Biological assessment of in-situ rehabilitation of polluted soils by waste-derived Technosols

Soil pollution by potentially harmful elements (PHEs) is a major environmental problem. Metal mining is one of the main pollutant sources, especially due to complex waste management. Residual polluted soils in the Guadiamar Green Corridor (Seville, Spain) more than 20 years after the Aznalcóllar mine spill and the remedial actions undertaken are a striking example of the potential damage. This study aimed to evaluate, at short term, the effectiveness of two Technosols designed to remediate these affected soils. In particular, the remediation process was assessed focusing on measurements of soil enzyme activity (dehydrogenase, cellulase, acid phosphatase, and protease) and the recovery of natural vegetation. Both Technosols (T4 and T6) were composed of ex-situ polluted soil [60%] and two mining wastes (iron oxyhydroxides-rich sludge [2%] and marble sludge [20%]); together with an agro-industrial waste (solid olive-mill by-product [18%]) in T4, and an urban waste (vermicompost from gardening [18%]) in T6. About 25 cm of the Technosols were surface applied in situ on polluted soils in triplicate. After one year of application (t₁), Technosols (T4 and T6; depth: < 25 cm) and treated polluted soils (T4-PS and T6-PS; depth: 25-30 cm) were characterised (soil properties, total, soluble and bioavailable PHEs, and soil enzyme activities) and compared to initial conditions (t₀). Vegetation in the treatments was also monitored by measuring cover, specific richness and diversity index. These polluted soils (T4-PS_(t0) and T6-PS_(t0)), besides posing a significant environmental and human health risk due to their extreme characteristics (pH<4, high concentration in PHEs), also showed low microbiological activity measured by dehydrogenase activity (<2 µg TPF g soil^-1 16 h^-1). Technosols T4 and T6 showed optimal conditions to rehabilitate the polluted soils and recover the non-existent natural vegetation (neutral pH, high OC, CaCO₃ and nutrient content, loamy textures, and good structure). Microbiological activity also increased slightly in these soils compared to the polluted one. And over time it was strongly stimulated. Dehydrogenase increased 20-fold (~85 µg TPF g soil^-1 16 h^-1), phosphatase 2-fold and cellulase 4-fold in T4_(t1) compared to initial values. In T6_(t1), dehydrogenase increased 6-fold (~63 µg TPF g soil^-1 16 h^-1), phosphatase 2-fold and cellulase remained constant. In contrast, in both Technosols, protease activity was almost halved. Technosols treatment reversed the adverse conditions (strongly acidic pH neutralisation, 2-fold increase in OC, addition of CaCO₃, and slight increase in CEC), and, in general, significantly reduced solubility and bioavailability of As, Cd, Cu, Cr, Ni, and Zn (except for Sb). It also stimulated microbiological activity. Dehydrogenase, cellulase, acid phosphatase, and protease activity in the treated polluted soils (T4-PS_(t1) and T6-PS_(t1)) was higher compared to initial conditions. Furthermore, after one year, 100% vegetation cover was established with the application of both Technosols. However, greater biodiversity developed in T4 than in T6, with higher specific richness (13 vs. 9) and diversity index (2.93 vs. 2.51). Therefore, these Technosols were effective in rehabilitating these polluted soils, as they improved soil properties, reduced PHEs mobility and bioavailability, and also promoted microbiological activity and establishment of biodiverse natural vegetation.