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REMOVAL OF HIGH-LEVEL ARSENIC BY ZERO-VALENT IRON
Citation:
Lien, H. AND R T. Wilkin*. REMOVAL OF HIGH-LEVEL ARSENIC BY ZERO-VALENT IRON. Presented at First Across Strait Conf. on the Soil & Groundwater Remediation, Taipei, Tawwan, 09/4-5/2002.
Description:
The objectives of this study were to conduct batch and column studies to (i) assess the effectiveness of zero-valent iron for arsenic remediation in groundwater, (ii) determine removal mechanisms of arsenic, and (iii) evaluate implications of these processes with regard to the stability of arsenic and long-term remedial performance of the permeable reactive barrier (PRB) technology. A high concentration arsenic solution (50 mg/L) was prepared by using sodium arsenite (arsenic (III)) to simulate groundwater at a heavily contaminated Superfund site in the USA. Simulated groundwater also contained a high concentration of sulfate (1000 mg/L). Batch studies indicate that the removal of arsenic is a two-step reaction with fast initial disappearance of arsenite followed by a slow subsequent removal. Kinetic analysis suggested that arsenic removal behaves as a zero-order reaction at high arsenic concentrations while first-order kinetics were followed at low concentrations. Flow-through columns were conducted at a flow rate of 20 mL/hr under reducing conditions for seven months (approximately 400 pore volumes). The results further supported the zero-order kinetics for the removal of arsenic at high concentrations. Arsenic breakthrough was observed in the column study. Arsenic removal capacity of zero-valent iron was determined to be approximately 6.73 mg-As/g-Fe. However, an increase in removal capacity was found across the column, which implied the corrosion of iron leads to the continuous generation of new sites for arsenic adsorption. The arsenic removal mechanisms may involve both adsorption and precipitation. A carbonate green rust identified from the analysis of surface precipitates suggested that arsenite uptake by green rust may be a major mechanism responsible for arsenic remediation by zero-valent iron.