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.

Predicting the relative oral bioavailability of naturally occurring As, Cd and Pb from in vitro bioaccessibility measurement: implications for human soil ingestion exposure assessment.

Chestnut soils developed over mineralized areas of southwestern Spain are characterized by high baseline concentrations of geogenic trace elements, notably Pb (up to 14,562 mg kg-1), As (up to 346 mg kg-1) and Cd (up to 319 mg kg-1), which could pose an unacceptable risk to the health of the hand-harvest workers who are being exposed to surface soil by incidental ingestion and dermal contact. Oral bioaccessibility, as determined by simulating the human digestion process in a test-tube environment (Unified BARGE Method), followed the increasing order of As (3.1%) < Pb (21.5%) < Cd (35.6%) in the gastric phase, and As (3.4%) < Pb (4.5%) < Cd (13.2%) in the gastrointestinal extract. Relative bioavailability (RBA) of As (3.1-2.1%), Pb (17.8-17.5%) and Cd (34.4-23.3%), predicted from in vitro bioaccessibility measurement through linear regression models, seems to be influenced not only by the pH and composition of digestive solutions but also by geochemical partitioning of trace elements among the soil constituents. The integration of RBA data in the risk calculations had a considerable effect on the site-specific risk estimations. After RBA adjustment, the level of carcinogenic risk associated with As exposure (< 1.5E-06) and the hazard index for non-carcinogens (< 0.4) was within the regulatory limits, indicating that occupational risks are not of concern. Hence, it can be concluded that the use of a default value of 100% for bioavailability may dramatically overestimate the chronic exposure to geologically sourced trace elements.