腐殖酸对土壤铅镉吸附、赋存形态及生物可给性的影响

腐殖酸（HAs）对重金属土壤环境行为及其生物有效性的影响与其组分分子量大小关系密切.通过3种in vitro试验方法（PBET、SBRC、IVG）探讨了HAs不同分子量组分（F1：<5kDa、F2：5~10kDa、F3：10~30kDa、F4：>30kDa）对土壤镉（Cd）、铅（Pb）生物可给性的影响，同时结合批量等温吸附实验、化学连续提取形态分级方法，分析了土壤Cd、Pb吸附和形态转化与生物可给性的关系.结果表明：土壤对Cd、Pb吸附能力既受HAs添加量的影响，也受到HAs分子量大小的影响.HAs添加量为0.2% C~1% C条件下，<5kDa分子量组分促进了土壤对Cd、Pb的吸附，且随着HAs添加量的增加，促进作用更强.而>10kDa的其他组分则降低了土壤吸附Cd、Pb的能力.在本试验HAs添加量范围内，不同组分均促进了土壤中酸提取态Cd向残渣态转化，且低分子量组分对其促进作用更强.土壤Pb²⁺主要以可还原态的形式存在，占57%~89%，随着HAs组分分子量的增加，酸交换态Pb占比逐渐下降，HAs的添加促进了活性态Pb向难利用态Pb转化，从而降低了Pb²⁺活性.3种in vitro方法生物可给性测定结果表明，<5k和5~10k组分均提高了胃、肠阶段土壤Cd、Pb生物可给性（BA_Cd、BA_Pb），而其余组分则使土壤Cd、Pb生物可给性降低.Cd、Pb生物可给性取决于Cd、Pb在土壤中转化平衡后的赋存形态，酸交换态占比高生物可给性相应也越高，土壤对重金属的吸附固定难以反映生物可给性.

Effects of humic acids on the adsorption,chemical speciation,and bioaccessibility of soil lead and cadmium

Effects of humic acids (HAs) on the environmental behaviors of heavy metals in soil and their bioavailability are closely related to the molecular weight of HAs components. Three in vitro methods, i.e., PBET, SBRC and IVG, were adopted to explore the effects of different molecular weight components of HAs (F1:<5kDa, F2:5~10kDa, F3:10~30kDa, F4:>30kDa) on the bioaccessibility of soil cadmium (Cd) and lead (Pb), and the bioaccessibility of soil Cd and Pb in relation to their adsorption, transformation in soils was analyzed in combination with batch adsorption experiment and fractionation by continuous extraction. The results showed that:Soil adsorption capacity of Cd and Pb was affected not only by the amount of HAs added, but also by the molecular weight of HAs. F1fraction (HAs component with molecular weight < 5kDa) promoted the adsorption of Cd and Pb in soil, and the promoting effect increased with the increase of HAs added at the dosage of 0.2% C~1% C, while other fractions with molecular weight > 10kDa decreased the adsorption the capacity of Cd and Pb in soil. All fractions promoted the transformation of soil Cd and Pb from acid-extractable form to residue form regardless of the amounts of HAs added, and the promotion potential of low molecular weight components was stronger. Soil Pb²⁺ mainly existed as reducible form, accounting for 57%~89%. The proportion of acid-exchangeable Pb in soil gradually decreased with the increase of molecular weight of the HAs components added. The addition of HAs facilitated the transformation of soils Pb from active form to inactive form, thus reducing the bioactivity of Pb²⁺. The bioaccessibility of soil Cd and Pb in gastrointestinal stage was enhanced by the addition of fractions with molecular weight < 5k and 5~10k, whereas decreased by other fractions. The bioaccessibility of Cd and Pb in soil as influenced by different HAs fractions depended on their final existing forms when transformation reached to equilibrium:higher proportion of acid-exchangeable form meant a higher bioavailability, while the soil adsorption ability of heavy metals was less relevant to their bioavailability.