Coagulation effect of polyaluminum-titanium chloride coagulant and the effect of floc aging in fluoride removal: A mechanism analysis

Abstract

Excessive discharge of fluoride can lead to environmental pollution and human hazards. Coagulation is a widely used process for the removal of fluoride. Existing research on the interaction between coagulants and fluoride has primarily concentrated on the removal of fluoride and the selection of coagulants, while studies on floc aging are lacking. Understanding the aging process of flocs can provide valuable insights into the mechanisms of interaction between coagulants and fluoride and further provide guidance for coagulation as well as sludge return processes. This study compares the fluoride removal efficiencies of two coagulants: polyaluminum-titanium chloride (PATC) and polyaluminum chloride (PACl), and examines the change patterns of fluoride ion during aging (0–120 h). Additionally, the study characterizes the impact of Ti or Al-Ti on the aging process using FTIR, XPS, XRD, and ESI-TOF-MS analyses. The results of the study demonstrate that the fluoride removal efficiency of PATC is 20.2% higher than that of PACl at a dosage of 0.8 mmol/L. The introduction of titanium provides additional binding sites for F−, resulting in a more intricate floc structure. Moreover, titanium promotes the hydrolytic polymerization of aluminum, increasing its positive charge and making it more attractive to F−. Upon the complete precipitation of the flocs, F− in the supernatant of PATC (0–6 h) and PACl (6–72 h) competes with -OH for binding sites, while F− attacks the Al-O or Al-O-Ti bond, leading to the disruption of the polymer. As time progresses, PATC flocs (6–120 h) are less affected by aging than PACl flocs (72–120 h). Throughout the aging process, PATC releases less F− at a slower rate, presumably because the introduction of titanium inhibited the dehydration and crystallization of the flocs to delay the release of F−.