Remediation of soils polluted by metal(loid)s based on waste valorization and bioremediation by symbiotic and saprobic microorganisms

Abstract

Soil pollution by potentially toxic elements (PTEs) (e.g., Pb, As, Zn, Cu, Cd, and Sb) is a significant environmental problem worldwide, mainly associated with anthropogenic sources and activities, including the mining industry. These elements, while naturally occurring in the environment, can reach toxic concentrations and persist for long periods, causing severe damage to ecosystems and posing serious risks to human health. In scenarios where extensive and severe soil pollution occurs, active soil remediation actions are needed to mitigate environmental damages and protect humans and other living organisms health. The Guadiamar Green Corridor (GGC) (Seville, SW Spain) represents an exemplary scenario of a natural ecosystem severely damaged over the long term by anthropogenic activities, and more specifically by PTEs pollution. This area was severely affected by the Aznalcóllar mining spill occurred in 1998, which resulted in high levels of persistent soil pollution despite initial cleanup efforts. Still nowadays, 25 years after the accident, residual polluted areas remain evident along the GGC by the existence of bare highly acidic soil patches with elevated PTEs concentrations. These residual polluted soils represent a risk not only to the environment but also to human health, requiring continuous monitoring and the development of comprehensive protocols and effective and feasible remediation strategies. This thesis addresses the need to develop sustainable and effective strategies for remediation PTE-polluted soils, using the GGC as a case study. The research is focused on the valorization of wastes and by-products from anthropogenic activities and the application of symbiotic microorganisms as viable strategies for remediation of soils polluted by PTEs. The aim of this thesis is to evaluate the effectiveness and feasibility of a set of cost-effective and environmentally friendly soil remediation techniques and strategies for the ecological remediation of PTE-polluted soils. For this purpose, various soil remediation treatments, including inorganic liming amendments (gypsum mining spoil and marble sludge), organic amendments (vermicompost and biotransformed dry olive residue (DOR) by saprobic fungi), and physical techniques such as landfarming and biopiles, were applied in situ to polluted soils in the GGC. Furthermore, a soil bioremediation approach based on arbuscular mycorrhizal fungi (AMFs) was implemented under greenhouse conditions to evaluate the potential of this bioremediation strategy to enhance the effectiveness of the physicochemical treatments. The thesis evaluates the impact of the treatments applied on the main soil properties and pollution levels over time, as assessed in chapters 2, 3, and 4. Liming treatments, based on gypsum mining spoil and marble sludge, were highly effective in neutralizing the strong acidity of the polluted soils, particularly marble-based treatments, which led to complete pH neutralization. Water-soluble and EDTA-extractable fractions of the PTEs were measured to assess the changes in soil mobility and bioavailability of PTEs following the application of the treatments to the polluted soil (PS). Liming treatments were also the most effective in reducing both fractions of these elements. This was associated with a significant increase in soil pH, resulting in the effective immobilization of highly mobile elements such as Cu, Zn, and Cd. However, excessively high pH levels could limit the immobilization of other PTEs such as As, thus increasing its bioavailability. Resolubilization of As and Pb could also occur in the presence of organic matter by competing mechanisms for sorption sites in the soil. Therefore, the doses of liming and organic amendments should be accurately estimated to effectively control the mobility of PTEs in polluted soils. In chapter 2, an ecotoxicological assessment of soil pollution in the treated soils was carried out using a variety of bioassays selected as indicators of PTEs stress in polluted soils, to evaluate the effectiveness of the remediation treatments applied to the PS. A set of ecotoxicological tests were performed using both the solid and liquid phase of the soil, involving target organisms from different taxonomic groups and trophic levels, including microorganisms, plants, and invertebrates. The results showed that marblebased treatments were the most effective in reducing soil toxicity, primarily due to their strong pH neutralization, which reduced PTEs solubility and minimized toxicity risks. Moreover, bioassays using the liquid phase showed higher sensitivity to toxicity compared to those using the solid phase, thus providing a better estimation of soil toxicity. The remediation treatments evaluated aimed to facilitate and accelerate natural ecological succession in areas of the GGC where it was hindered by the extremely limiting conditions. The success of these treatments in triggering succession and facilitating the recovery of biological communities in the highly degraded soil was assessed by analyzing the status of