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| Cd and Pb distribution characteristics and risk assessment in agricultural soil aggregates surrounding a zinc smelter |
| Received:October 18, 2022 |
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| KeyWord:zinc smelter;agricultural soil aggregate;cadmium;lead;soil physicochemical property;speciation;environmental pollution risk |
| Author Name | Affiliation | E-mail | | WEI Xiaoli | College of Nature Resources and Environment, South China Agricultural University, Guangzhou 510642, China | | | ZHANG Shan | College of Nature Resources and Environment, South China Agricultural University, Guangzhou 510642, China | | | ZHANG Yulong | College of Nature Resources and Environment, South China Agricultural University, Guangzhou 510642, China | | | YE Zihao | College of Nature Resources and Environment, South China Agricultural University, Guangzhou 510642, China | | | CHEN Huayi | College of Nature Resources and Environment, South China Agricultural University, Guangzhou 510642, China | | | LI Yongtao | College of Nature Resources and Environment, South China Agricultural University, Guangzhou 510642, China | | | LI Wenyan | College of Nature Resources and Environment, South China Agricultural University, Guangzhou 510642, China | lily1984191@scau.edu.cn |
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| Abstract: |
| To understand the distribution and chemical speciation of heavy metals（Cd and Pb）in soil aggregates and factors affecting these processes, agricultural soil surrounding a zinc smelter in Huludao, Liaoning Province, China, was analyzed. Five-step sequential extraction based on the Tessier method was used to determine the chemical speciation of Cd and Pb in soil aggregates in different size classes. The index of geo-accumulation（Igeo）, ratios of secondary and primary phases（RSP）, and risk assessment coding（RAC）methods were used to evaluate the environmental risk of heavy metal contamination of soil. Correlation and redundancy analyses were applied to determine the key factors affecting the distribution and speciation of Cd and Pb in soil aggregates. Cd and Pb contents in soil increased initially, followed by a decrease, with increasing distance from the center of the zinc smelter. Cd and Pb contents in the topsoil were 57.40 mg · kg-1 and 219.56 mg · kg-1, respectively, which were significantly higher than those in the subsurface layer（55.06 mg · kg-1 and 135.99 mg · kg-1, respectively）. With decreasing aggregate particle size, Cd and Pb contents tended to increase, with Cd showing greater enrichment than Pb. Cd was predominantly detected in exchangeable and carbonate-bound forms, accounting for 60%–94% of the total Cd. Meanwhile, Pb was predominantly detected in Fe – Mn oxide - and carbonate-bound forms, accounting for 45% – 93% of the total Pb. The proportions of exchangeable and carbonate-bound Cd forms as well as of Fe-Mn oxide - and carbonate-bound Pb forms in aggregates fractions with particle size <0.002 mm were significantly higher than those in the remaining aggregate fractions with larger particle sizes. The three independent evaluation methods yielded consistent results that the risk of heavy metal contamination was higher in aggregates with smaller particle size. The RAC method considered both speciation and total amount of heavy metals; thus, its evaluation results were the most accurate. RAC results indicated that Cd contamination in the study area poses a very high risk, whereas Pb contamination poses a moderate risk. Soil organic matter（SOM）, surface area（SA）, cation exchange capacity（CEC）, and Fe–Mn oxides were the major factors underlying differences in the characteristics of aggregate fractions with particle size <0.002 mm. SOM exerted a highly significant effect on both residual and organically bound Cd and Pb in aggregates with particle sizes of 0.25–2.00, 0.053–<0.25 mm, and 0.002–<0.053 mm. CEC exerted a significant effect on the distribution of exchangeable Cd in aggregate fractions with particle size <0.002 mm. Fe – Mn oxides exerted a certain effect on Cd and Pb speciation in aggregate fractions with particle sizes of 0.25–<0.053 mm and <0.002 mm and a greater effect on exchangeable, carbonate-bound, and Fe-Mn oxide-bound forms. The higher active forms of Cd and Pb in contaminated farmlands can be easily enriched and distributed in aggregates of particle size <0.002 mm, posing a high environmental risk, and SOM, CEC, and Fe– Mn oxides are the key factors affecting these Cd and Pb enrichment processes. |
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