ADSORPTION OF ARSENIC BY NATURAL AQUIFER MATERIAL IN THE SAN-ANTONIO EL TRIUNFO MINING AREA, BAJA-CALIFORNIA, MEXICO

Several experiments of arsenic (As) adsorption by aquifer material of the San Antonio-El Triunfo (SA-ET) mining area were conducted to test the feasibility of this material acting as a natural control for As co ncentrations in groundwater. This aquifer material is mineralogically complex, composed of quartz, feldspar, calcite, chlorite, illite, and magnetite/hematite. The total iron content (Fe2O3) in the fine fractio n is similar to 12%, whereas Fe2O3 in the coarse fraction is < 10 wt%. The experimental percent total As adsorbed vs. pH curves obtained mat ch the topology of total As adsorbed onto iron oxi-hydroxides surface (arsenate + arsenite; high adsorption at low pH, low adsorption at hig h pH). A maximum of about 80% adsorbed in the experiments suggests the presence of arsenite in the experimental solutions. The experimental adsorption isotherm at pH 7 indicates saturation of surface sites at h igh solute concentrations. Surface titration of the aquifer material i ndicates a point of zero charge (PZC) for the adsorbent of about 8 to 8.5 (PZC for iron oxyhydroxides = 7.9-8.2). Comparison between experim ental and modeled results (using the MICROQL and MINTEQA2 geochemical modeling and speciation computer programs) suggests that As is being a dsorbed mostly by oxyhydroxides surfaces in the natural environment. B ased on an estimated retardation factor (R), the travel time of the As plume from the SA-ET area to La Pat and Los Planes is about 700 to 50 00 years.