Tamarugite, NaAl(SO4)2·6H2O, as a valuable indicator of soil degradation in a Spanish coastal wetland receiving acidic leachates from spilled mine waste

This paper documents the first occurrence of tamarugite in a Spanish coastal wetland registered as a UNESCO Biosphere Reserve, and discusses its origin and environmental significance. Tamarugite occurs as bladed crystals in efflorescent coatings on the topsoil of marshy areas, alongside an abandoned mining railroad. Tamarugite formation is described as a sequential process involving: (1) Oxidative dissolution of pyrite ore spilled on the former railway tracks and generation of acidic sulfate-rich waters; (2) decomposition of layer minerals under acidic conditions and release of Al3+ ions into solution; (3) interaction between acid discharges and estuarine water during flood tidal periods; and (4) precipitation of tamarugite and associated sulfate salts (sideronatrite, epsomite, pickeringite) under strongly evaporative conditions. These transient minerals have potential to release acid and associated major elements (Al, Fe, Mg, Na, S) and trace metals (e.g. Zn) into the solution causing detrimental effects. The occurrence of tamarugite is, therefore, a valuable indicator of environmental degradation in wetland ecosystems.The author acknowledges the reviewers for their constructive comments, especially for suggesting and providing information on the classification of soils in accordance to the World Reference Base for Soil Resources (WRB). Funding for open access charge: Universidad de Huelva / CBUA

Mineralogical and Crystal-Chemical Constraints on the Glauconite-Forming Process in Neogene Sediments of the Lower Guadalquivir Basin (SW Spain)

Glaucony is a significant green marine facies in the northwestern passive margin of the Guadalquivir Basin (Spain), where glauconite formed authigenically on a sediment-starved continental shelf, with fecal pellets and benthic foraminiferal tests being the main glauconitized substrates. Results from a study using XRD, TGA-DSC, SEM-EDS, and EPMA have revealed that glauconite is remarkably heterogeneous in mineral composition and chemical maturity, even in a single grain, reflecting a complex interaction of micro-environmental factors, substrate influences and post-depositional alterations. In its early stage, the glauconitization process is consistent with the slow precipitation of a Fe-rich smectite phase, most likely intergrade between nontronite and Fe-montmorillonite end-members, which evolved to a regularly interstratified glauconite-smectite (Gl/S). The Fe-smectite-to-Gl/S transformation is interpreted as a diffusion-controlled reaction, involving sufficient Fe availability in pore water and the constant diffusive transport of seawater K + and Mg2+ ions towards the substrate. The pelletal glauconite is actually a highly evolved Gl/S consisting almost totally of mica layers, with 0.74 ± 0.05 apfu of K+ in the interlayer, while the Gl/S occurring as replacements of foraminiferal tests contains a mean of 7% of expandable layers in the walls and 16% in the chamber fillings, due to rate-limited ion diffusion.We thank Fernando Muñiz (University of Seville) for its collaboration during fieldwork, Josep Tosquella (University of Huelva) for helping with microfossil identification, and Jesús Díaz (University of Huelva) for assistance with thermal analysis. The authors greatly appreciate the quick and valuable review of the manuscript by three anonymous referees

Tracing Soil Contamination from Pre-Roman Slags at the Monte Romero Archaeological Site, Southwest Spain

Soil serves as a repository of human history, preserving artifacts within its horizons. However, the presence of chemically reactive remnants, such as ancient slags, can significantly impact the surrounding soil environment. This paper addresses this scarcely explored issue by focusing on soil contamination arising from pre-Roman slag deposits at the Monte Romero archaeological site in southwest Spain, dating back to the Tartessian period (c. 7th century BC). Through the high-resolution microscopy examination of slag wastes and the trace element analysis of soil samples by ICP-OES, this study evaluated current contamination status using a multi-index approach. The results revealed markedly high levels of Pb (>5000 mg kg−1), Cu (up to 2730 mg kg−1), and As (up to 445 mg kg−1) in the soil compared to a control sample. The identification of secondary complex compounds like Cu arsenates and Pb arsenates/antimonates within slag cavities suggests post-depositional weathering processes, leading to the dispersion of potentially toxic elements into the surrounding soil. Assessments through indices of contamination and potential ecological risk highlighted severe contamination, particularly concerning Ag, Pb, Sb, Cu, and As. This study underscores the importance of addressing potential environmental hazards associated with archaeological sites hosting remnants of metal production

