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Sorption model for dissolved and particulate aluminium in the Conway estuary, UK
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Estuarine, Coastal and Shelf Science 76 (2008) 914e919 www.elsevier.com/locate/ecss
Short communication
Sorption model for dissolved and particulate aluminium in the Conway estuary, UK S. Upadhyay1 School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Received 14 August 2007; accepted 15 August 2007 Available online 24 September 2007
Abstract Dissolved Al carried in river water apparently undergoes a fractional removal at the early stages of mixing in the Conway estuary. On the other hand, dissolved Al behaves almost conservatively in high salinity (>13) estuarine waters. In order to understand the geochemistry of Al in these estuarine waters, simple empirical sorption models have been used. Partitioning of Al occurs between solid and solution phases with a distribution coefficient, Kd, which varies from 0.67 105 to 3.38 106 ml g1 for suspended particle concentrations of 2e64 mg l1. The Kd values in general decrease with increasing suspended particulate matter and this tendency termed the ‘‘particle concentration effect’’ is quite pronounced in these waters. The sorption model derived by previous workers for predicting concentrations of dissolved Al with changing suspended sediment loads has been applied to these data. Reasonable fits are obtained for Kd values of 105, 106 and 107 ml g1 with various values of a. Further, a sorption model is proposed for particulate Al concentrations in these waters that fits the data extremely well defined by a zone with Kd value 107 ml g1 and C0 values 16 106 mg ml1 and 92 106 mg ml1. These observations provide strong evidence of sorption processes as key mechanisms influencing the distribution of dissolved and particulate Al in the Conway estuary and present new insight into Al geochemistry in estuaries. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: dissolved aluminium; particulate aluminium; sorption model; Conway estuary
1. Introduction The geochemical behaviour of dissolved Al in estuarine waters is only poorly defined. Previous studies have yielded diverse results and it may be that dissolved Al behaves somewhat differently in different estuarine systems. It appears that a number of factors and combination of processes influence Al geochemistry in estuaries. There is evidence for its highly interactive behaviour in many estuaries (Hosokawa et al., 1970; Mackin and Aller, 1984a,b, 1986; Morris et al., 1986; Vanbeusekom, 1988; Benoit et al., 1994; Upadhyay and Sengupta, 1995; Takayanagi and Gobeil, 2000; Xu et al., 2002; Ren et al., 2006) while it behaves almost conservatively,
E-mail address: [email protected] Present address: Department of Marine Sciences, Goa University, Goa 403 206, India. 1
0272-7714/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2007.08.021
i.e. riverine dissolved Al undergoes a simple linear mixing inversely with Al-poor seawater as a function of salinity in most parts of the Conway estuary (Hydes and Liss, 1977), Zaire and Rhone river plumes (Vanbennekom and Jager, 1978; Chou and Wollast, 1997) and a Scottish sea loch (Hall et al., 1999). Considering the geochemical dynamics of estuaries and complex solution chemistry of Al, it is not unexpected that dissolved Al may behave conservatively in some estuarine environments while being nonconservative in most others. The present study is an attempt to reinvestigate the geochemical behaviour of Al in the Conway estuary in the UK, an estuary where Hydes and Liss (1977) conducted one of the first studies of the estuarine behaviour of dissolved Al. This paper reports new data on particulate Al and Kd values, in addition to the distribution of dissolved Al in these waters. Solubleeparticulate interactions have been invoked to explain some aspects of the aluminium geochemistry in this estuary and the results presented here suggest that this approach may offer
S. Upadhyay / Estuarine, Coastal and Shelf Science 76 (2008) 914e919
a generalized explanation for the geochemical behaviour of Al in estuaries. 2. Methods During December 1995, a survey was made of the Conway estuary. Samples from the river/estuary were obtained at the mid-channel from the bridges over a whole tidal cycle, allowing the collection of a suite of samples covering most of the saline range. Surface (approximately 0.5 m depth) water samples were collected using a clean bucket which was rinsed with the sample at least once. The bottles for the storage of samples were rinsed twice with the sample water before being filled. Samples were stored unfiltered in acid-cleaned polyethylene bottles and were kept in the dark in a cool box in the field. They were transported to the laboratory in batches and the filtration was completed within 4e6 h of collection. Ideally, filtration should be performed in situ or immediately after collection. Since removal of Al due to sorption onto particulate material is time dependent, some alteration in Al speciation might have occurred as a result of storage of samples prior to filtration. Dissolved Al species are operationally defined by filtration through acid-cleaned 0.45 mm Millipore membrane filters in an acid-cleaned all plastic Millipore filtration apparatus. The fractions retained by these filters are termed suspended particulate matter (SPM). The filters were dried in an oven at a constant temperature of 60 C for 72 h, to a constant weight, and then weighed for the determination of SPM. Salinity was measured (precision 1%) using the Mohre Knudsen chlorinity titration method (Grasshoff et al., 1983). The analytical technique employed in the determination of dissolved Al was the fluorimetric lumogallion method of Hydes and Liss (1976). The samples were analyzed within a week of collection using a standard addition technique on triplicate of 5 ml each. Further details are described elsewhere (Upadhyay and Sengupta, 1995; Upadhyay et al., 2002). The reagent blank was determined by the difference of the fluorescence yields between portions of deionised water containing normal and twice the normal amounts of reagents. Reagent blanks corresponded to approximately 0.05 mg l1 Al. The estimated standard deviation of the reagent blank was 0.02 mg l1 Al for 10 replicates. This yields a detection limit of about 0.06 mg l1 Al (3 SD of the blank) for the method. The precision of analysis based on triplicate measurements was usually within 5%. Because of the low blanks relative to sample concentrations, the analytical uncertainties are expected to be minimal. Particulate Al was determined on the SPM retained by a 0.45 mm filter using an acetic acid leaching technique. This method has been applied previously (Orians and Bruland, 1986; Moran and Moore, 1988) for the determination of surface-bound exchangeable (particulate) Al for open ocean and coastal waters. For further testing the method, experiments were carried out to investigate the kinetics of dissolution of the particulate Al from the SPM obtained from estuarine waters. In the method, the filters were treated with 25% acetic acid (pH w 2.2) for 24 h and the leachate (5 ml) was neutralized
915
to pH 7.2 0.1 using ammonium hydroxide. The mixture was then diluted to 50 ml and centrifuged at 3000 rpm for 30 min to sediment the particles, and the supernatant was analyzed in the same manner as for dissolved Al. The particulate Al fraction so-determined is presented here as the surfacebound exchangeable Al which is assumed to participate in dissolvedeparticulate interactions (Upadhyay et al., 2002). The blanks for particulate Al were determined on three blank filters for each set of analyses being treated similarly, except that deionised water was filtered instead of sample. Blanks for particulate Al were typically about 2 mg l1 Al and the standard deviation of the blanks was similar to that measured (0.02 mg l1 Al) in dissolved Al analysis. Thus, the precision of the analysis was good (7% 4 mg l1 > 5%), as particulate Al concentrations were generally high.
