Synthesis And Application Of Magnetic Chlorapatite Nanoparticles For Zinc(Ⅱ),Cadmium(Ⅱ) And Lead(Ⅱ) Removal From Water Solutions

China is currently facing very serious environmental problems,the factors that cause environmental problems are multifaceted,including the pressure of the population,constitute the greatest threat to China’s ecological environment.The late starting China’s industrialization and the rapid development of the economy are easy to make people only focus on immediate interests and ignore the long-term interests.Most of the people pay much attention to the economy and ignore the ecological problems.To develop the economy for a moment of prosperity at the expense of damaging the environment makes the situation more serious.Environment is an important factor that relevant to our life and it would influence the living quality of human in both a good or bad way.Therefore,everyone is involved in the improvement and maintenance.It is urgent to overcome the environment problems for human beings.Water is essential to human life and we need to have clean water in order to have a good health.It is also a basic component in for the aquatic animals.In nowadays,with the growth and the development of economic and technology,the expansion of the industry has resulted in an over-emission of waste to the environment,leading to a severe and difficult environmental problem.Therefore,how to treat heavy metal polluted water with an efficient way becoming an attractive topic for researchers.Heavy metal pollution is one of the most important environmental problems today.Various industries（including mining and smelting of metalliferous,surface finishing industry,energy and fuel production,fertilizer and pesticide industry and application,metallurgy,iron and steel,electroplating,electrolysis,electro-osmosis,leather working,photography,electric appliance manufacturing,metal surface treating,aerospace and atomic energy installation etc）produce and discharge wastes contained different kinds of heavy metals into the environment.Thus,as a kind of shortage resource,heavy metals also bring about serious environmental pollution,becoming a threat to human and ecosystem.Toxic metals,such as Hg,Cr,Pb,Zn,Cu,Ni,Cd,As,Co,Sn,etc.,precious metals such as Pd,Pt,Ag,Au,Ru etc.and radionuclides such as U,Th,Ra,Am,etc.,are all potential pollutants in environment.There are many methods for removing heavy metal ions from aqueous solution.The conventional technologies for the removal of heavy metal ions from aqueous solution include chemical precipitation,oxidation-reduction,ferrite,filtration and ion exchange resins etc.But all the methods above not only have the drawback of large investment,high cost and complicated operating management and the potential of secondary pollution,but also cannot reuse the heavy metals and water.Currently,adsorption has been widely used during the application.With the superiority of wide resource,low cost and great efficiency,adsorption method attracts the eyes of people all the time.In recent years,using nano-materials and inorganic materials as adsorbents for heavy metal removal from wastewater has become an attractive topic.Chlorapatite nanoparticles is an ideal material which has been widely used for heavy metal removal owing to its high biocompatibility and adsorption properties.The synthesis of magnetic chlorapatite nanoparticles（MNCLAP）adsorbents was proposed to remove Zn²⁺,Cd²⁺and Pb²⁺from water solutions.The characterizations of MNCLAP were obtained by transmission electron microscopy（TEM）,scanning electron microscopy（SEM）,X-ray diffraction（XRD）analysis,energy-dispersive X-ray spectroscopy（EDS）and magnetization curves.The different experiment parameters of heavy metal adsorption such as equilibration time,initial heavy metal concentration of heavy metal ions,the amount of MNCLAP,pH values and competition adsorption were investigated.Moreover,desorption experiment has also been studied.The SEM micrograph of MNCLAP showed that the synthesized MNCLAP was in spherical shape with a rough surface.TEM image of synthesized MNCLAP demonstrated that the average diameter of these particles was estimated at approximately 20 nm.The EDS spectra of MNCLAP adsorbents unloaded and loaded with Zn²⁺,Cd²⁺and Pb²⁺indicated that MNCLAP adsorbents were consisted of calcium,phosphorus,iron and oxygen.Iron and oxygen were successfully presented in MNCLAP adsorbents with a remarkable content and the ratio of Ca/P is about 1.43.Comparing the spectra of loaded MNCLAP