Dynamic release and transformation of metallic copper colloids in flooded paddy soil: Role of soil reducible sulfate and temperature
Graphical abstract
Introduction
Paddy soils are usually polluted by multiple heavy metals, such as Cu, Cd, Pb, and As, that cause great environmental concern about food safety and human health worldwide (Ma et al., 2008). Periodic flooding can significantly affect the bioavailability and mobility of trace metals by restricting O2 diffusion into the soil. Some investigations reported the formation of metal colloids in the flooding condition which maintained a high mobility (Xia et al., 2018). Moreover, mobile natural colloids can serve as carriers for metal contaminants and assist their transport in the environment (Won and Burns, 2018; Wang et al., 2013). Therefore, for a proper risk assessment, it requires an in-depth understanding of the formation and fate of mobile natural metal colloids in flooded soils.
The transition from aerobic to anaerobic conditions facilitates the formation and stability of natural colloids. Metal ions can be bound to natural organic matter (NOM) and be present as colloidal species. Liao et al. (2017) found that NOM-Fe colloids are formed at the anoxic-oxic interfaces by means of the complexation and coagulation. Meanwhile, soil microorganisms are considered to play important roles in the release of colloids or nanoparticles by mediating various biochemical reactions under anoxic conditions. A previous study reported that endomycorrhizal fungi assisted wetland plants to transform Cu(II) to Cu(0) nanoparticles at the soil-root interface (Manceau et al., 2008), while bacteria dispersed in the porewater induced the formation of Cu(0) colloids via biomineralization under flooded conditions (Weber et al., 2009a).
In addition to metallic colloids, various investigations also provided sufficient evidences for the release of metal sulfide colloids in flooded soils since microbial sulfate reduction can significantly influence the transformation of metal species in the environment (Weber et al., 2009a). The occurrence and extent of metal sulfidation is highly dependent on the amount of sulfate available for microbial reduction in the soil. Cu(0) and Cu(I)-Sorg were the predominant species at sulfate depleted conditions over the flooding period of soil, whereas in soils containing enhanced sulfate levels, Cu(0) was transformed into Cu-sulfide upon sulfate reduction (Fulda et al., 2013a). When the concentration of reducible sulfate is limited, different metal contaminants may compete with each other for the reaction with biogenic sulfide. Weber et al. (2009b) reported that Cu, Cd, and Pb were easily sequestered in metal sulfide phases, whereas Zn, Ni, and Fe were hardly. Despite the numerous studies on the dynamics of metal contaminants in their dissolved and solid state in paddy soils with various sulfate contents, there is barely any information available about the effect of sulfate availability on formation and behavior of metal colloids in porewater under flooded conditions.
The chemical speciation and fate of heavy metals in soil are dependent on a series of biogeochemical processes that are mainly mediated by soil microorganisms (Chen et al., 2019). Differences in temperature as caused by seasonal variations or climate change no doubt affect such biogeochemical reactions, thus influencing the dynamics of trace metals in paddy soil. High temperature accelerated and enhanced the microbial reductive dissolution of Fe(III) and Mn(III, IV) (oxyhydr)oxides, which could further promote the release of dissolved and colloidal trace metal in the flooded soil (Hofacker et al., 2013b). The rate and extent of microbial sulfate reduction were reported to increase upon increasing temperature in rice paddy soil (Van Bodegom and Stams, 1999). In addition, temperature could also affect the abiotic processes in the environment. For example, the reaction rate increases by 1.5–3 times for every 10 °C increase in temperature (Brezonik, 1994). A change in temperature leads to numerous reactions that make the dynamics of metal contaminants in the environment elusive. On the one hand, reductive dissolution promotes the release of dissolved and colloidal metals, originally adsorbed onto minerals, into porewater. On the other hand, the reduction of large amounts of sulfate could in turn sequester mobile heavy metals into metal sulfide precipitation. Till now, the effects of temperature on the metal colloids dynamics in the flooded soils and associated mechanisms remain unclear.
With the hypothesis that sulfate and temperature can promote the natural colloids release and transformation, this study systematically investigated the impact of sulfate availability and temperature on the release, stability and transformation of copper colloids in flooded paddy soil. Special attention was paid to (1) monitor the dynamic release of Cu colloids as well as other chalcophile metals, including Cd, Pb, and Zn, in the porewater during soil flooding to analyze the competition effect between these chalcophile metals; (2) measure the change of colloid particle sizes using dynamic light scattering (DLS) to evaluate their stability upon increasing soil reduction; (3) determine the speciation transformation of copper colloids at different sulfate content or temperature using X-ray absorption spectroscopy. The findings of this study will expand our understanding of the formation and fate of metallic Cu and metal sulfide colloids in flooded paddy soil, thus assisting the evaluation of the risk of trace metals for rice food safety.
Section snippets
Soil and synthetic river water material
A large amount of topsoil (0–10 cm depth) was collected from a rice paddy field in the northern Guangdong Province, South China (Dabaoshan, Shaoguan, China) (24°31′37″N; 113°42′49″E). The soil was dried and gently crushed with a pestle and sieved to a soil aggregate size of <2 mm. The basic physical and chemical properties of the soil were characterized and they are listed in Table S1. Synthetic river water was prepared. It was composed of 1.0 mM CaSO4, 0.5 mM Mg(NO3)2, and 1.9 mM NaCl,
Dynamic release of metal colloids affected by initial sulfate content
The changes in porewater pH and dissolved concentration of sulfate in high sulfate (HS), medium sulfate (MS) and low sulfate (LS) treatments during soil flooding are shown in Fig. 1. Porewater pH of the three sulfate treatments showed a similar trend of an initial fast increase (day1−15) followed by a slower increase (day 15–30). HS treatment induced a lower pH (approximately 4.4) but a faster rate of pH increase, causing the pH value of the three treatments to be similar from day 5 until the
Conclusions
The present study revealed that reducible sulfate availability and temperature affected the formation and fate of Cu colloids thus controlling the metal dynamics and mobility in flooded paddy soils. The amount of sulfate limited the formation of sulfide during the flooding, reduced the release of metal colloids, and constrained the transformation of colloids into metal sulfide species. The metal sequestration was primarily consistent with the predicted thermodynamically sulfate ladder, which
Author contribution statement
HQ, BX, and EH conceived the idea and designed the experiment; BX and HX performed the experiment; HX, HQ, and EH wrote the manuscript; RQ, WP, LZ, XX, and XC revised the manuscript; HQ and EH raised the funding.
Declaration of Competing Interest
There is no competing interest to declare.
Acknowledgements
This study was sponsored by the National Key R&D Program of China (No. 2018YFC1800600, No. 2018YFD0800700), the National Natural Science Foundation of China (No. 41877500, No. 41701571, No. 41701573, and No. 41977115), the Research Fund Program of Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (No. 2018K01), and Shanghai Rising-Star Program (No. 20QA1404500).
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