Problems related to hexavalent chromium have been attracting public attention recently. The problems are divided into two categories. The first one is hexavalent chromium generated from plating and chemical conversion processes. The other is hexavalent chromium remaining in chemical conversion coatings. In this volume, we will look into the first category.

(1) Constant effluent

(1) Reduction method

As in the case with cyanogen, it is necessary to detoxify a constant effluent and concentrated effluent solution containing hexavalent chromium. A constant effluent discharged from the chromate processing after chrome plating or zinc plating is generally treated by the reduction or ion-exchange method as described here.

1. Acidize effluent by adding sulfuric acid (pH2 to 3), and then add reducing agents such as sodium bisulfite to reduce hexavalent chromium (chromic acid) to trivalent chromium (chromium sulfate).
   4H₂CrO₄ + 6NaHSO₃ + 3H₂SO₄ → 2Cr₂(SO₄) ₃ + 3Na₂SO₄ + 10H₂O
2. When chromium sulfate, which is trivalent chromium, is neutralized (pH9) by adding alkali, it is precipitated as chromium hydroxide and you can separate it.
   Cr₂(SO₄) ₃ + 6NaOH → 2Cr(OH) ₃ + 3Na₂SO₄

(2) Ion-exchange method

This method adsorbs and removes anion (= chromic acid) using anion exchange resin. [Fig.1] is an example of the flow diagram.
The ion exchange resin used to adsorb chromic acid will eventually become saturated and no longer able to adsorb more ions. It is important to replace the resin before hexavalent chromium starts leaking.
The saturated ion-exchange resin will be collected by a chromic acid manufacturer, who regenerates the resin and recycles the eluted chromic acid. The regenerated ion-exchange resin is returned to the surface treatment plant.

(2) Concentrated effluent containing chromic acid

It is difficult for the plants to perform in-plant treatment of concentrated effluent containing chromic acid, such as waste chromium plating solution and waste chromate solution. Therefore, waste management system has been established where the chromic acid manufacturers collect such effluent and detoxify or recycle it.