Arsenic Adsorption to N anoparticulate Magnetite in Natural Waters: Batch versus Column-Specific Effects
Farrell, Jesse Walter
Tomson, Mason B.
Doctor of Philosophy
Increasingly, cities in Latin America recognize the importance of drinking water quality on public health. A water assessment of Guanajuato, Mexico, and surrounding areas found arsenic in wells above the Mexican drinking water standard (251lg/1). A collaborative effort was initiated to develop and field test a new arsenic removal method using high surface area sorbents. Nanoscale magnetite, previously shown to effectively adsorb arsenic in batch systems, was packed in sand columns to create a continuous treatment process. Design and operating conditions were assessed in bench-scale columns, and subsequently, a pilot column with 456 g ($2.50 US) of commercially available, food-grade magnetite demonstrated removal of the equivalent arsenic contained in 1,360 liters of Guanajuato groundwater. However, strong interferences were present in natural waters as breakthrough of arsenic in laboratory columns was delayed> 1 Ox with a synthetic feed solution as compared to groundwater at the same pH. Adsorption isotherms conducted with pretreated Guanajuato groundwater helped deduce the species of utmost interference: silica. By the removal or addition of silica, adsorption isotherms confirmed silica's strong effect. Low-level geothermal waters with high silica concentrations are common throughout central Mexico and other parts of the world presenting a major challenge for arsenic adsorbents. Arsenic adsorption improved through pH reduction in batch; however, pilot-scale column experiments showed no improvement with the same treatment. Silica preloading, deep-bed redox processes, and influent water impurities provided plausible explanation for the column observations. Breakthrough was monitored closely in columns sampled from 4 locations along their length. Synthetic solution with silica, in contrast to a baseline without silica, showed decreasing arsenic adsorption with distance through the column, characteristic of pre1oading, and a regression in breakthrough suggested oxidation at the magnetite surface. Calcium has been shown in batch systems to improve arsenic adsorption kinetics but not equilibrium partitioning in the presence of silica. In column experiments, the addition of calcium substantially increased arsenic adsorption in the presence of silica beyond batch model predictions confirming column-specific enhancements. The column-specific effects of silica, calcium, and redox would not be observable from adsorption isotherms but have critical importance to arsenic treatment by nanomagnetite columns.
Civil engineering; Environmental engineering