This study focuses on the effects of p H and fluoride at different molar ratios of fluoride to Al（R F：Al） on the removal of cadmium（Cd^（2＋）） and phosphate by Al coagulation. Fluoride at R F：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 R F：Al of 10：1 whereas at lowered R F：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 p H 7 and 8 and that of phosphate at p H 6. Fluoride decreases the ζ-potential in both systems, and the decreasing extent is positively correlated to the elevated R F：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）n F m. Al-F complex formation inhibits Al hydrolysis and increases residual Al levels, and a more significant increase was observed at lower p H. Al-F complexes at high R F：Al complicate the coagulation behavior of Al towards both negative and positive ionic species. Moreover, fluoride at low R F：Al shows little effect on Al coagulation behavior towards Cd^2＋and phosphate, and the spent defluoridation adsorbent,i.e., aluminum（Al） hydro（oxide） with adsorbed fluoride at R F：Al of below 0.1：1, may be reclaimed as a coagulant after being dissolved.
Ruiping LiuBao LiuLijun ZhuZan HeJiawei JuHuachun LanHuijuan Liu
Although Al-based coagulation and adsorption processes have been proved highly efficient for fluoride （F） removal, the two processes both generate large amount of AI（OH）3 solid waste containing F （Al（OH）3-F）. This study aimed to investigate the feasibility of utilizing AI（OH）3-F generated in AI（OH）3 adsorption （Al（OH）3-Fads） and coagulation （Al（OH）3-Fcoag） for the adsorption of cadmium ion （Cd（II））. The adsorption capacity of AI（OH）3- Fads and Al（OH）a-Fcoag for Cd（II） was similar as that of pristine aluminum hydroxide （AI（OH）3）, being of 24.39 and 19.90 mg·g^-1, respectively. The adsorption of Cd（II） onto AI（OH）a-Fads and AI（OH）a-Fcoag was identified to be dominated by ion-exchange with sodium ion （Na^＋） or hydrogen ion （H^＋）, surface microprecitation, and electrostatic attraction. The maximum concentration of the leached fluoride from Al（OH）a-Fads and Al（OH）3-Feoag is below the Chinese Class-I Industrial Wastewater Discharge Standard for fluoride （〈 10mg·L^-1）. This study demonstrates that the AI（OH）3 solid wastes generated in fluoride removal process could be potentially utilized as a adsorbent for Cd（II） removal.
