Effects of cyanide and polyphosphates on adsorption of metals from simulated and real mixed-metal plating wastes

Abstract

This study evaluated the effects of chromate, cyanide, pyrophosphate and tripolyphosphate on the adsorption of a suite of metals (Cd, Cu, Cr, Zn, Ni and Pb) onto iron oxyhydroxide. All metals were present simultaneously, along with one of the anions. In the absence of complexing agents, adsorption of the strongest sorbing metals (Pb, Cu, Cr) slightly inhibited that of the remaining metals when all were present at concentrations of 10 −4 M and 10 −3 M total iron was available as the adsorbent. The presence of up to 10 −3 M CrO 4 had no significant effect on removal of the cationic metals from solution. Cyanide strongly inhibited sorption of Ni and Cu, but did not affect the other metals. These effects can be explained based on changes in metal speciation when the CN was added. Under the experimental conditions, CN formed strong soluble complexes with Ni. Also, reduction of Cu 2+ to Cu + by CN, and subsequent formation of Cu +-CN complexes, was thermodynamically favorable. The remaining metals in the system did not form strong complexes under the experimental conditions and were therefore unaffected. At low concentration, pyro- and tripolyphosphate enhanced sorption of all the metals, but at high concentration they led to a qualitative change in the metal sorption behavior. Whereas in the absence of the phosphate ligands, metal sorption is strongly pH dependent, at high polyphosphate concentration, this dependence almost disappears. Simultaneously, some of the Fe adsorbent dissolves as Fe-P complexes. The changes in metal behavior with increasing ligand concentration reflect the the totality of interactions in the system, which includes complexation, adsorption of free metal and complexes, iron dissolution and changes in the surface charge associated with the various sorption reactions. The effects of metal, chromate and CN concentration do not appear to be major impediments to the utilization of adsorption as a treatment process. The effects of P-containing ligands are more complicated: they may either increase or decrease metal sorption depending on the particular water under study. Further study to elucidate the interactions in these latter systems appears warranted.