these communities at various levels. In chapter 3, the vegetation status and spontaneous recolonization of the reference and treated soils by native plants were measured. In general, the treatments were effective in promoting spontaneous vegetation growth by improving soil properties and reducing PTEs availability. Among them, those based on vermicompost showed the greatest vegetation cover and species richness, approaching the conditions of the adjacent naturally recovered soils (RS). These treatments significantly increased soil organic carbon content and improved water retention capacity, essential for facilitating plant recolonization and growth in polluted soils. In the studied soils, two native plant species, Lamarckia aurea and Spergularia rubra, acted as pioneer species colonizing the soil, exhibiting remarkable ability to accumulate Pb and As in their roots. They can be considered key species in the area, as they not only serve as pioneers in recolonizing the degraded soils but also facilitate the subsequent recolonization by other species less tolerant to high levels of pollution. Thus, the success of the remediation strategy can be promoted by the early recolonization of the soil by these highly pollution-tolerant species, which enhance further evolution of the soil physical, biological, and chemical conditions. The presence of pioneer plant species in treated soils can enhance microbial activity by providing soil microbial communities with suitable habitat, along with essential organic carbon and nutrient contents, which are crucial for key soil processes and overall ecosystem functioning at the local scale. For this reason, when evaluating the rate of ecological succession driven by remediation treatments, it is important to take into account not only aboveground processes, such as vegetation response, but also those occurring belowground. In this regard, the soil microbiological status of both reference and treated soils was evaluated in chapter 4. The results showed that PS had low total abundances, community structure, and diversity of microbial communities, confirming that microbial biomass and taxonomic diversity of soil communities were significantly affected by the PTEs concentrations. On the other hand, the soil treatments evaluated for their microbiological status, specifically marble sludge and biopile, showed effectiveness in restoring soil quality. The abundance and structure of microbial populations under both treatments were restored to levels proximate to those found in RS. The application of the remediation treatments to PTE-polluted soils in the GGC led to significant improvements in soil quality, reducing the mobility and toxicity of PTEs, and allowing the establishment of vegetation. However, despite these improvements, the conditions and quality of the treated soils remained less favorable compared to the RS, with a persistent potential risk of PTEs remobilization that requires to be monitored over time. This highlights that physicochemical techniques for the remediation of polluted soils alone may not be enough to permanently alleviate pollution risks. Consequently, soil bioremediation processes could be implemented in parallel to promote the remedial effects of other techniques. In this sense, Chapter 5 explores the addition of a biological element, AMFs inoculation, to the soil remediation treatment that showed the highest overall effectiveness in the field-based insitu remediation approach (marble sludge), combined with organic amendments. Biotransformed DOR showed high effectiveness in improving soil physicochemical and biological status when combined with marble sludge, promoting plant growth and survival, and reducing PTEs toxicity and plant uptake. Thus, DOR biotransformed by saprobic fungi can represent an efficient organic amendment for remediating PTE-polluted soils. Furthermore, the combined application of marble sludge and DOR along with AMF inoculation, further enhanced PTEs immobilization in polluted soils by stimulating the phytostabilization process induced by AMFs. This bioremediation approach improved plant protection and significantly increased the overall effectiveness of the remediation process, showing potential as a sustainable bioremediation strategy for restoring soil functions and reducing toxicity in areas polluted by PTEs. In conclusion, this thesis provides valuable insights into the remediation of PTE-polluted soils. Overall, the results demonstrated that combining physicochemical treatments with a bioremediation approach, incorporating AMF inoculation and organic amendments, significantly enhanced the effectiveness of soil remediation processes. This approach not only improves soil health and reduces PTEs mobility, but also supports the re-establishment of vegetation and microbial communities in the degraded soils. These findings contribute to the development of more effective and sustainable soil remediation strategies that can be applied to similar polluted sites worldwide. Moreover, the results emphasize the importance of a holistic approach to soil remediation that considers not only the soil physicochemical status but also its biological health and capacity for ecological recovery.