Zeolite is a porous aluminosilicate crystal with skeleton structure, which has high cation exchange ability. In recent years, the use of cation exchange capacity of zeolite to remove ammonia nitrogen from water has attracted the attention of many researchers. The surface of zeolite aluminosilicate skeleton is negatively charged, so the adsorption ability of zeolite to phosphoric acid anion in water is very poor. In order to improve the adsorption capacity of phosphate in water, it is necessary to properly treat the surface of zeolite.
Phosphorus is a key nutrient element that can cause eutrophication of surface water. If the wastewater containing phosphorus produced by humans is discharged directly into surface water without treatment, the concentration of phosphorus in surface water will increase, thus increasing the risk of eutrophication. Therefore, effective removal of phosphate from wastewater containing phosphorus is very important for the prevention and control of surface water eutrophication. The main removal methods of phosphate in wastewater include biological method, chemical precipitation method and adsorption method. Among them, adsorption method is considered to be an effective method to remove phosphate in wastewater, especially for the removal of low concentration phosphate in wastewater shows obvious advantages.
I. Test method
Lanthanum chloride (LaCl3·6H2O) was purchased from a limited company and was analytically pure. Potassium dihydrogen phosphate (KH2PO4), ascorbic acid, ammonium molybdate, sulfuric acid, sodium bicarbonate, sodium chloride, sodium sulfate, hydrochloric acid, sodium hydroxide, antimony potassium tartrate and other agents were purchased from Sinopsin Group Chemical reagent Co., LTD., are pure reagents for analysis. Natural zeolite produced in Weichang County, Hebei Province. X-ray diffraction analysis shows that the main mineral components of the natural zeolite are clinoptilolite, mercerite and silica. Deionized water was used in all experiments.
In this study, natural zeolite mainly consists of Si, O, Al, K, Ca, Na and other chemical elements. Table 1 shows the chemical element content (mass fraction) of lanthanum-zeolite composites prepared under different precipitation pH values. It can be seen that lanthanum-zeolite composites contain not only the chemical composition of natural zeolite, but also lanthanum. This indicates that lanthanum-zeolite composites have been successfully prepared by the method used in this study.
Lanthanum-zeolite composites have strong buffering capacity. When the pH value of the solution is low (pH < pHPZC), the lanthanum-zeolite composite will consume H+ in the solution, resulting in an increase in the pH value of the solution. When the pH value of the solution is high (pH > pHPZC), the lanthanum-zeolite composite will consume the OH- in the solution, resulting in a decrease in the pH value of the solution. This shows that natural zeolite is suitable to be used as the carrier material of lanthanum hydroxide to prepare efficient and economical adsorbent for phosphorus removal
In addition, the unit adsorption capacity of phosphate in water by lanthanum-zeolite composite decreases with the increase of the amount of adsorbent. This is mainly because when the volume and concentration of the solution remain unchanged, with the increase of the dosage of adsorbent, the unsaturated sites on the surface of the adsorbent will increase, so the unit adsorption capacity will decrease.
When the precipitation pH value is 5 ~ 7 or 13, the adsorption capacity of phosphate in water is poor. When the precipitation pH value is controlled from 9 to 12, the phosphate adsorption capacity of the composite material is better. When the precipitation pH value increases from 9 to 11, the phosphorus absorption capacity of the composite material increases obviously. When the precipitation pH value continues to increase from 11 to 12, the phosphorus absorption capacity of the composite material is basically unchanged.