Preparation And Adsorption Performance Of Sodium Alginate Based Composite Gel For Heavy Metal Ions Removal From Water

Nowadays,heavy metal ions can frequently occur in different kinds of polluted water bodies.Due to their high toxicity and difficult to degrade,biological amplification and accumulation,it inevitably poses a grave threat to human health.Adsorption method has been widely used in the removal of heavy metal ions from aqueous solution because of its high cost-effectiveness,simple operation and strong versatility.Adsorbent is the key part and core foundation of adsorption method,which also largely determine the adsorption performance.The gel material with porous structure is conducive to the diffusion of small molecules into the its internal network easily,and it can swell and absorb?retain?water possibly even much larger than its own mass.It has great potential in the field of heavy metal pollution treatment.Sodium alginate?SA?is a natural anionic organic compound.It has many advantages,such as wide range of sources,low cost and environmental friendliness.It can gently and simply form hydrogel with three-dimensional network through ionic crosslinking.It has better metal ions binding ability and thus has attracted wide attention.However,the traditional ionic crosslinked SA hydrogel is limited by itself,which has the defects of weak mechanical strength,relatively poor water stability and relatively limited adsorption capacity,especially for the removal of anionic heavy metal ions.In view of this,the purpose of this study is to achieve its functionalization by surface modification,multiple blending and encapsulation modification,and to make up for its defects.Novel SA composite hydrogel beads based on functional design is developed and used to remove heavy metal ions from water.Combined with Fourier transform infrared?FT-IR?,scanning electron microscopy and energy-dispersive X-ray spectrometry?SEM-EDS?and X-ray photoelectron spectroscopy?XPS?etc.characterization methods,the adsorption mechanism of SA based composite hydrogel beads for heavy metal ions were explored,which will provides a new idea for effective removal of heavy metals in water and large-scale application of SA based gels as well.The main research results are as follows:?1?A facile route of poly?vinyl alcohol?/alginate@polyacrylamide?PVA/SA@PAM?for Cu?II?adsorption was prepared by crosslinking with PAM onto PVA/SA beads,which complemented both benefits of natural and synthetic polymers,resulting from the design of core-shell/bead-like structure and polymer blending.The PVA/SA@PAM composite gel beads showed improved adsorption swelling performance as well as good water stability With the increasing concentration of PAM solution used for surface modification,the adsorption capacity of Cu?II?increased firstly and then decreased.The results showed the optimal pH value was 5 and the coexistent ions in the water have little effect on the Cu?II?removal.The batch experimental data fitted well with the Freundlich isotherm model and the pseudo-second-order model,indicating that the adsorption behavior of Cu?II?is mainly heterogeneous multi-layer adsorption and chemical adsorption;thermodynamic studies revealed the adsorption process was spontaneous and endothermic in nature;Thomas model was fitted well with breakthrough curves under different initial Cu???concentration and flow rates in a column system.Besides,its removal ability only decreased slightly even after eleven cycles in reusability tests,indicating that it had good regeneration.The results revealed that complexation and cation-ion exchange are the dominant adsorption mechanism.?2?The low esterified pectin produced from grapefruit peel was blended with SA as stable matrix,while the biochar derived from grapefruit peel was acted as special functionalities of encapsulated material to prepare biochar/pectin/alginate hydrogel beads?BPA?and Cu?II?removal from aqueous solution.The role of biochar and pectin contents on the adsorption performance and compressive strength of BPA were investigated,and the optimum synthetic conditions were determined.Factors like solution pH,contact time,initial concentration of Cu?II?and coexisting inorganic salts or organic ligands on the adsorption removal were systematically investigated in batch mode.The adsorption isotherms were fitted