Enrichment and Fractionation of Rare Earth Elements in an Estuarine Marsh Soil Receiving Acid Discharges from Legacy Sulfide Mine Wastes

This paper provides new insights into the geochemical cycling of rare earth elements (REEs) in acid sulfate soils developed on salt marsh sediments of the Huelva estuary (Spain) as a result of sulfide mineral oxidation in abandoned ore stockpiles. The study was aimed at determining the REE abundance, fractionation pattern and mineralogical control of the dispersal and retention of REEs in the soil system. Forty-one samples were collected at 13 core sampling sites along two transects extending across the degraded marshland, and they were subjected to XRD, ESEM-EDS and ICP-MS analyses. Measurements revealed that the soil receiving acid discharges has relatively high concentrations of SREEs (174.77 +- 19.77 mg kg1) compared to local baseline concentrations. Shale-normalized REE patterns are generally flat, but a slight middle REE (MREE) enrichment is consistently apparent in all soil samples, involving relatively low LaN/GdN ratios (0.83 +- 0.08) and GdN/LuN ratios up to 1.42. The convex-upward REE pattern supports the possibility that iron oxy-hydroxide minerals play an important role in MREE retention through adsorption and co-precipitation mechanisms. Efflorescent sulfate salts left on the topsoil by the evaporation of acid waters show a strong depletion of light REEs (LaN/GdN = 0.16 +- 0.10) and act as a temporary reservoir of labile MREEs and heavy REEs during dry periods.This work has been partially supported by the Andalusian Regional Government (Spain) through the Research Group on Geology and Environmental Geochemistry (RNM-347)

Soil Acidification, Mineral Neoformation and Heavy Metal Contamination Driven by Weathering of Sulphide Wastes in a Ramsar Wetland

Past waste disposal practices have left large volumes of sulphidic material stockpiled in a Ramsar wetland site on the Atlantic coast of southwestern Spain, leading to severe land degradation. With the aim of addressing this legacy issue, soil core samples were collected along two transects extending from the abandoned stockpiles to the adjacent marshland and subjected to XRD, SEM-EDS, ICP-OES and ICP-MS analyses. Sulphide oxidation has been shown to be a major driver of acid generation and metal leaching into the environment. The marsh soil receiving acid discharges from the sulphide wastes contains elevated levels (in mg kg−1 ) of Pb (up to 9838), As (up to 1538), Zn (up to 1486), Cu (up to 705), Sb (up to 225) and Tl (up to 13), which are retained both in relatively insoluble secondary minerals (mainly metal sulphates and oxides) and in easily soluble hydrated salts that serve as a transitory pool of acidity and available metals. By using a number of enrichment calculation methods that relate the metal concentrations in soil and their baseline concentrations and regulatory thresholds, there is enough evidence to conclude that these pollutants may pose an unacceptable risk to human and ecological receptorsThis work has been partially supported by the Andalusian Regional Government through the Research Group on Geology and Environmental Geochemistry (RNM-347). : We thank Jesús de la Rosa (University of Huelva) for his collaboration during fieldwork and for assistance with chemical analysi

Oral bioaccessibility and human health risk assessment of trace elements in agricultural soils impacted by acid mine drainage

Cultivated soils around the historic mine site of Tharsis (Spain) contain elevated concentrations of As (up to 621 mg kg−1), Cu (752 mg kg−1) and Pb (2395 mg kg−1), exceeding the regional background levels and the statutory limits set for agricultural use. A site-specific health risk assessment of occupational and environmental exposures was conducted using an approach based on guidelines from regulatory agencies, refined by combining bioaccessibility and bioavailability data. Oral bioaccessibility, as determined by simulating the human digestion process in vitro (Unified BARGE Method), was largely related to total trace element concentrations in soil. Arsenic seemed to be evenly distributed among the gastric and gastro-intestinal phases (about 31%), whereas the bioaccessible fraction of pH-dependent metal cations, like Pb and Zn, was noticeably higher in the stomach (nearly 50%) than in the gastro-intestinal tract (less than 10%). Bioaccessibility assessed by single extraction with 0.43 M HNO3 was overestimated by a factor of 1.2–1.4 relative to that obtained from the BARGE method. Site-specific relative bioavailability (RBA) values of As (27.7%) and Pb (42.6%), predicted from bioaccessibility measurements through linear regression models, had little effect on the overall risk estimates. For the ingestion pathway, the RBA-adjusted cancer risk values (9.7E-05 to 2.0E-04) exceeded the regulatory threshold in all plots, and the hazard index re-calculated after adjustment of oral dose was also above the allowable limit, with values ranging from 2.5 to 4.8. However, no detrimental health effects are expected to occur through inhalation of soil particles in nearby residents.Departamento de Geologí

