Sequential fractionation and plant uptake of As, Cu, and Zn in a contaminated riparian wetland

https://doi.org/10.1016/j.envpol.2020.115734Get rights and content

Highlights

  • Examined seasonal changes, fractionation and pollution risk of metals in sediment.

  • The riparian wetland sediment was highly-polluted with As, Cu, and Zn.

  • 62–94% of As, Cu, and Zn in the sediment was in the residual fraction.

  • Reed plants were inefficient in taking up As, Cu, and Zn from the sediment.

  • Sediment was highly-polluted, but with low metal availability and plant uptake.

Abstract

Sediment serves as a sink for metals, thus it is critical to assess its contamination and associated risk. A typical riparian wetland close to a Zn-smelting operation in karst areas in southwest China was investigated. Sediment and reed plant (Phragmites australis) samples from wet and dry seasons were analyzed for total As, Cu, and Zn concentrations. Metal pollution in the sediment was assessed based on geoaccumulation index (Igeo). Further, metals in the sediment were fractionated into exchangeable, water and acid-soluble, reducible, oxidizable, and residual fractions based on the BCR sequential extraction. The results showed that the As, Cu, and Zn concentrations in the sediment were significantly higher than the background values (740–4081, 96–228, and 869–3331 vs. 10, 22, and 70 mg kg−1). With the Igeo being 10–17, the data indicate that the sediment was highly-polluted. While total As, Cu and Zn in the sediment increased from dry to wet season, their available concentrations decreased except Cu. With 62–94% of As, Cu, and Zn being in the residual fraction, metal availability in the sediment was low based on fractionation data. The data are consistent with low metal uptake by reed as their concentration ratios in plant roots to the sediment were 0.01–0.32. The results suggest that the riparian sediment was highly-polluted with As, Cu and Zn, but showing low metal availability and limited plant uptake.

Introduction

Riparian wetlands are located in river flood plains, which provide transition zones from the aquatic to the terrestrial ecosystem. Wetland sediment often accumulates metals via adsorption and sedimentation processes (Du Laing et al., 2009), thereby helping to protect water quality. There are increasing concerns on pollution in wetland sediments; however, compared to terrestrial soils, fewer studies are available, especially for riparian wetlands (Romic et al., 2012; Bai et al., 2016).

Anthropogenic activities including mining, smelting and wastewater irrigation have all contributed to metal accumulation in riparian wetlands (Anapgc et al., 2010; Fatih et al., 2015). Wetland sediment acts as both sources and sinks of metals, which plays an important role in contaminant remobilization in aquatic ecosystems and varies with seasons, thereby impacting sediment metal concentrations and plant uptake (Shaheen et al., 2019). Understanding the potential mobilization of metals in sediments is of importance for a more accurate risk assessment and reliable evaluation of restoration processes, particularly under different conditions. Total metal concentration is a poor indicator for sediment contamination, as it cannot provide information about the mobilization, bioavailability, and the associated risk (Wang et al., 2018). Compared to total concentration, available metal concentrations better reflect their potential mobilization, environmental behaviors and plant uptake (Degryse et al., 2009). Therefore, different geochemical fractions of metals have been used to evaluate their mobilization, potential risk, and plant uptake, which is important in determining their geochemically-reactive pools (Matong et al., 2016). To analyze metal fractionation, BCR sequential extraction has been widely used for metals in different matrix, which separates them into four fractions (exchangeable, water and acid-soluble F1, reducible F2, oxidizable F3, and residual F4), with F1 being most available and F4 being least available to plants (Sutherland, 2010; Sundaray et al., 2011).

Since aquatic plants are efficient in taking up metals from sediments, they have been used in ecological restoration of metal-contaminated wetlands (Sharma et al., 2015). Among plants, large perennial grass reed (P. australis) has attracted increasing attention, partially due to its strong adaptability, large biomass and long growth cycle (Bonanno, 2011). It has been used in wetlands for treating wastewater (Lesage et al., 2007; Vymazal et al., 2007). So far, reed plants show high tolerance to metalloid arsenic (As) and can accumulate metals like Zn (Fatih et al., 2007; Bonanno and Giudice, 2010).

