Phosphate removal in marine electrolytes by zeolite synthesized from coal fly ash

The aim of this study was to examine the influence of seawater electrolytes on removal of phosphate by zeolite synthesized from fly ash (ZFA). A low-calcium ZFA was initially saturated with Na+, Mg2+, Ca2+, Al3+, and Fe3+. Al- and Fe-ZFA showed nearly complete removal of phosphate regardless of the major seawater electrolytes, pH, and salinity. This result was explained primarily on the basis of the adsorption mechanism through the formation of inner-sphere complexes. The remaining ZFAs showed lower phosphate removal performance, in general with the order of Ca-ZFA > Mg-ZFA > Na-ZFA. Compared with pure water, increase of electrolyte concentration or salinity initially enhanced phosphate uptake but then reduced phosphate removal. The individual presence of major seawater electrolytes all facilitated the retention of phosphate, with CaCl2 being the most effective. The mechanism for phosphate removal by Na-, Mg-, and Ca-ZFA was due mostly to precipitation. Repeated batch equilibration experiments indicated that, compared with pure water, ZFA had greater sorptive capacity for phosphate (except for Ca-ZFA, whose capacity decreased slightly) and had lower reversibility for sorbed phosphate in marine electrolytes. In conclusion, our results suggest that presence of seawater electrolytes had roughly no effect, or even positive effects, on the removal of phosphate by ZFA.