Fluoride Pollution and its Control in Mining Areas

Abstract

Fluoride is also found in natural water at some concentration level. In seawater, fluoride is found 1 mg/l. In river and lakes, less than 0.5 mg/l. of fluoride is found and high concentration of fluoride occurs in groundwater. According to Indian standard drinking water specification and WHO, the maximum permissible limit of fluoride in drinking water is 1.5 ppm and highest desirable limit is 1.0 ppm. Fluoride has an advantageous effect on teeth at low concentration in drinking water. However, at high concentrations it has adverse effect on health. Excessive amount of fluoride ingested for a long period by human being causes a crippling disease known as fluorosis, which is not curable. The crippling skeletal fluorosis cause increases morbidity in a number of regions in the world. A number of studies have been done which shows an acute effect due to overdose of fluoride exposure. Naturally, fluoride present in drinking water have long term effects due to exposure and other natural sources which is the major concern to human health.
Recently, there have been reports of increase in the concentration of fluoride in both surface and ground waters in and around Talcher Coalfield. Therefore, Talcher coalfield was chosen as a case study for the assessment and control of fluoride. The untreated waste water contaminated with fluoride discharged from industrial units are either allowed to accumulate in the lagoon or discharged to the river without adequate treatment. The water from the mining region mix with the Nandira and Bangaru nallah, finally terminating with Brahmani river. The ground water also gets contaminated by percolation through the soil. A large number of villagers have reported fluoride induced symptoms like pain in the lower leg. The domestic animals like castles have shown bone related problems in this region. While Talcher is among the worst affected, there are other districts where the problem has assumed serious proportions.
In the present study, a few locations in Talcher coalfield was selected for assessment of fluoride concentration. It may be noted that in this region, fly ash disposal is being carried out in some of abandoned quarries and mines sumps. One of the biggest problems due to disposal of large quantities of coal ash is the possible leaching of different hazardous pollutants, including fluoride. Therefore an attempt has been made to investigate the concentration of fluoride in the selected location and to find out their source. Additionally an attempt has been made to find out an effective treatment method as well as optimum dose for removal of fluoride from the collected samples.
In this study, removal of fluoride by coagulation process using Alum and Nalgonda technique; and adsorption by activated carbon was studied. The effect of each adsorbent has been examined and discussed. The effect of the coagulants on turbidity and pH were also studied. It was found that 100% of fluoride can be removed with 0.3gm/l of fluoride. In order to reduce the cost, the treatment can also be carried out with 0.1 gm/l of alum, since the fluoride concentration comes below the permissible limit after 1 hour of treatment.
Nalgonda technique is a simple and economical method for fluoride removal. It is addition of alum, lime and bleaching powder followed by rapid mixing, flocculation, filtration and disinfection. Aluminium is added to the sample and it is responsible for removing fluoride. Increase in alum when fluoride is increase. 1/20th to 1/25th of that of the dose of the alum is the dose of lime. Bleaching powder is added at the rate of 3 mg/l for disinfection. It is preferred over the rest because of its low price and ease with which it is handled. In the present case, it was seen that with addition of 0.3 g/l of alum, 0.001 gm of lime and 3 gm/l of bleaching powder, 100% removal of fluoride can be carried from both the ash pond and central sump water after one hour.
Activated carbon is a cost effective material, and is being used for widespread application including removal of different harmful contaminants present in water. It was seen that with the addition of 0.8g/l of activated carbon, the fluoride concentration comes below the permissible limit after 1 hour of treatment. The reduction in fluoride concentration thereafter is very minimal. Therefore o.8 g/l can be considered as the optimum dose for treatment.
A major finding of this study is the reduction of pH after treatment. In all the cases the pH concentration falls drastically, making the water highly acidic. This may cause acid mine drainage. The reason for such behaviour has not been found out. Therefore, while carrying out treatment of fluoride contaminated water in coal mining areas, the neutralization pH is also to be kept in mind.