A study of the accumulation toxicity and determination of platinum group metals in plants

Abstract

A range of analytical techniques were investigated and compared for the determination of platinum group metals in plant tissues. These included ICP, DC arc AES and PlXE. Particular attention was paid to problems associated with determination by ETA AAS and INAA. PIXE was suitable as a routine method and gave results in good agreement with those from INAA. The ICP method was satisfactory only at relatively high concentrations.

The biological effects of platinum group metals on Eichhornia crassipes were studied. At an applied metal concentration of 10ppm, the relative toxicity was found to be: Pt2+ ≈ Pd2+ > Os 4+ ≈ Ru3+ > Ir3+ > Pt4+ " Rh3+. The metals were deposited mostly in plant roots though varying amounts were transported to the tops. Toxic symptoms of Pt 2+ complexes included the appearance of reddish-brown streaks in the leaves, whilst Rh3+ appeared to exhibit a tonic effect. In contrast, the grass Setaria verticillata was growth stimulated by Pt2+ at low levels.

Two complexed forms of platinum were investigated further because of the difference in relative toxicity. Platinum applied as the anti-tumour complex cis [Pt(NH₃)₂Cl₂] was found to be toxic at high levels. When applied at low levels some 47.9% of the platinum in the leaves was associated with alpha-cellulose and lignin; 16.1% was removed by the proteolytic enzyme pronase, and 20.8% found with water soluble pectates. A similar distribution was found in the floats of Eichhornia crassipes and in the plant roots the values were 35%, 3-5% and 14.2% respectively. In the roots however, a further 23.1% was removed with low molecular weight alcohol soluble materials and 12.0% with polar water soluble materials. The amino acid constituents of control and platinum treated plants are also presented.

When applied as Pt4+, platinum appeared relatively non-toxic. Analytical electron micrographs revealed Pt deposits concentrated in the epidermis of the root with lesser amounts extending up to the endodermis. XPS confirmed the deposits on the root surface as Pt and the binding energy suggests it is Pt4+. Ruthenium also has been detected in the epidermis of Ru treated root samples.