Color Removal by Ion Exchange
By C.F. ‘Chubb’ Michaud, CWS-VI and Francis Boodoo
Identifying the color problem
The presence of color in water can stem from several sources. Although often termed tannins (organics), color can have inorganic origins as well, such as iron or manganese. Simple field tests can confirm Fe++ or Mn++ and subsequent removal is rarely done by ion exchange. If, however, the color problem is determined to be organic, ion exchange provides an excellent and easy solution.
Natural organic matter (NOM) comes largely from decaying vegetation and can be completely soluble or suspended (colloidal or particulate). Soluble organics are generally acidic in nature (much like the extracts from tea or coffee) and may be polar or non-polar. These molecules are very large (Figure 1) and complex and may be hydrophilic (water loving) or hydrophobic (water hating). The unique nature of organics (being polar, non-polar or particulate) may explain why attempts to remove them often fail. Not all organic scavengers remove all organics. Some work better on some waters, which leads to regional treatment technique preferences (but not universal). Understanding the nature of the organic helps in choosing a proper treatment.
Organics in water
The term tannin refers to a family of organic acid plant extracts (fulvic, humic and tannic acids) that are weakly acidic and readily soluble. Tannins are polar (ionically charged) and can be removed by salt regenerated ion exchange quite effectively. Organics can also be non-polar (not charged) in nature, making them more hydrophobic. The parallel of hydrophobic is oleophilic (oil loving). Non-polar organics are best removed by adsorption. Treatment may include granular activated carbon (GAC) or again, salt regenerated anion resin.
During the degradation process, vegetable matter may break down to an insoluble particulate which can be removed by normal filtration, or to a colloidal (sub-micron) suspended particle (which cannot). Colloidal particles can be flocculated to form larger particles (for filtration). This might involve cumbersome equipment and chemical feeders. An alternative is to use a colloidal scavenger—a filter media capable of filtering submicron particles. Again, there are salt regenerated anion exchangers which have been designed for just that purpose.
Narrowing the treatment choices
Research done some years ago (reference lost) compared GAC, SBA (strong base) and WBA (weak base) anion resins for the reduction of organics in high-purity water production. It was found that each treatment reduced the organics by 30 to 50 percent but that they did not remove the same organics to the same degree. GAC works best on hydrophobic organics and ion exchange works best on hydrophilic. Colloidal removal may be done by either resin or carbon (if properly selected) but only the resin can be regenerated by a simple brine solution. Weak-base resins require a complex regeneration with caustic and would be ruled out for residential use. So, we are left to explore the pros and cons of GAC and salt-regenerated, strong-base anion exchange.
GAC has been used for de-colorization for over a century. The best carbons for that job are wood- or lignite-based because they generally have the larger-sized pores. Shell and coal carbons are not regarded as good color removers. Wood and lignite GACs work well but are unusually soft with low abrasion resistance. They also require regeneration (re-activation) with chemical (caustic or solvent) or heat (or both), which is impractical for home use. Nonetheless, these carbons would find utility with properly sized flow rates (three minute minimum EBCT or 2.5 gpm/cu ft) and annual change outs.
Anion exchange has been used for more than 50 years in the removal of color from potable water and many industrial processes. Resins are plastic adsorbents with a controllable balance of hydrophilic/hydrophobic properties and they are functionally reactive (ion exchangers for acids) and regenerable with salt. Resins can also be gel (continuous solid phase) or macroporous (both continuous solid and pore phases with a surface area equaling GAC). The pores can be very small or large enough to trap colloids (see Figure 2). During regeneration with brine, the resins shrink, squeezing out adsorbed hydrophobic organics. Chlorides push off the hydrophilic organics and the backwash and rinse steps dislodge trapped colloids.
Ion exchange resins are produced by suspending droplets of monomer (with agitation) in water and polymerizing that droplet while it is suspended to form a perfect plastic sphere (a bead). That bead can then be functionalized through a series of chemical reactions to give it anion exchange properties that allow for the exchange of acid groups (negatively charged ions) for chlorides (from salt regeneration). The reactions in Figure 3 show the backbone formation and the functionalizing steps of chloromethylation and amination.
If only monomer suspended in water is used, gel resin beads result. Despite their appearance as solids, the functionalized resins actually contain over 50 percent water, which becomes the transport medium for ion diffusion in and out of the bead. If the monomer is first mixed with a material that is soluble in the monomer (known as porogen) but not in the polymer as it forms, the result is a macroporous resin bead (see Figure 2).
