Chemical fractionation of heavy metals and zinc isotope source identification in sediments of the Huangpu River, Shanghai, China

Background The Huangpu River serves as a vital water source for around 24 million individuals residing in the metropolitan area of Shanghai. Despite this, elevated levels of heavy metals persist in the sediments of the river, with their chemical fractionation and sources remaining inadequately understood. Results To improve the management of heavy metal contamination, sequential extractions and zinc (Zn) isotopic compositions were utilized to evaluate pollution levels in the Huangpu River. The findings reveal that the majority of heavy metals in the river sediments are present in residual fractions, constituting an average of 67.5% for Cd, 57.6% for Cu, 60.6% for Ni, 56.2% for Pb, and 74.4% for Cr, with the exception of Zn (33.8%). Furthermore, a substantial portion of Zn, exceeding 66%, was found in acid-exchangeable, reducible, and oxidizable fractions, indicating a high potential for Zn release into aquatic ecosystems. Conclusion Further analysis of Zn isotopes pinpointed traffic emissions, including exhaust fumes and tire wear particles (account for ~ 34.0%), along with anthropogenic emissions and fertilizer (~ 31.7%), as the major culprits behind this contamination. These findings highlight the critical need for stricter regulations to control heavy metal contamination from traffic and domestic sources within the Huangpu River basin.


Introduction
Heavy metal contamination is a growing environmental concern due to their toxicity in the environment [31,45].While essential for some aquatic organisms in low concentrations, these metals exhibit a dramatic increase in toxicity when exceeding a specific threshold [42].Resistant to degradation, they accumulate in the environment, posing a significant threat to environmental health as they reach harmful levels [47,52].Furthermore, some heavy metals have a strong tendency to be absorbed and accumulated in crops, posing a substantial risk to human health [25,40].Therefore, research focusing on the distribution patterns and tracing the sources of heavy metals is imperative for the prevention and management of environmental contamination.
Over the past few decades, the environment has faced escalating exposure to heavy metals as a result of rapid industrialization and urbanization [7,68].The Huangpu River, has long served as a vital source of drinking water for Shanghai, China [61].It traverses the urban expanse of Shanghai before merging with Changjiang and ultimately flowing into the East China Sea.Several studies have focused on heavy metals contamination along the Huangpu River, including the types, concentrations, and pollution assessments of heavy metals [4,12,34,54,65,71].Meanwhile, it has been demonstrated that the river sediments surrounding high density of factories showed environment risks in Huangpu River [4].Nevertheless, the chemical fractionation, sources of heavy metals and contribution of each sources are still not well understood, which may hinder the development of effective policies for controlling and treating heavy metal contamination.
Zinc (Zn) is ubiquitous, present in soils, plants, and biota, acting as both an essential and a toxic element depending on its concentration and fractionation in solution [13].Concerning human health, a surplus of zinc is associated with oxidative stress and is a contributing factor in many chronic diseases [56].Zn isotope systematics has gained significant attention in the last two decades due to the notable Zn isotope fractionation observed in various biogeochemical processes, including chemical weathering, ocean circulation, and plant physiological processes [32,64].In particular, stable Zn isotopic compositions have widely used to examine the sources of Zn in sediments, soils, and water [18,53].This research focused on investigating heavy metal contamination in Huangpu River, with two primary objectives: (1) to analyze the chemical fractionation and evaluate the contamination levels of heavy metals in the river, and (2) to determine the sources of Zn in river sediments by examining isotopic compositions.The authors believe that the results can contribute to a better understanding of heavy metal transport in river ecosystems and offer technical assistance for managing heavy metal pollution in Huangpu River.

