Arsenic Removal by Nanoscale Magnetite in Guanajuato, Mexico

Abstract

Increasingly, cities in Latin America are recognizing the importance of drinking water quality on public health. A water assessment of Guanajuato, Mexico, and surrounding areas indicated naturally occurring arsenic in some wells above the Mexican drinking water standard of 25 μg/L and the World Health Organization recommendation of 10 μg/L. This initiated a collaborative effort with the city to evaluate a new arsenic removal method using high surface area magnetite sorbents. Nanoscale (20 nm) magnetite particles, previously shown to effectively adsorb arsenic in batch systems, were packed in sand columns to create a continuous treatment process. Design and operating variables were evaluated to confirm that magnetite-to-sand ratio and residence time most significantly affected arsenic breakthrough profiles. Subsequently, a pilot column with 456 g (ca. $2.50 USD) of a commercially available, food-grade magnetite (98 nm effective particle diameter) from a pigment manufacturer demonstrated removal of the equivalent arsenic contained in 1,360 L of Guanajuato groundwater. Although pH reduction dramatically improved arsenic adsorption in batch isotherms, no improvement in arsenic removal efficiency was observed when applied to pilot-scale, field columns in Guanajuato. Interference effects (e.g., from background silica) and changes to surface species over time may impact adsorption differently in column versus batch systems. Overall, this work represents one of the first pilot studies of a nanotechnology-enabled water treatment system, and it demonstrates the potential and additional challenges for taking nanoscale magnetite or other highly researched nanomaterials into a complex full-scale setting.