Synthesis, Characterization and Development of Aluminium Hydroxide–Based Adsorbents for Defluoridation of Drinking Water
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Date
2014-06-05
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Addis Ababa University
Abstract
In this study, the fluoride removal potential of aluminium hydroxide–based adsorbents; aluminium hydro(oxide) (AO), aluminium oxide–manganese oxide (AOMO) composite material and nanoscale aluminium oxide hydroxide (nano–AlOOH) have been investigated. The adsorbents were synthesised and characterized. A series of batch adsorption experiments were carried out to assess parameters that influence the adsorption process. The parameters considered were contact time and adsorbent dose, initial fluoride concentration, and raw water pH. The adsorption isotherms and kinetics models were used to determine adsorption parameters.
AOMO was prepared from manganese(II) chloride and aluminium hydroxide. The surface area of AOMO was found to be 30.7 m2/g and its specific density was determined as 2.78 g/cm3. Detailed investigation of the adsorbent by inductively coupled plasma–optical emission spectrometry (ICP–OES), inductively coupled plasma–mass spectrometry (ICP–MS) and ion chromatography (IC) (for sulfate only) showed that it contains Al, Mn, SO4, and Na as major components and Fe, Si, Ca and Mg, as minor components. Thermogravimetric analysis (TGA) was used to study the thermal behaviour of AOMO. X–ray diffraction (XRD) analysis showed that the adsorbent is poorly crystalline. The point of zero charge was determined as 9.54.
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Batch experiments (by varying the proportion of MnO) showed that fluoride removal efficiency of AOMO varied significantly with % of MnO with an optimum value of about 11% of manganese oxide in the adsorbent. The pH for optimum fluoride removal was found to be in the range 5 to 7. The adsorption data was analyzed using linear and non–linear Freundlich, Langmuir, and D–R models. The minimum adsorption capacity obtained from non–linear Freundlich isotherm model was 4.94 mg/g and the maximum capacity from Langmuir was 19.2 mg/g. Non–linear methods were found to best represent the adsorption characteristics, than their linearized counter–parts. The non–linear Freundlich isotherm most closely represented fluoride adsorption on AOMO. Kinetic studies showed that the adsorption is well described by a non–linear pseudo–second order reaction model with an average rate constant of 3.1 x 10-2 g/min.mg.
Results from experiments with nano–AlOOH showed that most of the adsorption took place during the first 30 min; and equilibrium was reached at one hour contact time with an adsorbent dose of 1.6 g/L and an initial fluoride concentration of 20 mg/L. Maximum adsorption occurred at around pH 7 at an initial fluoride concentration of 20 mg/L. The adsorption data were well fitted to the Langmuir isotherm model with a maximum adsorption capacity of 62.5 mg/g. The kinetic studies showed that the adsorption of fluoride by nano–AlOOH obeys a pseudo–second order rate equation with an average rate constant of 2.22 x 10-3 g/min.mg.
The AO adsorbent was synthesized at different OH:Al ratios to optimize the material, characterized and tested in batch and column experiments. The maximum fluoride uptake was achieved for the AO adsorbent synthesized at OH:Al ratio between 2.5 and 2.7. The surface area of the AO (OH:Al = 2.7) was found to be 37.7 m2/g. The composition was determined to be 90% Al(OH)2.7(SO4)0.1 (or 78.3% Al(OH)3 plus 10.7% Al2(SO4)3) with 10% Na2SO4 (as an impurity).
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The material is X–ray amorphous and scanning electron microscopic (SEM) studies show AO to be a network of fibers with a size range of 200 to 300 nm. The FTIR spectrum of AO indicated the presence of Al–OH and Al–O bonds, sulfate and water, but could not shed light on the nature of aluminium hydro(oxide). However, it is possible to postulate the presence of basaluminite, because spectral comparison of AO with basaluminite has very similar features in many aspects.
At an OH:Al ratio of 2.7, the surface site concentration of AO determined by acid–base titrations is 0.5 meq/g (equivalent to a surface site concentration of 8 sites/nm2) and an acidic component of 1.4 meq/g. At an OH:Al ratio of 3, the acidic component is no longer present and the uptake capacity of AO significantly reduced. The role of the acidic component may be the control of pH, but may also be the exchange of sulfate with fluoride.
In experiments to test the competitive adsorption of fluoride with ions typically found in natural waters, uptake was found to be unaffected by sodium salts of chloride and sulfate in concentrations up to 500 mg/L. A reduction of fluoride uptake with increasing concentrations of hydroxide and bicarbonate was ascribed to the pH dependence of fluoride sorption (as the addition of these ions caused an increase in pH), while phosphate appeared to compete with fluoride for sorption sites.
Continuous packed column experiments with AO (OH:Al = 2.7) showed that at a flow rate of 100 empty bed volumes (eBV) per day using deionized water, the fluoride adsorption capacity was 26.2 mg/g. The pH of treated water ranged between 4.4 and 7.0. In solutions representing buffering conditions of Ethiopian groundwaters (pH 8 ± 0.2, 10 mM NaHCO3, 3000 ppm CO2) uptake capacities at 100 and 10 eBV/day were 4.65 and 9.0 mg/g respectively. Aluminium was
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initially released in concentrations ranging from 0.6–2.0 mg/L in the experiments using deionized waters when the pH was less than 5. With the introduction of calcite post–column treatment, the pH was maintained in the range of 7.5–8.5, which significantly reduced sulfate concentrations due to gypsum precipitation and the prevention of early aluminium release were achieved.
The comparative performance of AO with activated alumina (AA) and pseudoboehmite (PB) was evaluated in terms of surface acidity and surface site concentrations, fluoride adsorption capacity, and potential for repetitive regeneration. The fluoride removal capacity of the adsorbents determined from mini–column studies was found to be 10.6, 1.9, and 2.4 mg/g for AO, AA, and PB, respectively. This significant difference in fluoride adsorption capacity is strongly related to the surface acidity and surface site concentration. Regeneration experiments showed, however, that AA and PB can be regenerated for more than 3 cycles; whereas the potential of regeneration of AO for more than 3 cycles is limited. This is due to the loss of the acidic component during regeneration with NaOH.
The AO was pilot tested in a rural community in the Ethiopian Rift Valley where groundwaters are heavily enriched with fluoride. The capital and operation cost of AO defluoridation plant were estimated based on information collected from field experience. The result from community the defluoridation plant showed that fluoride in the feed water (8–10 mg/L) is removed below 0.1 mg/L. The average adsorption capacity was determined to be 2.11 mg/g based on continuous field monitoring results obtained until the fluoride content in the treated water exceeds the breakthrough value of 1.5 mg/L. The capital and operational costs of the 1200 L/day defluoridation plant was estimated at approximately Birr 161,987 (9700 USD) and Birr
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220 (13 USD) per m3 of treated water, respectively. No major operational problems and complaints from the beneficiaries were experienced during operation.
Due to its high adsorption capacity compared to all commercially available aluminium hydroxide–based adsorbents for fluoride removal, AO is a highly promising material for defluoridation of drinking water both at household and community levels.
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Keywords
Based Adsorbents for Fluoride Removal, , AO is a Highly Promising Material for Defluoridation of Drinking Water Both at Household and Community Levels