To understand the influence of the pre-treatment of bentonite with Na and Ca on the adsorption of phosphate on zirconium-modified bentonite, three kinds of adsorbent materials including zirconium-modified raw, Na-pretreated and Ca-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 Ca. The maximum phosphate adsorption capacity calculated from the Langmuir isotherm model for zirconium-modified Ca-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 Ca 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 Ca pre-treatment could be mainly due to the increase in the amount of exchangeable Ca. Results of this work suggest that the zirconium-modified Ca-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.
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