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J. Ocean Univ. China (Oceanic and Coastal Sea Research) DOI 10.1007/s11802-015-2772-0 ISSN 1672-5182, 2015 14 (3): 417-424 http://www.ouc.edu.cn/xbywb/ E-mail:[email protected] An Experimental Study on Major Element Release from the Sediments in the Changjiang (Yangtze River) Estuary GUO Yanwei*, and YANG Shouye State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, P. R. China (Received September 30, 2014; revised January 20, 2015; accepted March 2, 2015) © Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2015 Abstract With the enhanced warming and acidification of global ocean, whether and to what extent the naturally-weathered fluvial sediment into the sea can release elements and thus influence the geochemical process and ecosystem of global ocean remain to be resolved. In this contribution, an experimental study was carried out to examine the release rates of major elements (Ca, K, Mg and Al) from the surface sediments in the Changjiang (Yangtze River) Estuary under the pH values of 4.0, 6.0 and 7.0. The two studied sediments consist primarily of quartz, plagioclase, calcite and clay minerals, with the BET (Brunauer, Emmett and Teller) surface areas of 61.7 m2 g−1 and 23.1 m2 g−1. Major elements of Ca, K, Mg and Al show different release rates under different solution pH values. With the decreasing solution pH, the release rates of Ca and K increase obviously, while the release rates of Mg and Al increase with the initial solution pH varying from 6.0 to 7.0. The different release rates of these elements are closely related to the original mineral composition of the sediments and the reaction kinetics. Based on the experimental observation, quartz and clay minerals that have low dissolution rates may dominate the major element release to the aqueous phase. This study reveals that the enhancing ocean acidification could cause considerable release of major elements from natural terrigenous sediments into the ambient marine environment, which has to be considered carefully in the future study on global change. Key words Changjiang Estuary; major elements; release rate; pH value; sediment 1 Introduction Riverine transport from land to sea is a major process for the global cycling of elements that are delivered to the sea in dissolved and particulate forms (Gislason et al., 2006). The total flux of particulate material is at least an order of magnitude greater than the dissolved component, and thus, dominates the cycling of most elements in continental margin (Oelkers et al., 2011; Jeandel et al., 2011). To what extent the particulate matter influences the chemical evolution of seawater depends on its dissolution rate after it reaches the ocean. Therefore, the interaction of riverine particulate material with seawater has to be considered carefully, and some research methods were developed (Jones et al., 2012; Oelkers et al., 2012; Pearce et al., 2013). Jones et al. (2012) mixed riverine bedload material and estuarine sediments from western Iceland with open ocean seawater to assess the dissolution of riverine particulate material in seawater. Gherbi et al. (2010) operated closed-system batch reactors in laboratory to calculate the dissolution rates of sediments collected from the Oued Cherf Reservoir. In addition, the release of heavy metals from various sediments has been examined by the * Corresponding author. Tel: 0086-21-65982208 E-mail: [email protected] batch experiments in laboratory (Cappuyns and Swennen, 2005; Chakraborty et al., 2012; Yacoub et al., 2014). At present, the socio-economic development and enhanced human activities have caused increasing pressure on marine environment, such as ocean acidification and warming, which is generated by abundant CO2 emissions through fossil fuel combustion (Doney, 2010). Ocean acidification would affect biogeochemical and ecological processes of the coastal and estuary areas, and further influence the stability of marine sediment with sources from land, causing the release of elements from the seafloor sediments to the ambient seawater. Thus, the cycling of elements in coastal marine environment in response to oceanic environmental change is very important for the chemical evolution of global ocean (Jeandel et al., 2011); however, the related research is scare thus far. Most of previous studies focus on the dissolution of natural or authigenic minerals under different pH values and temperatures (Köhler et al., 2003; Gislason and Oelkers, 2003; Harouiya et al., 2007; Chaïrat et al., 2007; Parthasarathy et al., 2013; Ramos et al., 2014). The Changjiang (Yangtze River) is the largest river in China and one of the largest in the word in terms of its huge water and sediment discharges, which plays a critical role in terrestrial material cycle and ecosystem health in the East China Sea (Saito et al., 2001; Yang et al., 2003; Wang et al., 2008; Zhai and Dai, 2009). Based on the 418 GUO et al. / J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2015 14: 417-424 long-term hydrologic observation, the water and sediment discharges of the Changjiang average at 8964×108 km3 