[Effect of the Combined Use of Denitrifying Bacteria, Calcium Nitrate, and Zirconium-Modified Zeolite on the Mobilization of Nitrogen and Phosphorus in Sediments and Evaluation of Its Nitrate-Nitrogen Releasing Risk].

In this work, the influence of an integrated method based on calcium nitrate, denitrifying bacteria, and zirconium-modified zeolite (CN+DB+ZZ) on the transport and transformation of nitrogen (N) and phosphorus (P) in sediments was investigated, and the risk of nitrate release from the calcium nitrate-injected sediment was evaluated. The effects of the single calcium nitrate injection (CN), calcium nitrate, and denitrifying bacteria combined treatment (CN+DB) and the combined treatment using calcium nitrate injection and zirconium-modified zeolite capping (CN+ZZ) on the mobilization of N and P in sediment were compared, and the nitrate releasing risk of these methods was also evaluated. The results indicated that although CN treatment could effectively control the P release from the sediment, this method could not effectively control the release of ammonium-nitrogen from sediment and has a high risk of releasing nitrate-nitrogen. The CN+DB combined method not only could effectively control the liberation of sedimentary P but also reduce the risk of nitrate-nitrogen release from the calcium nitrate-injected sediment compared with the single CN method. However, the CN+DB combined method could not effectively control the release of ammonium-nitrogen from the sediment. The CN+ZZ combined treatment not only could effectively prevent the release of sedimentary P but could also greatly reduce the release of ammonium-nitrogen from the sediment. However, the CN+ZZ combined method could result in a substantial release of nitrate-nitrogen from the calcium nitrate-injected sediment. The CN+DB+ZZ combined technology could effectively control the release of P from sediment as well as greatly reduce the risk of ammonium-nitrogen release from the sediment. Furthermore, the CN+DB+ZZ combined method resulted in a significant reduction of nitrate-nitrogen released from the calcium nitrate-injected sediment compared with the CN and CN+ZZ treatment methods. The prevention of the dissolution of the P-bound iron oxide/hydroxide in the sediment, the reduction of redox-sensitive P in sediment, and the improvement of the phosphate and ammonium adsorption abilities of sediment by the CN+DB+ZZ combined method is critical to control the release of phosphorus and ammonium-nitrogen from sediment using this method. Results of this study reveal that the CN+DB+ZZ combined technology could be a promising method for the control of phosphorus and ammonium-nitrogen release from sediments.

[Combined Use of Zirconium-Modified Bentonite Capping and Calcium Nitrate Addition to Control the Release of Phosphorus from Sediments].

In this study, sediment incubation experiments were carried out to investigate the efficiency and mechanism of the control of phosphorus (P) release from sediments. The results showed that under anoxic conditions, P could be released from the sediment into the pore water first and then the dissolved P in the pore water could be transported into the overlying water, leading to high concentrations of soluble reactive P (SRP) and diffusive gradient in thin-films (DGT)-labile P in the overlying water. However, the combined use of calcium nitrate (CN) addition and zirconium-modified bentonite (ZB) capping could effectively control the release of P from sediment, resulting in the low concentrations of SRP and DGT-labile P in the overlying water. Furthermore, the combined use of CN addition and ZB capping could significantly decrease the concentrations of SRP and DGT-labile P in the sediment. In addition, the combined utilization of CN addition and ZB capping also could result in a reduction of redox sensitive P (BD-P) in the uppermost sediment layer. The reduction of pore water SRP, DGT-labile P, and BD-P in sediment solids is of great importance to the control of sediment-P liberation by the combined use of CN addition and ZB capping. The reduction efficiency of overlying water SRP by combined CN addition/ZB capping technology was higher than that of single CN addition technology. Compared to that of single CN addition technology, the reduction efficiencies of pore water SRP, SRP diffusion flux across the sediment/overlying water interface (SWI), and BD-P in the sediment by the combined use of CN addition and ZB capping were also higher. The combined technology based on CN addition and ZB capping had a higher reduction efficiency of overlying water SRP during the late stage of sediment remediation than the single technology based on ZB capping, and the former had higher reduction efficiencies of pore water SRP, DGT-labile P, and SRP diffusion flux across the SWI and apparent P diffusion flux through the SWI than the latter. The results of this work indicate that the combined use of CN addition and ZB capping is a very promising method for the control of P release from sediments.

[Use of Iron-modified Calcite as an Active Capping Material to Control Phosphorus Release from Sediments in Surface Water Bodies].