Shake a few drops of water on a clean plastic surface and the water beads up. This is because of the hydrophobic nature of most plastics. There are, however, different base resin backbones that can be used. Some are more hydrophobic than others.
Styrene has been the workhorse backbone polymer for ion exchange resins since their invention in the 1940s. Styrene was readily polymerized, inexpensive and easily functionalized. Styrene is an aromatic compound containing a benzene ring and cross-linked with divinyl benzene (S/DVB), also containing a benzene ring. This makes the resin quite hydrophobic. As a result, organics love styrene. In fact, organics stick to styrene so well, they can foul it beyond recovery. It is very difficult to remove the organics from a styrene ion exchange resin using just salt brine regeneration. Organics migrate deeper and deeper into the bead matrix, entangling with the resin’s own chemistry. Although styrenic resins make excellent scavengers, they are not widely used for organic removal because of the difficulties in regenerating them.
Acrylic chemistry can also be used in the manufacture of anion resin. It is more expensive than styrene and is used by fewer resin manufacturers. Acrylates do not contain a benzene ring (although still cross-linked with DVB) and are far less hydrophobic. They hold organics less tenaciously but adequately and release them readily with brine regeneration. Acrylics are made in both gel and macro forms and have often been referred to as Type 1½ because they are intermediate to Type I and II styrenics in anion strength. Acrylic SBA resins have gained widespread use for both municipal and residential water treatment because their unique chemistry makes them ideal for organic and color removal and their ease of regeneration gives them excellent longevity.
Much work has been done in the use of anion exchange for the removal of organics from municipal water to avoid the formation of disinfection byproducts (DBPs), notably trihalomethanes (THMs). These are brought about by the reaction of chlorine with the organics. Municipalities have found it much more efficient to remove the organics prior to disinfection (with chlorine) than to remove toxic THMs after disinfection. The harmless precursors can be safely discharged along with the brine, whereas using an adsorbent to remove the DBPs could become a hazardous waste disposal issue. Anion resin can also provide five to eight years usable life compared to carbon that may need replacing after three to six months.
Municipal systems using SBA resins for organic removal are typically designed with 3.5 to five minutes EBCT and minimum bed depths of 36 inches. No polisher vessel is used but flow is usually split between several lead vessels in parallel so that one can be taken out for regeneration without shutting down the whole plant. Run volumes vary with the feed quality but have shown reductions of 70 to 90 percent for 30,000 gallons per cubic foot of anion resin before any significant leakage increase was measured. Municipalities have an advantage in that they can use harsher regeneration techniques (using caustic and flexible regeneration cycles). Because of continuous operation, they see the capacity break in three to six days. Experience tells us that for ‘salt only’ regeneration (no caustic), we should not let organics sit on resin that long. Some migrate into the interior of the resin over time and are held so tightly they will not come off. The result is a gradual fouling of the resin with permanent capacity loss.
Designing residential color removal systems
For residential uses, anion resin can be layered on top of softening resin to create a single tank mixed bed for both organic and hardness removal. This should be limited, however, to feeds with fairly low organic loads (not heavy color) and total hardness levels of less than 10 grains. If either the hardness or the organics are moderate to high (>10 grains hard or >1 ppm organics), separate beds should be used. For most applications, a one cubic foot unit should suffice. A typical capacity for organic scavengers is 2,000 mg/ft3. This means that if the level of organics is one ppm (one mg/L), then one cu. ft. of resin will treat 2,000 L of water (about 525 gallons). For the typical family, this means regenerating every other night with a dose of eight lbs. of NaCl. A better regeneration is more frequent with lower salt (every day with three lbs.).
There are advantages to pre-softening the water feeding an organic scavenger. First, the anion unit is regenerated with soft water (which avoids precipitation of calcium carbonate or calcium sulfate) and second, it allows the use of low levels of caustic in the brine tank (0.1 percent NaOH in with NaCl greatly improves the solubility of organics and aids in removing them from the spent resin). Brine makeup is also with soft water which minimizes the precipitation of lime in the brine tank if caustic is used.
