Environ. Sci. Technol. 2009, 43, 7192–7197
Metal Complexation by Humic Substances in Seawater R U J U N Y A N G †,¶ A N D C O N S T A N T M . G . V A N D E N B E R G †,* Department of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 3GP, United Kingdom, and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, 266100, P. R. China
Received January 21, 2009. Revised manuscript received May 19, 2009. Accepted May 20, 2009.
We determined the complex stability of copper, zinc, cobalt, and aluminum with humic acid (HA) and fulvic acid (FA) in pH 8 seawater. The method is based on metal competition against iron, for which the complex stability with humic substances (HS) in seawater had been calibrated previously against EDTA. The conditional stability constants, log K ′Mn+HS values, were found to decrease in the order of Cu > Zn > Co and Fe > Al. The complex stability of the HA species was greater than the FA species, but all determined complex stabilities are sufficiently high for significant complexation of the examined metals by HS in seawater. Data modeling shows a good data fit over the entire titrations for Cu and Co and for low levels of Zn. A second site on the HS appears to bind higher levels of Zn. The Al data suggest that Fe is exchanged for Al at a ratio different from 1:1. The data suggests that HS may be an important ligand for these metals in seawater.
Introduction Metals tend to occur complexed with organic matter in natural waters, including seawater. Little is still known of the nature of these ligands, but it is well-known that humic substances (HS) (humic and fulvic acids) play an important role in freshwaters because of their abundance. This has caused a large number (103) of papers to be published on this topic. HS are composed of a mixture of macromolecules with a nonrepetitive complex structure, which is still to be described even though their elemental composition is wellknown (1, 2). They include a complex mixture of carboxylated and fused alicyclic structures, which are expected to constitute strong ligands for metal binding (3). Acidity constants for HS fall into two groups: (1) carboxyl groups, with pKa values of ∼3.8 (FA) and ∼4.4 (HA) (4), and (2) phenolic groups with pKa values of 8.60 (FA) and 7.98 (HA) (5). So, in spite of the heterogeneity (differences in carboxyl/phenolic group ratios) of HS of different sources, proton binding is modeled to fall into just two distinguished groups. The same carboxyl and phenolic groups can be expected to play a role in metal complexation, and the stability of metal-HS species has been determined using various methods because of their importance to metal speciation in freshwaters. Early examples include work by the group of Buffle using ion-selective electrodes (ISE) (6), and studies into the interaction of metals with humic substances are still * Corresponding author e-mail:
[email protected]. † Department of Earth and Ocean Sciences. ¶ College of Chemistry and Chemical Engineering. 7192
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continuing today (7). With time, methods have become refined, but in principle the ISE is perhaps the best method to determine metal speciation because it measures the free metal ion concentration without affecting the equilibrium condition. However, a drawback of ISEs is the requirement to work at relatively high metal and ligand concentrations, much greater than typical for natural waters. Because of the heterogeneity of HS, a range of complex stabilities can be expected for the binding of individual metals. During a titration with metal, strong sites are saturated first, leaving weaker sites at higher concentrations. This is the cause for different complex stabilities obtained by techniques that operate within a different detection window (8, 9). The detection window is defined by the R-coefficient of the competing ligand in competitive cathodic stripping voltammetry (CSV) and by the R-coefficient of the natural complex as fixed by the metal and ligand concentrations at which the measurements are carried out. This explains why methods using high concentrations of HS and metals have found weak metal complexation (log K ′ values for copper of 5-6), while a relatively small number of stronger binding sites were overlooked. More recent measurements at a higher detection window found quite strong metal complexation such as log K ′ values of 1011 and 108 for HS in river water with cadmium and copper respectively (10). The constants (K ′ values) used in this work are “conditional stability constants”, which are based on concentrations of the ions (not the activities) and conditional upon the pH and major ion composition of the water. Very few measurements have been made of metal interactions with HS in seawater. This is not surprising as terrestrial HS are thought to precipitate in estuarine waters (11), suggesting that little escapes into the open sea or remains dissolved in seawater. Furthermore, early work using HS extracted from seawater suggested that complexation with trace metals (Mn, Co, Ni, Cu, Zn, Cd, and Hg) followed the Irving-Williams order but was weak because of competition with Ca and Mg, suggesting that