Study On The Adsorption Of Phosphorus In Water And Sediment By Using Zirconium-modified Clay And Lanthanum-modified Zeolite

Phosphorus is a key nutrient element which can lead to eutrophication of surface waters. Wastewater produced by humans contained phosphorus if discharged directly into surface water bodies without treatment, cause the increase of phosphorus concentration in surface water. Thus the risk of eutrophication of water body will increase. Sediment is not only an important reservoir of phosphorus, but also a source of phosphorus in surface water. Therefore, the control of phosphorus pollution in the surface water should be carried out from two aspects: exogenous pollution and endogenous pollution. Adsorption method is an effective method to remove the phosphorus pollution of surface water, and in situ modification technique is an effective method to inhibit the release of phosphorus from polluted sediments. Finding the suitable adsorbent material is the key of successlly applicating of these two technologies. Zirconium oxide is a widely used inorganic material, and it is non-toxic, stable physical and chemical properties, low water solubility. Zirconium oxide is found to have a strong ability in adsorbtion of phosphate in water and wastewater. The ability of lanthanum hydroxide to phosphate in water has a good adsorption capacity. However, the use of pure zirconium oxide or lanthanum hydroxide as phosphorus adsorbent is not cost-effective, because the cost is relatively high. Bentonite, kaolin and zeolite clay minerals, has a large surface area, high cation exchange capacity, stable physical and chemical properties, low cost and extensive sources. It is hope to form a composite with zirconium oxide or lanthanum hydroxide. Zirconium modified kaolin, zirconium modified bentonite and lanthanum modified zeolite will be a good adsorption, having low cost solid phosphorus adsorption material.The zirconium-modified kaolin clay was prepared. By batch adsorption experiments investigated the adsorption properties of the zirconium-modified kaolin clay in water. Results showed that the zirconium-modified kaolin clay exhibited excellent adsorption performance for phosphate in water. The phosphate adsorption capacity for the zirconium-modified kaolin clay increased with the loading amount of zirconium in the zirconium-modified kaolin clay. The phosphate adsorption capacity for the zirconium-modified kaolin clay increased with the increase of the preparated pH value from 8 to 10, remained basically unchangeable with the increase of the preparated pH value from 10 to 11, but decreased with the increase of the preparated pH value from 11 to 12. The equilibrium adsorption data of phosphate on the zirconium-modified kaolin clay prepared under preparated pH value 10 conditions could be better described by the Langmuir isotherm model than the Freundlich isotherm model. Sequential extraction of P from the phosphate adsorbed zirconium-modified kaolin clay showed that most of phosphate-P bound by the zirconium-modified kaolin clay(about 81% of total P) existed in the form of the metal oxide bound P(NaOH-P), which was unlikely to be released under reducing and common pH(5-9) conditions. Zirconium-modified kaolin clay used as a sediment amendment to control phosphorus(P) release from sediments in heavily polluted rivers under low dissolved oxygen(DO) conditions. The fluxes of phosphate-P and total P(TP) from sediments into the overlying water column was greatly reduced by the addition of the zirconium-modified kaolin clay into sediments under low dissolved oxygen conditions. The zirconium-modified kaolin clay amended sediments exhibited much higher phosphate adsorption capacity than the unamended sediments, and the former had much lower phosphate adsorption/desorption equilibrium concentration(EPC0) than the latter. Results of this work suggest that the zirconium-modified kaolin clay can be used as an efficient adsorbent to remove phosphate from surface waters and as a successful sediment amendment for controlling P release from sediments in heavily polluted rivers under low dissolved oxygen conditions.The zirconium-modified bentonite(ZrBT) was prepared. By batch adsorption experiments investigated the adsorption properties of the ZrBT in water. Results showed that Zr BT exhibited excellent adsorption performance for phosphate in water owing to its wide effective pH range, high adsorption capacity and rapid adsorption kinetics. The kinetics of phosphate adsorption onto ZrBT well followed the pseudo-second order kinetic model, supporting the adsorption was due to chemosorption. The equilibrium adsorption data of phosphate on the Zr BT could be better described by the Langmuir isotherm model than the Freundlich isotherm model. The presence of Cl- and SO42- in phosphate solution had no negative effect on phosphate adsorption onto the ZrBT, while the presence of HCO3- slightly inhibited the adsorption of phosphate. The mechanism for phosphate adsorption onto ZrBT was explained by the ligand exchange process and inner-sphere complexing mechanism. Most of phosphate bound by ZrBT(about 87% of bound phosphate) was unlikely to be released under common pH and reducing conditions. The soluble reactive phosphorus(SRP) and dissolved total P(TP) fluxes from sediments into overlying waters were significantly reduced by ZrBT addition under anaerobic conditions. The phosphate adsorption capacity for the ZrBT-amended sediment was much higher than that for the unamended sediment. These results suggest that ZrBT can be used as an efficient adsorbent to remove phosphate from surface waters and as a successful sediment amendment for controlling P release from sediments.The lanthanum-modified natural zeolite was prepared by a co-precipitation method. By batch adsorption experiments investigated the adsorption properties of the lanthanum-modified zeolite to remove phosphate from aqueous solution. The phosphate adsorption capacities for different composites prepared with different precipitated pH values were compared in batch mode. The adsorption characteristics of phosphate from aqueous solution on the lanthanum-modified zeolite prepared in pH value 11 was investigated using batch experiments. Results showed that the lanthanum-modified zeolite prepared with the precipitated pH values 5~7 and 13 had a low adsorption capacity for phosphate in aqueous solution, but the lanthanum-modified zeolite prepared in pH values 9~12 exhibited much higher phosphate adsorption capacity than those in pH values 5~7 and 13. The phosphate adsorption capacity for the lanthanum-modified zeolite increased with the increase of the precipitated pH value from 9 to 11, but remained basically unchangeable with the increase of the precipitated pH value from 11 to 12. The equilibrium adsorption data of phosphate from aqueous solution on the lanthanum-modified zeolite prepared in pH value 11 could be described by the Langmuir isotherm model. The kinetic data of phosphate adsorption from low concentration phosphate solution on the lanthanum-modified zeolite prepared in pH value 11 well followed a pseudo-second-order model. The presence of Cl- and SO42- in low concentration phosphate solution had no negative effect on phosphate adsorption onto the lanthanum-modified zeolite prepared in pH value 11, while the presence of HCO3- slightly inhibited the adsorption of phosphate. Coexisting humic acid had a negative effect on the adsorption of phosphate at low concentration on this adsorbent. The mechanism for phosphate adsorption onto this adsorbent was explained by the ligand exchange process. Results show that the lanthanum-modified zeolite prepared in pH value 11 is a promising adsorbent which can removal of low concentration phosphate from surface waters.