Characteristics and metal leachability of natural contaminated soil under acid rain scenarios

Contamination of soil with heavy metals has become a worldwide environmental problem, and receives great attention. In this study, we aim to investigate soil pollution level affected by an industrial district nearby. The total amount of typical heavy metals in the soils (Hengyang Songmu Industrial Park, Hunan Province, China) was analyzed. In addition, the fraction analysis and laboratory simulation leaching via different pH rainwater was carried out to study the migration and transformation of heavy metals. The main results show that the contents of Cu, Zn, Pb, Cr and Cd in the samples were higher than the soil background values in Hunan Province. The heavy metals forms, analyzed by sequential extraction method, show that the proportion of the unstable form of Cd, Zn and Pb was more than 50%. Igeo values indicate that the heavy metal pollution degree of soil sample #5 at the investigated area is recorded in the order of Cd(6.42), Zn(2.28), Cu(1.82), Pb(1.63), and Cr(0.37). Cu, Zn, Pb, Cr and Cd in this area could pose a potential leaching risk to the environment which may affect the food chain and constitute a threat to human health. It would be necessary to take steps to stabilize and monitor the heavy metals in soil. 92 W. Tan, Y. Li, L. Ding, Y. Wang, J. Li, Q. Deng, F. Guo, X. Xiao the Department of Defense sites, and 55% of the Department of Energy sites. In Europe, heavy metal contaminated soils encompass several million hectares, accounting for about 34.8% of the total contaminated soil (Panagos et al. 2013). Acid rain may enhance the release of heavy metal from soils due to the cation exchange in soil with major cations (e.g., H+, Ca2+, and Mg2+) accompanied by acid deposition (Huang et al. 2005, Wen et al. 2013). With the extension of leaching time in acid rain, metals were leached, hence contaminating the groundwater and deteriorating the terrestrial and aquatic ecosystems (Ding et al. 2011). Previous studies primarily focused on the assessment of soil environment pollution with heavy metals content by means of chemical analysis, or analysis of heavy metals’ behaviors from different carriers, including heavy metal adsorption by different types of materials. However, chemical monitoring alone does not always reveal the real threat connected with the presence of heavy metals in the soil environment, and the effects of acid rain on the mobility and potential risks of heavy metals based on naturally contaminated soil around IP still need further study (Baran et al. 2015). In this study, the overall novel objective lies in the following: i) batch column experiments conducted under simulated acid rain scenarios to evaluate desorption and potential risks of widely concerned heavy metals in naturally contaminated soil, and ii) using heavy metals digestion methods combined with geochemical indicators analysis, and fraction analysis methods combined with leaching experiments to reflect a more believable pollution situation of the heavy metals in naturally contaminated soil. The experimental results obtained highlight the importance of preliminary laboratory tests in evaluating the infl uence of such basic parameters as the types of contaminants, soil composition and leaching agents on the effi ciency of soil washing, which can provide basis references and scientifi c evidence for regulators’ decisionmaking. Materials and methods Soil collection We aimed to investigate soil pollution level affected by an industrial district (Fig. 1) in Hengyang, China. Consequently, a total of eleven points were evenly distributed and selected as sampling sites surrounding the industrial concentration area. Soil samples were collected with a probe at about 1–2 m depth after shoveling out the topsoil to avoid the external interference. Three soil samples were obtained at each sampling site, which were air-dried, ground to go through a 2 mm nylon sieve, and homogenized, before preparing the soil column (Li et al. 2004). Sub-samples were further ground with an agate grinder to pass through a 0.15 mm nylon sieve, which were then used to determine selected heavy metals. The sampling area has a typical subtropical monsoonal climate, characterized by a mean annual temperature of 23.2°C to 23.7°C and