Controlling Arsenic Contamination Through Bentonite Clays: a Batch Sorption Study

Arsenic (As) contamination of the bodies of water, especially drinking water sources, is a crucial environmental issue with direct implications on human health. In developing countries such as Pakistan, many inhabitants are exposed to contaminated drinking water. For several decades, there has been a search for cost-effective and efficient methods to treat As-contaminated water. Bentonite clays are a cheap potential candidate that could be effective in decontaminating As-polluted drinking water on an industrial scale. Four bentonite deposits in Khyber Pakhtunkhwa, Pakistan, were investigated in detail for their mineralogical composition and potential use as As adsorbents. It was hypothesized that smectite-rich local bentonite clays are effective in decontaminating As-contaminated drinking water. Thus, the specific objective was to evaluate the potential of indigenous bentonite clay deposits in Khyber Pakhtunkhwa for As decontamination and adsorption mechanism. Bentonite deposits in Khyber Pakhtunkhwa Province from Nowshera, Spin Kana, Spin Khak, and Peshawar were sampled. Along with other physicochemical properties, the detailed mineral composition of the clays was determined through X-ray diffraction analysis, total elemental composition through X-ray fluorescence analysis, and surface area through BET. The As adsorption capacity was determined through a batch sorption experiment by equilibrating uniform clay mass with different concentrations of As. Adsorption data were fitted to Langmuir, Freundlich, Dubinin–Radushkevich, and Temkin equations for adsorption parameters. Results showed that Nowshera bentonite was fine in texture, while Peshawar bentonite was rich in quartz (coarse texture) and cation exchange capacity (CEC), and specific surface area was greater for Nowshera bentonite followed by Spin Kana, Spin Khak, and Peshawar clays. Smectite occurred as the dominant mineral in Nowshera bentonite, while mica, quartz, feldspar, and calcite were the major mineral impurities in all the bentonites. The Nowshera bentonite rich in smectite adsorbed the maximum As (1040 mg kg−1) from the solution with greater affinity. Freundlich equation had a better fit compared to other adsorption equations indicating chemisorption of As on heterogeneous surfaces. It can be inferred from the study that Nowshera bentonite has the greatest As adsorption capacity and adsorption affinity. Arsenic sorption on smectite clays was mainly through chemisorption; bentonites with greater mineral impurities had lower As adsorption potential. The clay quarry of Nowshera could be utilized for the effective purification of As-polluted drinking water.