Adsorption Of Phosphate From Aqueous Solution By Siderite And ASO:Preparation,Performance And Mechanism

The current status of water in China and even in the world presents a very serious pollution situation.The pollution problems caused by the eutrophication of water are particularly prominent.In order to curb and improve the pollution status,regarding the treatment of phosphorous-containing wastewater,low concentration,namely deep treatment,has overtaken high concentration.As a traditional iron mineral resource,siderite（FeCO₃）was used for smelting iron and steel.And the siderite is particularly rich in China.Here,we report a detailed investigation of the use of natural non-activated siderite and activated siderite ore（ASO）for removal and recovery of phosphate from aqueous solution.And the natural siderite and ASO were characterized by X-ray diffraction（XRD）,scanning electron microscope（SEM）and Fourier Transform infrared（FT-IR）.The main conclusions of the present work are showed as follow:1.The mineralogical component analysis with XRD,SEM,TGA,X-ray fluorescence and chemical phase analysis,reveals that the natural siderite ore sample is composed of66.3 wt%siderite,22.4 wt%clay minerals and quartz,and 11.3 wt%goethite.According to the thermal gravimetric analysis,it can be concluded that the decomposition of siderite starts at about 430 ^oC,experiences a maximum decomposition rate temperature at about 510 ^oC and afterward ends at about 600 ^oC.Moreover,it can also be inferred that the activated siderite ore is more likely to be prepared within a temperature range from 450 ^oC to 550 ^oC.2.The largest adsorption capacity was achieved with a contact time of 24 h（0.05-0.1mm）,It is also found that the decrease of pH value facilitates the adsorption of phosphate on siderite,indicating that the dissolution of siderite and further the formation of iron oxyhydroxides might find place.Furthermore,the coexisting anions have minor effects on phosphate removal within the concentration ranges(15-100mg·L^-1).Batch experiments were carried out with an initial phosphate concentration of1 mg/L.The concentration of effluent has already reached the standards of surface water quality ClassⅡ（GB3838-2002,0.025 mg/L,China）.3.Adsorption analysis reveals the adsorption kinetics of phosphate on siderite satisfies the pseudo-second-order kinetic model（R²>0.98）.Using the Langmuir model,the maximum adsorption capacity of siderite on phosphate was determined to 6.2 mg·g^-1at a pH of 6.8 and 333 K and the highest desorption percentage was 93.61%at 0.05 M NaOH.Thermodynamics analysis reveals that the adsorption of phosphate onto siderite is an endothermic and spontaneous process.4.Dissolved oxygen play an important role in adsorption.Results of TEM and FTIR analysis suggested that under the condition of pH=6 and phosphate concentration（50mg/L）,two different monoprotonated phosphate complexes exist at siderite,with one surface complex coordinated in a bidentate binuclear（bridging）fashion,and the other as a monodentate mononuclear complex.5.The nano-porous material with the great specific surface area can be prepared through annealing siderite at 450–500 ^oC.The results show that divalent iron in siderite is oxidized to ferric iron,forming a conjugate at 430 ^oC and releasing a small amount of CO₂ simultaneously.As a result,the porous structure（beaded string）occurs on the surface of siderite,which the pore sizes are in nanoscale.When the temperature increases to 460 ^oC,the conjugates release a great amount of CO₂ and transforms into hematite.In addition,annealing temperature and time both affect the specific surface area and pore structure of the corresponding annealed product as the temperature changes from 400 to 600 ^oC and the time ranges from 10 to 60 min.In particular,the specific surface area of annealed product increases to 57.5 m²/g at 470 ^oC.The grain size of hematite increases with increasing the temperature and time.The decreased internal voids and the increased aperture increase with increasing the temperature and time,leading to the decreased specific surface area.6.Activated siderite ore（ASO）was prepared by annealing siderite ore at different temperatures and different times.The largest adsorption capacity was achieved with a contact time of 24 h（0.05-0.1 mm）,and phosphate can be removed efficiently by ASO in a relatively wide pH range between 3.0 and 11.0.The effects of coexisting anions were not significant within the experimental concentration ranges except HCO₃^-.The adsorption process of phosphate on ASO fitted pseudo-second-order kinetics model.ASO showed a high-adsorption capacity of 9.24 mg/g estimated from Langmuir isotherm at 303K.The desorption percentage was 96%at 0.05 mol/L NaOH.Thermodynamic studies indicated that phosphate adsorption on ASO was spontaneous,endothermic and favorable.Results of TEM and FTIR analysis suggested that two different monoprotonated phosphate complexes exist at ASO,with one surface complex coordinated in a monodentate binuclear（bridging）fashion,and the other as a monodentate mononuclear complex.7.Compared with other adsorbent,siderite and ASO had a strong ability of phosphate adsorption.Batch dynamic column experiments were carried out with an initial phosphate concentration of 1 mg/L and 10 mg/L for siderite and ASO,respectively.The concentration of effluent has already reached the standards of surface water quality ClassⅡ（GB3838-2002,0.025 mg/L,China）,the adsorption capacity of siderite and ASO are 0.48 mg/g and 2.72 mg/g,respectively.8.Phosphate desorption tests were conducted using ASO,the results show that ASO samples first stage:the adsorption capacity of ASO is 2.72 mg/g,the desorption rate was 90%;The second stage:the adsorption capacity of ASO is 2.43 mg/g,the desorption rate was 93%;The third stage:the adsorption capacity of ASO is 2.34 mg/g,the desorption rate was 92%;The average equivalent of ASO:single adsorption capacity of 2.5 mg/g,the desorption rate was 90%.The high phosphate adsorption and desorption capacities of the siderite and ASO demonstrate its potential for reducing eutrophication.We believe that these results are beneficial to guide the future use of siderite to control eutrophication.