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Geochemical Mechanism for the Enrichment of Heavy Metals in a Weathering Profile of Black Marlstone in the Northwestern Hunan Province, China (Vol.47, No.4, Tot No.330) TEXT SIZE: A A A

BAI Jialing1,2,FENG Zhigang1,2,MA Qiang1,2,LIU Xuanzhi1,2,JING Chenxin1,2

(1. School of Resource & Environment and Safety Engineering, University of South China, Hengyang, 421001, China;
2. Hengyang Key Laboratory of Geological Theory and Technology for the Nuclear Fuel Cycle, University of South China, Hengyang, 421001, China)

Abstract: To understand the underlying geochemical mechanism of the heavy metal element (HME) enrichment in weathering profiles of black marlstone, this study characterized 8 kinds of HME, including Cr, Ni, Cu, Zn, Cd, Pb, Th and U, along a weathering profile of black marlstone (DM profile) in the northwestern Hunan Province, China, and compared with two weathering profiles of black shales (XT and HZ profiles) from the same region. The results showed that: (1) Geochemical characteristics of HMEs in processes of black marlstone weathering and pedogenesis could be summarized as relatively low background values and high enrichment, compared to black shale cases. (2) The weathering profile of black marlstone was the outcome of further evolution of acid-insoluble material after the rapid dissolution of carbonate bedrock, in which the acid-soluble content occupied 78.57%. In this process, HEM without exogenous inputs, such as Cr, Ni, Cu, Zn, Pb, Th and U, would be leached or even strongly leached if they existed in the acid-soluble component of the bedrock (among them, Th is relatively inert), or would be enriched remarkably due to large amounts leaching of carbonates from the bedrock if they existed in the acid-insoluble component, therefore, the enrichment of these elements in the profile was a comprehensive result of both their absolute (strong) loss and significant relative enrichment during weathering and pedogenesis of black marlstone. Cd, the exogenous origin element, might be entered as water-soluble form and deposited in the lower part of the profile. (3) In the weathering profile of black marlstone, organic matter (OM) was not the main medium associated heavy metals, therefore its impact on the geochemical behaviors of HME might be limited. (4) The average contents of Cu and Cd in soils developed from black marlstone were 54.58 mg/kg and 0.58 mg/kg, respectively, which are higher than the risk screening values for soil contamination in agricultural lands, especially Cd, which has adverse effects on soil ecological environment and should be paid more attention.

Key words: black marlstone; weathering; heavy metal elements (HME); enrichment; geochemical mechanism

EARTH AND ENVIRONMENT Vol.47, No.4, Tot No.330, 2019, Page 436-447

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