yr−1 and 390×106 t yr−1 respectively, which makes Changjiang the largest terrigenous source contributing to the ECS. However, the sediment flux of the Changjiang has been rapidly decreasing since the impoundment of the Three Gorges Reservoir in 2003, averaging at only about 155×106 t yr−1 over the last ten years (2003–2013) (Changjiang Water Resources Commission, http://www.cjw.com. cn/). In recent years, the Changjiang delivered huge amount of nutrients into the estuarine and coastal areas, resulting in the deterioration of marine environment and the development of eutrophication, red tide and hypoxia in the East China Sea (Li et al., 2002; Chen and Chen, 2003; Wang, 2006; Chen et al., 2007; Cai et al., 2011). Unfortunately, most previous studies put the emphasis on the effects of ocean acidification, warming and hypoxia on the ecosystem and biogeochemical cycle of the Changjiang Estuary and the East China Sea, while how the estuarine and shelf sediments that are originally weathered from the land respond to the rapid changes of oceanic environment has never been revealed. However, this may significantly influence the material cycle and biogeochemical process in the East China Sea and the west Pacific. In this study, an experimental study was carried out in laboratory to investigate the release rates of major elements (Ca, K, Mg and Al) from the surface sediments of the Changjiang Estuary in the acidic environment with different solution pH values. This study will provide new insight into the current understanding of ocean acidification affecting biogeochemical process and element cycling in the coastal sea. 2 Materials and Methods Two surface sediment samples named A6-5 and F6 were respectively taken from the Changjiang Estuary in March 2013 and March 2012 (Fig.1), and the top 2 cm was taken from the box corer. The geographic coordinates are 122˚37.71΄E and 30º50.77΄N for A6-5, and 122º45΄E and 31º17.25΄N for F6. The sediment samples were stored in the refrigerator on board and land, and then dried at 60℃ in laboratory. Sediments grain size was analyzed by the laser size analyzer (Coulter LS230) at the State Key Laboratory of Marine Geology in Tongji University, after processing the samples with 10% H2O2 and 1 mol L−1 HCl to remove organic matter and carbonate, respectively. The analytic error for grain size composition is ≤ ±1%. The BET surface areas of the sediments were analyzed by N2 adsorption-desorption at 77 K on a Quanta Chrome Autosorb-IqMP Analyzer at the State Key Laboratory of Pollution Control and Resource Reuse at Tongji University. The contents of major minerals in the sediments were measured in a Rigaku DMAX2500 X-ray diffractometer (XRD) at the First Institute of Oceanography in Qingdao. For geochemical analyses, the contents of major oxides in the bulk samples were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and trace elements by inductively coupled plasma-mass spectrometry (ICP-MS) in the State Key Laboratory of Marine Geology at Tongji University. Sediment powder was first ignited under 600℃ in a muffle furnace to remove organic matter, and then 30 mg was dissolved in 1 mL 1:1 HNO3 and 3 mL pure HF for 24 h, heated to dryness. Attacked again by 1 mL of 1:1 HNO3 and 3 mL pure HF for 7 days, heated to dryness, and further digested by 4 mL of 1:1 HNO3. This digestion procedure results in complete dissolution of the particulate samples. Uncertainties determined by the replicate samples and geostandards GSD9 do not exceed ±5%. Fig.1 Study area with the sampling locations. The batch experiment on element release was performed in a closed system for nearly two months, with the method modified from Köhler et al. (2003), Yadav et al., (2008) and Jones et al. (2012). Approximately 0.4 g of powdered sample was placed into the polypropylene batch reactor, together with 500 mL of simulated seawater with different pH values (4.0, 6.0 and 7.0). Then, the reactors were sealed and placed at a shaker with rotating speed of 150 rpm (revolutions per minute) under room temperature of 25℃. This setting allows the solution complete mixing while not disturbing the particulate-water interface. For elemental determination, 20 mL of supernatant was regularly taken from the reactors through 0.22 μm cellulose acetate membrane filters. This sample was divided into two sub-samples: one for pH analysis and the other for elemental concentration measurement using ICP-AES and ICP-MS. In order to get the actual release rates of major elements from the sediments, avoiding the possible influence from other elements of real water, the simulated seawater was prepared by dissolving ultrapure NaCl to Milli-Q water to get the solution containing 3.5% of NaCl, which corresponds to the average salinity of normal seawater. The pH of simulated seawater was controlled by 0.1 mol L−1 HCl or 0.1 mol L−1 NaOH solutions. The pH of simu- GUO et al. / J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2015 14: 417-424 lated seawater was allowed to vary during the experiment due to the chemical reaction process between sediments and simulated seawater (Pearce et al., 2013). The pH of the supernatant was measured immediately after the sampling by using a standard glass electrode previously calibrated against the 4.01, 6.86 and 9.18 pH buffers. 