at most 10% of Cu in seawater might be bound and an insignificant fraction of the other metals (12). So, even if any HS escaped the estuarine trap, it would still not be able to bind metals. However, H-nuclear magnetic resonance spectroscopy (NMR) of marine organic matter has shown that a major component has the properties of HS (13), suggesting that HS do occur in seawater. Marine fulvic acid could be an important ligand for trace metals, possibly binding zinc stronger than copper or cadmium (14), and more recent experiments have shown that terrestrial HS forms relatively strong complexes with copper (15) and iron (16) in seawater with log K ′ values of 10-12 (Cu) and 11 (Fe). Iron-binding HS are now known to occur abundantly in coastal waters and at low concentrations in the deep ocean, and their concentrations are thought to be stabilized by the formation of soluble Fe-HS complexes (16). These various data suggests that a reassessment of trace metal complexation with HS in seawater is necessary to evaluate whether HS could be an important ligand for other metals as well as for iron. The complex of iron with HS (Fe-HS) gives a voltammetric signal in the presence of bromate due to the Fe-HS species acting as catalyst for the electrochemical reduction of bromate (17). Terrestrial FA and HA and marine HS occurring in seawater produce the same CSV response, indicating that they bind iron in a similar manner. Comparisons showed excellent agreement between the concentration of HS in coastal waters determined from the Fe-HS signal by CSV and UV spectrometry (16, 17), indicating that the Fe-binding 10.1021/es900173w CCC: $40.75
2009 American Chemical Society
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component is an integral part of the terrestrial FA, HA, and marine HS. HS extraction from seawater gives a poor recovery (18). Therefore, and because of its apparent similarity to marine HS, we use here terrestrial FA and HA as model compounds to determine the complex stability of copper (Cu), zinc (Zn), cobalt (Co), and aluminum (Al) with HS in seawater. Use is made of metal competition for the same strong binding site on HS between added metals and iron, while specifically monitoring the Fe-HS signal by catalytic CSV. This method may have wider importance because it can be used for any trace metal, including those that do not give a voltammetric response, and is used here for the first time to determine complex stabilities for HA and FA with metals in pH 8 seawater. Zn, Cu, and Co were selected for this study because they are biogenic elements (important to biota) and known to occur complexed with organic matter in seawater (19-22). Aluminum was selected because it is an element with the second highest abundance in crustal rock (23), it is threevalent like iron, and it is thought to occur at least partially complexed with organic matter in seawater (24). Because of its abundance, it may have importance as a competing element.
Materials and Methods Equipment and Reagents. The previously described voltammetric apparatus (17) included a mercury drop working electrode (Metrohm VA696), glassy carbon counter electrode, a silver-silver chloride reference electrode, and a voltammeter (µAutolabIII). Samples (10 mL) were placed in a quartz voltammetric cell and stirred by a rotating PTFE rod. Water used for the preparation of solutions was purified using an Elixir/Milli-Q apparatus with UV digestion (Millipore). Hydrochloric acid, methanol, and ammonia were purified by coldfinger, silica, distillation. UV-digested seawater (UVSW) was prepared fresh using a home-built system with a 100 W, high-pressure, mercury vapor lamp in 30 mL quartz tubes with a 1 h irradiation time. The seawater used for UVSW preparation was from open ocean origin (Pacific) with a dissolved iron concentration of 0.5 nmol L-1 and a salinity of 34. Metal stock solutions were diluted from BDH atomic absorption spectrometry standard solutions (1 mg L-1) in acidified Milli-Q (pH 2.1). The pH buffer was 0.5 mol L-1 POPSO [piperazine-N,N′-bis-(2-hydroxypropanesulfonic) acid, Sigma-Aldrich], set to pHNBS 8.1 by addition of ammonia (∼0.5 mol L-1). The bromate stock solution contained 0.4 mol L-1 bromate (AnalaR, BDH). Contaminating iron in the bromate and buffer solutions was removed by adsorption onto MnO2 and filtration (25). Fulvic acid (FA) and humic acid (HA) were from the Suwannee River (International Humic Substances Society, IHSS) and were dissolved in MQ and stored under refrigeration when not in use. CSV with DHN (2,3-dihydroxynaphthalene, Fluka) (26) was used to determine the background iron concentration in the seawater used for the experiments. Metal Competition Method to Determine Complex Stabilities. The complex stability of Fe-HS (for FA and HA) in seawater has previously been determined by ligand competition against EDTA (ethylenediaminetetraacetic acid) giving values for log K ′Fe′FA ) 10.6 and log K ′Fe′HA ) 11.1. The ligand concentration of 1 mg L-1 of the HS was put on a molar scale by making use of its binding capacity for iron (16). The same concentration of HS was used here to determine the complex stability of other metals. The complex stability of other metals with HS was determined relative to that of Fe-HS by titration of Fe-HS in seawater with the other metals, while monitoring the