Regional background
Shanghai is one of the most comprehensively industrial and commercial cities in China, which is surrounded by Jiangsu and Zhejiang Provinces in the north and west, and Hangzhou Bay in the east (Fig. 1a).It should be noticed that Shanghai locates mostly on the delta region of Changjiang and with the area of ~ 6340 km 2 .The elevation of Shanghai is low where the eastern is slightly higher than the western.The strata in Shanghai are mainly deposited in the Quaternary, except for several volcanic massifs, such as Sheshan, Xiaokunshan and Tianmashan [66].Meanwhile, the land use types in Shanghai are complex and can be divided into the industrial, agricultural, commercial and living areas (Fig. 1b).Huangpu River, which origins from Dianshan Lake with the length of ~ 113 km, is the most important and largest river flowing through Shanghai (Fig. 1).Meanwhile, Huangpu River had been the major drinking water source of Shanghai where several water supply points are in the upstream of river [4].However, its 83 km course through the densely populated Shanghai metropolitan area makes it vulnerable to pollution from various anthropogenic activities, including industrial and domestic wastewater discharge [62].Besides, Huangpu River is an important shipping artery in Shanghai, and hence the

Sample collection
Twelve bulk river sediment samples along the mainstream of Huangpu River were collected in 2017 (Fig. 1b).The river sediments were collected by an extendable grab bucket.The large stones/garbage in the sediments were removed and then the sediments were immediately transported to the laboratory of Shanghai Normal University and stored in an airtight cooler.

Chemical analysis Heavy metal fractionation analysis
The present study employed previously established methods [11,21,22] to extract and determine the chemical fractionation of heavy metals.The BCR (Community Bureau of Reference) sequential extraction procedure, as described by Wijaya et al. [59], was utilized and consisted of four operationally defined chemical fractions: (F1) Acid exchangeable fraction, which refers to easily soluble and exchangeable metals, (F2) Reducible fraction, which pertains to metals bound to manganese and iron oxides [49,70], (F3) Oxidizable fraction, which can be combined with organic matter or sulfide through chelation and complexation; and (F4) Residual fraction, which is embedded in the mineral lattice and is characterized by stable properties, weak biological activity, and weak ecological risk [14].The detailed geochemical fractionation procedure of sediment is presented in Supplementary Table S1.

Heavy metal concentration
The methods for heavy metal sediment pretreatment and concentration measurement were followed by [54].In summary, the sediment samples were subjected to grinding using an agate mortar and screened with a 100-mesh plastic screen.Subsequently, approximately 0.25 g of the sediment was weighed and placed in a Teflon microwave digestion tube.A series of concentrated acids, including 6 mL HNO 3 , 3 mL HCl, and 2 mL HF, were sequentially added to the tube.The samples were then subjected to a three-step temperature increase process and subsequently heated at 50 °C to remove excess acid [54].The data were acquired through the implementation of a tripartite experimental procedure, followed by the computation of the mean value.The relative standard deviation (RSD) of three replicate analyses was normally lower than 5%.The heavy metal concentrations were determined using inductively coupled plasma optical emission spectrometry (ICP-OES, icap 7400) at Shanghai Normal University.

Zn isotopic composition
Approximately 0.1 g of sediment was placed into a digestion tube and dissolved using concentrated HF and HNO 3 (1:1) at 140 °C for approximately 24 h.The resulting solution was evaporated on an electric heating plate at 140 °C and then treated with aqua regia (HCl:HNO 3 = 3:1).The crucible was heated at 80 °C for 1 h, followed by heating at 120 °C for 24 h.The samples were then evaporated at 50 °C and treated with 1 mL HCl at 120 °C until completely dissolved.The dissolved samples were transferred to a 7 mL beaker for further evaporation.Next, 1 mL 6 mol/L HCl and 0.001% H 2 O 2 were added and evaporated at 120 °C.Finally, Zn was separated and purified using anion exchange chromatography with AG MP-1 resin (100-200 mesh, Bio-Rad).The resin was loaded onto a column and eluted with 10 mL 8 mol/L HCl to remove matrix elements.Cu and Fe were then collected in the following 24 mL 8 mol/L HCl + 0.001% H 2 O 2 and 18 mL 2 mol/L HCl, respectively.Finally, Zn was collected in the subsequent 15 mL 0.5 mol/L HNO 3 .Zn isotopic compositions were measured by MC-ICP-MS (Neptune plus, Thermo Scientific) at Nanjing University.The Zn isotope was expressed in δ 66 Zn (‰) based on the following equation: δ 66 Zn = [( 66 Zn/ 64 Zn) sample / ( 66 Zn/ 64 Zn) standard -1] × 1000.The certified Zn standard is IRMM-3702 in present study [36].The SSB (standardsample bracketing) method was used to avoid the mass bias of the measurement [37].

