Mercury isotope compositions in large anthropogenically impacted Pearl River, South China
Introduction
Mercury (Hg) is a globally distributed toxic heavy metal. It can be transported globally in the atmosphere and deposited via dry or wet deposition (Driscoll et al., 2013; Selin, 2009). The Hg deposited in the aquatic ecosystem can be converted into neurotoxic methylmercury (MeHg), a more toxic Hg form than inorganic Hg, by microbial processes. The MeHg is readily bio-accumulated by low trophic level organisms and further bio-magnified during trophic transfer in aquatic food webs, posing potential threats to the aquatic biota and human health (Beckers and Rinklebe, 2017; Driscoll et al., 2013; Leopold et al., 2010). Therefore, understanding the Hg cycle in aquatic system become necessary to further constrain their impact on the MeHg formation. Rivers are important freshwater systems as they enable the transport of large amount of materials derived from both natural and anthropogenic sources to the ocean (Amos et al., 2014; Emmerton et al., 2013; Zhen et al., 2016). There is an increasing number of studies on Hg concentration and speciation in rivers (Baptista-Salazar et al., 2017; Carroll et al., 2000; Guo et al., 2008; Hissler and Probst, 2006; Wang et al., 2004), but the sources and processes affecting Hg in the river especially large rivers remain poorly unraveled emphasizing the need to constrain them to better understand the Hg global cycle.
Mercury isotope compositions play a critical role in tracing the sources and processes of Hg in the environment. Previous studies have reported mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes in atmospheric precipitation (Chen et al., 2012a, Foucher et al., 2013, Sherman et al., 2010, Wang et al., 2015, Yuan et al., 2015, Yuan et al., 2018), lake water (Chen et al., 2016), aquatic organisms (Blum et al., 2014; Donovan et al., 2016), glacier (Zdanowicz et al., 2016), seawater (Štrok et al., 2015), dissolved and suspended loads and sediments of contaminated rivers (Baptista-Salazar et al., 2018; Blum et al., 2014; Demers et al., 2018; Donovan et al., 2016; Foucher et al., 2013; Washburn et al., 2017, 2018). Nearly all physical, chemical and biological processes cause MDF (represented by δ202Hg, see details in method) (Blum et al., 2014), but MIF is only caused by a small number of processes. MIF of odd-mass isotopes (odd-MIF) can be caused by magnetic isotope effect (MIE) and nuclear volume effect (NVE) (Bergquist and Blum, 2007; Cai and Chen, 2016), MIE is mainly observed in photochemical reduction of Hg(II) and photo-demethylation of MeHg, whereas NVE is found in some non-photochemical processes such as abiotic dark reduction and liquid-vapor evaporation (Bergquist and Blum, 2007; Estrade et al., 2009; Zheng and Hintelmann, 2009, 2010a, 2010b). Interestingly, the mechanisms responsible for MIF of even-mass isotope (even-MIF) reported in many atmosphere-derived samples remain unknown (Chen et al., 2012a; Sherman et al., 2010; Wang et al., 2015; Yuan et al., 2015, 2018), although some studies have suggested it might be related to photochemical oxidation of elemental Hg(0) in the tropopause (Cai and Chen, 2016; Chen et al., 2012a). Despite these uncertainties, Hg isotopes remain a powerful tool for tracing both the sources and redox pathways of Hg in rivers. Due to the very low Hg concentration (possibly less than < 1 ng/L) in natural water which was a great challenge to the routine Hg isotope analysis, previous studies primarily focused on the sources and reactions affecting Hg in contaminated rivers using the isotope approach. Understanding Hg isotopes in large rivers may provide critical insight to Hg cycling in rivers and potentially the contribution of Hg from rivers to global ocean ecosystems.