concentration of residual Fe-HS by CSV. Terrestrial FA and HA were used in this work as they have been found to be a good model for the HS occurring in coastal seawater (16). FA or HA was added to UVSW in a Teflon bottle (100 mL) to a concentration of 1 mg L-1 and 10 mmol L-1 POPSO buffer (pH 8.1). Iron was added to 30 nmol L-1 for the FA experiment and 40 nmol L-1 for the HA experiment, in excess of the binding capacities of these HS and causing the concentration of unbound HS to be negligible. This mixture was equilibrated overnight prior to the titrations to allow iron in excess of the HS to form Fe hydroxide and precipitate. Twenty milliliters of the seawater mixture was put in a quartz voltammetric cell, and bromate (0.4 mol L-1) was added. The solution was purged (5 min) with N2 gas, and the HS were titrated with metal ions. The concentration of Fe-HS was measured by CSV using the square-wave modulation (20 Hz), scan rate 40 mV s-1, using a deposition time of 120 s. Five minutes equilibration time was allowed after each metal addition, after which the residual [Fe-HS] was measured in triplicate. The first three measurements with zero-added metal were used to obtain a value for the initial peak height of the experiment (ip0). Theory. The principle of the method is that the complex stability of the HS complex with the added metals is determined from their competition with iron for the same binding site on the HS. The binding site is relevant for iron complexation at realistic, low, ambient iron concentrations and forms strong complexes with iron in seawater. The complexation sites for various metals on HS are assumed to be similar to those for iron, involving at least partially the same groups that bind iron as otherwise no competition would occur. As the iron-binding sites are strong complexation sites, this study looks only into strong complexation of the other trace metals. Weaker binding sites are not detected in this experimental setup. For this reason, the concentrations of HS have been kept low and realistic for coastal waters (1 mg HS L-1), much less than those in freshwaters. It is assumed that the competitive binding of HS by the added metals is matched by an equivalent decrease in the concentration of Fe-HS. A special point is where the concentration of Fe-HS has been exactly halved, at which point the complex stability of the competing metal with HS matches that of Fe with HS. This point was used to obtain a first estimate for the complex stability (R-coefficient) of each of the metal-HS species, which was used to verify that the data was modeled correctly. Subsequently, the R-coefficients were converted to conditional stability constants assuming a site (ligand) concentration equal to the original concentration of Fe-HS. The metal speciation is therefore normalized to the concentration of the Fe-binding sites on the HS. The reaction mechanism for the metal competition experiments is as follows Fe-HS + M′ T M-HS + Fe′
(1)
where M′ is the free, inorganic, metal concentration (not complexed with HS). The competition between the metals for HS (FA and HA) is described by the ratio X X ) [Fe-HS]/CHS ) [Fe-HS]/([Fe-HS] + [M-HS]) (2) where CHS is the total concentration of metal binding sites of HS. The concentration of Fe-HS in the absence of competing metals approximately equals CHS. X is directly obtained from the CSV measurements as the peak height (nA) is directly related to the concentration of Fe-HS at each point of the titration with metals via the sensitivity VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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X ) ip /ip0
Here ip ) peak height (nA), and ip0 ) peak height (nA) without metal competition at the beginning of the titration. The sensitivity has been omitted from eq 3 as it occurred above and below the division line. The ratio X differs from that used before (25) in that here competing metal species occur in the denominator instead of competing ligand species. Therefore, use has to be made of R-coefficients of the ligands (RHS(Fe) and RHS(M) for FA and HA, respectively) instead of the metals. X is a measure of the relative magnitude of the R-coefficients for HS X ) RHS(Fe) /(RHS(Fe) + RHS(M))
(4)
RHS(M) ) [M-HS]/[HS′]
(5)
RHS(Fe) ) [Fe-HS]/[HS′]
(6)
where
and [HS′] is the concentration of HS not bound by the metals. At the halfway point of the titration, half of the initial Fe-HS is exchanged to M-HS, so [M-HS]/CHS ) [Fe-HS]/ CHS ) 0.5 and [M-HS] ) [Fe-HS], which means that then the complex stabilities are equal: RHS(M) ) RHS(Fe) As the value for RHS(Fe) is known from calibration against EDTA (16), and this point is then convenient to calculate a first estimate for the unknown RHS(M). The value for RHS(M) can then be used to obtain a value for the unknown conditional stability constant, K ′M-HS RHS(Fe) ) [Fe′]K′Fe-HS
(7)
RHS(M) ) [M′]K′M-HS
(8)
and
The calculation is slightly complicated by the fact that RHS(Fe′) varies during the titrations due to the displacement of Fe′ from Fe-HS. [Fe′] was therefore calculated at each point of the titration from [Fe′] ) (1 - X)C HS + [Fe′i]