Enrichment factor
Enrichment factor (EF) was used to determine the magnitude of the metal pollutions.Al, Fe, Li, Co, Sc, Ti and Cs are often used as reference elements [16].Ti was selected as the reference element in present study.EF of river sediments was calculated using Eq.1: where Me is the heavy metal measured, and Re is a reference element mainly combined in silicate minerals, which is geochemically conservative and not easy to chemically change in processes of the earth surface system.In this study, Ti was used as a reference element for EF calculation.(Me/Ti) Sample is the metal to Ti ratio for the analyzed sample; (Me/Ti) Background is the natural background value of metal to Ti ratio.The Ti concentration of the UCC (Upper Continental Crust) as the background value [44].

Geo-accumulation index
Another criterion to evaluate the heavy metal pollution is the geoaccumulation index (I geo ) proposed by Müller (Me/Re) background [38], by comparing current concentrations with preindustrial levels and can be calculated by the following equation: where C i is the measured concentration of the heavy metal; Bi is the geochemical background value of the metal.Factor 1.5 is the background matrix correction factor due to lithogenic effects [17].We adopted the UCC values in geoaccumulation index calculation [44].Seven classes of geoaccumulation index are distinguished to evaluate heavy metal pollution degrees (Table 1) [38].
The highest class reflects 100-times enrichment above the background values [17].

Quality control and assurance
The centrifuge tubes and conical tubes used for determining the total amount of heavy metals and conducting fractionation experiments were thoroughly cleaned with 50% high-grade pure HNO 3 .They were then soaked in 20% HNO 3 to remove insoluble impurities.Blank samples and parallel samples were included in the experiment.The content of heavy metals was measured using ICP-OES, with three repetitions and the average value were used.The detection limit for each heavy metal ranged from 0.001 mg/L to 0.004 mg/L (0.002, 0.002, 0.001, 0.001, 0.002 and 0.004 mg/L for Pb, Zn, Cu, Cr, Cd, and Co, respectively), with a relative standard deviation of less than 5%.For the determination of heavy metal fractionation, the recovery rate was tested using the formula: Recovery Rate = [(F1 (2) In this study, the recovery rate ranged from 80 to 120%.

Heavy metal composition in Huangpu River sediments
Heavy metal concentrations in Huangpu River sediments are presented in Table 2. Zn is the most abundant element among all of the heavy metals, with the highest and average concentration of 170.2 mg/kg and 127.3 mg/kg respectively, followed by Cr, Pb, Cu, and Co.The concentrations of heavy metals in sediments of Huangpu River exceed the soil background levels of Shanghai (Fig. 2a), as reported by Dai and Li [10].Specifically, the concentrations of Cr and Cd were found to be significantly higher than the background values.These findings suggest that heavy metal concentrations in the sediments of Huangpu River may reach contamination levels.A strong correlation was observed between Cr and other heavy metals (p ≤ 0.05), while Zn exhibited a notable correlation with Cr and Cu (p ≤ 0.05) (Fig. 3).Moreover, the majority of heavy metal concentrations in the downstream area of Huangpu River were higher compared to the midstream and upstream regions, with the exception of H12 located in the river estuary.Furthermore, certain heavy metals, including Cd, Pb, and Zn, exhibit significant enrichment at specific sampling sites along Huangpu River (Fig. 2b  and c).

