Abstract:

The application of biochar (BC) for the ecological remediation of soils contaminated with heavy metal(loid)s has emerged as a prominent research focus in ecology and environmental sciences. Despite its potential, the effects of BC on the mobility and phytoavailability of certain heavy metal(loid)s, such as arsenic (As) and antimony (Sb), remain inconclusive and are highly debated in the literature. Some isolated studies have demonstrated that BC can reduce the mobility and phytoavailability of As and Sb. However, the majority of relevant studies have reported contrasting results, particularly with pristine BC, which may enhance their mobility. We believe that the main reason for this discrepancy is the lack of systematic investigation into the dosage-dependent effects of BC, which is critical factor for determining its efficacy in soil remediation. To address this knowledge gap, a comprehensive pot experiment was designed and conducted to evaluate the impact of BC addition on the phytoavailability of As and Sb in tailings-contaminated soils. Considering that mine tailings were identified as the primary source of As and Sb contamination, this experiment utilized a soil mixture composed of forest soil, mine tailings, and varying concentrations of BC, with a total weight of (3000±100) g per pot. To ensure consistent initial conditions, 500 g of mine tailings were uniformly mixed into each pot, and an additional 1.00 g of antimony potassium tartrate was added to compensate for the relatively low Sb content in the tailings. Six treatment groups were established, each with distinct BC mass ratios (0, 1.5%, 3%, 4.5%, 6%, and 7.5%, labeled BC0, BC1.5, BC3, BC4.5, BC6, and BC7.5, respectively). Pennisetum purpureum, a commonly used plant species in phytoremediation studies, was cultivated in the treated soil for a growth period of 60 d. At the end of the experiment, the concentrations of As and Sb in the soil (determined through water extraction and sequential fractionation) and plant tissues were measured to assess the influence of BC on the mobility and phytoavailability of these two elements. The pot experiment results revealed that BC addition significantly altered the physicochemical properties of the tailings-contaminated soil by measuring changes in key indicators. Specifically, pH, organic carbon content, and phosphorus (P) concentration increased notably with increasing BC dosage. Concurrently, the addition of BC also promoted the release of water-soluble As and Sb, such that their concentrations increased with increasing BC dosages and reached significant levels in the high-dose biochar treatment group. The water-soluble As content rose from (2.70±0.70) mg·kg⁻¹ in the control group (BC0) to (10.24±1.19) mg·kg⁻¹ in the highest BC dose group (BC7.5), while the water-soluble Sb content increased from (11.76±1.97) mg·kg⁻¹ in BC0 to (16.58±1.04) mg·kg⁻¹ in BC7.5 (p=0.031). Sequential extraction analysis further demonstrated that the mild acid-soluble and oxidizable As fractions increased with BC amendment, whereas the reducible and residual fractions remained relatively unchanged. In contrast, the fractions of Sb showed no significant variation across the treatment groups, likely due to the uniform addition of antimony potassium tartrate. The increase in water-soluble and mild acid-soluble As and Sb fractions supports the view that BC enhances the mobility of As and Sb; however, the uptake of these metal(loid)s by Pennisetum purpureum did not show a corresponding enhancement, although water-soluble and mild acid-soluble metal(loid) fractions have been commonly used as indicators of phytoavailability. The As content in plants generally decreased from (74.13±23.47) mg·kg⁻¹ (BC0) to (18.34±4.73) mg·kg⁻¹ (BC7.5), but intermediate dosages (e.g., BC3 and BC6) showed no significant differences from the control, suggesting a nonlinear dose-response relationship. In addition, the Sb content in the plants (13.64-23.94 mg·kg⁻¹) showed no significant differences among the treatment groups, likely due to the uniform distribution of antimony potassium tartrate. Despite the above element contents in the soil and plant, the bioconcentration factor (BCF, K_B) and transformation factor (TF, K_T) were also used to determine the ability to be repaired by phytoextraction. K_B for As were highest in the control (BC0) and decreased significantly with increasing BC dosage, while K_T showed an opposite, increasing trend, peaking at BC7.5. Similar trends were observed for Sb, although the differences among the treatment groups were not significant. Both K_B and K_T for As and Sb were below 1, which is generally used to assess whether plants can be applied to the remediation of metal(loid)-contaminated soil, indicating that Pennisetum purpureum does not exhibit hyperaccumulation traits under the experimental conditions. However, BC addition promoted the translocation of As and Sb from the roots to the aerial parts of the plant, which is beneficial for phytoremediation. The discrepancy between the water-soluble concentrations of As and Sb and their actual uptake by Pennisetum purpureum suggests that phytoavailability may vary with plant growth stages and may not be fully captured during harvest. Additionally, the increase in oxidizable As and decrease in residual As indicate a potential enhancement of As availability with BC addition, although this was not reflected in the plant uptake. These findings underscore the complex and dynamic interactions of BC addition on the phytoavailability of As and Sb in contaminated soil. They also underscore the necessity for additional studies focused on optimizing BC dosage, timing, and application methods. In conclusion, this study provides crucial evidence on the effects of BC addition on the phytoavailability of As and Sb in tailings-contaminated soil. While BC addition significantly altered soil properties and increased the water-soluble fractions of As and Sb, its impact on plant uptake was not proportional. These findings underscore the importance of considering both the short- and long-term effects of BC application and the need for further investigation into the factors influencing the phytoavailability and translocation of heavy metal(loid)s in plants. These results contribute to the growing body of knowledge on the use of BC for soil remediation and serve as a basis for targeted future studies aimed at addressing the scientific and technical challenges associated with the remediation of As- and Sb-contaminated soils.

Key words: arsenic, antimony, biochar, phytoavailability, Pennisetum purpureum

摘要：

生物炭（BC）在（类）重金属污染土壤生态修复中的应用已成为生态环境领域的研究热点，但其对砷（As）和锑（Sb）的修复效果仍存在争议。为探究BC及其用量对污染土壤中As和Sb植物有效性的影响，设置了一系列BC添加量处理组（0、1.5%、3%、4.5%、6%、7.5%，分别记为BC0、BC1.5、BC3、BC4.5、BC6、BC7.5），并培植甜象草（Pennisetum purpureum）60 d。通过测定土壤和植物中As和Sb的含量，分析BC添加对As和Sb植物有效性的影响。结果表明，BC的添加显著提高了尾砂污染土壤的pH值、有机质和磷（P）含量，同时促进了水溶态As和Sb的释放。水溶态As和Sb含量分别从对照组的 (2.70±0.70) mg·kg⁻¹和 (11.76±1.97) mg·kg⁻¹增加至最高剂量组的 (10.24±1.19) mg·kg⁻¹和 (16.58±1.04) mg·kg⁻¹。然而，BC的添加并未显著影响甜象草对As和Sb的吸收和富集特性。尽管As含量有所降低，但各组间Sb含量差异不显著（13.64－23.94 mg·kg⁻¹），且甜象草中As和Sb含量与BC用量之间未呈现明显的规律性。值得注意的是，评估As和Sb植物有效性的两个指标（水溶态浓度与植物实际吸收量）表现出不一致性。这一结果提示，研究者需关注BC添加的短期与长期效应，同时也表明利用BC和植物吸收修复尾砂污染土壤中的As和Sb仍面临诸多科学与技术挑战。

关键词: 砷, 锑, 生物炭, 植物有效性, 甜象草