ABSTRACT
This study focuses on the effects of pH and fluoride at different molar ratios of fluoride to Al (RF:Al) on the removal of cadmium (Cd2+) and phosphate by Al coagulation. Fluoride at RF:Al≥3:1 inhibits the removal of Cd over wide Al dose ranges from 5 to 10 mg/L as Al. The removal of phosphate decreases significantly at high RF:Al of 10:1 whereas at lowered RF:Al (i.e., ≤6:1), an adverse effect is observed only at insufficient Al doses below 2 mg/L. Fluoride shows inhibitive effects towards the removal of Cd at pH7 and 8 and that of phosphate at pH6. Fluoride decreases the ζ-potential in both systems, and the decreasing extent is positively correlated to the elevated RF:Al. The Al fluoride interactions include the formation of Al-F complexes and the adsorption of fluoride onto Al(OH)3 precipitates, i.e., the formation of Al(OH)nFm. Al-F complex formation inhibits Al hydrolysis and increases residual Al levels, and a more significant increase was observed at lower pH. Al-F complexes at high RF:Al complicate the coagulation behavior of Al towards both negative and positive ionic species. Moreover, fluoride at low RF:Al shows little effect on Al coagulation behavior towards Cd2+ and phosphate, and the spent defluoridation adsorbent, i.e., aluminum (Al) hydro(oxide) with adsorbed fluoride at RF:Al of below 0.1:1, may be reclaimed as a coagulant after being dissolved.