In service, organics are picked up very rapidly by resin. However, regenerating them back off is a long and slow process. It isn’t the amount of brine used that is important, it is the length of the contact time. Using smaller injectors in the brining lengthens the contact time of the brine on the resin and improves the removal of the organics. Ideally, resins used for scavenging organics should be regenerated every night using low levels of salt (three lbs.) along with a pinch of caustic (0.1 percent) and a pulsed or paused regeneration technique. With most modern control valves, there are hidden addresses that can be used to close a solenoid valve midway through the brine draw. This effectively shuts down the unit, trapping the brine inside the resin for an extended period (30 to 50 minutes). Then another address reopens the solenoid, allowing the regeneration to proceed through the rinse and brine refill steps. There is as much cleaning done during the 30 minute soak period than would be done if brine was continually fed through the unit for an additional 30 minutes! That’s good to know. It minimizes brine expense and, of course, brine discharge. The timeline for this system might go as illustrated in Table 1.
Summary and conclusions
To address the removal of different forms of organic content in feedwater, one has to employ different forms of scavenger resins. Both macroporous and gel resins can remove both polar and nonpolar contaminants. However, the macroporous resin would be more highly crosslinked and exhibit higher selectivity and holding capacity. In addition, organics penetrate less deeply into the macro’s solid phase matrix, making regeneration quicker. Acrylic anions are more hydrophilic (or less hydrophobic) than styrenic anions and are the preferred choice for salt (only) regenerated residential units. If the organic problem is serious enough to treat, a one cu. ft. system should be considered. Don’t let the organics sit on the resin too long. It is best to regenerate with low salt doses every night than to use a heavy dose once a week. A pinch of caustic added to the brine tank with each bag of salt greatly improves the efficiency and life of scavenger resins.
Colloidal organics can only be removed with a colloidal scavenger resin. Large-pore, macroporous resins can be blended with other scavenger resins to gain this ability. There are some single-package products available that combine the properties of acrylic macros and large-pore styrenic macros and do a fine job in addressing all three forms of organics encountered.
About the authors
C.F. ‘Chubb’ Michaud (corresponding author) is the CEO and Technical Director of Systematix Company, Buena Park, Calif,, which he founded in 1982. An active member of the Water Quality Association, Michaud has been a member of its Board and of the Board of Governors and past Chair of the Commercial/Industrial Section. He is a Certified Water Specialist Level VI. He serves on the Board of Directors of the Pacific WQA and Chairs its Technical Committee. A founding member of WC&P’s Technical Review Committee, Michaud has authored or presented over 100 technical publications and papers. He can be reached at Systematix Inc., 6902 Aragon Circle, Buena Park CA 90620; telephone (714) 522-5453 or via email at cmichaud@systematixUSA.com.
Francis Boodoo, Technical Sales Manager for The Purolite Company, Bala Cynwyd, Pa., has over 25 years’ experience in industrial, commercial and potable water treatment. Boodoo holds a Bachelor’s Degree in business administration. Reach him at Purolite, 150 Monument Road, Bala Cynwyd, PA 29004; telephone (610) 668-9090; fax (610) 668-8139 or email him at firstname.lastname@example.org.
About the products
The resins referred to in this article are available as: Styrenic Gel: Purolite A400, A300; Styrenic Macro: Purolite A500, A500P, A510; Acrylic Gel: Purolite A850; Acrylic Macro: Purolite A860; Large Pore Macro: Purolite A501P and Combination: TANEX.
About the company
Purolite® manufactures an extensive range of resins covering the full scope of technology platforms, functionality as well as different resin structures. With market-leading manufacturing capability, broad product range and record of innovative research, Purolite has a unique commitment to the food and beverage, hydrometallurgical, metals finishing, chemical and petrochemical, pharmaceutical, sugar and sweeteners, ground and potable water, nuclear, softening and industrial water, semiconducter, power and a host of other industries. Contact the company for information on these and other resins at www.puroliteUSA.com.
Francis Boodoo, Technical Sales Manager for The Purolite Company, Bala Cynwyd, Pa., has over 25 years’ experience in industrial, commercial and potable water treatment. Boodoo holds a Bachelor’s degree in chemical engineering from the University of the West Indies and a Master’s degree in business administration from Colorado State University. Reach him at Purolite, 150 Monument Road, Bala Cynwyd, PA 29004; telephone (610) 668- 9090; fax (610) 668-8139 or email him at email@example.com.