Phytoaccumulation of trace elements (As, Cd, Co, Cu, Pb, Zn) by Nicotiana glauca and Euphorbia segetalis growing in a Technosol developed on legacy mine wastes (Domingo Rubio wetland, SW Spain)

Sulfidic mine wastes have the potential to generate acid mine drainage (AMD) and release acid leachates containing high levels of iron, sulfate and potentially toxic elements (PTEs). Soils receiving AMD discharges are generally devoid of vegetation. Only a few metal-tolerant plant species can survive under such adverse soil conditions. This work investigates two plant species, Nicotiana glauca and Euphorbia segetalis, that have successfully colonized an AMD-impacted wetland area in south-western Spain. The uptake of PTEs from the soil by roots and their transfer and accumulation in the above-ground biomass were quantified. Results showed that these pioneer plants grew in patches of neutral soil within the wasteland despite the high concentrations of PTEs in the rhizosphere soil (up to: 613 mg kg-1 As, 18.7 mg kg-1 Cd, 6370 mg kg-1 Cu, 2210 mg kg-1 Pb and 5250 mg kg-1 Zn). The target organs of As, Cu and Pb accumulation were: root > leaf > stem in N. glauca, and root > stem > leaf in E. segetalis. Zinc and Cd showed a significant decrease in roots relative to aerial parts of N. glauca, and Co was preferentially partitioned in stems of N. glauca and leaves of E. segetalis. The soil-plant transfer coefficient values of PTEs in all parts of both plants were well below unity with the only exception of Cd in leaves of N. glauca (1.254), suggesting that roots acted as a barrier limiting the uptake of PTEs by plants. Interestingly, under the same soil conditions, N. glauca absorbed Cd in considerable proportions from soil and accumulated it in its leaves, while E. segetalis was not effective in transferring PTEs from roots shoots except for Co. In conclusion, soil pH and plant-related factors greatly influence the stabilization of PTE in the rhizospheric soil and produce inconsistencies in PTE phytoavailability. The findings of this study provide criteria to assist in natural remediation in other legacy contaminated sites worldwide.Funding for open access publishing: Universidad de Sevilla/CBUA. This research was partially supported by the Regional Government of Andalusia (Spain) and the European Regional Development Fund Andalusia 2014–2020 through the Project P-18-TP-3503

Successful plant growth in acid mine drainage-impacted soil using pot-based experiments with waste amendments

