Iron Ions Contaminate Cation Exchange Resins

Iron ion contamination of cation exchange resins. Cation exchange resins are susceptible to iron ion contamination, especially in water treatment systems using well water as a source. Iron ion contamination of the resin occurs in three different ways:

① If iron ions appear as colloidal suspensions, they can leak through the filter and contaminate the cation exchange resin.

② Iron is exchanged onto the resin as ferrous (Fe2+) ions, which are then oxidized to ferric (Fe3+) ions, forming a gel-like, water-insoluble iron hydroxide on the resin particles.

③ Ferrous (Fe2+) ions exchanged onto the resin may be directly converted to ferric (Fe3+) ions on the resin's exchange groups, but this cannot be completely removed during regeneration and remains in the resin.

If the first scenario occurs, backwashing can be used to remove the accumulated colloidal suspensions from the resin bed. If iron ions accumulate throughout the resin bed, the resin can be treated with sodium sulfite or sodium bisulfite. This reduces the ferric (Fe3+) ions to the more soluble ferrous (Fe2+) ions, which have a lower affinity for the resin. The degree of iron and other ion contamination in resin can be determined by burning the resin or analyzing the iron content of the wet resin. The contaminated resin is cleaned with deionized water, soaked in a 10% saline solution for 30 minutes, the saline solution is discarded, and the resin is cleaned again with deionized water. Approximately one-tenth of the resin sample is taken and placed in a test tube. Then, twice the resin volume of 6 mol/L hydrochloric acid solution is added, the tube is sealed and shaken for 15 minutes, and the acid solution is poured into another clean test tube. One drop of saturated ammonium thiocyanate is added. The intensity of the Prussian blue color (from light blue to opaque brownish-black) indicates the severity of iron contamination in the resin.

It is worth noting that iron ions in water can form complex complexes with organic matter or silicon, and these complexes are negatively charged. These complexes can pass through cation exchange resins and contaminate the subsequent anion exchange resins.