REGENERATION OF HEAVY METAL EXHAUSTED CATION EXCHANGE RESIN WITH RECOVERABLE CHELATING AGENT

Ion exchange is an important process for removal of heavy metals from metal plating and metal finishing wastewaters. Use of ion exchange has been limited, due to high costs for regenerant and problems with disposal of regenerants. The studies investigated a process for recovery of a chelating agent regenerant for a heavy metal exhausted cation exchanger. Initial exchange studies evaluated chelating agents (EDTA, NTA, and Cit) for regeneration of a Cu, Zn, Ni, or mixture of metals for an exhausted exchanger. Continued studies dealt with recovery of the chelating agents from regenerants via hydroxide or sulfide precipitation or electromembrane procedures. Final studies involved the overall process of regeneration, recovery, and reuse of chelating agents as regenerants. The optimum pH range for all chelating agent regenerant solutions with a strong cation exchange resin, Dowex 50W-X10, was 8 to 9. EDTA and Cit were better than NTA, because the number of Na(\u27+) ions available for exchange per mole was greater than for NTA. Cit had another advantage because of its lower molecular weight. A Cu exhausted strong acid cation exchanger required 375 g (0.91 mole) of Na(,3)-EDTA/liter with a 55% (1.10 mole Cu removed/mole EDTA) chemical efficiency, or 317 g (1.35 mole) of Na(,2)-NTA/liter with a 49% (0.73 mole Cu removed/mole NTA) chemical efficiency, or 283 g (0.96 mole) of sodium citrate/liter with a 68% (1.02 mole Cu removed/mole Cit) chemical efficiency to achieve 90% regeneration. Results of recovery studies showed that only Cit could be recovered effectively by metal hydroxide precipitation. Greater than 99% recovery of Cit and associated heavy metal could be attained at a pH of 12.5 for Cu- or Ni-Cit complexes and between pH 11 to 12 for Zn-Cit complex. About 90% or more EDTA, NTA, or Cit and associated Cu or Zn could be recovered by sulfide precipitation or electromembrane separation. Recovery of Ni by NiS precipitation or electrodeposition was not possible. Cyclic reuse of Cit (via hydroxide precipitation) or EDTA (via sulfide precipitation or electromembrane separation) for a Cu exhausted strong cation exchanger was effective beyond five cycles. The studies showed that cyclic operation of an ion exchanger with reuse of a chelating agent regenerant was technically feasible for Cu and Zn but not Ni