Studies of Iron Removal in Drinking Water Using Limestone as Adsorbent

Abstract

The study assessed the suitability of two limestone samples (EKLD01 and EKLR02) for use as adsorbent to remove excess iron in drinking water through geochemical and mineralogical characterization using an ion beam linear accelerator and petrographic thin sectioning respectively. The radiological safety (activity concentration) of the limestone was evaluated through measurement of the Naturally-Occurring Radioactive Materials (NORMs) using a High-Purity Germanium (HPGe) -ray Semi-conductor Detector. In addition, the study monitored the iron adsorption efficiency (sorption capacity and percentage adsorption) of limestone with respect to Sample Size, Adsorbent Dose, Residence Time and pH. Agreement between the adsorption isotherms obtained from this study and model adsorption isotherms proposed by Langmuir (1916) and Freundlich (1909) assessed. The iron removal technique was developed using limestone grain sizes (500-1000 μm, 1000-2000 μm and 2000-6350 μm) through Batch Adsorption experiments using Iron standard solutions, followed by Column Adsorption experiments using iron rich/contaminated ground water from the Ashongman Estates, in the Ga East Municipal of the Greater Accra Region of Ghana. The geochemical and mineral characterization indicated that Sample EKLR02 was the most suitable limestone sample for the development of technique based on its relative high calcium content (56.14%). Radiologically, both limestone samples were safe as their calculated mean annual effective dose (0.2 mSv/yr for both samples) did not exceed the UNSCEAR (2000) recommended value of 0.4 mSv/yr. In the Batch experiment, the extent of iron adsorption with respect to time, pH, and dosage of limestone (20-100 g in 200 mL of iron standard solution) were studied. Maximum adsorption was attained at pH 12, limestone dosage of 100g/200 mL, time of 60 minutes, and particle size of 500-1000 μm. Consequently, these were adopted and used for the Column experiments. University of Ghana http://ugspace.ug.edu.gh vi In the Column experiments, optimal conditions obtained during the Batch experiments were adapted and used to analyze fourteen (14) water samples from boreholes and hand-dug wells from the Ashongman Estates. The measured iron concentrations of the sampled water from Ashongman Estates ranged from 0.20 to 4.66 mg/L prior to the Column experiment. At the end of the Column experiment, the measured iron concentrations ranged from 0.19 to 0.28 mg/L [below the World Health Organization (WHO) recommended level of 0.3 mg/L]. There was linear agreement (RL =1) between the adsorption data obtained from the study with Langmuir’s Model Adsorption Isotherm (1916) at iron concentrations of 1 mg/L and above (in the ground water samples). The adsorption data obtained from the study did not correlate well (R2 = 0.0138) with the Freundlich model adsorption isotherm (1909); and the Freundlich constant, Kf , was found to be negative -5.2367 instead of a positive value.