with unloaded MNCLAP,the zinc peak,cadmium peak and lead peak could be observed in loaded MNCLAP,which suggested that Zn²⁺,Cd²⁺and Pb²⁺had been adsorbed on the surface of MNCLAP successfully.The main components of MNCLAP were apatite（chlorapatite）,magnetite and maghemite.The magnetic hysteresis loop diagram of the MNCLAP has a saturation value of40.68 emu g^-1,suggesting that there is a strong magnetic response to a magnetic field.It has been known that the saturation magnetization,which indicated the maximum magnetic strength,is a most important parameter magnetic separation.Therefore,in our study,the synthesized MNCLAP has the enough ability to be separated from solutions.At room temperature（25±1℃）we studied different experimental parameters of heavy metal adsorption.In order to determine the adsorption rate and equilibrium time of MNCLAP adsorbent,adsorption kinetics experiments were carried out in this study.an amount of adsorbent 0.1 g L^-1 were added to 20 m L of 2×10^-3 mol L^-1 initial concentration solution of three heavy metals with the pH value 5±0.1 and then used Constant Temperature Water Bath Oscillator to vibrate in different time intervals ranging 15 min,25 min,35 min,45 min,105min,165 min,285 min,24 h and 48 h.After the completion of the reaction,using magnet to separate adsorbents from the solutions and then the residual heavy metal concentrations were measured by atomic absorption spectrophotometer.The adsorption of Zn²⁺,Cd²⁺and Pb²⁺by MNCLAP adsorbents was divided into two distinct stages.In the first two hours of adsorption quickly and then slowly increase the amount of adsorption until adsorption equilibrium.The adsorption equilibrium time was about 24 hour,though there was a slight increase,but did not significant effect,so 24 hours was selected as the contact time in the following experiment.The kinetic process of the experiment is in good agreement with the pseudo second-order kinetic model（R²=0.9999）,indicating that quasi-second-order kinetic model is suitable for describing the adsorption of Zn²⁺,Cd²⁺and Pb²⁺by MNCLAP adsorbents.To determine the adsorption capacity of MNCLAP for Zn²⁺,Cd²⁺and Pb²⁺,0.1g L^-1 adsorbent was added to 20 mL Zn²⁺,Cd²⁺and Pb²⁺containing solution,and the solution concentration was its solution concentration gradient was 10^-4 mol L^-1,2×10^-4 mol L^-1,4×10^-4 mol L^-1,6×10^-4 mol L^-1,8×10^-4 mol L^-1,10^-3 mol L^-1,3×10^-3 mol L^-1,6×10^-3 mol L^-1,9×10^-3 mol L^-1,10^-2 mol L^-1 three heavy metals solution.Adjust pH value 5±0.1 and then reaction for 24 hours by shaking on Constant Temperature Water Bath Oscillator.After that using magnet to separate MNCLAP from the solutions and then the residual heavy metal concentrations were measured by atomic absorption spectrophotometer.The results are in good agreement with Langmuir isotherms and the correlation coefficients（R²）of Zn²⁺,Cd²⁺and Pb²⁺are 0.9998,0.9997 and 0.9996,respectively.The maximum adsorption capacities for Zn²⁺,Cd²⁺and Pb²⁺are:1.1769 mmoL g^-1,1.1022 mmoL g^-1 and 1.1546 mmoL g^-1,respectively.Zn²⁺,Cd²⁺and Pb²⁺by Langmuir constants are:0.3047×10³ L mol^-1,0.3215×10³ L mol^-1 and 0.4098×10³ L mol^-1,respectively,demonstrating that the adsorption intensity of MNCLAP was in the order:Pb²⁺>Cd²⁺>Zn²⁺.To determine the best adsorption amount of Zn²⁺,Cd²⁺and Pb²⁺for MNCLAP,0.05 g L^-1,0.07 g L^-1,0.1 g L^-1,0.2 g L^-1,0.3 g L^-1,0.5 g L^-1 were added to 20 mL of 2×10^-3 mol L^-1 concentration of three metals solution with the pH value 5±0.1 and then reaction for 24 hours by shaking on Constant Temperature Water Bath Oscillator.After that using magnet to separate adsorbents from the solutions and then the residual heavy metal concentrations were measured by atomic absorption spectrophotometer.Results demonstrated that the adsorbed heavy metals,including Zn²⁺,Cd²⁺and Pb²⁺,were increased with the increasing of MNCLAP amount at the beginning.Then the adsorption amount reached to the maximum at the MNCLAP dosage of 0.1 g L^-1.After that,the adsorption amount was experienced a sharply decrease with the increasing of MNCLAP amount.The possible explanation was that at the initial stage with a quantitative amount of heavy metals,the adsorption process was accelerated by providing more adsorption sites through increasing the dosage of MNCLAP before reached to the maximum（saturation）.However,when the adsorption reached to the equilibrium,the heavy metal retained in solutions cannot adsorb on the available adsorption sites of added MNCLAP since the same cations