A mesoporous adsorbent is synthesized by optimized acid-base treatment of abundantly available laterite. Surface area analyzer analyzes the pore volume and B.E.T surface area of treated laterite (TL) and values are found to be in the range of 0.36 mL/g and 183 m2/g, respectively. These values are much higher compared to raw laterite. Barret-Joyner-Halenda (BJH) pore size distribution of TL is obtained in the range of 3 to 20 nm, which lies into mesoporosity region. The mineral phase analysis of TL reveals the formation of Fe/Al oxyhydroxides, those are responsible for higher fluoride ion adsorption. A batch mode adsorption of fluoride ion is performed with varying process parameters like temperature, initial concentration of fluoride ion, adsorbent particle size, contact time, presence of different competitive ion. Adsorption isotherm model of Langmuir, Freundlich, Temkin, Redlich-Petersen are used to analyze experimental adsorption data of fluoride ion on TL. Among all isotherm models used, best correlation with experimental data is obtained for Redlich-Petersen model. In this study, the increasing order of regression coefficient for four isotherm models is obtained as Temkin < Freundlich < langmuir < Redlich-Petersen. The Langmuir maximum uptakes of fluoride ion on TL using synthetic solution are obtained as 36.30, 37.98 and 39.09 mg g-1 at temperature 283, 305 and 315 K, respectively. The fluoride ion adsorption performance of TL is also compared with well-known fluoride adsorbent like activated alumina (AA). The maximum adsorption of fluoride on AA is found to be in the range of 16.65 mg/g, which is almost half compared to fluoride adsorption capacity of TL. The ΔG0 values at three different temperatures are obtained from Langmuir adsorption constant related with adsorption energy. The ΔG0 values at temperature 283, 305 and 315 K are -22.96, -25.17 and -26.85 kJ/mol, respectively. The ΔG0 is plotted as function of temperature (K). A straight line plot (r2: 0.9951) is obtained. From intercept and slope the values of ΔH0 and ΔS0 are evaluated as 18.15 KJ/mol and 0.143 KJ/mol-K. The kinetic data is fitted to 1st order and pseudo-second order kinetic model. The pseudo-second kinetic model provides the best fit with experimental data. A comparative adsorption of fluoride ion is performed in presence of lead and arsenic(V) ion separately. In presence of lead and arsenic(V) ion, the adsorption rate of fluoride on TL slightly increases and decreases, respectively. 95 % fluoride removal is achievable in batch mode of operation for initial fluoride concentration of 10 mg/L with TL dosing 0.75 g/L at 305 K.
Keywords: Treated laterite, fluoride, arsenic (V)
