Study On Phosphorus-modified Polyvinyl Alcohol Reinforced Removal Of Composite Heavy Metals Pollution Soil By Leaching

The problem of soil heavy metal pollution should not be underestimated,and remediation of compsite heavy metal soil pollution has become a research hotspot for soil pollution prevention and control.Currently,soil remediation techniques mainly include physical,chemical,biological methods.Compared with other soil remediation techniques,soil-washing offers extremely cost-effective and time-efficient,and can permanently remove heavy metals from soil.The selection and use of leaching agents is the key to this treatment method.Composite washing broken some limitations of single leaching and can be applied to remediate complex heavy metal contaminated soil.On the one hand,by adding acid-resistant heavy metal adsorbent materials to the drenching system,as most of the washing systems will remove heavy metals under acidic conditions,the amount and rate of desorption of heavy metals from the soil can be increased on the one hand,and thus the amount of drencher used can be reduced;on the other hand,it can partially restore the elution capacity of the leaching and achieve leaching recycling.Therefore,in this study,the phosphorus-modified polyvinyl alcohol（P-PVA）material with good heavy metal adsorption performance was added to the mixed system of leaching agents and soil to simulate cadmium（Cd）,copper（Cu）,lead（Pb）contaminated soil as a leaching object,and P-PVA can overcome the influence of soil on heavy metal adsorption,promote the desorption of heavy metals from the soil surface,and reduce the heavy metal content in the elution waste solution.P-PVA can reduce the difficulty of elution waste liquid treatment or realize the circulation of the eluent directly,saving the amount of eluent used,greatly improving the elution efficiency and reducing the elution cost.The main conclusions were as follows:（1）A simple one-pot method was successfully prepared for the P-PVA material.This material has strong heavy metal adsorption capacity and can achieve rapid adsorption of heavy metals in acidic solutions.The study of the adsorption capacity of P-PVA showed that coexisting heavy metals would compete with each other for adsorption,thus reducing the saturation adsorption capacity of each heavy metal.In the binary system,the competitive adsorption between Cd（Ⅱ）and Cu（Ⅱ）had the greatest impact,and the saturated adsorption capacity decreased by 59.12%and 59.48%,respectively.In the ternary system,the saturated capacities of Cd（Ⅱ）,Cu（Ⅱ）and Pb（Ⅱ）decreased by 64.78%,63.05%and 48.68%,respectively.The results of their adsorption kinetics and adsorption isotherms for heavy metals showed that the adsorption of P-PVA for Cd（Ⅱ）and Cu（Ⅱ）was consistent with quasi primary kinetic fitting and for Pb with quasi secondary kinetic fitting,and the adsorption of all three heavy metals was dominated by monolayer adsorption.The maximum adsorption capacities of 210.21mg/g,140.76 mg/g and 235.24 mg/g for Cd（Ⅱ）,Cu（Ⅱ）and Pb（Ⅱ）,respectively,were obtained by fitting the Langmuir model.According to the results of SEM,FTIR,XPS,EDS spectra,the phosphate group was successfully grafted on the surface of P-PVA,and the specific surface area was increased.P-O and P=O bonds in P-PVA played a major role in the adsorption of heavy metals,and they formed chelates with heavy metals and thus adsorbs them.（2）Salinity,soil soluble substances and leaching agents including cations and humic acids had some influence on the ability of P-PVA to adsorb heavy metals.In the soil leaching system,high concentrations of calcium,potassium and magnesium ions and Fe Cl₃ inhibited the adsorption of heavy metals by P-PVA.But the effect of calcium,potassium and magnesium ions was not obvious at low concentrations(391 mg/L for K⁺,200 mg/L for Ca²⁺,243 mg/L for Mg²⁺);the presence of humic acid（HA）and citric acid（CA）did not significantly affect the adsorption.In summary,when p H>3,the complex liquid composed of salt-based cations,HA and eluents（CA,Fe Cl₃）had little effect on the adsorption of heavy metals by P-PVA.（3）Compared with a single leaching,the composite leaching agents leached the three heavy metals best,and the addition of P-PVA promoted the elution rate of heavy metals and transferred the metal ions to P-PVA.The study of soil complex heavy metal elution using different leaching agents showed that the best elution time was 720minutes,and the optimal solid-liquid ratio for Fe Cl₃ leaching was 1:8,and the optimal solid-liquid ratio for CA and composite leaching agents leaching was 1:10.The three elution methods（0.02 mol/L Fe Cl₃,0.08 mol/L CA,0.04 mol/L Fe Cl₃+0.02 mol/L CA）all had good elution effects.Among them,the composite leaching had better elution effects on the two heavy metals.The removal rates of Cd and Cu reached 94.80%and86.96%,respectively,and the elution rate of Pb was 57.70%.At this time,the dissolved organic matter transferred from soil in the eluent was less than that in the single eluent.After the addition of 20%P-PVA,the solid-liquid ratio was changed to 1:4,CA was still able to achieve 97.27%,78.69%and 50.54%elution rates for Cd,Cu and Pb in soil by leaching.P-PVA could enhance the removal of heavy metals under partial leaching conditions.But P-PVA did not contribute significantly to the elution of heavy metals Pb under Fe Cl₃ and composite leaching agent washing conditions and Cu under CA and composite leaching agent washing conditions.Under most of the elution conditions,heavy metals in soil were transferred to P-PVA,which reduced the difficulty of post-treatment of leaching waste solution and realized the direct recycling of drenching agent.The addition of P-PVA reduced soil acidification and increased the water retention properties of the soil.