[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.

[Adsorption of phenanthrene from aqueous solution on cetylpyridinium bromide (CPB) -modified zeolite]

Surfactant-modified zeolites (SMZs) with different coverage types were prepared by loading of different amounts of cetylpyridinium bromide (CPB) onto natural zeolites and were used as adsorbents to remove phenanthrene from aqueous solution. The adsorption of phenanthrene from aqueous solution on monolayer and bilayer SMZs as a function of adsorbent dosage, initial phenanthrene concentration, contact time, and temperature was investigated using batch experiments. Results showed monolayer and bilayer SMZs were effective for the removal of phenanthrene from aqueous solution. The phenanthrene removal efficiency of SMZs increased with increasing adsorbent dosage, but the amount of phenanthrene adsorbed on SMZs decreased with increasing adsorbent dosage. The adsorption kinetics of phenanthrene on SMZs well followed a pseudo-second-order kinetic model. The equilibrium adsorption data of phenanthrene on SMZs at a low concentration of phenanthrene in solution could be described by the Linear equation and Freundlich equation. The main mechanism for phenanthrene adsorption onto monolayer SMZ is hydrophobic interaction, and the main mechanism for phenanthrene adsorption onto bilayer SMZ is organic partitioning. The calculated thermodynamic parameters such as Gibbs free energy change (deltaG(theta)), enthalpy changes (deltaH(theta)), and entropy change (deltaS(theta)) showed that the adsorption process of phenanthrene on SMZs is spontaneous and exothermic in nature. When the CPB loading amount of bilayer SMZ was twice as much as that of monolayer SMZ, the phenanthrene adsorption capacity for bilayer SMZ was slightly higher than that for monolayer SMZ. In a conclusion, both monolayer and bilayer SMZs are promising adsorbents for the removal of phenanthrene from water and wastewater, and monolayer SMZ is a more cost-effective adsorbent for phenanthrene removal than bilayer SMZ.

[Evaluation of in situ capping with lanthanum-modified zeolite to control phosphate and ammonium release from sediments in heavily polluted river]

The efficiency and mechanism of sediment capping with an active barrier system (ABS) using lanthanum-modified zeolite (LaMZ) to simultaneously prevent phosphate and ammonium release from sediments in heavily polluted river under low dissolved oxygen conditions were investigated using batch and sediment incubation experiments. The adsorption kinetics of phosphate and ammonium on LaMZ followed a pseudo-second-order model. The equilibrium adsorption data of phosphate on LaMZ could be described by the Langmuir isotherm model. The equilibrium adsorption data of ammonium on LaMZ could be described by the Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models. The mechanisms for phosphate adsorption onto LaMZ at pH 7 included ligand exchange, Lewis acid-base interaction and electrostatic attraction. The mechanism for ammonium adsorption onto LaMZ at pH 7 was cation exchange. The fluxes of phosphate and ammonium from the sediment to the overlying water were significantly reduced by the ABS using LaMZ under low dissolved oxygen conditions. Higher LaMZ dosage was found to be favorable for the prevention of ammonium release from the sediments using the ABS. Sequential extraction of phosphorus from phosphate-adsorbed LaMZ indicated that most of phosphate immobilized by the ABS using LaMZ was stable and unlikely to be released under low dissolved oxygen conditions. Results of this work indicate that LaMZ is a suitable capping material for preventing phosphate and ammonium release from sediments in heavily polluted river under low dissolved oxygen conditions.

[Removal of nitrate from aqueous solution using cetylpyridinium chloride (CPC)-modified activated carbon as the adsorbent]

Surfactant-modified activated carbon (SMAC) was prepared by loading cetylpyridinium chloride (CPC) onto activated carbon and used as adsorbents to remove nitrate from aqueous solution. The SMAC was effective for removing nitrate from aqueous solution. The SMAC exhibited much higher nitrate adsorption capacity than that of the unmodified activated carbon. The nitrate adsorption capacity for SMAC increased with increasing the CPC loading. The adsorption kinetics of nitrate on SMAC followed a pseudo-second-order kinetic model. The equilibrium adsorption data of nitrate on SMAC could be described by the Langmuir isotherm model. Based on the Langmuir isotherm model, the maximum nitrate adsorption capacity for SMAC with CPC loading amount of444 mmol per 1 kg activated carbon was determined to be 16.1 mg x g(-1). The nitrate adsorption capacity for SMAC decreased with the increasing solution pH. The presence of competing anions such as chloride, sulfate and bicarbonate reduced the nitrate adsorption capacity. The nitrate adsorption capacity for SMAC slightly decreased with the increasing reaction temperature. Almost 95% of nitrate molecules adsorbed on SMAC could be desorbed in 1 mol x L(-1) NaCl solution. The main mechanisms for the adsorption of nitrate on SMAC are anionic exchange and electrostatic attraction. The results of this work indicate that SMAC is a promising adsorbent for removing nitrate from aqueous solution.