With its upstream runs through a Zn-smelting operation, Xiaobai River in southwest China was polluted with metals including As, Cu and Zn, posing a potential health risk (Wang et al., 2018). Studies showed that the As concentrations in the river water were up to 0.14 mg L−1, higher than Chinese surface water standard at 0.10 mg L−1 (SEPA, 2002). To control its metal pollution, an ecological restoration wetland (5794 m2) was established in 2012, with aquatic plant reed being planted. With continued plant litter decomposition, wetland sediment is rich in organic matter, helping metal retention and reducing their transport to river systems (Komiyama et al., 2008). Besides, metals in sediments may be remobilized and changed under certain conditions, thus there is a threat to water quality and health risk to humans (Yan et al., 2016). Therefore, in addition to determine metalfractionation, it is necessary to investigate their variation under different conditions (dry and wet seasons), which helps to better understand their behaviors and the associated risks.

Though studies have been conducted on metal pollution in wetlands (Yu et al., 2007; Ding et al., 2009), limited information is available on riparian wetlands. This study focused on a metal-polluted riparian wetland in karst areas of southwest China, which has been ecologically-restored by growing reed plants for 3 years. The main objectives were to: 1) determine total concentrations of As, Cu, and Zn in the sediment and read plant samples during dry and wet seasons; 2) assess metalcontamination in the sediment using geoaccumulation index (Igeo); and 3) determine metal availability via As, Cu and Zn fractionation in the sediment using the BCR method. The results help to better understand metal accumulation in the sediment and plants, thereby helping to develop more efficient restoration plan for metal-contaminated wetlands.

Section snippets

Study area and sample collection

The riparian river (104°32′–104°33′ E, 22°53′–22°55′ N) is located in a typical karst area in southwest China (Fig. 1). As an international river, Xiaobai River originates from Maanshan reservoir, flows through a Zn-smelting operation and is adjacent to Vietnam. It has a warm-temperate and subtropical monsoon climate, with 1345 mm annual precipitation, 1414 mm annual evaporation, and 16.9 °C mean temperature. The study area is dry during November to April (256 mm precipitation) and wet during

Concentrations and pollution of As, Cu, and Zn in the riparian sediment

The total As, Cu, and Zn concentrations in the sediment were 740–4081, 96–228, and 869–3331 mg kg−1 (Table 1), much higher than the background values of river sediments at 10, 22, and 70 mg kg−1 (Li et al., 2018). Besides, their concentrations were higher than those in the sediment of an estuarine wetland in Hainan Island, China at 10, 15 and 58 mg kg−1, with Cu and Zn concentrations being higher than those in Pearl River estuary sediments at 11–65 and 64–200 mg kg−1 in China (Qiu et al., 2011

Conclusions

The pollution characteristic of a contaminated riparian wetland in southwest China was investigated. Metal enrichment in the sediment and potential pollution risk was analyzed with geoaccumulation index (Igeo). Results show significant enrichment and high-pollution (Igeo>5), with As, Cu, and Zn concentrations being significantly higher than sediment background values (740–4081, 96–228 and 869–3331 vs. 10, 22, and 70 mg kg−1). Most As, Cu, and Zn in the sediment was in the residual fraction

Author statement

Huijuan Zhang: Conceptualization, Methodology, Formal analysis, Investigation. Qi Wang: Formal analysis, Investigation. Qijing Xu: Methodology, Formal analysis. Wumei Xu: Methodology, Investigation. Silin Yang: Methodology. Xue Liu: Writing–original draft; Writing–reviewing and editing. Lena Q. Ma: Supervision, Writing–reviewing and editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported in part by National Key Research and Development Program (2018YFC1800504), National Natural Science Foundation of China (41867066, 41907129, 41967023), Yunnan Basic Research Program Foundation (2019FB032), Yunnan Provincial Innovation Team Project (YNQR-CXTD-2018-006), Yunnan Provincial Key Lab Project (2019DG013), Yunnan Education Department Science Research Foundation, Scientific Research Foundation of Southwest Forestry University, and Scientific Research Foundation

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