Complexation of platinum, palladium and rhodium with inorganic ligands in the environment

Abstract

Platinum (Pt), palladium (Pd) and rhodium (Rh) are emitted by vehicle exhaust catalysts (VECs) and their concentrations have increased significantly in various environmental compartments, including airborne particulate matter, soil, roadside dust, vegetation, rivers and oceanic environments, over the last two decades as the use of VECs has increased. However, data on the chemical speciation of the platinum-group elements (PGEs) and their bioavailabilities are limited. In this paper, the thermodynamic computer model, HSC, has been used to predict the interactions of Pt, Pd and Rh with different inorganic ligands and to estimate the thermodynamic stability of these species in the environment. Eh-pH diagrams for the PGEs in aqueous systems under ambient conditions (25 C and 1 bar) in the presence of Cl, N and S species have been prepared. The results indicate that Pt, Pd and Rh can form complexes with all of the inorganic ions studied, suggesting that they are capable of mobilizing the PGEs as aqueous complexes that can be transported easily in environmental and biological systems and that are able to enter the food chain. Hydroxide species can contribute to the transport of PGEs in oxidizing environments such as road-runoff waters, freshwater, seawater and soil solutions, whereas bisulphide complexes could transport Pt and Pd in reducing environments. Ammonia species appear to be significant under near-neutral to basic oxidizing conditions. Chloride species are likely to be important under oxidizing, acidic and saline environments such as seawater and road-runoff waters in snowmelt conditions. Mixed ammonia-chloride species may also contribute to the transport of Pt and Pd in highly saline solutions.

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).…”

Immobilisation of Platinum by Cupriavidus metallidurans

“…Potential Pt(II) complexes present in the reactions systems included Pt(II), Pt(OH)n −2+n and PtCln −2+n . Likely Pt(IV) compounds included Pt(IV), Pt(OH)n −4+n and PtCln −4+n , but very little data exists for these species [29,30,34,37,53]. EXAFS confirmed platinum-chloride to be the dominant aqueous species prior to (A) XANES spectra of bacteria reacted with 5000 µM Pt(II) and Pt(IV) solutions for up to 28 days.…”

“…Under surficial weathering conditions, the predicted oxidation state of platinum in aqueous solutions is Pt(II) or Pt(IV) [29]. Based on thermodynamic calculations, platinum is likely to occur as free aqueous ions only in oxidising and acidic environments, therefore, under most surficial conditions, platinum occurs as stable colloids/nanoparticles or is bound by coordination complexes to organic and inorganic ligands [29][30][31][32].…”

“…Platinum commonly forms complexes with chlorides, hydroxides and thiosulphates, but amorphous organo-platinum and platinum-hydroxide colloids also occur [29,30,[32][33][34][35][36]. Aqueous platinum is transported in the surficial environment until conditions favour its (biogeo)chemical transformation and/or chemical precipitation.…”

Comparison of Neural Networks and Regression Time Series in Estimating the Development of the Afternoon Price of Palladium on the New York Stock Exchange

“…Palladium together with rhodium, ruthenium, osmium, iridium and platinum forms a group of elements referred to as platinum-group metals. According to Colombo et al (2008), palladium is a shiny, silver-white metal with a cubic structure. At normal temperatures, this element is highly resistant to corrosion in air and to acids.…”