Combined application of EDDS and EDTA for removal of potentially toxic elements under multiple soil washing schemes
Graphical abstract
Introduction
A recent national soil quality survey in China, covering 70% of its total area, revealed that over 82.4% of the contaminated soil samples were contaminated by potentially toxic elements (PTEs) (mainly Cd, As, Hg, Cu, Pb, Cr, Zn, and Ni) based on the national standards of China (CMEP, 2014; Zhao et al., 2015). Increases in anthropogenic activities along with urban development have exacerbated the emission of PTEs and their ecological and health impact. Unregulated processing and storage of waste electrical and electronic equipment (WEEE or e-waste), for example, has led to substantial emissions into air and water, and the resultant accumulation in soil that tends to act as sink for the PTEs. Poorly controlled recycling processes of e-waste, including leaching with strong acids and direct burning in open air, have resulted in the release of high levels of PTEs that now poses adverse effects on the environment and threatens human health (Huo et al., 2007; Liu et al., 2013).
The use of chelants in enhancing extractions of PTEs from soils by soil washing (Beiyuan et al., 2017a, 2018), soil leaching (Hu et al., 2014; Qiao et al., 2017), phytoextraction (Hseu et al., 2013; Shaheen and Rinklebe, 2015), and pretreatment to immoblization (Beiyuan et al., 2016; Sanderson et al., 2017) have been extensively studied. Although the efficacy of chelants in mobilizing metals is well studied, the process itself is complicated, for example, chelant-enhanced phytoextraction showed initial positive results in pot experiments, however, with time the metal uptake by plants changed annually with field conditions (Antoniadis et al., 2017; Sidhu et al., 2017). The ex-situ chelant-assisted washing poses fewer risks compared to in-situ technologies, because it directly extracts the PTEs from soils and mitigates secondary off-site contamination (Bolan et al., 2014; Tsang and Yip, 2014). Ethylenediaminetetraacetic acid (EDTA) has been widely used but its poor biodegradability is of concern, because the persistence and mobilization of EDTA and metal-EDTA complexes in the environment can pose adverse ecological effects (Voglar and Lestan, 2013; Freitas and Nascimento, 2016), compromise soil aggregate stability and water retention properties (Jelusic and Lestan, 2014; Zupanc et al., 2014), as well as decrease crop yield and plant biomass (Bloem et al., 2017). As such, reduced dosage of EDTA has been recommended for its application.
As a substitute for EDTA, EDDS ([S,S]-ethylene-diamine-disuccinic-acid) has been widely studied because of its higher biodegradability and lower toxicity in field soils (Wang et al., 2012; Luo et al., 2015). While EDDS effectively extracted Cu and reduced its residual leachability and bioaccessibility in the treated soils (Hartley et al., 2014; Beiyuan et al., 2017b), the low complex capacity of EDDS with Pb (which has a higher toxicity and carcinogenicity compared to Cu) hampered its efficacy for the remediation of soils contaminated with multiple PTEs (Polettini et al., 2007; Begum et al., 2012). In comparison to EDDS or EDTA alone, the use of mixed chelants achieved a higher extraction efficiency of Pb under chelant-deficient conditions (Yip et al., 2010), and enhanced translocation of Pb from the plant roots to shoots (Luo et al., 2006). Recent studies also showed that combined application of chelants and/or chemical reagents was required for enhancing extraction of multiple PTEs from contaminated soils (Guarino and Sciarrillo, 2017; Guo et al., 2017). Thus, the mixture of EDDS and EDTA appears to have a good potential for cleaning up the soils contaminated by high contents of Cu and Pb while using a reduced dosage of EDTA. However, the interactions of multiple PTEs with mixed chelants and their resultant speciation are not well understood, which is important for informing the design of washing schemes for the optimum utilization of mixed chelants.
Change of washing procedures, for example, dividing a single-step continuous extraction into several times of washing in a smaller dosage of chelants with shorter washing time could enhance the metal extraction efficiency (Zou et al., 2009; Lo et al., 2011). Extraction by multiple washing steps mitigated the dissolution of minerals from the treated soils, such as Fe oxides, but also reduced the ability to remove target metals from strongly bound fractions (Finzgar and Lestan, 2007). An additional step of intermittent water washing after each chelant-assisted extraction step was found useful for removing adsorbed/entrapped metal-chelant complexes and free chelants from the soil particles (Lo et al., 2011), which may alleviate potential risks of metal remobilization afterward. Thus, in this study, a total of five different washing schemes were designed: 1) multi-pulse A washing scheme (by a constant dosage of chelant and intermittent water washing); 2) multi-pulse B washing scheme (by a constant dosage of chelant but no water washing); 3) step-gradient chelant washing scheme (by gradient-decreasing concentration of chelant); 4) 24-h continuous washing at the same dosage of a single washing step; and 5) 24-h continuous washing at a dosage equivalent to the total chelant consumption in all washing steps. This allowed a holistic evaluation of the metal speciation in mixed chelants solution and the leachability and bioaccessibility of residual metals in the treated soils under different washing schemes.
A field contaminated e-waste soil with coexistence of Cu, Zn, and Pb was selected for studying the extraction efficacy of equimolar mixture of EDDS and EDTA under the five different washing schemes. The equilibrium speciation of multiple metals and mixed chelants in solution was calculated by Visual MINTEQ software. The leachability and bioaccessibility of residual Cu, Zn, and Pb were assessed in the treated soils. All the results were compared with those of individual use of EDDS or EDTA, respectively.
Section snippets
Soil characteristics
The soil samples were collected from surface layer (0–15 cm) at an e-waste incineration site in Qingyuan of Guangdong Province, China. The soil was well mixed, air dried, and sieved through a 2-mm sieve. The soil pH (5.3) was measured at a solution-to-soil ratio of 2.5 L kg−1. It contained only 12.5% silt and clay, based on particle size distribution, and was therefore deemed suitable for soil washing. The soil organic matter was determined as 4.81%, which was measured by a total carbon
Efficacy of combined use of EDDS and EDTA under different washing schemes
Washing by EDDS alone showed lower extraction efficiencies of all target metals (Cu, Zn, and Pb) than those by EDTA in this study (Fig. 2 & Fig. S1), especially for Pb extraction. The difference in the Pb removal efficiencies by EDDS and EDTA was associated with the stability constants of Pb-EDDS2- (log K = 12.7) and Pb-EDTA2- (log K = 17.9) (Tandy et al., 2004; Luo et al., 2006). However, the stability constants of metal-chelants are not the only factor for metal extraction from contaminated
Conclusions
Although soil washing by EDDS alone gave relatively low extraction efficiency of the target metals, the combined use of EDDS and EDTA achieved comparable effectiveness of washing by EDTA alone but reduced half dosage of EDTA. The EDDS and EDTA had distinctive priorities to complex with different metals, as verified by the calculated speciation results, which allowed complementary use of mixed chelants under various soil washing schemes in this study. Marginal increments were found in the use of
Acknowledgement
The authors appreciate the financial support from the Hong Kong Research Grants Council (PolyU 15222115 and 15223517) for this study.
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