Removal of arsenic and cadmium with sequential soil washing techniques using Na2EDTA, oxalic and phosphoric acid: Optimization conditions, removal effectiveness and ecological risks
Introduction
Soil contamination by heavy metals has been and will continue to be a priority throughout the world. This holds particularly true for developing countries, where there has been an abrupt increase in mining activities due to rapid industrialization (Wang et al., 2012). The total area of soil contaminated by heavy metals was as high as 2 million hectares in China, which covered 1/6 of the total cultivated land (Yao et al., 2012). These toxic contaminants can be released to streams, accumulated in crop plants (Lee et al., 2006b) and eventually ingested by human beings, all of which are harmful for human health. During the past decades, various soil remediation technologies have been developed for heavy metal removal and stabilization, primarily including physical, chemical, and biological remediation as well as combinations of these technologies, such as chemical fixation, electrokinetic remediation, phytoremediation, and biological remediation (Mulligan et al., 2001). Among them, soil washing with acid, chelators, surfactants or organic ligands is considered to be one of the most suitable ex-situ techniques to remove heavy metals from the ecosystem (Oustan et al., 2011, Qiu et al., 2010) due to its advantages of high efficiency, cost effectiveness and contaminated soil reduction (Jang et al., 2007). However, soil washing is a very complicated process, which is reflected not only in its distinction as a phenomenon but also in its mechanisms. The factors and conditions affecting its performance are still unclear. Therefore, there is a need for investigations of optimization conditions, heavy metal removal effectiveness and distribution for soil washing with optimal washing agents.
Previously, a number of studies were conducted that addressed the influencing factors of heavy metal removal efficiency for soil washing with various washing agents (Arwidsson et al., 2010, Oh et al., 2012, Yin et al., 2014). Most of the studies were focused on a single reagent and/or single-step washing. However, various metals (cationic metals and anionic metals) often coexist at contaminated sites. These coexisting metals present difficulties to soil washing; thus, single reagent and/or single-step washing is insufficient to achieve remediation goals. For example, Na2EDTA is recognized as the most effective synthetic chelating agent to remove cationic metals but not in anionic metals (Udovic and Domen, 2010, Finžgar and Leštan, 2007). By contrast, organic acids (e.g., oxalic acid, citric acid, tartaric acid) and some inorganic acid (e.g., phosphoric acid and nitric acid) attain much higher anionic metals extraction from soils, sediments, and mine wastes (Drahota et al., 2014). It has been reported that repeated washing and rinsing steps are conducted to improve heavy metal removal efficiency, which can also reduce the consumption of washing agents and washing costs (Sun et al., 2001, Torres et al., 2012). For example, Finžgar and Leštan (2007) reported that the removal efficiency of Pb and Zn was improved when EDTA was used in multi-step leaching. Similarly, Gusiatin and Klimiuk (2012) reported that triplicate washing led to an increase in both efficiency of metal removal and percentage content of their stable forms. Generally, the studies discussed above were mainly focused on enhancing metal removal efficiency and redistribution during single/multi-washings with single reagents. Sequence washing in triplicate using multiple washing agents under optimal washing conditions has seldom been conducted but is necessary for fundamental research and actual remediation.
Therefore, the objective of this study is to identify the optimal washing conditions for soil washing with multiple washing agents, including the typical chelating agent (Na2EDTA), organic acid (oxalic acid), inorganic weak acid (phosphoric acid), and to investigate the influence of sequential washing on arsenic and cadmium removal and stability in combined arsenic-cadmium contaminated soils close to mining areas. Five influencing factors, including agent concentration, stirring speed, washing time, liquid/solid ratio and temperature, are considered based on the results of previous theoretical and experimental studies on soil washing. The modified Commission of the European Community Bureau of Reference (BCR) sequential extraction procedure (for cadmium) and the phosphor sequential extraction protocols (for arsenic) are applied to facilitate speciation analysis. Finally, the potential ecological risk index (PERI) assistant is utilized to evaluate removal efficiency of target metals. Furthermore, metal bioavailability and potential mobility in soils are also evaluated. The obtained results can provide useful information and technical support for actual ex-situ soil washing technology for remediation of soil contaminated by heavy metals.
Section snippets
Soil samples and washing agents
The studied soil was collected from the surface layer (0–20 cm) of a village close to a mining area southwest of Jiangxi, China. The collected soil was air-dried and ground before sieving to remove >2 mm gravels and <0.075 mm fine particles (particle-size cut points based on the previous studies), and then homogenized and stored until analysis and subsequent experiments. Selected physicochemical characteristics and contamination of the soil are listed in Table 1. The values of TOC, CEC and Eh
Optimization conditions of soil washing
The removal efficiency of two target heavy metals showed slightly different trends under the six influencing factor conditions (as shown in Fig. 1). Concentration, liquid/soil ratio, mixing time, stirring speed and pH of washing agents had notable influence on removal, while temperature had minor effect.
Conclusion
Currently, most studies on soil washing focus on enhancing metal removal efficiency and redistribution during single/multi-washings with single reagents. Sequence washing using multiple washing agents under optimal washing conditions has seldom been conducted but is necessary. Therefore, removal effectiveness and ecological risks of arsenic and cadmium with sequential soil washing techniques using Na2EDTA, oxalic and phosphoric acid were evaluated.
The results demonstrate that the tested six
Acknowledgements
This work was supported by the Research Fund for the Doctoral Program of Higher Education (20120003110033). The authors would like to extend special thanks to the editor and the anonymous reviewers for their constructive comments and suggestions in improving the quality of this paper.
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