Feasibility of self-sustaining smoldering for the preparation of constructed Technosols from acidic mine tailings

Self-sustaining smoldering has been widely investigated for treating organic solid waste, whereas its potential to transform acidic mine tailings into constructed Technosols (CTs) for rehabilitation remains unclear. Here, self-sustaining smoldering was evaluated as a method for preparing CTs from acidic mine tailings, with a focus on smoldering behavior, acid-generation potential, arsenic (As) risk, physicochemical properties, dissolved organic matter (DOM) characteristics, revegetation potential, and mineralogical transformation. At tailings/wheat straw ratios of 3:1-6:1 and airflow rates of 0.2-0.9 m³/h, smoldering was self-sustaining, increasing the net neutralization ratio from 1.65 to values in the range 3.06-6.32 and indicating a transition to nonacid-generating behavior. Smoldering increased pH and nutrient availability, with the contents of organic matter and available nitrogen, phosphorus, and potassium increased by 2.02-3.97, 1.29-3.56, 1.87-45.08, and 50.62-85.69 times their respective initial levels. Smoldering also promoted DOM humification; increased condensed aromatic and tannin-like compounds; depleted sulfur-containing DOM; induced pyrite depletion, goethite-to-hematite transformation, and kaolinite dehydroxylation; and increased surface roughness. Although total As concentrations decreased from 64.50 to 52.87-64.14 mg/kg and the optimal treatment met the relevant landscaping-soil limit, smoldering increased As bioavailability and leachability in several treatments, necessitating subsequent risk management. However, plant tests of the resulting loamy-sand CTs confirmed improved revegetation potential, with greater ryegrass germination, shoot growth, and dry biomass compared with untreated tailings, particularly under tailings/wheat straw ratios of 3:1. This study advances the application of smoldering and provides a low-cost and energy-efficient method for preparing CTs for widespread acidic mine-tailings rehabilitation.