Kinetic and equilibrium studies of copper-dissolved organic matter complexation in water column of the stratified Krka River estuary (Croatia)

TLDR: In this paper, an experimental methodology was used based on the differential pulse anodic stripping voltammetric (DPASV) determination of labile copper species by titrating the sample using increments of copper additions uniformly distributed on the logarithmic scale.

About: This article is published in Marine Chemistry.The article was published on 2009-05-20 and is currently open access. It has received 78 citations till now. The article focuses on the topics: Copper & Dissolved organic carbon.

Figures

Fig. 3. Salinity (A), DOC (B) and total dissolved copper (C) depth profiles of samples taken in 2007 (△) and in 2008 (▲); marked area represents the depth range where FSI layer occurred for both 2007 and 2008 sampling events. Insets: Same parameters in relation to salinity; Full and dashed lines represent parabolic and conservative variations of DOC (B) and total dissolved copper (C), respectively.

Fig. 4. Ligand concentrations (A) and stability constants (B) as a function of depth for strong (◯,●) and weak (△,▲) ligands for samples taken in 2007 (◯,△) and in 2008 (●,▲). Dotted line: salinity profile for 2008; full line: sum of total ligand concentrations for 2008. (C)–Calculated inorganic (■) and organic (X) copper fractions according to depth profile for samples from 2008. Inset: free copper concentration (◆) and the toxicity limit of 10 pM (according to Sunda et al., 1987) represented by dotted line. Marked area represents the depth range where FSI layer occurred for both 2007 and 2008 sampling events.

Fig. 1. Locations of sampling sites: 1–Freshwater sample of Krka River, 2–Depth profile samples (Krka River estuary), 3–Seawater sample.

Fig. 5. Comparison of the complexing parameters (A–ligands concentrations and B–stability constant) obtained by the kinetic and the at-equilibrium approaches for samples taken in 2007 (◯,△) and 2008 (●,▲). Parameters obtained for strong (◯,●) and weak (△,▲) complexes.

Fig. 6. (A)–comparison of total copper concentration profiles for 2008 (▲) and those obtained by Omanović et al. (2006) (X). Dashed line represents projection of total copper concentration profile (see text for details). (B)–corresponding free copper concentration for 2008 (♦) and for the projected profile (◊); dotted line represents toxicity limit (see text). (C)–reaction time of organically complexed Cu (calculations according to the projected profile),–kinetics for 50% (♦), and for 99% (◊).

Fig. 2. Sample of salinity 13 (2008) titrated with copper in logarithmic increments. (A)–at-equilibrium obtained experimental points (□), dotted line: non-complexing line, full line: fitting curve. (B)–fitting of the kinetic experimental data (●) for the copper concentration indicated on the plot (A), dashed line: total metal, full line: fitting curve,□–data used for at-equilibrium fitting, △–estimated Culabile value at 0 s of equilibration time.

Frequently asked questions

Q1. What have the authors contributed in "Kinetic and equilibrium studies of copper-dissolved organic matter complexation in water column of the stratified krka river estuary (croatia)" ?

In this paper, the authors present an open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. 

Q2. What are the future works mentioned in the paper "Kinetic and equilibrium studies of copper-dissolved organic matter complexation in water column of the stratified krka river estuary (croatia)" ?

Two comparative approaches were used to study copper-dissolved natural organic matter ( DNOM ) interaction in thewater column of the stratified Krka River estuary: ( i ) an at-equilibrium and ( ii ) a kinetic approach. 

Q3. What was used as a counter electrode?

The workingelectrodewas a static mercury drop electrode (SMDE) (size 1, 0.25mm2 of area), while a platinum wire was used as a counter electrode. 

Q4. What was taken into account to calculate the inorganic speciation of copper?

Inorganic chemical composition of solution and pH was taken into account to calculate the inorganic speciation of copper, and so the α value for each experimental data point, using thermodynamic stability constants from MINEQL and MINTEQ databases. 

Q5. What is the kinetics of trace metal speciation in aquatic systems?

Systems such as rivers, lakes, estuaries and oceans surface could be under permanent, highly fluctuating physicochemical changes, generating metal speciation far from equilibrium (Hering and Morel, 1989, 1990; Langford and Cook, 1995). 

Q6. How long does the equilibrium time of copper complexation last?

Relatively slow formation rates of copper complexation (k11:6.1– 20×103 (M s)−1, k21: 1.3–6.3×103 (M s)−1) indicate that in the water column of the Krka River estuary, an achievement of the pseudoequilibrium (99%) of copper complexation (induced by copper concentration increase) could last up to 4 h30 min. 

Q7. How is the concentration of copper in the Krka River measured?

Concerning total dissolved copper concentrations, it can be underlined that in the freshwater sample taken in the Krka River, measured concentration is very low, about 2.4±0.1 nM. 

Q8. What is the purpose of this study?

The aim of this study is to develop and to explore an analytical procedure and a modelling scheme in order to determine and compare copper-DNOM complexing parameters under assumption of equilibrium conditions and under kinetic regime. 

Q9. Why is the kinetics of DNOM binding difficult to study in natural environments?

Due to low organic carbon content (8–83 µMC; Vetter et al., 2007) associated with trace levels of metals (rarely higher than some nM), the study of DNOM binding properties in unpolluted systems remains complex, needing convenient analytical techniques usable at very low metal and carbon concentrations. 

Q10. What is the kinetic of copper association with ligands?

According to the equation describing the kinetic copper complexationwith ligands (see Eq. (5)), they defined association constants as, ki⁎=ki1×Li assuming that Li corresponds to the total concentration of ligand (LiT) and not the free one, which dramatically simplifies the fitting procedure. 

Q11. What is the reason for the higher concentration of dissolved organic carbon in the FSI layer?

Knowing that the major part of the DNOM in the aquatic environment is due to degradation or exudation from living organisms (Buffle, 1988), this exacerbated biological activity can explain the higher amount of dissolved organic carbon found in this layer. 

Q12. What is the average flow of the Krka River?

TheKrka River is amedium sized non-contaminated river of 49 kmof length with an average flow measured over the last 50 years varying between 40 and 60m3 s−1 (Bonacci et al., 2006). 

Q13. Why did Omanovi et al. (2006) observe increased copper concentration in the surface?

Omanović et al. (2006) observed increased copper concentration in the surface layer during the summer, probably due to traffic of tourist's boats, with total copper concentrations higher than 12 nM (Fig. 6A, crosses). 

Q14. What is the reason for the slow association rate of the ligands?

The slow association rate of the ligands analysed in their study could be equally linked to a competition between copper andmajor divalent cations vs. the DNOM binding sites. 

Q15. Why are the corresponding binding parameters often inaccurately determined?

it has been shown in a theoretical study that the corresponding binding parameters are often inaccurately determined (Garnier et al., 2005) due to experimental and fitting uncertainties. 

Q16. Why is the apparent stability constant of copper expected to decrease from the river to the sea?

This high apparent stability constant is expected to decrease from the river to the sea, due to the increase of major divalent cations competition effect.