Sewage Sludge Ash and Electricity Team Up to Neutralize Acid Mine Drainage

Deep inside abandoned coal mines, a slow-motion chemical reaction never stops. Leftover pyrite, exposed to water, oxygen and microorganisms, keeps oxidizing long after the last shift has ended, churning out hydrogen ions and dissolved iron and manganese that seep into surrounding waterways as acid mine drainage. This legacy pollution from closed coal mines is notoriously difficult to treat: the water is acidic, its pollutant composition is complex, and its environmental impacts persist for decades. Now a research team from China University of Mining and Technology and Shanghai Datun Energy has reported a coupled treatment process that simultaneously strips out ferrous iron and manganese while pushing the pH back toward neutral, using nothing more exotic than electricity and ash recovered from sewage sludge pyrolysis.

The study, published in Frontiers in Environmental Science, describes a laboratory-scale reactor in which aluminum plates are wired in a monopolar parallel configuration and immersed in synthetic acid mine drainage. When direct current flows through the plates, the sacrificial aluminum anodes dissolve, releasing metal cations that react with hydroxide ions generated by cathodic water electrolysis to form amorphous aluminum hydroxide flocs. These freshly generated flocs sweep through the solution, capturing dissolved metals through adsorption, surface complexation, precipitation and coprecipitation. The twist is the addition of pyrolysis ash, a mineral-rich solid produced by heating dewatered waste activated sludge to 600 degrees Celsius under oxygen-limited conditions for two hours. Ground and sieved to below 0.15 millimeters, this ash supplies alkalinity, mineral exchange sites and heterogeneous nucleation surfaces that the electrocoagulation process alone cannot provide.

The researchers began with single-factor experiments designed to isolate the influence of each operating parameter. Electrode material proved decisive. Iron electrodes achieved a maximum manganese removal efficiency of 90.15 percent, outperforming the 81.50 percent obtained with aluminum, but their performance on ferrous iron told a different story. With iron plates, iron removal peaked at 67.89 percent after roughly 30 minutes and then declined to 51.01 percent at 60 minutes, because the dissolving iron anode continuously reintroduces ferrous and ferric species into solution, offsetting the removal of the original dissolved iron. Aluminum electrodes, by contrast, sustained iron removal at 81.96 percent after an hour and raised the pH to 6.31, making them the clear choice for the coupled experiments.

Electrode spacing emerged as a second critical variable. When the gap between plates widened from 10 to 20 millimeters, removal efficiencies for both metals and the final pH all declined, reflecting increased ohmic resistance, longer ion migration distances and weakened mass transfer between electrodes. Tighter spacing improves performance by shortening the distance ions must travel, but going too narrow risks floc accumulation between plates, concentration polarization and short-circuiting. Current density, which governs the rate at which sacrificial anodes dissolve and hydroxide flocs form, showed a pronounced quadratic effect. Manganese removal exceeded 80 percent whenever current density surpassed 15 milliamperes per square centimeter, but pushing the current too high induced electrode passivation, bubble coalescence and floc fragmentation that reduced current utilization efficiency.

The pyrolysis ash dosage experiment provided the most striking demonstration of the coupled process. Holding all other conditions constant, raising the ash dosage from zero to 0.4 grams per liter increased ferrous iron removal by roughly 8 percentage points and manganese removal by roughly 22 percentage points within 30 minutes. The ash also raised the pH of the acidic wastewater, which started at 3.89, toward neutrality within a relatively short reaction period. Beyond the optimal dosage, however, removal efficiencies plateaued or declined slightly, because excessive particle loading causes agglomeration, partial blockage of adsorption sites and increased mass transfer resistance. The ash functions as a consumable mineral-assisted coagulant aid rather than a conventional packed-bed adsorbent, becoming incorporated into the composite flocs through precipitation, coprecipitation and sweep flocculation.

To move beyond trial and error, the team employed response surface methodology using a Box-Behnken design with three factors and three levels, treating ferrous iron removal, manganese removal and final pH as response variables. The resulting quadratic regression models were highly significant, with F-values of 52.92, 64.19 and 76.26 for the three responses and p-values below 0.0001, while nonsignificant lack-of-fit tests confirmed the models adequately described the experimental data. All pairwise interactions among electrode spacing, current density and ash dosage proved statistically significant for every response, revealing genuine synergies that single-factor experiments could not capture. The response surfaces showed that ferrous iron removal generally decreased with wider electrode spacing, rose and then fell with increasing current density, and climbed to a plateau with increasing ash dosage.

