天然有机质不同分子量组分对紫色土镉吸附-解吸的影响

Effects of different molecular weight fractions of natural organic matter on the adsorption and desorption of cadmium in purple soil

环境中广泛分布的天然有机质(NOM)对重金属土壤环境行为具有重要影响。为深化对NOM调控重金属土壤环境行为的认识,本文采用批量平衡试验方法,研究了NOM不同分子量组分(F0,未分级NOM;F1,<5000;F2,5000~10 000;F3,10 000~30 000;F4,>30 000)对紫色土Cd吸附-解吸的影响,分析了NOM效应与其组成和性质的关系。结果表明,添加NOM及其组分(0.1%、0.2%、0.5%C)均增加了土壤对Cd的吸附性能,与未添加NOM对照相比,土壤Cd吸附量增加6.25%~61.22%,而解吸率降低16.22%~56.52%;NOM组分添加量越高,对土壤Cd吸附能力的增加作用越显著;等添加量条件下(0.5%C),F1、F3、F4组分土壤Cd的吸附量明显高于F0,而F2的作用则相反,其中F1处理的土壤Cd吸附量较F0高约37.98%,解吸率降低了约38.63%,而F2则使土壤Cd吸附量降低了约6.68%,解吸率增加18.02%;随NOM组分分子量的增加,其C、H元素含量增加,而O含量减少,O/C、(N+O)/C元素比值降低,反映出高分子量组分氧化程度更低,极性更弱。研究表明,NOM组分对土壤Cd吸附的影响与其分子量大小的变化并不存在线性依存关系,低分子量组分F1可能主要通过影响土壤pH而改变对Cd的吸持能力,而高分子量组分则还受其结构复杂性的影响。

The widely distributed natural organic matter(NOM)in the environment has important influences on the environmental behavior of heavy metals in soils.In this study,bulk NOM was divided into four fractions according to their molecular weight(F0:bulk NOM as the control;F1:fraction with molecular weight<5000 Dalton;F2:5000~10 000 Dalton;F3:10 000~30 000 Dalton;and F4:>30 000 Dalton),and the effects of different NOM fractions on Cd adsorption and desorption in purple soil in relation to their structure and properties were studied using a batch experiment.The results showed that NOM and its fractions significantly enhanced the Cd adsorption ability of the soil.The Cd adsorption rates of the soil increased by 6.25%to 61.22%,while the desorption rates decreased by 16.22%to 56.52%with the addition of 0.1%,0.2%,and 0.5%C of NOM or its fractions to the soil.The promoting effect increased with the increase in NOM addition.At the same addition amount(0.5%C)of NOM fractions,the amount of Cd adsorption for F1,F3,and F4 was significantly higher than that of the bulk NOM(F0),while F2 showedthe opposite effect;the amount of Cd adsorption with the addition of F1 was about 37.98%higher than that of F0,and the desorption rate was about 38.63%lower.F2 decreased the amount of Cd adsorption by 6.68%and increased the desorption rate by 18.02%.The contents of C and H increased in the NOM fractions with the increase in their molecular weight,while the O content decreased.Accordingly,the ratios of O/C and(N+O)/C in the NOM samples decreased,thereby showing a lower oxidation state and a weaker polarity for the fraction with higher molecular weight.However,no linear dependence existed between the molecular weight of the NOM fractions and their effects on soil Cd adsorption.F1,which was a low molecular weight fraction,promoted Cd adsorption by soil mainly by altering soil pH,while the effects of fractions with higher molecular weight were possibly also relevant to their structural complexity.