Enhanced Phosphate Adsorption Research Articles

Confined La2O3 particles in mesoporous carbon material for enhanced phosphate adsorption.

Novel phosphate adsorbents with confined La2O3 inside mesoporous carbon were fabricated by the solid-state grinding method using pristine mesoporous carbon material CMK-3 (PCMK-3) and oxidized CMK-3 (OCMK-3) as the matrixes (denoted as La2O3@PCMK-3 and La2O3@OCMK-3). Compared with pure La2O3, La2O3@PCMK-3 and La2O3@OCMK-3 exhibited higher normalized phosphate adsorption capacity, indicative of efficient loading of La2O3 inside the mesopores of the carbon materials. Furthermore, La2O3 loading led to substantially enhanced phosphate adsorption. The adsorption capacities of La2O3@OCMK-3 samples were higher than those of La2O3@PCMK-3 samples, possibly owing to the oxygen-containing groups forming in OCMK-3 during HNO3 oxidation, which enhanced the dispersion of La2O3 in the mesopores of OCMK-3. The adsorption capacities of La2O3@PCMK-3 and La2O3@OCMK-3 increased with the La2O3 loading amount. Phosphate adsorption onto La2O3(14.7)@PCMK-3 followed the pseudo-second-order kinetics with respect to correlation coefficient values (larger than 0.99). As pH increased from 3.4 to 12.0, the phosphate adsorption amounts of La2O3(14.7)@PCMK-3 and La2O3(15.7)@OCMK-3 decreased from 37.64 mg g−1 and 37.08 mg g−1 to 21.92 mg g−1 and 14.18 mg g−1, respectively. Additionally, La2O3@PCMK-3 showed higher adsorption selectivity towards phosphate than coexisting Cl−, and . The adsorbent La2O3(14.7)@PCMK-3 remained stable after five regeneration cycles.

Fe-loaded biochar obtained from food waste for enhanced phosphate adsorption and its adsorption mechanism study via spectroscopic and experimental approach

The re-utilization of food waste is an eco-friendly method for valorizing waste. In this study, food waste was blended with iron (Fe-FW) and optimized using the response surface methodology (RSM) to produce Fe-loaded food waste biochar (Fe-FWB); moreover, the waste was utilized to probe the adsorption of phosphate in water, where pyrolysis time (1.0, 2.5, and 4.0 h), temperature (300, 450, and 600 °C), and Fe concentrations (0.1, 0.3, and 0.5 M) were set as independent variables. After optimizing the Fe-FWB, batch experiments were performed to examine the phosphate sorption characteristics of Fe-FW and Fe-FWB. A pseudo-second order and Elovich kinetic model thoroughly explained the adsorption kinetics, which was indicative of the rate-limited sorption via diffusion or surface coverage after the rapid initial adsorption. The Freundlich and Redlich–Peterson isotherm models more accurately simulated the adsorption of phosphate onto Fe-FW and Fe-FWB than the Langmuir isotherm model. The thermodynamic results presented a positive value of ΔG0, clearly indicating that the reaction was not spontaneous; positive values of ΔH0 and ΔS0 affirmed the endothermic characteristic of phosphate uptake into Fe-FW and Fe-FWB, with an increase in randomness. The adsorption of phosphate onto Fe-FW and Fe-FWB decreased as the solution pH increased from 3 to 11. In the presence of interfering anions, phosphate adsorption onto Fe-FWB was influenced by the coexistence of HCO3-, SO42-, and NO3−. These results suggest that the synthesized Fe-loaded food waste biochar can be used as an emerging adsorbent for phosphate removal from aqueous solutions.