Zn isotopic composition of Huangpu River sediments
Zn is an essential micronutrient for aquatic organisms, however, elevated levels can be toxic to both organisms and surrounding environment.The Zn concentration in sediments of Huangpu River is approximately two times higher than the background level in Shanghai, suggesting the presence of contamination.The concentrations of Zn in midstream and downstream are higher than upstream, except for site H12 situated at the river estuary (Fig. 4).Besides, Zn isotopic composition of Huangpu River varied by approximately 0.12‰, with δ 66 Zn varies from 0.25‰ to 0.37‰ (Fig. 4).In comparison, the Zn isotopic composition in Huangpu River sediment is lower than that of the East China Sea (average δ 66 Zn value of 0.43‰) as reported by Zhang et al. [72], but higher than that of the Pearl River (average δ 66 Zn value of 0.23‰) and the Erren River (average δ 66 Zn value of 0.18‰) in China as reported by Zeng and Han [69] and Tu et al. [55].

Chemical fractionation of heavy metals in Huangpu River sediments
The total concentration of heavy metals provides valuable information on their environmental dynamics (Hochella Jr et al., [24]).However, the different chemical fractionation of heavy metals can have varying effects on aquatic ecosystems [3], Kim et al., [29]).Heavy metal fractionation is primarily determined by sediment mineralogy and chemistry [23].Therefore, the BCR sequential extraction techniques were used to extract different chemical fractions to understand heavy metal fractionation in Huangpu River sediments [41,43].
Based on sequential extraction results, residual fractions are the predominant component for heavy metals in Huangpu River sediments, except for Zn.Residual fractions account for an average of 67.5%, 57.6%, 60.6%, 56.2%, and 74.4% for Cd, Cu, Ni, Pb, and Cr, respectively (Fig. 5a).This suggests that most heavy metals in the sediments are stable and not easily released into the aquatic environment.However, the non-residual fractions of Zn account for 66.2%, which means that Zn in the sediments can be easily released, potentially leading to significant environmental contamination if its concentration exceeds a certain threshold (Figs.2d and 5a).Zn contamination in rivers is a global issue, as seen in the Lot River in France [2], Wenrui Tang River in China [63], and Ganga River in India [39].In this study, the chemical fractionation analysis reveals that Zn in Huangpu River sediments is primarily composed of acid-exchangeable and reducible fractions, with levels exceeding 50% except at the H11 site (Fig. 5b).This implies that Zn in river sediments has the potential to be easily released, thereby posing a threat of environmental contamination in Huangpu River.

Relationship between heavy metal contamination and urbanization
As mentioned above, Zn in river sediments might be a significant heavy metal contamination in Huangpu River.Abundant previous studies have shown a positive correlation between heavy metal concentrations in soils and factors such as population density, traffic volume, and urbanization [1,4].It should be emphasized that Huangpu River flows through both urban (Minhang, Xuhui, Huangpu, Hongkou, and Yangpu) and rural (Qingpu, Songjiang, and Baoshan) areas of Shanghai (Fig. 1, [8]).As shown in Fig. 6a, the levels of zinc in Huangpu River sediments are significantly elevated in areas with higher urbanization, indicating that urbanization may play a significant role for the contamination of zinc in Shanghai.This finding aligns with previous studies by Ajmone-Marsan and Biasioli [1], Bai et al. [4] and Shi et al. [48], which observed higher heavy metal concentrations in urban areas compared to rural areas in Shanghai.
Figure 6b indicates that there is a positive correlation between population density and Zn concentrations in Huangpu River sediments.This suggests that urbanization, with its associated increase in municipal sewage and domestic waste, is a contributing factor to elevated Zn levels [50,67].Additionally, heavy metal contamination is influenced by traffic volume, as vehicular emissions contribute significantly to environmental pollution.Urban areas with high traffic volume exhibit increased Zn concentrations, as depicted in Fig. 6c.Moreover, Zn concentrations in the rural regions of Qingpu and Songjiang surpass the baseline levels of Shanghai, with agricultural practices in Qingpu and industrial processes in Songjiang being identified as the primary sources [4].These anthropogenic activities contribute to the release of Zn into the surrounding environment.Overall, heavy metal contamination in Shanghai is linked to urbanization, with potential sources including traffic emissions, and other sources like natural processes and urban wastewater.