The use of calcite (CA) as an active capping material has high potential for controlling the release of phosphorus (P) from sediments, but its efficiency still needs to be enhanced. To address this issue, an iron-modified CA (Fe-CA) was prepared, the removal performance of phosphate from aqueous solution by Fe-CA was studied, and the efficiency of the use of Fe-CA as an active capping material to prevent the liberation of P from sediments was investigated. The results showed that Fe-CA exhibited much higher phosphate removal ability than CA. The phosphate removal efficiency of Fe-CA increased with an increase in the Fe-CA dosage. Increasing the initial phosphate concentration gave rise to an increase in the amount of phosphate removed by Fe-CA, and the maximum amount of phosphate removed by Fe-CZ reached 3.09 mg·g-1. Sediment capping with Fe-CA could effectively control the release of soluble reactive P (SRP) from the sediment into the overlying water, leading to a very low concentration of SRP in the overlying water. Additionally, the Fe-CA capping also resulted in the transformation of a small amount of redox-sensitive P (BD-P) and metal-oxide-bound P (NaOH-rP) in sediments to residual P (Res-P), leading to a slight increase in the stability of P in the sediment. The overwhelming majority (90.8%) of P bound by the Fe-CA capping layer existed in the form of NaOH-rP, calcium-bound P (HCl-P), and Res-P, which are relatively very stable. Furthermore, the percentage of bioavailable P (BAP) as a proportion of total extractable P in the P-bound Fe-CA capping layer was very low, and the bound P was re-released with difficulty into the water column for algae growth. Compared to CA capping, the efficiency for the control of sedimentary P release into the overlying water by Fe-CA capping was much higher, and the stability of P bound by the Fe-CA capping layer was also higher. The results of this work indicate that Fe-CA is a very promising active capping material for the interception of the release of P from sediments into the overlying water.

[Efficiency of Magnesium Hydroxide Capping and Amendment to Control Phosphorus Release from Sediments].

Eutrophication of freshwater bodies has become a global environmental problem, and phosphorus (P) has been identified as one of the key limiting factors responsible for this eutrophication problem. Reducing internal P release is crucial to the control of the eutrophication of freshwater bodies besides reducing the input of external P. To control sedimentary P release, magnesium hydroxide[Mg(OH)2] was applied as a capping and amendment material in this study. The adsorption performance of phosphate on Mg(OH)2 was investigated in batch mode, and the effect of Mg(OH)2 capping and amendment on the mobilization of P in sediments was studied using sediment incubation experiments. Results showed that Mg(OH)2 exhibited good adsorption performance toward phosphate. The phosphate removal efficiency of Mg(OH)2 increased with increasing adsorbent dosage. The adsorption equilibrium data of phosphate on Mg(OH)2 could be better described by the Freundlich and Dubinin-Radushkevich isotherm models compared to the Langmuir isotherm model. Mg(OH)2 capping and addition both could effectively control the release of reactive soluble P (SRP) from sediments into the overlying water, resulting in a low concentration of SRP in the overlying water under Mg(OH)2 capping and amendment conditions. Mg(OH)2 capping and amendment both could reduce pore water SRP in the uppermost sediment (0-10 mm), which played a key role in the control of the release of SRP from sediments into the overlying water. The as-prepared Mg(OH)2 possessed a much higher phosphate adsorption ability than commercial Mg(OH)2, and the former also had a higher controlling efficiency of sedimentary P release than the latter. In summary, Mg(OH)2 is a promising capping and amendment material for the control of internal phosphorus release in freshwater bodies.

[Regulating Effect and Mechanism of Calcite/Chlorapatite Mixture Addition on Transformation and Transport of Phosphorus in Sediments].

Understanding the effect of calcite and chlorapatite mixture (CA/ClAP) addition on the mobilization of phosphorus (P) in sediments is crucial to the application of CA/ClAP as an amendment material to control the release of P from sediments. To address this issue, batch experiments were conducted to investigate the removal performance of phosphate by CA/ClAP, and sediment incubation experiments were carried out to study the effect of CA/ClAP addition on the mobilization of P in sediments. The results showed that the removal ability of phosphate by CA/ClAP was much higher than those by calcite and chlorapatite, and the kinetics data of phosphate removal by CA/ClAP followed a pseudo-second-order kinetics model. Increasing calcite and chlorapatite dosages would be favorable for the removal of phosphate by CA/ClAP, and coexisting Ca2+ enhanced the phosphate removal. CA/ClAP addition not only reduced the concentration of soluble reactive P (SRP) in the overlying water, but also decreased the concentration of SRP in the pore water. The addition of CA/ClAP in sediments caused an increase in the content of P in the sediments, but the increased P mainly existed in the form of calcium-bound P (HCl-P), which was difficult to be re-released into the water column under anoxic and common pH (5-9) conditions. The reduction of SRP in the pore water after the addition of CA/ClAP played an important role in the prevention of sedimentary P liberation into the overlying water by the CA/ClAP amendment. The results of this work indicate that CA/ClAP can be used as an amendment material for interception of the release of P from sediments into overlying water.

[Adsorption of Phosphate on Mg/Fe Layered Double Hydroxides (Mg/Fe-LDH) and Use of Mg/Fe-LDH as an Amendment for Controlling Phosphorus Release from Sediments].