Effect of Hydrogen Ion Presence in Adsorbent and Solution to Enhance Phosphate Adsorption

In this paper, the effect of hydrogen ions on the adsorption onto granular activated carbon (GAC) with the inorganic contaminant phosphate, which exists as a form of four species depending on the solution pH, is investigated. Various batch isotherm and kinetic experiments were conducted in an initial pH 4 as an acid, a pH 7 as neutral, and a pH 9 solution as a base for the GAC conditioned with deionized water and hydrochloric acid, referred to as GAC and GACA, respectively. The physical properties, such as the total surface area, pore volume, pore size distribution, and weight of the element, obtained from Brunauer–Emmett–Teller (BET) and scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM–EDX) represent no significant differences. However, the hydrochloric acid (HCl) condition results in an alteration of the pH of the point of zero charge from 4.5 to 6.0. The optimized initial pH was determined as being acid for the GAC and as being neutral for the GACA. According to the Langmuir isotherm, the relatively high Qm was obtained as being acid for the GAC and clearly distinguishes the pH effect as being the base for the GACA. An attempt was made to assess the adsorption mechanism using the pseudo-first-order (PFO), the pseudo-second-order (PSO), and the intraparticle diffusion models. The higher R2 for the PSO in the entire pH range indicated that chemisorption was predominant for phosphate adsorption, and the pH did not change the adsorption mechanism. A prolonged Bed Volume (BV) for the GACA demonstrated that the hydrogen ions on the surface of the GAC enhanced phosphate adsorption.

Facile synthesis of polyethyleneimine@Fe3O4 loaded with zirconium for enhanced phosphate adsorption: Performance and adsorption mechanism

Facile synthesis of polyethyleneimine@Fe3O4 loaded with zirconium for enhanced phosphate adsorption: Performance and adsorption mechanism

Key roles of the crystal structures of MgO-biochar nanocomposites for enhancing phosphate adsorption

The affinity of biochar (BC) adsorbing phosphate was weak, while generation of magnesium oxide (MgO)-BC nanocomposites that transformed the crystal structures of BC would change the adsorption processes in improving the phosphate adsorption. Hereon, four different crystal structure of absorbents were selected to illustrate why the crystal structures and surface properties of absorbents were of great importance for the phosphate adsorption. The results showed that MgO/KBC with higher combination degree between MgO and KBC could change the normal crystal structure (MgO/KBC1, MgO phase (dominant)) to C-Mg-O phase (dominant). Therefore, MgO/KBC could achieve highest adsorption rate (k2, 8.059 g mg−1 min−1) and qm (maximal adsorption capacity, 121.950 mg g−1) for phosphate adsorption among absorbents, and even it had high anti-interference capacity for anions and natural organic matter (NOM). The mechanisms of MgO/KBC for phosphate adsorption were hydrogen-bond interaction, inner-sphere complexation and surface chemical adsorption; adsorption of phosphate on MgO/KBC1 was mainly controlled by inner-sphere complexation (Mg-O-PO3H2−, Mg-O-PO3H2− species). In addition, the adsorbability of MgO/KBC for phosphate could be restored after recalcination, which further proved that an efficient nanocomposite, calcinated from waste biomass (fallen leaves), was proposed to control eutrophication.

Influence of Ca2+ on phosphate removal from water using a non-core-shell Fe3O4@ZIF-67 composites

Excessive phosphate in surface water can spur the growth of organism, which put a threat on the bio-safety of drinking water. Different from previous study, sodium acetate (NaAc) was used as modification agent of Fe3O4, then the magnetic Fe3O4@ZIF-67 composites with a non-core-shell sturture were synthesized. According to ZIF-67 loading and phosphate adsorption of synthesized Fe3O4@ZIF-67, the Fe3O4@ZIF-67 composites with a Co2+/Fe3O4 molar ratio of 1:2 was taken into further evaluation. Fe3O4@ZIF-67 composites exhibited satisfactory adsorption performance and easy magnetic recovery, and the sorption capacity reached 116.59 mg P/g within 180 min (C0 = 20 mg P/L, 298 K). The maximum adsorption capacity of phosphate was obtained at the neutral pH, and not significantly interfered with ions of K+, Mg2+, NO3− and SO42-. Remarkably, Fe3O4@ZIF-67 described enhanced phosphate adsorption ability in the presence of Ca2+. To investigate the role of Ca2+, two modes (preloading and addition) of Ca2+ dosing were examined and the improved adsorptive performance could be owing to the synergistic eff ;ect of adsorption and Ca-P micro-precipitation on the surface of Fe3O4@ZIF-67. During the regeneration process, the performance of desorbent was evaluated by phosphate releasing and secondary adsorption, and the absence or presence of Ca2+ would have a significant impact on the selection of desorption solution.

