Elsevier

Chemosphere

Volume 170, March 2017, Pages 161-168
Chemosphere

Combined effects of low-molecular-weight organic acids on mobilization of arsenic and lead from multi-contaminated soils

https://doi.org/10.1016/j.chemosphere.2016.12.024Get rights and content

Highlights

  • Low-molecular-weight organic acids (LMWOAs) are reactive constitutes in root exudates.

  • LMWOAs drive mobilization of soil-borne potentially toxic trace elements (PTTEs).

  • Batch experiment to examine the combined effects of LMWOAs on mobility of PTTEs.

  • Important insights into the complication of multiple LMWOAs on PTTEs were gained.

  • Implications for better understanding mobility of PTTEs by LMWOAs in field.

Abstract

A batch experiment was conducted to examine the combined effects of three common low-molecular-weight organic acids (LMWOAs) on the mobilization of arsenic and lead in different types of multi-contaminated soils. The capacity of individual LMWOAs (at a same molar concentration) to mobilize soil-borne As and Pb varied significantly. The combination of the organic acids did not make a marked “additive” effect on the mobilization of the investigated three elements. An “antagonistic” effect on element mobilization was clear in the treatments involving oxalic acid for some soils. The acid strength of a LMWOA did not play an important role in controlling the mobilization of elements. While the mobilization of As and Pb was closely associated with the dissolution of soil-borne Fe, soil properties such as original soil pH, organic matter contents and the total amount of the element relative to the total Fe markedly complicated the mobility of that element. Aging led to continual consumption of proton introduced from addition of LMWOAs and consequently caused dramatic changes in solution-borne Fe, which in turn resulted in change in As and Pb in the soil solution though different elements behaved differently.

Introduction

Low-molecular-weight organic acids (LMWOAs) are active components in root exudates of plants (Carson et al., 1992, Gerke et al., 1994, Reichard et al., 2007). LMWOAs could therefore play an important role in the mobilization of nutrients and potentially toxic trace elements in rhizospheric soils (Marschner et al., 1987, Gobran and Huang, 2011). Except for extremely acidic soils such as acid sulfate soils in coastal lowlands and mine sites (Lin et al., 2008), this process may, to a significant degree, control the availability of trace elements for plant uptake in soils. For example, it was demonstrated that the LMWOAs-mediated iron dissolution in the rhizosphere had a potential role in root iron uptake (Jones et al., 1996); Tao et al. (2006) showed that oxalate enhanced uptake of As by wheat plant; Ma et al. (2001) found that LMWOAs could affect the availability of Al to plants; and work by Chen et al. (2015) suggested that Cd uptake by rice plant is influenced by LMWOAs. The mobilized trace elements could also have adverse impacts on soil microbial metabolism, and consequently affect nutrient supply (Liang and Tabatabai, 1978, He et al., 2005) and degradation of organic matter (Gerringa, 1990), including organic pollutants (Perrin-Ganier et al., 2001) in soils. In some circumstances, the LMWOAs-mobilized trace elements can be further transported from the soils to the surface water and groundwater, reaching off-site receptors (Zinder et al., 1986, Slowey et al., 2005, Perelomov et al., 2011). Therefore, understanding the mobility of trace elements by LMWOAs is essential for assessing the phyto-availability of soil-borne trace elements, microbial toxicity of the trace elements in the soils, and the potential for translocation of trace elements from soils to water environments. This is particularly relevant to agricultural, urban and industrial lands that are heavily contaminated by heavy metals and metalloids.

Plant root exudates contain multiple LMWOAs though the dominant LMWOA types may vary with plant species and change over time (Jones, 1998, Ash et al., 2016a, Ash et al., 2016b). In the past decade or so, there has been increasing research into mobilization of heavy metals and metalloids by various LMWOAs (Cieśliński et al., 1998, Van Hees et al., 2000, Liu et al., 2008, Wang and Mulligan, 2013, Rocha et al., 2015). An extensive review of the relevant literature indicates that the vast majority of available published papers limited their experiments to the examination of trace element mobilization by individual LMWOAs (Examples are shown in Table 1). This is not sufficient for assessing LMWOAs-driven mobilization of soil-borne trace elements in field systems where multiple LMWOAs are concurrently present in the same space. The combination of different LMWOAs may result in additive, synergistic or antagonistic effects on mobilization of different trace elements. This represents a major knowledge gap that needs to be filled in order to better understand the LMWOAs-driven mobilization of heavy metals in rhizospheric soils.

In this study, the capacities of various combinations of three common LMWOAs (citric acid, oxalic acid and malic acid) to mobilize arsenic and lead were compared using six multi-contaminated soils with different soil properties. The objectives were to understand (a) the integrative effects of the selected LMWOAs on each of the investigated trace elements; (b) whether different types of trace elements respond differently to the exposure of various LMWOA combinations; and (c) how the mobilization of the trace elements is complicated by other soil properties.

Section snippets

The soil samples

A total of 6 contaminated soil samples (M1, M2, M3, M4, M5 and M6) were used for the experiment in this study. These samples were collected from the Moston Brook closed landfill site in the Greater Manchester region, northwestern England. Information about the sampling site was documented in Mukwaturi and Lin (2015). After collection, the soil samples were oven-dried at 40 °C for two days in the laboratory and then ground with a mortar and a pestle to pass through a 2 mm stainless steel sieve.

Extractable iron in various treatments on the 7th day

The data on Fe extracted by the 7 extracting solutions (refer to Table 3) for the 6 soil samples (M1–M6) are shown in Fig. 1. Citric acid (T1) extracted the largest amount of Fe (statistically significant at P < 0.05) among the three individual acid treatments for all the 6 samples. Oxalic acid (T2) extracted significantly more Fe than did malic acid (T3) for M2 and M3 while malic acid extracted much more (significant at P < 0.05) Fe than did oxalic acid for M1, M4, M5 and M6 from the soil (

Discussion

Under the set experimental conditions, the capacity of individual organic acids (at a same molar concentration) to mobilize soil-borne heavy metals varied significantly. This is consistent with work by others (e.g. Cieśliński et al., 1998, Wu et al., 2003, Vítková et al., 2015). The chemical mechanisms responsible for dissolution of soil-borne heavy metals by organic acids are mainly through acidification, complexation and reduction (Bienfait et al., 1982, Jones and Darrah, 1994, Schwab et al.,

Conclusion

Under the set experimental conditions in this study, the capacity of individual LMWOAs (at a same molar concentration) to mobilize soil-borne As and Pb varied significantly. The amount of the investigated element extracted by the mixed LMWOA solutions was much less than the sum of that element extracted by the individual LMWOA solutions, and in some cases, even less than one of the individual organic acid extraction. Where oxalic acid is involved, an “antagonistic” effect on element

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