1. Introduction
Oxides of iron (Fe), manganese (Mn), and aluminium (Al) are effective sorbents for trace elements [
1,
2]. Therefore, they play an important role as the sinks for potentially toxic heavy metals and metalloids in various environmental compartments such as soils, sediments, and aerosols, etc. [
3,
4,
5,
6]. In soils that are contaminated by potentially toxic elements, the presence of soil-borne Fe, Mn, and Al oxides immobilize pollutants entering soils from external sources, and consequently reduce their bioavailability. This represents a major chemical mechanism through which the toxic effects of trace elements entering a soil system can be attenuated [
7]. However, upon changes in environmental conditions, the trace elements bound to these oxides can be released. Soil acidification is an important driver for dissolution of metal oxides [
8,
9,
10]. For manganese and iron that have variable valence, a drop in redox potential could also destabilize Fe and Mn oxides [
11,
12].
In vegetated soils, low-molecular-weight organic acids (LMWOAs) contained in root exudates of plants play an important role in mobilizing trace elements bound to Fe, Mn, and Al oxides in rhizospheric soils [
13,
14,
15,
16]. While localized acidification in rhizospheric soils due to secretion of organic acids from plant roots and rhizobacteria solubilizes Fe, Mn, and Al oxides, complexation of Fe, Mn, and Al with organic ligands is likely to play a more important role in the dissolution of Al, Mn, and Fe oxides [
3,
17,
18,
19]. In addition, certain types of organic acids such as oxalic acid also have a strong capacity to cause reductive dissolution of iron oxides [
20,
21].
In soil systems, Al, Mn, and Fe oxides, together with other soil minerals, compete for available LMWOAs, resulting in consumption of free organic ligands, and acid neutralization, which causes the increase in solution pH [
7,
22]. This could, in turn, lead to re-immobilization of the mobilized Al, Mn, and Fe [
7,
23], and consequently re-immobilization of the previously released trace elements in the soil solutions. So far, there have not been detailed investigations on (a) competitive dissolution among Al, Mn, and Fe oxides upon attack by LMWOAs, (b) temporal variation in LMWOAs-mobilized Al, Mn, and Fe, and (c) effects of (b) on trace elements in soil solutions. The aim of this study was to close the above knowledge gaps.
2. Materials and Methods
2.1. The Contaminated Soil Used for the Experiment
The selected soil 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 [
24]. 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. This is done to achieve a very fine and homogenous sample prior to analysis. Samples were later stored in an air-tight re-sealable laboratory polythene bags for further use.
Prior to the batch experiment, the soil was characterized and some major chemical characteristics of the soil samples are given in
Table 1. The soils had a pH of 7.1 and an electrical conductivity (EC) value of 0.039 dS/m. The concentrations of Cd, Cu, Ni, Pb, and Zn exceeded the guideline values for soils with plant uptake [
25,
26].
2.2. Experimental Design
Seven treatments were set to observe the release of Fe, Mn, and Al, other trace elements in the presence of three selected organic acids (citric acid, oxalic acid, and malic acid) and their combinations, as shown in
Table 2.
A batch experiment was conducted with 125 mL plastic bottles being used as batch reactors. In each bottle, 10 g of the soil were mixed with 100 mL of a relevant solution (refer to
Table 2). The bottle with contents was shaken by hand for 1 min. and then placed in a paper box at room temperature. Various physical and chemical parameters in the extracting solution were monitored during a period of 15 days. In-situ measurement of pH, EC, and Eh was made 1, 3, 5, 7, and 15 days after the commencement of the experiment. Solution samples were also collected for determination of Fe, Mn, Al and other trace elements after each in-situ measurement. Before each in-situ measurement and sampling operation, the bottle with contents was shaken by hand for 1 min. and then allowed to stand for 1 h. An aliquot of supernatant (5 mL) was taken from each bottle. The supernatant was centrifuged for 10 min at 3500 rpm and then passed through a 0.45 µm filter with polytetrafluoroethylene (PTFE) membrane prior to analysis.
