1. Introduction
The soils that are used for planting vegetables in China are faced with soil acidification, secondary salinization, nonpoint source pollution and heavy metal pollution [
1]. Heavy metals cannot be degraded in the environment and easily accumulate along the food chain, they thus ultimately endanger human health [
2,
3]. With improved living standards, humans have higher requirements for food safety and the green production of agricultural products. However, at present, the problem of heavy metal pollution in farmland soil is becoming increasingly prominent [
4]. The migration of soil heavy metals through the food chain is an important source of human heavy metal exposure [
5]. In particular, vegetables provide an indispensable source of food in the daily lives of humans, and heavy metals accumulate in the human body after the vegetables they pollute are consumed. Therefore, it is urgent to remove heavy metal pollution in vegetable planting soils to ensure food safety.
Karst landforms are unsuitable for agricultural development due to their rugged surfaces, the small size of their cultivated land areas, thin soil layers and infertile soils [
6].
Spinacia oleracea L., a common vegetable in Guizhou Province, southwest China, is highly adaptable to various soil types and maintains water and fertility [
7]. However, several studies have demonstrated that stem and leaf vegetables, including
S. oleracea have strong abilities to accumulate heavy metals in soil [
8,
9]. Ru et al. [
10] studied the characteristics and control measures of the absorption and accumulation of the heavy metals Cu, Zn, Pb and Cd in
S. oleracea. They suggest that the contents of Cu, Zn, Pb and Cd increased significantly in
S. oleracea with increases in the soil contents. With the growth of
S oleracea, the trend of increasing heavy metal contents in soil becomes significant. With the addition of heavy metals to soil, only a small portion of the heavy metals in the soil are able to be absorbed by
S. oleracea roots, with most Cu and Pb accumulating in the roots after their absorption by
S. oleracea. Sun et al. [
11] studied the accumulations of heavy metals in vegetables and greenhouse soils in Hebei Province. They show that
S. oleracea and scallion and garlic seedlings had relatively large enrichment coefficients for five heavy metals. The average bioaccumulation factor for different vegetable categories were ranked in the order of leaf vegetables > root vegetables > bulbous vegetables > fruit vegetables and for different heavy metals, were ranked in the order of Cd > Ni > Cr > As > Pb. Zubair et al. [
12] investigated the concentrations of two heavy metals, Hg and As, in
Spinacia oleracea L. and rhizosphere soil. They suggest that
S. oleracea is a good accumulator of heavy metals and has shown significant levels of both As and Hg accumulations. Their concentrations in plants increased with increasing soil concentrations. The abovementioned studies have shown that
S. oleracea has a strong enrichment ability for heavy metals in soil. Unfortunately, the heavy metal distribution in the karst area in Guizhou Province is anomalous, and its geological background values are higher than those in the other provinces of China [
13]. Therefore, especially for areas with higher background values of heavy metals, it is necessary to find low-cost and effective methods to reduce their accumulation through
S. oleracea and its rhizosphere soil, thereby decreasing the level of harm to humans.
Biochar is a high-quality energy and soil conditioner, reducing agent, slow-release carrier of fertilizer and is a carbon dioxide storage agent [
14]. It has been widely used in carbon fixation and emissions reduction, water purification, heavy metal adsorption and soil improvement [
15], especially for heavy metal remediation in soils [
16]. Dhiman et al. [
17] reported the use of a polyacrylamide superabsorbent polymer hydrogel and of its mixture with pyrolyzed plantain peel biochar for use as a soil amendment to reduce heavy metal uptake by wastewater-irrigated
S. oleracea. Malandrino et al. [
18] evaluated the effectiveness of vermiculite treatments to reduce the availability of pollutants in two plants,
Lactuca sativa and
S. oleracea, in pot experiments. They suggest that the addition of vermiculite significantly reduced the uptake of metal pollutants by plants, which confirmed the possibility of using this clay in amendment treatments of metal-contaminated soils. Tan et al. [
19] explored the effects of municipal sludge composting on
S. oleracea growth and the soil environment. The application of sludge compost had a significant effect on the heavy metal contents in
S. oleracea and significantly increased the nutrient and organic matter contents in its rhizosphere soils. There was a significant correlation between the amount of sludge compost and the heavy metal contents in sludge. However, biochar, as a low-cost and green material, is rarely reported with respect to remediating heavy metals in common edible vegetables and the rhizosphere soils in karst areas. Therefore, the objective of the present study is to (1) evaluate the degree of heavy metal pollution (e.g., As, Cd, Cr, Cu, Ni, Pb and Zn) in
S. oleracea and its rhizosphere soils with different biochar dosages by using the potential ecological risk index; (2) determine the nutrient relationships between
