贫Cd碳酸盐岩发育土壤Cd的富集与超常富集现象——以贵州岩溶区为例

通过对贵州岩溶区19条风化剖面Cd分布特征的研究, 初步揭示了贫Cd碳酸盐岩发育的土壤中Cd的富集机制及Cd含量的约束因素。①贫Cd基岩发育的土壤同样可以导致Cd的明显富集, 甚至超常富集, 且含量峰值通常位于土壤层底部(T1)。②Cd在基岩中通常优先赋存在酸不溶相, 另一方面, 由于基岩酸不溶物含量一般极低, 酸溶相的Cd占全岩Cd的比例仍处于绝对优势。在富Cd的基岩酸不溶物基础上, 叠加了来自酸溶相中Cd的贡献, 是岩溶区土壤Cd普遍富集的原因。③土壤中Cd的含量与其在基岩或基岩酸不溶物中的含量均不相关。T1中Cd的含量受制于Cd在基岩酸不溶物中的含量、基岩酸溶相Cd占全岩中Cd的质量百分数、T1中Cd的亏损率的共同约束。有利于Cd在T1中超常富集的最佳条件是: Cd在基岩酸不溶物中含量高、基岩酸溶相Cd占全岩中Cd的比例大、T1中Cd的亏损率小。另外, 对于由基岩酸不溶物残余累积形成的土壤层, T1为其发育起点, Cd在T1中含量高, 通常在土壤层中也高, 反映出一般风化剖面的发育特征。研究结果可以深化对岩溶环境Cd地球化学行为的认识, 为区域上基于地质成因开展Cd污染风险评价及建立其清洁水平提供参考。

Enrichment and supernormal enrichment phenomenon of Cd in soils developed on Cd-poor carbonate rocks: A case study of karst areas in Guizhou,China

Based on the study on the distribution characteristics of Cd for 19 weathering profiles in the karst areas of Guizhou, China, this work preliminarily revealed the enrichment mechanisms of Cd and the restrictive factors of Cd contents in soils developed on Cd-poor carbonate rocks.The main conclusions are as follows: ① Soils derived from Cd-poor bedrocks can also contain obvious enrichment or even supernormal enrichment phenomenon of Cd, and the summit content of Cd is usually located at the bottom of the soil layer(i.e., T1).② Cd generally preferentially occurs in acid-insoluble phase in bedrocks, and on the other hand, owing to very low mass percentage of acid-insoluble residues in bedrocks, the proportion of Cd of acid-soluble phase in the bulk rocks still has an abnormal advantage.Thus, on the basis of Cd-rich acid-insoluble phase in bedrocks, combined with the contribution of Cd from acid-soluble phase in bedrocks, it creates a fact that Cd is universally rich in soils in the karst areas.③ The content of Cd in soils is not directly related to its content in bedrocks or acid-insoluble residues of bedrocks, and it is constrained by the content of Cd in acid-insoluble residues of bedrocks, the mass percentage of Cd of acid-soluble phase in the bulk rock and the loss rate of Cd in T1 together.The optimum conditions conducive to the extraordinary enrichment of Cd in T1 contain three aspects as high level of Cd in acid-insoluble residues of bedrocks, large proportion of Cd of acid-soluble phase in the bulk rock and low loss rate of Cd in T1.In addition, for the soil layer formed by the accumulation of acid insoluble residues in bedrocks, T1 is the starting point for its development and evolution.The higher the content of Cd is in T1, the higher it is usually in the soil layer, reflecting the development characteristics of general weathering profile.This study might deepen the understanding of geochemical behavior of Cd in karst environment, and provide reference for regional Cd pollution risk assessment based on geological genesis and establishing its cleaning level.