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Natural occurrence of nanocrystalline Al-hydroxysulfates: Insights on formation, Al solubility control and As retention

  • Nanocrystalline basaluminite [Al4OH10(SO4)(H2O)3–5] and Aggregation of the e-Keggin polyoxocation [Al12(AlO4)(OH)24(H2O)12]7+, referred to as Al13, have both been described to form in acid mine Drainage environments. Although the chemical composition is quite similar, their crystalline varieties significantly differ, demonstrating that various types of Al-hydroxysulfates can form under similar conditions and that their respective formation is not fully understood yet. Here, we report the occurrence of nanocrystalline precipitates that form naturally in a small alpine catchment in Switzerland where an acidic mountainous stream (pH 4) is neutralized successively after mixing with several neutral tributaries. The stepwise neutralization in conjunction with the large amount of precipitates provide an ideal setting for obtaining new insights into (i) the structure of naturally forming Al-hydroxysulfates, (ii) their formation mechanism, (iii) their role in controlling the solubility of Al,Nanocrystalline basaluminite [Al4OH10(SO4)(H2O)3–5] and Aggregation of the e-Keggin polyoxocation [Al12(AlO4)(OH)24(H2O)12]7+, referred to as Al13, have both been described to form in acid mine Drainage environments. Although the chemical composition is quite similar, their crystalline varieties significantly differ, demonstrating that various types of Al-hydroxysulfates can form under similar conditions and that their respective formation is not fully understood yet. Here, we report the occurrence of nanocrystalline precipitates that form naturally in a small alpine catchment in Switzerland where an acidic mountainous stream (pH 4) is neutralized successively after mixing with several neutral tributaries. The stepwise neutralization in conjunction with the large amount of precipitates provide an ideal setting for obtaining new insights into (i) the structure of naturally forming Al-hydroxysulfates, (ii) their formation mechanism, (iii) their role in controlling the solubility of Al, and (iv) their ability to lower the mobility of As. Synchrotron-based high-energy X-ray diffraction and subsequent pair distribution function analyses demonstrate that these precipitates are structurally identical to basaluminite samples obtained from acid mine drainage sites. In contrast, only minor amounts of tetrahedrally coordinated Al, as present in Al13, were identified by nuclear magnetic resonance spectroscopy. The precipitates are further characterized by elevated As concentrations up to 600 lg/g, whereas other heavy metals are at background concentrations only. Given the low As concentrations in the stream from which precipitation occurs (<0.03 mg/L), high As concentrations confirm that basaluminite serves as a highly efficient As sink, which is attributed to its high anion-exchange capacity. Chemical analysis of streamwater samples in combination with geochemical modeling show that precipitation occurs instantaneously upon mixing with neutral streams. Moreover, our data reveal that the precipitation of basaluminite exerts a strong solubility control on dissolved Al concentrations as manifested by the quasi-constant basaluminite ion activity product observed during neutralization from pH 5 to pH 5.9. We hypothesize that in our field system, high fluoride and sulfate concentrations on the order of 100 and 1–2 mg/L,zeige mehrzeige weniger

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Metadaten
Autor*innen:C. Wanner, R. Poethig, S. Carrero, A. Fernandez-Martinez, Christian Jäger, G. Furrer
Dokumenttyp:Zeitschriftenartikel
Veröffentlichungsform:Verlagsliteratur
Sprache:Englisch
Titel des übergeordneten Werkes (Englisch):Geochimica et Cosmochimica Acta
Jahr der Erstveröffentlichung:2018
Organisationseinheit der BAM:6 Materialchemie
6 Materialchemie / 6.3 Strukturanalytik
Verlag:Elsevier Ltd.
Jahrgang/Band:238
Erste Seite:252
Letzte Seite:269
DDC-Klassifikation:Naturwissenschaften und Mathematik / Chemie / Analytische Chemie
Freie Schlagwörter:Acid rock drainage; Al13; Aluminum; Anion exchange; Arsenic retention
Basaluminite
Themenfelder/Aktivitätsfelder der BAM:Chemie und Prozesstechnik
DOI:10.1016/j.gca.2018.06.031
ISSN:0016-7037
ISSN:1872-9533
Verfügbarkeit des Dokuments:Datei im Netzwerk der BAM verfügbar ("Closed Access")
Datum der Freischaltung:08.10.2018
Referierte Publikation:Ja
Datum der Eintragung als referierte Publikation:05.10.2018
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