**1.2. Principles of NOM and arsenic removal using strongly basic ion-exchanging resins**

Ion exchange is a process used to remove ionic components dissolved in water, the process being based on their high affinity to bind to the ion-exchange resin. As a consequence, ions are captured and exchanged for the resin's ions. When the resin capacity for exchanging particular ions is reached (exhaustion), it is usually regenerated, so that it can be used in repeated cycles. It is reported that the application of special macroporous strongly basic anionic resins is suitable for the removal of NOM from natural waters [25]. These resins are very effective in removing the water coloring matter (humins) and function in the regime of regeneration with sodium chloride [26, 27]. The main principle of NOM removal by these macroporous resins is based on the finding that more than 90% of NOM in groundwater are in fact weak acids represented mostly by molecules and also by dissociated anions and often cations of alkaline earth elements. Molecules of humic acids are large and complex, composed of benzene rings and other interconnected chemical structures. Every NOM molecule contains a number of carboxylic groups. Like carbonic acid, humic acids do not fully dissociate at a neutral pH. Depending on the number of carboxylic functional groups in their molecules, NOM can act as polyvalent species. These NOM properties make, in fact, the basis for their removal from the anionic resin by a saturated solution of sodium chloride. Strong base anionic macroporous exchange resins efficiently exchange chloride for all the anions present in raw water. The anions from water interact with the resin functional groups in dependence of their relative affinity, as well as the rate and dynamics of the ionic exchange. Under ideal conditions, when the resin is saturated, sulfate and anions of organic acids occur adsorb on the surface, while the other anions, such as nitrate, chloride and bicarbonate, are sorbed in the interior of the resin beads. In the initial cycle of ionic exchange, the resin plays the role of a dealkalizer and removes bicarbonates from the raw water. Also, the beginning of the working cycle is characterized by a decrease in the pH of the effluent, which often falls below pH 6. In the further course of the work cycle, the resin adsorbs anions that have a higher affinity than bicarbonate, which is released into the effluent. This is accompanied by a pH increase, which may attain a value that is higher than that of the raw water. In the further course of the working regime, chloride and nitrate are released and their places occupy anions of organic acids and sulfate.

Effect of Extremely High Specific Flow Rates on the Ion-Exchange Resin Sorption Characteristics http://dx.doi.org/10.5772/60586 77

**Figure 1.** Possible mechanism of reversible sorption of organic molecules on ion exchange resin

arsenic in groundwater may be due to anthropogenic activities. Namely, many insecticides and fungicides, wood-protecting agents, chemicals used in semiconductor production, and additives to various alloys and glasses contain arsenic compounds [21]. It is known that prolonged exposure of the human organism to small doses or short exposure to high doses of arsenic causes skin disease and serious disturbance of internal respiratory and digestive organs, blood circulation, and of the nervous system. Also, Ng et al. proved that arsenic is

The EU Drinking Water Directive from 1998 recommended that the maximum tolerable concentration (MTC) of arsenic in drinking water should be 10 μg/L [23]. Three years later, in its revision of the corresponding standard, the US EPA adopted also the same MTC value of 10 μg/L [24]. Because of the geological and mineralogical origin of arsenic in groundwater, its

**1.2. Principles of NOM and arsenic removal using strongly basic ion-exchanging resins**

Ion exchange is a process used to remove ionic components dissolved in water, the process being based on their high affinity to bind to the ion-exchange resin. As a consequence, ions are captured and exchanged for the resin's ions. When the resin capacity for exchanging particular ions is reached (exhaustion), it is usually regenerated, so that it can be used in repeated cycles. It is reported that the application of special macroporous strongly basic anionic resins is suitable for the removal of NOM from natural waters [25]. These resins are very effective in removing the water coloring matter (humins) and function in the regime of regeneration with sodium chloride [26, 27]. The main principle of NOM removal by these macroporous resins is based on the finding that more than 90% of NOM in groundwater are in fact weak acids represented mostly by molecules and also by dissociated anions and often cations of alkaline earth elements. Molecules of humic acids are large and complex, composed of benzene rings and other interconnected chemical structures. Every NOM molecule contains a number of carboxylic groups. Like carbonic acid, humic acids do not fully dissociate at a neutral pH. Depending on the number of carboxylic functional groups in their molecules, NOM can act as polyvalent species. These NOM properties make, in fact, the basis for their removal from the anionic resin by a saturated solution of sodium chloride. Strong base anionic macroporous exchange resins efficiently exchange chloride for all the anions present in raw water. The anions from water interact with the resin functional groups in dependence of their relative affinity, as well as the rate and dynamics of the ionic exchange. Under ideal conditions, when the resin is saturated, sulfate and anions of organic acids occur adsorb on the surface, while the other anions, such as nitrate, chloride and bicarbonate, are sorbed in the interior of the resin beads. In the initial cycle of ionic exchange, the resin plays the role of a dealkalizer and removes bicarbonates from the raw water. Also, the beginning of the working cycle is characterized by a decrease in the pH of the effluent, which often falls below pH 6. In the further course of the work cycle, the resin adsorbs anions that have a higher affinity than bicarbonate, which is released into the effluent. This is accompanied by a pH increase, which may attain a value that is higher than that of the raw water. In the further course of the working regime, chloride and