3. Results and discussion 3.1. Riverine input of dissolved Al and its behaviour in estuarine waters Freshwater samples in the Conway river were collected at a site twice with an interval of about 8 h between them. Dissolved Al (w8 mg l1) content was almost identical on both sampling occasions. Another freshwater sample was obtained about 2 km further downstream and had dissolved Al of 13.6 mg l1. It is not clear from these results whether there is a systematic increase in dissolved Al downstream in the tidal, non-saline and more turbid section of the Conway or the increase from 8 to 13.6 mg l1 represents short-term fluctuations in freshwater chemistry. Therefore, in subsequent discussion a range from 8 to 13.6 mg l1 for the freshwater endmember is considered. The distribution of dissolved Al in estuarine waters is shown in Fig. 1. Over much of the saline range dissolved Al shows conservative behaviour. At low salinity there may be nonconservative behaviour, depending on the assumed freshwater endmember concentration. The only other study made previously on dissolved Al behaviour in this estuary is by Hydes and Liss (1977). In their study, they observed removal at low salinity in two surveys, and more complex behaviour in another. However, the three estuarine profiles they have obtained are similar to each other, and strikingly, the estuarine dissolved Al profile of the present study displays a great degree of resemblance to theirs. Dissolved Al profiles of the Conway estuary are different from observations in most of the estuaries. The only other studies on estuarine dissolved Al profiling that have some similarities to the Conway are from the Zaire and Rhone river plumes and a Scottish sea loch. Dissolved Al is nearconservative at higher salinities in these estuarine waters: in the Rhone it is apparent from salinity >1, in the Zaire from salinity >10, in the Scottish loch from salinity >20, and in the Conway in this study the corresponding situation occurs from salinity >13. For one-dimensional, tidal-averaged conditions, the fractional loss of a dissolved constituent within the estuary can
S. Upadhyay / Estuarine, Coastal and Shelf Science 76 (2008) 914e919
916
the observed trend. Such interactions may be especially important because particulate Al (13e85 mg l1) represents a large component of the total Al. The solidesolution partitioning of a trace metal in natural waters is often described by a conditional parameter, in terms of an empirical distribution coefficient, Kd (ml g1), defined as
10
Kd ¼
5
0 0
5
10
15
20
25
30
35
Salinity Fig. 1. Dissolved Al as a function of salinity in estuarine waters (solid line represents the theoretical dilution line (TDL) and the broken line is the best-fit line drawn by the method of least squares linear regression through the data above salinity 13, extrapolated back to salinity 0 to obtain river input value, Cr ).
be calculated as below (Hydes and Liss, 1977; Officer, 1979): % removal ¼ Cr Cr Cr 100 where Cr denotes the riverine concentration and Cr is the river input (obtained by extrapolation of the best-fit line drawn by the method of least squares linear regression through the data above salinity 13 in the mid and lower reaches of the estuary) into the adjoining sea. Using this formula and the riverine endmember concentration of 13.6 mg l1, the loss of dissolved Al in the Conway estuary amounts to about 31%. If the freshwater dissolved Al concentrations are averaged, this yields removal of approximately 4% which is, however, barely significant. On the contrary, an endmember of w8 mg l1 hints at addition of dissolved Al at low salinity. Given the fact that the freshwater region just above the immediate riveresea confluence had dissolved Al of 13.6 mg l1, the appropriate riverine endmember is 13.6 mg l1. This suggests removal of dissolved Al at the early stages of estuarine mixing, rather than addition through mobilization of reversibly associated Al from river-borne organic material upon mixing with seawater (Teien et al., 2006).
