It was believed that when hydroxyapatite (HAP) was used to remediate heavy metal-contaminated soils, its effectiveness seemed likely to be affected by its particle size. In this study, a pot trial was conducted to evaluate the efficiency of two particle sizes of HAP: nanometer particle size of HAP (nHAP) and micrometer particle size of HAP (mHAP) induced metal immobilization in soils. Both mHAP and nHAP were assessed for their ability to reduce lead (Pb), zinc (Zn), copper (Cu), and chromium (Cr) bioavailability in an artificially metal-contaminated soil. The pakchoi (
Heavy metal pollution in the soil has become a serious environmental problem in China, particularly following the rapid industrialization of the nation. Heavy metal in soil readily accumulates in plants and is then transported through the food chain, thus becoming a major threat to human health [
Supplementation of phosphate-based materials has been found to have a great effect on immobilizing Pb in contaminated soil and has recently become a commonly used technique due to its cost-effectiveness and decreased disruptive nature [
With the rapid development of nanotechnology there has been an increased usage of HAP nanoparticles put into use in wastewater and for soil remediation. Nanometer size particle HAP (nHAP) has a larger specific surface area than micrometer sized particle HAP (mHAP). Moreover, theoretically nHAP has larger reactivity and sorption capacities than that of common size of HAP [
The soil samples were derived from vegetable gardens. After being air-dried, the soil samples were grounded to pass through a 10 mm sieve for the pot trial. Soil pH was measured in 1 : 2.5 soil water suspensions with a combination pH electrode. Soil organic matter was determined by wet digestion with K2Cr2O7 and H2SO4; available N, P, and K were determined according to standard methods recommended by Lu [
Basic properties of the soil.
pH | Organic matter (g·kg−1) | Available (mg·kg−1) | Total (mg·kg−1) | |||||
---|---|---|---|---|---|---|---|---|
N | P | K | Pb | Zn | Cu | Cr | ||
4.95 | 14.5 | 88.9 | 11.2 | 95.0 | 42.0 | 60.7 | 23.8 | 55.2 |
Two soil amendments were used in this study: mHAP (micrometer hydroxyapatite, bought from Nanjing Emperor Nano Material Co., ltd., China, average particle diameter = 12
The soil was left to equilibrate for 20 days before planting pakchoi (
After harvest, the pakchois were removed from the pots and cut into two parts: shoots and roots. The shoots and roots were washed three times by deionized water, then put into the oven to dry at 70°C, and passed through 2 mm sieve for further experiment. The soil samples were taken from the pots after harvesting the vegetables and were air-dried at room temperature, followed by passing through 0.149 mm sieve.
Pb, Zn, Cu, and Cr in pakchoi were extracted by using acid digestion mixture (HNO3 and HClO4). The pakchoi samples were heated with HNO3 and HClO4 mixture until the color became clear, filtered, reconstituted to the desired volume, and measured by the inductively coupled plasma optical emission spectrometry (ICP-OES) for Pb, Zn, Cu, and Cr content. For the analysis of Pb, Zn, Cu, and Cr in soil, 0.3 g of homogenized soil samples was digested with HNO3, HClO4, and HF. The samples were heated until the color became clear, dissolved with several drops of 1% HNO3, filtered, diluted to a volume of 50 mL with distilled water, and analyzed for the content of Pb, Zn, Cu, and Cr [
The method of sequential extraction developed by BCR sequential extraction procedure [
The means and standard deviations (SD) were calculated by Excel 2003. Statistical analysis including the analysis of variance was conducted using SPSS version 17.0 software (SPSS Inc., USA) and differences (
The biomass of the pakchoi shoots and roots was significantly decreased by metal application at T2 and T4 treatment levels but increased at T1 and T3 treatment levels compared to the control treatment (Table
Biomass of pakchoi (
Treatment | Shoot (g·pot−1 DW) | Root (g·pot−1 DW) |
---|---|---|
T0 (CK) |
|
|
T0 + 1.5% mHAP |
|
|
T0 + 3% mHAP |
|
|
T0 + 1.5% nHAP |
|
|
T0 + 3% nHAP |
|
|
T1 (250/100 mg·kg−1 Pb/Zn) |
|
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T1 + 1.5% mHAP |
|
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T1 + 3% mHAP |
|
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T1 + 1.5% nHAP |
|
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T1 + 3% nHAP |
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T2 (500/200 mg·kg−1 Pb/Zn) |
|
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T2 + 1.5% mHAP |
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T2 + 3% mHAP |
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T2 + 1.5% nHAP |
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T2 + 3% nHAP |
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T3 (100/25 mg·kg−1 Cu/Cr) |
|
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T3 + 1.5% mHAP |
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T3 + 3% mHAP |
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T3 + 1.5% nHAP |
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T3 + 3% nHAP |
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T4 (200/50 mg·kg−1 Cu/Cr) |
|
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T4 + 1.5% mHAP |
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T4 + 3% mHAP |
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T4 + 1.5% nHAP |
|
|
T4 + 3% nHAP |
|
|
Mean values denoted by the same letter in a column do not differ significantly according to the Duncan test.
The application of mHAP and nHAP significantly reduced the concentration of Pb, Zn, Cu, and Cr in the shoots and roots of the pakchoi grown in the contaminated soil (Tables
Concentration of Pb and Zn in pakchoi (
Pb | Zn | |||
---|---|---|---|---|
Shoot |
Root |
Shoot |
Root | |
T0 (CK) |
|
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T0 + 1.5% mHAP |
|
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T0 + 3% mHAP |
|
|
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T0+ 1.5% nHAP |
|
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T0 + 3% nHAP |
|
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|
T1 |
|
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T1 + 1.5% mHAP |
|
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T1 + 3% mHAP |
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T1 + 1.5% nHAP |
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T1 + 3% nHAP |
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T2 |
|
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T2 + 1.5% mHAP |
|
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T2 + 3% mHAP |
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T2 + 1.5% nHAP |
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T2 + 3% nHAP |
|
|
|
|
Mean values denoted by the same letter in a column do not differ significantly according to the Duncan test.
