Study On Adsorption Of Phosphate, Ammonium And Phenanthrene From Aqueous Solution And Immobilization Of Phosphate And Ammonium In Sediment By Modified Zeolite

Large quantities of untreated pollutants such as phosphate, ammonium andphenanthrene are discharged into lakes directly, resulting in water is pulluted. Thecontaminated sediment could act as an interior polluted source, and may pool thepollutants to the overlying water. Natural zeolite is a low-cost and high efficiencyadsorbent. Natural zeolite has high selectivity to the cationic pollutants such asammonia nitrogen due to its high cation-exchange ability. In order to improve thephosphate and phenanthrene sorption capacities for zeolite, modified zeolites withdifferent chemical reagents were prepared. The sorption characteristics and mechanismof phosphate, ammonium and phenanthrene from aqueous solution withlanthanum-modified zeolite (LaMZ), zirconium/magnetite/zeolite composite(Zr/Fe3O4/Zeo) and cetylpyridinium bromide (CPB)-modified zeolite were investigatedusing batch experiments. The immobilization of phosphate and ammonium in sedimentby LaMZ was also investigated. The main conclusions are as follows:Lanthanum-modified zeolite (LaMZ) was used as an adsorbent to removephosphate and ammonium from aqueous solution. Results showed that LaMZ waseffective for the removal of phosphate and ammonium from aqueous solution. Thekinetic studies showed that the adsorption of phosphate and ammonium onto LaMZcould be described by a pseudo-second-order kinetic model. The equilibrium adsorptiondata of phosphate on LaMZ fitted well with the Langmuir isotherm model, and theequilibrium adsorption data of ammonium on LaMZ fitted well with the Langmuir,Freundlich and Dubinin-Radushkevich isotherm models. The thermodynamic studiesrevealed that the adsorption of phosphate and ammonium on LaMZ was spontaneousand endothermic process. The phosphate adsorption capacity for LaMZ decreasedgradually with increasing pH from3to10, but decreased greatly with increasingsolution pH from10to12. The ammonium adsorption capacity for LaMZ was relativelyhigh at pH3-8，but decreased with increasing solution pH from8to12. The presence ofcoexisting anions such as Cl-, HCO3-and SO42-had negligible influence on phosphateremoval by LaMZ. Coexisting Mg2+had negligible influence on ammonium removal byLaMZ, but coexisting cations such as K+, Na+and Ca2+had negative influence onammonium removal by LaMZ. The main mechanisms for phosphate removal by LaMZinclude electrostatic interaction, ligands exchange and Lewis acid-base interaction depending on solution pH. The main mechanism for ammonium removal by LaMZ iscation exchange．Through batch sorption tests, effect of LaMZ on phosphate sorption of Taihu Lakesediments was investigated. Langmuir and Freundlich equations were found to be quiteuseful to describe equilibrium of phosphate (1-15mg·L-1concentration) on Taihu Lakesediments in the lake. According to the Langmuir isotherm model, the maximumphosphate sorption capacity of the original Taihu Lake sediment was791mg·kg-1, andthat of the Taihu Lake sediments amended with10,25and50g·kg-1were937,1037and1505mg·kg-1, respectively. The pseudo-second-order model could be used todescribe phosphate sorption kinetics of the Taihu Lake sediments in the lake. Thephosphate sorption capacity of the Taihu Lake sediments decreased with increasingsolution pH. The sorption of phosphate on the Taihu Lake sediments was a spontaneousendothermic process. The phosphate adsorption/desorption equilibrium concentration ofthe Taihu Lake sediments amended with10-50g·kg-1LaMZ was0.129and0.241mg·L-1, which was much lower than that of the original Taihu Lake sediments (0.386mg·L-1). The phosphate adsorption/desorption equilibrium concentration of theLaMZ-amended Taihu Lake sediments decreased with increasing LMZ dosage.Sequential extraction of phosphorus from phosphate-sorbed original Taihu Lakesediments and LaMZ-amended Taihu Lake sediments indicated that most of thephosphate adsorbed by LaMZ in the sediments existed in the forms of NaOH-P andHCl-P, which are quite stable and unlikely to get released under anaerobic condition.In-situ sorbent amendment is a promising contaminated sediment remediationtechnology. The immobilization of phosphorus (P) in Taihu Lake sediment-watersystems using LaMZ as a novel amendment was investigated by conducting a series ofexperiments. P could be released from unamended and LaMZ-amended Taihu Lakesediments at a very low initial concentration of P in water. The amount of P releasedfrom LaMZ-amended Taihu Lake sediment was less than that from Taihu Lakesediment. The content of NaOH-P (P extracted with NaOH) or HCl-P (P extracted withHCl) in LaMZ-amended Taihu Lake sediment was higher than that in Taihu Lakesediment. The content of BD-P (P extracted with bicarbonate dithionite) inLaMZ-amended Taihu