We determine the efficiency and mechanism of Mg/Fe layered double hydroxides (Mg/Fe-LDH) addition for the control of phosphorus (P) release from sediments by studying the adsorption behavior and mechanism of phosphate from an aqueous solution on Mg/Fe-LDH. The impact of Mg/Fe-LDH addition on the mobilization of P in sediments as well as the adsorptive removal of phosphate by sediments is investigated, and the stabilization of P bound by Mg/Fe-LDH is also evaluated. Results showed that the kinetics data of phosphate adsorption onto Mg/Fe-LDH fitted better with the Elovich kinetics model than with the pseudo-first-order and pseudo-second-order kinetics models, and that the Freundlich and Dubinin-Radushkevich models were more suitable for describing the adsorption isotherm behavior of phosphate on Mg/Fe-LDH than the Langmuir model. Phosphate adsorption possessed a wide effective pH range of 4-10. Coexisting Ca2+ and Mg2+ enhanced phosphate adsorption onto Mg/Fe-LDH, while coexisting Na+, K+, and Cl- had negligible impacts on the phosphate adsorption. The presence of SO42- and HCO3- in aqueous solution inhibited the adsorption of phosphate on Mg/Fe-LDH. The phosphate adsorption mechanisms were deduced to be anion exchange, electrostatic attraction, ligand exchange and inner-sphere complex formation. The addition of Mg/Fe-LDH into sediments not only greatly reduced the concentration of reactive soluble P (SRP) in the overlying water, but also significantly decreased the level of SRP in the pore water. In addition, Mg/Fe-LDH addition also increased the adsorption capacity for the sediments, and the phosphate adsorption ability for the Mg/Fe-LDH-amended sediments increased with increased amendment dosage. Almost half of the phosphate bound by Mg/Fe-LDH existed in the form of relatively stable P, i.e., metal oxide-bound P (NaOH-rP), which was difficult to release back into the water column under normal pH and anoxic conditions. Nearly half of the phosphate bound by Mg/Fe-LDH existed in the form of easily released P, i.e., NH4Cl extractable P (NH4Cl-P) and redox-sensitive P (BD-P), which had a high risk of re-releasing into the water column. We conclude that it is very necessary for Mg/Fe-LDH to be recycled from the sediments after the application of Mg/Fe-LDH as an amendment to control sedimentary P liberation.

[Comparison of the Control of Sedimentary Phosphorus Release Using Zirconium-, Lanthanum-, and Lanthanum/Zirconium-Modified Zeolites as Sediment Amendments].

In this study, the adsorption characteristics of phosphate on zirconium-modified zeolite (ZrMZ), lanthanum-modified zeolite (LaMZ) and lanthanum/zirconium-modified zeolite (LaZrMZ) were comparatively investigated, and the effect of ZrMZ, LaMZ, and LaZrMZ addition on the mobilization of phosphorus (P) in sediments was comparatively studied. Results showed that the phosphate adsorption capacity decreased in the order of LaZrMZ > LaMZ > ZrMZ. The addition of LaZrMZ, LaMZ, and ZrMZ in sediments all could effectively reduce the concentrations of soluble reactive P (SRP) in the overlying and pore waters. Furthermore, the addition of LaZrMZ, LaMZ and ZrMZ all could decrease the amount of mobile P in sediments, and the reduction rate decreased in the order of LaMZ > LaZrMZ > ZrMZ. The amendment of sediments with LaZrMZ, LaMZ, and ZrMZ all could lead to a decrease in the amount of water-soluble P (WSP), algal available P (AA-P) and iron oxide-filter paper extractable P (FeO-P) in the sediments. The reduction rate of WSP and AA-P decreased in the order of LaMZ > LaZrMZ > ZrMZ, and the reduction rate of FeO-P decreased in the order of LaZrMZ > ZrMZ > LaMZ. The addition of LaZrMZ and LaMZ both could reduce the content of readily desorbed P (RDP), and the reduction rate decreased in the order of LaMZ>LaZrMZ. The LaZrMZ and ZrMZ amendments both could decrease the concentration of NaHCO3 extractable P (Olsen-P) in sediments, and the reduction rate decreased in the order of LaZrMZ>ZrMZ. The results of this study demonstrate that the addition of ZrMZ, LaMZ, and LaZrMZ in sediments all could effectively intercept the upward mobilization of sedimentary P to the overlying water, and LaZrMZ is a very promising amendment for the control of the internal P loading from the point of view of both phosphate adsorption ability and bioavailable P immobilization efficiency.

[Effect of Zirconium-Modified Zeolite Addition on Migration and Transformation of Phosphorus in River Sediments Under Static and Hydrodynamic Disturbance Conditions].

