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.