419 samples of A6-5 and F6 calculated using the multi-point BET method, are 61.74 m2 g−1 and 23.06 m2 g−1, respectively. These two samples are dominated by silty clay, with the mean grain sizes of 11.9 μm and 11.7 μm. The contents of SiO2 in A6-5 and F6 samples are about 55.2% and 59.5% respectively, which corresponds well with the dominance of quartz in the samples, accounting for 34.1% and 41.7% respectively. The contents of Al2O3 in the samples are around 14.0%. The other major minerals include feldspar (about 16.5%), calcite (about 5.0%) and dolomite (about 2.9%), while illite dominates the clay minerals in the sediments, averaging at 26.1%. 3 Results The grain size, bulk compositions of elements and minerals as well as BET surface areas of the samples are listed in the Table 1 and Table 2. The surface areas of the Table 1 BET surface areas, bulk compositions and grain size compositions of the investigated samples Samples Surface area Md Mz Clay Silt Sand SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O TiO2 K2O P2O5 S. D. (μm) (μm) (%) (%) (%) (%) (%) (%) (%) (%) (m2 g−1) (%) (%) (%) (%) (%) A6-5 61.7 8.5 11.9 11.2 23.6 76.6 0.02 55.2 14.9 5.94 F6 23.1 8.6 11.7 13.4 22.6 76.5 0.9 59.5 13.7 5.43 Notes: Md, median grain size; Mz, mean grain size; S.D., standard deviation. 0.10 0.09 2.76 4.01 1.35 0.77 2.80 0.11 2.55 2.83 1.92 0.68 2.75 0.68 Table 2 Contents of major minerals in the A6-5 and F6 samples (%) Samples Quartz Anorthite K-feldspar Calcite Dolomite Illite Chlorite Kaolinite A6-5 F6 34.1 41.7 11.1 12.2 4.0 5.3 7.8 2.1 3.0 2.7 25.9 26.4 11.5 8.1 2.6 1.4 In this study, the pH values and major elemental concentrations in the supernatant were measured with the reaction time for the calculation of the release rates of major elements. The flux of each element released during the experiment was determined as follows (Gherbi et al., 2010): M i ci ,rVr ci , nVn , (1) n where Mi stands for the mass of the ith element in the supernatant phase; ci,r corresponds to the concentration of this element in the supernatant present in the reactor; ci,n represents the concentration in the nth supernatant sample, and Vr and Vn denote the mass of supernatant present in the reactor and taken as the nth sample. The release fluxes of Ca, K, Mg and Al from A6-5 and F6 samples under the solution pH of 4.0 change substantially with the onset of the experiments (Fig.2), showing a rapid increase while the sediments reacting with the solution. For Ca and Mg, both elements are released from the bulk samples continuously with time and then reach the equilibrium after 400–600 h of the experiment. In comparison, K in the samples is released fast in the first 200 h, reaching the highest value of 4.5 mg, and then the release flux decreases and the equilibrium occurs after about 300 h, with the mass of around 1.5 mg. Element Al, however, shows some different trends with time, overall increasing until the experiment time of 1400 h. The release fluxes of Ca and Mg show obvious difference between A6-5 and F6 samples. The Ca release flux in A6-5 is less than that in F6 at the first 300 h of the experiment. However, as the reaction continues, A6-5 releases more Ca than F6, with the equilibrium flux of around 7.0 mg in A6-5 and about 6.0 mg in F6. The release flux of Al in F6 is overall higher than that in A6-5 sample throughout the experi- ment. This study also reveals that the solution pH in both samples changes with the reaction time, showing the increasing at first and then a decreasing trend (Fig.3). For instance, within the first 100 h of the experiment, the solution pH in A6-5 increases from initial solution pH 7.0 to 8.1, then decreases to 7.7 and finally reaches about 8.0. Pearce et al. (2013) found the similar change in solution pH while studying the reaction of basaltic samples with seawater. 4 Discussion 4.1 pH Effect on the Release Rates of Major Elements In order to evaluate the pH effect on the release rates of major elements of interest, we calculate the percentages of these four major elements released from the bulk samples relative to their bulk concentrations in F6 sample (Fig.4). Obviously, these four elements display different release percentages with the changes in solution pH. Overall, Ca has the highest release percentages (>50%) under various solution pH values than the other elements, while Al has the lowest release percentages (<1%). In addition, with the decrease of solution pH, the release percentages for specific elements (Ca, K, Mg) increase, especially for the solution pH value of 4.0, which implies that under more acidic environment, more major elements such as Ca, K and Mg are released more from the sediment. The alkali earth metals Ca and Mg show overall increasing release percentages with time under different solution pH values, and the release percentages are close under the solution pH of 6.0 and 7.0. This suggests that the solution pH near neutral condition has weak effect on Ca 420 GUO et