In this study, the effect of the addition of zirconium-modified zeolite (ZrMZ) on the migration and transformation of phosphorus (P) in river sediments under static and hydrodynamic disturbance conditions was studied using sediment core incubation experiments. Results showed that, whether under static or hydrodynamic disturbance condition, the ZrMZ amendment suppressed the release of SRP from sediments into the overlying water. Furthermore, the addition of ZrMZ to the upper sediment (0-10 mm) not only resulted in the decrease of the dissoluble reactive P (SRP) concentration in the overlying water at a depth of 0-30 mm, but also led to the decrease of the diffusion flux of SRP from the pore water to the overlying water across the sediment-water interface (SWI). In addition, the ZrMZ amendment induced the transformation of the redox-sensitive P (BD-P) and HCl extractable P (HCl-P) into the metal oxide-bound P (NaOH-rP) and residual P (Res-P), thus resulting in the reduction of mobile P (sum of NH4Cl extractable P and BD-P) in the top 10 mm of sediment. In addition, the addition of ZrMZ into the top 10 mm of sediment resulted in reduction of the content of mobile P in 10-20 mm of sediment. Furthermore, the effect of ZrMZ addition on the migration and transformation of P in sediments under hydrodynamic disturbance condition had a certain difference from that under static condition. The presence of hydrodynamic disturbance enhanced the immobilization efficiency of SRP in the pore water at a depth of 0-20 mm by the ZrMZ amendment, and also increased the reduction efficiency of the SRP diffusion flux from the pore water to the overlying water across the SWI by the ZrMZ amendment. However, the efficiency of the control of SRP release from sediments to the overlying water by the ZrMZ amendment was slightly reduced by the hydrodynamic disturbance. The reductions of mobile P in the top sediment, SRP in the pore water as well as the diffusion flux of SRP from the pore water to the overlying water across the SWI played a key role in the control of SRP release from sediments to the overlying water by the ZrMZ amendment. Results of this work indicate that ZrMZ is a very promising amendment for the control of SRP release from river sediments under static and hydrodynamic disturbance conditions.

[Effect of Magnetic Zirconium/Iron-Modified Bentonite Addition on Phosphorus Mobilization and Species Transformation in River Sediments].

A magnetic zirconium/iron-modified bentonite (ZrFeBT) was prepared, and the effect of ZrFeBT addition on the mobilization and species transformation of P in river sediments was investigated using incubation sediment core experiments. The results showed that, under anoxic conditions, P could be released from river sediments into the pore water, and then P in the pore water could be released into the overlying water. The addition of ZrFeBT into river sediments could greatly suppress the release of P from river sediments into the pore water under anoxic conditions. Therefore, the release of P from the pore water into the overlying water could be significantly suppressed by the addition of ZrFeBT. After the addition of ZrFeBT into river sediments, the transformation of loosely sorbed P (Labile-P) and BD extractable P (BD-P) to NaOH extractable P (NaOH-rP) and residual P (Res-P) in the sediments was observed. The decrease of bioavailable P (BAP) including water soluble P (WSP), readily desorbable P (RDP), NaHCO3 extractable P (Olsen-P), algal available P (AAP), and Fe oxide-paper extractable P (FeO-P) in the sediments was also observed. A certain amount of P in the ZrFeBT after the incubation experiment was present in the form of mobile P (Labile-P and BD-P), Olsen-P, and FeO-P, which could be re-released into the pore water and overlying water when the environmental conditions change in the future. The control of P release from river sediment into the overlying water by the addition of ZrFeBT could be mainly attributed to the reduction of P in the pore water as well as the reduction of mobile P and BAP in the sediments after ZrFeBT amendment. The results of this study inidcated that ZrFeBT is a promising amendment for the regulation of P release from river sediments into the overlying water.

[Influence of Calcium Ion Pre-treatment on Phosphate Adsorption onto Magnetic Zirconium/Iron-modified Bentonite].

Two kinds of magnetic zirconium/iron-modified bentonites (ZrFeBTs), including magnetic zirconium/iron modified raw bentonite (ZrFeRBT) and magnetic zirconium/iron-modified Ca2+-pretreated bentonite, (ZrFeCaBT) were prepared and characterized. Their phosphate adsorption characteristics were compared to determine the effect of the Ca2+ pre-treatment on the adsorption of phosphate onto ZrFeBTs. The results showed that the as-prepared ZrFeBTs contained Fe3O4 and Zr, and the content of exchangeable Ca2+ in ZrFeCaBT was much higher than that in ZrFeRBT. The adsorption isotherm data exhibited good agreement with the Langmuir isotherm model, with maximum monolayer phosphate adsorption capacities of 8.70 mg·g-1 and 11.5 mg·g-1 for ZrFeRBT and ZrFeCaBT, respectively. The isotherm and kinetics studies showed that the adsorption of phosphate on ZrFeBTs was a chemisorption process. The phosphate adsorption capacities for ZrFeBTs decreased with increasing solution pH. The ZrFeBTs exhibited a high selective adsorption for phosphate in the presence of anions and cations, including Cl-, HCO3-, SO42-, NO3-, Na+, K+, Mg2+, and Ca2+. Furthermore, coexisting Ca2+ greatly enhanced the adsorption of phosphate onto ZrFeBTs. The pre-treatment of raw bentonite with Ca2+ significantly improved the adsorption of phosphate onto ZrFeBTs.