a mean annual rainfall of 1300–1800 mm (Li et al. 2009). Fig. 1. Diagram of soil sampling locations in Hengyang Songmu Industrial Park, China Characteristics and metal leachability of natural contaminated soil under acid rain scenarios 93 Preparation of simulated rain Simulated acid rain (SAR) was prepared to reflect the characteristics of real acid rain in southern China (Huang et al. 2009). At the sampling sites, SO4 2− and NO3 − in the acid rain were 50–26 and 38–39 μmol/L, respectively, while the pH of rainfall varied from 4.18 to 7.43 (Chen et al. 2014). SAR was designed according to main ion composition and pH of the local rain water. Synthetic acid rain with the pH of 4.1, 5.6, and 7.0 was prepared from a stock H2SO4 – HNO3 solution mixture (1:1.3, v/v). The concentrations of K+, Na+, Ca2+, Mg2+, NH4 +, Cl−, SO4 2−, NO3 −, and F− were 6, 30, 29, 5, 41, 30, 38, 38 and 3 μmol/L, respectively. Leaching experiment Batch column experiments were conducted to evaluate desorption and potential risks of widely concerned heavy metals in polluted soil around IP under SAR with a range of pH values. Column experiments were performed using PVC cylinders (ø100 mm × 1000 mm), and three parallel leaching columns were made for each experiment which were showed in Fig. 2. Glass beads (ø6 mm) and quartz sand (ø1.0–2.0 mm) approximately 5 cm and 10 cm high, respectively, were packed in columns and supported on a Teflon filter (pore diameter of 5 mm). Then homogenized contaminated soil (35 cm high) was packed in after being covered with approximately 10 cm of quartz sand (ø1.0–2.0 mm) and 5 cm of glass beads (ø6 mm), respectively. Each functional layer was isolated using 200 mesh nylon insulation. All materials that fi lled in columns were subjected to high-temperature sterilization in advance. The injection solution was delivered by a high-level water tank at a flow rate of 0~60 mL/h (Darcy flux of 12.2 cm/h) controlled by accommodating the valve. The column was fl ushed with SAR solution. Three parallel experiments were designed to make clear the relationship of all the factors. Data collection and chemical analysis The total contents of heavy metals (Cd, Pb, Cu, Zn and Cr) in soil were digested (0.5 g dry sample) with a mixture of concentrated 3 mL nitric acid, 3 mL hydrofluoric acid, and 6 mL hyperchloric acid in a polyvinyl fluoride crucible via microwave digestion (CEM MARS 5, Matthews, USA). After digestion, the sample solution was allowed to air-cool and then diluted with deionized water, followed by graphite furnace atomic absorption spectrophotometry (GFAAS-AA800, PerkinElmer Inc.) for Cd and Pb and flame atomic absorption spectrophotometry (Hitachi Z-5000) for Cu and Zn, respectively. The concentration of Cr(VI) in aqueous solution was analyzed with UV/Visible Spectrophotometer at 540 nm (Shimadzu UV-1240). The chemical partitioning of heavy metals in soils was performed using the modified four-step BCR procedure as previously described (Li et al. 2010). According to the sequential extraction method (Rauret et al. 1999), four operationally defined chemical forms of metals were isolated as HOAc extractable, reducible, oxidizable, and residual fractions. The metal content of extracts was analyzed as described previously. The cumulative sum of four fractions was compared with the total concentration to check the recovery, and the recovery values were found satisfactory. Leachate Fig. 2. Schematic diagram of the leaching experimental setup Table 1. Classification of geoaccumulation index Igeo Class Sediment quality ≤0 0 Absence of contamination 0–1 1 From absent to moderately contaminated 1–2 2 Moderately contaminated 2–3 3 From moderate to heavily contaminated 3–4 4 Heavily contaminated 4–5 5 From heavily to extremely contaminated ≥5 6 Extremely contaminated samples were collected at every 24 hour intervals from the bottom of columns. The total organic carbon (TOC) of soil was tested with TOC 5000A Organic Carbon Instrument. Each sample of the soil material was analyzed in three duplicates to determine the mean value and standard deviation (≤±5%). Statistical analyses were performed using SigmaPlot (v. 13.0 for Windows) and Origin 8.5 (v8.5.1 SR2). Geochemical indicators The Geoaccumulation Index (Igeo) developed initially by Muller (1979), is given by the following equation: