Modeling of manganese removal from groundwater by filteration through manganese oxide coated media

Abstract

High intake of manganese has shown to be toxic with adverse health impacts, and therefore, WHO (2004) recommended a health-based guideline value of 0.4 mg/l for drinking water. WHO (2011) eliminated the health-based guideline value noting that this value is well above the concentrations of Mn normally found in drinking-water. However, well water Mn concentration in many regions in Bangladesh exceeds 0.4 mg/l. Bangladesh, therefore, needs a low cost and efficient water treatment technology to remove Mn from groundwater. Adsorption of soluble manganese onto manganese oxide coated sand is one of the effective treatment methods for removal of Mn from drinking water. This research work focused on understanding the mechanism of Mn removal from water during filtration through Mn oxide coated media, and developing a model to simulate adsorption and surface oxidation of Mn during filtration. Laboratory column experiments were carried out to assess the Mn removal by Mn-oxide coated filter media (commercially available green sand) under various water quality (e.g., different pH, initial Mn, Dissolved Oxygen, Bicarbonate) and system (e.g., flow rate) conditions. Experimental results suggest that in the absence of Bicarbonate in the influent water, the removal of Mn is characterized by only adsorption, and the system gradually approaches the breakthrough point since the adsorption sites on the media are exhausted with increasing filter run time. In the absence of Bicarbonate, effluent Mn concentration increased gradually from 13% to nearly 71% of influent Mn concentration (10.2 mg/l) during filter run time of up to 350 minutes. However, in the presence of Bicarbonate (200 mg/l) in the influent water, Mn removal efficiency of Mn-oxide coated media was found to increase significantly and the system did not approach the breakthrough point; effluent Mn concentration stabilizes at nearly 38% of influent Mn concentration. Results of laboratory column experiments suggest that the dominant mechanism for the removal of Mn(II) is continuous regeneration of Mn-oxide coated media, caused by the surface mediated oxidation of adsorbed Mn by DO in the presence of Bicarbonate, rather than MnCO3(s) precipitation. Results of multi-port column experiments also support the concept of regeneration of Mn-oxide coated media. Results of multi-port column experiments provided useful insights on the effects of flow rate, initial Mn concentration and pH on the removal of Mn within the filter media. Mn removal has been found to increase with decreasing flow rate (due to higher contact time), with increase in initial Mn concentration of the influent water (due to increase in linear driving force), and with increase in pH value of influent water (which promotes Mn oxidation). Under the experimental conditions, maximum removal efficiency of 98% was found for an initial Mn concentration of 4.2 mg/l at flow rate of 1 ml/min.cm2. A model developed by Zuravnsky (2006) was modified to predict the soluble Mn removal via adsorption and surface oxidation onto Mn-oxide coated media under continuous media regeneration by DO. A number of model parameters were estimated from laboratory batch experiments and from empirical formulations. The model was calibrated (to find out the value of oxidation rate constant, kr by curve fitting) using experimental data under various operating conditions (flow rate of 1-3 ml/min.cm2, initial Mn concentration of 1.15-4.83 mg/l, and pH of 6-8 of influent water), and subsequently used to predict soluble Mn removal. The model was able to predict Mn removal reasonably well. The sensitivity analysis suggests that flow rate, Freundlich isotherm constants (K, n) and kr have a significant effect on the model predicted bulk-water Mn concentration profile, while the effects of axial dispersion coefficient (DL), mass transfer coefficient (kf) and specific surface area of media (Av) are not significant.