Complex Adsorption Of Heavy Metals In Soil And The Effect Of Organic Acid

Taken Shaanxi Province typical agricultural soil - Lou soil, Loessial soil, Dark loessial soil, Yellow cinnamon soil and Sandy soil as the sample, the systematic study of the equilibrium sorption characteristics of Cd²⁺, Cu²⁺, Zn²⁺ in the single and combined conditions in five soils, of equilibrium sorption characteristics of Cd²⁺, Zn²⁺ in the single and combined conditions with oxalic acid coexist in five soils were carried out, the interaction mechanisms between the various heavy metals, between the heavy metals and the soil composition, and the relationship between heavy metals and physical-chemical properties of soil were discussed, the effects of oxalic acid on heavy metals complex adsorption mechanisms were proved also. The main conclusions are as follows:Cd²⁺, Cu²⁺, Zn²⁺ in the tested soils, the isotherms are presented as L-type, III type and L-type respectively. For Cd²⁺, Dark loessial soil always have the strongest adsorption to Cd²⁺, while the sandy soil on the adsorption of Cd²⁺ is always the worst. In addition to ST, the Cu²⁺ have a greatest effect on Cd²⁺ adsorption in the other four kinds of soil, while the Zn²⁺ effect is relatively small. Effect of Cu²⁺ on Cd²⁺ adsorption has a certain offset results against effect of Zn²⁺; for Cu²⁺, when the temperature is 20℃or 30℃, the Cu²⁺ has stronger adsorption on Loessial soil, Dark loessial soil and Lou soil, Sandy soil (ST) on Cu²⁺ adsorption is always the worst. Zn²⁺, Cd²⁺ resulted in a decrease of Cu²⁺ adsorption in different soils at 20℃. Furthermore, , Zn²⁺ or Cd²⁺ can promote Cu²⁺ adsorption in five soils, while Zn²⁺ and Cd²⁺coexistence reduces Cu²⁺ adsorption at 30℃; for Zn²⁺, when at 20℃, the different soils adsorption to Zn²⁺ showed a Dark loessial soil, Lou soil adsorption maximum, and Sandy soil the smallest adsorption, Cd²⁺ has a weak inhibitory effect, while Cu²⁺ has stronger inhibitory effect on Zn²⁺ adsorption. In both single treatments of Cd²⁺, Cu²⁺, Zn²⁺ and complex treatment of Cd+Zn+Cu, the three kinds of metal ions adsorption ordered by Cd> Zn> Cu.The Freundlich adsorption isotherms model is the best model for describing Cd²⁺, Cu²⁺, Zn²⁺ adsorption in Cd²⁺, Cu²⁺, Zn²⁺ single and combined treatment conditions. Mechanism study shows that, electrical charge adsorption, resulted from carboxyl hydroxyl of organic matter (OM) and electrical charge of clay minerals, is the mainly mechanism of Cd²⁺adsorption, Cu²⁺ has its impact on Cd²⁺adsorption through its chemical adsorption of complexing with the soil organic matter, while Zn²⁺ impacted mainly through electrical attraction of organic carboxyl, hydroxyl on OM and the competitive adsorption to the charged adsorption sites on clay minerals; Cu²⁺ sorption mechanism includes physical adsorption mechanism, including the ion exchange on clay mineral surface and surface energy form, and chemical adsorption mechanism, this is mainly mechanism including Cu(OH)2, CuCO3 precipitation generation and strong complex between Cu²⁺ and the hydroxyl and carboxyl of organic matter. Cd²⁺ coexisted in soil can compete with Cu²⁺ adsorption by charged adsorption. The coexistence of Zn²⁺ promote Cu²⁺ adsorption may be due to supply of some physical adsorption site of Zn²⁺, which released by some Zn²⁺ transformed into a chemical adsorption,, for the Cu²⁺ by the use of results; As same as Cu²⁺ adsorption mechanism, Zn²⁺ adsorption mechanism is the coexistence of both chemical and physical adsorption, i.e. ion exchange adsorption and surface energy on the clay mineral surface, and the strong complex effect (main mechanism) with hydroxyl and carboxyl groups in OM,. Cd²⁺ effects on Zn²⁺ adsorption mainly by competitive adsorption on electrostatic adsorption sites, while Cu²⁺ can produce competition by multi-factors, such as electrical attraction and complex adsorption.The effect of oxalic acid on Cd²⁺, Zn²⁺ adsorption were similar, it can reduce the Cd²⁺, Zn²⁺ adsorption in Cd²⁺, Zn²⁺ in single treatment or the combined treatment. In different soils, Lou soil, Dark loessial soil showed best adsorption capacity, while the Sandy soil is weakest. The reason that oxalic acid does not change the Cd²⁺, Zn²⁺ mutual antagonism is that it can form complexes with Cd²⁺, Zn²⁺, while the pH in soil solution decreased, therefore, the adsorption ability of soil can be reduced by way of decrease of electrical attraction on Cd²⁺, Zn²⁺. Since oxalic acid has big impact on electrical attraction for adsorption Cd²⁺, Zn²⁺, it does not change the interaction mechanism between Cd²⁺, Zn²⁺ antagonize.