Abstract: The isothermal curves of Pb ²⁺ adsorption-desorption by soil colloids followed Freundlich and Langmuir equation. The adsorption constant Ka showed the trend of old manured loessal soil >yellow loessal soil>dark cultivated loessal soil>yellow cinamon soil, which was consistent with surface charge density σ₀, an indicator of the strength of Pb ²⁺ adsorption by each soil colloid. Low molecular- weight organic acids (LMWOAs) made Pb ²⁺ adsorbed amount decreased and adsorption affinity increased, and the effect of citric acid was larger than that of oxalic acid. The constant K_d of residual Pb ²⁺ adsorption after desorption showed the trend of old manured loessal soil > yellow cinamon soil > dark cultivated loessal soil > yellow loessal soil, which reflected the adsorption strength of residual Pb ²⁺ after desorption, and was related to the contents of organic matter and free ferric oxide. The isothermal curves of Pb ²⁺ desorption from soil colloids were obvious higher than those of Pb ²⁺ adsorption. In NaNO₃ and oxalic acid background solution, the isothermal curves of Pb ²⁺ adsorption were far away from those of Pb ²⁺ desorption, showing that the hysteretic degree between adsorption and desorption was very large. The isothermal curves of Pb ²⁺ desorption from soil colloids were closed to those of Pb ²⁺ adsorption in citric acid background solution, which showed that the Pb adsorption and desorption was reversible.

Key words: Constant charge soil colloid, Adsorption, Desorption, Lead(Ⅱ), Low-molecular-weight organic acids