were distributed on greater amount of surface.Thus the optimum adsorption amount of MNCLAP for metal ions removal is the 0.1 g L^-1.In order to investigate the effect of initial pH on the adsorption of heavy metal ions,an amount of adsorbent 0.1 g L^-1 was added to 20 m L of 2×10^-3 mol L^-1 concentration of three metals solution with the pH value4,5,6,7,8,9,10 respectively and then reaction for 24 hours by shaking on Constant Temperature Water Bath Oscillator.After that using magnet to separate MNCLAP from the solutions and then the residual heavy metal concentrations were measured by atomic absorption spectrophotometer.The amount of heavy metal ions increased with the increase of pH value,especially the heavy metal adsorption capacity began increased dramatically after the pH value exceeds 8,this phenomenon because of the heavy metal ions solution began to precipitate resulting in substantial reduction.Therefore,the optimal pH value of MNCLAP for heavy metal ions is 4-8.In competitive adsorption experiments,the target heavy metal ion concentration was fixed at 2×10^-3 mol L^-1,while the other heavy metal ion concentrations varied within the range of 10^-4 to 10^-2 mol L^-1 for the other two heavy metal ions.The experimental results showed that when three kinds of heavy metal ions coexist,the adsorption capacity of MNCLAP for the three metal ions Zn²⁺,Cd²⁺and Pb²⁺are lower than the adsorption capacity of the three kinds of metal ions in a single solution.In addition,the experimental results also show that:when the concentration of Zn²⁺fixed at 2×10^-3 mol L^-1unchanged,the concentration of Cd²⁺and Pb²⁺was varied in the range of 10^-4 mol L^-1-10^-2mol L^-1.The equilibrium adsorption capacity of Zn²⁺decreased from 0.9556 mmol g^-1 and0.9168 mmol g^-1 to 0.1326 mmol g^-1 and 0.0976 mmol g^-1,respectively;when the concentration of Cd²⁺fixed at 2×10^-3 mol L^-1 unchanged,the concentration of Zn²⁺and Pb²⁺was varied in the range of 10^-4 mol L^-1-10^-2 mol L^-1.The equilibrium adsorption capacity of Cd²⁺decreased from 0.9386 mmol g^-1 and 0.9017 mmol g^-1 to 0.1535 mmol g^-1 and 0.1053mmol g^-1,respectively;when the concentration of Pb²⁺fixed at 2×10^-3 mol L^-1 unchanged,the concentration of Zn²⁺and Cd²⁺was varied in the range of 10^-4 mol L^-1-10^-2 mol L^-1.The equilibrium adsorption capacity of Pb²⁺decreased from 0.9725 mmol g^-1 and 0.9872 mmol g^-1 to 0.2219 mmol g^-1 and 0.2168 mmol g^-1,respectively.These results indicate that the coexistence of other heavy metal ions may greatly reduce the one type metal adsorption by MNCLAP since the maximum adsorption capacities of MNCLAP adsorbent towards single Zn²⁺,Cd²⁺or Pb²⁺are 1.1769,1.1022 and 1.1546 mmol g^-1,respectively.This fact indicated that Zn²⁺,Cd²⁺and Pb²⁺were adsorbed in the same sites and they would compete for the adsorption sites on MNCLAP absorbent in the binary system.Results also indicated that the adsorption capacity of Zn²⁺and Cd²⁺were reduced more apparently with the presence of Pb²⁺in the binary system.That may attribute to the higher Langmuir constant of Pb²⁺than Zn²⁺and Cd²⁺during the adsorption.Consequently,owing to the higher Langmuir constant,effects of the coexistence of Zn²⁺and Cd²⁺on Pb²⁺adsorption were more slightly and the reduction of Pb²⁺adsorption capacity induced by Cd²⁺was more apparent than Zn²⁺.It confirmed that the adsorption affinity of MNCLAP was in the order of:Pb²⁺>Cd²⁺>Zn²⁺.By using desorption experiments,Ca（NO₃）₂,EDTA,HCl and NaOH in four different eluents desorption experiments found that the EDTA solution may induce the biggest heavy metal ions desorption amount from the metal-loaded MNCLAP absorbent.This phenomenon may attribute to the complex formed by EDTA and metal ions which possesses lower sorption affinity for MNCLAP adsorbents.It can be found that whether under the extreme acid or alkaline conditions,only trace amounts of metal ions can be desorbed,and the desorbed Zn²⁺and Cd²⁺were almost negligible under the alkaline condition.Additionally,the desorption rates of Zn²⁺,Cd²⁺and Pb²⁺in Ca（NO₃）₂ solutions were 15.96%,14.15%,and 15.13%,respectively.This may due to the reversible process of ion-exchange between the MNCLAP adsorbents and the solution:Ca²⁺may concentrate on the surface of MNCLAP adsorbents while the concentration of Ca²⁺in solutions was higher than that of MNCLAP adsorbents,leading to the desorption of heavy metals.The results show that the prepared MNCLAP adsorbent has obvious advantages in the removal of heavy metals in aqueous solution.