Regulating Effect And Mechanism Of Zirconium Modified Granular Zeolite On Transport And Transformation Of Phosphorus In Sediments

With the fast economic development and the urbanization advance,large amounts of landscape water bodies have been constructed in China for the beautification of a living environment or entertainment.Additionally,most of the landscape water bodies in China are in the state of closed or semi-closed drainage.It is widely accepted that the high phosphorus（P）concentration in the overlying water in closed or semi-closed water bodies easily results in their eutrophication,which is often linked to the overgrowth of harmful algae and the deterioration of water quality.Furthermore,the endogenous release of phosphorus（P）from sediments into the overlying water（i.e.,internal P loading（IPL））generally makes a significant contribution to the eutrophication of closed or semi-closed water bodies when external inflows of P（e.g.,P-containing wastewater discharge）are substantially reduced.Therefore,to control eutrophication of closed or semi-closed landscape water bodies,it is critical for an administrator to suppress the release of P from sediments.Currently,restoration techniques for sedimentary P release control comprise sediment dredging,in situ treatment of sediment with Ca（NO₃）₂,in situ addition of flocculation agents such as Al and Fe salts into sediment,in-situ oxygenation,in-situ covering with inert materials such as sand and in-situ active capping/mixing technology and so on.More recently,in-situ active capping/mixing technology（CMT）,which includes the covering of P-sorbent material on the top of sediments（in-situ active capping technology）or the mixing of P-sorbent material with sediments（in-situ reactive mixing technology）,has attracted more and more interest by scholars,because this method has a high potential for sedimentary P release control.Generally,the effective control of sediment P release by CMT is closely related to whether the selected reactive capping/mixing materials（RCMMs）have a good performance for immobilizing aqueous and sedimentary P.Among various RCMMs,zirconium-modified zeolite（ZMZ）shows high efficiency for immobilizing P in sediments.What’s more,since Zr oxide is non-toxic material and zeolite is an environmentally friendly material,the composite of Zr oxide andZMGZshould be safe for landscape water bodies.Commonly,landscape water bodies in China are shallow.For shallow surface water bodies,sediments are often suffered from hydrodynamic disturbance induced by water flow or wind action as well as suffered from macrozoobenthic bioturbation.When the P-sorbent is laid on the top of sediments as a reactive covering material or added into sediments as a reactive mixing material,RCMMs are expected to be subjected from hydrodynamic disturbance and bioturbation.In such case,it is preferable to use the RCMMs in the form of granule rather than powder.Due to the RCMMs in the form of a granule should have a stronger ability to resist being disturbed by the water flow,wind action and activities of invertebrates in sediments than that in the form of a powder.Additionally,the RCMMs in the form of a granule is more conveniently added onto the surface of sediment than that in the form of powder because the former has a higher setting rate than the latter.Thus,for the effective control of sediment P release in shallow landscape water bodies,there is a need to develop the RCMMs in the form of a granule.As far as we know,no investigation has been carried out to examine the effectiveness of the ZMZ in the form of a granule（i.e.,zirconium-modified granular zeolite（ZMGZ））to control the release of P from sediments.In the present study,a ZMGZ was prepared,characterized and used as reactive capping and mixing materials to control the release of P from sediments in landscape water bodies.The features and mechanisms of phosphate sorption to ZMGZ were investigated in batch tests.Microcosm incubation experiments were conducted to explore the impact of ZMGZ covering and mixing on the release of P from sediments to the overlying water.Results of this investigation will contribute to the practical use of ZMGZ as reactive capping and mixing materials to control sediment P release in shallow closed or semi-closed landscape water bodies.Firstly,the adsorption of phosphate and ammonium from aqueous solution on zirconium-modified granular zeolite（ZMGZ）was investigated using batch experiments.The adsorption kinetics data of phosphate and ammonium on ZMGZ could be well described by a pseudo-second-order model.The adsorption equilibrium data of phosphate on ZMGZ fitted well to the Langmuir isotherm models.Based on the Langmuir isotherm,the maximum monolayer phosphate and ammonium adsorption capacities for ZMGZ were 6.87 mg/g at 303 K.Thermodynamic parameters showed that the adsorption of phosphate on ZMGZ was spontaneous and endothermic in nature.The phosphate adsorption capacity for ZrMZ decreased with increasing pH from 5 to 10.When the initial solution pH was 10,ZMGZ still had a high phosphate uptake capacity(4.45 mg PO₄^3-/g).This suggests that ZMGZ possesses a wide pH range for the effective adsorptive removal of phosphate from water.The presence of Na⁺、K⁺、SO₄^2-、Ca²⁺、HCO₃^-and SO₃^2-in aqueous solution had a slight effect on phosphate adsorption onto ZMGZ,while coexisting HCO₃^-reduced the phosphate adsorption capacity for ZMGZ.There was a rapid decrease in the phosphate concentration from 8.52 to 0.30 mg PO₄^3-/L with the augment in the ZMGZ concentration ranging from 2 to 5 g/L.By contrast,an