Adsorption Of Heavy Metal From Aqueous Media By Using Magnetic Hydrogels

Heavy metal contaminated wastewater has been regarded as a serious threat to the environmental safety and human health,and it is of great practical significance to remove heavy metal before being discharged into our environment.Adsorption has been widely used for treating heavy metal wastewater because of abundant adsorption materials and high efficiency.However,adsorption materials generally difficult to be re-collected for recycle.In order to overcome these defects,the main objective of this study was?1?to synthesize a series of magnetic hydrogel beads by using carboxylated chitosan?CMC?,sodium alginate?SA?,polyethyleneimine?PEI?and chitosan?CS?as the raw materials,and?2?to evaluate their adsorption performance and mechanism for treating wastewater containing copper or chromium.The main studies and conclusions are summarized as follows:?1?Magnetic hydrogel beads Fe₃O₄@CMC/SA were prepared by using CMC,SA and Fe₃O₄ as the raw materials.The synthesized magnetic hydrogel beads were then applied in Cu???removal from aqueous solution.Results showed that the gel beads have good adsorption performance toward Cu???.The adsorption kinetics and thermodynamic behavior could be well described by using pseudo second-order kinetic equation and Langmuir isotherm adsorption model.Meanwhile,the effects of pH,ion strength and the number of reuse cycles on the adsorption performance of hydrogel were studied.It was found that the adsorption capacity increased with the increase of pH value,and its maximum adsorption capacity reached 55.22 mg·g^-1 at pH 5.0.Competitive adsorption was observed with the addition of cationic ions,resulting in lower adsorption capacity.After 5 cycles,the adsorption capacity could still reach 32.44 mg·g^-1,suggesting its good recyclability.?2?Magnetic hydrogel beads Fe₃O₄@CMC/SA/PEI were prepared by introducing PEI molecular chains into the Fe₃O₄@CMC/SA hydrogel.The synthesized magnetic hydrogel beads were characterized by scanning electron microscopy?SEM?and infrared spectroscopy?FTIR?,and their adsorption kinetics and thermodynamics toward Cr???removal in aqueous media were then investigated in detail.Moreover,the effects of PEI content,pH,anion and the number of reuse cycles on adsorption performance were also studied.It was found that the adsorption performance enhanced at first and then leveled off with increasing amount of PEI.When the PEI content was kept at 72.5%based on the dry hydrogel,the synthesized hydrogel breads exhibited regular spherical shape and superior Cr???adsorption performance.The adsorption kinetics could be well fitted by using pseudo-second-order rate kinetic model,while the adsorption isotherms could be well described by using Langmuir isotherm model.The adsorption capacity increased with the decrease of pH value,and its adsorption capacity reached 225.37 mg·g^-1 at pH 2.0.The synthesized magnetic hydrogel breads could be easily synthesized and recycled.After 5 cycles,the adsorption capacity could still reach 115.96 mg·g^-1,suggesting its good recyclability.?3?Magnetic hydrogel beads Fe₃O₄/CS/PEI were firstly prepared by using CS and PEI as the raw materials via precipitation method;thereafter,magnetic hydrogel beads Fe₃O₄/CS/PEI/GA with excellent removal performance were successfully synthesized by grafting gallic acid?GA?on the surface of Fe₃O₄/CS/PEI.The synthesized hydrogel beads were characterized by SEM,FTIR and X-ray photoelectron spectroscopy?XPS?.The total Cr adsorption performances and the removal efficiency of Cr???were compared by using the two magnetic hydrogel beads.It was found that the hydrogel Fe₃O₄/CS/PEI/GA has higher adsorption capacity for total Cr and stronger reduction ability toward Cr???,and their adsorption kinetics and thermodynamic behavior could be well described by using pseudo second-order kinetic equation and Langmuir isotherm adsorption model.The total Cr adsorption capacities of the two hydrogel beads were 285.52 and 319.04 mg·g^-1 at pH 2.0,respectively.When the pH increased to 7 or 12,both the total Cr adsorption performance and the removal efficiency of Cr???decreased significantly.