1. Introduction
Heavy metal pollution in soil is a critical issue worldwide, particularly in agricultural fields due to food safety concerns [
1,
2,
3,
4]. Various sources can contribute to the release of heavy metals into the environment, including industrial areas [
5], smelting activities [
6,
7], and abandoned mines [
8,
9,
10]. Heavy metal pollution in soil or water can be a concern because of its toxicity, persistency, bioaccumulation, and leaching into groundwater [
11].
The bioavailable fraction of heavy metals differs from the total heavy metal concentration. The total heavy metal concentration can be used to determine the level of soil contamination, whereas the bioavailable fraction of heavy metals can provide information on the mobility and bioavailability of the heavy metals in soil [
12,
13,
14,
15]. Various chemical extractants have been used to estimate the bioavailable fraction of heavy metals in soil. The sequential extraction method is divided into different fractions, such as exchangeable fraction, bound to carbonate fraction, bound to Fe-Mn oxide fraction, bound to organic fraction, and residual fraction depending on ionic strength of the extractants [
16]. Among the five different fractions, the exchangeable and bound to carbonate fractions are considered to be the bioavailable fractions of heavy metals [
17]. Other extractant includes the toxicity characteristic leaching procedure (TCLP) for conducting ecological risk assessments, Mehlich-3, EDTA, CaCl
2, and NH
4NO
3 [
18,
19,
20,
21,
22,
23].
The application of chemical amendments or adsorbents is a common technique for heavy metal remediation in soil. The main purpose of applying chemical amendments to soil is to immobilize heavy metals, thereby reducing their mobility and bioavailability in soil [
24,
25,
26,
27,
28,
29,
30]. Many studies have tried to find amendments or adsorbents that are low cost and have a high remediation efficiency, such as lime, steel slag, coal ash, biochar, and red mud [
12,
13,
31,
32,
33,
34]. Soil amendments have different mechanisms of heavy metal reduction; their reduction efficiencies vary depending on their characteristics and the soil environment. For example, soil pH is one of the factors controlling the mobility and bioavailability of heavy metals. As the soil pH increases, the concentrations of cationic heavy metals in the soil, such as Cd
2+, Pb
2+, Pb
2+, and Cu
2+ decrease, while the concentration of As increases [
12,
35,
36]. In addition, soil organic matter (SOM) plays an important role in immobilizing the bioavailable fractions of heavy metals. SOM has a high affinity for heavy metals with various functional groups and forms precipitates [
31,
37,
38]. Iron-based materials including iron oxide and zero valent iron have also been used for heavy metal remediation due to their high reactivity, large surface area, and sorption ability [
27,
35].
Paddy soils provide a unique environment in which anaerobic conditions are maintained during flood periods. Soils with anaerobic conditions have different characteristics than those with aerobic conditions. Prolonged flood conditions in paddy soils can change the microbial activity in soils owing to the anaerobic fermentation of SOM [
37]. Under anaerobic conditions, reducing the redox potential can cause the soil pH to become neutral regardless of whether the soil was previously was acidic or alkaline [
39]. Consequently, the mobility and bioavailability of heavy metals in paddy soil are different from those of upland soil in terms of the formation of chemical complexes. For example, sulfide, a reduced form of sulfate, can form a metal sulfide complex [
40].
Heavy metals including arsenic (As) and lead (Pb) can be accumulated in rice (
Oryza sativa L.) and cause adverse effects on human health such as cancer, cardiovascular, neurological, and respiratory disease [
41]. Heavy metal concentration in paddy soil can be varied depending on the source of pollutants. When groundwater irrigation was the source of as in paddy soil, the concentration of As in soil was ranged 3.1–61.04 mg kg
−1 in Bangladesh, India, Vietnam, and Nepal [
42,
43,
44]. Mine deposition or industrial wastewater can also be a source of As in paddy soil and concentration of As in soil was ranged 2.5–172.07 mg kg
−1 in China and 3.05–34.0 mg kg
−1 in Korea [
45,
46]. The concentration of Pb in paddy soil was also varied depending on the pollutant source and ranged 24.8–1,486 mg kg
−1 when mine waste, groundwater, and irrigation surface water were sources of pollutants in paddy soil [
34,
47,
48].
Coal plant by-product such as fly ash and bottom ash has been used for heavy metal remediation in soil because of high porosity and large specific surface area [
49]. However, fly ash and bottom ash have a disadvantage in terms of low efficiency for heavy metal remediation in soil when they are used as an original form. For this reason, modification of coal plant by-products such as a hydrothermal method or synthesizing with zeolite adsorbent has been studied to enhance the efficiency of heavy metal remediation [
50,
51,
52,
53].
The main purpose of this study was to evaluate the use of recycled waste materials, named artificial lightweight material (ALM), as adsorbents of heavy metal in paddy fields. In addition, ALM combined with FeO and lime was examined for its synergetic effects on the reduction of bioavailable fractions of heavy metals in soil and crops.
2. Materials and Methods
2.1. Soil Amendments
The main soil amendment, named “Artificial Lightweight Material (ALM),” was provided by a coal power plant located in Incheon province, South Korea. To manufacture ALM, 20% bottom ash, 50% low-quality unburned carbon, and 30% dredging sand were mixed, heated to a temperature of 550–600 °C, cooled to 25 °C, and heated again to 1100–1200 °C in a heating chamber. To evaluate the effects of different combinations of soil amendments, ALM was applied with either 0.5% lime (ALM10+L) or 0.5% iron oxide (ALM10+FeO) in a paddy field. Lime was purchased from a commercially available market, and iron oxide was purchased from Sigma Aldrich.
