Removal of fluoride using various adsorbents
3.7 Summary
The initial concentration of fluoride present in water indicates the contamination level of the source water and depending on that the treatment time and adsorbent mass come into the picture as one of the most dependent variables. Adsorption is mainly a combination of mass transfer and ion exchange process and it mainly depends on the surface area, ionic charge of the adsorbent and the adsorbate molecules. During adsorption process, pH is the most important parameter which depicts the overall effect of different ion-ion interactions. Hence, several parameters like initial fluoride concentration, duration of experiment, adsorbent mass, temperature, stirring speed, pH and other ions effect were investigated during adsorption treatment of fluoride contaminated water. The excerpts obtained from the experimental results are presented below separately for four different adsorbents.
Pyrophyllite
The current study highlighted that pyrophyllite had considerable potential for fluoride removal from aqueous solution. It was found that pyrophyllite showed significant fluoride removal efficiency (85%) at pH 4.9. Maximum fluoride adsorption capacity was found to be 2.2 mg/g at an initial fluoride concentration of 10 mg/L. The experimental data fitted well with Langmiur adsorption isotherm and the value of equilibrium parameter “RL” suggested that fluoride adsorption by pyrophyllite was favorable. Presence of other ion effects the fluoride removal efficiency. Kinetic study reveals that fluoride adsorption by pyrophyllite follow pseudo second order kinetic model and both intra-particle diffusion as well as surface diffusion steps was involved during
fluoride uptake. The mass transfer coefficient increased from 4.52 × 10 to 9.34 × 10 when initial fluoride concentration increases from 4 mg/L to 10 mg/L. The negative values of Goand Hovalues suggest that the fluoride adsorption by pyrophyllite was a spontaneous process and exothermic in nature. Low cost of pyrophyllite made this material a potential candidate for defluoridation of water.
Acidic alumina
The present work showed that alumina which is acidic in nature had a considerable potential for fluoride removal from aqueous medium. Around 94 percentage fluoride was adsorbed by alumina at pH 4.4. Kinetic study revealed that fluoride adsorption process was controlled by pseudo-second-order rate equation and intraparticle diffusion was not the rate-controlling step. From EDX study, `it was concluded that fluoride adsorption onto alumina was mostly due to surface adsorption. Thermodynamic study concluded that the fluoride adsorption process was spontaneous, feasible and endothermic nature with activation energy of 95.13 kJ/mol which were fallen in physical adsorption process range. The equilibrium data was fitted with Langmuir and Freundlich isotherm models. Langmuir isotherm model was fitted well to this system with an adsorption capacity of 8.4 mg/g of adsorbent. The performance of alumina towards fluoride was decreased with the presence of other ions. The interference of carbonate and bicarbonate ions was more than that of chloride, nitrate and sulfate ions. The spent adsorbents were regenerated with basic solutions and regenerated adsorbents showed very good adsorption efficiencies. Finally, the process calculation shown here will be helpful in designing a batch adsorber.
Schwertmannite (Sh)
It has been successfully demonstrated that Sh can be potentially used for fluoride removal from aqueous solutions by adsorption. The permissible limit defined by WHO for defluoridation was achieved by 3 g/L adsorbent dose in 90 minute contact time at a pH 3.6. Kinetic study revealed that fluoride adsorption process was controlled by pseudo second order rate equation and intraparticle diffusion was not the rate-controlling step.
Thermodynamic study concluded that the fluoride adsorption process was spontaneous, feasible and exothermic nature. The equilibrium isotherm data was best fitted with Langmuir and Temkin isotherm models with an adsorption capacity of 12.5 mg/g of adsorbent. The performance of Sh towards fluoride adsorption was decreased with the presence of phosphate and bicarbonate ions. The spent adsorbents were regenerated with basic solutions and regenerated adsorbents showed good adsorption efficiencies.
Nanomagnetite schwertmannite (NMSh)
In this study, batch adsorption experiments were carried out to remove fluoride from aqueous solution using synthesized NMSh. NMSh was prepared by introducing nanomagnetite in ordinary schwertmannite, which is a composite of ferromagnetic and paramagnetic materials. This new property improves the fluoride adsorption efficiency.
The permissible limit defined by WHO for defluoridation was achieved by 2 g/L adsorbent dose in 90 minute contact time at a pH 5.73. Kinetic study revealed that fluoride adsorption process was controlled by pseudo-second-order rate equation and intraparticle diffusion was not the rate-controlling step. Thermodynamic study concluded that the fluoride adsorption process was spontaneous, feasible and endothermic nature
with activation energy of 73.19 kJ/ mol which were fallen in physical adsorption process range. The equilibrium isotherm data was best fitted with Langmuir and Temkin isotherm models with an adsorption capacity of 17.24 mg/g of adsorbent. The performance of NMSh towards fluoride was decreased with the presence of other ions. The interference of phosphate, carbonate and bicarbonate ions was more than that of chloride and sulfate ions. The spent adsorbents were regenerated with basic solutions and regenerated adsorbents showed good adsorption efficiencies. Finally, the process calculation shown here will be helpful in designing a batch adsorber. This study will also help to run a continuous fluoride adsorption set up which gives the real life of the adsorbent. The results of magnetic removal of fluoride using the new adsorbent indicated that nano magnetite aggregation process not only improves the magnetic property, but also provides a highly-promoted fluoride adsorption capacity compared to the other adsorbents.
Although several studies such as study of temperature dependent isotherms, mathematical modeling, detailed adsorption mechanisms have not covered, but the above preliminary investigations will be helpful for selecting adsorbents and designing a fluoride removal unit in real practice.