Study On Adsorption Of Heavy Metals By Polymer Soft Capsules

Adsorption method was a kind of green,environmentally friendly,simple and efficient method for wastewater treatment.In recent years,researchers had devoted themselves to developing cheaper and more effective natural polymers as adsorbents.Among them,alginate,starch,chitosan,cyclodextrin,etc.because of its particular molecular structure,physicochemical properties and excellent selectivity for heavy metal ions,had been widely concerned.In this paper,a titration-curing wrap method was used to encapsulate anionic polyacrylamide?PAM?with sodium alginate and trihexyltetradecylphosphonium chloride?CYPH@IL101?with chitosan to prepare calcium alginate@polyacrylamide?PAM@Alg?capsules and chitosan@trihexyltetradecylphosphonium chloride?Chitosan@CYPH@IL101?softgels.As Cd???,Pb???and Cr?VI?were the target pollutant,the influence of pH,dosing quantity,initial concentration,contact time,temperature,and coexisting ions on the absorption of heavy metal ions of the two materials were studied.The main research contents and results are summarized as follows:?1?The CaCl₂ and polyacrylamide?PAM?mixed solution was added dropwise to the sodium alginate?SA?solution,SA reacted with Ca²⁺to form calcium alginate,which was cured to form a film,and the polypropylene droplets were wrapped to form PAM@Alg soft capsules.Electron micrographs showed that the surface of the PAM@Alg soft capsule was uniform and was a spherical material with a double layer structure.The static adsorption experiments showed that the adsorption amount of Pb?II?and Cd?II?in the solution gradually increased with the increase of pH value in the range of pH=2-5 or pH=2-6.5.The isothermal adsorption experiments and adsorption kinetics fitting results showed that the adsorption process of heavy metal ions by PAM@Alg soft capsules conformed to the pseudo-second-order kinetic model and the Langmuir isotherm model,indicating that the adsorption process was dominated by chemical adsorption.At 298 K,the maximum adsorption capacities were q_cd?II?=281.69 mg·g^-1 and q_Pb?II?=595.24 mg·g^-1,respectively.The adsorption thermodynamic constants??G^?,?H^?,?S^??analysis showed that the higher the temperature,the greater the adsorption capacity,indicating that the adsorption process was a spontaneous endothermic process.When C₀=10 mg·L^-1,and the dosage of adsorbent was 2g·L^-1,the residual concentration of Cd²⁺in the solution was less than 0.05 mg·L^-1,met to standard emissions;and when C₀=10 mg·L^-1,the dosage of adsorbent was0.5g·L^-1,the residual concentration of Pb²⁺in the solution was less than 0.1 mg·L^-1,up to standard emissions.The kinetics of mixed cation adsorption experiments showed that the adsorption order of PAM@Alg soft capsules was Pb²⁺>Cu²⁺>Cd²⁺>Zn²⁺.It might be that the outer layers of Pb²⁺have more electrons,more deformability and easier adsorption.?2?Using potassium dichromate as the target pollutant,the adsorption properties of chitosan@CYPH@IL101 soft capsules to Cr?VI?were studied.Static adsorption results were as follows:Chitosan@CYPH@IL101 soft capsules adsorption of Cr?VI?was a slow adsorption,and slow balanced process.When the pH=2,dosage of adsorbent was 1g·L^-1,adsorption equilibrium can be reached in 10h.At 298K,the experimental results of adsorption kinetics and isothermal adsorption showed that increasing the oscillation speed of the shaker was conductive to the increase of the adsorption rate.Under the higher rotational speed(r=150·min^-1 or r=250·min^-1),the experimental results were more in line with Pseudo-second-order kinetic model and Langmuir isothermal adsorption model,the fitting value of adsorption capacity was 102.46mg·g^-1,indicating that the adsorption process was mainly controlled by chemical action.The common anions PO₄^3-,SO₄^2-and NO₃^-all inhibited the adsorption of Cr?VI?in solution on Chitosan@CYPH@IL101 soft capsules.When the dosage of adsorbents was 2.4g·L^-1,the remaining Cr?VI?concentration was 0.29mg·L^-1,meeting the national wastewater discharge standards.?3?The results of competitive adsorption between anions and cations indicated that PAM@Alg soft capsules had selective adsorption to heavy metal cations,which was consistent with the results of FTIR and XPS.PAM@Alg soft capsules relied mainly on the carboxylate?-COO^-?on the surface of calcium alginate and?-NH₂?on polyacrylamide to adsorption of Cd???and Pb???in solution by electrostatic attraction and complexation.Under strong acidic conditions,the amino groups which were on the surface of the chitosan@CYPH@IL101 soft capsule were protonated as-NH₃⁺and CYPH@IL101?R₄P⁺?,would repel the metal cation in the solution,so it would selective adsorption of Cr?VI?which the solution in anionic form(HCrO₄^-,Cr₂O₇^2-and CrO₄^2-).