Cp Cd
ð1Þ
where Cp (mg g1) and Cd (mg ml1) are the metal concentrations in the particulate and solution phases, respectively. This distribution coefficient has been used in several studies to address the solubleeparticulate balance in the exchange of metal species in aqueous systems. The distribution coefficient (Kd) for Al in the Conway estuary varies from 0.67 105 to 3.38 106 ml g1 for suspended particle concentrations of 2e64 mg l1, which is very similar to other observations (Benoit et al., 1994; Upadhyay and Sengupta, 1995; Xu et al., 2002). This may reflect the broad similarity of the geochemical characteristics of Al in these estuarine waters. The relationship between log10 Kd and log10 [SPM] (Fig. 2) exhibits a good linear correlation (r ¼ 0.81; n ¼ 21) which is significant well above the 99.9% confidence level. The slope of the regression line is 0.69 and is consistent with observations in riverine and estuarine systems (Benoit et al., 1994; Upadhyay and Sengupta, 1995; Xu et al., 2002; Upadhyay et al., 2002). This tendency of declining Kd values with increasing suspended particle concentrations has been termed the ‘‘particle concentration effect’’ (O’Connor and Connolly, 1980) and may be linked with particle concentration, composition and characteristics and/or other processes.
7.0 r = -0.81; n = 21; P << 0.001 Slope = -0.69 6.5
Log [ Kd (ml g-1) ]
Dissolved Al (μg l-1)
15
6.0
5.5
5.0
3.2. Solid-solution partitioning of Al As there is some evidence from this survey, the earlier surveys of the Conway by Hydes and Liss (1977) and from other estuaries of nonconservative behaviour of dissolved Al, it is considered here if particleewater interactions can explain
4.5 0
0.5
1.0
1.5
Log [ SPM (mg l-1) ] Fig. 2. Log10Kd vs. log10 [SPM] in the Conway estuary.
2.0
S. Upadhyay / Estuarine, Coastal and Shelf Science 76 (2008) 914e919
3.3. Sorption model for dissolved Al in estuarine waters
0.5
ð2Þ
where Cr and Cd are the equilibrated dissolved Al concentrations (mg ml1) in the river water and estuarine water, respectively. Cp(r) and Cp are the equilibrated concentrations (mg g1) of particulate exchangeable aluminium in the river water and estuarine water, respectively. Cp(a) is the exchangeable aluminium concentration (mg g1) on the (re-)suspended sediment particles. SPMr is the suspended particulate load (g ml1) carried by the river. SPMa is the additional suspended particulate load (g ml1) generated by sediment resuspension in the estuary. Defining a as the fractional proportion of exchangeable aluminium on the (re-)suspending sediment particles relative to final equilibration, i.e. a ¼ Cp(a)/Cp, and substituting Kd ¼ Cp(r)/Cr and Kd ¼ Cp/Cd into Eq. (2) gives: Cd ½1 þ Kd SPMr ¼ Cr ½1 þ Kd SPMr þ Kd SPMa ð1 aÞ
Cd / Cr
1.0
To investigate the change in sorptive equilibrium in river water induced by the addition of (re-)suspended particles for metals in solution and the exchangeable fraction bound to particles, Morris et al. (1986) have applied a mass balance approach. Assuming the river water is the only significant source of dissolved Al and the pore water dissolved Al contributions are negligible, this formulation for the estuary yields: Cr þ CpðrÞ SPMr þ CpðaÞ SPMa ¼ Cd þ Cp SPMr þ SPMa
917
ð3Þ
Variations in the concentration ratio (Cd/Cr) for dissolved Al with changes in the (re-)suspended particulate load (SPMa) in the Conway estuary with the assumption of dissolved Al concentration in the riverine endmember (Cr) as 13.6 mg l1 are shown in Fig. 3. For removal to occur, a < 1, i.e. the added particles must have undersaturated adsorption sites with respect to particulate equilibrated metal. Using Eq. (3) for the observed riverine suspended load of 2.6 mg l1, curves are generated to fit these data for Kd values of 105, 106 and 107 ml g1 with various values of a. Reasonable fit embracing most of the data points is obtained for Kd value of 105 ml g1 with a values of 0.11 (the broken line) and 0.86 (the upper solid line). On the other hand, a much closer fit embracing all the data points is obtained for Kd values of 106 and 107 ml g1 with a values of 0.55 and 0.96 and 0.66 and 0.97, respectively. The lower solid line represents for Kd value of 106 ml g1 and a value of 0.55 with the line representing the couple Kd ¼ 107 ml g1 and a ¼ 0.66 being almost superimposed on it while the lines representing the couples Kd ¼ 105 ml g1 and a ¼ 0.86; Kd ¼ 106 ml g1 and a ¼ 0.96; and Kd ¼ 107 ml g1 and a ¼ 0.97 are superimposed as the upper solid line. It is difficult to obtain a precise value for a. However, considering the small magnitude of removal observed in the Conway estuary, these arbitrary limits appear to be reasonable, especially for higher Kd values. Indeed, Kd values of this magnitude were observed toward the head of the estuary (see Fig. 2). The model fitting of the field data supports the proposition that the sorption
0 0
50
100
SPMa (mg l-1) Fig. 3. Variations in the concentration ratio (Cd/Cr) for dissolved Al with changes in the (re-)suspended particulate load (SPMa) in the Conway estuary. The lower solid line representing the couple Kd ¼ 106 ml g1 and a ¼ 0.55, the broken line representing the couple Kd ¼ 105 ml g1 and a ¼ 0.11 and the upper solid line representing the couples Kd ¼ 105 ml g1 and a ¼ 0.86; Kd ¼ 106 ml g1 and a ¼ 0.96; and Kd ¼ 107 ml g1 and a ¼ 0.97 encompass the area for sorption model predictions.
