Research On The Adsorption Kinetics Of Heavy Metal Elements (Cu, Zn, Cd) In Red Soils

Soil heavy metal pollution has received extensive attention around the world.Adsorption is a crucial process，which can lead to heavy metals accumulation in soils,and also determine the mobility, toxicity and biological validity of metals. Studyingthe adsorption of heavy metal pollutants in soils is essential for effectively predictingthe environmental effect of heavy metals and providing a scientific basis for health riskassessment of heavy metals. Therefore, red soil was collected, which is a typical soil insouth area of China. Three kinds of heavy metals, Cu, Zn and Cd, were involved.Effects of the multiple environmental factors (water-to-soil ratio, temperature, ionconcentrations of heavy metals) on the single metal adsorption in soils, comparison ofthe simultaneous competitive adsorption of binary mixture solutions of Cu, Zn, Cd ions,and the influence of the adding orders on heavy metals adsorption in red soils werestudied by using the method of batch test. The main conclusions were as follows:(1) The increase of water-to-soil ratio and ion concentrations reduced the singleCu, Zn, Cd ions adsorption rates. The equilibrium capacities of single Cu, Zn, Cd inred soils increased as the temperature went up. The first-order and second-orderkinetics equations were used to fit the adsorption process of the single Cu, Zn, Cd ionsin different temperatures, and results indicated that the fitting curve obtained by thetwo equations could well coincide with the experimental data, the fitting correlationcoefficient R reached up to0.99, and values of qecaculated by the two kineticsequations were very close to the the value of qein experiments.(2) Compared the competitive adsorption kinetics of binary mixture solutions(Cu+Zn, Zn+Cd, Cu+Cd) of Cu, Zn, Cd ions with their single metal adsorptionkinetics in red soils, it could be found that the equilibrium adsorption times of Cu2+,Zn2+and Cd2+in simultaneous competitive adsorption were different from theequilibrium adsorption times of single metal adsorption, and the equilibrium amountsof Cu, Zn, and Cd when heavy metals coexisted in red soils were less than that of theirsingle one. The soil distribution coefficient Kdvalues of Cu ions in the simultaneouscompetitive adsorption were greater than0.2, the Kdvalues of Zn ions were between0.1~0.16, while the Kdvalues of Cd ions were less than0.1. Therefore, the selectivitysequence of heavy metals in simultaneous competitive adsorption was Cu> Zn> Cd.(3) Six treatments (Cu→Zn, Zn→Cu, Zn→Cd, Cd→Zn, Cu→Cd and Cd→ Cu) were added to red soils. It could be found that Cu, Zn, and Cd ions were hardly toreach the state of the balance when metals were added to red soils orderly, and the totalamounts of heavy metals adsorbed by soils under this condition were less than the totalamounts of heavy metals entering into soils at the same time. The selectivity sequenceof heavy metals in sequential competitive adsorption was Cu> Zn> Cd, which wasdifferent from that of metals in simultaneous competitive adsorption.