At a dosage of 0.5 grams, the biochar removed 77.45% of chemical oxygen demand, or COD, and 79.30% of total phenols, expressed as gallic acid equivalents, from the wastewater. Increasing the dose tenfold from 0.05 grams produced only a modest gain, indicating that removal efficiency had approached a plateau. The wastewater presented a demanding treatment challenge because it contained a high concentration of organic matter in an acidic liquid.

After particles were removed by decanting and cloth filtration, the stock sample had a pH of 4.40–4.65, total COD of 67.4 grams of oxygen per liter and soluble COD of 66.6 grams of oxygen per liter. Its five-day biochemical oxygen demand was 8.3 grams of oxygen per liter, giving a BOD5-to-COD ratio of 0.123 and indicating that a substantial fraction of the organic matter was relatively resistant to biodegradation. Total phenols measured 721.5 milligrams per liter, while turbidity was 1,124 nephelometric turbidity units.

Filtration was an important boundary on the findings: the experiment principally measured adsorption from the liquid phase, rather than combined removal of suspended particles, dissolved contaminants and other wastewater constituents. Slow pyrolysis at 600 degrees Celsius produced a strongly alkaline, carbon-rich powder. The biochar had a measured pH of 9.5 and a point of zero charge of 8.49, while its measured zeta potential was negative 29.89 millivolts.

Elemental analysis found 67.21% carbon, 26.544% oxygen, 3.123% nitrogen and 3.123% hydrogen. Its low H/C ratio of 0.047 and O/C ratio of 0.395 were consistent with substantial carbonization, while the retained oxygen content indicated that some oxygen-bearing functional groups remained after heating. The biochar yield was 18.18% of the dry feedstock mass, its ash content was 10.83%, and its moisture content was 4.18%.

These measurements describe material made under one set of pyrolysis conditions. They do not establish how the adsorbent would perform across other feedstocks, furnaces, mills or operating conditions. Porosity and structural analysis showed how the material could capture organic pollutants.

Its BET specific surface area was 135.23 square meters per gram, although the reported Langmuir surface area was lower at 14.84 square meters per gram and the total pore volume was 0.0055 cubic centimeters per gram. The average pore width was 21.95 angstroms, placing the structure near the boundary between microporous and mesoporous material. Scanning electron microscopy showed that pyrolysis transformed the compact, heterogeneous raw residue into a more fragmented structure with visible cavities and roughened surfaces.

Fourier-transform infrared spectroscopy indicated increased aromatic carbon structures after heating: a band at 1,552 reciprocal centimeters was assigned to aromatic carbon-carbon bonding, a band at 1,051 reciprocal centimeters indicated residual carbon-oxygen functionality, and a band at 862 reciprocal centimeters was associated with aromatic carbon-hydrogen bending. X-ray diffraction showed that the carbon structure was predominantly amorphous, with a broad feature around 2 theta of 22–25 degrees and a nearby peak associated with an inorganic mineral phase, particularly calcite, from the biomass ash. The biochar was not chemically inert.

Potassium occurred at 11,800 milligrams per kilogram, calcium at 5,570 milligrams per kilogram, phosphorus at 316 milligrams per kilogram, iron at 284 milligrams per kilogram and magnesium at 235 milligrams per kilogram. Calcium can participate in coagulation and flocculation, while magnesium, iron and other mineral constituents can influence ionic and redox processes in wastewater. Their presence may therefore have contributed to the material’s behavior, but the batch results did not isolate the contribution of each element.

Likewise, the biochar’s alkalinity was observed alongside the adsorption results, but the experiments did not separate adsorption from every other change that could occur in the wastewater matrix. The findings therefore establish the combined performance of the material under the tested conditions rather than a single universal mechanism for COD and phenol removal. Contact-time experiments distinguished the behavior of the two measured pollutants.

Total-phenol adsorption followed a pseudo-second-order kinetic model with an R-squared value reported as 1, while the pseudo-first-order model fitted poorly, with an R-squared value of 0.0322. The experimental equilibrium uptake was 2,058.65 milligrams per gram, closely matching the pseudo-second-order estimate of 2,058.012 milligrams per gram. This close correspondence and strong model fit were consistent with surface interaction controlling the observed phenol uptake, although model fit alone cannot identify a unique molecular mechanism.

COD behaved differently: its pseudo-first-order R-squared value was 0.7483 and its pseudo-second-order R-squared value was 0.76, with a pseudo-second-order equilibrium estimate of 157.1 milligrams per gram compared with an experimental value of 160 milligrams per gram. The moderate fit suggested that COD removal involved multiple rate-controlling processes, potentially including surface interaction, intraparticle diffusion and mass-transfer limitation. Temperature produced distinct and non-monotonic responses across the three tested conditions.