al. / J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2015 14: 417-424 Fig.2 The release flux of major elements as a function of elapsed time with the initial solution pH of 4.0. For the calculation, please refer to the Eq. (1) in the text. Fig.3 Changes in solution pH during the experiment. The numbers in brackets indicate initial pH values. and Mg release. Based on the XRD results, we infer that Ca and Mg in the studied sediments primarily reside in calcite and dolomite. Considering the semi-quantitatively estimation of major mineral contents in the sediments by the XRD method, this study cannot make a stoichiometric calculation on how many calcite and dolomite were dissolved during the batch experiment and how they contributed to the released Ca and Mg in the supernatant. Nevertheless, this study reveals that these carbonate minerals are poorly attacked in the neutral solution compared to the case of pH 4.0. Silicate minerals such as anorthite in the sediments can also make some contribution to Ca and Mg release, although it is difficult to quantitatively calculate their influence. The more detailed characterization of residual sediments after the dissolution experiment may be necessary to reveal the possible roles of carbonate and silicate minerals in Ca and Mg release. Both Ca and Mg are essential elements required for marine organisms, and the continuous input of riverine particulate material is fundamental to maintaining the vitality of near coastal ecosystems (Bruland et al., 1991; Jeandel et al., 2011). Therefore, any subtle environmental change may alter the Ca and Mg availability in coastal waters, which probably results in a significant impact on marine ecosystem and seawater chemical evolution. It is noteworthy that under the solution pH of 4.0, K is released rapidly in the first 200 h and then stays at 40%–50% till 300 h. After about 300 h, the release percentage rapidly decreases to about 20%, and then reaches equilibrium of adsorption and dissolution. Under the solution pH of 6.0 and 7.0, the K release percentages are almost the same, showing very low values (<6.5%) and weak increasing trends. This probably indicates that under acidic condition, some K-bearing minerals, e.g., illite, kaolinite and K-feldspar in this study, may dissolve during the first stage of the experiments, while under neutral condition, they are relatively resistant to alteration in marine environment. The large change in K release percentage with time under the solution pH of 4.0 is probably caused by the subsequent incorporation of released K into GUO et al. / J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2015 14: 417-424 the existing clay minerals or a newly-precipitated secondary phase. Jones et al. (2012) also found that when riverine particulate material weathered from basalt interacts with seawater, the K concentration in the seawater also decreases obviously after the initial increase. Further experiments on mineral compositions in the residual sediment samples after the dissolution will be necessary to testify this speculation. In comparison, Al shows obviously different variations of release percentages with changing solution pH values, yielding the overall highest release percentage in the solution pH of 7.0 and the lowest in the solution with pH 4.0 (Fig.4). Reitzel et al. (2013) found that in the solution pH range of 7.5 to 9.5, while the uppermost Al-treated sediment layer in a lake was resuspended, a significant 421 proportion of Al can be dissolved and the dissolution rate increases with increasing pH. Some studies suggest that the presence of aqueous Al3+ can decelerate the dissolution of aluminosilicates because of Al-inhibition, decreased chemical affinity, or both (Gautier et al., 1994; Oelker et al., 1994; Chen and Brantley, 1997). For example, Chen and Brantley (1997) suggested that the decrease in dissolution rate of albite in the presence of dissolved aluminum may be attributed to increasing adsorption of Al3+, which further inhibits the albite dissolution. Therefore, the mechanism for Al release from the sediments under variable solution pH values seems to be complicated, involving the dissolution of aluminosilicates minerals in the sediment and the Al inhibition of other aluminosilicates. Fig.4 The effect of solution pH on the release of Ca, Mg, K and Al from F6 sample. 4.2 Dissolution Rates of Major Elements Under Different Solution pH The release rates (ri) of elements can be estimated by the following equation (Gherbi et al., 2010): ri 1 SBET M i , t (2) where t denotes time, and SBET is the surface area of the sediments present in the reactor; SBET can be calculated by multiplying the mass of powder in the reactor by the powder’s initial specific BET surface area. The slope of the regression curve of Mi versus time t, i.e., ∂Mi/∂t has been used to calculate element release rates based on Eq. (2). In this study, the linear regression of element release flux versus reaction time was obtained using the software of Origin pro 8.0. The coefficients R2 of the linear regression for the element under different pH values range widely from 0.30 to 0.95, and p ranging between 0.005 and 0.225. This regression method has been accepted and used extensively in calculating pure mineral and sediment dissolution rates (Köhler et al., 2003; Yadav et al., 2008; Gherbi et al., 2010). 