2.2. Determining Application Rate of Soil Amendment
Prior to determining the application rate of ALM in the field, sorption batch experiments were conducted for As and Pb. One hundred mL of each stock solution (100 mg L−1), made with NaAsO2 and Pb(NO3)2, was mixed with three different ALM ratios (5%, 10%, and 20% (w/v)) in a 250 mL flask. The flask was shaken at 120 rpm for 48 h, centrifuged at 3000× g for 5 min, and filtered through a 0.45 μm filter paper for ICP-OES analysis. The sorption efficiency of each ALM ratio was determined by calculating the difference between the initial concentration of metals and the remaining concentration in the solution.
2.3. Field Experiment Setup
A total of seven, 10 m × 6 m (L × W) plots were constructed in a paddy field and arranged in a completely randomized block design. A 15 cm high guard row was built around each plot to prevent cross-contamination. Each plot was subjected to the following treatments: Control: no amendment; ALM10: ALM 10% (w/w); ALM10+L: ALM 10% + 0.5% lime; ALM10+FeO: ALM10% + 0.5% FeO. The plots were thoroughly plowed to make the soil homogeneous and equilibrated for 8 weeks without water supply. The plots were then flooded with water containing trace concentrations of heavy metals (data not shown). A week after flooding, two stands of 50-day old seedling with 3–4 plants in each stand were transplanted, and the flooded condition was maintained for 28 weeks. After 3 weeks of drying, rice was harvested. The rice cultivar named “Oh-Dae”, Oryza sativa L., was selected for the experiment as it is a common rice cultivar in the local area. During rice cultivation, conventional cultivation practices, fertilization, and weed control using pesticides were conducted.
2.4. Soil and Plant Sampling
Soil samples were collected at 0, 8, 16, and 32 weeks after soil amendment application. Five soil samples were collected from different locations within the treatment area using a hand auger at a depth of 20 cm. These five soil samples were thoroughly mixed to form one composite soil sample, which was placed in a plastic sample bag. The homogenized soil sample was air-dried at room temperature (25 °C) and passed through a 2 mm and then a 0.15 mm sieve for chemical and heavy metal analysis.
Every third stand of rice sample from each treatment plot was collected and air-dried at room temperature (25 °C) until the moisture reached approximately 15%. The rice grains were separated from the plants, and 100 rice grains in each treatment were weighed for growth comparison.
2.5. Chemical and Heavy Metal Analysis in Soil and Plant
Soil pH (H2O 1:5 w/v) and electrical conductivity (EC) were measured using a pH meter (MP220, Mettler Toledo, Columbus, OH, USA) and EC meter (S230, Mettler Toledo, Columbus, OH, USA) after shaking 10 g of soil and 50mL of distilled water for 1 h. Soil organic matter (SOM), available P2O5, and cation exchange capacity (CEC) were measured following the Walkley–Black, Bray No1, and ammonium acetate exchange methods and are summarized in Table 1.
Both As and Pb were extracted by digesting the soil sample with aqua regia (HNO3:HCl (v/v) = 1:3) in a heating block (Block Heating Sample Preparation System, Ctrl-M Science). The bioavailable fractions of the heavy metals in the soil were extracted with the Mehlich-3 extractant (M3).
Digestion of rice grains was conducted in a heating block for 2 h after 24 h of stagnation in an HNO3 solution. The heavy metal concentrations were measured using an ICP-OES (ICAP 7000series, Thermo Fisher, Waltham, MA, USA). For quality assurance and quality control purposes, blank and spiked samples were measured every 50 samples. In addition, certified reference material for heavy metal contaminated soil (BAM, Germany) was analyzed, and the mean recovery ratios for As and Pb were 102% and 98%, respectively. All glassware and polyethylene bottles were soaked overnight in a 0.5% HNO3 solution and rinsed with deionized water before the experiment. The total content of organic carbon (TOC) and nitrogen (TN) of artificial light material was measured with an elemental analyzer (EA1112, Thermo Fisher Scientific, Waltham, MA, USA).
2.6. Statistical Analysis
All measurements were conducted in triplicate, and statistical analysis (ANOVA test) was performed using SPSS software (Version 20.0) [
54]. The ANOVA test was conducted using Tukey’s method at a significance level of
p < 0.05.
4. Conclusions
ALM produced from recycled waste, bottom ash, unburned coal, and dredged sand was examined for heavy metal remediation in soil. The application of ALM combined with FeO and lime was examined to determine their synergetic effects in reducing the bioavailable fraction of heavy metals. When ALM was applied to paddy fields, the bioavailable fractions of As and Pb decreased by 22.7% and 52.4%, respectively. However, when ALM was combined with FeO and lime, the reduction efficiencies of As and Pb significantly increased by 52.8% and 65.7%, respectively, compared to those in the control. In addition, the uptake of As and Pb also decreased by 52.1% and 79.7%, respectively, when ALM was combined with FeO and lime.
Correlation analysis showed that soil pH, EC, P2O5, and SOM were the main factors that reduced the bioavailable fractions of heavy metals, and opposing correlations were observed for As and Pb. When the soil pH and EC increased, the reduction efficiency of As decreased, whereas the reduction efficiency of Pb increased as the soil pH and EC decreased. In the case of SOM, As and Pb concentrations decreased when SOM concentration was increased.
Although the main reduction mechanism was not confirmed, the increases in soil pH, EC, and CEC concentrations may affect to reduce the heavy metal concentrations in the soil through ionic exchange and chemical complexation. In addition, combining FeO and lime with ALM showed a synergetic effect in reducing the bioavailable fractions of As and Pb in soil. However, we have not examined the longevity of applied amendments in the field and in the future, long-term monitoring of the mobility and bioavailability of heavy metals should be conducted to assess the longevity of amendments in soil.