2.3. Analytical Methods
For the initial soil characterization, pH/Eh, and EC of the soil samples were measured in a 1:5 (soil:water) extract using a calibrated Mettler Toledo 320 pH/Eh meter and a Mettler Toledo EC meter (Leicester, UK), respectively. Total element concentration was determined using a Niton XL2 Gold Hand-held XRF Analyzer (Winchester, UK). The instrument was calibrated by firstly analysing the 73,308 standard reference materials prior to sample analysis. To ensure accuracy and reliability of the results obtained, all analyses were performed in duplicates and the analysis time was set at 240 s. The pH and Eh in the solutions for the incubation experiment were measured using a Mettler Toledo 320 pH/Eh meter. The EC in the solutions for the incubation experiment was measured using a Mettler Toledo EC meter. Concentrations of Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn in the filtrate were determined using a Varian 720 ES inductively coupled plasma optical emission spectrometer (ICP-OES, Palo Alto, CA, USA).
2.4. QA/QC and Statistical Analysis
All the chemicals used in the experiment are of analytical grade. The experiments were performed in triplicate (i.e., each of the 7 treatments in the experiment was independently repeated three times). Repeatability analysis shows that the mean RSD for pH, Eh, EC, Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn was <2.80%, <0.75%, <2.82%, <10.6%, <12.1%, <15.3%, <10.5%, <8.00%, <10.2%, <10.3%, <8.39%, <12.8%, and <4.96%, respectively. Statistical analysis of the experimental data was performed using one-way analysis of variance (ANOVA) and the means compared using significant difference (Duncan) method at 5% level (IBM SPSS software Version 17.0). “Descriptive statistics” followed by “explore” were used to test the data for normality. The “sig” value of Shapiro–Wilk is greater than 0.05. The data in this study is consistent with the test of variance homogeneity using Levene statistic. All experimental data were presented as mean ± standard error (n = 3). Pearson coefficient of correlation was used to determine the relationship between Al, Mn, or Fe and each of the trace elements investigated in this study.
4. Discussion
The clear trend that the pH in soil solutions increased over time for all the 7 treatments suggests that proton consumption took place continuously during the period of the 15-day incubation experiment after mixing the soil with either the single or combined LMWOA solutions. This was accompanied by the enhancement of reducing conditions over time, as indicated by the consistent trend that the solution Eh decreased with increasing duration of the experiment. The temporal variation in both solution pH and Eh appeared to markedly complicate the mobilization-immobilization of various soil constituents, resulting in irregular variation in the EC value. Different organic acids or combinations of organic acids had differential effects on these chemical processes. However, as a whole, the EC value tended to decrease over time, and at the end of the experiment (the 15th day), EC dropped to a very low value for all the treatments. This suggests that, at this point, hydrolytic polymerization of metal complexes dominated as a result of rising pH [
27,
28]. For the single organic acid treatments (T1–T3), no removal of Fe, Mn, and Al from the solutions took place at this point because their concentrations remained high in the solutions. However, for the combined organic acid systems, removal of these metals from the solutions occurred, as evidenced by the facts that their concentrations tended to decrease after either 5 or 7 days of the experiments. This suggests that the presence of multiple organic acid ligands enhanced the precipitation of solution-borne Fe, Mn, and Al.
The lower initial pH for oxalic acid treatment can be attributed to the greater acid dissociation constant (Ka) or lower pKa, as compared to the other two individual acid treatments (Equations (1)–(7)) [
29].
The sharp increase in pH during the first 3 days for the oxalic acid treatment indicates rapid consumption of H
+ generated from dissociation of oxalic acid (Equations (1) and (2)). This could involve protonation of variably charged organic and inorganic soil colloids (including oxides of Fe, Mn, and Al) and reactions with carbonate and silicate minerals [
30,
31]. The slight decrease in solution-borne Fe, Mn, and Al prior to the 5th day of experiment suggests that release of soil-borne Fe, Mn, and Al to the solutions was inhibited.