In this study, we report the Hg isotope compositions of dissolved load in surface water, spring water, groundwater, rainwater, wastewater, rock, leaf litter, soil and PM2.5 from the whole Pearl River Basin (PRB), the second largest river in China with a length of 2240 km. Located in a subtropical zone, the Pearl River originates from the Maxiong Mountain of Qujing City in Yunnan Province and flows through seven provinces in southern China and ends into the South China Sea. It has an average annual precipitation of 1470 mm and an average annual runoff of 3.2 × 1011 m3, with a drainage basin estimated to be 4.5 × 105 km2 (Liu et al., 2017a; Zhen et al., 2016). As an important water resource for industrial and domestic uses in Pearl River Delta (PRD), the most densely industrialized and urbanized region in China, the Pearl River is seriously polluted by industrial activities and domestic sewage (Liu et al., 2011, 2012; Zhen et al., 2016). Despite numerous studies have been conducted for Hg content, speciation and bioavailability in various materials from the PRD, such as sediments (Liu et al., 2011; Shao et al., 2011; Shi et al., 2010), soils and vegetables (Chen et al., 2012b; Shao et al., 2013), aquatic organisms (Shao et al., 2011, 2013) and river water (Liu et al., 2012; Zhen et al., 2016), the sources and processes affecting Hg in the Pearl River remain poorly understood. Because solar radiation, rainfall, water discharge and hydrodynamic condition would be obviously different at various flowrate stage and impact to different extent the sources and processes of Hg in rivers, samples collected both at flood and low water stages are investigated. Thus, the goals of this study are 1) to use Hg isotope compositions to identify the source of dissolved Hg in the Pearl River and its tributaries rivers 2) to determine the possibly key transformation processes that affect Hg transportation in the Pearl River.
Section snippets
Reagents and materials
Millipore-Q water (18.2 MΩ cm) and analytical grade reagents (HCl, HNO3, H2SO4, NH2OH·HCl, SnCl2, KBr, KBrO3, L-cysteine) from both Sinopharm Chemical Reagent Co., Ltd (China) and Sigma-Aldrich (USA) were used throughout the experiment. The Hg standard solutions NIST SRM 3133 and NIST SRM 3177 were used as Hg isotope reference materials. The thallium solution NIST SRM 997 was used for instrumental mass bias correction (Blum and Bergquist, 2007; Chen et al., 2010). Most of the vessels were
Results
The Hg concentrations and isotope compositions of all dissolved load in surface river water, natural and anthropogenic samples in PRB are listed in Tables S1, S2, S3 along with other parameters (T, pH, DOC, Cl−, TSS and monthly discharge).
Effect of tributaries on mainstream Hg budget
As shown in Fig. 2, dissolved Hg concentrations (Hgd) and Hg isotope compositions of the mainstream could be influenced by the input of tributaries. The effect of Hg isotope compositions from the tributaries input on the mainstream (predominantly via the flux of dissolved Hg) is estimated following:where Mi and Mi-1 are the ith and i-1st
Conclusions and implications
This study reports for the first time Hg isotope compositions in the Pearl River, a highly anthropogenic-influenced large river. All dissolved load measured during flood and low water periods are characterized by negative δ202Hg, slightly positive Δ200Hg, and mostly positive Δ199Hg, which are similar to the Hg isotope compositions reported in the local atmospheric precipitation. The isotope compositions suggest that atmospheric precipitation is the major Hg source to surface water. Furthermore,
CRediT authorship contribution statement
Yuanyuan Zhang: Conceptualization, Methodology, Software, Validation, Investigation, Writing - original draft. Jiubin Chen: Conceptualization, Resources, Data curation, Writing - review & editing, Supervision, Project administration, Funding acquisition. Wang Zheng: Writing - review & editing. Ruoyu Sun: Writing - review & editing. Shengliu Yuan: Software, Validation, Formal analysis, Investigation, Visualization. Hongming Cai: Validation, Formal analysis, Investigation, Visualization. David Au
Acknowledgements
This research was financially supported by National Natural Science Foundation of China (41561134017, 41625012, U1612442, 41830647, U1301231), “ten thousand talent” project of Ministry of Science and Technology of the People's Republic of China, “hundred talent” project of Guizhou Science and Technology Department.
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