Acceleration and centralization of a back-diffusion process: Effects of EDTA-2Na on cadmium migration in high- and low-permeability systems

Xueji You; Shuguang Liu; Chaomeng Dai; Guihui Zhong; Yanping Duan; Yaojen Tu

doi:10.1016/j.scitotenv.2019.135708

Acceleration and centralization of a back-diffusion process: Effects of EDTA-2Na on cadmium migration in high- and low-permeability systems

Sci Total Environ. 2020 Mar 1:706:135708. doi: 10.1016/j.scitotenv.2019.135708. Epub 2019 Nov 25.

Authors

Xueji You¹, Shuguang Liu², Chaomeng Dai³, Guihui Zhong¹, Yanping Duan⁴, Yaojen Tu⁵

Affiliations

¹ Department of Hydraulic Engineering, College of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China.
² Department of Hydraulic Engineering, College of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China; The Yangtze River Water Environment Key Laboratory of the Ministry of Education, Tongji University, 1239 Siping Road, Shanghai 200092, China.
³ Department of Hydraulic Engineering, College of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China. Electronic address: daichaomeng@tongji.edu.cn.
⁴ School of Environmental and Geographical Sciences, Shanghai Normal University, No. 100 Guilin Road, Shanghai 200234, China. Electronic address: duanyanping@shnu.edu.cn.
⁵ School of Environmental and Geographical Sciences, Shanghai Normal University, No. 100 Guilin Road, Shanghai 200234, China.

PMID: 31787287
DOI: 10.1016/j.scitotenv.2019.135708

Abstract

Pollutant accumulation in the low-permeability zones (LPZs) in groundwater systems is regarded as a secondary source, and its consequent back-diffusion can extend the timeframe of pump-and-treat remediation. However, the bioavailability and mobility of heavy metals and the medium characteristics can be changed during the process. This study investigated the accumulation and back-diffusion law of toxic metals and the effects of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) on them by implementing a series of tank experiments. In these experiments, a cadmium solution was injected first, and deionized water or EDTA-2Na constantly washed the system consisting of different medium layers. The experimental results showed that the cadmium breakthrough curves had some concentration gradient reverse points where the curves fluctuated with elution by deionized water, which did not exist when EDTA-2Na was the eluent. In these scenarios, the mass of accumulated cadmium in the media before elution was large, with a value of 931 mg (153 mg/kg), when the low-permeability medium was clay. However, when EDTA-2Na was injected together with cadmium, the value dropped to 319 mg (52.3 mg/kg), greatly reducing the cadmium accumulation. Additionally, the use of EDTA-2Na as an eluent resulted in the appearance of a secondary peak in the breakthrough curve, showing that EDTA-2Na accelerated and centralized the back-diffusion. Notably, the reduced cadmium accumulation in LPZs with the elution by EDTA-2Na was partly due to a reduced adsorption capacity of the clay minerals. The above results can advance the technology related to pump-and-treat remediation.

Keywords: Back-diffusion; Clay; EDTA; Heavy metals; High- and low-permeability zones; Plume tail.