According to the fitted model, the optimal operating conditions were an electrode spacing of 5 millimeters, a current density of 20.73 milliamperes per square centimeter and a pyrolysis ash dosage of 0.46 grams per liter. Validation experiments run at a rounded current density of 21 milliamperes per square centimeter delivered average removal efficiencies of 83.62 percent for ferrous iron and 92.71 percent for manganese, with the final effluent pH climbing to 7.04. Relative errors between predicted and measured values were just 1.30 percent, 0.13 percent and 0.85 percent for the three responses, demonstrating close agreement between model and experiment. Under these conditions, the process transferred 85.71 milligrams of iron and 3.606 milligrams of manganese out of each liter of treated water, leaving residual concentrations of 16.79 and 0.284 milligrams per liter respectively.

A preliminary cost assessment suggests the process could be economically viable. At the optimized current density and an effective anodic area of roughly 440 square centimeters, the operating current is 9.24 amperes, corresponding to a specific electrical energy demand of 23.1 to 69.3 kilowatt-hours per cubic meter depending on cell voltage. Faraday’s law puts the theoretical aluminum consumption at approximately 1.55 kilograms per cubic meter, while the ash dosage corresponds to 0.46 kilograms per cubic meter. Using scenario prices of 10 cents per kilowatt-hour for electricity, 2.50 dollars per kilogram for aluminum and 10 to 50 cents per kilogram for ash preparation, the estimated operating cost falls between 6.23 and 11.04 dollars per cubic meter, excluding depreciation, labor and solids management.

Characterization of the recovered composite flocs revealed the underlying removal mechanisms. Scanning electron microscopy showed heterogeneous, irregular agglomerates with rough, flocculent surfaces and compact granular domains, consistent with the aggregation of electro-generated hydroxide flocs and ash particles. Energy-dispersive X-ray spectroscopy detected oxygen, iron, carbon, manganese, silicon, calcium and aluminum, with iron and manganese accounting for 47.28 and 1.92 weight percent of the analyzed region. X-ray diffraction identified major peaks assigned to FeOOH and Fe3O4, alongside silicate and carbonate mineral phases from the ash, and weak peaks suggesting MnOOH and Mn3O4. Together these observations support a combined mechanism in which electro-generated hydroxide flocculation, adsorption and surface complexation, precipitation and coprecipitation, and heterogeneous nucleation promoted by the ash all contribute to metal immobilization.

The authors acknowledge that their study used synthetic acid mine drainage rather than field-collected water, so the results should be regarded as laboratory-scale evidence of feasibility rather than direct validation of field performance. Real mine drainage carries high sulfate concentrations, dissolved organic matter and coexisting ions that may influence conductivity, metal speciation, electrode passivation and competitive adsorption. Future work will need to validate the process on water from different mine sites and seasonal conditions, integrate continuous-flow operation, measure energy demand and electrode consumption, and conduct standardized leaching evaluations of the recovered solids. Still, the vision is compelling: a treatment train that turns two waste streams, sewage sludge and abandoned mine water, into a single solution, powered by electricity and guided by statistical optimization.

Subject of Research: Experimental study on the coupled treatment of acid mine drainage from closed coal mines by electrocoagulation and pyrolysis-ash adsorption

Article Title: Experimental study on the coupled treatment of acid mine drainage from closed coal mines by electrocoagulation and pyrolysis-ash adsorption

Article References: Yuan, D., Chang, Y., Li, X., Qin, B., Zhang, M., Gao, C., & Wu, X. (2026). Experimental study on the coupled treatment of acid mine drainage from closed coal mines by electrocoagulation and pyrolysis-ash adsorption. Frontiers in Environmental Science, 14, Article 1964982. https://doi.org/10.3389/fenvs.2026.1964982

Image Credits: AI Generated

DOI: 10.3389/fenvs.2026.1964982

Keywords: Experimental, coupled, treatment, acid, mine, drainage, closed, coal, mines, electrocoagulation, pyrolysis-ash, adsorption