Chemical stabilization of heavily contaminated mine soils using technosols made from non-hazardous industrial wastes

Abandoned mine lands often have lost their natural resilience and adaptive capacity to retain potentially toxic trace elements (PTE) and, therefore, can act a secondary source areas of acidic effluents and harmful contaminants into surface and pore waters. To address this issue, a batch leaching test was conducted to assess the effects of tailored combinations of non-hazardous industrial wastes on the immobilization of PTE in three mine soils (MS₂, MS₃, MS₄) of the historic Rio Tinto mining district (Spain). To do this, different Technosols (T) were prepared by mixing, at a ratio of 60:40 (w/w), organic (R2: sludge from the clear treated water) and inorganic waste materials with acid-neutralizing capability (R4 and R5: slags from the iron and steel making industry) and adsorbent properties (R6: red gypsum). The Technosols tested were made of: (T0) exclusively MS₂, MS₃ and MS₄ (controls); (T1) 80%MS₄+20%[R2+(R5+R6)]; (T2) 75%MS₃+25%[R2+(R5+R6)]; (T3) 75%MS₂+25%[R2+R4]; (T4) 75%MS₂+25%[R2+R5]; (T5) 75%MS₃+25%[R2+R5]; and (T6) 75%MS₄+25%[R2+R5]. The leaching test was performed according to the standard EN-12457-4, at a liquid-to-solid ratio of 10 L kg^-1 with constant agitation for 24 h. The leaching conditions were recorded in terms of pH and Eh values, and the PTE concentrations in the leachate solutions were analyzed by ICP-MS. Results showed that the leachates from untreated MS were ultra acid (pH 2.5-3.5) and had concentrations of Cu, Pb and Zn as high as 556 mg kg^-1 (MS₃), 90 mg kg^-1 (MS₄) and 28 mg kg^-1 (MS₃), respectively, while the released concentrations of Cr, Ni, As and Cd were below 1 mg kg^-1. In relative terms, the most mobile PTE were Cd, Cu and Zn, which accounted for 12.6%, 10.3% and 5.4% of its total concentration, respectively. The Technosol application was able to buffer the soil pH to average values of 7.6, thus enhancing the attenuation of PTE by chemical fixation. The mobile fraction of PTE was drastically reduced or even rendered virtually negligible (<0.10%) except for Ni, which was leached from the T1 up to 0.79% of its total concentration, and for Cd (0.49% from the T2). In particular, T3 was the most effective for reducing the mobility of Cd, and T4 and T5 showed promise for assisting in the attenuation of Cu. Chemical speciation calculations predict that most of the PTE leached from untreated MS are in the form of sulfate complexes, comprising 62-77% of the dissolved fraction, and to a lesser extent as free ions. Upon treatment, the activity of such species decreased noticeably with increasing pH, while the contribution of hydroxyl complexes was shown to increase. In conclusion, the use of Technosols was effective in reducing the acid generation potential of the MS and for boosting the PTE stabilization processes.