At 25 degrees Celsius, COD removal was 75.61%, total-phenol removal from real wastewater was only 0.1223%, and gallic-acid removal was 60.64%. Raising the temperature to 35 degrees Celsius increased COD removal to 79.46%, wastewater-phenol removal to 19.15% and gallic-acid removal to 72.28%. At 45 degrees Celsius, COD removal declined slightly to 76.32%, wastewater-phenol removal rose further to 24%, and gallic-acid removal fell to 62.15%.

The researchers related the temperature effects to changes in diffusion and molecular mobility and considered the possibility that excessive heat affected accessible adsorption sites or surface properties. The divergent responses for phenols in real wastewater and in a controlled gallic-acid solution also showed that a single synthetic compound could not reproduce the behavior of the full wastewater matrix. The two phenolic measurements therefore need to be read separately rather than as interchangeable tests of treatment performance.

Equilibrium modeling indicated that phenol adsorption became increasingly heterogeneous as temperature rose. The Freundlich model gave an R-squared value of 0.8544 at 25 degrees Celsius, 0.9372 at 35 degrees and 0.9833 at 45 degrees, compared with Langmuir values of 0.7612, 0.7460 and 0.2700. The estimated Langmuir maximum capacity increased from 93.75 milligrams per gram at 25 degrees to 100.95 milligrams per gram at 35 degrees and 117.98 milligrams per gram at 45 degrees.

Although the Freundlich exponent remained above one at every temperature, consistent with favorable adsorption under the fitted model, the combined fit statistics favored Freundlich behavior more strongly at the two elevated temperatures. That pattern is more consistent with adsorption across sites of differing energies than with a single idealized mechanism. The capacity values were model estimates from batch isotherms, and they do not demonstrate that heating would improve performance in a continuous-flow or field-scale treatment system.

Thermodynamic analysis estimated phenol adsorption as spontaneous and endothermic across the tested temperatures. The standard enthalpy change was positive 71.37 kilojoules per mole, the entropy change was negative 144.96 joules per mole-kelvin, and calculated Gibbs free-energy changes ranged from negative 27.75 kilojoules per mole at 298.15 kelvin to negative 24.80 kilojoules per mole at 318.15 kelvin. The negative entropy value was interpreted as increased ordering of phenol molecules at the biochar surface as they became associated with the adsorbent.

The estimated maximum capacity of 93.75–117.98 milligrams per gram placed the unmodified material within the range reported for other non-chemically activated agricultural biochars, but below values around 194–215 milligrams per gram for chemically activated rice-husk carbons and approximately 201 milligrams per gram for one two-step-pyrolysis material. This comparison establishes a laboratory performance range, not a cost-effectiveness ranking, because the study did not calculate production costs, regeneration cycles, biochar loss or performance in full-scale treatment units. The combined result is a waste-to-waste strategy with clear experimental limits.

Olive mill solid waste served as the feedstock for an alkaline, porous adsorbent that removed much of the measured COD and phenols from pretreated olive mill wastewater, while a dosage of 0.1 grams was selected for subsequent tests because it balanced removal performance with material use. The work used filtered batch samples from one Jordanian mill and did not test continuous flow, repeated regeneration, pollutant selectivity, leaching, downstream water quality or ecotoxicity. The highest temperature condition also required heating the wastewater to 45 degrees Celsius, but the researchers did not conduct an energy-cost analysis.

Even so, the study demonstrates a measurable and technically interpretable form of waste valorization: under controlled laboratory conditions, slow-pyrolysis biochar from olive mill solid waste adsorbed organic contaminants from olive mill wastewater, with phenol uptake governed more clearly by a pseudo-second-order model than COD uptake and with capacity estimates that varied with temperature and equilibrium model.

Subject of Research: Laboratory testing of olive mill waste biochar for removing COD and total phenols from olive mill wastewater

Article Title: Transforming olive mill solid waste: innovative biochar production for sustainable organic pollutant removal in olive mill wastewater treatment

Article References: Al-Karablieh, E. K., Hamadneh, I., Al-Qodah, Z., Bubba, M. D., Al-Zghoul, T. M., Aziz, F., Mokaizh, A. A. B., Al-Khraisha, A., & Jamrah, A. I. (2026). Transforming olive mill solid waste: innovative biochar production for sustainable organic pollutant removal in olive mill wastewater treatment. Frontiers in Sustainable Food Systems, 10, Article 1977076. https://doi.org/10.3389/fsufs.2026.1977076

Image Credits: AI Generated

DOI: 10.3389/fsufs.2026.1977076

Keywords: biochar, olive mill wastewater, olive mill solid waste, chemical oxygen demand, total phenols, adsorption, slow pyrolysis, agricultural waste, Jordan, wastewater treatment, Transforming, olive