STUDIES ON REMOVAL OF FLUORIDE AND ARSENIC FROM CONTAMINATED WATER

Abstract

Water is a vital component of life and also an important building block for the ecosystem. In the last few decades, due to growth in human population and industries, this component of the ecosystem is facing some serious challenges and has become scarce in its usable form as drinking water. Groundwater is most important source of drinking water and irrigation for more than 50 % of the global population. The rapid increase in global population and industrial activities has caused overexploitation of groundwater, which in turn has resulted in a drastic degradation of its quality. More and more cases of groundwater contamination with inorganic pollutants, salinity, heavy metals, etc., are coming into light day by day. Arsenic and fluoride are two of such contaminants, which have posed greatest threat to the human beings. It is estimated that, worldwide the number of people suffering from high arsenic and fluoride content in drinking water is about ten million and a hundred million, respectively. The consumption of drinking water having excess quantities of these contaminants results in several types of diseases and health related problems like bone and skeletal fluorosis due to fluoride and different types of cancers due to arsenic. Because of these probable severe adversities in drinking water, WHO has issued guideline value of 10 μg/L and 1500 μg/L for arsenic and fluoride, respectively. Also, the maximum permissible limits, as per the Indian Standard (IS 10500) are recommended as 10 μg/L and 1500 μg/L for arsenic and fluoride, respectively. There are many countries around the world like India, China, Mexico, Argentina, and Pakistan, where these contaminants are reported to coexist in groundwater. The co-occurrence of both these contaminants in the groundwater of many areas in Rajnandgaon District, Chhattisgarh, India has been reported to be well above the permissible limits. Co-occurrence of these pollutants may create more serious effects on human health. Natural clays have excellent capacities to accumulate both anionic as well as cationic pollutants either through ion exchange or adsorption or both. Laterite soil, in particular, is considered as good adsorbent as it has high content of aluminum oxide (Al2O3 ~ 21%), iron oxide (Fe2O3 ~ 47%) and silica (SiO2 ~ 28%) as well as has arsenic and fluoride removal capacity. The removal efficiency of laterite can also be improved by its surface modification. Further, both arsenic and fluoride can exist as negatively ii charged species in solution under certain conditions but their speciation chemistry is different. Thus, the presence of both species in solution may influence the individual removal of these species. In many of the recently published papers, metal oxides and metal hydroxides are reported to be potential candidates for the adsorptive removal of arsenic and fluoride from water. These adsorbents are abundantly available in nature in the form of various types of minerals and can also be synthesized in a laboratory easily. In particular, oxides and hydroxides of aluminum are studied extensively for the remediation of arsenic and fluoride bearing water. Alumina is one of the most broadly studied adsorbents for the removal of fluoride from water. It is also reported in many of the literature that the spent adsorbent can be reused several times with the help of regeneration process; however, the limitation of the regeneration is that it subsequently decreases the adsorption capacity of the material with every cycle of usage and it needs to be discarded after certain number of cycles. Further, regeneration also creates some pollutants. The regeneration step can be neglected in the case of low cost adsorbents as they are mostly made from such raw materials that have almost no commercial value and their regeneration can be even more costly than the actual cost of the production. Thus, the concerns related to the disposal of spent adsorbent still remain unanswered. Apparently, it seems that the solidification of spent adsorbent derived from low cost material may be an attractive route for its management. In reality, spent adsorbent management issue is not well studied, which is very important for the applicability of the adsorbents. Frequently, it has been observed that the spent adsorbent/sludge is disposed on the ground leading to possible contamination of surface water and groundwater sources through seepage. Further, surface modification increases the cost of the adsorption process and produces many environmental consequences. Thus, to get a suitable adsorbent for sustainable utilization, the life cycle analysis of the adsorbent is important. In the present study, adsorptive removal of arsenic and fluoride has been carried out with the help of 2 different types of adsorbents, i.e., Acid base treated laterite soil (ABTL) and aluminum oxide/hydroxide nanoparticles (AHNP). For both the adsorbents, iii the effects of various process parameters like initial pH of the solution, dose of adsorbent, contact time and initial concentration of ions have been studied. In case of ABTL as adsorbent, optimum conditions for maximum removal of both the contaminants (arsenic and fluoride) in the single component system are found as pH 5, adsorbent dose 20 g/L, contact time 300 min with an initial concentration of arsenic as 500 μg/L and fluoride as 10000 μg/L, respectively. Adsorption isotherm data was well described with Langmuir model and under the optimum conditions, the Langmuir maximum adsorption capacity of ABTL is found to be 769 