Study On Removal Of Low Concentration Of Heavy Metals By Poly-γ-glutamic Acid And Its Complexes

With the rapid pace of industrialization and urbanization,China’s heavy metal pollution has become increasingly prominent.Various types of mining waste,electroplating wastewater,metallurgical and metal processing waste water were discharged disorderly,making heavy metal pollution incident emerge in an endless stream.China’s heavy metal pollution risk remains high,and exploring the feasibility of heavy metal treatment is necessary.In this paper,adsorption effects and characteristics of heavy metals including Cu²⁺,Pb²⁺ and Cd²⁺ on poly-γ-glutamic acid（γ-PGA）were studied.The best adsorption p H of Cu²⁺,Pb²⁺ and Cd²⁺ were 4,6 and 6,respectively,for γ-PGA,and the optimum temperature range from 15 to 20 degree.Adsorption balance is achieved in about 60 minutes.The adsorption capacity of Cu²⁺,Pb²⁺ and Cd²⁺ with initial concentration of 100 mg/g was 1158 mg/g,1129 mg/g and 301 mg/g,and the removal rates were 92.6%,90.3% and 24.07%,respectively.In addition,the adsorption rate follows Cu²⁺> Pb²⁺> Cd²⁺.As a new type of biological adsorbent,γ-PGA has the advantages of large adsorption capacity,fast adsorption rate and no secondary pollution.However,γ-PGA is easy to dissolve in water,which is not conducive to the treatment of heavy metal wastewater in the mobile environment,neither to its reuse.The precipitate floc is easy to desorb and the adsorption stability is poor.In view of these deficiencies,the modification of γ-PGA not only exerts its excellent adsorption ability to heavy metals,but also enhances its practicability in practical wastewater treatment.The γ-PGA/gelatin were prepared by crosslinking γ-PGA with gelatin,of which the optimum adsorption p H for Cu²⁺,Pb²⁺ and Cd²⁺ was 4,6 and 6,respectively.Compared with the Freundlich isothermal equation,the experimental data are more consistent with the Langmuir isothermal equation,and the adsorption capacities of Cu²⁺,Pb²⁺ and Cd²⁺ are 234.44,199.80 and 82.78 mg/g at 20 ℃.Compared with the pseudo-first-order kinetics model,the experimental data are more consistent with the pseudo-second-order one.And the adsorption rate follows Cu²⁺> Pb²⁺> Cd²⁺.The removal rates of Cu²⁺,Pb²⁺ and Cd²⁺ with initial concentration of 100 mg/L were 96%,99% and 45%,respectively.From the comparison of the infrared spectrum,it can be seen that the amount of COO-after the reaction is greatly reduced,indicating that the-COOH in γ-PGA plays a major role in the adsorption process,which can combine and fix the free metal ions.Natural resolution of the three metal ions was only 0.12%,0.7% and 1.9% after 24 hours.And the adsorption rate of γ-PGA/gelatin about Cu²⁺,Pb²⁺ and Cd²⁺ was more stable than of γ-PGA.The desorption rates of Cu²⁺,Pb²⁺ and Cd²⁺ were 78.05%,74.16% and 77.24%,respectively,under the condition of HCl elution,which were beneficial to the recovery of heavy metals.The γ-PGA/gelatin can be recovered about 65% after adsorption and elution,of which the mechanical strength has been greatly improved compared to γ-PGA.The γ-PGA/Fe₃O₄ magnetic particles were prepared by coating γ-PGA on the surface of Fe₃O₄ magnetic nanoparticles.The scanning electron microscopy and infrared spectroscopy showed that γ-PGA had been successfully coated on the surface of Fe₃O₄.The γ-PGA/Fe₃O₄ exhibits good paramagnetism under the external magnetic field,and it can realize the efficient separation of γ-PGA/Fe₃O₄ from the solution.The optimum adsorption p H of Pb²⁺ and Cd²⁺ for γ-PGA/Fe₃O₄ is 5 and 6,respectively.The experimental data are more accurately accorded with the Langmuir equation and pseudo-second-order kinetics equation than Freundlich equation and pseudo-first-order kinetics.The adsorption capacity of Pb²⁺ and Cd²⁺ at 27 ℃ was 27.90 mg/g and 13.45 mg/g,respectively.The adsorption rate of Pb²⁺ was higher than that of Cd²⁺.The recoveries of Pb²⁺ and Cd²⁺ were 94.33% and 92.65%,respectively,after hydrochloric acid desorption.Without adding the desorption agent,the desorption does not occur after 24 hours,which indicates the good adsorption stability of γ-PGA/Fe₃O₄.After the first treatment,desorption and regeneration cycle,the recovery rate of γ-PGA/Fe₃O₄ reached about 98%,basically no loss.The secondary adsorption capacity of Pb²⁺ and Cd²⁺ reached 83.4% and 72.6% of the first one respectively,showing a higher recycling potential.