De-Fluoridation of Ground Water Using Raw and Modified Bentonite Clay
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Date
2022-06-12
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Addis Ababa University
Abstract
The presence of fluoride in groundwater sources of drinking water has posed a concern to global public health, particularly in the East African Rift Valley (EARV). The study's ultimate objective was twofold. First, investigate low-cost, locally available adsorbents for fluoride adsorption that could be used in Ethiopia's Rift Valley (which is the country's most fluorotic region), and second, contribute to the search for an appropriate and long-term fluoride removal technology for the treatment of fluoride-contaminated groundwater for drinking water for developing countries. Due to cultural beliefs and the terrible odor they emit, the use of bone char and the Nalgonda process for de-fluoridation is being rejected by users in the study area. Several researchers have advocated for the use of non–conventional low–cost adsorbents, such as natural materials and waste/byproducts from agriculture and industry, as effective adsorbents for the removal of fluoride from aqueous solution in recent years. Low–cost adsorbents, such as clay minerals, have emerged as a viable remediation technique for removing fluoride from polluted ground water. However, only a few clay minerals have been studied and investigated for their usefulness in removing fluoride from polluted water, either as is or after slight alteration. As a result, a new, cost-effective technology of fluoride removal is required. Therefore, in this study, the feasibility of fluoride adsorption from aqueous solutions using naturally available bentonite clay in both modified and unmodified forms is investigated. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy analysis was applied to describe the structure and nature of modified and unmodified bentonite clay. The physicochemical characteristics of the adsorbent were also investigated for moisture content, pH, apparent density, specific surface area, cation exchange capacity and its point-of-zero charge. Results obtained from these studies are presented and discussed. These effects of treatment or modification have been discovered. The EDX analysis reveals a significant silica and alumina content, as well as trace amounts of Fe+3, Ca+2, and Mg+2. The presence of the primary
minerals, silica and alumina, with a minor amount of hematite, was revealed by XRD analysis. Furthermore, the silica and alumina levels have increased due to modification of the original material. SEM scans revealed considerable alterations in the original pore structure. The research involves a series of adsorption experiments in a column mode to evaluate the ability of the adsorbents for fluoride removal from polluted water. The column operations were used to investigate the practical application of the produced low–cost adsorbents for removal effectiveness at greater fluoride concentrations under an ideal pH setting of 7.2, which is suitable for drinking. Under the optimized values of the process parameters of initial fluoride concentration (mg/L), Flowrate (mL/min) and bed depth (cm) under continuously flowing fixed bed column was determined. At optimized conditions, RB have shown very low fluoride removal efficiency (47.19%), whereas, modification of the clay surface with HCl and aluminum oxide, on the other hand, increased fluoride removal efficiency to 79.77% and 94.38%, respectively for the 5mg/L of initial fluoride concentration, 10cm bed depth and 15mL/min flowrate. The statistical model, central composite design (CCD) and mathematical models were applied to evaluate the column adsorption performance. The adsorption modelling study reveals that for all models such as Thomas, Clarck, Yoon-nelson and Adam-Bohart, the lower the flow rates with higher bed height leads to maximum fluoride uptake on to adsorbent. The analysis of variance was used to determine the importance of independent variables and their interactions on adsorption capacity and Fluoride removal. All the R2 values indicate that the models match the experimental data well. Moreover, the study investigated the effects of co-existing ions in water on the performance of ALUM-MBENT, in column mode, the effects of anions on fluoride adsorption were investigated using concentrations of each anion of 0.1, 10, and 100 mg/L. When competing anions are present, the adsorption capacity of the adsorbent reduces, according to the findings. Multi-valent anions are more easily absorbed than monovalent anions. Carbonates and phosphate are the main anions that have the greatest influence on the fluoride adsorption. The fluoride adsorption is strongly affected by the concentrations of
competing anions. Phosphate, bicarbonate, and sulfate greatly reduced fluoride adsorption. However, chloride had little effect on the sorption. The impact of major anions on fluoride adsorption followed the order of Cl- <SO42- <PO43- <CO32. The last objective reports on regeneration of the used adsorbent for fluoride adsorption in column mode. The addition of 0.1MNaOH was able to desorb fluoride. Meanwhile, the regenerated adsorbent showed reasonable removal of fluoride even after three consecutive cycles of experimentation. The degree of desorption varied between 94.5% and 68.5% from cycle 1 to cycle 3. When scaled up, the optimized ALUM-MBENT/ATB can be employed for fluoride treatment from polluted groundwater in EARV. Whatever the current fluoride adsorption capacity of ATB/ALUM-MBENT is encouraging, it is too early to conclude that ATB/ALUM-MBENT is directly applied in the field for treatment of fluoride`, since the adsorbents are not tested at large scale in the field and may not sure about its regenerablity after field application. Therefore, it is recommended that ATB/ALUM-MBENT should be tested at field level and hence possibly be developed further and it can most likely contribute to the provision of safe drinking water to some of the 16 million people still using unimproved sources, especially those living in rural fluoritic areas of RVLB.
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Keywords
De-Fluoridation, Ground Water, Using Raw and Modified, Bentonite Clay