S. oleracea and its rhizosphere soil; and (3) reveal the effects of heavy metal exposure in
S. oleracea and its rhizosphere soil with different biochar levels on the health of children and adults.
4. Discussion
That pH values in soil increase with the addition of different proportions of biochar has already been reported [
14], and this phenomenon in the present study is therefore not significant. Liu et al. [
28] report the effect of lychee biochar on the remediation of heavy metal contaminated soil that was used to grow sunflowers. The sunflower plants significantly decreased the Pb, Cd, As, and Zn concentrations in contaminated soil (
p < 0.05), with decreases of 12.4, 11.0, 4.35, and 8.17%, respectively, when compared with soil samples without sunflowers planted. The addition of biochar to heavy metal contaminated soil significantly enhanced the heavy metal remediation that was affected by sunflowers. Chen et al. [
29] studied whether Cd bioavailability was affected in soils containing biochar and crop straw. The results show that biochar amendments enhanced the peanut biomass and physiological quality, and biochar had a greater impact on immobilizing Cd in the soil. In addition, biochar was more significant (
p < 0.05) in decreasing the Cd bioavailability and improving the biomass of peanuts. Wang et al. [
30] demonstrated a facile pyrolytic synthesis of biochar/ZnO passivators, their use in Cu(II) immobilization, and the mechanism of that process. Biochar was prepared from waste pomelo peel that was combined with ZnO to immobilize Cu(II) in contaminated soil, and the maximum adsorption capacity was 216.37 mg/g. In addition, mechanistic investigations indicated that Cu(II) bound to biochar/ZnO was primarily in a nonbioavailable state (75.6%). The heavy metal concentrations in lime soil were reduced by biochar, something that may be caused by several mechanisms including Π–Π electron interactions, ion exchange, surface complexation, precipitation reactions, and redox reactions [
31]. Especially for the Π–Π interactions, the biochar surface is highly aromatic and has dense Π electron clouds, which easily combine with the Π-bonds of heavy metals. The Π electrons on the biochar surface can serve as electron donors and interact with metal electron acceptors with d-orbitals and form Π–Π electron interactions [
32]. Lin et al. [
33] revealed the mechanism for the adsorption of Cd
2+ on corn straw scale biochar samples that were produced at 350 °C and 700 °C, and cation-Π interactions were one of the main mechanisms. Cation-Π interactions are relatively complex and mainly depend on the degree of aromaticity on the biochar surface and on the properties of the metals. The degree of biochar aromaticity gives rise to a common aromatic structure. The stronger the electron donor ability of the biochar and the lower the d orbital energies of the metals are, the more obvious the effect.
Ion exchange may be the other main reason for the different adsorption capacities of biochar for heavy metals in soils. The adsorption capacities of different metal ions depend on their own holding forces; hence, the adsorption capacity of biochar for ions is different if multiple metal ions coexist. In addition, depending on the raw materials and temperatures employed, different amounts of oxygen-containing functional groups, such as -OH, -CO, -O, and -COOH, may form on the biochar surfaces [
28]. It has been reported that the functional groups of biochar have a high adsorption affinity for Cu. Therefore, Jiang et al. [
34] further explored the adsorption and fixation of Cu by rice straw biochar and evaluated the effects of the alkalinity levels and functional groups on the fixation of Cu. The results show that both the alkalinity and oxygen-containing functional groups lead to Cu fixation; however, adsorption by the oxygen-containing biochar functional groups was the main mechanism. Additionally, Jiang et al. [
35] studied the effects of biochar on soil surface charges and on the adsorption of Pb
2+ through batch experiments. They found that electrostatic and non electrostatic interactions coexisted in the adsorption process and that the formation of complexes between the functional groups and Pb
2+ was the main mechanism for adsorption. These mechanisms may be the other reason for the different heavy metal adsorption capacities of biochar that were determined in the present study.
Our study shows that the heavy metal concentrations in soils decreased continuously with the addition of increased biochar amounts, and this tendency terminated with the addition of 400 g of biochar. Several researchers have also found that additional biochar contents and the adsorption of heavy metals are often positively correlated; however, the adsorption capacity for heavy metals reaches saturation when the additional amount of biochar reaches a certain value [
36,
37]. Gan et al. [
38] used 10, 30, 50, 70 and 100 g/kg of straw biochar in a heavily polluted area of an electroplating plant to determine the changes in the total amounts and speciation of heavy metals in soil. They found variations in the stabilizing effects of different biochar levels on heavy metals. Liu et al. [
39] used several different crops as raw materials to prepare biochars and studied the adsorption of Cd
2+ and Pb
2+ in solution. Their results indicate that the removal efficiency for Pb increased with increasing amounts of biochar materials that were produced by corn straw, wheat straw and peanut shells. Additionally, the pH levels were different for different soil types, which can significantly affect the forms of heavy metals that are present and the surface charge distributions of biochar [
40]. Biochar is normally alkaline and could thus increase the soil pH levels at higher application rates, particularly in acidic soils, which is consistent with the results of our study. It was also found that such pH changes could in turn facilitate the hydrolysis of heavy metals, which would thereby enhance their adsorption by soil and accelerate the transformations of the oxidizable and residual fractions of heavy metals [
41]. Due to the differences in raw materials, technologies and pyrolysis conditions used, biochar shows variations in its physical and chemical properties, such as structure and particle size distribution, pore volume, apparent density, specific surface area, pH, volatile matter content, ash content and water holding capacity [
42]. The raw materials and pyrolysis temperatures that are used to synthesize biochar are significant factors that affect the characteristics and properties of biochar. The mean particle sizes and size distributions of biochar are dependent on the properties of the raw materials, and the particle size of biochar determines the surface area [
42,
43]. Furthermore, the present study demonstrates that the amount of biochar applied is an important factor that determines the different adsorption capacities for heavy metals, especially in soils that are contaminated with mixtures of heavy metals.
