The Influence of Acetate and Oxalate as Simple Organic Ligands on the Behavior of Palladium in Surface Environments

Abstract

Acetate and oxalate are simple organic ligands that occur naturally in surface waters, groundwaters, and soils; they may play a role in the transport of palladium in such environments. Mass transfer of Pd in surface environments has implications for geochemical exploration as well as the environmental impact of the platinum-group elements. The complexation of divalent palladium and acetate was studied via measurement of the solubility of amorphous Pd(OH) 2 in 1 molal NaClO 4 as a function of concentration of acetate (0.0001 to 0.1 molal) at 25°C. From these data, the predominant Pd 2+ acetate species was determined to be Pd(ac) 2 0 (where ac denotes the acetate anion), with a conditional stability-constant of log ␤ 2 * = 9.3 ± 0.3. Palladium oxalate complexes were investigated using UV-visible spectrophotometry. Experiments were conducted at 25°C as a function of concentration of NaCl (0.1 to 2.0 molal) and oxalate (0.1 to 100 millimolal). The results demonstrate that oxalate-bearing Pd species become important at relatively low concentrations of oxalate. At 1 molal NaCl, the existence of an isosbestic point indicates an equilibrium between one oxalate-bearing Pd species and PdCl 4 2-. At lower concentrations of NaCl, more than two Pd species may be present under the experimental conditions. Calculation of speciation as a function of pH in a model soil solution shows that Pd(ac) 2 0 never exceeds 40% total Pd, even in the absence of oxalate and at acetate concentrations as high as 5 millimolal. At lower concentrations of acetate or in the presence of oxalate, the proportion of Pd(ac) 2 0 is negligible. However, Pd(ox) 0 appears to be an important Pd 2+ species, predominating at low pH, with its field of predominance expanding with increasing concentration of oxalate.

C14–22 n-Alkanes in Soil from the Freetown Layered Intrusion, Sierra Leone: Products of Pt Catalytic Breakdown of Natural Longer Chain n-Alkanes?

“…Other Pt species may occur in natural systems. Evidence for the conditions under which inorganic ligands, such as Pt(OH), Pt(OH) 2 , and Pd(OH) 2 , and organic siderophiles can occur have been provided by experimental studies [40][41][42][43][44][45]. The initial study reported here did not find Pt(OH) or Pt(OH) 2 to have a strong effect on the structure of nC 31 (e.g., Figure 6g).…”

The influence of ethylenediamine tetra acetic acid (EDTA) on the transformation and solubility of metallic palladium and palladium(ii) oxide in the environment

“…17,18 Increasing concentrations of Pd in the environment over time reect the shis from Pt and Rh to Pd in the consumption of this element by the catalytic converter industry. [32][33][34][35][36][37][38] Few studies have quantitatively determined the chemical mass transfer of PGEs in the presence of complexing agents to better understand their potential behavior in the environment. 20 Accordingly, it is assumed that Pd is also emitted mainly in particulate form from catalytic converters.…”

“…Humic acids, as well as simple organic analogues like acetate, salicylate, and amino acids, have been shown to form complexes with PGEs and enhance their mobility. [32][33][34][35][36][37][38] Few studies have quantitatively determined the chemical mass transfer of PGEs in the presence of complexing agents to better understand their potential behavior in the environment. 28,[39][40][41] For instance, Wood and Van Middlesworth 38 conducted experiments with amorphous Pd(OH) 2 and acetate solutions with concentrations ranging from 0.0001 to 0.1 mol kg À1 and determined a stability constant of log b 2 ¼ 9.3 AE 0.3 for Pd 2+ .…”

Extra- and intra-cellular accumulation of platinum group elements by the marine microalga, Chlorella stigmatophora

“…The inorganic speciation of Pd(II), Pt(II) and Pt(IV) in sea water is dominated by chlorides and mixed hydroxychlorides (Gammons, 1996) ] w 10 À13 at equilibrium; . Because of their strong interactions with soft ligands, cations of both metals are predicted to complex readily with natural organic ligands and surface functional groups (Wood, 1990;Wood and Middlesworth, 2004). However, the slow rearrangements in the coordination spheres of Pt(II) and Pt(IV) mean that reactions involving this metal are kinetically hindered compared with those involving Pd .…”