Arsenic Adsorption to N anoparticulate Magnetitein Natural Waters:Batch versus Column-Specific Effects

[摘要] Increasingly, cities in Latin America recognize the importance of drinking waterquality on public health. A water assessment of Guanajuato, Mexico, and surroundingareas found arsenic in wells above the Mexican drinking water standard (251lg/1). Acollaborative effort was initiated to develop and field test a new arsenic removal methodusing high surface area sorbents. Nanoscale magnetite, previously shown to effectivelyadsorb arsenic in batch systems, was packed in sand columns to create a continuoustreatment process. Design and operating conditions were assessed in bench-scalecolumns, and subsequently, a pilot column with 456 g ($2.50 US) of commerciallyavailable, food-grade magnetite demonstrated removal of the equivalent arseniccontained in 1,360 liters of Guanajuato groundwater.However, strong interferences were present in natural waters as breakthrough ofarsenic in laboratory columns was delayed> 1 Ox with a synthetic feed solution ascompared to groundwater at the same pH. Adsorption isotherms conducted withpretreated Guanajuato groundwater helped deduce the species of utmost interference:silica. By the removal or addition of silica, adsorption isotherms confirmed silica'sstrong effect. Low-level geothermal waters with high silica concentrations are commonthroughout central Mexico and other parts of the world presenting a major challenge forarsenic adsorbents. Arsenic adsorption improved through pH reduction in batch;however, pilot-scale column experiments showed no improvement with the sametreatment. Silica preloading, deep-bed redox processes, and influent water impuritiesprovided plausible explanation for the column observations. Breakthrough wasmonitored closely in columns sampled from 4 locations along their length. Syntheticsolution with silica, in contrast to a baseline without silica, showed decreasing arsenicadsorption with distance through the column, characteristic of pre1oading, and aregression in breakthrough suggested oxidation at the magnetite surface. Calcium hasbeen shown in batch systems to improve arsenic adsorption kinetics but not equilibriumpartitioning in the presence of silica. In column experiments, the addition of calciumsubstantially increased arsenic adsorption in the presence of silica beyond batch modelpredictions confirming column-specific enhancements. The column-specific effects ofsilica, calcium, and redox would not be observable from adsorption isotherms but havecritical importance to arsenic treatment by nanomagnetite columns.

[发布机构] Rice University

[效力级别] engineering