Preparation Of Lignin-Iron Oxide Composite And Its Immobilization Performance On Cd/Pb In Co-contaminated Soil

Heavy metal pollution in soil is a great threat to human health and ecological environment,so it is of great significance to effectively solve this problem in China.Among many remediation technologies,immobilization remediation technology is a simple,rapid and effective technology,and the key is to seek green,efficient and materials which have long-team effect of immobilization.In this paper,lignin-iron oxide composite(GLS)was prepared by co-precipitation of lignin,which has the properties of anti-biodegradation and heavy metal adsorption and fixation,and the typical mineral iron oxide in red soil of South China.Based on the characterization of GLS,the immobilization properties and mechanism of Cd and Pb in soil were investigated.(1)The preparation conditions of GLS co-precipitation method were optimized by taking adsorption capacity as the index.The results showed that when p H=13,ageing time was 60hours,ageing temperature was 90?,and lignin mass ratio was 10%,the maximum adsorption capacity of GLS10 on Cd²⁺and Pb²⁺were up to 49.87 and 120.24 mg/g,respectively.With the increase of lignin proportion,the crystal structure of GLS transformed from crystalline to weak crystalline or amorphous,and formed agglomerates composed of nanoparticles.Besides,specific surface area,total pore volume and C/Fe ratio of GLS also increased.The specific surface area,total pore volume and C/Fe ratio of GLS10 were 335.20 m²/g,0.30 cm³/g and3.52,respectively.The evaluation of oscillatory stability of acid solution showed that GLS10had better acid resistance than iron oxide(GLS0).(2)GLS10 has a wide p H range adaptability to the adsorption properties of Cd²⁺and Pb²⁺,and conformed to the pseudo-second-order kinetics model,the initial adsorption rate were17.60 and 3.61 times of GLS0,respectively.According to Langmuir and Freundlich isotherm adsorption models,the maximum monolayer adsorption capacities of GLS10 to Cd²⁺and Pb²⁺were 68.03?77.52 mg/g and 152.73?178.57 mg/g,respectively.The competitive adsorption of Cd²⁺and Pb²⁺on GLS10 were in accordance with the Extended Freundlich model and the Extended Langmuir model,respectively.Pb²⁺had a greater inhibition on the adsorption process of Cd²⁺.By binding with GLS10 surface hydroxyl group and C-OH,Cd²⁺and Pb²⁺formed Cd-O and Pb-O and adsorbed on GLS10.(3)The mobility factor(IM)values of Cd and Pb in soil decreased rapidly with the increase of the addition amount of GLS10.When the addition amount was 5%,the IM values of Cd and Pb reduced by 49.37%and 80.90%,respectively.Meanwhile,the contents of soil organic mater,cation exchange,ammonium nitrogen,available phosphorus and available potassium were increased.GLS10 reduced the IM values of Cd and Pb in soil by 58.94%and78.15%respectively after 120 days with the dosage of 3%,which was about 5 times of GLS0.Under acid rain leaching condition,the IM value still keeps a downward trend,and the immobilization effect was relatively stable.(4)The study on the morphological distribution and transformation of Cd and Pb in soil samples was conducted by using Zeien-Brümmer six steps continuous classification extraction method.The results showed that GLS10 transformed the easily soluble and exchangeable Cd/Pb into the organic binding form and the amorphous hydroxyl iron binding form,and also affected the transformation behavior of the manganese oxide binding form.When the dosage was 5%,the proportions of easily soluble and exchangeable Cd,Pb in soil decreased by17.10%,0.24%and 12.21%,23.20%,respectively.The higher adsorption properties of GLS10make the drop of easily soluble Cd and Pb greater than that of GLS0 in soil.Not only can GLS10 rapidly transform the easily soluble Cd and Pb,and reached into equilibrium at 15 and30 days respectively with the dosage of 3%,but promoted the further transformation of the exchangeable Cd and Pb into the residue form.After 120 days,the contents of Cd and Pb in soil increased by 12.37%and 7.76%respectively,which were greater than GLS0.Basic on the stability of GLS10,the organic binding form and amorphous hydroxyl iron binding form which formed by immobilization can resistance to acid rain dissolution,and the manganese oxide binding form of Cd and Pb can also maintain stability.