Research On The Fabrication Of Sodium Alginate Based Gel Beads Adsorbents For Adsorption Of Heavy Metal Ions

With the rapid development of population and modern economy, how to effectively treat heavy metal wastewater has become a research focus in the field of water treatment.Compared with traditional technologies for the removal of heavy metals, adsorption has the following characteristics:Relatively low cost, simplicity,recyclability, high efficiency, and selectively removingheavy metal ions with low concentrations.In this study, sodium alginate (SA) based macromolecule gel beads (SA-CMC, SA-PEG and SA-ILS) were synthesized by the method of blending and crosslinking, namely sodium carboxymethyl cellulose (CMC), polyethylene glycol (PEG) and ionic liquids (ILS) were added into the raw material (SA) with the cross-linking agent of CaC12.Single factor experiment was conducted to study the removal effect of heavy metals (Pb2+ and Cu2+) existed in simulation wastewater by prepared gel beads adsorbents. The effect of different experimental parameters, including initial pH, adsorbent dosage, reaction temperature, contact time, and initial metal ion concentration on the adsorption of heavy metalswere discussed. Further, the adsorption isotherm of Langmuir, Freudlich and Dubinin-Radushkevich (D-R),pseudo-first-order,pseudo-second-order,and particle diffusion kinetics models, and thermodynamic models were used to study and analyze the adsorption behavior.At the same time, desorption and regeneration of the gel beads were also studied. The structure of the gel beads was characterized and the adsorption mechanism was explored.Static adsorption experiments and specific surface area, pore size analysis were performed for SA-CMC gel beads.It was founded that the best quality ratiowas 1:1.The maximum adsorption of Pb2+was obtained under thefollowing conditions:pH=4.0-5.0; initialconcentration=5 mg/L; temperature=35±2℃;the dosage of sorbent-0.8 g; oscillation time=18h. Isothermal studies showed that the adsorption equilibrium data of Pb2+ was most consistent with the Langmuir isotherm model, which indicated that the adsorption of Pb2+ on SA-CMC gel beads was monolayer.Kinetic studies showed that the adsorption data was more in line with the pseudo-second-order kinetic model, suggesting that the adsorption was mainly based on chemical adsorption.Static adsorption experiments were conducted for SA-PEG and SA-ILS gel beads, the results showed that the maximum adsorption of Pb2+and Cu2+ was attained under the following conditions:pH=5.0; initialconcentration=5 mg/L; temperature=20℃; the dosage of sorbent =0.8 g; oscillation time=300 min.In isothermal studies, the adsorption equilibrium data of Pb2+ and Cu2+ was most consistent with the Freudlich isotherm model and then followed by D-R model, which indicated that the adsorption of Pb2+ and Cu2+on SA-PEG and SA-ILS gel beads was multilayer.Moreover, it was concluded that the adsorption was mainly chemical adsorption by calculating the average adsorption energy E of D-R model.In kinetic studies, the adsorption data was more in line with the pseudo-second-order kinetic model, showing that the adsorption was mainly based on chemical adsorption. Furthermore, the data of the particle diffusion model indicated that particle diffusion was not the only rate controlling step in adsorption process, it could also be influenced by surface adsorption and liquid membrane diffusion.In terms of thermodynamics, the calculated Gibbs free energy ΔG° were all less than 0, and the fitting of the entropy change ΔS° and enthalpy change ΔH° were both less than 0 under different temperature, indicating that the adsorption process was spontaneous and exothermic. Furthermore, the fitting of enthalpy change ΔH° also proved that the Pb2+ and Cu2+ adsorption by two gel beads was mainly chemical adsorption. The data of gel beads desorption and regeneration showed that 0.1 mol/L HCl solution could be used to desorption and regeneration. Gel beads still exhibited a high removal efficiency (approximately 90%) for Pb2+, even after five desorption.The main conclusions about the studies of characterization were shown as follows:The FTIR spectrum of gel beads before and after adsorption showed that the peaks of O-H bond and COO-bond increased after adsorption, suggesting that the O-H bond and COO- bond on gel beads might react with Pb2+ and Cu2+ in the adsorption process. The scanning electron microscope (SEM) pictures showed the surface characteristics of the gel beads, which indicated that the structure of SA-CMC, SA-PEG and SA-ILS gel beads was network-like. The X-ray energy spectrum (EDS) proved that the heavy metal ions were adsorbed on the gel beads. To this end, the adsorption mechanisms can be concluded as follows:(1) Physical adsorption,namely the gel beads modified by CMC, PEG and ILS endowed more surface and internal pore structure, which provided more adsorption sites for Cu2+and Pb2+;(2) Chemical adsorption,namely due to the solitary electronicof O existed in COO-and-OHof gel beads and the space orbitprovidedby Pb2+ and Cu2+, complex coordinationoccurred betweengel beadsand Pb2+ and Cu2+;(3) Electrostatic adsorption, which generatedbecause of the negatively charged surface of gel beads, and positively chargedPb2+ and Cu2+.