Effects Of Low Molecular Weight Organic Acids On Pb Release From Two Contaminated Soils

With the impact of environmental factors, lots of specific organism in soils will be secreted from plants, of which low molecular weight organic acids (LMWOAs) with relatively high steady-state concentrations are the important components. LMWOAs have the potential to enhance metal mobility in soil profiles by reducing soil pH and forming complexes with heavy metals, so as to reduce or control the soil pollution.In this study, Batch experiments were carried out to study the effects of low molecular weight organic acids (LMWOAs) on Pb release from two contaminated soils (red soil and brown soil). The Pb release amount was evaluated as a function of kind and concentration of LMWOAs, incubation temperature, contact time and initial organic acid pH, and the Pb forms after reaction were also determined. The results indicated:1. Compared to the control, the Pb release from both soils treated by LMWOAs with different kinds and concentrations increased significantly and the Pb release amount increased as well with the concentration of organic acids increasing. After the reaction, the soil pH tended to increase but the increasing amount seemed to be smaller with the concentration getting higher.2.As the concentration was from5mmol/1to100mmol/1, the Pb release amounts from red soil were:from9.3mg/kg to104.3mg/kg for acetic acid; from14.2mg/kg to200.8mg/kg for oxalic acid; from25.5mg/kg to107.7mg/kg for citric acid, and the Pb release amount from brown soil were:4.6mg/kg to51.4mg/kg for acetic acid;22.9mg/kg to165.8mg/kg for oxalic acid;14.0mg/kg to75.0mg/kg for citric acid. Generally speaking, the order of the organic acids to make Pb release from soil was: oxalic> citric>acetic, and red soil could release more Pb than brown soil under the same conditions.3. All the mixture of organic acids could release Pb significantly (p<0.05) than the control, and red soil released more Pb than brown soil under the same conditions. When the concentration was100mmol/l, mixture of acetic and citric acid could make red soil release Pb125.9mg/kg,39.3%of the total amount. However, mixed acids failed to make more Pb release from soils than single acid did.4. With the temperature from15to35℃, the Pb release amount from the two soils treated by all the LMWOAs increased remarkably, and the most release amount was232.9mg/kg at35℃for red soil with100mmol/1, which was72.7%of the total Pb amount in the soil. It was also found that when the temperature was35℃, the Pb release amount from brown soil increased sharply with the increasing organic acid concentration.5. Pb release tended to increase first and then decrease as time went on for the two soils, and the14th day was found to be the infection point, when no remarkable changes appeared from than on.6.As pH rose from2to5, Pb release from red soil tended to reduce but no such regularity was found for brown soil;The Pb release amount from brown soil treated by oxalic with different pHs was:6.18(pH4),16.34(pH5),5.48(pH2) and1.46(pH3) mg/kg, respectively. The Pb form results after the reaction illustrated that, for both the soils, the reducible Pb took the biggest proportion of the changing amount among the four kinds of forms, the second one was the acid-extractable Pb, and the amount of the oxidizable Pb and the residual Pb was relative lower. Oxalic and citric acid could make similar changing tend while citric acid would lead to larger change for the Pb forms. In addition, acetic acid also played an important role in making the changes of the Pb forms.Therefore, LMWOAs have an important role in making Pb release from soils and the release effects are closely related with different external conditions, which can be summarized as:concentration>temperature> initial pH>cultivation time. If used rationally, it will help greatly reduce or control soil pollution.