Identification of the pollution sources with Zn isotopic composition
The isotopic composition is a valuable tool for studying the origins and processes controlling the cycling of Zn in river sediments and soils [9,51,53].Hence, Zn isotopic and elemental ratios were utilized to trace the origins of Zn contamination in Huangpu River.Meanwhile, the contribution rates of each endmembers were calculated by IsoSource (Version 1.3.1).This approach can offer crucial theoretical and technical assistance for the management and remediation of heavy metal contamination.
The Zn isotopic compositions versus Cu/Zn ratios of source endmembers and Huangpu River sediments were illustrated in Fig. 7.According to the findings of source apportionment analysis, in addition to natural and plant sources (~ 34.3%), the heavy metal contamination in Huangpu River sediments is predominantly attributed to traffic-related emissions, including vehicle tire wear and exhaust emissions, accounting for approximately 34.0% (Fig. 7a and b).Moreover, anthropogenic activities and fertilizer use may also play a significant role, contributing to around 31.7% of the contamination (Fig. 7a and  b).Shanghai is an international metropolis with high population density (over 24 million) and traffic volume in the urban areas [33,46].The high population density accompanied by a large amount of domestic sewage and wastewater, which would induce serious heavy metal contamination [15,58].Meanwhile, the city roads corresponded to high traffic volume were densely distributed in Shanghai, especially in urban areas.Hence, the primary sources of Zn contamination in Shanghai are the aerosol and particulate matter (PM) that originate from exhaust emissions and tire debris resulting from the frictional heat between the tire and paved road surface [30,35,60].By using Zn isotopic compositions, the traffic emission and the human activities can be identified as the main source of Zn in Huangpu River sediments.It further suggested that the government should pay more emphasis on traffic and anthropogenic emissions for heavy metal control.

Conclusions
This investigation aims to reveal the heavy metal contamination and trace the sources of contamination with the proxy of Zn isotopic composition in Huangpu River.The major conclusions are summarized as follows.Among the 12 collected samples, Zn concentrations in Huangpu River sediments are mostly higher than the soil background value of Shanghai.The heavy metals in Huangpu River sediments are mainly composited by residual fractions which account for 67.5%, 57.6%, 60.6%, 56.2%, and 74.4% in average for Cd, Cu, Ni, Pb, and Cr, respectively.However, the non-residual fractions of Zn are high (account for 66.2%), which suggests that Zn can be easily released from sediment into the aquatic environment and may further induce serious environmental contamination.Our results also indicated that the highly urbanized areas with dense population and high traffic volume have much higher Zn concentrations.Besides, caused by agricultural activities and industrial factories in Qingpu and Songjiang, the Zn concentrations of Qingpu and Songjiang are higher than the background value, even though they are countryside areas.Based on the Zn isotopic compositions and Cu/Zn ratios, the Zn contaminations in Huangpu River are mainly induced by traffic emissions and human activities: human activities include the anthropogenic emissions and fertilizer while traffic emissions indicate the aerosol and particulate matter (PM) originated from the exhaust emissions and the tire debris caused by the frictional heat between the tire and paved road surface.In other words, traffic emissions and human activities are the most important sources for Zn contamination in Shanghai.[20], Jeong [26], Jeong et al. [27], and Wang et al. [57]

Fig. 2 Fig. 3 Fig. 4
Fig.2The concentration of heavy metal from the upstream to downstream (a).Data of backgrounds are from Dai and Li[10].The EF, I geo and RAC of river sediments in Huangpu River (b, c and d)

Fig. 5
Fig. 5 Chemical fractionation of heavy metal for sediment in Huangpu River (a).Chemical fractionation of Zn from upstream to downstream (b)

Fig. 6 Fig. 7
Fig.6 The concentration variation of Zn between countryside and urban area (a).Zn concentration and population density of Shanghai (b).Zn concentration and traffic of Shanghai (c).Data of backgrounds are from Dai and Li[10], data of urbanization are from Cui and Shi[8]

Table 2
Concentration of heavy metals in Huangpu River sediment [10]unit is mg/kg.BV is background value.Background value of Shanghai are from Dai and Li[10]