Ion Exchange for Heavy Metal Removal
"Basic Concepts" section reprinted from the EPA SUMMARY REPORT "Control and Treatment Technology for the Metal Finishing Industry – Ion Exchange," EPA 625/08-81-007.
Ion exchange is a reversible chemical reaction wherein an ion (an atom or molecule that has lost or gained an electron and thus acquired an electrical charge) from a wastewater solution is exchanged for a similarly charged ion attached to an immobile solid particle. These solid ion exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins. The synthetic organic resins are the predominant type used today because their characteristics can be tailored to specific applications.
An organic ion exchange resin is composed of high molecular weight polyelectrolytes that can exchange their mobile ions for ions of a similar charge from the wastewater. Each resin has a distinct number of mobile ion sites that set the maximum quantity of exchanges per unit of resin.
Most plating process water is used to cleanse the surface of the parts after each process bath. To maintain quality standards, the level of dissolved solids in the rinse water must be regulated. Fresh water or recycled water is added to the rinse tank accomplishes this purpose, and the overflow water is treated to remove pollutants and then is discharged or reduced in volume and recycled using a vacuum distillation process. As the metal salts, acids, and bases used in metal finishing are primarily inorganic compounds, they are ionized in water and could be removed by contact with ion exchange resins. In a water deionization process, the resins exchange hydrogen ions (H+) for the positively charged ions (such as nickel, copper, and sodium) and hydroxyl ions (OH-) for negatively charged sulfates, chromates and chlorides. Because the quantity of H+ and OH- ions is balanced, the result of the ion exchange treatment is relatively pure, neutral water.
Ion exchange reactions are stoichiometric and reversible, and in that way they are similar to other solution phase reactions. For example:
NiSO4 + Ca(OH)2 = Ni(OH)2 + CaSO4
In this reaction, the nickel ions of the nickel sulfate (NiSO4) are exchanged for the calcium ions of the calcium hydroxide Ca(OH)2 molecule. Similarly, a resin with hydrogen ions available for exchange will exchange those ions for nickel ions from the wastewater solution. The reaction can be written as follows:
(R–SO3H)2 + NiSO4 = (R-SO3)2Ni + H2SO4
R indicates the organic portion of the resin and SO3 is the immobile portion of the ion active group. Two resin sites are needed for nickel ions with a plus 2 valence (Ni+2). Trivalent ferric ions would require three resin sites.
As shown, the ion exchange reaction is reversible. The degree the reaction proceeds to the right will depend on the resins preference or selectivity for nickel ions compared with its preference for hydrogen ions. The selectivity of a resin for a given ion is measured by the selectivity coefficient K, which in its simplest form for the reaction
R–A+ + B+ = R–B+ + A+
is expressed as: K = ( concentration of B+ in resin/concentration of A+ in resin) x (concentration of A+ in solution/concentration of B+ in solution.)
The selectivity coefficient expresses the relative distribution of the ions when a resin in the A+ form is placed in a solution containing B+ ions. Table 1 gives the selectivity's of strong acid and strong base ion exchange resins for various ionic compounds. It should be pointed out that the selectivity coefficient is not constant but varies with changes in solution conditions. It does provide a means of determining what to expect when various ions are involved. As indicated in Table 1, strong acid resins have a preference for nickel over hydrogen. Despite this preference, the resin can be converted back to the hydrogen form by contact with a concentrated solution of sulfuric acid (H2SO4):
(R–SO3)2Ni + H2SO4 –> 2(R-SO3H) + NiSO4
This step is known as regeneration. In general terms, the higher the preference a resin exhibits for a particular ion, the greater the exchange efficiency in terms of resin capacity for removal of that ion from the wastewater solution. Greater preference for a particular ion, however, will result in increased consumption of chemicals for regeneration.
Resins currently available exhibit a range of selectivity's and thus have broad application. As an example, for a strong acid resin the relative preference for divalent calcium ions (Ca+2) over divalent copper ions (Cu+2) is approximately 1.5 to 1. For a heavy metal selective resin, the preference is reversed and favors copper by a ratio of 2,300 to 1.
Table 1: Selectivity of ion exchange resins in order of decreasing preference.
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