removal is a technologically very complicated and demanding operation.

nitrate are released and their places occupy anions of organic acids and sulfate.

carcinogenic [22].

76 Ion Exchange - Studies and Applications

Group of authors showed that in a complex solution containing humic acids and arsenic, complex compounds are formed between arsenic and carboxylic groups of humic acids [28]. By investigating this interaction, Warwick et al. found that the deprotonated functional groups of humic acids and arsenic form associates, depending on the pH, ionic strength, and arsenic concentration [29].

In the process of removal of organic ions, such as those of NOM using anion exchange resin, two sorption mechanisms may be involved, and these are ion exchange and physi‐ cal adsorption (Figure 1). Ionic exchange includes the transfer of the ions from the ionic exchanger and electrostatic interaction between the ions of functional groups, presented as quaternary ammonium ion-exchanging groups and participation of the carboxylic groups. This interaction is of donor-acceptor type. Quaternary ammonium groups [-N(CH3) 3+] are specific active groups which determine the intensity of the ionic exchange. Physical adsorption takes place as the van der Waals interaction between the non-polar (hydropho‐ bic) groups present in the NOM molecules and central structure of the polymer resin to which substituents are attached [30, 31].

The adsorption of arsenic ion on the surface of strongly basic ion exchanging resin may be presented as follows:

$$R-Cl + H\_2AsO\_4^- \rightarrow R-H\_2AsO\_4 + Cl^- \tag{1}$$

$$2\text{R}-\text{Cl} + \text{HAsO}\_4^{2-} \rightarrow \text{R}\_2-\text{HAsO}\_4 + 2\text{Cl}^-\tag{2}$$

Since the affinity of the acrylic resin with quaternary ammonium groups is higher to doubly charged ions than to the singly charged ones, the efficiency of the ionic exchange is higher at higher pH values, when doubly charged arsenate ions are dominant [32].

Removal of As(III) by ionic exchange is less effective, since at the pH < 9 it occurs in water in the form of the molecule H3AsO3 [20]. On the other hand, As(V) is present in the form of the anions H2AsO4 and HAsO4 2-, which makes the ionic exchange more probable. It is just the specific phenomenon of the occurrence of arsenous acid as an uncharged species that cannot be removed from water which allowed the development of procedures for distinguishing between arsenite and arsenate [33, 34]. Therefore, in order to remove arsenite from water, it has to be oxidized to arsenate. The regeneration of the ion-exchange resin proceeds according to the following reaction:

$$\mathrm{RH}\_{2}\mathrm{AsO}\_{4}^{-}+\mathrm{Cl}^{-}\rightarrow\mathrm{RCl}+\mathrm{H}\_{2}\mathrm{AsO}\_{4}^{-}\tag{3}$$

Regeneration can be done with HCl and NaCl. The use of HCl yields arsenic acid, H3AsO4, which has no influence on the equilibrium of ion exchange and the regeneration is more efficient [32]. The strongly basic resin that has been pretreated by anions, e.g. chloride, is capable of removing a wide spectrum of anions from water, depending on their relative affinities, shown by the following order [35]:

$$\text{CrO}\_4^{2-} \gg \text{SeO}\_4^{2-} \gg \text{SO}\_4^{2-} \gg \text{HSO}\_4^- \gg \text{NO}\_3^- \gg \text{Br}^- \text{HAsO}\_4^{2-} \gg \text{SeO}\_3^{2-} \gg \text{HSO}\_3^{3-} \gg \text{NO}\_2^- \gg \text{Cl}^-$$