[Adsorption Behavior of Phosphate from Water on Zirconium-loaded Granular Zeolite-amended Sediment].

In this study, a zirconium-loaded granular zeolite (ZrGZ) was prepared, characterized and used as a sediment amendment to control internal phosphorus (P) loading in water samples from a 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 a 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. The coexistence of SO42- and HCO3- inhibited the adsorption of phosphate on ZrGZ-amended sediment, while coexisting Na+, K+, Mg2+ and Ca2+ enhanced the phosphate adsorption, and this 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, and 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-rP and Res-P, which was relatively unreactive. These results indicated that ZrGZ addition enhanced the phosphate adsorption capacity of river sediment, and that ZrGZ was a promising amendment for controlling the release of P from river sediment.

[Effect of Zirconium-modified Zeolite Addition on Phosphorus Release and Immobilization in Heavily Polluted River Sediment].

The effect of zirconium-modified zeolite (ZrMZ) addition on the release and immobilization of phosphorus in heavily polluted river sediment was investigated using microcosm incubation experiments. Results showed that addition of ZrMZ to sediment greatly reduced concentrations of P in pore water and overlying water, also reducing the release flux of P across the interface between overlying water and sediment. The addition of ZrMZ to sediment resulted in the transformation of NH4Cl extractable P (NH4Cl-P), Na2S2O4/NaHCO3 extractable P (BD-P), and HCl extractable P (HCl-P) into NaOH extractable P (NaOH-rP) and residual P (Res-P) in sediment, thereby leading to the reduction of mobile P (sum of NH4Cl-P and BD-P) in sediment. Content of bioavailable P (BAP) including water soluble P (WSP), readily desorbable P (RDP), iron oxide paper strip extractable P (FeO-P), and anion resin extractable P (Resin-P) in sediment also declined following addition of ZrMZ. Control of P release from sediment by ZrMZ could be due to reduction of P in pore water and immobilization of P in sediment. Results of this work indicate that ZrMZ is very promising for controlling P release from sediments in heavily polluted rivers.

[Adsorption of Phosphate from Aqueous Solutions on Sediments Amended with Magnetite-Modified Zeolite].

Understanding the characteristics of phosphate adsorption onto magnetite-modified zeolite (MZ)-amended sediment is helpful for knowing the exchange behavior and process of phosphorus at the interface between the overlying water and MZ-amended sediment. Furthermore, it is helpful for the application of MZ as an amendment to control phosphorus release from sediment. To achieve this goal, the adsorption of phosphate on unamended and MZ-amended sediments was comparatively investigated using a series of batch experiments, and the fractionation of phosphorus in the phosphate-adsorbed MZ was studied using a sequential extraction process. The kinetic data of phosphate adsorption onto unamended and MZ-amended sediments were more suitably fitted to the Elovich model than to the pseudo-first-order and pseudo-second-order models. The equilibrium adsorption data of phosphate adsorption onto the unamended and MZ-amended sediments were well described by the Langmuir, Freundlich, and Dubinin-Radushkevic isotherm model. The phosphate adsorption performance of the unamended and MZ-amended sediments decreased with increasing solution pH from 4 to 11. The presence of cations, such as K+, Mg2+, and Ca2+, enhanced the adsorption of phosphate on the unamended and MZ-amended sediments, and the promoting effect decreased in the order of Ca2+ > Mg2+ > K+, whereas the presence of HCO3- inhibited the adsorption of phosphate. The mechanisms for phosphate adsorption onto the unamended and MZ-amended sediments involved electrostatic attraction and ligand exchange, while the mechanism for the adsorption of phosphate on MZ in the amended sediment involved ligand exchange. The sequential extraction analysis of phosphate-adsorbed MZ showed that 49.4% of phosphorus in MZ existed in the mobile form (NH4Cl-P, BD-P, and NaOH-nrP), which could be easily released from MZ. Therefore, the used MZ should be recovered from sediment using external magnetic fields after its application. The results of this study indicated that MZ is a promising sediment amendment for the control of internal loading in rivers.

[Adsorption of Phosphate from Aqueous Solution on Hydrous Zirconium Oxides Precipitated at Different pH Values].

