Thanks to a slew of recent media reports, the controversy surrounding chromium-6 in drinking water has once again been brought to the forefront of public debate. Let’s review the scientific data and examine some ways water treatment professionals can be sure to protect their customers.

• The treatment of wastewater or groundwater that has been contaminated by the chromate ion can be accomplished by the use of anion resins. Success has been achieved by the use of strong or weak base anion resins, or selective anion resins.

• Cr(VI) is a problematic environmental pollutant because it is a strong oxidant and much more toxic than Cr(III). Cr(VI) exists as the chromate ion in basic solutions and as the dichromate ion in acidic solutions.

• Type 1 anion resin had been used in the past for the treatment of cooling-tower blowdown. The resin is regenerated with brine at 6 or 7 pounds per cubic foot of resin plus 2 pounds per cubic foot of sodium hydroxide. Exchange capacities of up to 3 to 4 pounds per cubic foot can be attained, and the chromate eluted off the resin can be recovered and reused.

• A Type 1 anion resin in the chloride form would collect the chromate anion on the exchanger, from which it could be stripped with NaCl in a concentrated form for disposal or reclamation. The same resin, but regenerated with NaOH, would also collect the chromate anion on the bed, but would yield the bases of all the salts present. Passage of this effluent through a cation resin in the hydrogen form would yield deionized water.

• A macroporous type 1 anion resin capacity for Cr⁺⁶ is somewhere between 0 and 3 pounds per cubic foot (average 1.5 pounds per cubic foot) depending on what else is in the water. The average of 1.5 pounds per cubic foot is based on 20 parts per million (ppm) influent chromate and 1 ppm effluent with less than 100 ppm sulfate present and a pH around 5. The maximum capacity of 3 pounds per cubic foot is achieved by operating at pH 4.5 with a primary and a polisher such that the primary leakage can go up to 50 percent of inlet level.

• Weak-base anion resins can exhibit high chromate capacities and low chromate leakages and are more resistant to oxidation. They are economically regenerated with 3 to 5 bed volumes of 4 percent NaOH, but may require an acid conditioning step before resuming service. The complicated regener.ation scheme and the handling and disposal of these regenerant streams must be taken into account when considering this resin.

• The obvious drawback to using one of the above resins is the problem of how to dispose of the spent regenerant. Plating operations may have conventional wastewater treatment equipment available to precipitate the accumulated chrome from the regenerant wastewater. Groundwater treatment applications may prefer to regenerate offsite at a resource recovery facility, or use a higher capacity selective resin and simply dispose of the resin.

• A specialty resin has been developed with the capability to selectivity remove hexavalent chrome from polluted groundwaters, provided the pH is adjusted to below 6.5. If the pH is too high, the removal is not good. Capacities as high as 6 or 7 pounds of chromate per cubic foot of resin are possible with this resin. It is typically used as a nonregenerable media for groundwater remediation and trace chrome removal.

• As effluent discharge regulations become more stringent, the ion exchange method of chromate removal using a selective, high-capacity, single-use resin becomes more attractive. Chromate pollution over the years from plating operations and corrosion control has contaminated many groundwater sources. A selective chromate resin is an economical remediation technique in many of these applications.