well by the Freundlich model,suggesting that the adsorption of Cu?II?by BPA is mainly multi-layer adsorption.Kinetic process was described well by the pseudo-second-order model and film diffusion primarily governed the overall adsorption rate,followed by intraparticle diffusion.Thermodynamics analysis suggested spontaneous feasibility and endothermic nature of adsorption behavior.BPA has some certain selectivity for different heavy metal ions.The coexistence of inorganic salts and organic ligands could inhibit and promote the removal of Cu?II?,which is closely related to their concentration in water.After five adsorption-desorption experiments,the adsorption capacity of BPA was stable,which indicated that BPA had regeneration ability.It was found that the adsorption synergistic effect and interfacial interaction enhanced the adsorption,mechanical and thermal stability of the BPA.Based on these,we proposed the enhanced adsorption mechanism of Cu?II?.?3?In order to improve the adsorption affinity for Cr?VI?,polyaniline-polypyrrole?PAN-PPy?surface modification of graphene oxide?GO?through in-situ polymerization as functional filler,SA and CaCO₃ as matrix raw materials and pore forming agents,graphene oxide@polyaniline-polypyrrole/SA?GO@PAN-PPy?porous composite hydrogel beads were prepared for Cr?VI?and Cu?II?removal from water.The results showed that encapsulation,surface modification and porous structure could promote the adsorption performance of Cr?VI?and Cu?II?.The adsorption performance for Cr?VI?and Cu?II?varied with different AN and Py monomer ratios in modified fillers;moreover,the adsorption capacity of Cr?VI?increased with the increase of filler embedment,while that of Cu?II?increased firstly and then decreased.In a single system,the effects of solution pH,contact time,pollutant concentration and ionic strength on the adsorption of Cr?VI?or Cu?II?were investigated in batch mode.Within a certain range,NaCl in aqueous solution could effectively promote the adsorption of Cr?VI?while significantly inhibit the adsorption of Cu?II?.The Freundlich model described well the experimental adsorption data of Cr?VI?and Cu?II?by composite beads.And their adsorption kinetics were fitted well by the pseudo-second-order and the intraparticle diffusion models.Mechanical process.Both of competitive and cooperative adsorption were found in the In binary system of Cr?VI?and Cu?II?.Pre-swelling adsorption experiments showed that increasing swelling could enhance the adsorption performance for Cr?VI?and Cu?II?.Moreover,the 5 adsorption-desorption cycles of Cr?VI?and Cu?II?by composite beads were investigated in a single/binary system.Based on the above results,the enhanced adsorption mechanism of Cr?VI?and Cu?II?was proposed.?4?With CaCO₃ as the core material and pore-forming agent,crosslinked chitosan layer coated on the surface of CaCO₃ can serve as a reactive base for further modification with triethylenetetramine?TEAE?.By introducing multi-functionalized chitosan microbeads into alginate matrix and then removing CaCO₃ after gelation,a novel triethylenetetramine?TEAE?modified chitosan microbeads with multicavity structure encapsulated in alginate backbone?named as HCTA?was fabricated for Cr?VI?removal from aqueous solution.The effects of solution pH,coexisting ions,contact time,initial Cr?VI?concentration and temperature on the Cr?VI?removal by HCTA were investigated in batch mode.Based on this,Langmuir and Freundlich isothermal models,pseudo-first-order and second-order kinetic models,intraparticle diffusion model and thermodynamic analysis were used to analyze the adsorption behavior and speed-limiting steps of HCTA for Cr?VI?removal.The features of breakthrough curves of Cr?VI?removal by HCTA under different influent flow rate and desorption curves with different eluents were investigated in dynamic adsorption experiments.It was found that highly efficient Cr?VI?removal in a wide pH range?2–10?,whereas Na⁺,Ca²⁺,Al³⁺,Cl^-,NO₃^-and SO₄^2-had minimal effects on the removal performance.Good reusability HCTA beads were found after 9 cycles.Compared with SA beads,HCTA beads showed good chemical and structure stability for alkali resistance even at high saline conditions.The adsorption process involves many mechanisms,such as electrostatic interaction,reduction reaction,chelation and cation exchange.