In this study, hydrous zirconium oxide (HZO) samples precipitated at different pH values were prepared, characterized and used as adsorbents to remove phosphate from aqueous solution. The adsorption characteristics and mechanisms of phosphate on these HZO samples were investigated. The results showed that the presence of Na+ slightly enhanced the adsorption of phosphate on HZO samples prepared at precipitation pH of 4.8 and 8.0, but it greatly enhanced the adsorption of phosphate on HZO prepared at precipitation pH of 10.6. The presence of Ca2+ slightly enhanced the adsorption of phosphate on HZO prepared at precipitation pH of 4.8, but it significantly enhanced the adsorption of phosphate on HZO samples prepared at precipitation pH of 8.0 and 10.6. The presence of HCO3- or SO42- inhibited phosphate adsorption onto HZO, and the inhibitory effect of these anions on phosphate adsorption onto HZO precipitated at pH 4.8 was much higher than that on phosphate adsorption onto HZO samples precipitated at pH 8.0 and 10.6. The phosphate adsorption was dependent upon solution pH, and it decreased with increasing solution pH. The Langmuir, Freundlich and Dubinin-Redushckevich (D-R) isotherm models fitted well to the adsorption equilibrium data of phosphate on HZO samples precipitated at pH 4.8, 8.0 and 10.6. In the presence of Na+ but in the absence of Ca2+, there was no significant difference of the maximum phosphate monolayer adsorption capacity derived from the Langmuir isotherm model among HZO samples prepared at precipitation pH of 4.8, 8.0 and 10.6. In the presence of Ca2+, the maximum phosphate monolayer adsorption capacity derived from the Langmuir isotherm model for HZO precipitated at pH 8.0 or 10.6 was much higher than that for HZO precipitated at pH 4.8. The mechanism for phosphate adsorption onto HZO mainly obeyed the inner-sphere complexing mechanism. The surface chloride and hydroxyl groups played the key role in the adsorption of phosphate on HZO precipitated at pH 4.8 or 8.0, while only the surface hydroxyl groups played the key role in the adsorption of phosphate on HZO precipitated at pH 10.6. Results of this work demonstrated that the HZO precipitated at pH 8.0 or 10.6 was a more promising adsorbent for removing phosphate from wastewater than the HZO precipitated at pH 4.8.

[Comparison of Phosphate Adsorption onto Zirconium-Modified Bentonites with Different Zirconium Loading Levels].

In this study, zirconium-modified bentonites (ZrMBs) with different zirconium loading levels were prepared, and the adsorption behaviors of phosphate on these ZrMBs were comparatively investigated using batch experiments. The results showed that the kinetic process of phosphate on ZrMBs well followed the pseudo-second-second kinetic model. The kinetic process was divided into three stages, including a rapid external surface adsorption stage, a gradual adsorption stage where both the intra-particle diffusion and film diffusion were rate-controlled, and a final equilibrium adsorption stage. The equilibrium adsorption data of phosphate on ZrMBs could be well described by the Langmuir, Freundlich, Sips and Dubinin-Radushkevich isotherm models. Phosphate adsorption onto ZrMBs was more favorable under strongly acidic condition than under weakly acidic or neutral condition, while phosphate adsorption onto ZrMBs under weakly acidic or neutral condition was more favorable than that under alkaline condition. Coexistence of Na+ and K+ slightly enhanced phosphate adsorption onto ZrMBs, while coexisting Ca2+ greatly enhanced the phosphate adsorption. The presence of HCO3- or SO2-4 inhibited the adsorption of phosphate on ZrMBs. The mechanism for phosphate adsorption onto ZrMBs followed the ligand exchange and inner-sphere complexing mechanism. The phosphate adsorption capacity for ZrMB increased with increasing loading level of zirconium, while the amount of phosphate adsorbed on unit mass of ZrO2 in ZrMB decreased with increasing loading amount of zirconium in ZrMB. When the loading amount of ZrO2 in ZrMB increased from 3.61% to 13.15%, the maximum phosphate adsorption capacity (MPAC) for ZrMB increased from 3.83 to 9.03 mg·g-1, while a further increase in the ZrO2 loading amount to 19.63% resulted in a slight increase of MPAC to 9.66 mg·g-1. However, an increase in the loading amount of ZrO2 in ZrMB from 3.61% to 19.63% caused a decrease of the MPAC for the ZrO2 located in ZrMB from 106 to 49.2 mg·g-1. Considering both cost and adsorption capacity of adsorbent, the ZrMB with 13.15% of zirconium loading amount could be more suitably used as an adsorbent to remove phosphate from aqueous solution than the other ZrMBs.

[Effect of Zirconium Modified Kaolin-Based Cap on Migration and Transformation of Phosphorus Between Sediment and Overlying Water].