(9)
where [Fe′i] is the iron initially present in the water. The [Fe′i] was low by equilibrating the Fe and HS in the seawater overnight prior to the metal additions causing the excess Fe′ to hydrolyze to colloidal species and was negligible compared to the amount of Fe′ released by the competing metal additions. CHS was used in this equation as all HS was complexed with Fe at the start of the experiment. Values for [M′] at each point of the titration were calculated from the metal mass balance [M′] ) CM - [M-HS]
(10)
and [M-HS] was obtained from the decrease in [Fe-HS] during the titration [M-HS] ) (1 - X)CHS
(11)
A first estimate for K ′M-HS was obtained from K ′M-HS ) K ′Fe-HS [ Fe′]/[M′] at the halfway point of the titration, which was used to check the value obtained from a further calculation using all data points of the titrations. Therefore, values were calculated for RHS(M) from each point of the titration from RHS(M) ) RHS(Fe′)(1 - X)/X
(12)
from which K ′ values were calculated from 7194
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K′M-HS ) RHS(M) /[M′]
(3)
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(13)
The values for K ′M-HS obtained at each point of the titration (eq 3) were averaged and used to obtain the experimental standard deviation.
Results Kinetics of the Metal Exchange Reaction. The seawater with the Fe and HS was equilibrated overnight prior to the metal titrations to allow the Fe in excess of the binding capacity to be removed and to obtain a constant Fe-HS concentration and stable response. The excess inorganic iron is known to precipitate during the equilibration causing a gradual decrease of the signal of Fe-HS until the residual Fe is kept in solution by complexation with the HS (16). The rate at which equilibrium was reached for the exchange reaction of metals with HS-bound Fe was established in preliminary experiments by measuring the concentration of Fe-HS species as a function of time after the addition of metal. The metal additions were found to cause an immediate decrease in the peak height of Fe-FA and Fe-HA for all metals tested. Each measurement required about 2 min, which defined the resolution of these experiments. These preliminary experiments showed that the metal exchange reaction was rapid, reaching equilibrium within 5 min for all metals tested. Possibly much longer reaction times might be required to achieve equilibrium with some other complexation sites on the HS. However, it is assumed here that the strong sites are occupied by iron, so an equilibration time of 5 min was considered appropriate. The preliminary experiments established (a) that the equilibration time with the added metals was 1. Our measurements show that the complex stability with the HA is sufficiently great to bind the tested metals in seawater, which means that HA, when present, is likely to bind at least a proportion of these metals in seawater. FA forms somewhat weaker complexes, binding iron when present at subnmol L-1 concentration and other metals when present at 10 s of nmol L-1 (Table 1). HS have been shown to occur in coastal waters at levels of 40-400 µg L-1, equivalent to 1-10 nmol L-1, and at 1 nmol L-1 level in deep ocean waters (16). The complex stability of marine HS with Fe and the number of binding sites per mg of HS are more similar to that of HA than FA (16), suggesting that HA is the dominant HS in the marine system. It is therefore appropriate to use the complex stability of the HA species in a model for metal interaction with marine HS in seawater. It should be noted that the values for the stability constants determined here are conditional upon the seawater composition, which is characterized by high concentrations of Ca2+ (0.01 M) and Mg2+ (0.05 M) and a pH of 8.1. The stability of organic complexes (probably HS) in freshwaters is known to be affected by pH (33), Ca2+ (34), and the metal-to-ligand ratio (35). For this reason, it is difficult to compare these conditional stability constants for seawater to pre-existing values originating from freshwater experiments. Most literature values are freshwater-derived, except for values for log K ′Cu2+HA in seawater of 10 and 12 (15), compared to the value of 10.7 found here. We intend to further evaluate the importance of the new stability constants in a complexation model such as WHAM and to help extend this model to metal species in seawater. However, the values of the conditional stability constants (Table 1) are sufficiently large to expect significant complexation of various metals in seawater. Collection of representative marine HS is required for further experimental work to compare the metal complexing behavior of marine and terrestrial HS.
Acknowledgments We acknowledge the influence of the work of Jacques Buffle on this work and our work in the past. The visit of R. Yang to Liverpool was financially supported by the Chinese National Program for High Technology Research and Development (2007AA09Z106). We gratefully acknowledge helpful comments of the referees.
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