mechanism is important in affecting the concentrations of dissolved Al in the Conway estuary. 3.4. Sorption model for particulate Al in estuarine waters The relationship between particulate Al (normalized to the total mass of SPM (mg g1)) and SPM is shown in Fig. 4. A strong interrelationship between particulate Al and SPM is evident in the Conway estuary. The broken line is a least squares non-linear regression of the concentration of metal in solid phase against SPM ([Part. Al] ¼ 42.54 [SPM]1.035) in these estuarine waters. These data exhibit an excellent correlation (r ¼ 0.90; n ¼ 21; P 0.001), suggesting that sorption processes would influence the distribution of particulate Al in the Conway estuary. A sorption model for particulate Al concentrations in these waters similar to the sorption model for dissolved Al in freshwaters derived by Upadhyay et al. (2002) is presented here. Let C0 be the concentration of metal in solution (mg ml1) in the absence of suspended particulate matter, Ca is the concentration of metal (mg ml1) adsorbed (particulate) onto the SPM (g ml1) in equilibrium with the solution, and Cd is the equilibrium concentration of the metal (mg ml1) in solution. The assumptions are that sorption exchange reactions occur and an equilibrium is attained between Al on solid phase and in solution. Applying mass balance to the sorptive equilibrium, C0 ¼ Ca þ Cd
ð4Þ
S. Upadhyay / Estuarine, Coastal and Shelf Science 76 (2008) 914e919
918
SPM < 2 mg l1 fit extremely well in the steeply rising part of the model (the data at greater SPM levels are not available for comparison). It may, however, be stated that more data representing seasonality from the same system and from systems covering wide spectrum of geological terrains will be essential for further validation of this model. The observations of very high particulate Al (normalized to particulate matter masses) at low SPM and low particulate Al at high SPM levels in the Galveston and Yalujiang estuaries (Benoit et al., 1994; Xu et al., 2002) generally conform with the model proposed. Considering the broad range of geochemical characteristics of these estuaries, these results are interesting and present new insight into Al geochemistry in estuaries.
50
[ Part. Al ] = 42.54 [ SPM ] -1.035 r = -0.90 ; n = 21; Ρ << 0.001
Particulate Al (mg g-1)
40
30
20
Acknowledgements Professors T.D. Jickells and P.S. Liss have contributed immensely to this work. This research was supported by the Commonwealth Scholarship Commission in the United Kingdom.
10
References 0 0
10
20
30
40
50
60
70
SPM (mg l-1) Fig. 4. Particulate Al (normalized to particulate matter masses) vs. SPM in the Conway estuary (the data from the streams flowing into the Conway river are symbolized as -). The solid lines are constructed using Eq. (6).
Using Eq. (4), C0 is calculated for individual samples which ranges between 16 106 and 92 106 mg ml1. Conceptually, Ca =SPM ð5Þ Cd Eqs. (4) and (5) can then be rearranged to calculate particulate Al concentrations (mg g1) at varying concentrations of SPM: Kd ¼
Ca =SPM ¼
C0 SPM þ Kd1
ð6Þ
Using Eq. (6), curves are generated to fit these data (Fig. 4) with Kd values of 105, 106 and 107 ml g1 and C0 values ranging from 16 106 to 92 106 mg ml1. The closest of the fits is obtained with Kd value 107 ml g1 and C0 values 16 106 mg ml1 (the lower solid line) and 92 106 mg ml1 (the upper solid line) those representing the lower and upper limits, respectively, of the metal concentration in solution in the absence of SPM in the present case. The model fitting of these data would imply that sorption processes are the key mechanisms affecting the partitioning of Al between solid phase and in solution in the Conway estuary. The model output is tested by using data from the streams flowing into the Conway river. These data points with
Benoit, G., Oktay-Marshall, S.D., Cantu, A., Hood, E.M., Coleman, C.H., Corapcioglu, M.O., Santschi, P.H., 1994. Partitioning of Cu, Pb, Ag, Zn, Fe, Al and Mn between filter-retained particles, colloids and solution in six Texas estuaries. Marine Chemistry 45, 307e336. Chou, L., Wollast, R., 1997. Biogeochemical behaviour and mass balance of dissolved aluminium in the western Mediterranean Sea. Deep-Sea Research II 44, 741e768. Grasshoff, K., Ehrhardt, M., Kremling, K., 1983. Methods of Seawater Analysis. Verlag Chemie, 419 pp. Hall, I.R., Hydes, D.J., Statham, P.J., Overnell, J., 1999. Seasonal variations in the cycling of aluminium, cadmium and manganese in a Scottish sea loch: biogeochemical processes involving suspended particles. Continental Shelf Research 19, 1783e1808. Hosokawa, I., Ohshima, F., Kondo, N., 1970. On the concentrations of the dissolved chemical elements in the estuary of the Chikugogawa river. Journal of Oceanographic Society of Japan 26, 1e5. Hydes, D.J., Liss, P.S., 1976. A fluorimetric method for the determination of low concentrations of dissolved aluminium in natural waters. Analyst 101, 922e931. Hydes, D.J., Liss, P.S., 1977. The behaviour of dissolved aluminium in estuarine and coastal waters. Estuarine, Coastal and Marine Science 5, 755e769. Mackin, J.E., Aller, R.C., 1984a. Dissolved Al in sediments and waters of the East China Sea: Implications for authigenic mineral formation. Geochimica et Cosmochimica Acta 48, 281e297. Mackin, J.E., Aller, R.C., 1984b. Processes affecting the behaviour of dissolved Al in estuarine waters. Marine Chemistry 14, 213e232. Mackin, J.E., Aller, R.C., 1986. The effects of clay minerals on dissolved Al distributions in sediments and waters of the Amazon continental shelf. Continental Shelf Research 6, 246e262. Moran, S.B., Moore, R.M., 1988. Temporal variations in dissolved and particulate Al during a spring bloom. Estuarine, Coastal and Shelf Science 27, 205e215. Morris, A.W., Howland, R.J.M., Bale, A.J., 1986. Dissolved Al in the Tamar estuary, Southwest England. Geochimica et Cosmochimica Acta 50, 189e197. O’Connor, D.J., Connolly, J.P., 1980. The effect of concentration of adsorbing solids on the partitioning coefficient. Water Research 14, 1517e1526.
