Removal of manganese and arsenic from groundwater using manganese oxide coated sand

Abstract

Manganese (Mn) and arsenic (As) are common natural groundwater contaminants in Bangladesh. WHO (2004) recommended health-based drinking water guideline values of 0.4 mg/l and 0.01 mg/l for Mn and As, respectively. WHO (2011) eliminated the health-based guideline value for Mn citing that this value (i.e. 0.4 mg/l) is well above concentrations of Mn normally found in drinking water. Obviously this logic is not valid for Bangladesh, since well water Mn concentration in many regions of Bangladesh exceeds 0.4 mg/l by a large margin. Bangladesh therefore needs a low cost and efficient water treatment technology to remove Mn (as well as As) from groundwater. Removal of dissolved Mn by filtration through granular media (sand) could be an effective treatment method for Mn removal from drinking water. This research work focused on understanding the mechanism of Mn(II) oxidation/removal from water during filtration through granular media (sand), and evaluation of the effectiveness of different Mn-oxide coated media (synthetic/prepared media, “green sand”) in removing Mn and As from water. The oxidation/ removal of Mn(II) in granular media (sand) was carefully evaluated through controlled laboratory experiments. The removal of Mn(II) during filtration through a granular media (sand) has been found to involve adsorption of Mn(II) onto filter media, followed by oxidation of adsorbed Mn(II) in the presence of bicarbonate, creating/ regenerating Mn-oxide coating, thus increasing Mn removal capacity of the media with increasing filter run time. The removal/oxidation of Mn(II) is favored at higher pH values and lower flow rates. Thus, removal of Mn(II) during filtration through granular media (sand) could be achieved without the addition of any oxidizing agent [for oxidation of Mn(II) to Mn(IV)], provided the water contains bicarbonate ion. Laboratory investigations have been carried out to assess removal of As and Mn in three different types of Mn-oxide coated media: (a) “prepared” Mn-oxide coated filter media (prepared by passing Mn-bearing water through filter columns made up of Sylhet sand), (b) “synthetic” Mn-oxide coated media (prepared by treating Sylhet sand with MnNO3, H2O2 and NaOH), and (c) commercially available Mn-bearing “green sand”. The average Mn content of the “prepared” filter media was about 7,550 mg/kg; average Mn contents of “synthetic” media and “greensand” were 25,250 mg/l and 14,400 mg/kg, respectively. The three types of Mn-oxide coated filter media have been found to be very effective in removing dissolved Mn from groundwater over a wide range of initial Mn concentration and pH. Results of both column and batch experiments showed that Mn removal efficiency increases with increasing Mn-content of the filter media. Mn removal efficiency also increases with increasing Mn in the influent water and increasing pH (in column experiments). Synthetic Mn-coated media, with the highest Mn content among the 3 media, performed best in removing Mn at higher flow rates. However, the other media (“prepared” media and green sand) also performed well at slower flow rates. A contact time of about 1.5 minutes was found to be sufficient for almost complete removal of Mn under the experimental conditions employed in this study. Removal of arsenic [As(III)] in Mn-oxide coated filter media has been found to be strongly dependent on pH. Removal of As decreased with increasing pH, especially at pH beyond 7.5 and virtually no As removal took place at pH 9 and beyond. Removal of As(III) by Mn-oxide solids is governed by oxidation of As(III) to As(V) by MnO2, and subsequent adsorption of As(V) onto MnO2 solids; the efficiency of both these processes are significantly reduced at higher pH values. In general, As removal increased with increasing Mn-content of the filter media. At relatively lower pH values (less than 7.5), arsenic appears to compete with bicarbonate ions for adsorption onto Mn- coated media. Presence of phosphate significantly reduced arsenic removal due to competitive adsorption. The leaching or release of Mn from different Mn-oxide coated media has been found to increase with decreasing pH, though the leaching was not found to be significant; release of Mn was relatively higher for the “prepared” and “synthetic” media, compared to greensand. Under anaerobic condition created by oxidation of organic matter (which also reduces pH of water significantly), Mn is leached/released from Mn-coated media; Mn leaching/ release from freshly “prepared” Mn-oxide coated media was significantly higher compared to the “synthetic” media and green sand. This suggests that creation of anaerobic condition in a filter bed could result in release of significant Mn from the media.