Chrysotile asbestos is a soil pollutant in many countries. It is a carcinogenic mineral, partly due to its surface chemistry. In chrysotile, Fe II and Fe III substitute Mg octahedra (Fe[6]), and Fe III substitutes Si tetrahedra (Fe[4]). Fe on fiber surfaces can generate hydroxyl radicals (HO . ) in Fenton reactions, which damage biomolecules. To better understand chrysotile weathering in soils, net Mg and Si dissolution rates over the pH range 3.0–11.5 were determined in the presence and absence of biogenic ligands. Also, HO . generation and Fe bulk speciation of pristine and weathered fibers were examined by EPR and Mössbauer spectroscopy. Dissolution rates were increased by ligands and inversely related to pH with complete inhibition at cement pH (11.5). Surface‐exposed Mg layers readily dissolved at low pH, but only after days at neutral pH. On longer timescales, the slow dissolution of Si layers became rate‐determining. In the absence of ligands, Fe[6] precipitated as Fenton‐inactive Fe phases, whereas Fe[4] (7 % of bulk Fe) remained redox‐active throughout two‐week experiments and at pH 7.5 generated 50±10 % of the HO . yield of Fe[6] at pristine fiber surfaces. Ligand‐promoted dissolution of Fe[4] (and potentially Al[4]) labilized exposed Si layers. This increased Si and Mg dissolution rates and lowered HO . generation to near‐background level. It is concluded that Fe[4] surface species control long‐term HO . generation and dissolution rates of chrysotile at natural soil pH.

Background: Fibrous chrysotile has been the most commonly applied asbestos mineral in a range of technical applications. However, it is toxic and carcinogenic upon inhalation. The chemical reactivity of chrysotile fiber surfaces contributes to its adverse health effects by catalyzing the formation of highly reactive hydroxyl radicals (HO • ) from H 2 O 2 . In this Haber-Weiss cycle, Fe on the fiber surface acts as a catalyst: Fe 3+ decomposes H 2 O 2 to reductants that reduce surface Fe 3+ to Fe 2+ , which is back-oxidized by H 2 O 2 (Fenton-oxidation) to yield HO • . Chrysotile contains three structural Fe species: ferrous and ferric octahedral Fe and ferric tetrahedral Fe (Fe 3+ tet ). Also, external Fe may adsorb or precipitate onto fiber surfaces. The goal of this study was to identify the Fe species on chrysotile surfaces that catalyze H 2 O 2 decomposition and HO • generation. Results:We demonstrate that at the physiological pH 7.4 Fe 3+ tet on chrysotile surfaces substantially contributes to H 2 O 2 decomposition and is the key structural Fe species catalyzing HO • generation. After depleting Fe from fiber surfaces, a remnant fiber-related H 2 O 2 decomposition mode was identified, which may involve magnetite impurities, remnant Fe or substituted redox-active transition metals other than Fe. Fe (hydr)oxide precipitates on chrysotile surfaces also contributed to H 2 O 2 decomposition, but were per mole Fe substantially less efficient than surface Fe 3+ tet . Fe added to chrysotile fibers increased HO • generation only when it became incorporated and tetrahedrally coordinated into vacancy sites in the Si layer. Conclusions:Our results suggest that at the physiological pH 7.4, oxidative stress caused by chrysotile fibers largely results from radicals produced in the Haber-Weiss cycle that is catalyzed by Fe 3+ tet . The catalytic role of Fe 3+ tet in radical generation may also apply to other pathogenic silicates in which Fe 3+ tet is substituted, e.g. quartz, amphiboles and zeolites. However, even if these pathogenic minerals do not contain Fe, our results suggest that the mere presence of vacancy sites may pose a risk, as incorporation of external Fe into a tetrahedral coordination environment can lead to HO • generation.

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Background and aimsGraminaceous plants exude phytosiderophores (PS) for acquiring Fe. Adsorption of PS and its metal complexes to the soil solid phase reduces the FePS solution concentration and hence Fe uptake. In this study we aimed to quantify adsorption, and to determine to what extent adsorption depends on the complexed metal and on soil properties. Furthermore, we examined if adsorption is a reversible process.MethodsAdsorption and desorption of PS and metal-PS complexes were examined in batch experiments in which the PS 2′-deoxymugineic acid (DMA) and its metal-complexes (FeDMA, CuDMA, NiDMA and ZnDMA) interacted with several calcareous soils.ResultsAdsorption of DMA ligand (0–1000 μM) and metal-DMA complexes (0–100 μM) was linear in the concentration range examined. Adsorption varied by a factor ≈2 depending on the complexed metal and by up to a factor 3.5 depending on the soil. Under field-like conditions (50 % water holding capacity), 50–84 % of the DMA was predicted to be retained to the soil solid phase. Alike adsorption, desorption of metal-DMA complexes is fast (approximate equilibrium within 1 hour). However, only a small fraction of the adsorbed FeDMA (28–35 %) could be desorbed.ConclusionsDespite this small fraction, the desorbed FeDMA still exceeded the amount in solution, indicating that desorption of FeDMA from soil reactive compounds can be an important process buffering the solution concentration.Electronic supplementary materialThe online version of this article (doi:10.1007/s11104-016-2800-x) contains supplementary material, which is available to authorized users.