Lake sediment was less than that in Taihu Lake sediment. Theequilibrium sorption of phosphate from aqueous solution at a high initial concentrationof P (0.5~15mg/L) could be described by the Langmuir isotherm model. The phosphatesorption capacity for LaMZ-amended Taihu Lake sediment was much higher than thatfor Taihu Lake sediment. The phosphate sorption capacity for LaMZ-amended TaihuLake sediment decreased with increasing aging time. Sequential extraction of P fromphosphate-sorbed Taihu Lake sediment and LaMZ-amended Taihu Lake sedimentindicated that most of sorbed phosphate onto LaMZ existed in NaOH-P and HCl-Pforms, and was relatively stable and unlikely to be released under anaerobic condition.The efficiency and mechanism of sediment capping with an active barrier system(ABS) using LaMZ to simultaneously prevent phosphate and ammonium release fromsediments in heavily polluted river under low dissolved oxygen conditions wereinvestigated using batch and sediment incubation experiments. The adsorption kinetics of phosphate and ammonium on LaMZ well followed a pseudo-second-order model.The equilibrium adsorption data of phosphate on LaMZ could be described by theLangmuir isotherm model. The equilibrium adsorption data of ammonium on LaMZcould be described by the Langmuir, Freundlich and Dubinin-Radushkevich(D-R)isotherm models. The mechanisms for phosphate adsorption onto LaMZ at pH7includeligand exchange, Lewis acid-base interaction and electrostatic attraction. Themechanisms for ammonium adsorption onto LaMZ at pH7include cation exchange.The fluxes of phosphate and ammonium from the sediment to the overlying water weresignificantly reduced by the ABS using LaMZ under low dissolved oxygen conditions.Higher LaMZ dosage was found to be favorable for the prevention of ammoniumrelease from the sediments using the ABS. Sequential extraction of phosphorus fromphosphate-adsorbed LaMZ indicated that most of phosphate immobilized by the ABSusing LaMZ was stable and unlikely to be released under low dissolved oxygenconditions.A novel zirconium/magnetite/zeolite composite (Zr/Fe3O4/Zeo) was prepared andused as an adsorbent to remove phosphate and ammonium from aqueous solution.Results showed that the phosphate adsorption capacity for Zr/Fe3O4/Zeo decreased withincreasing solution pH. The equilibrium adsorption data of phosphate by Zr/Fe3O4/Zeocould be described by the Langmuir and Dubinin-Radushkevich (D-R) isotherm models.The equilibrium adsorption data of ammonium by Zr/Fe3O4/Zeo could be described bythe Langmuir, Freundlich, and D-R isotherm models. The adsorption kinetic data ofphosphate and ammonium by Zr/Fe3O4/Zeo well followed a pseudo-second-order model.The adsorption process of phosphate by Zr/Fe3O4/Zeo was endothermic andspontaneous in nature. The regeneration experiments showed that about91%ofphosphate adsorbed by Zr/Fe3O4/Zeo could be desorbed in0.2mol/L NaOH solution,and about92%of ammonium adsorbed by Zr/Fe3O4/Zeo could be desorbed in0.2mol/L NaCl solution. Ligand exchange was the main mechanism for phosphateadsorption by Zr/Fe3O4/Zeo. Cation exchange was the main mechanism for ammoniumadsorption by Zr/Fe3O4/Zeo.Surfactant-modified zeolites (SMZs) with different coverage types were preparedby loading of different amounts of cetylpyridinium bromide (CPB) onto natural zeolitesand were used as adsorbents to remove phenanthrene from aqueous solution. Resultsshowed monolayer and bilayer SMZs were effective for the removal of phenanthrenefrom aqueous solution. The phenanthrene removal efficiency of SMZs increased withincreasing adsorbent dosage, but the amount of phenanthrene adsorbed on SMZsdecreased with increasing adsorbent dosage. The adsorption kinetics of phenanthrene onSMZs well followed a pseudo-second-order kinetic model. The equilibrium adsorptiondata of phenanthrene on SMZs at a low concentration of phenanthrene in solution couldbe described by the Linear equation and Freundlich equation. The main mechanism forphenanthrene adsorption onto monolayer SMZ is hydrophobic interaction, and the mainmechanism for phenanthrene adsorption onto bilayer SMZ is organic partitioning. Thecalculated thermodynamic parameters such as Gibbs free energy change (ΔG),enthalpy changes (ΔH), and entropy change (ΔS) showed that the adsorption process of phenanthrene on SMZs is spontaneous and exothermic in nature. When the CPBloading amount of bilayer SMZ was twice as much as that of monolayer SMZ, thephenanthrene adsorption capacity for bilayer SMZ was slightly higher than that formonolayer SMZ.The above conclusions revealed that LaMZ and Zr/Fe3O4/Zeo are efficient andpotential adsorbents for removing phosphate and ammonium; CPB-modified zeolite hashigh selectivity to phenanthrene from aqueous solution; LaMZ used as an amendmentand capping material could prevent phosphate and ammonium release from sedimentseffectively.