μg/g and 526 μg/g for arsenic and fluoride, respectively. Adsorption followed pseudo second order kinetics for both the contaminants in single component. Further, binary adsorption experiments were also performed at the same optimum conditions with varying concentrations of arsenic and fluoride. In the binary adsorption studies, the adsorption of arsenic does not get affected much as the concentration of fluoride is increased, while fluoride shows antagonistic behavior and its adsorption decreases as the concentration of arsenic is increased. The extended Freundlich model is found to best represent the apparent equilibrium adsorption phenomena in binary system. In case of AHNP adsorbent, the optimal conditions for maximum removal of arsenic and fluoride are found as pH 7, adsorbent dose 2 g/L and 8 g/L for arsenic and fluoride, respectively, and contact time 300 min for the single component system. The adsorption process is well explained by Langmuir isotherm and follows pseudo second order kinetics for both arsenic and fluoride. The Langmuir maximum adsorption capacity of AHNP is found as 833.33 μg/g for arsenic and 2000 μg/g for fluoride at optimum conditions. Binary adsorption study was performed at the same optimum conditions using AHNP also with varying the concentrations of arsenic and fluoride. In the binary adsorption studies, arsenic showed slightly synergistic behavior as the concentration of fluoride is increased, while fluoride shows antagonistic behavior and its adsorption decreases as the concentration of arsenic is increased. Among different bi-component isotherms, the modified competitive Langmuir isotherm is found to well describe the bi-component system. The performance of both the adsorbents has also been tested successfully with the help of a real groundwater sample having arsenic 512 μg/L and fluoride 6300 μg/L along with other ions in batch mode of operation. iv The performance of both the adsorbents has also been tested in the column mode of operation. For this study, a column made of Perspex was used which had an internal diameter of 1 cm and the influent was introduced in up flow direction by a peristaltic pump. The bed height was taken as 20 cm for both the adsorbents and initial concentrations of arsenic and fluoride were taken as 500 μg/L and 10000 μg/L respectively. In these studies, the effect of flow rate of the influent was studied from 17 ml/hr to 50 ml/hr for ABTL adsorbent and from 17 ml/hr to 100 ml/hr for AHNP. To further describe the adsorption mechanism, various column adsorption kinetic models were applied to fit the experimental data. Thomas model and Yoon Nelson model showed good correlation for the adsorption of both arsenic and fluoride for both the adsorbents. The Thomas model estimated the maximum adsorptive capacity of the ABTL adsorbent as 60.37 μg/g for arsenic and 384 μg/g for fluoride whereas for AHNP adsorbent it was 18323.7 μg/g for arsenic and 13390.89 μg/g for fluoride in the column mode of operation. The Yoon Nelson model estimated the time required to reach 50 % breakthrough curve (τ). For ABTL adsorbent, the value of τ are found to be 9292.87 min for arsenic and 2719.83 min for fluoride, whereas, for AHNP adsorbent, values of τ are found as 44153.5 min for arsenic and 12102.75 min for fluoride. The Adam-Bohart model assessed the adsorption performance of the columns at different flow rates of the influent and predicted the adsorption capacity coefficient and thus the column performance. The increase in flow rate of the influent speeded up the exhaustion of the column. After adsorption, the spent adsorbents have been stabilized as clay bricks. The effects of concentration of spent adsorbent and sintering temperature have been investigated on the properties of bricks and leaching of arsenic and fluoride. The bricks have been tested for various properties like density, percentage water absorption, shrinkage, compressive strength, and efflorescence. The maximum values of density and shrinkage of the bricks formed are found as 2.3 g/cm3 and 10.2 % respectively, whereas the percentage water absorption and compressive strength of the bricks are found to range between 11 % to 14 % and 35 kgf/cm2 to 150 kgf/cm2 respectively. All the test results are in accordance with the criteria set by Indian Standards for building bricks. The leaching test of arsenic and fluoride from the bricks reveals that their maximum v values in leachate are 510 μg/L and 2100 μg/L respectively, which are below the permissible limits as per USEPA standards. Life cycle assessment (LCA) of defluoridation of water using laterite soil based adsorbents has been carried out. The scope of LCA study consists of cradle to grave approach (i.e., starting from the acquisition of raw materials to the management of spent adsorbent). Environmental impacts associated with the defluoridation process are interpreted with the help of CML 2001 and TRACI methods using GaBi 6.0 software. All calculations are based on the amount of adsorbent required to reduce the fluoride concentration from 10000 μg/L to 1500 μg/L of 720 L water. The results from life cycle impact assessment reveal that the overall impacts are highest for TTL followed by ABTL and ATL. The fluoride adsorption capacity of adsorbents is found as the key factor influencing environmental impacts. Further, through sensitivity analysis, loading capacity of the vehicle and the distance between the mining and the processing site are found to play important role in environmental degradation, which can be reduced by selecting a vehicle with lower loading capacity due to its higher fuel economy