This paper addresses the challenge of remediating soil impacted by acid mine drain- age (AMD) using an innovative and sustainable Technosol-based approach to stabi- lize soil and facilitate vegetation recovery. The study assessed the effectiveness of Technosols made from recycled organic (water clarification sludge) and inorganic (siderurgical slags and red gypsum) wastes in mitigating the detrimental effects of AMD on soil properties, pore water chemistry, and plant growth through a 4-month pot experiment. Technosols significantly improved soil health by neutralizing net acidity (296 mmol H+ kg 1), raising pH levels from extremely acidic (3.3) to mildly alkaline (7.7–8.0), and limiting the mobility of potentially toxic elements (PTEs). Dis- solved Cu and Zn concentrations dropped from 80.21 and 72.08 mg L 1, respec- tively, to below 1 mg L 1 by the end of the monitoring period. The experiment identified several concomitant mechanisms of PTE retention, such as decreased dis- solution of metal-bearing minerals, precipitation reactions and adsorption onto Fe and Al (oxy)hydroxides. Aqueous speciation modelling indicated a decline in toxic metal forms (e.g. Al3+, AlSO4+, Cu2+, Zn2+ and H2AsO4 ) in soil pore water after treatment, thus reducing phytotoxicity. Additionally, waste amendments enhanced nutrient availability, with nitrate concentrations reaching up to 417 mg L 1, support- ing seed germination and seedling establishment. The most effective Technosol, com- bining water treatment sludge and white steel slag (60:40 w/w), enabled robust growth of Brassica juncea. Principal component analysis showed a strong correlation between healthy plant responses (survival rate, plant height, leaf number, biomass production) and improved soil pore water parameters (pH, PTEs, aluminium, calcium, bicarbonate and nitrate ions), highlighting the benefits of waste amendments. These findings underscore the potential of waste-derived Technosols in stabilizing AMD- impacted soils and promoting thriving plant growth. However, further validation in field trials with diverse plant species is recommended for real-world applications.This research received financial support from the Regional Govern- ment of Andalusia (Spain) and the European Regional Development Fund Andalusia 2014–2020 under Project P-18-TP-3503, in partner- ship with DSM Soluciones Medioambientales. It was also funded by EU project 101071300 Sustainable Horizons (HORIZON) Horizon Europe Framework Programme- HORIZON-WIDERA-2021-ACCESS- 05. We express our gratitude to Nereida Pascual, Lourdes Vales, and Alicia Rodríguez (DSM, Centro de Nerva) for their invaluable assis- tance in collecting waste samples and providing essential information about the waste materials. Funding for open access charge: Universi- dad de Huelva / CBUADepartamento de Geologí

Unveiling a Technosol-based remediation approach for enhancing plant growth in an iron-rich acidic mine soil from the Rio Tinto Mars analog site

This paper explores the potential of Technosols made from non-hazardous industrial wastes as a sustainable solution for highly acidic iron-rich soils at the Rio Tinto mining site (Spain), a terrestrial Mars analog. These mine soils exhibit extreme acidity (pHH2O = 2.1–3.0), low nutrient availability (non-acid cation saturation < 20 %), and high levels of Pb (3420 mg kg−1), Cu (504 mg kg−1), Zn (415 mg kg−1), and As (319 mg kg−1), hindering plant growth and ecosystem restoration. To address these challenges, the study systematically analyzed selected waste materials, formulated them into Technosols, and conducted a four-month pot trial to evaluate the growth of Brassica juncea under greenhouse conditions. Technosols were tailored by adding varying weight percentages of waste amendments into the mine Technosol, specifically 10 %, 25 %, and 50 %. The waste amendments comprised a blend of organic waste (water clarification sludge, WCS) and inorganic wastes (white steel slag, WSS; and furnace iron slag, FIS). The formulations included: (T0) exclusively mine Technosol (control); (T1) 60 % WCS + 40 % WSS; (T2) 60 % WCS + 40 % FIS; and (T3) 50 % WCS + 16.66 % WSS + 33.33 % FIS. The analyses covered leachate quality, soil pore water chemistry, and plant response (germination and survival rates, plant height, and leaf number). Results revealed a significant reduction in leachable contaminant concentrations, with Pb (26.16 mg kg−1), Zn (4.94 mg kg−1), and Cu (2.29 mg kg−1) dropping to negligible levels and shifting towards less toxic species. These changes improved soil conditions, promoting seed germination and seedling growth. Among the formulations tested, Technosol T1 showed promise in overcoming mine soil limitations, enhancing plant adaptation, buffering against acidification, and stabilizing contaminants through precipitation and adsorption mechanisms. The paper stresses the importance of tailoring waste amendments to specific soil conditions, and highlights the broader implications of the Technosol approach, such as waste valorization, soil stabilization, and insights for Brassica juncea growth in extreme environments, including Martian soil simulants.This research received financial support from the Regional Government of Andalusia (Spain) and the European Regional Development Fund Andalusia 2014–2020 under Project P-18-TP-3503, in partnership with DSM Soluciones Medioambientales. It was also funded by EU project 101071300 Sustainable Horizons (HORIZON). We express our gratitude to Nereida Pascual, Lourdes Vales, and Alicia Rodríguez (DSM, Centro Ambiental de Nerva) for their invaluable assistance in collecting waste samples and providing essential information about the waste materials. Funding for open access charge: Universidad de Huelva / CBUA.Departamento de Geologí