Fluoride-contaminated water remediation using biochar derived from dairy processing sludge

Seeking a technically reliable disposal platform for milk sludge has drawn particular concern considering its hazardous potential to our environment. Milk sludge from the dissolved air flotation process has abundant calcium because of the naturally available calcium in milk and lime addition as a coagulant. No previous study on its ability to remove fluoride based on its abundant calcium has been reported. As a strategic measure to minimize the environmental impacts, milk sludge was pyrolyzed. Milk sludge biochar was used as an adsorbent to remove fluoride from water. To delineate the close relationships between the fluoride removal capacity and the physico-chemical properties of biochar, milk sludge biochar was fabricated at the temperature range from 500 to 800 ˚C. In an effort to align the experimental/theoretical data, adsorption kinetics, isotherms, column studies, and equilibrium modelling using visual MINTEQ 3.1 were conducted. In addition, milk sludge biochar was characterized using field-emission scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. Biochar fabricated at 800 °C (MS800) exhibited the highest fluoride removal capacity of 485.9 mg/g. Only MS800 mainly composed highly soluble Ca(OH)₂ while the other temperature-prepared biochar mainly composed of less soluble CaCO₃. Consequently, MS800 is providing more Ca²⁺ to the system. At the equilibrium, MS800 releases Ca²⁺ ∼200 mg/g into water within 24 h. X-ray diffraction of the spent biochar shows that fluoride was removed by forming a CaF₂ precipitation. All experimental findings of this study confirmed that milk sludge converted into a value-added, efficient environmental media to remediate fluoride-contaminated wastewater.