[Adsorption of Congo red from aqueous solution on hydroxyapatite]

The adsorption of Congo red (CR) from aqueous solution on hydroxyapatite was investigated using batch experiments. The hydroxyapatite was effective for CR removal from aqueous solution. The adsorption kinetics of CR on hydroxyapatite well followed a pseudo-second-order model. The equilibrium adsorption data of CR on hydroxyapatite could be described by the Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models. Thermodynamic parameters such as Gibbs free energy change, enthalpy change and entropy change were calculated and showed that the adsorption of CR on hydroxyapatite was spontaneous and exothermic in nature. The CR adsorption capacity for hydroxyapatite decreased significantly with increasing pH from 8 to 10. Thermal regeneration showed that hydroxyapatite could be used for six desorption-adsorption cycles with high removal efficiency for CR in each cycle. The mechanisms for CR adsorption on hydroxyapatite with pH value below the pH at point of zero charge (pH(PZC)) include electrostatic attraction, hydrogen bonding and Lewis acid-base interaction. The mechanisms for CR adsorption on hydroxyapatite with pH value above its pH(PZC) include hydrogen bonding and Lewis acid-base interaction. Results of this work indicate that hydroxyapatite is a promising adsorbent for CR removal from aqueous solution.

[Influencing factors of calcite active barrier system to control phosphorus release from sediments]

Influencing factors of calcite active barrier system to control phosphorus release from sediments were researched, including barrier thickness, Ca2+ of overlying water, temperature, calcite's grain size and artificial aeration. The results show that the calcite barrier can effectively control phosphorus release from sediments under anaerobic conditions, and its efficiency was influenced by barrier thickness, Ca2+ concentration of overlying water, temperature, calcite's grain size and artificial aeration. The efficiency of calcite barrier to control phosphorus release from sediments will increase with the increment of Ca2+ concentration of overlying water and barrier thickness. Application of 12.7 kg/m2 calcite active barrier system resulted in 56% reduction of phosphorus flux from sediment for 72 days; however 99% reduction was obtained by 38.2 kg/m2 calcite active barrier system. When the Ca2+ concentration of overlying water increased from 1 mmol/L to 5 mmol/L, the phosphorus concentration was reduced by about 36% in the 72nd day. In comparison to low temperature, high temperature was disbennifit to control phosphorus release from sediments by a thin calcite barrier. The smaller the grain sizes of calcite, the more efficient the calcite barrier. The ammonia and phosphorus release rate from sediments under calcite barrier will be decreased by aeration.

[Efficiency and mechanism of compound barrier with HCl modification zeolite and calcite to control nitrogen and phosphorus release from sediments]

A novel compound barrier constructed with HCl modification zeolite and calcite was reported to control phosphorus and nitrogen release from sediments. Using short-term batch experiments and long-term sediment incubation experiments, the efficiency and mechanism of the compound barrier under anaerobic condition was investigated. For sediment incubation experiments, five kinds of active barrier conditions were applied respectively. The results showed that the calcite barrier reduced the phosphorus release from the sediment effectively, but not for the ammonia release from sediments. The compound barrier constructed with natural zeolite and calcite can reduce both the phosphorus and the ammonia release from sediments. In comparison to natural zeolite, the novel compound barrier constructed with HCl modification zeolite and calcite can control phosphorus released from sediments more effectively. Application of 40 g natural zeolite and 100 g calcite barrier resulted in 84% reduction of phosphorus flux from sediments for 80 days, but 91% reduction was obtained by compound barrier with 40 g HCl modification zeolite and 100 g calcite. In comparison with the natural zeolite, the duration of ammonia release control from sediments was shortened to two-thirds for the compound barrier with HCl modification zeolite. Reduction of Na+ and increment of H+ released from HCl modification zeolite were the possible reasons for the higher efficiency of the compound barrier to control the phosphorus release from the sediments.

[Influencing factors of phosphorus release control from sediments by compound barrier constructed with zeolite and calcite]

A novel compound barrier technology for the control of phosphorus and nitrogen release from sediments was reported. The compound barrier was constructed with zeolite and calcite. Using this compound barrier technology, various influencing factors of phosphorus release from the sediments were investigated. The results showed that the compound barrier with calcite and zeolite can not only control the ammonia release from the sediment, but also reduce the phosphorus release from the sediment effectively. The efficiency of the compound barrier to immobilize the phosphorus of sediments increased with the increment of the quantity of zeolite or calcite. The efficiency was also affected by the grain size of the zeolite. The less the grain sizes of the zeolite, the more the efficiency. The construction mode of the compound barrier played an important role in the control efficiency of phosphorus release from the sediments. It was found that the efficiency of barrier mixed with zeolite and calcite was slightly less than that of the barrier made up of zeolite (underlayer) and calcite (superstratum). The compound barrier with zeolite (underlayer) and calcite (superstratum) can control phosphorus release much more efficiently than the barrier with zeolite (superstratum) and calcite (underlayer).