Technosol genesis: identification of organo-mineral associations in a young Technosol derived from coking plant waste materials

Background, aim and scope Technosols form a new soil group in the World Reference Base for Soil Resources (WRB) (FAO, World Soil Resour Rep 132:145, 2006) mainly characterised by anthropogenic parent material of organic and mineral nature and which origin can be either natural or technogenic. The increasing number of sites affected by technogenic materials and their impact on the environment as growing media for plants or as source of pollutants requires an understanding of their functioning and evolution. Among the parameters describing soil functions, the structure is a key property, encompassing physical, chemical and biological soil processes, which is not yet usually studied in Technosols. The presence of high levels of complex technogenic materials and pollutants (polycyclic aromatic hydrocarbons [PAHs] and trace elements) is likely to determine aggregation and induce a specific soil structure. This work was undertaken to test this hypothesis and assess whether technogenic material would contribute to the formation of water-stable aggregates in Technosols. Materials and methods The hypothesis was tested using material collected from a former coking plant site. Samples were submitted to water-granulodensimetric fractionation to quantify the water-stable organo-mineral associations in five fractions. Each fraction was analysed for C, PAHs and trace elements. To assess whether specific organo-mineral associations or particles were present in the material, granulometric fractions were observed at a photonic scale, and the main constituents were morphologically and analytically characterised at an electronic scale by scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDX). The 0-2 µm fraction was also analysed by transmission electron microscopy (TEM)/EDX. Results The comparison of the weight distribution of water-stable associations with the texture revealed the presence of water-stable organo-mineral associations in the material. These associations were mostly in the coarse fractions (50-2,000 µm, corresponding to 74% of the total material) and resulted from the association of finer particles (<50 µm). Organic C was mainly of technogenic origin, i.e. hydrocarbons, and it was not distributed homogeneously among the fractions: the coarser fractions (50-2,000 µm) contained 80% of the total organic C and had the highest C/N ratio. The remaining 20% was primarily associated with the 2-20 µm fraction and exhibited a low C/N ratio. PAHs represented 2% of the total soil C and were associated mostly with the 2-20 and 200-2,000 µm fractions. The SEM/EDX analyses showed five main classes of particles: quartz particles, Fe oxide beads, Fe and Al oxides, anthropogenic organic matter mainly associated with S and slag (complex material). The three last classes were mainly present in the 2-20 and 20-50 µm fractions. Gypsum was the main constituent of the 0-2 µm fraction and only a few aluminosilicates (of the montmorillonite group) were observed. Fe, Si and Ca were quantitatively the three major elements but were differently distributed among the fractions. The 200 µm-2 mm fraction presented the highest amounts of trace elements (Cr, Zn, Ba, Pb), with Zn and Pb also being present in the 2-20 µm fraction. Discussion This work showed specific water-stable organo-mineral associations in the coking plant material, resulting from both the former industrial activities (i.e. slag) and a neo-aggregation phenomenon involving technogenic organic matter. Characterisation of the constituent elements allowed one to make analogies between the technic material and soils and, thus, to specify the constitution and the fate of organo-mineral associations in such material. Organic matter of industrial origin, i.e. hydrocarbons, was probably involved in these associations, as shown by the high C/N ratio of the coarse fractions and confirmed by the presence of PAHs. Also, uncommon particles were identified, i.e. Fe oxide beads and complex mineral material such as slag residues, resulting from industrial processes at high temperature. Elements like Ca and Fe were recovered at strong concentrations. Fe was present in stable compounds (e.g. Fe oxide beads), but Ca was associated with S and more easily dissolved as observed during the water fractionation process. The fate of the high levels of Fe and Ca present in this soil was probably different to that of natural soils. Other elements, i.e. heavy metals, were present at insufficient rates to act as aggregation factors. However, the material structure would control their localization and availability in the soil particles. Conclusions This work showed specific organo-mineral associations in the coking plant material, resulting from both the former industrial coking activity and a neo-aggregation phenomenon involving PAHs. Technic C played a primary role in the aggregation process in the near absence of humic compounds and clay minerals. The approach used allowed one to characterise the material by specifying and localising the primary organic and mineral constituents within the granulometric fractions and the distribution of PAHs and trace elements. Hence, standard methods are proved to be useful to characterise Technosols, allowing the identification of new types of associations. Recommendations and outlook Most Technosols are deeply affected by technic materials which control the main processes including aggregation. It is expected that all associations and particles identified in this work confer a specific function to the Technosol, making their characterisation relevant. The evolution of the newly formed associations should be thoroughly studied as they contain several pollutants, particularly PAHs, in higher concentrations than in the bulk soil, in order to assess environmental risk of contaminant release. Thus, monitoring of aggregation will be useful to specify the capacity of Technosols to fulfil certain functions as growing media in the case of urban planning or site remediation.