422 GUO et al. / J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2015 14: 417-424 The logarithm values of release rates of Ca, K, Mg and Al show that with the increasing of solution pH value, the release rates of Ca and K decrease rapidly (Fig.5). This may be related to the dissolution kinetics of different minerals in the sediment. Under more acidic condition, minerals such as calcite, dolomite and even some silicate minerals in the sediments are more easily to be dissolved, causing more Ca and K to be released to the solution. As for Mg and Al, the pH effects on their release rates are not obvious. The reason may be that elemental release rates obtained in this research are normalized to the surface area of the sediments, while the surface areas are dominated by those of clay minerals rather than by other minerals that contain Mg and Al (Sverdrup and Warfvinge, 1993). In addition, mineral dissolution rates depend strongly on saturation state and solution composition. Furthermore, the calculation of release rate by Eq. (2) has been widely used for the dissolution experiments of minerals and lake sediments (Oelkers and Poitrasson, 2002; Köhler et al., 2003; Gherbi et al., 2010), which is based on the assumption that the element release flux has the linear relationship with reaction time. However, the complicated mineral composition of the studied estuarine sediments may cause the difference in the release fluxes between different elements. Besides, under the pH range of 4.0, 6.0 and 7.0, F6 has always higher element release rates than A6-5 sample (Fig.5), which can be explained by different clay mineral contents and surface areas between two studied samples. According to Eq. (2), the higher surface area causes slower release rates, and thus we suggest that clay mineral content and assemblage and surface area of mineral grain are important factors influencing element release rate. The comparison between element release rates generated in this study with dissolution rates of individual minerals can provide insight into the water-sediment interaction. The logarithms of dissolution rate are higher than −6 mol cm−2 s−1 for halite (Alkattan et al., 1997), about −9.8 mol cm−2 s−1 for calcite (Cubillas et al., 2005), and about −17.7 mol cm−2 s−1 for quartz (Brady and Walther, 1990). For those of clay minerals, montmorillonite has the logarithm of dissolution rate of about −18.2 mol cm−2 s−1 (Rozalen et al., 2008) with that for kaolinite being about −18.2 mol cm−2 s−1 (Huertas et al., 1999). The measured logarithms of dissolution rates of the four elements in this research are in the range of −14.5 to −18.2 mol cm−2 s−1, which is close to that for the slowly-dissolving minerals including quartz and clay minerals, suggesting that these minerals may dominate the chemical evolution of the aqueous phase during the experiment. The dissolution rates of lake sediments calculated by Gherbi et al. (2010) are close to those of quartz and caly minerals, also suggesting that these minerals may control the dissolution process of siliciclastic sediments. Fig.5 Logarithm of element dissolution rates as a function of pH. For the detailed calculations, please refer to the Eq. (2) in the text. 5 Conclusions A batch experiment was designed in this study for the investigation of major element release from the Changjiang estuarine sediments in the simulated seawater with different acidic conditions. The studied sediments are siltdominant and consist of quartz, plagioclase, calcite, dolomite, illite and chlorite. The experimental results suggest that the major elements of Ca, K, Mg and Al are all released from the sediments under the solution pH of 4.0, 6.0 and 7.0, albeit with variable releasing rates and fluxes. Under more acidic condition (pH 4.0), more Ca, Mg and K are released from the sediments, while the neutral solution (pH 6.0 and 7.0) causes weak release. Al, however, shows somewhat increasing release under neutral condition with pH 7.0. With the long-term reaction of the naturally-weathered riverine sediments with saline water under different pH values, Ca, K, Mg and Al exhibit variable release fluxes and rates, which is not only determined by the solution pH, but also tightly related to reaction kinetics and the mineral characters (assemblage, surface area and grain size, etc.) in the sediments. Overall, quartz and clay minerals that have low dissolution rates may dominate the major element release to the aqueous phase with the ex- GUO et al. / J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2015 14: 417-424 perimental time extending longer. This study reveals that with the enhanced simulated seawater acidification, the naturally-weathered terrigenous sediment may undergo further dissolution, causing the release of major elements (Ca, Mg, K and Al) into sea water. Our study may provide new insight on the elemental cycle in a changing ocean. 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