Analysis of solid waste discharged from water treatment plant as a fluoride-absorbing functional material

Solid waste from a surface water treatment plant (WTP sludge) was used as a fluoride adsorbent material for the treatment of water from underground sources by applying the concept of circular flow of materials. The sludge can be used as an alternative economic mineral resource and can be employed in drinking water collection, treatment, and distribution services. Physical and chemical characterization of the WTP sludge was performed via the Brunauer–Emmett–Teller (BET) method, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal and thermogravimetric analysis (DTA/TGA), and particle size analysis. The structural characterization indicated that the dominant phases of WTP sludge were quartz, kaolinite, and illite. Chemical analysis demonstrated that the distributions of elements such as aluminum, iron, and silicon in the WTP sludge were identical to those in a clayey residue. Thermal analysis revealed a mass loss of approximately 30% in the sample. The WTP sludge exhibited a fluoride removal ability of approximately 28% as determined by adsorption assays. The Langmuir and Freundlich models were both adequate for representing the removal, particularly the Langmuir model. The pseudo-second-order kinetic model represented the results well, with R2 = 0.9986 and an experimental adsorption capacity (qe) of 0.206 mg/g. The experimental design was based on three factors (pH, initial fluorine concentration, and initial ion concentration) and showed (95% confidence, α = 0.05) that the interaction between the three factors was significant in the response variable. However, these individual parameters were not statistically significant. This study demonstrates the potential of WTP sludge as a groundwater fluoride adsorbent.