Irreversible uptake of palladium from aqueous systems using l-cysteine methyl ester physisorbed on carbon black

Abstract

The uptake of palladium from aqueous systems by L-cysteine methyl ester-modified carbon black (CysOMe-CB) has been investigated. A solution containing 20 ppm PdCl2 could be reduced to below the limit of detection within 15 min by using 2 mg carbon black per mL water. The Pd(II) was spontaneously reduced to Pd(0) on the carbon black (CB) surface, likely due to the known reducing nature of oxygen-containing carbonaceous materials, although the possible concurrent oxidation of cysteine to cystine is discussed. The presence of Pd(0) was demonstrated by X-ray Photoelectron Spectroscopy (XPS), with ca. 6 sulfur atoms detected for every Pd atom. CysOMe was predominately present on the CB as physisorbed material, and leaching of CysOMe back into solution was determined to be minor (ca. 0.4 mol% per mL water per mg CB over 24 hours). Attempted removal of Pd from the CB surface by extraction with hydrochloric acid or sodium acetate solutions was found to labilise other metal ions already present in the CB subsurface, and accumulate them at the CB surface.