Bicarbonate concentrations affect arsenic release from arsenopyrite and nanoscale iron(iii) (hydr)oxide formation: importance of unconfined aquifer carbonate chemistry

Abstract

Managed aquifer recharge (MAR) is an important engineering solution for achieving sustainable groundwater management. Unfortunately, if not operated properly, MAR can cause undesirable arsenic mobilization in groundwater. To avoid unexpected arsenic mobilization, we need a better understanding of the evolving water chemistry and nanoscale mineral-water interfaces in MAR systems. Bicarbonate is a ubiquitous groundwater component, but its effect on arsenic mobilization in MAR is not fully understood. Hence, we examined the effects of bicarbonate concentrations (0.01 mM, 0.1 mM, 1.0 mM, and 10 mM) on the dissolution of arsenopyrite and the nanoscale secondary mineral formation in both open systems (mimicking shallow unconfined aquifers) and closed systems (mimicking deep confined aquifers) over 7 days. In the open system, owing to pH evolution and the subsequent formation and growth of iron(iii) (hydr)oxide nanoparticles, the arsenic mobilization decreased with increasing bicarbonate concentrations. However, the increase from 1.0 to 10 mM formed surface complexation and aqueous arseno-carbonate complexes and did not further reduce the arsenic mobilization. In the closed system, arsenic mobilization and iron(iii) hydr(oxide) nanoparticle formation were similar for all conditions. This study highlights bicarbonate-controlled nanoparticle formation and arsenic mobilization in MAR systems, providing valuable insights for enabling safer and more sustainable MAR operations.