The reaction between the soil-borne Fe/Mn/Al oxides/hydroxides and individual organic acids could lead to the release of these metals into the solution, as illustrated in the following example for the reaction of aluminium hydroxide with citric acid:
In Equation (8), the attack of aluminium hydroxide by citric acid results in the formation of insoluble aluminium citrate, which further reacts with another citrate ion to form soluble aluminium citrate complex (Equation (9)). Therefore, Equation (9) represents the rate-limiting step for the organic acid-driven dissolution of aluminium hydroxide. At least before the 7th day of the experiment, the reaction equilibrium was not reached because the concentration of dissolved Al kept increasing from the 7th day to the 15th of the experiment. The malic acid treatment showed a similar trend to the citric acid, suggesting that both citric acid and malic acid behaved similarly in terms of solubilizing the Al in the investigated soil. The delayed release of Al in the oxalic acid treatment could be attributed to the smaller acid dissociation constant (Ka), as compared to the other two organic acids [
27].
The close correlation among the solution-borne Fe, Mn, and Al suggests that these metals were solubilized consistently from the oxides of these metals upon contacts with the organic acids. The reduced correlation coefficient during the later stage of the experiment may reflect differential rate of precipitation for these metals. By comparison, the amount of three metals mobilized during LMWOA extraction was in the following decreasing order: Al > Fe > Mn. This was consistent with the total concentration of these three metals contained in the investigated soil (
Table 1), suggesting the strong control of the soil-borne metals on the solution-borne metals. However, when considering the ratio of solution-borne metal to soil-borne metal (M
solution:M
soil), it is clear that Mn had much stronger affinity to the LMWOAs, as compared to either Al or Fe; M
solution:M
soil was more than 0.23 for Mn while M
solution:M
soil for either Al or Fe was only 0.07.
The fact that solution-borne Pb was not related to any of the solution-borne Fe, Mn, and Fe suggests that Pb was not predominantly bound to the oxides of these metals. This is not in agreement with many other findings that show Pb is favourably bound to oxides of iron and manganese [
32,
33,
34,
35,
36]. Previous investigation suggested that Pb in contaminated soils at this site was mainly in the form of lead sulfate [
37]. Consequently, the mobilization of Pb was not related to the dissolution of Fe, Mn, and Al oxides. In contrast, As, Cr, Zn, and Ni showed a close correlation with Fe, Mn, or Al, suggesting the LMWOA-driven dissolution of Fe, Mn, or Al had a major control on the mobilization of these elements of potential toxicity. It is interesting to note that the correlation coefficient for Cu vs. Fe/Mn/Al and Cd vs. Fe/Mn/Al varied over time. During the earlier stage of the experiment, a close correlation was observed, suggesting that the solution-borne Cu and Cd were essentially of Fe/Mn/Al oxide sources. In particular, it is likely that Cd was mainly bound to Mn oxides given the closer correlation between these two metals. The poor correlation between Cd or Cu and Fe/Mn/Al during the later stage of the experiment may be attributed to different immobilization rate between Cd/Cu and Fe/Mn/Al.
5. Conclusions
Although the capacity of malic acid to mobilize Fe, Mn, and Al in the contaminated soil was weaker compared to citric and oxalic acids, there was a general trend showing that the concentration of dissolved Fe, Mn, and Al increased over time during the 15-day experiment with marked increase in metal concentrations only occurring after 5 or 7 days of the experiment. For the same LMWOA treatment, the three metals showed a very similar temporal variation pattern. The total amount of Fe, Mn, and Al contained in the soil had an important control on the concentration of the dissolved Fe, Mn, and Al, respectively. Manganese oxides appeared to be more prone to LMWOA attack. However, in the presence of multiple LMWOAs, the soil-borne Fe, Mn, and Al were mobilized rapidly within the first 5 or 7 days of the experiment and then tended to decrease. The co-existence of multiple LMWOAs appeared to enhance the formation of insoluble Fe, Mn, and Al organic complexes, leading to their precipitation. The LMWOA-driven dissolution of Fe, Mn, or Al had a major control on the mobilization of As, Cr, Zn, Ni, Cu, and Cd, but not Pb, which was not largely derived from oxides of iron, manganese, and aluminium in the investigated soil.