Surface Functional Group Engineering of CeO2 Particles for Enhanced Phosphate Adsorption.

Surface functional groups play a dominating role in determining the adsorption performance of metal oxide particles. The ability to manipulate the surface functional groups is vital in designing an effective adsorbent for water decontamination. In this study, a facile method is proposed for tuning the amount of the surface hydroxyl groups of CeO2 particles. The volume of water added during the ethylene glycol-mediated solvothermal synthesis of CeO2 particles can be used to adjust the amount of surface hydroxyl groups. By simple reduction in the volume of water, the number of surface hydroxyl groups of CeO2 particles can be increased and the phosphate adsorption capacity can be greatly improved. Our results show that the obtained CeO2 particles have high phosphate adsorption capacity at low phosphate concentrations, fast adsorption kinetics, and the ability to achieve an ultralow phosphate concentration in the real sewage effluent. This study provides an effective strategy for designing highly effective metal oxide adsorbents through surface functional group engineering.

Investigation into lanthanum-coated biochar obtained from urban dewatered sewage sludge for enhanced phosphate adsorption

Phosphate adsorption using metal-modified biochar has awakened much attention and triggered extensive research. In this study, the effect of lanthanum (La)-modified sludge using impregnation-co-precipitation was used for phosphate adsorption. Consequently, La-coated biochar at a pyrolysis temperature of 600 °C had the highest phosphate adsorption and the lowest heavy metal leaching potential. The treatment of virgin biochar with alkali before La loading was found to be beneficial for the increase of phosphate adsorption capacity. The adsorption kinetics was well depicted by the pseudo-second-order model, and indicating that intraparticle diffusion played a crucial role in the adsorption process. The good fitness between adsorption data and the Langmuir isotherm model showed a maximal adsorption capacity of 93.91 mg/g, where phosphate absorption was highly correlated to its concentration in the solution. The La-coated biochar showed high adsorption capacity when the solution pH varied from 3.0 to 6.0, and was insensitive to the coexisting chloride, nitrate, sulfate, bicarbonate and citrate. Moreover, the adsorption mechanism was further explored by using Zeta potential analysis, FTIR and XPS, indicating that the phosphate is adsorbed through electrostatic attraction in the form of the inner-sphere complexation. All the results suggested that the sludge-based biochar, as a support material for La, could serve as a promising adsorbent for phosphate in real applications.

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.

The removal of phosphate by thermally treated red mud from water: The effect of surface chemistry on phosphate immobilization

This study investigated thermal treatment of red mud (RM) and its effect on phase composition, surface property, and sorption capacity exemplified by phosphate. Dehydration (∼600 °C), decomposition of carbonate minerals (700 °C–800 °C), and silicate/aluminate formation (900 °C–1000 °C) occurred upon thermal treatment of RM. Grain growth and vitrification that rendered initial morphology changes and decreased the specific surface area of RM from 26.5 to 4.1 m2/g when treated from 600 to 1000 °C, respectively. Surface acidity, i.e., intrinsic acidity constant and surface acidity density, decreased as well after thermal treatment at 600 °C due to burnouts of organics then increased upon further elevated-temperature treatment because of phase transformation. Thermal activation enhanced phosphate adsorption density (μmol/m2). Multilayer sorption aided by leached metal ions was responsible for phosphate immobilization.

Polyethyleneimine-modified biochar for enhanced phosphate adsorption.

