INVESTIGATION OF ARSENATE ADSORPTION ON ALUMINUM AND IRON NANOPHASES

Arsenic (As) is a highly toxic and carcinogenic metalloid that is tested in many groundwater sources worldwide. Effective remediation of As from contaminated groundwater remains a significant challenge due in part to the redox transformations of the iron oxyhydroxides, the main substrate for As sequestration.

In this research we tested four inexpensive and widely available nanomaterials, namely amorphous aluminum hydroxide (am-AlH), amorphous iron oxyhydroxide (am-FeOx), native kaolinite (NK), and acid-activated kaolinite (AK), for their capacity to adsorb arsenate As(V) from solution. The As(V) stock solution, prepared in deionized (DI) water using Na₂HAsO₄*7H₂O (Alfa Aesar™), was diluted to four As concentrations (5, 1, 0.5, and 0.1 mM). Batch reactors were set up using 500 mg of NK or AK and 50 mg of am-AIH or am-FeOx reacted at room temperature with 20 mL of each As(V) solution for 24, 48, and 168 hours. The solids ware separated from supernatant by filtering and As concentrations were measured using LC-ICP-MS at Sandia National Laboratories.

The capacity to immobilize As varied among solids. The As uptake from 5 mM solution after 168 hrs was up to 30% for am-FeOx, 12% for am-AIH, and ~1.2% for AK and NK. The rate of uptake did also vary among solids, with As uptake by am-FeOx occurred relatively fast and did not changed while am-AIH was the only material that exhibited increased adsorptive capacity for the duration of these experiments. Both AK to NK exhibited variable rates of As immobilization. In case of the Al-bearing phases, our data points to the likely presence of multiple mechanisms for As immobilization, including adsorption and co-precipitation. These results are encouraging and suggest that Al-based nanomaterials may be a viable alternative to As sequestration from contaminated water. In contrast to Fe oxyhydroxides, Al compounds do not undergo redox transformation, and thus could provide a long-term solution to As remediation from contaminated waters.

Additional ongoing experiments will test the capacity of the same nanomaterials to immobilize As(V) over a wide range of pH (2-10) and ionic strength conditions. Together this data results will be used in predicting the fate and transport of As(V) in the environment and possibly finding stable, long-term efficient adsorbents for As from geofluids.