Arsenic sorption by carbonate-rich aquifer material, a control on arsenic mobility at Zimapan, Mexico

Arsenic retention by carbonate-rich aquifer material at Zimapan, Mexico, was studied to gain insight into the processes controlling arsenic mobility in natural systems. Batch experiments showed that retention of soluble As (V) on carbonate-rich aquifer material was 35.3-90% in the pH range of 7-9 found in Zimapan natural water. Chemical and mineralogical compositions, point of zero charge (PZC), and experimental arsenic retention at various pH of three composite samples of limestone (M1, M2, M3) showed that sorption (adsorption and coprecipitation) may be one of the main processes controlling arsenic mobility in the Zimapan As-polluted aquifer. The PZC values approximately corresponded to the PZC reported for the main minerals present in each sample: hydrous ferric oxides (M1), calcite (M2), and the range from hydrous ferric oxides and calcite (M3). The chemical and mineralogical compositions of each sample explain the obtained PZC values. Experimental and modeled arsenic retention at various pH values on sample M1 corresponded to reported arsenic adsorption onto hydrous ferric oxides. Coprecipitacion of complex Ca arsenates or arsenic adsorption onto calcite or clay minerals could be the main processes of arsenic retention on samples M2 and M3. Groundwater flow through the granular, carbonate-rich, shallow aquifer may decrease the water As content as a result of these interactions. A remediation method based on the promotion of polluted water flow into the shallow aquifer could be developed from these results.