Biochar, a low-cost porous carbonaceous adsorbent, has low adsorption capacity for anion contaminants. The objective of this study was to improve biochar's ability to adsorb phosphorus (P) through polyethyleneimine (PEI) modification to form an amine-functionalized biochar. Biochars prepared by pyrolysis of bamboo biomass, before and after PEI modification, were characterized using the Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), elemental analysis, and batch sorption experiments. The effects of pH, coexisting anions, and ionic strength on P adsorption by PEI-modified biochar were also investigated. Results indicated that PEI was successfully grafted onto biochar which resulted an increase in surface amine group and in P adsorption. The peak of P adsorption occurred at pH of three and adsorption of P was decreased with increasing of ionic strength and when coexisting ions, such as HCO3-, SO42-, NO3-, and Cl-, were coexisted. The electrostatic interaction between P and surface functional groups of PEI-modified biochar served as the primary mechanism controlling the adsorption process. These results indicate that chemically functionalized biochar with amine groups can enhance P adsorption.

Ce(III) nanocomposites by partial thermal decomposition of Ce-MOF for effective phosphate adsorption in a wide pH range

A series of hierarchical micro/nano Ce-based composites were derived from Ce-MOF via thermal treatment in N2 atmosphere. Different from conventional complete decomposed materials, forming cerium oxide in air, Ce-MOF that calcinated in N2 at lower temperatures (400 °C or 500 °C) showed a partial thermal decomposition with high percent content of Ce(III). Even though the complete decomposed products held higher surface areas, the partial decomposed samples exhibited extremely higher phosphate uptake, with working capacity 2–4 times higher than that of ceria. The results implied a predominant effect of different valence states on phosphate removal by Ce-based materials, in which Ce(III) species were demonstrated playing the major role to form binding with phosphate. The maximum adsorption capacity (189.4 mg/g) was achieved by Ce-MOF-500(S) with wide applicable scope of pH ranging from 2 to 12 and great selectivity for phosphate in the presence of competing anions. Remarkably, Ce-MOF-500(S) described obvious enhanced phosphate adsorption ability under alkaline condition. This was due to the fact that the hydrolyzed Ce(III) species brought more active sites in the form of hydroxyl groups for ligand exchange with phosphate. Furthermore, based on the analysis of FTIR, XPS, XRD and zeta potential, electrostatic attraction, ligand exchange and surface precipitation were confirmed as the main adsorption mechanisms for partial decomposed samples, while electrostatic attraction was the main mechanism for complete decomposed samples.

Enhancing phosphate adsorption by novel calcium-silicate composites: batch and column studies

Enhancing phosphate adsorption by novel calcium-silicate composites: batch and column studies

Enhanced phosphate adsorption on Ca-Mg-loaded biochar derived from tobacco stems.

Tobacco stems were used as precursors to prepare biochars (YGs) and develop Ca-Mg-loaded biochars (CMYGs) to enhance phosphate adsorption from aqueous solutions. Some influencing factors, such as pH, adsorption time, temperature, and structure characterization, were investigated. Fourier transform infrared (FTIR) spectra and X-ray diffraction (XRD) patterns showed several new peaks, indicating that Mg(OH)2 and MgO have been present on the surface of the CMYGs. The adsorption could reach equilibrium in 100 min reaction. The equilibrium data were well described by the Langmuir and Freundlich model. After five recycles, the phosphate removal capacity of CMYGs biochar retained over 50%. Moreover, the XRD and FTIR analyses showed that the phosphate sorption mechanisms involved surface electrostatic attraction, inner-sphere complexation and precipitation reactions. Overall, the soaking method could be used to effectively load Mg2+ onto the surface of YGs. The CMYGs synthesized at 750 °C is a promising adsorbent for phosphate removal with a high adsorption capacity for phosphate-polluted wastewater.

