Title : Brownfield remediation with phosphates: A nature-based and circular economy approach - a case study from Central Italy
Abstract:
Soil contamination by heavy metals (HMs) [or potential toxic elements (PTEs)] poses serious risks to ecosystems and human health. Moreover, cleaning up a brownfield is a hard task because HMs are non-degradable, persistent, and often difficult to remove. Heavy metal inputs arise from anthropogenic activities—mining, smelting, industrial manufacturing, fossil fuel combustion, waste disposal, and certain agricultural practices—as well as natural processes such as rock weathering, volcanic emissions, and wildfire deposition. Metals persist in the environment and can reach groundwater and freshwater as part of the food-chain. In soils, anthropogenic inputs dominate over geogenic sources. Metal mobility is strongly controlled by factors such as pH, mineralogy, and erosion processes that transport metal-bearing clay fractions. Wind and water can transport soil, mainly clay particles that can usually bind contaminants such as HMs. Using waste material is a tool suggested from the circular economy, so waste becomes a valuable resource. This study evaluates the immobilization efficiency of several heavy metals (Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn) using phosphate amendments—synthetic hydroxyapatite, phosphatic rock from Florida and Morocco—applied to a brownfield site in Central Italy. Soil samples were taken in the surroundings of the “Rieti basin” (Central Italy) where the factory “Nuova Rayon S.p.A.” (Rieti, Italy) and some plots of “Immobiliare SNIA S.r.l.” (Milan, Italy).
Heavy metal immobilization followed a two-step mechanism: first rapid surface complexation and secondly partial dissolution of hydroxyapatite and ion exchange with Ca, leading to the precipitation of metal-substituted hydroxyapatite phases. Synthetic hydroxyapatite generally shows the best efficiency, whereas phosphatic rocks were less effective but still provided a measurable immobilization. From a circular economy perspective, however, phosphatic rocks remain attractive due to their lower cost, availability, and waste-valorization potential. The study aims to (1) characterize how phosphate source properties—particularly particle size distribution and mineralogy—influence dissolution behaviour and P availability in representative soils; (2) quantify the resulting pathways of HMs immobilization, including precipitation versus sorption mechanisms; and (3) assess the persistence of immobilization under variable pH and redox regimes relevant to real-world sites.
