Effect of pre-treatment of bentonite with sodium and calcium ions on phosphate adsorption onto zirconium-modified bentonite. 
To understand the influence of the pre-treatment of bentonite with Na+ and Ca2+ on the adsorption of phosphate on zirconium-modified bentonite, three kinds of adsorbent materials including zirconium-modified raw, Na+-pretreated and Ca2+-pretreated bentonites were synthesized and characterized firstly, and afterward their adsorption performance and mechanism for phosphate were studied comparatively.  The phosphate adsorption ability for zirconium-modified bentonite decreased after the pre-treatment of bentonite with Na+, but it increased after the pre-treatment of bentonite with Ca2+.  The maximum phosphate adsorption capacity calculated from the Langmuir isotherm model for zirconium-modified Ca2+-pretreated bentonites (13.4 mg P/g) was much higher than that for the zirconium-modified raw bentonite (9.06 mg P/g).  The pre-treatment of bentonite with Na+ and Ca2+ did not change the interaction type between zirconium-modified bentonite and phosphate, i.e., the coordination of phosphate to zirconium.  The decreased phosphate adsorption capacity for zirconium-modified bentonite induced by the Na+ pre-treatment could be mainly attributed to the decrease of the specific surface area and the content of exchangeable Ca.  The increased phosphate adsorption capacity for zirconium-modified bentonite induced by the Ca2+ pre-treatment could be mainly due to the increase in the amount of exchangeable Ca.  Results of this work suggest that the zirconium-modified Ca2+-pretreated bentonite is more suitably used as an adsorbent for the removal of phosphate from wastewater than the zirconium-modified raw and Na+-pretreated bentonites.