In this study, microcosm incubation experiments were conducted to investigate the effect of zirconium modified kaolin (ZrMK)-based cap on the migration and transformation of phosphorus (P) between sediments collected from a heavily polluted river and overlying waters under anaerobic conditions. The results showed that a large amount of P was released from the sediments into the overlying water column under anaerobic conditions, and the overwhelming majority of P in the overlying water existed in the form of phosphate. The flux of P from the anaerobic sediments was slightly reduced by the kaolin-based cap, while significantly reduced by the ZrMK-based cap. Sequential extraction of P from the kaolin-based cap at the end of incubation experiments suggested that 29% of P adsorbed by kaolin existed as the bicarbonate-dithionite extracted P (BD-P), and 63% of adsorbed P existed as the residual P (Res-P). Sequential extraction of P from the ZrMK-based cap at the end of incubation experiments suggested that 90% of P adsorbed by ZrMK existed as the NaOH extractable P (NaOH-P) and Res-P, which were unlikely to be released under anaerobic conditions. Compared with no capping, sediments capping with ZrMK did not promote BD-P release from the sediments under anaerobic conditions, but promoted the formation of NaOH-P in the sediments. X-ray photoelectron spectroscopy (XPS) and solid state ³¹P nuclear magnetic resonance (NMR) analyses of ZrMK-based caps before and after sediment incubation experiments indicated that the adsorption of P by the ZrMK-based caps followed the ligand exchange and inner-sphere complexing mechanism. Results of this work indicate that ZrMK is a promising active capping material for controlling P release from sediments in heavily polluted rivers.

[Adsorption of Phosphate by Lanthanum Hydroxide/Natural Zeolite Composites from Low Concentration Phosphate Solution].

A series of composites of lanthanum hydroxide/natural zeolite ( La( OH) 3/NZ composites) were prepared by co-precipitation method, and these composites were used as adsorbents to remove phosphate from aqueous solution. The phosphate adsorption capacities of different composites prepared with different precipitated pH values were compared in batch mode. The adsorption characteristics of phosphate from aqueous solution on the La(OH)3/NZ composite prepared with the precipitated pH value of 11 was investigated using batch experiments. The results showed that the La(OH)3/NZ composite prepared with the precipitated pH values of 5-7 and 13 had a low adsorption capacity for phosphate in aqueous solution, while the La( OH) 3/NZ composites prepared with the precipitated pH values of 9-12 exhibited much higher phosphate adsorption capacity. The phosphate adsorption capacity of the La (OH)3/NZ composite increased with the increase of the precipitated pH value from 9 to 11, but remained basically unchanged with the increase of the precipitated pH value from 11 to 12. The equilibrium adsorption data of phosphate from aqueous solution on the La ( OH ) 3/NZ composite prepared with the precipitated pH value of 11 could be described by the Langmuir isotherm model with the predicted maximum phosphate adsorption of 44 mg x g(-1) (phosphate solution pH 7 and 30 degrees C). The kinetic data of phosphate adsorption from low concentration phosphate solution on the La(OH)3/NZ composite prepared with the precipitated pH value of 11 well followed a pseudo-second-order model. The presence of Cl- and SO4(2-) in low concentration phosphate solution had no negative effect on phosphate adsorption onto the La(OH)3/NZ composite prepared with the precipitated pH value of 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 the La(OH)3/NZ composite prepared with the precipitated pH value of 11. The mechanism for phosphate adsorption onto the La(OH)3/NZ composite prepared with the precipitated pH value of 11 was explained by the ligand exchange process. Results of this work indicate that the La(OH)3/NZ composite prepared with the precipitated pH value of 11 is a promising adsorbent material for the removal of low concentration phosphate from water/wastewater.

[Adsorption Characteristics of Nitrate and Phosphate from Aqueous Solution on Zirconium-Hexadecyltrimethylammonium Chloride Modified Activated Carbon].

A novel adsorbent material, i.e., zirconium-cationic surfactant modified activated carbon (ZrSMAC) was prepared by loading zirconium hydroxide and hexadecyltrimethylammonium chloride (CTAC) on activated carbon, and was used as an adsorbent for nitrate and phosphate removal from aqueous solution. The adsorption characteristics of nitrate and phosphate on ZrSMAC from aqueous solution were investigated in batch mode. Results showed that the ZrSMAC was effective for nitrate and phosphate removal from aqueous solution. The pseudo-second-order kinetic model fitted both the nitrate and phosphate kinetic experimental data well. The equilibrium isotherm data of nitrate adsorption onto the ZrSMAC were well fitted to the Langmuir, Dubinin-Radushkevich (D-R) and Freundlich isotherm models. The equilibrium isotherm data of phosphate adsorption onto the ZrSMAC could be described by the Langmuir and,D- R isotherm models. According to the Langmuir isotherm model, the maximum nitrate and phosphate adsorption capacities for the ZrSMAC were 7.58 mg x g(-1) and 10.9 mg x g(-1), respectively. High pH value was unfavorable for nitrate and phosphate adsorption onto the ZrSMAC. The presence of Cl-, HCO3- and SO4(2-) in solution reduced the nitrate and phosphate adsorption capacities for the ZrSMAC. The nitrate adsorption capacity for the ZrSMAC was reduced by the presence of coexisting phosphate in solution, and the phosphate adsorption capacity for the ZrSMAC was also reduced by the presence of coexisting nitrate in solution. About 90% of nitrate adsorbed on the ZrSMAC could be desorbed in 1 mol x L(-1) NaCl solution, and about 78% of phosphate adsorbed on the ZrSMAC could be desorbed in 1 mol x L(-1) NaOH solution. The adsorption mechanism of nitrate on the ZrSMAC included the anion exchange interactions and electrostatic attraction, and the adsorption mechanism of phosphate on the ZrSMAC included the ligand exchange interaction, electrostatic attraction and anion exchange interaction.

