Removal of manganese from groundwater by oxidation and adsorption processes

Abstract

Although significant research works have been carried out on removal of arsenic and iron from groundwater, relatively little work has been done on the removal of manganese from groundwater in Bangladesh, in spite of its existence at relatively high concentration in many areas of Bangladesh. The primary concern regarding presence of manganese in water stems from its capability to stain sanitary-ware and laundry at concentration> 0.1 mg/L and its deposition in water distribution system, even at concentration as low as 0.02 mg/L. Chemical oxidation followed by filtration, is by far the most widely used manganese removal technique. Although coagulation by iron and aluminum salts has been widely used in Bangladesh for arsenic removal, little information is available on its capability in removing manganese from water. The present study is focused on assessing removal of manganese from groundwater using two chemical oxidants (potassium permanganate and bleaching_powder) commonly available in Bangladesh. Besides, manganese removal by ~imple aeration, a method widely used for iron removal, has also been assessed. In addition possIble -manganese removal by coagulation with iron salts has also been evaluated. In this study, it has been found that potassium permanganate is capable to remove manganese from groundwater very effectively over a wide range of initial manganese concentration. In the near-neutral natural pH range of groundwater, maximum manganese removal by permanganate oxidation was achieved at a permanganate dose equal to that required from stoichiometric consideration. However, for an initial manganese concentration of 2.0 mg/L, greater than 95% removal was achieved for a dose fraction equal to 0.8 times that required from stoichiometric consideration. A slightly (20%) higher permanganate dose did not have any significant effect on manganese removal. Removal of manganese has been found to be highly dependent on pH. In general, removal increased as pH increased. Beyond pH of about 8, removal of manganese with permanganate is almost independent of pH. One drawback of permanganate oxidation is the development of color. It has been found that sand filters, with depths of 10 to 20 cm, are capable to reduce the color below Bangladesh drinking water standard. The sand filters were also found to be very effective in removing oxidized manganese solids. In case of oxidation by chlorine (bleaching powder), better manganese removal could be achieved only at pH values 10 and above. In the neutral pH range, removal of manganese was relatively poor (less than 50%) for initial manganese concentration ranging from 1.0 to 10.0 mg/L. However, removal increased significantly as pH increased and almost complete removal was achieved at pH 10. Significant manganese removal (over 95%) can also achieved by simple aeration at high pH value (>10). Thus, at higher pH values, oxidation of manganese by air may have contributed to the higher manganese removal by bleaching powder. In this study, experiments were carried out to evaluate the influence of dissolved iron on manganese removal by oxidation. It was found that when high amount of dissolved iron was present in water, manganese removal by chemical oxidation (with potassium permanganate) was very poor. This is partly due to the fact that a significant part of the. oxidizing agent added for oxidation of manganese may have been utilized for oxidation of VI dissolved ferrous iron. It was also found that when both manganese and iron are present in water, pre-oxidation of iron (e.g., by aeration) prior to the addition of the oxidizing agent would improve manganese removal. In this study, effectiveness of coagulation by iron salts in removal of manganese was evaluated. Coagulation experiments were carried out with and without the addition of an oxidizing agent (potassium permanganate) in order to ascertain the dominant removal mechanism (either oxidation or adsorption onto iron floes). From results obtained in this study, it appears that very little manganese could be removed by adsorption onto the coagulated floes of iron solids. The increased removal of manganese by coagulation with ferric chloride at higher pH values appears to be due the oxidation of manganese by atmospheric oxygen rather than adsorption of manganese on iron solids and their subsequent precipitation.