Adsorption-desorption Characteristics Of Copper,Lead And Cadmium In Ion-type Rare Earth Mining Soil

Gannan ion type rare earth mine is large-scale development and utilization,because of a wide range of minerals,widely distributed,rich reserves and so on.In the early stage of rare earth mining,the use of pool leaching and heap leaching technology resulted in serious environmental damage and resource loss.With the continuous improvement of the process,it is widely used in situ leaching process,which can reduce environmental damage and raise rare earth mining rate.However,heavy metal pollution of groundwater and soil is more serious.Therefore,it is of great significance to study the adsorption and desorption of heavy metals in soils in the field of ecological environment.In this paper,the soil（raw ore and tailings soil）of gannan ionic rare earth mining area was selected as the research object,and the current polluted Cu,Pb and Cd were the target pollutants.The effects of heavy metal concentration,adsorption time,pH,soil and water ratio,humic acid and temperature on the soil adsorption of single heavy metals were studied.Because the actual soil pollution is coexisting with many kinds of heavy metals,this paper studies the effect of soil on the competitive adsorption of binary and ternary heavy metals.The desorption characteristics of single heavy metal were studied by using HNO₃ and EDTA as desorbents,and the desorption characteristics of binary and ternary heavy metals were studied by EDTA as desorbent.The results showed that:（1）For single heavy metal,heavy metal concentration,adsorption time,pH,soil and water ratio,humic acid,temperature and other factors have an important influence on adsorption.The adsorption amount of heavy metal on the soil increased with the increase of equilibrium concentration,and then increased slowly and finally became gentle.The maximum adsorption capacities of Cu,Pb,and Cd on the ore soil were 0.42,1.43,and 0.83mg/g,respectively,and the maximum adsorption capacities on the tailings soil were 0.55,2.48,and 0.88 mg/g,respectively.According to the adsorption time,adsorption can be divided into two stages:rapid adsorption and slow adsorption.When the pH values were 3 and 5,the adsorption capacity of heavy metals in soil reached the minimum and maximum values,respectively.Visual MINTEQ software was used to study the different morphological distribution of heavy metals under different pH.When the pH was 6.5,Cu²⁺accounted for62%.When pH<6,Pb²⁺was present,and Pb²⁺was gradually decreasing with the increase of pH.When pH=9.5,Cd began to precipitate in the form of Cd（OH）₂,and the total precipitation reached 92%.The adsorption rates of Cu,Pb,and Cd for the two soils were lower than 35%,55%,and 40%under different soil-water ratios.When the content of humic acid was 5%,the adsorption amount reached the maximum.The maximum adsorption capacities of Cu,Pb,and Cd on the ore soil were 1.43,4.45,and 1.07 mg/g,respectively,and the maximum adsorption capacities on the tailings soil were 2.48,4.67,and 1.8 mg/g,respectively.The adsorption of heavy metals by ore and tailings increases with increasing temperature.（2）The adsorption curves of the three heavy metal ions in heavy metal binary mixtures（Pb-Cu,Pb-Cd,Cu-Cd）systems were extremely irregular in most lines.This phenomenonwas mainly caused by competitive adsorption sites and counteractions between heavy metals.In the soil,the order of adsorption of adsorption is Pb>Cu>Cd.In Pb-Cu-Cd ternary competition system,the distribution coefficients of Pb,Cu and Cd in ore-bearing soil were 0.023,0.005,and 0.013,respectively;the distribution coefficients of Pb,Cu,and Cd in tailings were 0.029,0.008,and 0.005,respectively.The total co-distribution coefficients of Pb-Cu-Cd ternary competitive adsorption in ore and tailings were 0.041 and 0.042,respectively,and the difference was not significant.（3）The desorption of a single heavy metal in the soil was performed using 0.1 mol/L HNO3 solution and 0.1 mmol/L EDTA solution.With the increase of the equilibrium concentration of heavy metals,the desorption amount also increased.The desorption amounts of Cu,Pb,and Cd ranged from 4.7 to 5.2,6.8 to 7.4,and 8.4 to 8.8 mg/g,respectively.Using EDTA as a desorption agent to study the two heavy metals,it was concluded that as the equilibrium concentration of heavy metals increases,the amount of desorption increases.The desorption amount of ternary heavy metals by EDTA was in the order of Cd>Cu>Pb,and the total distribution coefficients of ore and tailings were 4.11 and 4.459,respectively.