Effectiveness and mechanism of the in situ formed iron hydroxide (FeO_xH_y) towards the removal of phosphate by coagulation and adsorption

摘要

This study prepares the in situ formed iron hydroxide (In situ FeO_xH_y) by the interactions between Fe~(3+) and OH~- at different molar ratios of 1:0,1:1,1:2,and 1:3, and investigates the performance and mechanism involved in the removal of phosphate. The removal efficiency increases with elevated doses of FeO_xH_y and the maximum removal is observed to be in neutral pH ranges. The elevated ratios of OH~- to Fe~(3+) inhibit the removal of phosphate whereas at low alkalinity,the introduction of OH~- shows beneficial effects. The abovementioned four kinds of FeO_xH_y can rapidly adsorb most phosphate within 15 s, and the adsorption process can be well described by the Pseudo-second-order kinetic model. Freundlich model can well describe the adsorption of phosphorus onto these four FeO_xH_y. The adsorption of phosphate significantly decreases the zeta potential of these in situ FeO_xH_y. The MINITEQ software can indicate the species distribution of phosphate and iron salts, and the theoretical calculation indicates that the introduced OH~- can promote the hydrolysis of Fe~(3+) especially in low alkalinity system,which can easily adsorb H_2PO_4~- and HPO_4~(2-) and form the multinuclear hydroxy iron phosphate complexes thereafter.