S. Upadhyay / Estuarine, Coastal and Shelf Science 76 (2008) 914e919 Officer, C.B., 1979. Discussion of the behaviour of non-conservative constituents in estuaries. Estuarine, Coastal and Marine Science 9, 91e94. Orians, K.J., Bruland, K.W., 1986. The biogeochemisty of aluminium in the Pacific Ocean. Earth and Planetary Science Letters 78, 397e410. Ren, J.L., Zhang, J., Li, J.B., Yu, X.Y., Liu, S.M., Zhang, E.R., 2006. Dissolved aluminium in the Yellow Sea and East China Sea- Al as a tracer of Changjing (Yangtze River) discharge and Kuroshio incursion. Estuarine, Coastal and Shelf Science 68, 165e174. Takayanagi, K., Gobeil, C., 2000. Dissolved aluminium in the upper St. Lawrence estuary. Journal of Oceanography 56, 517e525. Teien, H.C., Standring, W.J.F., Salbu, B., 2006. Mobilization of river transported colloidal aluminium upon mixing with seawater and subsequent deposition in fish gills. Science of The Total Environment 364, 149e164.
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Upadhyay, S., Liss, P.S., Jickells, T.D., 2002. Sorption model for dissolved aluminium in freshwaters. Aquatic Geochemistry 8, 255e275. Upadhyay, S., Sengupta, R., 1995. The behaviour of aluminium in waters of the Mandovi estuary, west coast of India. Marine Chemistry 51, 261e276. Vanbennekom, A.J., Jager, J.E., 1978. Dissolved Al in the Zaire river plume. Netherlands Journal of Sea Research 12, 358e367. Vanbeusekom, J.E.E., 1988. Distribution of dissolved Al in the North Sea: Influence of suspended matter. Mitteilungen aus dem GeologischPala¨ontologischen Institut der Universita¨t Hamburg 65, 117e136. Xu, H., Zhang, J., Ren, J.L., Liu, C.L., 2002. Aluminium in the macrotidal Yalujiang estuary: partitioning of Al along the estuarine gradients and flux. Estuaries 25, 608e621.
Estuarine, Coastal and Shelf Science 76 (2008) 914e919 www.elsevier.com/locate/ecss
Short communication
Sorption model for dissolved and particulate aluminium in the Conway estuary, UK S. Upadhyay1 School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Received 14 August 2007; accepted 15 August 2007 Available online 24 September 2007
Abstract Dissolved Al carried in river water apparently undergoes a fractional removal at the early stages of mixing in the Conway estuary. On the other hand, dissolved Al behaves almost conservatively in high salinity (>13) estuarine waters. In order to understand the geochemistry of Al in these estuarine waters, simple empirical sorption models have been used. Partitioning of Al occurs between solid and solution phases with a distribution coefficient, Kd, which varies from 0.67 105 to 3.38 106 ml g1 for suspended particle concentrations of 2e64 mg l1. The Kd values in general decrease with increasing suspended particulate matter and this tendency termed the ‘‘particle concentration effect’’ is quite pronounced in these waters. The sorption model derived by previous workers for predicting concentrations of dissolved Al with changing suspended sediment loads has been applied to these data. Reasonable fits are obtained for Kd values of 105, 106 and 107 ml g1 with various values of a. Further, a sorption model is proposed for particulate Al concentrations in these waters that fits the data extremely well defined by a zone with Kd value 107 ml g1 and C0 values 16 106 mg ml1 and 92 106 mg ml1. These observations provide strong evidence of sorption processes as key mechanisms influencing the distribution of dissolved and particulate Al in the Conway estuary and present new insight into Al geochemistry in estuaries. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: dissolved aluminium; particulate aluminium; sorption model; Conway estuary
1. Introduction The geochemical behaviour of dissolved Al in estuarine waters is only poorly defined. Previous studies have yielded diverse results and it may be that dissolved Al behaves somewhat differently in different estuarine systems. It appears that a number of factors and combination of processes influence Al geochemistry in estuaries. There is evidence for its highly interactive behaviour in many estuaries (Hosokawa et al., 1970; Mackin and Aller, 1984a,b, 1986; Morris et al., 1986; Vanbeusekom, 1988; Benoit et al., 1994; Upadhyay and Sengupta, 1995; Takayanagi and Gobeil, 2000; Xu et al., 2002; Ren et al., 2006) while it behaves almost conservatively,
E-mail address: [email protected] Present address: Department of Marine Sciences, Goa University, Goa 403 206, India. 1
0272-7714/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2007.08.021
i.e. riverine dissolved Al undergoes a simple linear mixing inversely with Al-poor seawater as a function of salinity in most parts of the Conway estuary (Hydes and Liss, 1977), Zaire and Rhone river plumes (Vanbennekom and Jager, 1978; Chou and Wollast, 1997) and a Scottish sea loch (Hall et al., 1999). Considering the geochemical dynamics of estuaries and complex solution chemistry of Al, it is not unexpected that dissolved Al may behave conservatively in some estuarine environments while being nonconservative in most others. The present study is an attempt to reinvestigate the geochemical behaviour of Al in the Conway estuary in the UK, an estuary where Hydes and Liss (1977) conducted one of the first studies of the estuarine behaviour of dissolved Al. This paper reports new data on particulate Al and Kd values, in addition to the distribution of dissolved Al in these waters. Solubleeparticulate interactions have been invoked to explain some aspects of the aluminium geochemistry in this estuary and the results presented here suggest that this approach may offer
S. Upadhyay / Estuarine, Coastal and Shelf Science 76 (2008) 914e919
