## 1 Introduction  
Soils contaminated with potentially hazardous trace elements (PHE) arising from mining wastes and industrial discharges pose serious threat to human health and ecosystems [1; 2]. Management of these environmentally significant soils [3] may require the use of sustainable remediation strategies to mitigate the dispersion of contaminants and their associated risks. Conventional soil clean-up technologies [4; 5], especially those involving surface capping and landfilling, are often financially inefficient in extensive mine lands where treatments should be affordable and self-sustaining. In situ stabilization or chemical fixation of PHE induced by the addition of immobilizing agents, such as soil amendments made of waste materials and by-products [6; 7; 8], is a cost-effective and eco-friendly option to reclaim abandoned mine lands following circular economy principles and modern waste management trends. These amendments can be referred as Technosols according to the World Reference Base for Soil Resources [9], because they form a new soil layer [10]. The immobilization technique involves PHE removal from soil solution either through precipitation, adsorption, ion exchange, or complexation mechanisms [11], thus reducing to some extent contaminant uptake by plants and leaching into groundwater or nearby watercourses [12]. Since contaminant solubility is linked to its mobility and bioavailability, chemical stabilization has the potential to reduce environmental risk [13], although the immobilizing effects may not last long enough and, consequently, repeated amendment additions may be needed to sustain the stabilization process. Liming is a widely used practice for reducing the mobility of PHE in contaminated soils with different degrees of success [14; 15]. Most research on the use of lime-based waste materials and their efficiencies for PHE immobilization in soil has been conducted under laboratory conditions in short-term studies [11; 12], as the liming effects were observed only after the first few years of treatment. These results from lab-scale experiments need to be validated by further field work to verify long-term stability of immobilized PHE in amended soils [16; 17; 18], where natural processes controlling PHE mobility can be monitored over long time scales under realistic conditions. Long-term field monitoring provides direct evidence of the ageing process of amendments [19]. Specifically, the waste sludge (i.e., micronized calcium carbonate) generated by marble processing plants has been proven as an effective nanomaterial for acid soil neutralization and for enhanced PHE retention capacity of soils affected by mining activities [20; 21; 22; 23]. In fact, a field trial in the historic mining district of Tharsis (southwestern Spain) showed the potential of marble waste sludge (MWS) as a promising soil additive for assisting re-vegetation of legacy mine lands [24]. The most remarkable features of the MWS are alkalinity (pH = 8.8-9.2), fineness (mean particle size = 8.24 \(\upmu\)m), and purity (\(\sim\)95 wt% of calcite). Using this MWS, a single amendment of 22 cmol\({}_{\rm c}\) of lime per kilogram of soil was applied by [PERSON] and [PERSON] [24] to raise the pH from 3.2 to a pH level suitable for plant regrowth. The acid neutralizing effect of the soil amendment effectively reduced the concentrations of Al, Fe, sulfate and PHE, notably Cd, Cu, Pb and Zn, in the most labile and phytoavailable fraction enough to allow the settlement of spontaneous plant species (_Lolium preenne, Plantago coronopus_ and _Spergularia rubra_). Even though the reported early findings showed promise, substantial uncertainty exists regarding the long-term effectiveness of PHE stabilization in soil. To address this uncertainty, a mini plot field experiment was conducted to evaluate the sustainability of a single application of MWS on chemical immobilization of PHE in the mine soil of Tharsis, after a period of 12 years post-application.