Abstract:

Lead (Pb) soil pollution is a major issue in China. Therefore, a safe utilization technology for farmland contaminated with Pb must be developed urgently. The quantity and type of soil organic matter and iron oxides determine the chemical forms of Pb; however, the influence of different types of organic matter and iron oxides on the chemical forms of Pb remains unknown. In this study, three zonal soils in China were selected: namely red, cinnamon, and black soils. Soil culture experiments were conducted with the addition of Pb, straw, and bacteria. Samples were collected after four months of cultivation in greenhouse conditions. The chemical forms of Pb were determined using the sequential extraction method, the contents of different types of organic carbon and iron oxides were determined, and the contents of different types of organic carbon in the soil were obtained using the near-edge X-ray absorption spectrum of carbon. Through correlation analysis, the main factors influencing the chemical morphology of Pb in various soil types were identified. The research results showed that: 1) Pb in the soil mainly exists in the forms of specific adsorption /carbonate-bound, humic acid-bound, and iron-manganese oxidation states; in different treatments of red soil, cinnamon soil, and black soil, the percentages of these three forms of Pb in the total Pb content were 74.4%‒89.2%, 84.2%‒95.6% and 73.4%‒84.9%, respectively. 2) In the low Pb treatment, compared with the control, rice straw (RS) and straw+bacteria (RS+B) significantly reduced the ion-exchange Pb content in Red soil by 83.1%-88.1% in the low Pb treatment and decreased the carbonate-bound Pb content in Cinnamon soil by 15.1%‒16.2%. The humic acid-bound Pb contents in red, cinnamon and black soils were significantly increased by 20.1%‒43.7%, 43.7%‒43.9%, and 25.4%‒41.5%, respectively, and the residual Pb contents in red and cinnamon soils were significantly increased by 50.8%‒73.0% and 11.0%, respectively. 3) In the soil treated with high concentration of Pb, compared with the control, both RS and RS+B significantly reduced the water-soluble Pb content of red soil by 52.6%. It significantly reduced the content of ion-exchange state Pb in red and black soils by 83.0%‒86.1% and 50.2%‒52.6%, respectively. The contents of humic acid-bound Pb in Red and black soils were significantly increased by 53.8%‒56.4% and 29.4%‒33.1%, respectively. The content of Pb in the iron-manganese oxidation state in red soil was significantly increased by 17.4% and 16.0%, respectively, and the content of Pb in the strongly organically bound state was 9.9% and 11.7%, respectively. The Pb content in the residue state was significantly reduced by 20.5% and 26.4%, respectively. In Cinnamon soil, RS and RS + B significantly increased the Pb content in the ion-exchange state by 21.4% and 9.4%, the Pb content in the carbonate-bound state by 5.6% and 5.7%, and the Pb content in the residue state by 41.4% and 22.9%, respectively. The Pb content in the oxidation states of iron (Fe) and manganese (Mn) was significantly reduced by 7.5% and 15.7%, respectively. Straw treatment significantly reduced the content of strongly organically bound Pb by 24.6%. In Black soil, RS and RS+B significantly reduced the carbonate-bound Pb content by 6.4% and 6.6%, respectively. 4) Among the soils treated with the two Pb concentrations, the Pb mobility factors (MFs) of black soil were the lowest, which was related to the high content of organic matter and clay particles. In the soil treated with low Pb, compared with the control, in the treatments of RS and RS + B treatments, the Pb MFs decreased the most in red soil by 32.0% and 23.8%, respectively. The decrease in the Pb movement coefficient in cinnamon soil was 14.0% and 13.5%, respectively. The Pb MFs in Black soil decreased by 17.2% and 3.0%, respectively. In the soil treated with high Pb, the Pb MF of RS and RS+B treatments in cinnamon soil were slightly higher than those of the control, and their values increased by 1.67 and 2.68, respectively, compared with the control. RS and RS+B reduced the MFs of Pb. In the two treatments of red soil, it decreased by 16.3% and 14.7%, respectively, while in black soil, it decreased by 24.7% and 23.2%, respectively. 5) Among the low Pb treatments of the three soil types, the passivation effect of Pb was better because the Pb MFs in the low-concentration treatment were lower than those in the high Pb treatment. The mobility coefficients of red soil, cinnamon soil, and black soil treated with high Pb were 2.00, 1.35, and 2.45 times those of the low-Pb treatment, respectively. In Cinnamon soil, the Pb MFs in the high Pb treatment to that in the low Pb treatment was less than 2, indicating that the space for reducing Pb MFs in Cinnamon soil was higher than that in red and black soils. The possible reason is that among the three types of soil, Cinnamon soil has the highest pH value (7.43), and the ion-exchange state Pb and carbonate-bound state Pb in the soil are greatly affected by the pH values. Therefore, Pb ions complexed with hydroxyl ions reduced their solubility in the solution at a relatively high soil pH. The X-ray near-edge absorption spectrum results of carbon show that in the straw treatment of Cinnamon soil, the proportion of fatty carbon and carboxyl carbon of organic carbon is the highest, and the percentage of the two parts of organic carbon in the total organic carbon is 57%. Aliphatic and carboxyl carbons provide binding sites for Pb. The aromaticity of black soil organic carbon in the RS treatment was enhanced, thereby reducing the number of Pb binding sites. This is one of the reasons why the Pb MFs in the high Pb treatment/low Pb treatment were the highest among the three soil types. 6) Correlation analysis showed that pH and various types of organic carbon in red soil promoted Pb fixation. Various types of organic carbon and iron oxides mainly affect the Pb content in the ion-exchange, carbonate-bound, and humic acid-bound states in cinnamon and black soil. This study reveals the role of organic carbon and iron oxides in regulating the form of Pb in different soil types, providing a theoretical basis for the safe utilization and risk assessment of Pb-contaminated soil in different regions.

Key words: straw, bacteria, Pb chemical forms, soil organic carbon, iron oxides, influencing factors

摘要：

中国土壤铅（Pb）污染问题突出，亟待开发受铅污染耕地的安全利用技术。土壤有机物和铁氧化物的数量和类型决定了Pb的形态，但不同类型有机物和铁氧化物对Pb形态的影响尚不清晰。选取中国3种地带性土壤，设计添加Pb、秸秆和细菌的土壤培养试验，温室条件下培养4个月后取样。结果显示，1）土壤中Pb主要以专性吸附/碳酸盐结合态、腐殖酸结合态、铁锰氧化态形式存在；在红壤、褐土和黑土的不同处理中，这3种形态的Pb占总Pb含量的比例分别为74.4%－89.2%、84.2%－95.6%和73.4%－84.9%。秸秆、秸秆+细菌促进了低铅处理土壤中铅老化的进程；高Pb处理中Pb形态转化主要发生在离子交换态、专性吸附/碳酸盐结合、腐殖酸结合态及铁锰氧化态之间。2）高Pb处理红壤、褐土及黑土的移动系数分别为低铅处理的2.00、1.35和2.45倍，表明褐土中具有较充足的吸附点位可降低Pb移动性，可能与褐土的高pH值、脂肪碳和羧基碳比例有关。3）红壤中pH和各类型有机碳对Pb的固定起促进作用；各类型有机碳、铁氧化物主要影响褐土和黑土中离子交换态、碳酸盐结合态及腐殖酸结合态Pb含量。研究结果可为Pb污染土壤的安全利用及风险评估提供理论依据。

关键词: 秸秆, 细菌, 土壤铅形态, 有机物, 铁氧化物, 调控因素