Concentration of Cu and Cr in pakchoi (
Cu | Cr | |||
---|---|---|---|---|
Shoot |
Root |
Shoot |
Root | |
T0 (CK) |
|
|
|
|
T0 + 1.5% mHAP |
|
|
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T0 + 3% mHAP |
|
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T0 + 1.5% nHAP |
|
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|
T0 + 3% nHAP |
|
|
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|
T3 |
|
|
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|
T3 + 1.5% mHAP |
|
|
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T3 + 3% mHAP |
|
|
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T3 + 1.5% nHAP |
|
|
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T3 + 3% nHAP |
|
|
|
|
T4 |
|
|
|
|
T4 + 1.5% mHAP |
|
|
|
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T4 + 3% mHAP |
|
|
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T4 + 1.5% nHAP |
|
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T4 + 3% nHAP |
|
|
|
|
Mean values denoted by the same letter in a column do not differ significantly according to the Duncan test.
Some researchers [
pH value in the tested soil as affected by mHAP and nHAP amendments.
Treatment | pH |
---|---|
T0 (CK) | 4.95 |
T0 + 1.5% mHAP | 5.46 |
T0 + 3% mHAP | 5.99 |
T0 + 1.5% nHAP | 5.20 |
T0 + 3% nHAP | 5.76 |
T1 | 5.08 |
T1 + 1.5% mHAP | 5.76 |
T1 + 3% mHAP | 6.07 |
T1 + 1.5% nHAP | 5.42 |
T1 + 3% nHAP | 5.83 |
T3 | 5.11 |
T3 + 1.5% mHAP | 5.60 |
T3 + 3% mHAP | 5.99 |
T3 + 1.5% nHAP | 5.18 |
T3 + 3% nHAP | 5.74 |
T2 | 5.16 |
T2 + 1.5% mHAP | 5.72 |
T2 + 3% mHAP | 6.24 |
T2 + 1.5% nHAP | 5.45 |
T2 + 3% nHAP | 5.74 |
T4 | 4.97 |
T4 + 1.5% mHAP | 5.68 |
T4 + 3% mHAP | 6.19 |
T4 + 1.5% nHAP | 5.39 |
T4 + 3% nHAP | 5.87 |
pH was an important parameter which affected metal immobilization and dissolution in soil [
It was reported that when HAP was dissolved in deionized water and 0.1 mol/L KNO3 solution, the dissolution rate of HAP mainly depended on pH [
The distribution of Pb, Zn, Cu, and Cr in the uncontaminated soil and metal-contaminated soil as analyzed by the BCR sequential extraction method was shown in Figure
Relative percentages of Pb, Zn, Cu, and Cr in each fraction of the soils from the CK, mHAP, and nHAP treated pots.
The result showed that nonresidual fraction of Pb, Zn, Cu, and Cr decreased with the increase of soil pH, hinting that the pH values play an important role in decreasing the nonresidual fraction. The increase of pH values induced by HAP favored the precipitation of heavy metals. In addition, Gray et al. [
In this paper, we identified the effectiveness of mHAP and nHAP to reduce the amounts of heavy metals in contaminated soil. The efficiency of
We also found that the addition of mHAP and nHAP decreased the nonresidual fraction of Cu, Zn, and Cr in the soil. However, the immobilization efficacy was lower than Pb. We hypothesized two reasons for this phenomenon. Firstly, the solubility products of Cu, Zn, and Cr phosphate are known to be much greater than that of Pb phosphate. However, hopeite [Zn3(PO4)2], Cu3(PO4)2, and Cr2(PO4)7 are much more soluble than pyromorphite. As such, Cu, Zn, and Cr phosphate may not control the solubility of these heavy metals in this case [
The reduction of Pb content in the pakchoi shoots and roots can be attributed to the formation of pyromorphite in the soil. As to the decrease of Zn, Cu, and Cr in pakchoi shoots and roots, the main reason involved of Zn, Cu and Cr adsorption on the HAP and the higher solubility of [Zn3(PO4)2], Cd3(PO4)2 and Cr2(PO4)7 than pyromorphite. Moreover, the reduction of Pb, Zn, Cu, and Cr content in the pakchoi shoots and roots was as follows: Pb>Zn
Many studies have been conducted in recent years focused on the heavy metal remediation potential of nanomaterials in contaminated soil [
The effectiveness of two different sizes of HAP particle, nanometer size particle of HAP (nHAP) and micrometer size particle of HAP (mHAP), was assessed for their ability of reducing the bioavailability of Pb, Zn, Cu, and Cr. The results showed that both mHAP and nHAP had significant effect on reducing the uptake of Pb, Zn, Cu, and Cr by pakchoi. Furthermore, both mHAP and nHAP were efficient in covering Pb, Zn, Cu, and Cr from nonresidual into residual forms. However, mHAP was superior to nHAP immobilization of Pb, Zn, Cu, and Cr in metal-contaminated soil and reducing the Pb, Zn, Cu, and Cr by pakchoi. In addition, mHAP and nHAP were both more efficient in transferring bioavailable Pb into less bioavailable form than Zn, Cu, and Cr. This may be due to the fact that more Pb was formed into insoluble pyromorphite like minerals after treated with HAP. However, in this study, it was suggested that HAP with micrometer size was more effective immobilization soil metals than nanometer size HAP, possibly due to its higher dissolution rate.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the National Spark Program of China (no. 2012GA780051).