The influence of common groundwater major ions on arsenic (As) retention by native limestones was studied through column experiments. Columns were packed with rock particles (0.5-1.41 mm) and fed with solutions containing As, and chloride (Cl), sulfate, bicarbonate or fluoride (F) in concentrations similar to those measured in one of the most As-rich wells of Mexico. Besides, other solutions were also treated containing multiples or submultiples of those anion concentrations. Physico-chemical parameters, As, and each anion concentrations were determined weekly along 4 months. After the end of the experiments rocks collected from the top of the columns were analyzed by XRF, XRD, and SEM-WDS. Concentrations of As decreased from 1.2 mg/L to values below the Mexican drinking water standard (0.025 mg/L), since the first week in the solutions containing F or Cl keeping low values afterwards, indicating that they do not interfere with As removal. However, although As strongly decreased in the solutions containing sulfate since the first week, it started to increase from the 12th week in the highest concentrated solution. Bicarbonate was the anion affecting most As retention, since, after its decrease below 0.025 mg/L in the 2nd week for both solutions (30 mg/L and 300 mg/L), it started to increase since the 7th week in the most concentrated one, but maintained a low concentration in the least concentrated solution. Saturation index calculations and XRD analyses did not evidence the formation of As minerals. However, SEM elemental maps and XRF analyses showed the presence of As on the rocks after the treatments. Results indicate that As may be retained by sorption. Sulfate and bicarbonate compete with As for sorption sites. Results showed that native limestones are an option for treating As polluted water. Experiments also indicated that bicarbonate and sulfate may interfere with As removal depending on their concentrations.
Keywords: Arsenic; Filtration technique; Groundwater; Interfering anions; Limestone; Mexico; Treatment.
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