Determination of the Intrinsic Stability Constants of Toxic Divalent Metal Ions to Alginic Acid

Abstract

Alginic acid (AA), which is a polyuronide found in seaweeds, has a strong affinity for a variety of metal ions. 1,2Such a character has been utilized for removing toxic heavy-metal ions from waste water. 3,4 The stability constants of metal-AA complexes may be useful to calculate the distribution of metal species for a variety of ligand concentrations and pH. There have been numerous studies on the affinity of metal ions to AA. 5-7 Nevertheless, little has been known about the stability constants of the metal-AA complexes. Recently, Jang and coworkers have evaluated the conditional stability constants of the Cu(II)-and Zn(II)-AA complexes by using a Langmuir model. 8 On the other hand, AA has been known as a polyelectrolyte. There have been many studies on complexation between divalent metal ions and polyelectrolytes. [9][10][11][12] Gregor and coworkers have shown that Bjerrum's method may be applied to divalent metal-polyelectrolyte systems. 9,10 However, in evaluating the stability constants for metal-polyelectrolyte complexes, a large electrostatic field should be considered. Hence, the intrinsic stability constants have to be evaluated under an electrostatic-free condition. 13 Although the conditional stability constants of a few metal-AA complexes have been evaluated, 8 the intrinsic stability constants have not been reported.In a previous study, 14 we determined some physicochemical parameters in terms of the AA molecule that were utilized for applying to the electrostatic model. In the present study, the conditional stability constants of the divalent metal-AA complexes were determined based on a modified Bjerrum's method at a variety of ionic strengths. Subsequently, the intrinsic stability constants were evaluated by extrapolating the conditional stability constants to the electrostatic-free condition involving the electrostatic model. ExperimentalThe AA was purchased from Nacalai Tesque INC (Kyoto, Japan). Sodium nitrate (99% purity, Nacalai Tesque INC) was used to adjust the ionic strength (I). Cu(NO3)2·3H2O, Pb(NO3)2 (Wako Pure Chemicals, Osaka, Japan) and Ni(NO3)2·6H2O (Nacalai Tesque INC) were used to prepare the stock solutions of metal ions. The concentrations of the stock solutions were determined by ICP-AES (SPS 3000S type, Seiko Denshi). The standard solutions for Cu 2+ , Ni 2+ and Pb 2+ (1000 ppm) were purchased from Nacalai Tesque INC. The stock solutions of metal ions were freshly prepared in each experiment.To 50 mg of AA in a 100-ml ground-glass stoppered Erlenmeyer flask, a 30-ml volume of CO2-free water containing metal ions (1 mM) and NaNO3 (I = 0.01 -1.0) was added. After shaking for 12 h under an argon atmosphere at 25˚C, the mixture was placed into a polyethylene titration cell. Titration was carried out under an argon atmosphere, and the temperature of the cell was maintained at 25.00 ± 0.19˚C. An AUT-501 auto-titration system (TOA Electronics Ltd., Tokyo) was used by connecting with a GST-5311-type glass electrode (TOA Electronics Ltd.). The titrant was a 0.01 M NaOH aqueous solu...

Quantifying Pb and Cd Complexation by Alginates and the Role of Metal Binding on Macromolecular Aggregation

“…This observation is in good agreement with the known higher affinity of Pb 2+ for the AA carboxyl groups and is consistent with the existing literature on alginatecontaining biomasses. 6,8,12,44 It is difficult to compare the above stability constants with literature results, given that the experiments were not performed using the same alginates, nor in the same intervals of pH, ionic strength, or metal-to-ligand ratios. In particular, the metal binding properties of the AA are very dependent on its guluronic acid content.…”

“…Nonetheless, few studies have attempted to separate effects due to intrinsic chemical binding from the electrostatic or macromolecular (i.e., conformational) contributions. For the most part, metal binding studies have been performed at high metal concentrations (typically mM, e.g., ref ), in acidic pH (typically between pH 2.0 and pH 4.0, e.g., refs , ), and at high alginate concentrations (typically between 1 and 5 g L -1 , e.g., refs , , ). Quantitative information corresponding to low metal-to-alginate ratios, low Pb and Cd concentrations, circumneutral pH values, and lower ionic strengths are missing from the literature, despite their obvious environmental and biological relevance.…”

Plutonium (IV) complexation with citric and alginic acids at low PuTconcentrations

“…Due to non-integer slopes, a number of studies have used a modeling approach based on the assumption of formation of complexes with mixed stoichiometries. For example, Fukushima et al [18] show that divalent metal ions bind with alginic acid in terms of a mixture of 1 : 1 and 1 : 2 metal-alginic acid complexes. Since the 1 : 1 approach was not appropriate for our system, a non-linear regression was performed for the model ligands and Pu(IV) based on the formation of 1 : 1, 1 : 2 and 1 : 3 complexes using Eq.…”

“…Polysaccharides are highly dispersed and complex molecules with a substantial polyfunctional and polyelectroytic behavior [17,18]. The most common polysaccharides are cellulose and starch, derived from plants [13].…”

Binding of Pu(IV) to galacturonic acid and extracellular polymeric substances (EPS) fromShewanella putrefaciens, Clostridium sp. andPseudomonas fluorescens

“…Polysaccharides are highly dispersed and complex molecules with a substantial polyfunctional and polyelectroytic behavior [27,28]. The most common polysaccharides are cellulose and starch, derived from plants [29].…”