The researchers, led by Bernadetta Kaźmierczak, began with dried used coffee grounds sieved to a particle size of 600 to 400 micrometers. They pyrolyzed 120-gram batches in a chamber furnace at 600 degrees Celsius under a carbon dioxide protective gas flowing at 5 liters per minute, following a carefully staged temperature program with holding periods at 200, 550 and 600 degrees Celsius. This produced an unactivated reference biochar, designated BFK_600, with a yield of about 20 percent of the starting mass. For the activated variants, the team cut off the carbon dioxide supply once 600 degrees Celsius was reached and introduced water vapor instead, at flow rates of either 2 or 4 milliliters per minute, for activation times of 15, 30 or 45 minutes. Six activated biochars resulted, each named for its exact activation recipe.

Chemical characterization began with Fourier-transform infrared spectroscopy, which tracks the vibrational fingerprints of functional groups on the biochar surface. The spectra revealed a dramatic transformation during pyrolysis itself. The raw coffee grounds displayed broad hydroxyl bands between 3530 and 3300 inverse centimeters, aliphatic C-H bands at 2923 and 2856 inverse centimeters, and a suite of carbonyl, amide and ether signals. After pyrolysis at 600 degrees Celsius, most of these bands vanished, reflecting the dehydration, decarboxylation and decarbonylation reactions that strip oxygen from the carbon matrix at high temperature. In other words, the very process that creates the carbon skeleton also destroys many of the oxygen-containing groups that make a sorbent chemically active.

The striking finding was that water vapor activation partially reversed this loss. Activated biochars developed new hydroxyl bands in the 3800 to 3000 inverse centimeter region that were absent from the unactivated material, along with shifted C-O, C-N and C-O-C signals near 1144 to 1146 inverse centimeters and new bands attributed to C=C, N-H and N-COO structures. The effect depended on the steam dose. At a flow of 4 milliliters per minute, the biochars gained additional hydroxyl bands at 3619, 3608, 3361 and 3331 inverse centimeters, plus a band at 1240 inverse centimeters corresponding to C-O, C-OH and N-COO groups that appeared only in the 15-minute sample. Notably, the longest and most intense activation, 4 milliliters per minute for 45 minutes, produced no hydroxyl bands at all, suggesting that excessive steam exposure can push the surface chemistry past a useful point.

Surface area measurements told an equally nuanced story. The unactivated biochar had a BET specific surface area of just 7.4 square meters per gram, typical of a poorly developed pyrolysis char. Steam activation increased this figure in every case, but not linearly. At the 2 milliliter per minute flow, the best result was 37.9 square meters per gram after 15 minutes, falling to 17.4 after 45 minutes. At 4 milliliters per minute, the optimum shifted to 30 minutes, yielding 62.6 square meters per gram, roughly a ninefold expansion over the unactivated char, while the 15-minute sample at that flow reached only 12.9. The team concluded that steam first carves open new pores, but prolonged or overly intense treatment begins to destroy the porous framework it created. Nitrogen adsorption isotherms classified all the materials as mesoporous, with dominant pore diameters between 2 and 2.5 nanometers.

Scanning electron microscopy coupled with energy-dispersive X-ray analysis added morphological and elemental detail. Raw coffee grounds showed a flat, fibrous structure with no visible porosity. After pyrolysis, the material fragmented into a heterogeneous network resembling broken fibers, with recesses hinting at pores. Activated samples retained this rough, folded texture, with cavities confirming porosity. The elemental analysis was particularly revealing: pyrolysis raised the relative carbon content from 49.7 to 65.7 percent while oxygen fell from 32.7 to 9.7 percent, as volatile oxygen compounds escaped. Steam activation then pushed oxygen back up, by factors of 1.3 to 1.7 depending on the recipe, and nitrogen also rose relative to the unactivated char, likely because the water vapor blanket on the particle surface limits the volatilization of nitrogen species while oxidation-reduction reactions rebuild organooxygen groups.

With the surface chemistry mapped, the researchers turned to the practical question: could these biochars remove chromium(III) from water? They selected the biochar with the richest oxygen functionality, BFK_600_4ml_15min, and the one with the largest surface area, BFK_600_4ml_30min, and compared them against the unactivated reference. Adsorption tests used a chromium(III) solution of 120 milligrams per liter at pH 4 and 20 degrees Celsius, conditions chosen to keep the metal dissolved while mimicking the mildly acidic character of real tannery wastewater streams. Chromium concentrations were tracked by inductively coupled plasma mass spectrometry.

The results showed that surface chemistry mattered more than raw surface area. The biochar activated at 4 milliliters per minute for 15 minutes, despite its modest BET area, achieved the best performance: 51 percent chromium removal with a 0.5-gram sorbent dose and 56 percent with 1.0 gram, compared with 44 and 47 percent for the unactivated char. The high-surface-area 30-minute biochar reached 47 and 55 percent respectively. The team attributes the advantage of the 15-minute material to its hydroxyl bands and the distinctive 1240 inverse centimeter C-OH signal, which provide active binding sites for trivalent chromium ions. Kinetics followed a familiar pattern: adsorption was fastest in the first ten minutes and approached equilibrium by 210 minutes for activated samples, versus 150 minutes for the unactivated char. Statistical analysis confirmed a significant effect of contact time, with Pearson correlation coefficients between 0.87 and 0.99 linking contact time to removal efficiency.

The broader significance lies in the process itself. Physical activation with steam avoids the corrosive reagents such as potassium hydroxide or phosphoric acid used in chemical activation, which can generate surface areas above 3000 square meters per gram but leave behind liquid waste streams requiring careful disposal. Steam activation produces no such by-products, and the new study demonstrates that at 600 degrees Celsius it does something chemical activation often cannot: it regenerates the oxygen-bearing functional groups that high-temperature pyrolysis destroys, even if the resulting surface areas remain modest. Earlier work by the same group showed that at 700 degrees Celsius steam activation instead wipes out organooxygen groups and boosts surface area far more dramatically, so the activation temperature emerges as a dial for choosing between a chemically rich surface and a geometrically extensive one.

For a waste stream generated at a scale of millions of tons annually, the prospect of turning espresso dregs into a green sorbent for heavy-metal-laden wastewater is compelling, and the study’s careful dissection of steam flow and activation time gives future work a clear parameter map. The removal percentages reported here, in the 44 to 56 percent range under conservative single-pass conditions, are lower than the 90 to 99 percent figures reported in the literature for lead, cadmium and manganese on coffee biochars, and the authors are explicit that their experiments were comparative rather than optimized. Even so, the message is clear: the humble used coffee ground, activated with nothing but water vapor, can be engineered into a functional water-cleaning material, and the recipe that best restores its chemical appetite for chromium is a short, gentle blast of steam rather than a long, aggressive one.

Subject of Research: Water vapor activation of biochars produced from used coffee grounds for chromium(III) adsorption from water

Article Title: Water vapor activated biochars made from used coffee grounds

Article References: Kaźmierczak, B., Radulski, P., Osuch-Słomka, E., & Zięba, E. (2026). Water vapor activated biochars made from used coffee grounds. Results in Chemistry, 31, Article 103908. https://doi.org/10.1016/j.rechem.2026.103908

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103908

Keywords: used coffee grounds, biochar, water vapor activation, pyrolysis, chromium(III), adsorption, FTIR spectroscopy, BET surface area, SEM/EDS, wastewater treatment, circular economy, functional groups