[Efficiency of Sediment Amendment with Zirconium-Modified Kaolin Clay to Control Phosphorus Release from Sediments in Heavily Polluted Rivers].

A zirconium-modified kaolin- clay (ZrMK) was prepared and used as a sediment amendment to control the release of phosphorus (P) from sediments in heavily polluted rivers under low dissolved oxygen (DO) condition. Results showed that the ZrMK exhibited excellent adsorption performance of phosphate in water. The phosphate adsorption capacity of the ZrMK increased with the increasing of loading amount of zirconium in the ZrMK. The phosphate adsorption capacity of the ZrMK increased with the increase of the precipitated pH value from 8 to 10, remained basically unchangeable with the increase of the precipitated pH value from 10 to 11, but decreased with the increase of the precipitated pH value from 11 to 12. The phosphate equilibrium adsorption data of the ZrMK can be better described by the Langmuir isotherm model than the Freundlich isotherm model when the ZrMK was prepared with the precipitated pH value 10. Sequential extraction of P from the phosphate-adsorbed ZrMK showed that most of phosphate-P bound by the ZrMK (about 84% of total P) existed in the form of the metal oxide P (NaOH-P) and residual P (Res-P), which was unlikely to be released under hypoxia and common pH (5-9) conditions. The fluxes of phosphate-P and total P (TP) from sediments into the overlying water column were greatly reduced with the adding of ZrMK to sediments under low dissolved oxygen conditions. The ZrMK-amended sediments exhibited much higher phosphate adsorption capacity than the original sediments, and the former had much lower phosphate adsorption/desorption equilibrium concentration (EPC,) than the latter. Our findings suggest that the ZrMK can be used as an efficient sediment amendment for controlling P release from sediments in heavily polluted rivers under low dissolved oxygen conditions.

[Synthesis of hydroxyapatite/magnetite/zeolite composite for Congo red removal from aqueous solution].

In this study, a novel hydroxyapatite/magnetite/zeolite (HAP/Fe3O4/Zeo) composite was prepared, characterized and used as an adsorbent to remove Congo red (CR) from aqueous solution. The adsorption characteristics of CR from aqueous solution on the HAP/Fe3O4/Zeo composite were investigated using batch experiments. Results showed that the HAP/Fe3O4/Zeo composite was effective for the removal of CR from aqueous solution. The CR adsorption capacity for the HAP/Fe3O4/Zeo composite decreased with solution pH increasing from 3 to 4 or solution pH increasing from 7 to 11, and remained basically unchanged with pH increasing from 4 to 7. The CR removal efficiency of the HAP/Fe3O4/Zeo composite increased with increasing adsorbent dosage, while the amount of CR adsorbed on the HAP/Fe3O4/Zeo composite decreased with increasing adsorbent dosage. The adsorption kinetic data of CR on the HAP/Fe3O4/Zeo composite well fitted a pseudo-second-order model. The equilibrium adsorption data of CR on the HAP/Fe3O4/Zeo composite could be described by the Langmuir and Freundlich isotherm models. The maximum monolayer adsorption capacity for CR derived from the Langmuir isotherm model was determined to be 117 mg x g(-1) at pH 7 and 303 K. The adsorption process of CR on the HAP/Fe3O4/Zeo composite was spontaneous and endothermic. The main mechanisms for the adsorption of CR on the HAP/Fe3O4/Zeo composite at pH 7 included surface complexation, hydrogen bonding and Lewis acid-base reaction. Thermal regeneration showed that the HAP/Fe3O4/Zeo composite could be used for five desorption-adsorption cycles with high removal efficiency for CR in each cycle. X-ray diffraction (XRD) analysis revealed that the HAP/Fe3O4/zeolite composite contained Fe3O4, and this composite had relatively high saturation magnetization. The HAP/Fe3O4/Zeo composite adsorbed with CR could be collected from aqueous solution under an external magnetic field quickly. Results of this study suggested that the HAP/Fe3O4/Zeo composite should be applicable for the removal of CR from wastewater.