The growth and development of plants depend on a fine soil environment. However, there are numerous obstacles in soil that can restrict the growth of plants in nature. Biochar applications can significantly increase soil pH, which thus reduces the contents of exchangeable metals, e.g., Al, Cu and Fe, and increases the availability of essential elements, e.g., N, P, K, Ca, and Mg [
44]. This mechanism explains why the soil pH was positively correlated with the K and P contents in plants, as is shown by the redundancy analysis in the present study. Biochar can limit the damage that is caused by elements that are harmful to
S. oleracea growth. Therefore, the pH values of the lime soil were positively correlated with the Pb, P and K contents and negatively correlated with the As, Cr, Hg, Cd and N contents in
S. oleracea. Thus, the BCF values of Cr, As, Cd and Pb decreased as the pH values of the lime soil increased with added biochar. Khan et al. [
45] report that the application of HWB decreased the concentrations of Cr, Zn, Cu, Mn, and Pb in cilantro by 25.5%, 37.1%, 42.5%, 34.3%, and 36.2%, respectively, when compared with the control. In
S. oleracea, the concentration reductions were Cr 75.0%, Zn 24.1%, Cu 70.1%, Mn 78.0%, and Pb 50.5% when compared with those of the control. The bioaccumulation factor results also indicate that hardwood biochar inhibited the bioaccumulation of selected heavy metals in cilantro and
S. oleracea, which thus reduced the health risks. Song et al. [
46] demonstrated that biochars are rich in nutrient contents and improved garlic yields. Sewage sludge is a suitable biochar resource, especially for biochar that is produced at 450°C; this sludge has rich micropores, relatively stable functional groups in its structure and rugged surfaces that provide good contact with soil, all of which are conducive to its use as biochar. Zheng et al. [
47] revealed the effects of bean stalk and rice straw biochars on the bioavailability of metal(loid)s in soil and their accumulation in rice plants. Both biochars significantly decreased the Cd concentrations in iron plaque (35–81%), roots (30–75%), shoots (43–79%) and rice grains (26–71%). Following the biochar additions, the zinc concentrations in the roots and shoots decreased by 25.0–44.1 and 19.9–44.2%, respectively. In addition, biochar can enhance the uptake of nutrients and promote plant growth. Overall, biochar has a low content of mineral nutrients, and its ability to directly supply nutrients is limited. The promotion of crop growth by biochar is mainly due to the improvements in the physical and chemical properties of soil, enhanced soil nutrient availability, and changes in soil structure [
48].
In assessing the potential ecological risk index by using toxicity coefficients, the C-0 and C-160 levels presented moderate ecological hazards, while the C-320, C-800, C-400 and C-240 levels presented mild ecological hazards. The method of introducing toxicity coefficients focuses on evaluating the potential risk of heavy metal toxicity to the environment. However, although the potential ecological risk index is used to evaluate the pollution status of heavy metals in soil, it is impossible to comprehensively evaluate the adverse human health effects that are caused by harmful environmental factors [
19]. The carcinogenic and noncarcinogenic health risk models, which were developed by the USEPA, were used to link environmental pollution sources with the health of exposed humans by using certain evaluation criteria and technical routes [
49]. In the present study, the HI, HQ, CR and TCR values indicate that exposure to the heavy metals in lime soil and
S. oleracea pose a serious threat to human health and present an unacceptable cancer risk, with children being more threatened than adults. Zhang et al. [
50] investigated the characteristics of heavy metal pollution and health risk evaluations for soils and vegetables in various functional areas of lead-zinc tailings ponds. The heavy metal pollutants in the soils and vegetables in the studied tailings area posed a serious threat to the surrounding ecological environment and to the health of the residents. Lin et al. [
33] report that the noncarcinogenic and carcinogenic effects that were caused by heavy metal exposure were regarded as acceptable (except for those in Lingyun County). Lingyun County soils should receive more attention due to the unacceptable levels of carcinogenic risk. As, Cd, Hg, and Cr were regarded as the priority control metals, which is in accordance with our results. Sawut et al. [
51] demonstrated that when considering the noncarcinogenic risks for children, the HI values were larger than 1 in all areas, which indicates that local children face a higher potential noncarcinogenic risk. In addition, the carcinogenic risk that results from dermal contact with vegetable bases does not pose a high risk for residents. For children, the carcinogenic risks posed by As through inhalation and ingestion are the main exposure pathways. The TCR values for adults and children were >1 × 10
−4 (unitless) in Sishihu village, Anningqu town, and Qinggedahu village, and this degree of carcinogenic risk is unacceptable. The serious threats to human health that are caused by heavy metals in lime soils and
S. oleracea may be due to soil materials that are collected from polluted areas.