a generalized explanation for the geochemical behaviour of Al in estuaries. 2. Methods During December 1995, a survey was made of the Conway estuary. Samples from the river/estuary were obtained at the mid-channel from the bridges over a whole tidal cycle, allowing the collection of a suite of samples covering most of the saline range. Surface (approximately 0.5 m depth) water samples were collected using a clean bucket which was rinsed with the sample at least once. The bottles for the storage of samples were rinsed twice with the sample water before being filled. Samples were stored unfiltered in acid-cleaned polyethylene bottles and were kept in the dark in a cool box in the field. They were transported to the laboratory in batches and the filtration was completed within 4e6 h of collection. Ideally, filtration should be performed in situ or immediately after collection. Since removal of Al due to sorption onto particulate material is time dependent, some alteration in Al speciation might have occurred as a result of storage of samples prior to filtration. Dissolved Al species are operationally defined by filtration through acid-cleaned 0.45 mm Millipore membrane filters in an acid-cleaned all plastic Millipore filtration apparatus. The fractions retained by these filters are termed suspended particulate matter (SPM). The filters were dried in an oven at a constant temperature of 60 C for 72 h, to a constant weight, and then weighed for the determination of SPM. Salinity was measured (precision 1%) using the Mohre Knudsen chlorinity titration method (Grasshoff et al., 1983). The analytical technique employed in the determination of dissolved Al was the fluorimetric lumogallion method of Hydes and Liss (1976). The samples were analyzed within a week of collection using a standard addition technique on triplicate of 5 ml each. Further details are described elsewhere (Upadhyay and Sengupta, 1995; Upadhyay et al., 2002). The reagent blank was determined by the difference of the fluorescence yields between portions of deionised water containing normal and twice the normal amounts of reagents. Reagent blanks corresponded to approximately 0.05 mg l1 Al. The estimated standard deviation of the reagent blank was 0.02 mg l1 Al for 10 replicates. This yields a detection limit of about 0.06 mg l1 Al (3 SD of the blank) for the method. The precision of analysis based on triplicate measurements was usually within 5%. Because of the low blanks relative to sample concentrations, the analytical uncertainties are expected to be minimal. Particulate Al was determined on the SPM retained by a 0.45 mm filter using an acetic acid leaching technique. This method has been applied previously (Orians and Bruland, 1986; Moran and Moore, 1988) for the determination of surface-bound exchangeable (particulate) Al for open ocean and coastal waters. For further testing the method, experiments were carried out to investigate the kinetics of dissolution of the particulate Al from the SPM obtained from estuarine waters. In the method, the filters were treated with 25% acetic acid (pH w 2.2) for 24 h and the leachate (5 ml) was neutralized
915
to pH 7.2 0.1 using ammonium hydroxide. The mixture was then diluted to 50 ml and centrifuged at 3000 rpm for 30 min to sediment the particles, and the supernatant was analyzed in the same manner as for dissolved Al. The particulate Al fraction so-determined is presented here as the surfacebound exchangeable Al which is assumed to participate in dissolvedeparticulate interactions (Upadhyay et al., 2002). The blanks for particulate Al were determined on three blank filters for each set of analyses being treated similarly, except that deionised water was filtered instead of sample. Blanks for particulate Al were typically about 2 mg l1 Al and the standard deviation of the blanks was similar to that measured (0.02 mg l1 Al) in dissolved Al analysis. Thus, the precision of the analysis was good (7% 4 mg l1 > 5%), as particulate Al concentrations were generally high.
3. Results and discussion 3.1. Riverine input of dissolved Al and its behaviour in estuarine waters Freshwater samples in the Conway river were collected at a site twice with an interval of about 8 h between them. Dissolved Al (w8 mg l1) content was almost identical on both sampling occasions. Another freshwater sample was obtained about 2 km further downstream and had dissolved Al of 13.6 mg l1. It is not clear from these results whether there is a systematic increase in dissolved Al downstream in the tidal, non-saline and more turbid section of the Conway or the increase from 8 to 13.6 mg l1 represents short-term fluctuations in freshwater chemistry. Therefore, in subsequent discussion a range from 8 to 13.6 mg l1 for the freshwater endmember is considered. The distribution of dissolved Al in estuarine waters is shown in Fig. 1. Over much of the saline range dissolved Al shows conservative behaviour. At low salinity there may be nonconservative behaviour, depending on the assumed freshwater endmember concentration. The only other study made previously on dissolved Al behaviour in this estuary is by Hydes and Liss (1977). In their study, they observed removal at low salinity in two surveys, and more complex behaviour in another. However, the three estuarine profiles they have obtained are similar to each other, and strikingly, the estuarine dissolved Al profile of the present study displays a great degree of resemblance to theirs. Dissolved Al profiles of the Conway estuary are different from observations in most of the estuaries. The only other studies on estuarine dissolved Al profiling that have some similarities to the Conway are from the Zaire and Rhone river plumes and a Scottish sea loch. Dissolved Al is nearconservative at higher salinities in these estuarine waters: in the Rhone it is apparent from salinity >1, in the Zaire from salinity >10, in the Scottish loch from salinity >20, and in the Conway in this study the corresponding situation occurs from salinity >13. For one-dimensional, tidal-averaged conditions, the fractional loss of a dissolved constituent within the estuary can
S. Upadhyay / Estuarine, Coastal and Shelf Science 76 (2008) 914e919
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the observed trend. Such interactions may be especially important because particulate Al (13e85 mg l1) represents a large component of the total Al. The solidesolution partitioning of a trace metal in natural waters is often described by a conditional parameter, in terms of an empirical distribution coefficient, Kd (ml g1), defined as
10
Kd ¼
5
0 0
5
10
15
20
25
30
35
Salinity Fig. 1. Dissolved Al as a function of salinity in estuarine waters (solid line represents the theoretical dilution line (TDL) and the broken line is the best-fit line drawn by the method of least squares linear regression through the data above salinity 13, extrapolated back to salinity 0 to obtain river input value, Cr ).
