Washing Mechanism Of Citric Acid-modified Sodium Lignosulfonate To Remove Heavy Metals From Soil

Soil washing technology has become one of the hot topics in the field of soil heavy metal pollution remediation in recent years due to its advantages such as high efficiency,short cycle,flexible operation,and the ability to completely remove heavy metals.Screening environmentally friendly and efficient eluents is a decisive factor in the development and application of this technology.In this study,biodegradable citric acid(CA)modified papermaking waste sodium lignin sulfonate(LS)was used for washing to remove cadmium(Cd),lead(Pb),and zinc(Zn)heavy metals in contaminated soil,and the best parameters of the modification treatment were screened.The before and after modification were explored and compared.The material removes heavy metals from the soil under different washing conditions.The washing parameters were optimized through the design of the composite center,and the washing mechanism was further explored.At the same time,the washing was evaluated from the physical and chemical properties of the soil before and after washing,and the morphology and micromorphology of heavy metals Impact on the soil.The main research results showed as follows:(1)The optimal treatment conditions for citric acid-modified sodium lignosulfonate are:CA: LS mass ratio of 2:1,the amount of initiator is 0.25 g,the reaction time is 6.0 h,and the temperature is 60℃.Under these conditions,the yield of CA-LS and the conversion of CA were 99.008% and 98.35%,respectively.Fourier transform infrared spectroscopy,scanning electron microscopy,and energy spectroscopy analysis showed that the characteristic groups of the modified citric acid(–OH and –COOH)had been inserted into the backbone of sodium lignosulfonate,indicating that the citric acid successfully modified the ligninsulfonic acid sodium.The modified CA-LS can increase the removal rate of cadmium,lead and zinc by LS and CA by 22.52-64.56% and 7.46-17.56%,respectively.(2)The agent concentration,solution pH and washing time all had significant effects on the eluent efficiency of soil heavy metals(p < 0.05).With the increase of the concentrations of CA,LS and CA-LS,the washing efficiency of cadmium,lead and zinc increased as a power function(p < 0.05).Under the action of the three eluents,the removal rates of Zn and Pb decreased logarithmically with the increase of the pH value of the eluent.Removal maximums were reached at pH 5.0(12.96 and 60.03%).The washing rates of cadmium,lead,and zinc all increased exponentially with the increase of washing time(p < 0.05),and became gentle after 120.0 min.(3)The response surface method optimization results showed that the concentration of CA-LS,the pH value of the eluent,and the washing time all had significant effects on the removal of cadmium,lead,and zinc(p < 0.05).The pH value of the washing solution was greater than the eluent time,and there was a significant interaction between the concentration of CA-LS and the pH of the eluent solution(p < 0.05).The optimal elution parameters of cadmium,lead,and zinc were obtained by the optimized model.Under these conditions,the removal rates of cadmium,lead,and zinc reached 80.12,83.64,and 45.56%,respectively.(4)The CA-LS washing mechanism may be that hydrogen ions in the washing system promote the dissolution and desorption of metals from soil colloids,and that carboxyl and sulfonic acid groups are combined with metal ions through chelation and electrostatic adsorption.The improvement of the washing efficiency of CA-LS relative to the material before modification may be due to three reasons: firstly,the microspheres formed after modification increase the contact area between the eluent and the soil colloid;secondly,the density of carboxyl groups after modification is increased The ionization degree increases,which enhances the electrostatic adsorption in the electric field.The third is that the introduced carboxyl group enhances the coordination and chelating ability of the sulfonic acid group to heavy metals and forms a multi-dimensional network complex.(5)The physical and chemical properties of the soil changed significantly before and after washing.The degree of influence of washing materials on soil environmental quality was CA > CA-LS > LS.Among them,LS and CA-LS can supplement the soil organic matter content of 7.18%-10.88% after washing,and have no significant effect on soil total nitrogen,total phosphorus,total potassium,alkaline nitrogen and available potassium content(p > 0.05),However,the available phosphorus content and soil pH decreased significantly(p < 0.05).The physical and chemical properties of all soils were significantly reduced by 6.21-20.95%after CA washing.It shows that CA-LS washing can reduce the adverse effects of CA on soil fertility to a certain extent and supplement soil organic matter appropriately.Fourier infrared spectroscopy,scanning electron microscopy,and energy spectroscopy analysis further showed that the main structure of the soil was not damaged by the washing process.(6)CA-LS washing has a greater effect on the morphological distribution of Cd,Pb,and Zn in the soil.Washing mainly removes water-soluble,exchangeable,and carbonate-bound heavy metals in Cd,Pb,and Zn.Part of the Pb residue state is also removed,while Zn is firmly incorporated into the crystal lattice of the soil.Chemically enhanced washing may also be difficult to wash out.In short,the mobility of the three heavy metals after CA-LS washing was reduced and stabilized,and the soil environmental risk was reduced.