Characterization of the particle size fraction associated heavy metals in tropical arable soils from Hainan Island, China

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Abstract

Tropical arable soils were sampled from Hainan Island. The particle size fractions associated Cr, As, Cd and Pb were characterized and their leachability was analyzed as well. The pollution of heavy metals in the arable soils was not severe except Cr (204 mg kg 1) in Haikou. The distribution of heavy metals increased with decrease of particle size. Although the smallest fractions (< 53 μm) occupied only 5.08–9.57%, they had the highest distribution factor (DF) of 3.50 for Cd, 2.11 for Pb, 1.73 for Cr and 1.09 for As, respectively. The contributions of micro-aggregates (< 250 μm) to the total amount of heavy metals was 41.9% for Cr, 44.6% for As, 61.6% for Cd and 48.6% for Pb, respectively, and the second mass loading came from the particles of 250–1000 μm. The residues of Cr, Cd and Pb were correlated positively with the contents of organic carbon as well as Fe in fractions, while a large variation distribution of As was found in particles, indicating its high activity in soil microenvironment. The lowest leachability was found in the easily migratory micro-aggregates, which should be taken into account in the future environmental risk assessment and soil remediation.

Highlights

► Particle size fractions associated heavy metals in arable soils was characterized. ► Several factors were calculated to assess the distribution of heavy metals. ► The leachability of heavy metals in different particles was analyzed. ► The results will be helpful in the future risk assessment and soil remediation.

Introduction

Heavy metals in soil have attracted extensive attention worldwide since most of them are harmful to the crop quality and pose a potential threat to human health through the food chain (Huang and Jin, 2008, Quinton and Catt, 2007). In China, the harsh fact is that soil pollution by heavy metal is getting worse with the rapid urbanization for decades (Wei and Yang, 2010). Understanding the distribution characters of heavy metals in soil is the premise of risk assessment and soil remediation. Numerous studies have been conducted to determine heavy metals in both bulk soils and different particle size fractions. However, most previous studies are focused on urban soil, garden soil, dust and sediment (Acosta et al., 2009, Acosta et al., 2011, Ajmone-Marsan et al., 2008, Beamer et al., 2012, Sutherland, 2003) and only few data are available about the distribution of heavy metals in various particle size fractions from arable soil (Qian et al., 1996, Quenea et al., 2009, Zhang et al., 2003).

The mobility and biological effectiveness of soil heavy metals have a strong correlation with the size and composition of particle fractions (Zhang et al., 2003). Various size fractions have different composition and properties, which affected the behavior of pollutants in the soil microenvironment (Acosta et al., 2011). Generally, fine particles have a higher ability to carry the heavy metals due to the increase of specific surface area, the presence of clay minerals, organic matter, and Fe/Mn/Al oxides in the micro-aggregate (Cai et al., 2002, Ljung et al., 2006, Semlali et al., 2001, Sutherland, 2003, Wang et al., 2006), which would protect the heavy metals from remedial attempts. Moreover, since the fine soil fractions are often preferentially transported to deep soil, surface/ground water and air, they are more harmful to the environments (Farenhorst and Bryan, 1995, Uusitalo et al., 2001). Therefore, exploring the partition in soil particle sizes was very important for the assessment of mobility and bioavailability of heavy metals in soil (Wang et al., 2006).

Hainan Island is located in the South China Sea and it is becoming an international travel insight according to the national developing plan. The industrialization caused heavy metal accumulation in arable soil (Hao et al., 2009, Li et al., 2009). The concentration of Cr in the suburban soil in Haikou was up to 586.70 mg kg 1 (Li et al., 2009). The rich precipitation and strong biological action in the study area could result in a rapid cycle of organic carbons in soil. The distribution character of heavy metals in particle size fractions is still unknown in arable soils in Hainan Island. Moreover, in order to predict both potential mobility and their bioavailability to plants, it is necessary to assess the leachability of heavy metals in different particles. Up to now, only few data about water-leaching risk of heavy metals in different particle size fractions have been reported (Magnuson et al., 2002, Zhang et al., 2003).

In order to assess the environmental risk and take applicable measures in the future, it is important to know in which scale particles the heavy metals are preferred to distribute. The objectives of this study are to: (1) assess the contamination levels of toxic Cr, Pb, As and Cd in arable soils in Hainan Island; (2) investigate the distribution patterns and leachability of heavy metals in different particle size fractions; and (3) discuss the factors effecting the residues of heavy metals in different particle size fractions.

Section snippets

Study site and sampling

Hainan Island (18°10′–20°10′N, 108°37′–111°03′E) is separated from the mainland by a narrow strait. It features a typical tropical marine climate with an annual temperature of 23–26 °C and precipitation of 1500 mm. The island occupies an area of 33,920 km2 and the major soil type is latosol (pH  5.00) with a complex terrain of plains and hills staggered.

Three composite surface arable soils (0–20 cm) were collected in 2011 from Haikou (19°44′N, 110°34′E), Qionghai (19°22′N, 110°36′E) and Tunchang

Soil properties

The soil properties and content of heavy metals in arable soils are shown in Table 1. The surface soils were characterized with a dominant fraction of fine aggregates of 53–250 μm (32.4–35.4%) and coarse aggregates of 250–1000 μm (31.4–38.0%). The smallest fraction (< 53 μm) was only 5.08% in Haikou, 5.12% in Qionghai and 9.57% in Tunchang. The particles > 4000 μm was less than 6.00%.

The contents of TOC in bulk soils were 24.0 g kg 1 in Haikou, which was similar with the mean level in northeastern

Conclusions

The arable soils in Hainan Island have been polluted by Cr, As, Cd and Pb. The distribution of most heavy metals in different size particles was increased with decreasing particle size. The content of organic carbons as well as Fe played an important role in the distribution of Cr, Cd and Pb, while As had a different pattern. The mass loading of heavy metals in micro-aggregates was high while their leachability was very low, which should be paid attention during the environment risk assessment

Acknowledgments

We are grateful to people who helped with sampling. Thanks also to the financial supports from the Ministry of Land and Resources of PRC Foundation (1212011087123, 200911020) and the National Natural Science Foundation of China (11075171; 10905061).

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