be calculated as below (Hydes and Liss, 1977; Officer, 1979): % removal ¼ Cr Cr Cr 100 where Cr denotes the riverine concentration and Cr is the river input (obtained by extrapolation of the best-fit line drawn by the method of least squares linear regression through the data above salinity 13 in the mid and lower reaches of the estuary) into the adjoining sea. Using this formula and the riverine endmember concentration of 13.6 mg l1, the loss of dissolved Al in the Conway estuary amounts to about 31%. If the freshwater dissolved Al concentrations are averaged, this yields removal of approximately 4% which is, however, barely significant. On the contrary, an endmember of w8 mg l1 hints at addition of dissolved Al at low salinity. Given the fact that the freshwater region just above the immediate riveresea confluence had dissolved Al of 13.6 mg l1, the appropriate riverine endmember is 13.6 mg l1. This suggests removal of dissolved Al at the early stages of estuarine mixing, rather than addition through mobilization of reversibly associated Al from river-borne organic material upon mixing with seawater (Teien et al., 2006).
Cp Cd
ð1Þ
where Cp (mg g1) and Cd (mg ml1) are the metal concentrations in the particulate and solution phases, respectively. This distribution coefficient has been used in several studies to address the solubleeparticulate balance in the exchange of metal species in aqueous systems. The distribution coefficient (Kd) for Al in the Conway estuary varies from 0.67 105 to 3.38 106 ml g1 for suspended particle concentrations of 2e64 mg l1, which is very similar to other observations (Benoit et al., 1994; Upadhyay and Sengupta, 1995; Xu et al., 2002). This may reflect the broad similarity of the geochemical characteristics of Al in these estuarine waters. The relationship between log10 Kd and log10 [SPM] (Fig. 2) exhibits a good linear correlation (r ¼ 0.81; n ¼ 21) which is significant well above the 99.9% confidence level. The slope of the regression line is 0.69 and is consistent with observations in riverine and estuarine systems (Benoit et al., 1994; Upadhyay and Sengupta, 1995; Xu et al., 2002; Upadhyay et al., 2002). This tendency of declining Kd values with increasing suspended particle concentrations has been termed the ‘‘particle concentration effect’’ (O’Connor and Connolly, 1980) and may be linked with particle concentration, composition and characteristics and/or other processes.
7.0 r = -0.81; n = 21; P << 0.001 Slope = -0.69 6.5
Log [ Kd (ml g-1) ]
Dissolved Al (μg l-1)
15
6.0
5.5
5.0
3.2. Solid-solution partitioning of Al As there is some evidence from this survey, the earlier surveys of the Conway by Hydes and Liss (1977) and from other estuaries of nonconservative behaviour of dissolved Al, it is considered here if particleewater interactions can explain
4.5 0
0.5
1.0
1.5
Log [ SPM (mg l-1) ] Fig. 2. Log10Kd vs. log10 [SPM] in the Conway estuary.
2.0
S. Upadhyay / Estuarine, Coastal and Shelf Science 76 (2008) 914e919
3.3. Sorption model for dissolved Al in estuarine waters
0.5
ð2Þ
where Cr and Cd are the equilibrated dissolved Al concentrations (mg ml1) in the river water and estuarine water, respectively. Cp(r) and Cp are the equilibrated concentrations (mg g1) of particulate exchangeable aluminium in the river water and estuarine water, respectively. Cp(a) is the exchangeable aluminium concentration (mg g1) on the (re-)suspended sediment particles. SPMr is the suspended particulate load (g ml1) carried by the river. SPMa is the additional suspended particulate load (g ml1) generated by sediment resuspension in the estuary. Defining a as the fractional proportion of exchangeable aluminium on the (re-)suspending sediment particles relative to final equilibration, i.e. a ¼ Cp(a)/Cp, and substituting Kd ¼ Cp(r)/Cr and Kd ¼ Cp/Cd into Eq. (2) gives: Cd ½1 þ Kd SPMr ¼ Cr ½1 þ Kd SPMr þ Kd SPMa ð1 aÞ
Cd / Cr
1.0
To investigate the change in sorptive equilibrium in river water induced by the addition of (re-)suspended particles for metals in solution and the exchangeable fraction bound to particles, Morris et al. (1986) have applied a mass balance approach. Assuming the river water is the only significant source of dissolved Al and the pore water dissolved Al contributions are negligible, this formulation for the estuary yields: Cr þ CpðrÞ SPMr þ CpðaÞ SPMa ¼ Cd þ Cp SPMr þ SPMa
917
ð3Þ
Variations in the concentration ratio (Cd/Cr) for dissolved Al with changes in the (re-)suspended particulate load (SPMa) in the Conway estuary with the assumption of dissolved Al concentration in the riverine endmember (Cr) as 13.6 mg l1 are shown in Fig. 3. For removal to occur, a < 1, i.e. the added particles must have undersaturated adsorption sites with respect to particulate equilibrated metal. Using Eq. (3) for the observed riverine suspended load of 2.6 mg l1, curves are generated to fit these data for Kd values of 105, 106 and 107 ml g1 with various values of a. Reasonable fit embracing most of the data points is obtained for Kd value of 105 ml g1 with a values of 0.11 (the broken line) and 0.86 (the upper solid line). On the other hand, a much closer fit embracing all the data points is obtained for Kd values of 106 and 107 ml g1 with a