Study On The Adsorption Efficiency And Mechanism Of Sodium Alginate-based Magnetic Composites For Pb(Ⅱ) And Cu(Ⅱ)

Environmental problems are one of the inevitable consequence of the rapid development of global industry.Since the 21st century,environmental water pollution incidents led by heavy metal pollution have occurred frequently all over the world,which has aroused great concern from scientific experts and scholars.Heavy metals are difficult to be biodegraded in nature.Once they flow into the human body through the food chain,they can easily damage to the kidneys,liver,reproductive system,brain tissue and other organs.Therefore,a number of technical methods such as ion exchange,membrane separation,chemical precipitation,electrochemistry and adsorption have been applied to the treatment of heavy metal pollution in water.Among them,the adsorption method is favored because of the unique advantages of easy availability of raw materials,high adsorption efficiency and wide requirements for the concentration of heavy metal ions in raw water.Sodium alginate（SA）is considered to be a potential raw material for biomass adsorbent due to its rich active groups and good cross-linking and adsorption capacity.Magnetic Fe₃O₄particles are the most commonly raw material used for magnetic separation in recent years.The nano-level fine structure not only makes it easy to attach to the base material,but also enhances the stability of the base structure.L-cysteine（L-Cys）is one of the rare sulfur-containing non-essential amino acids.It contains mercapto（-SH）,amino（-NH₂）,and carboxyl（-COOH）groups,and has good heavy metal enrichment potential.This research will synthesize an environmentally friendly magnetic composite material by comprehensively utilizing the advantages of various raw materials.In order to further improve the adsorption efficiency,reduce the risk of secondary pollution,and increase its application potential in the treatment of heavy metal polluted water.The experiment uses sodium alginate,Fe₃O₄ and L-cysteine as raw materials,and uses calcium ion cross-linking method to prepare an environmentally friendly SA@L-Cys@Fe₃O₄ magnetic composite（abbreviation:SA@L-Cys@Fe₃O₄）.The suitable preparation process conditions of SA@L-Cys@Fe₃O₄ were explored through single factor condition experiment method.The surface morphology,internal structure,magnetic properties and thermal stability of the material were investigated by means of SEM,EDS,VSM and TGA.The adsorption parameters such as batch adsorption conditions,adsorption kinetics,adsorption isotherms and adsorption thermodynamics were also discussed.Finally,BET,XPS and flame atomic absorption spectroscopy were used to explain the adsorption mechanism of Pb（Ⅱ）and Cu（Ⅱ）by SA@L-Cys@Fe3O4.The main conclusions are as follows:（1）Through single-factor condition research,the suitable SA@L-Cys@Fe₃O₄preparation conditions and parameters were found as follows:sodium alginate concentration was 30.0 g·L^-1,L-cysteine concentration was 6.0 g·L^-1 The concentration of Ca Cl₂ is 2.5g·L^-1 and the concentration of Fe₃O₄ is 2.0 g·L^-1.The magnetic composite material prepared under this condition has a network gel structure and has a good removal rate of heavy metals Pb（Ⅱ）and Cu（Ⅱ）.Compared with zero addition of L-cysteine,The removal efficiency of Pb（Ⅱ）and Cu（Ⅱ）is increased by 30.27%and 27.81%respectively.SA@L-Cys@Fe₃O₄ has good magnetic separation ability,and the structural stability is improved compared with no added Fe₃O₄.（2）The results of adsorption conditions show that increasing the dosage,solution p H,adsorption temperature,initial target ion concentration and contact time within a appropriate range are more favorable for the adsorption of Pb（Ⅱ）and Cu（Ⅱ）by SA@L-Cys@Fe₃O₄.Among these factors,temperature has least influence on the adsorption of Pb（Ⅱ）and Cu（Ⅱ）by SA@L-Cys@Fe₃O₄.Initial p H:5,adsorption temperature:25°C,target ion initial concentration:100 mg·L^-1,contact time:120 min,SA@L-Cys@Fe₃O₄ dosage for Pb（Ⅱ）and Cu（Ⅱ）are under the conditions of 0.1 and 0.75g·L^-1,the removal efficiencys of Pb（Ⅱ）and Cu（Ⅱ）were as high as 76.78%and 98.08%.The adsorption process is more in line with the quasi-second-order kinetic equation and Langmuir isotherm adsorption curve,indicating that the adsorption of SA@L-Cys@Fe₃O₄ to Pb（Ⅱ）and Cu（Ⅱ）should be dominated by monolayer chemical adsorption.The maximum adsorption capacity of SA@L-Cys@Fe₃O₄for Pb（Ⅱ）and Cu（Ⅱ）is as high as 330.03 and 175.44mg·g^-1,which has greater adsorption advantages than many other types of adsorption materials.Moreover,after 5 cycles of adsorption-desorption experiments,the removal rate of Pb（Ⅱ）and Cu（Ⅱ）is still is still as high as 87.40%and 72.35%,and the iron dissolution rate is less than 1%of the total iron content of the adsorption material,and the risk of secondary pollution is extremely low.（3）After comprehensive analysis of BET,XPS and calcium ion release,it is found that the specific surface area of SA@L-Cys@Fe₃O₄ is 62.48m²·g^-1,which is larger than the specific surface area of ordinary calcium alginate gel.Larger specific surface area increases the effective adsorption sites.The adsorption of SA@L-Cys@Fe₃O₄ to Pb（Ⅱ）and Cu（Ⅱ）should be influenced by the combined action of physics and chemistry,and chemical adsorption is the main method.During the whole adsorption process,the coordination of amino group（-NH₂）and carboxyl group（-COOH）in SA@L-Cys@Fe₃O₄ and the exchange of calcium ions play a vital role.