Abstract
Construction and demolition fines (C&D-fines) and green waste compost (GWC) are two commonly generated urban waste materials that represent repositories of geochemical value. Here technosols were produced from volumetric mixtures of these materials ranging from 0–100% C&D-fines, with the remaining proportion comprised of GWC. Agronomic assessment was carried out by way of pot and rhizobox plant growth experiments with ryegrass, barley and pea to determine germination, plant mass and rooting behaviours. Geochemical and mineralogical evaluation was achieved by soil pore water solution measurements combined with X-ray powder diffraction analyses respectively, to characterise the technosols and their distinct deviations from a reference agricultural geogenic soil (soil). The results demonstrated that germination, growth and root mass/surface area of vegetation were up to 80-fold greater after 30-days in the technosol composed of equal volumes of the two materials (50% C&D-fines: 50% GWC) compared to the soil. High concentrations of Ca and Mg in pore waters (550–800 mg·L−1) were dominant features of the technosols, in contrast to the soil (<50 mg·L−1), resulting from gypsum and calcite enrichment of the C&D-fines. In contrast, the GWC represented a source of soluble K (450–1000·mg·L−1). Highly elevated Ca concentrations in extended leaching tests of the C&D-fines reflected ongoing gypsum dissolution, whereas soluble Mg and K were rapidly depleted from the GWC. In summary, short-term performance of the technosols as plant growth substrates was strong despite their geochemical and mineralogical distinction from soil. Gleaning additional geochemical value from combining urban wastes in this way is potentially suited to myriad scenarios where geogenic soils are contaminated, sealed or otherwise absent. Further assessment will now be needed to determine the geochemical longevity of the technosols before wider scale applications can be recommended.
Highlights
In recent years, increasing efforts have been made to repurpose urban wastes to produce fertile soil-like plant growth substrates that can be used for a variety of applications where access to soil is limited, such as where impermeable/sealed urban areas pervade or where topsoil is polluted or denuded [1,2,3,4]
One of the most recent analyses on the topic of constructed technosols stated that such substrates produced from a wide variety of waste materials have a high potential to provide multiple soil functions in urban areas [10], with applications not limited to greening, and including the growth of food crops [11]
The aims of this study were to: (1) manufacture a range of soil-like substrates using solely construction and demolition material (C&D)-fines and green waste compost; (2) assess key mineralogical and geochemical characteristics of these technosols distinguishing them from a reference agricultural geogenic soil; (3) test selected plant performance parameters; (4) and evaluate the results in relation to the use of waste materials to produce soil-like substrates for a variety of applications
Summary
In recent years, increasing efforts have been made to repurpose urban wastes to produce fertile soil-like plant growth substrates that can be used for a variety of applications where access to soil is limited, such as where impermeable/sealed urban areas pervade or where topsoil is polluted or denuded [1,2,3,4]. These types of artificial substrates, whose characteristics and pedogenesis are dominated by their technical origin, are sometimes called technical soils or technosols [5]. An additional advantage of urban applications arises when technosol components can be sourced locally to their end usage, avoiding long-distance transportation of wastes for disposal by other means [12], or similar transportation related costs incurred securing adequate supplies of soil ex-situ
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