values of 0.55 and 0.96 and 0.66 and 0.97, respectively. The lower solid line represents for Kd value of 106 ml g1 and a value of 0.55 with the line representing the couple Kd ¼ 107 ml g1 and a ¼ 0.66 being almost superimposed on it while the lines representing the couples Kd ¼ 105 ml g1 and a ¼ 0.86; Kd ¼ 106 ml g1 and a ¼ 0.96; and Kd ¼ 107 ml g1 and a ¼ 0.97 are superimposed as the upper solid line. It is difficult to obtain a precise value for a. However, considering the small magnitude of removal observed in the Conway estuary, these arbitrary limits appear to be reasonable, especially for higher Kd values. Indeed, Kd values of this magnitude were observed toward the head of the estuary (see Fig. 2). The model fitting of the field data supports the proposition that the sorption
0 0
50
100
SPMa (mg l-1) Fig. 3. Variations in the concentration ratio (Cd/Cr) for dissolved Al with changes in the (re-)suspended particulate load (SPMa) in the Conway estuary. The lower solid line representing the couple Kd ¼ 106 ml g1 and a ¼ 0.55, the broken line representing the couple Kd ¼ 105 ml g1 and a ¼ 0.11 and the upper solid line representing the couples Kd ¼ 105 ml g1 and a ¼ 0.86; Kd ¼ 106 ml g1 and a ¼ 0.96; and Kd ¼ 107 ml g1 and a ¼ 0.97 encompass the area for sorption model predictions.
mechanism is important in affecting the concentrations of dissolved Al in the Conway estuary. 3.4. Sorption model for particulate Al in estuarine waters The relationship between particulate Al (normalized to the total mass of SPM (mg g1)) and SPM is shown in Fig. 4. A strong interrelationship between particulate Al and SPM is evident in the Conway estuary. The broken line is a least squares non-linear regression of the concentration of metal in solid phase against SPM ([Part. Al] ¼ 42.54 [SPM]1.035) in these estuarine waters. These data exhibit an excellent correlation (r ¼ 0.90; n ¼ 21; P 0.001), suggesting that sorption processes would influence the distribution of particulate Al in the Conway estuary. A sorption model for particulate Al concentrations in these waters similar to the sorption model for dissolved Al in freshwaters derived by Upadhyay et al. (2002) is presented here. Let C0 be the concentration of metal in solution (mg ml1) in the absence of suspended particulate matter, Ca is the concentration of metal (mg ml1) adsorbed (particulate) onto the SPM (g ml1) in equilibrium with the solution, and Cd is the equilibrium concentration of the metal (mg ml1) in solution. The assumptions are that sorption exchange reactions occur and an equilibrium is attained between Al on solid phase and in solution. Applying mass balance to the sorptive equilibrium, C0 ¼ Ca þ Cd
ð4Þ
S. Upadhyay / Estuarine, Coastal and Shelf Science 76 (2008) 914e919
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SPM < 2 mg l1 fit extremely well in the steeply rising part of the model (the data at greater SPM levels are not available for comparison). It may, however, be stated that more data representing seasonality from the same system and from systems covering wide spectrum of geological terrains will be essential for further validation of this model. The observations of very high particulate Al (normalized to particulate matter masses) at low SPM and low particulate Al at high SPM levels in the Galveston and Yalujiang estuaries (Benoit et al., 1994; Xu et al., 2002) generally conform with the model proposed. Considering the broad range of geochemical characteristics of these estuaries, these results are interesting and present new insight into Al geochemistry in estuaries.
50
[ Part. Al ] = 42.54 [ SPM ] -1.035 r = -0.90 ; n = 21; Ρ << 0.001
Particulate Al (mg g-1)
40
30
20
Acknowledgements Professors T.D. Jickells and P.S. Liss have contributed immensely to this work. This research was supported by the Commonwealth Scholarship Commission in the United Kingdom.
10
References 0 0
10
20
30
40
50
60
70
SPM (mg l-1) Fig. 4. Particulate Al (normalized to particulate matter masses) vs. SPM in the Conway estuary (the data from the streams flowing into the Conway river are symbolized as -). The solid lines are constructed using Eq. (6).
Using Eq. (4), C0 is calculated for individual samples which ranges between 16 106 and 92 106 mg ml1. Conceptually, Ca =SPM ð5Þ Cd Eqs. (4) and (5) can then be rearranged to calculate particulate Al concentrations (mg g1) at varying concentrations of SPM: Kd ¼
Ca =SPM ¼
C0 SPM þ Kd1
ð6Þ
Using Eq. (6), curves are generated to fit these data (Fig. 4) with Kd values of 105, 106 and 107 ml g1 and C0 values ranging from 16 106 to 92 106 mg ml1. The closest of the fits is obtained with Kd value 107 ml g1 and C0 values 16 106 mg ml1 (the lower solid line) and 92 106 mg ml1 (the upper solid line) those representing the lower and upper limits, respectively, of the metal concentration in solution in the absence of SPM in the present case. The model fitting of these data would imply that sorption processes are the key mechanisms affecting the partitioning of Al between solid phase and in solution in the Conway estuary. The model output is tested by using data from the streams flowing into the Conway river. These data points with
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