalpy changes. The mechanism involves π-electron interactions between aromatic systems and metal cations, attraction by free electron pairs of oxygen from ethers, and formation of Hg-O₂ complexes through deprotonated alcohol groups.

Perhaps most compelling are the practical implications of this research. The production cost for SE-300 was calculated at approximately 18.98 USD per kilogram, with the total cost of mercury removal estimated at just 0.157 USD per gram—significantly lower than other adsorbents such as NH₂-functionalized magnetic graphene oxide at 0.780 USD per gram. The material demonstrated impressive reusability, maintaining mercury removal above 96 percent across five adsorption-desorption cycles with ultrasonic washing regeneration. When applied to real-world complex aqueous matrices, SE-300 achieved 98 percent removal in distilled water, 75 percent in river water from the Caquetá River, and 63 percent in simulated domestic wastewater. These results, while showing decreased performance in complex waters due to competition from cationic ions and suspended solids, reinforce the material’s potential for real-world applications, particularly for populations located in the Amazon region where mercury contamination from mining activities poses ongoing threats to environmental and public health.

Subject of Research: Environmental chemistry and sustainable mercury remediation using biomass-derived carbonaceous adsorbents

Article Title: Valorization of Amazonian Sacha Inchi shells into carbonaceous materials for efficient mercury removal from multiple water matrices

Article References: de los Rios-Montalvo, D.-R., Paredes-Laverde, M., Gallego, J. L., Villegas-Guzman, P., Manrique- Losada, L., & Acelas, N. (2026). Valorization of Amazonian Sacha Inchi shells into carbonaceous materials for efficient mercury removal from multiple water matrices. Heliyon, 12(16), Article e45591. https://doi.org/10.1016/j.heliyon.2026.e45591

Image Credits: AI Generated

DOI: 10.1016/j.heliyon.2026.e45591

Keywords: mercury removal, Sacha Inchi, carbonaceous materials, adsorption, circular economy, Amazon, water treatment, biomass valorization, Valorization, Amazonian, Sacha, Inchi