Fractionation and batch desorption experiments to understand the co-occurrence of arsenic and fluoride in Punjab, Pakistan

This study investigates the mechanisms governing the co-occurrence of arsenic (As) and fluoride (F−) in groundwater of Punjab, Pakistan. Three sites with contrasting groundwater chemistry were selected based on WHO (10 µg/L) and Pak. NEQs (50 µg/L) limits for arsenic and WHO and Pak. NEQs (1.5 mg/L) limits for fluoride in drinking water: Kotla Deva Maal (high As, low F−), Chah Fateh Walla (low As, high F−), and Kot Mega (high As and F−). Sediment analyses included physicochemical characterization, particle size distribution, X-ray diffraction (XRD), sequential extraction, and batch leaching experiments. Saturation indices were also calculated for major As and F− bearing minerals. Results showed that high arsenic concentrations were primarily associated with fine-grained sediments. XRD identified quartz as the dominant phase, followed by phyllosilicates and feldspars. Clay-rich phyllosilicates were more abundant in As-contaminated sites, while micas (chlorite, muscovite, biotite) dominated F− contaminated sites. Minor iron oxides and fluorite were detected in the As and F−contaminated sites but not at the co-contaminated site, suggesting their presence mainly in amorphous or poorly crystalline forms. Saturation indices indicated dissolution of As-bearing minerals across all sites, while F− behavior varied with calcite, fluorite, and fluorapatite equilibria. Sequential extraction revealed As was mainly associated with crystalline metal oxides, whereas batch leaching showed maximum As release at alkaline pH and maximum F− release at acidic pH. Overall, the findings suggest that As mobilization was driven by alkali desorption and reductive dissolution of hydroxides, with additional contributions from phyllosilicate through alkaline desorption or dissolution under acidic rainwater infiltration. Fluoride release was controlled primarily by dissolution of fluorite, fluorapatite, and micas, alongside secondary desorption via bicarbonates competition from phyllosilicates and other adsorptive sites. These mechanisms highlight the complex geochemical controls on As and F− co-occurrence in Punjab aquifers.