augment in the ZMGZ concentration from 2 to 5 g/L gave rise to a significant rise in the phosphate removal rate from 57.4 to 98.5%.When the ZMGZ concentration was above5 g/L,a further increase in the adsorbent concentration did not bring about the decrease of the phosphate concentration and the increase in the phosphate removal rate.Secondly,zirconium was coated onto the sueface of granular zeolite（GZ）by the solution-evaporation methodand the zirconium-loaded GZ（ZrGZs）with different zirconium loading levels of 5.53%,13.43%,27.89%（hereafter named as ZrGZ-1,ZrGZ-2 and ZrGZ-3,respectively）were synthesized.To determine the effect of zirconium loading amount on the adsorption of phosphate on ZrGZ,the adsorption characteristics and mechanism of phosphate on different ZrGZs were comparatively investigated using batch experiments.Results showed that the adsorption kinetics data of phosphate on ZrGZs could be well describedby a pseudo-second-ordermodel.The equilibrium adsorption data of phosphate on ZrGZs were found to be quite usefuldescribed by the Langmuir and Freundlich equilibrium adsorption models.The phosphate adsorption of ZrGZ-1showed little pHdependence in the pH range from 4 to9,but it decreased significantly decreased with the increase of pH from 9 to 10.The phosphate uptake ability for ZrGZ-2 and ZrGZ-3 decreased with increasing pH from 4to 10.On the basis of the Langmuir isotherm,the maximum monolayer adsorption capacities of phosphate for ZrGZ-1,ZrGZ-2 and ZrGZ-3were found to be 5.32,8.65and 10.3 mg·g^-1,respectively.The maximum phosphate uptake contents for the unit mass of zirconium in ZrGZ-1,ZrGZ-2 and ZrGZ-3 were calculated to be 96.2,64.4 and37.1 mg·g^-1,respectively.The desorption efficiencies of phosphorus from phosphate-loaded ZrGZ-1,ZrGZ-2 and ZrGZ-3 by 1 mol·L^-1 were 75.5%,66.2%and65.0%,respectively.From the point of view of the phosphate capacity for adsorbents,the use of ZrGZ with higher zirconium loading amount to remove phosphate is better than that with lower zirconium loading amount.From the viewpoint of reducing cost and improving the utilization efficiency of zirconium to adsorb phosphate,the application of ZrGZ with lower zirconium loading amount is better than that with higher zirconium loading amount.Then,a zirconium-loaded granular zeolite（ZrGZ）was prepared,characterized and used as a sediment amendment to control internal phosphorus（P）loading in heavily polluted river.The adsorption characteristics of phosphate on ZrGZ-amended sediment were investigated using batch experiments,and the stability of P in phosphate-adsorbed ZrGZ was evaluated using sequential chemical extraction method.Results showed that the Langmuir isotherm model was more suitable for describing the equilibrium adsorption data of phosphate on ZrGZ-amended sediment than the Freundlich and Dubinin-Radushkevich isotherm models.The adsorption process of phosphate on ZrGZ-amended sediment could be well described by the pseudo-second-order and Elovich kinetic models,and both film and intra-particle diffusion controlled the adsorption rate during the gradual adsorption stage.Coexisting SO₄^2-and HCO₃^-inhibited the adsorption of phosphate on ZrGZ-amended sediment.However,coexisting Na⁺、K⁺、Mg²⁺and Ca²⁺enhanced the phosphate adsorption,and the promoting effect decreased in the order of Ca²⁺>Mg²⁺>Na⁺（K⁺）.The ZrGZ-amended sediment exhibited a higher phosphate adsorption capacity than the unamended sediment.The maximum phosphate adsorption capacity derived from the Langmuir isotherm model was found to be 336 mg·kg^-1,which was higher than that for the unamended sediment(215 mg·kg^-1).Sequential tests showed that P in phosphate-adsorbed ZrGZ mainly existed in the form of NaOH-P and Res-P,which was difficult to be released.Results of this work indicate that ZrGZ addition enhances the phosphate adsorption capacity for river sediment,and ZrGZ is a promising amendment for controlling the release of P from river sediment.Finally,zirconium-modified granular zeolite（ZMGZ）was prepared and investigated for the suppression of the internal P loading in shallow landscape water bodies in this study.Results showed that the as-synthesized ZMGZ possessed high phosphate sorption capacity with a Langmuir maximum monolayer uptake capacity of6.87 mg PO₄^3-/g.Capping of landscape water body sediments with ZMGZ reduced the releasing rate of soluble reactive P（SRP）from the sediments into the overlying water,especially it significantly reduced the SRP releasing rate under anoxic condition.Furthermore,the sediment capping decreased the pore water SRP concentration,and it was difficult for most of P trapped by the ZMGZ covering layer to be re-released into the water column.When the ZMGZ-based covering layer was destroyed,ZMGZ became a sediment amendment.Mixing of landscape water body sediments with ZMGZ also resulted in the reduction of the SRP concentrations in the overlying and pore waters under the conditions of low and high dissolved oxygen,and it was also hard for most of P adsorbed by the added ZMGZ to be released back into the water column.Additionally,the addition of sediments with ZMGZ led to the decrease of the amounts of mobile and bioavailable P in the sediments.To sum up,we conclude that the capping/mixing combination method,which involves sediment capping with ZMGZ at first and then gradual mixing of ZMGZ with sediment via various natural disturbances（e.g.,physical disturbance and bioturbation）,is a very promising strategy for inhibiting the release of SRP from shallow landscape water body sediments into the overlying water.