不同分子量腐植酸的结构特征及其对土壤镉有效性的影响

为探究不同分子量腐植酸组分的结构特征,本研究利用褐煤粉提取腐植酸进行超滤分级,获得四个不同分子量的腐植酸组分。研究发现,分子量大于50 kDa和小于1 kDa的腐植酸分别占总腐植酸的49.66%、47.07%,中间两个组分占比很小；腐植酸的理化性质和功能主要是由1 kDa以下的腐植酸和50 kDa以上的腐植酸所决定的。1 kDa以下的腐植酸总酸性基、羧基和酚羟基官能团含量最高,酚羟基占总酸性基的比例也较高,腐植酸的芳香化程度、缩合度随分子量的增大而增大,含氧官能团含量、E4/E6值随分子量的增大而减少。通过盆栽试验,探索了不同分子量腐植酸对土壤镉有效性的影响,土壤对镉的吸附量随腐植酸分子量的增大而减少,各处理均在500 mg/L时达到吸附平衡；土壤镉的解吸量亦随腐植酸分子量的增大而减少。小分子量腐植酸的酸性官能团丰富,对土壤重金属的吸附络合能力更强,可以降低土壤对镉的吸持能力,提高其移动性和生物有效性；大分子腐植酸则主要起吸持和固定作用,降低重金属的移动性和生物有效性。

Structural Characteristics of Humic Acids with Different Molecular Weights and Their Effect on Cadmium Availability in Soil

To explore the structural characteristics of different molecular weight humic acid fractions, we used lignite powder to extract humic acid for ultrafiltration classification, and obtained four humic acid fractions with different molecular weights. It was found that phytic acid with molecular weight greater than 50 kDa and less than 1 kDa accounted for 49.66% and 47.07% of total phyphyic acid, respectively, and the middle two components were very little. The physicochemical properties and function of phytic acid were mainly determined by phyphyic acid below 1 kDa and above 50 kDa. Humic acid below 1 kDa had the highest contents of total acid group, carboxyl group and phenolic hydroxyl group. Compared with carboxyl group, phenolic hydroxyl group accounted for a higher proportion of total acid group. The degree of aromatization and condensation of humic acid increased with the increase of molecular weight, while the content of oxygen-containing functional groups and the value of E4/E6 decreased with the increase of molecular weight. The pot experiment was used to study the effects of different molecular weight humic acids on cadmium availability in soil. The adsorption amount of cadmium decreased with the molecular weight of humtic acid, reaching adsorption equilibrium at 500 mg/L. And the desorption of soil cadmium also decreased with the increase of the molecular weight of humtic acid. Small molecular weight humic acid rich in soil heavy metal adsorption and complexation ability, can reduce soil cadmium adsorption capacity, improve its mobility and bioavailability; Large molecular humic acid mainly acts as adsorbent and immobilization, reducing the mobility and plant availability of heavy metals.