不同温度下镉在典型农田土壤中的吸附动力学特征

吸附反应是重金属进入土壤后发生的重要过程,其直接影响着重金属的生物有效性。以我国12种典型农田土壤为实验材料,采用批次平衡法研究了镉在土壤中的吸附动力学特征,并采用动力学方程进行拟合。结果表明,在高、低两个温度条件下,镉在土壤中的吸附均存在两个不同的反应阶段,即快速吸附反应和慢速吸附反应阶段。25℃时,反应开始5min内的吸附量可达最大吸附量的72.69%以上,至0.5h时可达83.76%以上;40℃时分别可达最大吸附量的77.50%和87.02%。其中,快速吸附反应以化学吸附为主,慢速吸附反应则以物理吸附为主,以化学吸附为主的快速反应又主导着整个反应过程。不同类型农田土壤对镉的吸附动力学模型以Elovich模型较优。碱性土壤吸附速率和吸附量都高于酸性土壤,温度的升高也会促进土壤对镉的吸附速率和吸附量。pH是影响镉吸附速率的主要因子。

Effects of Temperature on the Adsorption Kinetics of Cadmium in Typical Agricultural Soils

Adsorption is the initial process when heavy metals enter soil.Adsorption is important because it directly influences the bioavailability of heavy metals in soil.In this experiment,batch techniques were used to study Cd adsorption by 12 typical agricultural soils from China.The adsorption data were fitted using kinetic equations.The results showed that Cd adsorption could be divided into two steps regardless of temperature:fast reaction and slow reaction.At 25 ℃,Cd adsorption reached 72.69% of the soil’s maximum adsorption capacity within the first 5 minutes of the reaction and 83.76% of the soil’s maximum adsorption capacity within the first 30 minutes of the reaction.At 40 ℃,Cd adsorption reached 77.50% of the soil’s maximum adsorption capacity within the first 5 minutes of the reaction and 87.02% of the soil’s maximum adsorption capacity within the first 30 minutes of the reaction.Chemical adsorption was the main adsorption mechanism for the fast reaction,whereas physical adsorption was the main adsorption mechanism for the slow reaction.Furthermore,the whole reaction mainly depended on the fast reaction,which is attributed to chemical adsorption.The Elovich model best described Cd adsorption by these soils.The adsorption rate and amount of Cd adsorption were both greater for alkaline soils than for acid soils.High temperatures also increased the adsorption rate and the amount of Cd adsorption by the soils.The Cd adsorption rate was mainly affected by pH.