Epsilon Archive for Student Projects

Abstract

In-situ injection of zerovalent Iron (ZVI) particles to remediate contaminated groundwater is a widely applied approach. However, studies assessing the remediation and immobilisation mechanisms of Arsenic (As) under anoxic field conditions with zerovalent iron particles are lacking, especially for nanosized sulphidated ZVI (S-nZVI) and oxidic shell free ZVI (OSF-nZVI). This study aimed to assess the suitability of nanosized S-nZVI and OSF-nZVI sorbents for the remediation of an anoxic aquifer contaminated with As from chromated copper arsenate (CCA) wood preservative solution and compare them with microsized S-mZVI and mZVI sorbents. Batch experiments were conducted under anoxic conditions by mixing sorbents with contaminated sediment and artificial groundwater, mimicking the conditions in the natural aquifer. Sorbent performance was assessed at uncontrolled pH and at pH acidified to field pH. Fractionation of As in the solid phase was assessed by extraction with phosphate and oxalate and by microwave-assisted aqua regia digestion of the samples. Based on this, three fractions were defined, namely specifically sorbed As, As associated with amorphous Fe-(hydr)oxides and residual As. Results showed As removal efficiencies of all four ZVI sorbents of at least 85 % at uncontrolled and acidified pH. All four ZVI sorbents showed resistance towards As release at acidification despite increased Fe dissolution. Fractionation results at uncontrolled pH showed a dominating As immobilisation mechanism for ZVI sorbents by formation of amorphous Fe-As-coprecipitates. In the S-mZVI treatment, formation of additional As-S compounds was indicated at uncontrolled pH. Acidification did not affect As immobilisation in the nanosorbent treatments. In the microsorbent treatment, As immobilisation changed upon acidification from coprecipitation towards specifically sorbed As and formation of recalcitrant As-Fe (mZVI) or As-S compounds (S-mZVI). All four tested ZVI sorbents were potentially suitable for the remediation of the contaminated anoxic aquifer. S-mZVI was assessed as the most suitable of the four ZVI sorbent due to the highest As removal efficiency and the highest As immobilisation strength.