Compound-Specific Isotope Analysis and Enantiomer Fractionation to Characterize the Transformation of Hexachlorocyclohexane Isomers in a Soil–Wheat Pot System

Abstract

The uptake by plants from soil is one of the first steps for hexachlorocyclohexane (HCH) isomers to enter the food web. However, the HCH transformation associated with the uptake process is still not well understood. Therefore, a soil−wheat pot experiment was conducted to characterize the HCH transformation during wheat growth using compound-specific isotope analysis (CSIA) and enantiomer fractionation. The results showed that the δ 13 C and δ 37 Cl values of β-HCH remained stable in soil and wheat, revealing no transformation. In contrast, an increase of δ 13 C and δ 37 Cl values of α-HCH indicated its transformation in soil and wheat. A shift of the enantiomer fraction (EF) (−) from 0.50 to 0.35 in soil at the jointing stage and 0.35 to 0.57 at the harvest stage suggested that the preferential transformation of enantiomers varied at different growth stages. Based on the dual element isotope analysis, the transformation mechanism in the soil−wheat system was different from that in wheat in hydroponic systems. The high abundance of HCH degraders, Sphingomonas sp. and Novosphingobium sp., was detected in the α-HCH-treated rhizosphere soil, supporting the potential for biotransformation. The application of CSIA and EF allows characterizing the transformation of organic pollutants such as HCHs in the complex soil−plant systems.

Soil from a Hexachlorocyclohexane Contaminated Field Site Inoculates Wheat in a Pot Experiment to Facilitate the Microbial Transformation of β-Hexachlorocyclohexane Examined by Compound-Specific Isotope Analysis

“…The BCFs observed in this study were much lower than those reported in a previous study. 16 This effect could be related to the possible transformation of β-HCH in the current study, which led to an only minor accumulation of β-HCH in the plant tissues.…”

“…Similar results were also found in a previous study. 16 The concentration of β-HCH in the soil of the unplanted control decreased during the whole wheat growth period from an initial concentration of 18 mg kg −1 to 9.3 ± 3.1 mg kg −1 at the jointing stage, to 9.7 ± 0.9 mg kg −1 at the heading stage, and to 7.8 ± 0.4 mg kg −1 at the harvest stage (Figure S2a). The loss of β-HCH in the unplanted control is most likely related to evaporation and possible transformation reactions in the soil.…”

“…In a previous experiment using garden soil and otherwise nearly identical conditions, no transformation of β-HCH was observed, indicating that the β-HCH degrading microbial community is already well developed in the contaminated field soil due to long exposure time to HCHs. 16 Additionally, compared to the results of the planted control, the increase of the δ 13 C values in the spiked treatment suggested that β-HCH degrading bacteria in the soil was stimulated, activated, and further developed by spiking the soil with a high concentration of β-HCH.…”

“…The carbon isotope fractionation caused by phase partitioning for chemicals, which has a similar physiochemical properties of HCH, was insignificant with a value of 0.3‰. 45,46 In addition, our previous garden soil pot and hydroponic experiments with β-HCH 16,17 showed that there are no isotopic changes of HCH observed during the volatilization, and compared to the fractionation caused by transformation, the possible fractionation caused by volatilization or phase partitioning could be ruled out to explain our observation. Therefore, the increase of δ 13 C values in the soil indicates that β-HCH was transformed in both the bulk and the rhizosphere soil.…”

“…34 Recently, CSIA was successfully applied to characterize the transformation of HCHs in garden soil− plant and hydroponic systems. 16,17 The investigation of the fate of organic pollutants in a complex system is always challenging since several degradation processes (e.g., aerobic and anaerobic degradation processes or biotic and abiotic degradation processes) are simultaneously active. 35,36 It is a challenge to identify the main process in such cases.…”