Removal of phosphate from aqueous solution by a novel Mg(OH)2/ZrO2 composite: Adsorption behavior and mechanism

In this work, Mg(OH)2 was incorporated into ZrO2 to obtain a novel Mg(OH)2/ZrO2 composite (MZ) by a simple co-precipitation method for phosphate adsorption. For comparison purpose, pure ZrO2 and Mg(OH)2 were also fabricated. The as-prepared MZ, ZrO2 and Mg(OH)2 were characterized by SEM, EDS, N2 adsorption/desorption and pHPZC, and their phosphate adsorption properties were comparatively investigated in batch mode. The MZ samples before and after phosphate adsorption were characterized by XPS, and the phosphate-adsorbed MZ was characterized by 31P NMR. The results showed that MZ was mainly composed of ZrO2 and Mg(OH)2. In addition, MZ had a larger specific surface area than single-component ZrO2 and Mg(OH)2. MZ exhibited a much higher affinity towards phosphate in aqueous solution than pure ZrO2 and Mg(OH)2. The Langmuir maximum phosphate adsorption capacity of MZ was 87.2 mg PO4/g at initial pH 7, which was 84% and 4.3 times higher than those of single-component ZrO2 and Mg(OH)2, respectively. Phosphate was adsorbed onto the MZ surface mainly via a ligand exchange between the ZrO2 phase and phosphate forming inner-sphere phosphate complexes as well as the reaction between the Mg(OH)2 phase and phosphate forming Mg-P compounds such as MgHPO4 and Mg3(PO4)2. The enhanced phosphate adsorption by the incorporation of Mg(OH)2 into ZrO2 could be attributed to: (i) the increase in the specific surface area of the adsorbent material; and (ii) the enhanced adsorption of phosphate on the ZrO2 component by the presence of Mg2+ that was dissolved from the Mg(OH)2 component. Results of this work indicate that MZ is a more promising adsorbent for the removal of phosphate from wastewater than Mg(OH)2 and ZrO2.

Development of rare earth element doped magnetic biochars with enhanced phosphate adsorption performance

To improve the biochar’s adsorption performance towards phosphate as well as to endow the biochar with magnetic property, novel rare earth element doped magnetic biochars, Ce/Fe3O4-BC and La/Fe3O4-BC, were prepared by separately co-precipitating cerium (Ce) and lanthanum (La) with FeCl3, FeCl2 and biochar. For comparison, Fe3O4-BC without doping Ce or La was also synthesized. The characterization results indicated that Ce and La ions were successfully doped into the magnetite. The La/Fe3O4-BC had higher magnetic saturation and point of zero charge than Ce/Fe3O4-BC. A series of batch experiment results demonstrated that the phosphate adsorption capacity of biochar has been greatly improved after Ce or La doping, especially La. The phosphate adsorption capacity of La/Fe3O4-BC was 20.5 mg/g at pH 6.5, which was 1.6 and 2.9 times than that of Ce/Fe3O4-BC and Fe3O4-BC, respectively. The adsorption kinetics and isotherms of phosphate onto La/Fe3O4-BC and Ce/Fe3O4-BC were best fitted by the pseudo-second order and Freundich model, respectively, indicating that the adsorption process was a multilayer process and controlled by chemical reaction. The combined results of batch experiments and physiochemical analyses revealed that the possible mechanisms were the formation of inner-sphere complex at neutral conditions and electrostatic attraction between positively charged adsorption sites and phosphate under acid conditions. The enhanced phosphate adsorption performance and endowed magnetic property indicated La/Fe3O4-BC could be used as a promising adsorbent in phosphate removal.

Enhanced phosphate adsorption performance by innovative anion imprinted polymers with dual interaction

It is therefore difficult to develop anionic imprinted polymers due to its intrinsic properties of anionic templates, which have much smaller charge size ratio, species diversity, and irregular geometry than cationic templates. Moreover, its synthesis is not suitable for traditional apolar media. Hence, well-defined recognition cavities formed by coordination are almost impossible for anion imprinted polymers. Synthesized anion imprinted polymers (IIP) by electrostatic attraction of positive and negative charges to enhance the stability of anion and functional monomer in the pre-polymerization process were proposed. Speciation of phosphate and surface potential at different pH on IIP have effect on phosphate capture were investigated. The resultant IIP has a high 78.88 mg g−1 at 35 °C while non-anion imprinted polymers (NIP) has only 33.35 mg g−1 adsorption capacity. In coexist experiments, all the selectivity coefficients (α) of IIP are greater than 1.8, and selectivity would rise with anion concentration increased. All these shows IIP possessed outstanding selectivity and specificity to phosphate. Adsorption energy E, and ΔH0 of IIP are 14.26 kJ mol−1 and 36.32 kJ mol−1, respectively. It indicated that phosphate adsorbed to IIP is ion exchange reaction. It is further verified by FTIR and XPS that recognition sites of anion imprinted polymers are N+ of AMD and P–O− in H2PO4−. They moved to imprinted cavities and form N+–O−, NH hydrogen bonded, which also contribute to adsorption.

Enhanced phosphate removal using nanostructured hydrated ferric-zirconium binary oxide confined in a polymeric anion exchanger

Phosphate removal is an important measure required to control eutrophication of water bodies as it inhibits excessive algal growth. Currently available phosphate removal methods have various limitations and function poorly under wide pH ranges. This study successfully prepared and characterized a novel nanocomposite adsorbent, nHFZO@I402, which has high capacity and specific affinity towards phosphate. The nHFZO@I402 constituted of nanostructured hydrated ferric oxide and hydrated zirconium oxide loaded on the surface and inside the pores of a polymeric anion exchanger. Batch experiments were conducted to investigate the effects of varied conditions on phosphate adsorption. The results demonstrated that nHFZO@I402 achieved high phosphate removal efficiencies over a wide pH range (4–11), which indicates higher adaptability towards varying environmental conditions. In addition, nHFZO@I402 exhibited enhanced phosphate adsorption capacity regardless of coexisting anions (chloride, nitrate, sulfate, and carbonate) at different concentrations. Kinetic analysis suggested that the adsorption process best-fitted with the pseudo-second-order kinetic model, suggesting chemical sorption mechanism. Furthermore, the underlying mechanism of phosphate adsorption by nHFZO@I402 was assessing through XPS analysis. Phosphate removal was effected by the replacement of hydroxyl groups by phosphate species and interaction between ammonium groups and phosphate. In general, these results indicated that nHFZO@I402 is a highly efficient adsorbent for phosphate removal with excellent adaptability to varying and undesirable environmental conditions.

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.

Enhancing phosphate adsorption capacity of SDS-based magnetite by surface modification of citric acid

In this study, citric acid (CA) was employed as a low-molecule organic acid to influence the adsorption performance of phosphorus by as-obtained magnetite. The factors including initial phosphate concentrations, dosage of citric acid, pH value, ion strength, contact time and temperature were examined in detail. Results indicated that the dissolution of anion sodium dodecyl sulfate (SDS) covering on surface of magnetite, a slight decrease of Fe level and a superior structure of magnetite after CA modification occurred. The pH-dependence of phosphate adsorption was impeded and the surface potential of magnetite positively increased at pH>5.0 when CA was added. Non-linear regression Langmuir-Freundlich model was fitted well in thermodynamics, and the opposite adsorption process as a function of temperatures with or without CA addition was due to the decrease of active energy and active mobility of phosphate ion. Finally, the declining adsorption efficiency with increasing cycles was observed while phosphate removal was approximately finished and had small change with 0.05 and 0.1M of CA addition. Those improvements of removal efficiency of phosphorus by modified iron oxide were because of the removal of anionic SDS that increased the surface positive charge, and especially the dissolution of element Fe into solution to form precipitate with phosphorus ions. The enhanced stability of magnetite by CA also promoted the high removal efficiency of phosphorus. These implications of CA on phosphate removal can be extended to the field where phosphate pollution is notorious but urgent.